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 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.
