Common variants in the BMP2, BMP4, and HJV genes of the hepcidin regulation pathway modulate HFE hemochromatosis penetrance

Most cases of genetic hemochromatosis (GH) are associated with the HFE C282Y/C282Y (p. Cys282Tyr/p. Cys282Tyr) genotype in white populations. The symptoms expressed by C282Y homozygotes are extremely variable. Only a few suffer from an overt disease. Several studies have suggested that, in addition to environmental factors, a genetic component could explain a substantial part of this phenotypic variation, although very few genetic factors have been identified so far. In the present study, we tested the association between common variants in candidate genes and hemochromatosis penetrance, in a large sample of C282Y homozygotes, using pretherapeutic serum ferritin level as marker of hemochromatosis penetrance. We focused on two biologically relevant gene categories: genes involved in non-HFE GH (TFR2, HAMP, and SLC40A1) and genes involved in the regulation of hepcidin expression, including genes from the bone morphogenetic protein (BMP) regulatory pathway (BMP2, BMP4, HJV, SMAD1, SMAD4, and SMAD5) and the IL6 gene from the inflammation- mediated regulation pathway. A significant association was detected between serum ferritin level and rs235756, a common single- nucleotide polymorphism (SNP) in the BMP2 genic region (P = 4.42 x 10(-5)). Mean ferritin level, adjusted for age and sex, is 655 ng/ml among TT genotypes, 516 ng/ml in TC genotypes, and 349 ng/ml in CC genotypes. Our results further suggest an interactive effect on serum ferritin level of rs235756 in BMP2 and a SNP in HJV, with a small additive effect of a SNP in BMP4. This first reported association between common variants in the BMP pathway and iron burden suggests that full expression of HFE hemochromatosis is linked to abnormal liver expression of hepcidin, not only through impairment in the HFE function but also through functional modulation in the BMP pathway. Our results also highlight the BMP regulation pathway as a good candidate for identification of new modifier genes.