Examination of ALAS2, ABC7, Rag1, and IL6 Genes as Candidate Modifiers of Iron Overload in HFE C282Y Homozygotes with Severe Iron Overload.

The variable penetrance of HFE hemochromatosis led us to examine candidate modifier genes in 24 adult probands homozygous for the HFE C282Y mutation with severe iron overload. Mutations of the X-linked genes ALAS2 and ABC7 are associated with sideroblastic anemia and are such candidate genes. One C282Y homozygote had a P520L mutation in ALAS2 and another had an an ABC7 ivs2+1 G->A mutation that would result in the loss of a splice site and predict skipping of exon 2 with an in-frame deletion of 27 amino acids from the mature protein. Examination of respective family members demonstrated that inheritance of the ALAS2 P520L mutation or the ABC7 ivs2+1 mutation alone was not associated with excessive iron storage or sideroblastic anemia. Only subjects homozygous for HFE C282Y and hemizygous or heterozygous for these ALAS2 or ABC7 mutations had iron overload. Because the families were small, it was not possible to establish conclusively that these mutations affected the severity of iron overload. Nevertheless, these data suggest that if ALAS2 and ABC7 mutations are modifers of HFE hemochromatosis, they occur at a low frequency and do not account for the majority of cases of severe iron overload in C282Y homozygotes. Mice expressing double knockouts of rag1 and hfe or beta 2 microglobulin exhibit more severe iron overload than mice with knockouts of either gene alone. Thus, we sought to determine if mutations in RAG1 were associated with severe iron overload in 20 HFE C282Y homozygotes with severe iron overload. Four non-synonymous SNPs were identified in the HFE C282Y homozygotes: R249H, R449K, K820R, and M1006V. These mutations occurred at allele frequencies of 0.3500, 0.0238, 0.1190, and 0.0227, respectively. In 364 control subjects, the allele frequencies of R249H (0.3750) and K820R (0.1133) did not differ significantly from those in HFE C282Y homozygotes. A promoter polymorphism in IL6 at nt −174 has been shown to be associated with IL-6 expression: the G allele with high expression and the C allele with low expression. Lower levels of IL-6 expression might result in lower levels of hepcidin expression, further decreasing the already lower levels of hepcidin resulting from HFE C282Y homozygosity. Analysis of 340 control whites disclosed 62 IL6 -174C/C subjects had a mean transferrin saturation of 28% and 107 IL6 -174G/G subjects had a mean transferrin saturation of 25% (p <0.05). We found that the C allele frequency was 0.4800 in C282Y homozygotes with severe iron overload; this did not differ significantly from the frequency of 0.4338 in white control subjects. These data suggest that the IL6 -174 C allele is not significantly associated with severe iron overload in HFE C282Y homozygotes. We conclude that mutations of ALAS2 and ABC7 may be rare causes of severe iron loading in HFE C282Y homozygotes, but it is unlikely that RAG1 and the IL6 promoter polymorphisms contribute significantly to the iron loading of such subjects.