Mapping genes responsible for strain-specific iron phenotypes in murine chromosome substitution strains

Abstract

The highly variable clinical phenotype observed in patients homozygous for the C282Y mutation of the hereditary hemochromatosis gene (HFE) is likely due to the influence of non-HFE modifier genes. The primary functional abnormality causing iron overload in hemochromatosis is hyper-absorption of dietary iron. We found that iron absorption in inbred mice varies in a strain-specific manner, as does the pattern of iron distribution to the liver and spleen. A/J mice absorbed approximately twice the amount of 59Fe delivered by gavage compared to the C57BL/6 strain. Genetic comparisons between A/J and C57BL/6 were facilitated by the availability of consomic chromosome substitution strains (CSS). Each CSS has an individual chromosome pair from A/J on an otherwise C57BL/6J background. We found that iron absorption and iron content in liver and in spleen were continuous variables suggesting that each trait is under multigenic control. No trait co-segregated among the CSS. Chromosome 5 from A/J, however, imparted the highest iron absorption phenotype and multiple CSS had absorption levels equivalent to A/J. Chromosomes 9 and X were associated with high spleen iron content. These data suggest that multiple genes contribute to the regulation of iron absorption and that individual organ iron phenotypes are independently regulated.

Figure 1. Iron measures in different strains of mice

a. Mice were fasted and given a measured dose of ⁵⁹Fe by gavage. Twenty-four h later, mice were sacrificed, dissected and the percent of the administered radiolabel remaining with the carcass minus the head and GI tract was measured. b. The hepatic iron content (black bars) and the percent Tf saturation (grey bars) were determined for the four strains. N values are the same for animals in Figures a and b. Error bars represent SEM. * P< 0.05 vs. C57BL/6. ** P< 0.05 vs. A/J.

Figure 2. Comparison of iron parameters in F1 mice

Black bars: Absorption of ⁵⁹Fe. Absorption was measured in A/J, C57BL/6 and F1 mice. Gray bars: Hepatic iron content in A/J, C57BL/6 and F1 mice. All mice were 8-9 week old males. Error bars represent SEM.

Figure 3. Iron measures in CSS mice

a. ⁵⁹Fe absorption was measured in CSS and ordered by mean percent of administered radiolabel absorbed. b. Liver iron concentration was measured and ordered by increasing value of the mean. c. Spleen iron was measured and ordered by increasing values of the mean. Student's T-test was applied to compare C57BL/6J to all other strains. Error bars represent SEM. * Mean values statistically different from C57BL/6 (dotted line) and A/J (solid line) when adjusted for the number of groups (p < 0.0024).