Several mutations of the HFE gene have been described. The most common mutation in patients with HH is the C282Y mutation. In the C282Y mutation, a base substitution leads to a change in the amino acid in position 282 from cysteine (C) to tyrosine (Y). The loss of the sulfhydryl-containing amino acid disrupts the tertiary structure of HFE so that it no longer binds to beta-2 microglobulin. Beta-2 microglobulin appears to act along with other proteins to chaperone the newly synthesized HFE out of the Golgi apparatus and to the cell surface where it can bind to TfR. In the C282Y mutation, HFE remains in the Golgi, never making it to the cell surface. The result is that transferrin binding to TfR is enhanced, and excessive amounts of iron enter the cells of the small intestine, liver, and other tissues.
Due to the C282Y mutation, HH is considered an autosomal recessive disorder with low penetrance. In other words, some individuals homozygous for the mutation (carry two copies of the gene) do not develop clinical symptoms of the disease. Among symptomatic patients with HH, the majority are homozygous for C282Y.
A second mutation, H63D, causes a histidine (H) residue in position 63 to be replaced by aspartic acid (D). The mechanism by which this mutation leads to increased iron uptake is less well understood when compared to the C282Y mutation. Unlike the C282Y mutation, the H63D mutation does not seem to affect the binding of beta-2 microglobulin and intracellular movement since detectable concentrations of the mutated protein are found on cell membranes. Some researchers speculate that the H63D mutation affects the binding of proteins involved in iron regulation and uptake at the cell surface. In rare cases, individuals who are homozygous for H63D or are heterozygous for C282Y and/or H63D may be diagnosed with HH.
A third mutation, S65C, leads to a serine-to-cysteine substitution in its associated protein. This mutation has been found in some compound heterozygotes for C282Y or H63D but is rarely associated with iron overload in HH.
Additional mutations of HFE have been identified, but their clinical significance is unclear.
Most laboratories perform molecular assay tests for only the C282Y, H63D, and S65C mutations.