DNA Testing for Hereditary Hemochromatosis

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 multi systemic 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 reversible 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 Centres 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.

 By:  Krishna P.Pudasaini
       Australian National University
       National Institute of Bioscience and Technology
       Department of Biotechnology (Genetic Engineering)
       E-Mail: biotech_krishna2004@yahoo.com
                    nib@anu.edu.au

References:

       a)      Molecular Genetic Testing in Mainstream

b)      On the guidance of john Johnson, MD(T),Australian National University

c)      Text book of molecular medicine

d)     Modern genetics and human