Phenotypic Variability in Genetic Hemochromatosis: Study of Five SNPs in a Cohort of 342 Compound Heterozygotes for the HFE Cys282Tyr / His63Asp Mutations

The HFE p.Cys282Tyr (C282Y) mutation is the disease-causing defect in most cases of genetic hemochromatosis, but involvement of the p.His63Asp (H63D) variant in the pathogenesis of the illness is still a matter of debate. Some authors consider C282Y/H63D (YD) subjects as at risk of developing hemochromatosis, whereas others argue that iron overload, seen in some of these subjects, is related to external factors. As for the C282Y homozygotes, genetic modifiers might contribute to the phenotypic variability of hemochromatosis in YD subjects. This work was thus aimed at assessing the possible influence of Single Nucleotide Polymorphism (SNPs) suspected to play a role in iron metabolism on disease expression in compound heterozygotes.

We genotyped five SNPs in 342 YD unrelated Caucasian patients (208 males, 134 females) from South France. Information on demographics, clinical manifestations, laboratory tests, viral status and alcohol consumption was obtained from the medical records of the participants. The selected SNPs were already described as modifiers of iron overload by previous studies. They are located in the TMPRSS6 (rs855791 and rs4820268), TF (rs3811647), CYBRD1 (rs884409) and GNPAT (rs11558492) genes. The role of GNPAT in iron metabolism is unknown but it has been recently associated with the severity of the clinical expression in C282Y homozygous subjects¹. Analysis of the SNPs was performed using Endpoint TaqMan Genotyping technology.

For each SNP, the additive effect of the minor allele on the logarithm of serum ferritin (SF) was first assessed through a linear regression, controlling for age, sex, alcohol consumption, and patient’s referral motive. We found a marginally significant association between rs884409 allele G of CYBRD1 and SF (p = 0.014).

In a second step, we restricted our analysis to the 77 most severe males (SF > 500 µg/l and no past or current history of alcohol consumption > 20g/day), and found that the GNPAT rs11558492 allele G was significantly enriched in this subset. Indeed, 8 subjects were homozygous for allele G and 40 were heterozygous AG. G allele frequency in these 77 males was 36%, which is higher than the 19% found in the individuals from Northwest Europe (CEU) used for the HapMap project (p<0.0001) and the 21% reported in 4300 European Americans from the National Heart, Lung, and Blood Institute (NHLBI) Exome Sequencing Project (p <0.0001). It is also higher than in the 26% found in the rest of the YD cohort (p=0.0029) which includes a large number of younger subjects recruited because of family history of hemochromatosis and of premenopausal women whose severity on the long term is difficult to assess.

In conclusion, our study draws attention to two SNPs (rs884409 and rs11558492) in genes DCYTB and GNPAT, respectively, that are appear to worsen the phenotypic expression of YD patients. These results also support the hypothesis that GNPAT plays a still undiscovered role in iron homeostasis.

1.   McLaren, C. E. et al. Exome sequencing in HFE C282Y homozygous men with extreme phenotypes identifies a GNPAT variant associated with severe iron overload. Hepatol. Baltim. Md (2015). doi:10.1002/hep.27851

The authors are grateful to the members of the “Centre de competence” on rare iron disorders of CHU de Montpellier