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| Hereditary
Hemochromatosis |
| Genes Can Cause Hemochromatosis |
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By Amanda
Ewart Toland, PhD
Reviewed
by Chris Friedrich,
MD, PhD
By definition, hereditary hemochromatosis
— an iron overload disorder — runs in families:
An individual develops the condition by inheriting altered,
or mutated, copies of the HFE gene
(which regulates iron absorption in our bodies) from
his or her parents. However, even though the disorder
is clearly inherited, it's not always easy to spot a
pattern of disease within families who have the mutation.
This is because people can be carriers
of a hemochromatosis mutation without having any symptoms
or complications. Thus, the fact that no one in your
immediate or extended family has symptoms of hereditary
hemochromatos does not mean you cannot develop it yourself.
Fortunately, it's relatively easy to find out whether
you have the disorder and then to treat it — if
it's caught early. (For news about hemochromatosis in
families, see Related News below.)
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When
Hemochromatosis Isn't Hereditary
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Although
the majority of people who develop hemochromatosis,
or iron overload, do so by inheriting mutations
in the HFE gene, there are others ways of
acquiring the disorder as well — for
example, by taking in excessive amounts
of iron via food, vitamins, or medication.
These
types of acquired iron overload are
called secondary hemochromatosis.
The focus of these pages, however, is hereditary
hemochromatosis; unless otherwise
stated, all mentions of hemochromatosis
here (and on other pages of the Genetic
Health site) refer to the hereditary kind.
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How
Hemochromatosis Runs in Families
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Although
approximately 1 out of every 200 people in the US is a
carrier of one of the genetic mutations that cause hereditary
hemochromatosis, not nearly that many people actually
develop symptoms or complications from the disorder. This
is because hereditary hemochromatosis is passed through
families in what is known as autosomal recessive
fashion, which means that an individual must inherit a
mutated copy of the HFE gene from both parents to develop
the disorder. (Like most genes, the HFE gene comes in
pairs, with one copy inherited from each parent.) Thus,
if both parents have a mutation in just one of their two
HFE genes, each of their children has a 25 percent chance
of inheriting two mutated HFE genes — and with them
hereditary hemochromatosis.
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More
on Basic Genetics and Patterns of Inheritance
(Coming Soon)
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When
Inheriting Just One Copy of a Mutated HFE Gene
Can Be a Problem
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Even
though an individual must inherit a mutated copy
of the HFE gene from both parents to develop hereditary
hemochromatosis, some people who have inherited
just one HFE gene with the C282Y mutation may
be at risk for cardiovascular disease due to small
increases in iron storage.
According
to recent premilimary studies, male carriers
of the C282Y mutation are twice as likely to
have heart attacks as men who do not have the
mutation, while female carriers of the C282Y
mutation are
1 1/2 times more likely to have heart attacks.
(Remember: You only need to have one mutated
copy of the HFE gene to be a carrier.) However,
more research is needed to determine if the
effect real.
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Different
Kinds of Risk for Different People
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To
further complicate matters, even individuals who have
inherited two mutated copies of the HFE gene have varying
degrees of risk for hemochromatosis. This is because
there are two distinct mutations —
C282Y and H63D (named for their locations on the HFE
gene) —
that can lead to the disorder, and their combination
determines both the chances of symptoms occurring and
their severity. For example, the C282Y mutation appears
to cause a more severe form of hemochromatosis (and
one that appears at an earlier age) than does the H63D
mutation. Thus, individuals who have inherited two copies
of the C282Y mutation will almost always develop symptoms
(and, if left untreated, complications) of hemochromatosis,
and individuals who have inherited two copies of the
H63D mutation rarely develop symptoms. Likewise, individuals
who have inherited one C282Y mutation and one H63D mutation
have a different level of risk than individuals who
have a pair of C282Y or H63D mutations.
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References
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Olynyk,
JK, Cullen, DJ, Aquilia, S, Rossi, E, Summerville, L,
Powell, LW. A population-based study of the clinical
expression of the hemochromatosis gene. New Engl.
J. Med. 341: 718-724 (1999).
Adams, PC. Nonexpressing homozygotes for C282Y Hemochromatosis:
Minority or majority of cases. Mol. Genet. And Metab.
71: 81-86 (2000).
Camaschella, C. Roetto, A., Cali, A., Gobbi, M. Garozzo,
G., Carella, M. Majorano, N., Totaro, A., and Gasparini,
P. The gene TFR2 is mutated in a new type of haemochromatosis
mapping to 7q22. Nat. Genet. 25: 14-15 (2000).
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