Find out if you are at risk - get tested
How does the test work?
The DNA test examines the HFE gene for mutations that cause the gene to become defective. The HFE gene has three known mutations which cause the gene to become defective, namely C282Y, H63D and S65C. These three mutations account for about 85% of all cases oh hereditary hemochromatosis. The C282Y mutation is the one most commonly found in individuals with hemochromatosis and is associated with a more severe phenotype, or clinical presentation. The H63D and S65C mutations are associated with a milder phenotype. All three mutations can be detected by DNA testing.
What is the HFE gene?
The HFE gene was first discovered as the causative gene for hemochromatosis in 1996. This discovery allowed DNA testing for the diagnosis of hereditary hemochromatosis.
The HFE gene is located on the short arm of chromosome 6. It is linked to the HLA locus on chromosome 6 and was originally called HLA-H (even though it is not a histocompatibility gene), and the modern designation for this gene is now HFE.
The HFE gene encodes an HLA class I-like protein. The normal HFE protein is expressed on the cell surface in association with β-2 microglobulin as a heterodimer. This protein heterodimer is responsible for regulating the absorption of dietary iron. HFE regulates iron uptake by interacting with the transferrin receptor, leading to a decreased affinity of the transferrin receptor for transferring iron and reducing iron uptake, thus limiting the amount of dietary iron absorbed by the body.
Genetics of Hemochromatosis
Hereditary hemochromatosis is an autosomal recessive disorder of iron metabolism. "Autosomal" means that the gene is on one of the first 22 pairs of chromosomes, and not on the X or Y chromosome (HFE is located on chromosome 6). Therefore, males and females are equally affected by the disease. "Recessive" means that two copies of the defective gene, one inherited from each parent, are necessary to have the condition. That means that you usually have to have two copies of the defective HFE gene before you are at risk of developing hereditary hemochromatosis.
We all have two copies of the HFE gene, one copy inherited from our mother and one copy inherited from our father. Our chances for developing hemochromatosis depends on whether we have inherited the normal or the defective HFE gene from our parents. Hereditary hemochromatosis can occur when a person inherits two defective copies of the HFE gene, one from each parent. Men and women have the same chance of inheriting two copies of the defective HFE gene.
A person who has inherited one defective HFE gene and one normal HFE gene usually does not develop the symptoms of hemochromatosis as the normal gene can balance out the defective HFE gene. In a small number of cases, inheriting only one defective gene may still eventually lead to iron overload. In these people, the iron overload may be triggered by a precipitating factor, such as hepatitis (inflammation of the liver) or alcohol abuse.
Risk of Iron Overload
Of the three mutations, C282Y carries the highest risk for iron overload when inherited in the homozygous state. The H63D and S65C mutations are less penetrant and have a lower chance of causing disease. The molecular diagnosis of hemochromatosis involves an assay to detect the three known mutations within the HFE gene.
The most significant mutation is C282Y, in which a G is changed to an A at nucleotide position 845 of the HFE gene. This results in a missense mutation which removes a highly conserved cysteine residue and substitutes it with a tyrosine amino acid residue at position 282 of the HFE gene product. Position 282 of the HFE gene product is the β-2 microglobulin binding domain of the protein, and the C282Y mutation destroys a key cysteine residue that is required for disulfide bonding with β-2-microglobulin. This mutation prevents the HFE gene product from binding to β-2 microglobulin and thus prevents the protein from being co-expressed on the cell surface. Since this protein complex is responsible for binding to the transferrin receptor on the surface of gastrointestinal epithelial cells to regulate dietary iron uptake, the absence of this protein from the cell surface leads to unregulated protein absorption by the body. This results in high dietary iron absorption and iron overload. Between 80% to 85% of patients with hereditary hemochromatosis have two copies of the C282Y mutation (homozygous for C282Y).
The second most significant mutation is H63D, in which a C is changed to a G at nucleotide position 187 of the HFE gene. This results in a missense mutation which removes a histidine residue and substitutes it with an aspartic acid residue at position 63 of the HFE gene product. This mutation is thought to alter a pH-dependent intramolecular salt bridge, possibly affecting interaction of the HFE protein with the transferrin receptor, thus preventing the proper regulation of iron uptake by the body.
The least important mutation is S65C, in which an A is changed to a T at nucleotide position 193 of the HFE gene. This results in a missense mutation which removes a serine residue and substitutes it with a cysteine residue at position 65 of the HFE gene product. The mechanism that this mutation prevents iron regulation is currently unknown.