Vol. 14, Spring 1997
Molecular Genetic Testing in Mainstream Medicine
DNA Testing for Hereditary Hemochromatosis
- Introduction
Hereditary hemochromatosis (HH) is regarded as the most common genetic disorder in Caucasians, with an estimated prevalence of 1/200-1/400 (homozygotes) and a carrier frequency (heterozygotes) of 1/8-1/10. It is an excellent example of a common genetic disorder with high morbidity and mortality which is preventable if diagnosed early. A DNA test for hemochromatosis has recently become available and will be a valuable aid for early detection of this disorder.
Hemochromatosis is characterized by increased iron absorption in the intestine, resulting in a gradual build-up of excess iron (iron overload). Most homozygotes develop symptoms in adulthood, although the age of onset has been as young as 2 years. Heterozygotes are usually asymptomatic.
Early symptoms in homozygotes are multisystemic and often non-specific, including malaise, fatigue, impotence, abdominal pain, or joint pain. As iron storage increases, deposits occur in organs such as the liver, heart, and pancreas, leading to organ damage and dysfunction.
Late sequelae include diabetes, arthritis, congestive heart failure, cirrhosis, and hypogonadism. Bronze discoloration of the skin occurs in some patients. There is high morbidity and mortality associated with untreated hemochromatosis. However, if diagnosed early and treated by phlebotomy, progression of iron storage is arrested.
Many symptoms are reversable and normal lifespan is possible. Therapy after late detection is less effective, particularly if cirrhosis is present, and prognosis is guarded. Therefore, early diagnosis is essential.
Until the recent development of a DNA test, early diagnosis has been difficult. Despite the high prevalence of HH, (one million Americans are affected), most cases remain undiagnosed. This is likely due both to the misconception that HH is a rare disorder and to the non-specific nature of the symptoms. Testing has been based upon measurements of serum iron, iron binding capacity, transferrin saturation, and ferritin concentration. However, these tests are imperfect, without clear "cut-offs" for results indicating affected status. Due to the benefits of early diagnosis and treatment, universal screening of all adults using these screening tests has been advocated by the Centers for Disease Control, a major HMO, and other groups. Limited population screening is occurring because iron and iron binding capacity are offered as components of some routine "chemistry panels". Liver biopsy has traditionally been the "gold standard" for definitive diagnosis, but this invasive procedure is not usually performed until late in the course of the disorder. In contrast, the DNA test is simple, non-invasive, and enables early diagnosis.
There are complexities in the genetic analysis of HH. Most, but not all homozygous individuals develop disease. Females exhibit a lower penetrance (i.e. not all homozygous females develop disease) likely due to iron loss through menses. In addition, although the disorder is classified as autosomal recessive,(requiring presence of two abnormal alleles to cause disease), in some families inheritance mimics an autosomal dominant pattern. This occurs when an affected parent has a carrier partner, in which case half of the children are affected.
- Key Points
- Prevalence of HH is high in Caucasians. 1/200-400 are homozygotes and 1/8-10 are carriers
- Early diagnosis is essential to avoid morbidity and mortality
- Early symptoms are non-specific
- DNA testing (Cys282Tyr mutation) is highly effective (detects over 85% of cases) with rapid turn-around
- Phlebotomy is simple and effective therapy
- The HH gene and mutations
Over a decade ago, the gene for HH was found to be linked to the major histocompatibility gene cluster (HLA) on the short arm of chromosome 6. Certain HLA types were found to be statistically associated with HH, allowing for predictions of risk based upon HLA type alone. The associated alleles are A3, B7, and B14. HLA linkage and haplotyping have been useful within affected families for predicting inheritance of the gene. Very recently, the linkage to HLA finally led to actual isolation of the gene, called "HLA-H" because of predicted resemblance of the gene product to HLA proteins. The function of the predicted HLA-H protein is unknown at present. The gene name is somewhat controversial and may be changed in the future.
A common defect, Cys282Tyr, has been discovered in the HLA-H gene. It is a single base mutation changing amino acid 282 in the protein from cysteine to tyrosine. Most affected patients are homozygous for this DNA mutation, with frequencies varying from 85-100% in early studies. A second potential mutation, His63Asp, (predicting a change from histidine to aspartic acid in amino acid 63), has been reported. There is some uncertainty as to the significance of this second mutation. In the homozygous state it does not cause HH. In combination with the Cys282Tyr mutation on the other chromosome it may contribute to HH in some cases. "Compound heterozygotes" with different mutations on the two chromosomes are found both in affected and in normal populations indicating reduced penetrance for His63Asp, if it is indeed clinically significant. Alternatively, it is possible that His63Asp is a benign sequence alteration (DNA polymorphism).
The DNA tests for these mutations are simple polymerase chain reaction (PCR) analyses of targeted gene regions. Individuals identified as Cys282Tyr homozygotes are at high risk for developing HH and should be followed closely for elevated iron levels. These recommendations include pediatric patients since the onset of iron storage occurs in childhood and the benefit of early therapy is clear in terms of producing normal longevity. Unfortuntaely, a small percentage of Caucasian HH patients have a defect unlinked to the HLA-H gene, and that will not be detected by the current targeted DNA testing. A significant contribution of other as yet unknown genes to HH is more likely in other ethnic groups.
- Indications for DNA testing include:
- Previous clinical diagnosis of HH
- Positive family history or partner with HH, especially if Cys282Tyr positive
- Elevated transferrin saturation (>60%) or serum ferritin concentration (>400 ng/ml in men and >200 ng/ml in women)
- Unexplained elevation of serum liver enzymes
- Cirrhosis, liver failure, or hepatocellular carcinoma
- Diabetes mellitus
- Non-specific compatible symptoms and signs: fatigue, abdominal pain, hepatosplenomegaly, joint pain, cardiac arrythmia, congestive heart failure, impotence, hypogonadism, hypothyroidism, hyperpigmentation
Iron storage occurs secondarily in other disorders, such as thalassemia and porphyria. Of interest, a recent study of patients with porphyria cutanea tarda found an increased frequency of HH mutations, with some patients homozygous for the Cys282Tyr mutation. These results indicate that HH mutations may contribute to iron overload in other conditions predisposing to this clinical problem.
Due to the wide range of early symptoms, patients may be diagnosed in a variety of clinical settings. Physicians in primary care and in all medical specialties should consider HH in the differential diagnosis of the above clinical presentations.
- Illustrative Case Report
A couple was counseled regarding their family history of HH and the risk for their 3-year-old child. An aunt to the father was recently diagnosed and was undergoing periodic phlebotomy. Her mother died of liver cancer, sometimes found as a late development in HH and a sign that she might have been affected. Her brother, the paternal grandfather to the child in question, died of a stroke but was not known to have HH. The risk that the father carries one HH gene is at least 1/3, and he has at least a 1/60 chance to be affected. He had normal iron screening studies but was interested in pursuing genetic testing.
His aunt was tested and was found to be homozygous for the common Cys282Tyr mutation. We are in the process of testing the father. Should he be homozygous or heterozygous for this mutation, we will proceed to testing of the spouse to see if the couple is at risk of having affected children. Although genetic testing is not generally advocated for children, exceptions include disorders with potential pediatric onset and benefit of early intervention. HH fits this paradigm and we will certainly test the child should both parents be carriers. Asymptomatic homozygotes identified in the family will be advised to undergo regular iron testing to detect overload and initiate phlebotomy.
contributed by John P. Johnson, M.D. (MT) and Annette K. Taylor, Ph.D. (CO)
The Genetic Drift Newsletter is not copyrighted. Readers are free to duplicate all or parts of its contents. The Genetic Drift Newsletter is published semiannually by the Mountain States Genetics Network for associates & those interested in Human Genetics. In accordance with accepted publication standards, we request acknowledgement in print of any article reproduced in another publication. The views expressed in the newsletter do not necessarily reflect local, state, or federal policy. For additional information, contact Carol Clericuzio, M.D., Editor, Department of Pediatrics, The University of New Mexico, Albuquerque, NM, 87131
Molecular Genetic Testing in Mainstream Medicine:
Table of Contents
Introduction
Venous Thrombosis and the Factor V (Leiden) Mutation
DNA Testing for Hereditary Hemochromatosis
APO E Genotype Testing for Broad Beta Disease (Type III Hyperlipoproteinemia)
Fetal Rh Testing for Maternal-Fetal Incompatibility
Type 1 (insulin-dependent) Diabetes Mellitus
Adult Onset Neurodegenerative Disorders: CAG Triplet Repeat Expansion Mutations
Genetic Testing for Prader-Willi and Angelman Syndromes
Clinical and Applied Molecular Genetics Laboratories - MoSt GeNe Region
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