Phenotype Variation in C282Y Homozygotes for the Hemochromatosis Gene

Background & Aims: Typical hemochromatosis patients are homozygous for the C282Y mutation of the HFE gene. However, approximately 50% of women and 20% of men do not have an increase in serum ferritin level. This study assessed factors, genetic and otherwise, that may modify biochemical expression in C282Y homozygotes. Methods: Hemochromatosis families that each had at least 2 untreated C282Y homozygotes were included. Nonexpressors were defined as having a ferritin level less than 300 μg/L for a male and a ferritin level less than 200 μg/L for a female. A multivariate linear stepwise regression analysis was performed to assess the contribution of sex, age, and the presence of a nonexpressor in the family on serum ferritin level. Heritability calculations were performed using variance component analysis. Results: Fifty-three pedigrees consisting of 148 C282Y homozygotes (92 males, 56 females) were identified. Twenty-nine homozygotes were nonexpressors (19.6%), of which 11 of 29 were male (37.9%) (P = .0054). Based on multivariate analysis, highly significant associations with increased ferritin levels in C282Y homozygotes were male sex (P = .00005), increasing age (P = .009), and absence of a nonexpressor in the family (P = .01). Variance component modeling using age, sex, and C282Y genotype as covariates in 39 pedigrees (n = 296) showed a residual heritability for serum ferritin of .35 ± .10 (P < .001). Conclusions: These findings suggest that male sex is the major clinical factor associated with an increased serum ferritin level in hemochromatosis. In addition, these results support the hypothesis that other genes contribute to the variance in serum ferritin concentration among C282Y homozygotes.