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.
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
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
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
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
By: Krishna P.Pudasaini
Australian National University
National Institute of Bioscience and Technology
Department of Biotechnology (Genetic Engineering)
Molecular Genetic Testing in Mainstream
the guidance of john Johnson, MD(T),Australian National University
book of molecular medicine
Modern genetics and human