Actinic Prurigo
Actinic Prurigo is a sunlight induced itchy, pimply, idiopathic photodermatoses. It is uncommon in Caucasian populations with preponderance in native populations in North, Central and South America. It is more frequent in females by a factor of 2 – 4 and has a childhood onset, with a mean age at presentation of less than 10 years.
Actinic Prurigo is triggered by UV radiation in genetically susceptible individuals. Some researchers have proposed that it is an autoimmune disorder, the putative antigen being an epidermal protein which is transformed by UV exposure. Family studies have indicated that the disorder has a strong association with the HLA class II gene DRB1*04:07, which is found in 60 – 70% of patients compared to 4 – 8% of DR4 controls. HLA-DRB1*04:01 is the next most prevalent gene associated with Actinic Prurigo, found in approximately 20% of patients.
Actinic Prurigo presents with symptoms very similar to polymorphous light eruption (PLE), which is another sunlight induced skin irritation. Genetic testing helps to distinguish the two as PLE has no HLA association. Care is needed in interpreting the results though as a small number of patients do present with both Actinic Prurigo and PLE.
Addison’s Disease
Addison’s disease is a disease of the Adrenal Cortex caused by autoimmune destruction of the Adrenal gland with fibrosis and mononuclear cell infiltration. Symptoms become evident when more than 90% of the Adrenal gland is destroyed. Addison’s disease is characterised by increased production of adrenocorticotropic hormone (ACTH) and adrenal insufficiency with reduced production of corticosteroids and androgens. Patients present with hypotension, weakness, fatigue, light headedness when standing up, anorexia, nausea, salt craving and increased melanin pigmentation of the skin. Autoantibodies to 21-hydroxylase are present in 90% of cases.
Family studies have shown that Addison’s disease has a genetic component with HLA class II the most strongly associated genetic region. Addison’s disease is associated with DRB1*03:01-DQB1*02 (DR17, DQ2) and DRB1*04-DQB1*03:02 (DR4, DQ8). The most strongly associated DRB1*04 allele is DRB1*04:04. The major histocompatibility complex class I related chain – A (MICA) is an additional risk factor. MICA genes are highly polymorphic with over 107 alleles described. MICA is a ligand for the Natural Killer (NK) cell receptor NKG2D which is important for thymic maturation of T cells.
Autoantibody testing for anti-21-hydroxylase is more diagnostic in Addison’s disease than genetic testing. Genetic testing does however contribute to a better understanding of the aaetiology of the disease.
Ankylosing Spondylitis
Ankylosing Spondylitis (AS) is one of the major forms of chronic inflammatory arthritis and is the prototypical example of the spondyloarthropathies, a group of chronic autoimmune joint diseases. Ankylosing Spondylitis has a global distribution, though rarer in Africans. It is characterised by arthritis affecting the spine and pelvis, specifically the sacroiliac joint, initially causing pain and reversible stiffness (stiffness in the mornings that goes away later in the day with exercise) but in a proportion of cases leading to progressive joint fusion and irreversible stiffness and deformity.
Twin studies have confirmed that susceptibility to AS is genetically determined. There is a strong association with the HLA class I molecule HLA-B27, found in upwards of 90% of patients. Association of AS to B27 is amongst the strongest genetic associations with a common disease, although the mechanism of action remains uncertain. Ankylosing Spondylitis is thought to be triggered by exposure to a common environmental pathogen. Proposed mechanisms include the ‘arthritogenic peptide’, ‘molecular mimicry’ and endoplasmic reticulum stress due to B27 misfolding and accumulation. The arthritogenic peptide theory proposes that disease results from an HLA-B27 restricted cytotoxic T cell response to a peptide or peptides found only in joints and other affected tissues. The molecular mimicry theory proposes that some invading pathogens share antigenic determinants with native cell surface antigens in the joint resulting in an autoimmune response.
Different B27 alleles have different strengths of association with AS, making genetic testing useful over and above serological testing. HLA-B*27:02 and B*27:05 are strongly associated. Until recently B*27:06 and B*27:09 were thought to be protective but a number of AS cases have now been reported in patients carrying these alleles, making these alleles protective only relative to the strongly associated alleles.
Family studies suggest that less than 50% of the overall genetic risk is due to HLA-B27. HLA-B27 is found in 8 – 10% of the population with only a minority of carriers going on to develop the disease. It is likely that other genes both within and outside the MHC are involved. Other MHC genes which have shown some association in studies include HLA-B*40:01, B*52 and B*38. One haplotype study looking at the HLA-B – DRB1 haplotype has suggested the existence of non B27 genes in AS carried by both B27 positive and B27 negative individuals though the actual association was not identified. A number of non HLA genes have been shown, through genome wide association studies, to be associated with AS. These include the Interleukin-23 Receptor (IL23R) and the protein cleaving enzyme Endoplasmic Reticulum Aminopeptidase 1 (ERAP1).
The strong association of HLA-B27 with AS makes B27 testing a useful component of the diagnostic work up which includes a physical examination, use of X rays MRI and a check for family history of AS. Waiting for a patient to fulfil all the classification criteria for a diagnosis of AS may been too late in that the damage to joints would already have happened. The value of B27 testing is that it allows a presumptive diagnosis and early treatment in patients showing some of the symptoms.
Behcets
Behcet’s disease is a systemic inflammatory vasculitis, characterised by multiple courses of remissions and relapse of oral ulcers, genital ulcers, skin lesions and ocular lesions. It can affect the arteries and veins of almost any system and has been shown to involve the gastrointestinal and neurological systems. It was first described by the Turkish dermatologist Dr Behcet in the 1930 and is endemic in Turkey and other parts of the old ‘Silk Route’ through Europe and Asia. The cause of Behcet’s is unknown but it is believed to be triggered by an infectious or environmental agent in a genetically predisposed individual. The most generally accepted theory is that the pathogen derived antigen ‘Heat Shock Protein 65 (HSP65)’, which has a high sequence homology with the human HSP60 results in cross-reactivity which leads to an autoimmune response.
The HLA class I molecule HLA-B51 is the most strongly associated risk factor and has been shown to be associated with Behcet’s in Turkish and Asian patients though the association in Caucasian patients is much weaker. This has lead to the search for other genes which may be the true disease markers. One study has found a potential association with HLA-B*57:01. Other MHC genes which may be involved include MICA, though this is in linkage disequilibrium with HLA-B51 and the TNF genes. Non MHC genetic systems which have been proposed to be involved in Behcet’s include the interleukin-1 (IL-1) gene and mutations in the Mediterranean fever gene (MEFV).
Testing for HLA-B51 is a useful, though not a diagnostic tool for Behcet’s as the vast majority of HLAB51 carriers do not develop Behcet’s. Diagnosis is instead based on the recurrence of symptoms including oral ulcers, genital ulcers, skin lesions and ocular lesions. Treatment options depend on organs involved. Available treatments include corticosteroids, azathioprine, Cyclophosphamide, cyclosporine A and anti-TNF agents.
Birdshot Chorioretinopathy
Birdshot Chorioretinopathy is a rare form of bilateral posterior Uveitis, accounting for between 1 – 3% of all Uveitis cases. A slight female predominance has been reported. The disease is uncommon in children. Birdshot Chorioretinopathy causes severe, progressive inflammation of both the choroid and the retina. The name of the disease stems from the hypopigmentation pattern of the lesion on the retina, which resembles the impacts from a shotgun. Symptoms of Birdshot Chorioretinopathy include retinal vasculitis, particulate matter in the vitreous or vitreous inflammation, macular oedema, flashing lights in the eyes, night blindness and loss of colour vision. A frequent prognosis is complete loss of visual acuity.
Birdshot Chorioretinopathy is the disease with the strongest association to a HLA class I antigen, with more than 95% of patients carrying the HLA-A29 antigen. The relative risk of HLA-A29 carriers developing Birdshot Chorioretinopathy has been estimated to be between 50 and 250. HLA-A*29:02, which is the most frequent A29 allele in the Caucasian population is also the allele most frequently associated with Birdshot Chorioretinopathy in Caucasians. The disease has however been observed in HLA*29:01 Caucasian patients. Birdshot Chorioretinopathy is extremely rare in Asia where the most frequent HLA-A29 allele is A*29:01. The mechanism by which HLA-A29 confers susceptibility to Birdshot Chorioretinopathy is unknown. Proposals include antigen specific molecular mimicry following an infection or a role for retinal S-Antigen.
The presence of HLA-A*29 alone is not sufficient for a diagnosis of Birdshot Chorioretinopathy, as there are many cases of patients who do not carry HLA-A*29. Indeed the prevalence of HLA-A29 in the Caucasian population (around 7%) is far higher than the disease frequency and it is likely, as with many HLA disease associations, that other factors are involved. Nonetheless the strong association suggests that genetic testing for HLA-A*29 and A*29 alleles, over serological testing, is useful as a supportive finding as part of the diagnosis.
Coeliac Disease
Coeliac Overview
Coeliac disease is an inflammatory disorder of the small intestine with an autoimmune component and strong heritability. It is characterised by diarrhoea, abdominal distension, poor weight gain and short stature. Coeliac disease is a lifelong condition, with the only effective treatment being complete exclusion of gluten from diet. This is effective in over 95% of cases.
Gluten is a dietary protein found in wheat, barley and rye. Gluten peptides pass through the epithelial barrier of the small intestine into the lamina propria, where they are deaminated by the enzyme transglutaminase to give negatively charged gluten peptides. In genetically predisposed individual, the deaminated gluten peptides trigger a cascade of innate and adaptive immune responses including lymphocyte infiltration into the proximal part of the small intestines leading to destruction of the intestinal epithelium and mucosa including villous atrophy and crypt hyperplasia.
The HLA class II antigens DQ2 and DQ8 are the major risk factors predisposing individuals to Coeliac Disease and account for over 35% of the genetic risk. Close to 90% of patients with Coeliac Disease express the HLA-DQ2 molecules with most of the remainder expressing the HLA-DQ8 molecule. There is a gene dosage effect so that those patients homozygous for the DQ2 or DQ8 genes or who are heterozygous for the DQ2 and DQ8 genes have a higher disease susceptibility. Deaminated gluten peptides bind strongly to HLA-DQ2 and DQ8 presenting a HLA-gluten peptide complex that activates CD4+ T cells which produce proinflammatory cytokines, including interferon gamma, which leads directly to tissue remodelling and flattening of the intestinal mucosa. The immune response also includes the development of antibodies against gluten and auto-antibodies to endogenous tissue transglutaminase. The best HLA genetic test for Coeliac Disease is now recognised to include HLA-DQA1 testing alongside the traditional HLA-DQB1 testing. The HLA-DQA1*05:01 – DQB1*02:01 encoded molecule is the DQ2 most associated with Coeliac Disease. A small percentage of Coeliac patient express the DQA1*02:01 – DQB1*02:02 genotype. The DQ8 alleles associated with Coeliac Disease are DQA1*03 – DQB1*03:02.
Presence of HLA-DQ2 and DQ8 on their own are not predictive of coeliac disease. Genome wide association studies have indicated a large number of non HLA genes which segregate with Coeliac Disease and may potentially be associated with it. Coeliac disease is therefore a complex disorder, requiring environmental trigger. Diagnosis of Coeliac Disease is based mainly on histology, though this is not always reliable. Genetic testing for HLA-DQ2 and DQ8 as a complement to histology has proved to be important for helping to confirm the diagnosis. Typing for HLA-DQ2 and DQ8 does provide good negative predictive value. However HLA-DQ2 and DQ8 typing does not provide any additional diagnostic benefit in patients already known to be tissue transglutaminase antibody positive.
Coeliac Reporting
Presence of HLA-DQ2 and DQ8 on their own are not predictive of coeliac disease. Genome wide association studies have indicated a large number of non HLA genes which segregate with Coeliac Disease and may potentially also be associated with it. Coeliac disease is therefore a complex disorder, requiring environmental trigger. Diagnosis of Coeliac Disease is based mainly on histology, though this is not always reliable. Genetic testing for HLA-DQ2 and DQ8 as a complement to histology has proved to be important for helping to confirm the diagnosis. Typing for HLA-DQ2 and DQ8 does provide good negative predictive value. However HLA-DQ2 and DQ8 typing does not provide any additional diagnostic benefit in patients already known to be tissue transglutaminase antibody positive.
HLA-DQA1*01:01, *02:01; DQB1*05:01, *03:03
This patient is NEGATIVE for HLA-DQ2 (and is DQA1*05 NEGATIVE) and NEGATIVE for HLA-DQ8 (DQA1*03, DQB1*03:02). Patients with this genotype have a VERY LOW RISK of predisposition to Coeliac disease
HLA-DQA1*05:01, *01:02; DQB1*01:01, *05:01
This patient is NEGATIVE for HLA-DQ2 (but is DQA1*05 POSITIVE) and NEGATIVE for HLA-DQ8 (DQA1*03, DQB1*03:02). Patients with this genotype have a VERY LOW RISK of predisposition to Coeliac disease
HLA-DQA1*03:02, *02:01; DQB1*02:01, *03:01
This patient is Heterozygous POSITIVE for HLA-DQ2 (but is DQA1*05 NEGATIVE) and NEGATIVE for HLA-DQ8 (DQA1*03, DQB1*03:02). Patients with this genotype have a LOW RISK of predisposition to Coeliac disease
HLA-DQA1*03:01, *03:02; DQB1*03:02, *03:01
This patient is NEGATIVE for HLA-DQ2 (DQA1*05, DQB1*02) and POSITIVE for HLA-DQ8 (DQA1*03, DQB1*03:02). Patients with this genotype have a HIGH RISK of predisposition to Coeliac disease though other factors are likely involved
HLA-DQA1*03:01, *01:02; DQB1*02:01, *03:02
This patient is POSITIVE for HLA-DQ2 (but is DQA1*05 NEGATIVE) and POSITIVE for HLA-DQ8 (DQA1*03, DQB1*03:02). Patients with this genotype have a HIGH RISK of predisposition to Coeliac disease though other factors are likely involved
HLA-DQA1*03:02, *05:01; DQB1*02:01, *03:01
This patient is POSITIVE for HLA-DQ2 (DQA1*05, DQB1*02) and NEGATIVE for HLA-DQ8 (DQA1*03, DQB1*03:02). Patients with this genotype have a HIGH RISK of predisposition to Coeliac disease though other factors are likely involved
HLA-DQA1*02:02, *02:02; DQB1*02:01, *02:01
This patient is Homozygous POSITIVE for HLA-DQ2 (but is DQA1*05 NEGATIVE) and NEGATIVE for HLA-DQ8 (DQA1*03, DQB1*03:02). Patients with this genotype have a HIGH RISK of predisposition to Coeliac disease though other factors are likely involved
HLA-DQA1*03:01, *03:01; DQB1*03:02, *03:02
This patient is Homozygous POSITIVE for HLA-DQ8 (DQA1*03, DQB1*03:02). Patients with this genotype have a HIGH RISK of predisposition to Coeliac disease though other factors are likely involved
HLA-DQA1*03:01, *05:01; DQB1*03:02, *03:01
This patient is NEGATIVE for HLA-DQ2 (but is DQA1*05 POSITIVE) and POSITIVE for HLA-DQ8 (DQA1*03, DQB1*03:02). Patients with this genotype have a HIGH RISK of predisposition to Coeliac disease though other factors are likely involved
HLA-DQA1*03:01, *05:01; DQB1*02:01, *03:02
This patient is Heterozygous POSITIVE for HLA-DQ2 (DQA1*05, DQB1*02) and POSITIVE for HLA-DQ8 (DQA1*03, DQB1*03:02). Patients with this genotype have a VERY HIGH RISK of predisposition to Coeliac disease though other factors are likely involved
HLA-DQA1*05:01, *05:01; DQB1*02:01, *02:01
This patient is Homozygous POSITIVE for HLA-DQ2 (DQA1*05, DQB1*02). Patients with this genotype have a VERY HIGH RISK of predisposition to Coeliac disease though other factors are likely involved
Colitis
Ulcerative Colitis is one of two major forms of Inflammatory Bowel Disease, the other being Crohn’s disease. Colitis literally means inflammation of the colon and ulcerative refers to the presence of ulcers. Colitis is primarily a disease of the large intestine (colon and rectum). The inflammation and ulcers are the cause of the common symptoms of diarrhoea mixed with blood and mucus seen with Ulcerative Colitis. Other symptoms include crampy pain in the abdomen, pain when passing stool and inflammation of the rectum. If flare ups persist then Ulcerative Colitis may be accompanied by fever, weight loss and feeling sick. Onset of Ulcerative Colitis is typically between the ages of 15 and 35 with a population prevalence of around 200 per 100,000 persons in Western populations.
The most consistently replicated association of Ulcerative Colitis is with the HLA class II allele HLADRB1*01:03. This association is particularly strong in patients with severe disease, as defined by a need to colectomy. Among patients who do require colectomy, HLA-DRB1*01:03 may be associated with a shorter mean time to surgery. HLA-DRB1*15:02 but not HLA-DRB1*15:01, has also been shown to be associated with Ulcerative Colitis in some populations though the strength of the association varies in different populations. In some populations HLA-DRB1*04 has some protective effect. HLA risk alleles however only contribute a minor part of the overall genetic picture. The advent of genome wide association studies have identified a large number of potential candidate genes which are being studied for association with Ulcerative Colitis, including the Interleukin-23 Receptor (IL23R).
The association between HLA and Ulcerative Colitis is of low specificity and sensitivity, limiting the value of genetic testing for diagnostic purposes. However HLA-DRB1*01:03 typing may be of value in predicting patients who may potentially require colectomy as potential candidates for more aggressive treatment in a bid to spare the colon.
Chron’s
Crohn’s is one of two major forms of Inflammatory Bowel Disease, the other being Ulcerative Colitis. Crohn’s differs from ulcerative colitis in that Crohn’s can affect any part of the gastrointestinal tract from the mouth to the anus, whereas ulcerative colitis mainly affects the large intestine. Crohn’s disease is characterised by abdominal pain, diarrhoea, which may be bloody, vomiting and weight loss. Crohn’s is often associated with complications outside of the gastrointestinal tract such as skin rash, arthritis, Uveitis, tiredness and lack of concentration. Onset of Crohn’s during childhood can have a significant effect on development.
The specific cause of Crohn’s is unknown but epidemiological studies do suggest a dysregulation of the immune response against the gut flora in a genetically susceptible individual, perhaps as a result of infection of the gut with an atypical microbe. Other environmental factors do perhaps play a part with some studies showing that smoking and appendectomy may act as triggers.
The most consistently replicated association of Crohn’s disease is with the HLA class II allele HLADRB1*07. The association is specifically in patients with ileal involvement. HLA-DRB1*01:03 has also been shown more recently to be associated with Crohn’s. As with ulcerative colitis, HLA-DRB1*01:03 may be associated with more severe disease, as defined by patients who require colectomy. HLADRB1*15:01 appears to confer protection against Crohn’s.
The advent of genome wide association studies have identified a large number of potential candidate genes which are being studied for association with Crohn’s. Of particular interest are the innate pattern recognition receptors ‘Nucleotide-binding Oligomerisation Domain containing 2’ / ‘Caspase Recruitment Domain family, member 15’ (NOD2/CARD15). NOD2/CARD15 is a member of the family of pattern recognition receptors that recognise microbial components. NOD2/CARD15 expression is high in the intestinal crypts.
The association between HLA and Crohn’s is of low specificity and sensitivity, limiting the value of genetic testing for diagnostic purposes. In addition, as Crohn’s shares some HLA gene susceptibility with ulcerative colitis, e.g. HLA-DRB1*01:03, it is not possible to use HLA typing to distinguish Crohn’s from ulcerative colitis. Knowledge of the HLA type of an already diagnosed Crohn’s patient may be of value in helping to predict disease course and may indicate treatment options. This may for example include the use of Infliximab as an anti-TNF agent for certain stages of Crohn’s disease.
Graves Disease
Graves Disease is an autoimmune disease of the thyroid gland. It is characterised by hyperthyroidism associated with goitre, palpitations, bulging eyes, sweating, heat intolerance, tremor, anxiety and weight loss. The immunological response in Graves Disease comprises diffuse lymphocyte infiltration into the thyroid gland with thyroid stimulating immunoglobulin (TSI) autoantibody production. The hyperthyroidism is caused by activation of the thyroid stimulating hormone receptor (TSHR) by binding of autoantibodies.
Monozygotic and dizygotic twin studies have shown a much higher concordance in monozygotic twins (20%) than dizygotic, indicating a genetic component to Graves Disease. The most strongly associated gene is the HLA class II gene. Implicated haplotypes include DRB1*03:01-DQA1*05:01DQB1*02:01 (DR17, DQ2) and DRB1*04:01-DQA1*03:01-DQB1*03:02 (DR4, DQ8). The highest risk is associated with DR17, DQ2. HLA-DRB1*07 (DR7) is protective for Graves Disease.
Diagnosis is based on thyroid function tests and biochemical and clinical manifestations of hyperthyroidism rather than on genetic testing. Genetic testing does however contribute to a better understanding of the aaetiology of the disease.
Hemochromatosis (HFE)
HFE Overview
Haemochromatosis is an adult onset disorder, characterised by inappropriately high iron absorption resulting in progressive iron overload. Increases in the body’s iron levels may be acquired by for example, blood transfusion or may be due to inherited disturbances to the mechanisms regulating intestinal iron absorption. Hereditary Haemochromatosis is an autosomal recessive disease with estimated prevalence of 2 in 1,000 in the Caucasian population, with lower incidence in other races. It is characterised by cirrhosis of the liver, diabetes, cardiomyopathy, arthritis, testicular failure and tanning or bronzing of the skin.
Haemochromatosis is usually related to the deficient synthesis or reduced activity of the peptide hormone Hepcidin (coded for by the ‘Hepcidin Anti-Microbial Peptide’ gene – HAMP), which is produced in the liver. Hepcidin normally down-regulates the entry of iron into the blood stream and therefore regulates iron homeostasis. Mutations in the gene for Hepcidin or in the genes which regulate Hepcidin synthesis such as the Human Haemochromatosis gene (HFE) and ‘Transferrin Receptor 2’ gene – (TFR2) can affect Hepcidin activity. More than 80% of Hereditary Haemochromatosis is due to mutations in such regulatory genes. In addition, mutations in HFE can affect the ability of HFE to bind the Transferrin receptor, thereby affecting ferritin levels.
Hereditary Haemochromatosis caused by mutations in the HFE gene is the most common and is termed Type 1 haemochromatosis. Type 2 haemochromatosis is caused by mutations to HAMP and Type 3 by mutations to TFR2. Other types exist.
The HFE gene is located on the short arm of chromosome 6. It codes for a HFE protein which is homologous to MHC class I and is composed of alpha 1 and 2 domains and an immunoglobulin like alpha 3 domain which associates with beta-2 microglobulin. HFE does not however present peptide to T-cells. Several mutations in the HFE gene have been described. The cysteine to tyrosine substitution at position 282 (C282Y) is present in the homozygous state in over 80–93% of Hereditary Haemochromatosis cases. In a further 5% of cases, the C282Y mutation is present as a compound heterozygous with a histidine to aspartic acid substitution at position 63 (H63D). These mutations result in excessive, chronic iron absorption in the gut, as well as a relative inability to store iron in macrophages. However, a significant proportion of people with these genotypes do not develop iron overload. Other genetic mutations as well as environmental factors are likely to contribute to iron overload in patient.
Genetic testing for HFE, while useful, is not completely diagnostic and other clinical factors such as iron levels and familial history need to be taken into account.
HFE Reporting
HHCC
This individual is HOMOZYGOUS for the NORMAL HFE gene at position 282 (C/C).
This individual is HOMOZYGOUS for the NORMAL HFE gene at position 63 (H/H).
This genotype makes a diagnosis of Type-1 (HFE-related) hereditary haemochromatosis VERY UNLIKELY.
Ifiron levels (transferrin saturation and serum ferritin) are currently RAISED, then ALTERNATIVE REASONS (e.g. alcohol consumption, fatty liver disease and/or metabolic syndrome) should be considered. In individuals with severe iron overload, other rare forms of hereditary haemochromatosis cannot be excluded. Ifiron levels are currently NORMAL, this individual is UNLIKELY to develop Type-1 hereditary haemochromatosis.
CARRIER TESTING or GENETIC COUNSELLING of relatives of an individual with this genotype is NOT RECOMMENDED.
HDCC
This individual is HOMOZYGOUS for the NORMAL HFE gene at position 282 (C/C).
This individual is A CARRIER for the MUTATION in the HFE gene at position 63 (H/D).
This genotype makes a diagnosis of Type-1 (HFE-related) hereditary haemochromatosis VERY UNLIKELY.
Ifiron levels (transferrin saturation and serum ferritin) are currently RAISED, then ALTERNATIVE REASONS (e.g. alcohol consumption, fatty liver disease and/or metabolic syndrome) should be considered. In individuals with severe iron overload, other rare forms of hereditary haemochromatosis cannot be excluded. Ifiron levels are currently NORMAL, this individual is at NO INCREASED RISK of developing Type-1 hereditary haemochromatosis.
CARRIER TESTING or GENETIC COUNSELLING of relatives of an individual with this genotype is NOT RECOMMENDED.
DDCC
This individual is HOMOZYGOUS for the NORMAL HFE gene at position 282 (C/C).
This individual is HOMOZYGOUS for the MUTATION in the HFE gene at position 63 (D/D).
This genotype makes a diagnosis of Type-1 (HFE-related) hereditary haemochromatosis UNLIKELY but maybe ASSOCIATED WITH a slight INCREASE in iron levels (transferrin saturation and serum ferritin).
Ifiron levels are currently RAISED, then ALTERNATIVE REASONS (e.g. alcohol consumption, fatty liver disease and/or metabolic syndrome) should be considered. In individuals with severe iron overload, other rare forms of hereditary haemochromatosis cannot be excluded. Ifiron levels are currently NORMAL, this individual is at NO INCREASED RISK ofdeveloping Type-1 hereditary haemochromatosis.
CARRIER TESTING or GENETIC COUNSELLING of relatives of an individual with this genotype is NOT RECOMMENDED.
HHCY
This individual is A CARRIER for the MUTATION in the HFE gene at position 282 (C/Y).
This individual is HOMOZYGOUS for the NORMAL HFE gene at position 63 (H/H).
This genotype makes a diagnosis of the Type-1 (HFE-related) hereditary haemochromatosis UNLIKELY. Ifiron levels (transferrin saturation and serum ferritin) are currently RAISED, then ALTERNATIVE REASONS (e.g. alcohol consumption, fatty liver disease and/or metabolic syndrome) should be considered. In individuals with severe iron overload, other rare forms of hereditary haemochromatosis cannot be excluded. Ifiron levels are currently NORMAL, this individual is at NO INCREASED RISK ofdeveloping Type-1 hereditary haemochromatosis.
CARRIER TESTING of relatives of an individual with this genotype is NOT RECOMMENDED but GENETIC COUNSELLING MAY BE CONSIDERED.
HHYY
This individual is HOMOZYGOUS for the MUTATION in the HFE gene at position 282 (Y/Y).
This individual is HOMOZYGOUS for the NORMAL HFE gene at position 63 (H/H).
Ifiron levels (transferrin saturation and serum ferritin) are currently RAISED, this genotype is COMPATIBLE WITH a diagnosis of Type-1 (HFE-related) hereditary haemochromatosis.
Ifiron levels are currently NORMAL, then this individual is AT RISK of developing Type-1 hereditary haemochromatosis and it is recommended that the indices of iron overload be MONITORED EVERY YEAR.
CARRIER TESTING and GENETIC COUNSELLING of adult first-degree relatives of an individual with this genotype SHOULD BE CONSIDERED.
HDCY
This individual is A CARRIER for the MUTATION in the HFE gene at position 282 (C/Y).
This individual is A CARRIER for the MUTATION in the HFE gene at position 63 (H/D).
This genotype makes a diagnosis of the Type-1 (HFE-related) hereditary haemochromatosis UNLIKELY but may PREDISPOSE to MILD/MODERATE IRON OVERLOAD.
Ifiron levels (transferrin saturation and serum ferritin) are currently RAISED, then ALTERNATIVE REASONS (e.g. alcohol consumption, fatty liver disease and/or metabolic syndrome) should be considered. In individuals with severe iron overload, other rare forms of hereditary haemochromatosis cannot be excluded. Ifiron levels are currently NORMAL, then this individual may be AT RISK of developing MILD-TO-MODERATE iron overload and it is recommended that the indices of iron overload be monitored EVERY 3 YEARS.
CARRIER TESTING of relatives of an individual with this genotype is NOT RECOMMENDED but genetic counselling MAY BE CONSIDERED.
HDYY
This individual is HOMOZYGOUS for the MUTATION in the HFE gene at position 282 (Y/Y).
This individual is A CARRIER for the MUTATION in the HFE gene at position 63 (H/D).
Ifiron levels (transferrin saturation and serum ferritin) are currently RAISED this genotype is COMPATIBLE WITH A DIAGNOSIS of Type-1 (HFE-related) hereditary haemochromatosis.
Ifiron levels are currently NORMAL, then this individual is AT RISK of developing Type-1 hereditary haemochromatosis and it is recommended that the indices of iron overload be MONITORED EVERY YEAR.
CARRIER TESTING and GENETIC COUNSELLING of adult first-degree relatives of an individual with this genotype SHOULD BE CONSIDERED.
DDCY
This individual is A CARRIER for the MUTATION in the HFE gene at position 282 (C/Y).
This individual is HOMOZYGOUS for the MUTATION in the HFE gene at position 63 (D/D).
This genotype makes a diagnosis of Type-1 (HFE-related) hereditary haemochromatosis UNLIKELY but maybe ASSOCIATED WITH a slight INCREASE in iron levels (transferrin saturation and serum ferritin).
Ifiron levels are currently RAISED, then ALTERNATIVE REASONS (e.g. alcohol consumption, fatty liver disease and/or metabolic syndrome) should be considered. In individuals with severe iron overload, other rare forms of hereditary haemochromatosis cannot be excluded.
Ifiron levels are currently NORMAL, this individual is at NO INCREASED RISK ofto developing Type-1 hereditary haemochromatosis.
CARRIER TESTING of relatives of an individual with this genotype is NOT RECOMMENDED but GENETIC COUNSELLING MAY BE CONSIDERED.
DDYY
This individual is HOMOZYGOUS for the MUTATION in the HFE gene at position 282 (Y/Y).
This individual is HOMOZYGOUS for the MUTATION in the HFE gene at position 63 (D/D).
Ifiron levels (transferrin saturation and serum ferritin) are currently RAISED, this genotype is COMPATIBLE WITH A DIAGNOSIS of Type-1 (HFE-related) hereditary haemochromatosis.
Ifiron levels are currently NORMAL, then this individual is AT RISK of developing Type-1 hereditary haemochromatosis and it is recommended that the indices of iron overload be MONITORED EVERY YEAR.
CARRIER TESTING and GENETIC COUNSELLING of adult first-degree relatives of an individual with this genotype SHOULD BE CONSIDERED.
Insulin Dependent Diabetes Mellitus
Diabetes Mellitus is a group of diseases in which the body fails to maintain normal glucose levels either because the body does not produce enough insulin (type 1) or because the cells of the body do not respond to the insulin that is produced (type 2). Type 1 diabetes is also known as Insulin Dependent Diabetes Mellitus (IDDM) as it requires patients to inject insulin.
IDDM is characterised by progressive infiltration of immune cells into the Islets of the pancreas and autoantibody production leading to autoimmune destruction of the insulin producing pancreatic Islet cells. Primary damage in IDDM is caused by a cellular immune response in which CD4+ Th1 helper T cells activate in situ CD8+ cytotoxic T cells directed against the beta cells. Clinical symptoms of diabetes occur when over 90% of an individual’s beta cells are destroyed. Autoantibody production pre dates the development of clinical symptoms. Antibodies are developed against most components of beta cells, including Insulin, Glutamic Acid Decarboxylase (GAD) and Islet-cell Antigen2 (IA-2).
IDDM is a polygenic disease where the concordance rate in twins is 30 – 50%. An as yet unknown trigger is required for genetically susceptible individuals to develop the condition. Environmental factors such as diet and viral infection have been shown to be associated with an increased incidence of IDDM. The HLA system contributes approximately 50% of the heritable risk of IDDM and is the most important susceptibility genetic region and has been named IDDM1. The next most important genetic region is the Insulin gene region on chromosome 11, which is thought to contribute approximately 10% of the heritable risk. This region has been named IDDM2. An additional 15 genes located on different chromosomes, with variable but small effects have been described and named IDDM3 – IDDM17.
More than 90% of Caucasian patients with IDDM carry the haplotypes DRB1*03:01, DQA1*05:01, DQB1*02:01 (DR17, DQ2) or DRB1*04:01, DQA1*03:01, DQB1*03:02 (DR4, DQ8), particularly where the age of onset is less than 15. Patients heterozygous for these haplotypes carry a greater susceptibility risk. The critical residues are thought to be DQ alpha Arg-52, DQ beta Asp-57 and DQ beta Arg-74. Absence of DQ beta Asp-57 and DQ beta Arg-74 is strongly associated with IDDM. Resistance to IDDM is conferred by some DQ6 alleles. DQA1*01:02, DQB1*06:02 confers protection in Caucasian populations.
Genetic testing is useful as an aid to diagnosis of IDDM. In addition, as IDDM is characterised by a variable length silent period before overt disease, genetic testing for disease susceptibility genes in siblings of IDDM patients may be a useful measure.
Juvenile Idiopathic Arthritis
Juvenile Idiopathic Arthritis (JIA) is a broad disease which comprises a set of clinically heterogeneous chronic arthropathies of unknown aaetiology which develop before the age of 16. It was previously called Juvenile Chronic Arthritis in Europe and Juvenile Rheumatoid Arthritis in the USA. Several subtypes of Juvenile Idiopathic Arthritis have been defined, including Systemic Juvenile Arthritis, Oligoarthritis, Polyarthritis, Enthesitis-related arthritis and Psoriatic arthritis. Each of these conditions has distinct methods of presentation and clinical symptoms and in some cases, different genetic background.
Systemic Juvenile Arthritis is a form of JIA which affects more than 5 joints and is typically accompanied by or preceded by fever and rash. Oligoarthritis affects 4 or fewer joints during the first 6 months of disease. Polyarthritis affects 5 or more joints. Patients may be rheumatoid factor pos or neg. Enthesitis related arthritis most commonly affects sites such as cranial insertion points and the Achilles tendon. Psoriatic arthritis is diagnosed by the presence of arthritis and a psoriatic rash as well as swelling of one or more fingers.
Both genetics and non genetic factors are believed to play a role in susceptibility to JIA. HLA class I and II genes are the most commonly linked genes in association studies, though non HLA genes have also been implicated. Systemic Juvenile Arthritis has been associated with a single nucleotide polymorphism in the regulatory region of the interleukin-6 gene. Oligoarthritis has been associated most frequently with HLA-A2, DRB1*08 and DRB1*11. HLA-DRB1*04 and HLA-DRB1*07 are thought to be protective for Oligoarthritis. Polyarthritis is associated in most studies with HLA-DRB1*04. Enthesitis related arthritis is associated with HLA-B*27 and Psoriatic arthritis is associated with HLAC*06:02 and with polymorphisms in the IL-23 receptor gene.
There is no cure for JIA and treatment is based mainly on early diagnosis and a combination of drug and physical therapy aimed at relieving symptoms. In this regard genetic testing is clinically relevant as a contributor to early diagnosis.
Multiple Sclerosis
Multiple Sclerosis (MS) is an idiopathic autoimmune neurodegenerative disease in which dysregulation of the immune system causes myelin sheath degradation. Multiple sclerosis is characterised by plaques or lesions in the brain and spinal cord. The location and size of the lesions is unpredictable. Symptoms of Multiple Sclerosis include affected coordination, balance and vision and disturbances in the bowel, bladder and sexual organs. Disease onset is typically between the ages of 20 and 40. Women have a 2 -3 fold higher incidence of MS than men. The distribution of MS inversely parallels the global distribution of UV light, suggesting a role for vitamin D in the disease.
First degree relatives of patients with MS are generally at 20 – 40 times greater risk of developing the disease themselves, compared to the general population, showing that the disease has a large genetic component. Family studies, including monozygotic and dizygotic twin studies have shown that HLA-DRB1*15:01 on the HLA-DRB1*15:01, DQA1*01:02, DQB1*06:02 haplotype, is a disease susceptibility gene, accounting for up to 35% of the heritability of the disease. Homozygosity for this haplotype increases disease risk six fold. Linkage disequilibrium initially made it difficult to confirm disease association with HLA-DR*15:01 rather than HLA-DQB1*06:02 but studies of MS in Black populations have confirmed HLA-DR*15:01 as the susceptibility gene. A role for other HLA class I and II alleles has been indicated by a number of studies but the overall effect of these remain small compared to HLA-DRB1*15.
The exact mechanism by which HLA increases the susceptibility to MS is unknown. A recent finding of a vitamin D response element in the promoter region of HLA-DRB1, which is completely conserved in HLA-DRB1*15:01 together with evidence showing that vitamin D is significantly lower in MS patients is suggestive.
A number of genome wide association studies have indentified other candidate genes other than HLA, including IL-7 Receptor alpha and the IL-2 Receptor alpha genes as MS disease susceptibility loci.
The main value of genetic testing in MS is to provide insights into the mechanism of the disease, thereby potentially suggesting strategies for prevention and treatment, rather than as a diagnostic tool.
Myasthenia Gravis
Myasthenia Gravis (MG) is a rare autoimmune neuromuscular disease, aimed at the neuromuscular junction. It is characterised by fluctuating muscle weakness and fatigability. It is a potentially life threatening condition when respiratory muscles are involved. In 85% of cases, autoantibodies directed against nicotinic acetylcholine receptors (AChR’s) in the postsynaptic membrane of the neuromuscular junction have been found. Autoantibodies directed against the Muscle Specific tyrosine Kinase (MuSK) also on the postsynaptic membrane have been found in 70% of cases. Together, autoantibodies to AChR and/or MuSK are present on over 90% of MG cases. Other autoantibodies are also often present.
The most frequent form of Myasthenia Gravis presents with thymus follicular hyperplasia and has an early onset. It has been shown to be associated with the ‘ancestral HLA haplotype’ A1 – B8 – Cw7 – DR17 – DQ2. The strong linkage disequilibrium had made it difficult to determine precisely which locus on this haplotype is most strongly associated. Early studies implicated HLA-B8 but this was later replaced with a stronger associated with HLA class II, specifically HLA-DR17. One consistent finding is that HLA-DR3 has a positive association with early onset Myasthenia Gravis and a negative association with late onset Myasthenia Gravis, whilst HLA-DR7 has the opposite, i.e. a negative association with early onset Myasthenia Gravis and a positive association with late onset Myasthenia Gravis. Another study using SNPs in a large group of patients found the strongest association to be with HLA-Cw7. The real disease association locus could even be another locus on the A1 – B8 – Cw7 – DR17 – DQ2 haplotype. One recent study has identified a region that overlaps the MHC class III and class I regions of the A1 – B8 – Cw7 – DR17 – DQ2 haplotype, which they termed MYAS1, that associates strongly with Myasthenia gravis.
Diagnosis of Myasthenia gravis does not depend on genetic testing. Tests to measure concentrations of serum autoantibodies are a more effective diagnostic tool. Genetic testing may however be an aid to diagnosis and is of value in providing insights into the mechanism of the disease, thereby potentially suggesting strategies for prevention and treatment.
Narcolepsy
Narcolepsy is a chronic, debilitating sleep disorder first described in the late 19th century which is characterized by excessive daytime sleepiness, cataplexy i.e. sudden transient muscular weakness, disturbed night sleep and sleep paralysis. Narcolepsy is caused by deficiencies in hypothalamic hypocretin (also known as orexin) neurotransmission, most likely through autoimmune-mediated postnatal cell death of hypocretin producing neurons. Hypocretin is a neuropeptide hormone that is responsible for controlling appetite and sleep patterns.
Prevalence studies have shown that genetic factors play a role on narcolepsy but are neither necessary nor sufficient to cause narcolepsy. Twin studies show that only 25 – 30% of twins are concordant. Family studies show that the risk of developing narcolepsy for a first degree relative of a patient is only 1 – 2%. This is however 30 – 40 times higher than the estimated prevalence of narcolepsy in the general population, showing that genetic heritability does play a role. One of the most important predisposing genetic factors is the HLA-DQB1*06:02 allele on the DRB1*15:01 – DQA1*01:02 – DQB1*06:02 haplotype. Between 85 and 95% of narcolepsy patients with cataplexy carry this haplotype. Persons homozygous for HLA-DQB1*06:02 carry a greater risk than heterozygous persons, though persons heterozygous for HLA-DQB1*03:01 and HLA-DQB1*06:02 are also at increased risk. HLA-DQB1*05:01 and DQB1*06:01 are thought to be protective for narcolepsy. HLA-DQB1*06:02 has a larger P4 pocket which would facilitate binding of larger hydrophobic residues compared to HLA-DQB1*06:01 and this may help explain the opposite effects these two allele have on narcolepsy susceptibility.
The genetic risk of developing narcolepsy is not however fully explained by HLA-DQB1*06:02 as there are many patients with narcolepsy who do not carry HLA-DQB1*06:02, pointing to the possibility of non HLA gene involvement. Candidate genes include the different hypocretin system genes, though initial studies have not shown an association and TNF alpha gene polymorphisms.
Genetic testing for narcolepsy, particularly, HLA-DQB1*06:02 typing is useful as an aid to diagnosis in patients with cataplexy but the association is not specific as there are many narcolepsy patients with HLA-DQB1*06:02 and many HLA-DQB1*06:02 persons who do not have narcolepsy.
Osteoarthritis
Osteoarthritis is the most common form of arthritis and is characterised by joint pain accompanied by varying degrees of functional limitation and reduced quality of life. Pain is usually in the large weight bearing joints such as the knees and hips. It also affects the joints of the thumb and fingers. Hand osteoarthritis has a good prognosis with most cases becoming asymptomatic after a few years. Involvement of the thumb base may have a worse prognosis. Knee osteoarthritis is very variable in its outcome. Over a period of several years about a third of cases improve, a third stay much the same and the remaining third develop progressive symptomatic disease. Hip osteoarthritis probably has the worst overall outcome of the three major sites though some hips heal spontaneously.
Osteoarthritis is a complex disorder caused by a combination of genetic predisposition and environmental factors. Environmental factors include lifestyle factors such as being overweight, a sedentary occupation, repetitive use of joints and history of trauma to affected joints.
The genes that predispose to Osteoarthritis remain to be clarified. Many studies have pointed to different HLA class I and II associations, perhaps indicating the heterogeneity of the condition. Several studies on generalized Osteoarthritis have revealed an association with HLA-B8. Linkage of HLA-B8 on the HLA-A1, B8, DR17 haplotype makes it difficult to be certain the association is actually with HLA-B8 and not with some other gene on the haplotype. A Japanese study showed an association with HLA-Cw4. Other studies have indicated associations with HLA-B35, B40, DR2 and DQ1 but again linkage disequilibrium makes it difficult to be certain what the true disease susceptibility gene is.
Diagnosis of Osteoarthritis is on the basis of persistent joint pain that is worse with use, age 45 years old and over and morning stiffness lasting no more than half an hour, rather than on genetic testing. The main value of genetic testing in Osteoarthritis is to provide insights into the mechanism of the disease, thereby potentially suggesting strategies for prevention and treatment, rather than as a diagnostic tool.
Polyarthralgia
Arthralgia literally means pain and Polyarthralgia means pain in several joints, anything from two upwards. Polyarthralgia encompasses Simple Arthralgia in which the main symptom is pain with no clinical features of inflammation in the joints or morning stiffness, Osteoarthritis in which pain is usually in the large weight bearing joints such as the knees and hips and in the joints of the thumb and fingers, Seronegative Arthritis and Rheumatoid Arthritis. Each of these has a distinct clinical presentations and genetic basis.
Simple Arthralgia refers to arthritis with no clinical manifestation of inflammation. It is often associated with fever or infection. It is thought to be associated with HLA-B*27 or HLA-DRB1*04.
Osteoarthritis is the most common form of arthritis and is characterised by pain in the large weight bearing joints such as the knees and hips as well s in the joints of the thumb and fingers. Both genetic and environmental factors contribute to the predisposition to Osteoarthritis. Environmental factors include lifestyle factors such as being overweight, a sedentary occupation, repetitive use of joints and history of trauma to affected joints. Genetic factors include HLA-B*08, though the strong linkage disequilibrium on the HLA-A1, B8, DR17 haplotype makes it difficult to be certain.
Seronegative arthritis refers to arthritis without the presence of Rheumatoid factor. It typically affects multiple joints. It is thought to be associated with HLA-DRB1*04.
Rheumatoid is a chronic inflammatory symmetrical arthritis (it affects both sides of the body equally) which affects multiple synovial lined joints. It is characterised by progressive articular damage leading to joint deformation and disability. Patients are Rheumatoid factor positive and up to 70% of patients develop autoimmunity to citrullinated protein. Rheumatoid Arthritis is associated with a set of shared epitopes at positions 70 – 74 in the genetic sequence of the HLA-DRB1 gene. These include the sequences QKRAA, QRRAA, RKRAA and RRRAA. HLA-DRB1 alleles which share these sequences include HLA-DRB1*01:01, HLA-DRB1*01:02, HLA-DRB1*04:01, HLA-DRB1*04:04, HLA-DRB1*04:05, HLA-DRB1*04:08, HLA-DRB1*10:01, HLA-DRB1*13:03, HLA-DRB1*14:02 and HLA-DRB1*14:06.
Genetic testing for Polyarthralgia is an aid to diagnosis rather than a strong diagnostic tool.
Polyarthritis
Polyarthritis is a form of arthritis which affects 5 or more joints simultaneously. It is usually associated with other autoimmune conditions such as psoriasis, colitis and crohns. Subcategories include Rheumatoid Arthritis (RA), Systemic Lupus Erythematosus (SLE), Seronegative Arthritis and Psoriatic Arthritis.
RA is a symmetrical arthritis that affects multiple synovial lined joints. It is characterised by progressive articular damage leading eventually to joint deformation. RA was initially thought to be associated with HLA-DR4 but is now seen to be more associated with a set of shared epitopes in positions 70-74 of the HLA-DRB1 locus. The epitopes include the amino acid sequences QKRAA, QRRAA, RKRAA AND RRRAA. HLA class II alleles which share these epitopes include HLA-DRB1*01:01, *01:02, *04:01, *04:04 and *04:05. Alleles which do not share these epitopes and which may be protective for RA include HLA-DRB1*03 and HLA-DRB1*04:03.
SLE is an arthritic condition characterised be repeated bouts of relapse and remission of symptoms including pain. It is associated with production of anti-nuclear autoantibodies, including anti-Sm, anti-Ro/SSA and anti-La/SSB. Presence of anti-Ro/SSA and anti-La/SSB autoantibodies is associated with the HLA class II DRB1*03:01 allele. Presence of anti-Sm autoantibodies is associated with the HLA class II DRB1*15:01 allele.
Seronegative Arthritis describes a set of arthritic conditions all of which are characterised by absence of rheumatoid factor (rheumatoid factor is an autoantibody against the Fc portion of IgG). These include the seronegative spondyloarthropathies and seronegative Rheumatoid Arthritis. Seronegative Polyarthritis is in some cases associated with other systemic autoimmune diseases including psoriasis, bowel and bladder symptoms and anterior Uveitis all of which have different HLA associations. The seronegative spondyloarthropathies are associated with HLA-B27. Uveitis is also associated with HLA-B27, while Behcet’s is associated with HLA-B51. Seronegative Rheumatoid Arthritis may be associated with HLA-DR4.
Psoriatic Arthritis is an inflammatory joint disease which is associated with a psoriatic rash and swelling of the fingers (Dactylitis). Symptoms include spinal pain and enthesitis. Anterior Uveitis is seen in about a third of cases, though symptoms are often temporary. Nail involvement is common. HLA types implicated in susceptibility to Psoriatic Arthritis include HLA-B*27, HLA-B*39 and HLAC*06:02.
Psoriasis Vulgaris
Psoriasis Vulgaris is the most common form (vulgaris means common) of Psoriasis. Psoriasis Vulgaris is a chronic, immune-mediated inflammatory skin disease which manifests as elevations or plaques of the skin that do not contain pus. The plaques are red or pink in colour and are covered by white or silvery scales. They may be thick or thin, large or small. Plaques appear most often on the elbows and knees and on the scalp. Psoriasis Vulgaris is associated with systemic disorders, including Crohn’s and IDDM. It is rarely life threatening but it does cause considerable psychological stress for patients.
Psoriasis Vulgaris presents with inflammatory infiltrate of leukocytes predominantly into the dermis and involves a cell mediated immune response. However the auto-antigen to which the immune response is directed has not been identified.
Population studies have shown that Psoriasis Vulgaris has a genetic component with relatives of patients at far higher risk of developing the disease than the general population. The risk of Psoriasis Vulgaris is up to three fold higher in monozygotic twins than in dizygotic twins.
The gene found by association studies to be most closely linked to Psoriasis Vulgaris and probably accounting for 35 – 50% of the heritable risk is HLA-C*06. Approximately 87% of Psoriasis Vulgaris patients carry this gene. Other non HLA genes which have shown association are functional polymorphisms in genes for factors that control inflammation such as TNFα.
Genetic testing, especially for HLA-Cw6 is a useful aid to diagnosis, although most diagnosis is done quite easily on the basis of the skin presentation.
Reactive Arthritis
Reactive Arthritis, formally known as Reiters syndrome, is an autoimmune condition that develops in response to an infection in another part of the body, generally the gastrointestinal tract but also the genitourinary tract. Reactive Arthritis shares some clinical characteristics with other spondyloarthropathies such as Ankylosing Spondylitis and Psoriatic Arthritis. Reactive Arthritis is characterised by predominantly lower limb arthritis and enthesitis (inflammation of tendon insertion sites) as well as inflammatory back pain, inflammation of the eye and in some cases genitourinary inflammation. A number of bacteria have been identified as the possible sources of infection leading to Reactive Arthritis. These include Campylobacter, Salmonella and Chlamydia species.
Reactive Arthritis affects both males and females equally and has an age on onset of between 20 and 50. The condition is usually self limiting within a few days though it has been known to go on for up to 4 months in some patients. Non steroidal anti-inflammatory drugs and physical therapy may be used for the pain. The bacterial infection may need to be treated with antibiotics.
Population studies have shown the condition to be one of the most strongly associated with HLAB27 with over 80% of patients carrying the HLA-B*27 gene. The exact mechanism of action is unknown but may include the arthritogenic peptide theory in which a peptide either from B27 itself or from the site of attack such as the joint is presented to T cells by HLA-B27 leading to an immune response. Other theories include activation of auto reactive T cells which escaped thymic selection, molecular mimicry with bacterial peptide mimicking B27 derived self peptide and aberrant B27 heavy chain homodimers on the cell surface in the absence of beta 2 microglobulin acting as targets for inflammation.
HLA-B27 typing in Reactive Arthritis is a useful aid to diagnosis.
Rhematoid Arhtritis
Rheumatoid Arthritis (RA) is a chronic inflammatory arthritis that affects multiple synovial lined joints. The natural history of the condition is one of progressive articular damage leading to joint deformation and disabilities. Rheumatoid Arthritis is associated with a number of co-morbidities particularly in the cardiovascular system. The onset of RA is preceded by a prearticular autoimmune phase, the presence of antibodies against citrullinated (deaminated) protein and the presence of rheumatoid factor. Autoimmunity to citrullinated protein appears to be highly specific for Rheumatoid Arthritis with anti – citrullinated protein antibodies detected in approximately 70% of Rheumatoid Arthritis patients.
The prevalence of RA seems to be relatively uniform around the world though prevalence is low in sub Saharan Africa. This might reflect underreporting. The risk of developing RA does however vary between ethnic populations in the same global region, demonstrating a genetic component to this disease. The prevalence of Rheumatoid Arthritis in the general population is less than 1% but rises to between 2 – 4% in siblings of RA patients. This is confirmed by studies in monozygotic twins.
Multiple genetic loci have been shown to contribute to the risk of developing RA. Of these, the HLA class II DRB1 is the most important and contributes 30 – 50% of the overall genetic susceptibility risk. The association was initially thought to be with HLA-DR4 but subsequent studies have shown an association between RA and multiple HLA-DRB1 alleles not all of them DR4. These alleles did however share common sequences at positions 70 – 74 in the sequence. This has lead to the shared epitope hypothesis. Amino acids in these positions influence both peptide binding and contact between MHC and T cell receptor. HLA-DRB1 alleles associated with RA, more specifically with anti – citrullinated protein antibodies RA, were shown to have the sequence Arg-Ala-Ala (RAA) at positions 72–74. Gln (Q) or Arg (R) at position 70 carries a higher risk than Asp (D) whilst Lys (K) or Arg (R) at position 71 carry a higher risk than Ala (A) or Glu (Q). The shared epitopes that carry an increased risk are therefore QKRAA, QRRAA, RKRAA and RRRAA. HLA-DRB1 alleles with one of these sequences include HLA-DRB1*01:01, HLA-DRB1*01:02, HLA-DRB1*04:01, HLA-DRB1*04:04, HLA-DRB1*04:05, HLA-DRB1*04:08, HLA-DRB1*10:01, HLA-DRB1*13:03, HLA-DRB1*14:02 and HLA-DRB1*14:06. There is a gene additive effect such that patient’s homozygous or compound heterozygosity for the disease susceptibility alleles are at increased relative risk. Alleles which do not share any of these sequences and may be protective for RA are HLA-DRB1*03, HLA-DRB1*04:03, HLA-DRB1*04:07, HLA-DRB1*07, HLA-DRB1*08 and HLA-DRB1*09.
The shared HLA-DRB1 epitope does not explain the full RA genetic heritability. Genome wide association studies have led to recognition of an association between RA and the Arg620 to Trp polymorphism in the Protein tyrosine Phosphatase, non-receptor type-22 (PTPN22) gene, which codes for a powerful inhibitor of T-cell activation. A number of other candidate genes which have a much smaller effect than HLA-DRB1 and PTPN22 have also been described.
Scleroderma
Scleroderma, also known as systemic sclerosis, is a chronic systemic autoimmune disease. It is characterised by fibrosis or hardening of the skin, lungs, gastrointestinal tract and/or heart with endothelial dysfunction and a proliferative vasculopathy primarily affecting small blood vessels and capillaries. Patients often present with multiple autoantibodies, including antitopoisomerase I, anticentromere and anti-RNA polymerase III antibodies. In addition, patients have elevated levels of Th2 cytokines. Skin biopsies of scleroderma patients show perivascular infiltrates of mononuclear inflammatory cells.
Scleroderma occurs in genetically susceptible individuals who have encountered specific environmental triggers. Genome wide associated studies have implicated a number of candidate genes in the pathogenesis of the disease. Principle amongst these are HLA class II genes. Strong correlations have been demonstrated between certain HLA-class II alleles and each of the Scleroderma specific autoantibodies.
Scleroderma with anticentromere autoantibodies has been shown to be associated with HLADQB1*05:01 and other DQB1 alleles encoding non-leucine residues at position 26 in the peptide binding groove.
Scleroderma with antitopoisomerase I autoantibodies has been shown to be associated with HLADRB1*11, especially the DRB1*11:04 and DQB1*03:01 in Caucasian subjects and DRB1*15:02 and DQB1*06:01 in Japanese and Korean subjects.
Scleroderma with anti-RNA polymerase III antibodies has been shown to be associated with HLADRB1*13:02 and DQB1*06:04.
A number of other candidate genes outside of HLA, such as connective tissue growth factor, have also been tested for association with scleroderma.
Genetic testing in Scleroderma may be of value in providing insights into the mechanism of the disease, thereby potentially suggesting strategies for prevention and treatment.
Sjogrens
Sjogrens is a late onset chronic autoimmune disease which affects the exocrine glands, mainly the salivary and tears glands, resulting in insufficient secretion by those glands. It is characterized by a progressive lymphoid and plasma cell infiltration of the salivary and tear glands, accompanied by the production of autoantibodies (such as the anti nuclear antibodies anti-Ro/SSA and anti-La/SSB) leading to mucosal (dry mouth) and conjunctival dryness (dry eyes). Hallmarks of Sjogrens are B cell hyperactivity as manifested by hypergammaglobulinemia, circulating autoantibodies and/or immune complexes. Sjogrens may occur alone (primary Sjogrens) or secondary to other autoimmune disorders such as SLE and rheumatoid arthritis. Primary Sjogrens generally affects females (9/10 patients are female) and has an onset at age of about 40. The reasons for the female bias are unknown though hormones such as prolactin have been implicated.
Multiple genetic loci have been shown to contribute to the risk of developing Sjogrens. Of these, the HLA class II DR and DQ are the most important. An association has been demonstrated between Sjogrens and multiple HLA-DRB1, DQA1 and DQB1 alleles. Two HLA types in particular have been implicated, the DRB1*03 – DQB1*02 – DQA1*05:01 and the DRB1*15 – DQB1*06 – DQA1*01:02 haplotypes. The DRB1*03 – DQB1*02 – DQA1*05:01 haplotype has been linked to Sjogrens patients with anti-Sjogrens A and B autoantibodies while the DRB1*15 – DQB1*06 – DQA1*01:02 haplotype has been linked to Sjogrens patients with anti- Sjogrens A autoantibodies only.
Two other non HLA genes have been shown to be significantly associated with genetic susceptibility to primary Sjogrens disease, Signal Transducer and Activator of Transcription 4 (STAT4) and Interferon Regulatory Factor 5 (IRF5).
Sjogrens is diagnosed on the basis of screening questions, eye tests and salivary flow rates tests rather than on genetic testing. Genetic testing in Sjogrens may be of value in providing insights into the mechanism of the disease, thereby potentially suggesting strategies for prevention and treatment.
Systemic Lupus Erythematosus
Systemic Lupus Erythematosus (SLE) is a systemic inflammatory autoimmune disease characterised by the production of autoantibodies (such as the anti nuclear antibodies anti-Sm, anti-Ro/SSA and anti-La/SSB) with patients presenting with a diverse array of different clinical manifestations including skin, joint, haematologic, neurologic, renal and other organ involvement. SLE is predominately a female disease, affecting females and males in a ration of 9:1. Onset is usually between puberty and menopause with onset outside this range less common. Patients often undergo repeated cycles of flare ups and remissions. Complications of the disease seen in nearly half of all patients include glomerulonephritis and impaired renal function and/or neurological symptoms such as seizures, psychiatric symptoms, peripheral neuropathies or stroke.
The exact aaetiology of SLE is unknown, though it is thought to have a genetic component as well as environmental and hormonal components. The genetic component of SLE is supported by twin and family studies. The concordance of SLE in monozygotic twins in approximately 25 – 50% compared to only 5% in dizygotic twins. Population studies have revealed that susceptibility to SLE is associated with HLA class II, in particular with haplotypes bearing the DRB1*03:01, DQB1*02:01 and the DRB1*15:01, DQB1*06:02 alleles. These alleles were present in over 65% of SLE cases. The DRB1*03:01 allele carries a higher risk than the DRB1*15:01. Individuals homozygous for DRB1*03:01 or compound heterozygous for DRB1*03:01 and DRB1*15:01 carry the highest risk. The DRB1*03:01 allele is associated with production of anti-Ro/SSA and anti-La/SSB autoantibodies while the DRB1*15:01 allele is associated with production of anti-Sm autoantibodies.
Genome wide association studies have also identified a number of non HLA genes that are risk factors for SLE. These include genes for the Fc receptors for immunoglobulin G which mediate clearance of immune complexes. The Arg620 to Trp polymorphism in the Protein tyrosine Phosphatase, non-receptor type-22 (PTPN22) gene (which codes for a powerful inhibitor of T-cell activation) may also have a role in SLE.
Genetic testing for SLE has significant clinical relevance for efforts to more fully characterise the aaetiology and pathway of development of SLE and may lead to improved diagnostic and prognostic tools. Developments in genetic testing for SLE may also lead to the development of more specific therapies for SLE and related conditions.
Uveitis
Uveitis is the most common form of inflammatory eye disease and is a significant cause of visual impairment and blindness. Uveitis specifically refers to inflammation of the layer of the eye known as the ‘Uvea’ (the iris, ciliary body, and choroid) but is commonly used to describe any inflammatory process involving the interior of the eye. Uveitis is classified anatomically into Anterior, Intermediate, Posterior and Pan-Uveitis. Anterior Uveitis involves inflammation of the anterior chamber and the iris and accounts for more than 90% of Uveitis referrals. Intermediate Uveitis involves inflammation of the vitreous cavity and Posterior Uveitis involves inflammation of the retina and choroid. Pan-Uveitis involves inflammation of all layers of the Uvea.
Uveitis onset is usually between the ages of 20 to 50 and is relatively uncommon in the very young (< 10 years of age) and in the elderly (> 70 years of age). Uveitis is often associated with a number of other seronegative spondyloarthropathies such as Ankylosing Spondylitis, Reactive Arthritis, Psoriatic Arthritis and Inflammatory Bowel Disease.
Twin and family studies have shown a strong association between susceptibility to Uveitis and the HLA-B*27 gene. HLA-B27 positive relatives of Uveitis patients are at higher risk of developing the disease than B27 positive individuals without an affected relative. However not all B27 positive individuals develop Uveitis, showing that HLA-B27 is often necessary but is not a sufficient genetic predisposing factor for the disease. Other non HLA genes may also be involved.
Studies of the frequencies of different HLA-B*27 alleles in HLA-B27 associated Uveitis in patients with other systemic diseases have revealed no significant difference in the frequencies of the alleles. In a recent study, the HLA*27:04 allele was found at a significantly lower frequency in Uveitis patients without other systemic diseases.
The association between HLA-B27 and Uveitis is not 100% which slightly limits its value as a diagnostic tool. However HLA-B27 testing may be of value in identifying a previously undiagnosed or misdiagnosed spondyloarthropathy among patients with recurrent Uveitis.