Reply
Authors: Christine E. McLaren, Mary J. Emond, V. Nathan Subramaniam, Pradyumna D. Phatak, James C. Barton, Paul C. Adams, Lawrie W. Powell, Lyle C. Gurrin, Grant A. Ramm, Gregory J. Anderson, Gordon D. McLaren
Journal: Hepatology (2015)

Our work was supported, in part, by grant 1R24DK093433‐01 from the National Institute of Diabetes and Digestive and Kidney Diseases, grant P30 CA‐62203 from the National Cancer Institute, and funds from the Department of Veterans Affairs. This work was also supported, in part, by a Project Grant (APP1031325) from the National Health and Medical Research Council (NHMRC) of Australia (to V.N.S.). V.N.S., G.A.R., and G.J.A. are supported by Senior Research Fellowships from the NHMRC of Australia. *C.E.M., M.J.E. and V.N.S. contributed equally to this study and are joint first authors. [Correction added October 22, 2015, after first online publication: this note was missing from original version.] Author names in bold designate shared co‐first authorship. We appreciate the response of Bardou‐Jacquet et al. to our report of a positive association of GNPAT p.D519G (rs11558492) with severe iron overload in males with HFE C282Y homozygosity and low alcohol consumption.1 Their genome‐wide association study of 474 unselected French male and female C282Y homozygotes found no significant association of p.D519G with iron overload. Briefly, these differences in our studies can be linked to the design and power of our study and to inclusion criteria. Power Considerations The power of our extreme phenotypes study should be greater than that of many larger clinical studies.2 Bardou‐Jacquet et al. do not provide power calculations nor confidence intervals (CIs) with effect sizes for association with serum ferritin (SF). Biologically/clinically significant effect sizes could be covered by 95% CIs. As with others,3 we observed a strong positive association between SF and alcohol consumption. Adjusting for alcohol provides greater power to detect associations with modifier alleles, but it is not apparent whether Bardou‐Jacquet et al. made such an adjustment. Their subset of 36 males who consumed <20 g/day of alcohol has insufficient power to detect a p.D519G association without an extreme phenotypes design. Thus, the observations of Bardou‐Jacquet et al. may not be incompatible with ours. Because the SF frequency distribution is skewed, log transformation provides greater power and more‐reliable type I error rates. Log transformation is not mentioned by Bardou‐Jacquet et al. Sample Considerations Bardou‐Jacquet et al. apparently did not seek participants with extreme high and low iron phenotypes, weakening their ability to demonstrate an association of p.D519G with severe iron overload. Logistic regression analyses by Bardou‐Jacquet et al. did not reveal significant association with severe fibrosis in any group. However, severe hepatic fibrosis is not necessarily present in C282Y homozygotes with severe iron overload, and the fibrosis risk is influenced by noniron, nonalcohol factors. Our participants were selected based on iron overload severity, rather than hepatic fibrosis. Six homozygotes with severe iron overload in our study did not have p.D519G, suggesting that other modifier alleles remain undiscovered. It is unknown whether GNPAT p.D519G protein, or a closely linked locus, augments iron absorption. However, a larger, multinational study that we are presently conducting should be informative. Any results may also differ from those in France owing to haplotypic and broader genetic differences between populations. We agree with Bardou‐Jacquet et al. that: "More studies with diverse populations are needed to shed light on these discrepancies."1