GNPAT polymorphism rs11558492 is not associated with increased severity in a large cohort of HFE p.Cys282Tyr homozygous patients

Dysregulated hepcidin response to dietary iron in male mice with reduced Gnpat expression

Exome sequencing has identified the glyceronephosphate O-acyltransferase (GNPAT) gene as a genetic modifier of iron overload in hereditary hemochromatosis (HH). Subjects with HFE (Homeostatic Iron Regulator) p.C282Y mutations and the GNPAT p.D519G variant had more iron loading compared with subjects without the GNPAT variant. In response to an oral iron challenge, women with GNPAT polymorphisms loaded more iron as compared with women without polymorphisms, reinforcing a role for GNPAT in iron homeostasis. The aim of the present study was to develop and characterize an animal model of disease to further our understanding of genetic modifiers, and in particular the role of GNPAT in iron homeostasis. We generated an Hfe/Gnpat mouse model reminiscent of the patients previously studied and studied these mice for up to 26 weeks. We also examined the effect of dietary iron loading on mice with reduced Gnpat expression. Gnpat heterozygosity in Hfe knockout mice does not play a role in systemic iron homeostasis; Gnpat^+/− mice fed a high-iron diet, however, had lower hepatic hepcidin (HAMP) mRNA expression, whereas they have significantly higher serum iron levels and transferrin saturation compared with wildtype (WT) littermates on a similar diet. These results reinforce an independent role of GNPAT in systemic iron homeostasis, reproducing in an animal model, the observations in women with GNPAT polymorphisms subjected to an iron tolerance test.

Increased frequency of GNPAT p.D519G in compound HFE p.C282Y/p.H63D heterozygotes with elevated serum ferritin levels

Glyceronephosphate O-acyltransferase (GNPAT) p.D519G (rs11558492) was identified as a genetic modifier correlated with more severe iron overload in hemochromatosis through whole-exome sequencing of HFE p.C282Y homozygotes with extreme iron phenotypes. We studied the prevalence of p.D519G in HFE p.C282Y/p.H63D compound heterozygotes, a genotype associated with iron overload in some patients. Cases were Australian participants with elevated serum ferritin (SF) levels ≥300μg/L (males) and ≥200μg/L (females); subjects whose SF levels were below these cut-offs were designated as controls. Samples were genotyped for GNPAT p.D519G. We compared the allele frequency of the present subjects, with/without elevated SF, to p.D519G frequency in public datasets. GNPAT p.D519G was more prevalent in our cohort of p.C282Y/p.H63D compound heterozygotes with elevated SF (37%) than European public datasets: 1000G 21%, gnomAD 20% and ESP 21%. We conclude that GNPAT p.D519G is associated with elevated SF in Australian HFE p.C282Y/p.H63D compound heterozygotes.

Gnpat does not play an essential role in systemic iron homeostasis in murine model

The GNPAT variant rs11558492 (p.D519G) was identified as a novel genetic factor that modifies the iron‐overload phenotype in homozygous carriers of the HFE p.C282Y variant. However, the reported effects of the GNPAT p.D519G variant vary among study populations. Here, we investigated the role of GNPAT in iron metabolism using Gnpat‐knockout (Gnpat^−/−), Gnpat/Hfe double‐knockout (Gnpat^−/−Hfe^−/− or DKO) mice and hepatocyte‐specific Gnpat‐knockout mice (Gnpat^fl/fl;Alb‐Cre). Our analysis revealed no significant difference between wild‐type (Gnpat^+/+) and Gnpat^−/− mice, between Hfe^−/− and DKO mice, or between Gnpat^fl/fl and Gnpat^fl/fl;Alb‐Cre with respect to serum iron and tissue iron. In addition, the expression of hepcidin was not affected by deleting Gnpat expression in the presence or absence of Hfe. Feeding Gnpat^−/− and DKO mice a high‐iron diet had no effect on tissue iron levels compared with wild‐type and Hfe^−/− mice, respectively. Gnpat knockdown in primary hepatocytes from wild‐type or Hfe^−/− mice did not alter hepcidin expression, but it repressed BMP6‐induced hepcidin expression. Taken together, these results support the hypothesis that deleting Gnpat expression has no effect on either systemic iron metabolism or the iron‐overload phenotype that develops in Hfe^−/− mice, suggesting that GNPAT does not directly mediate iron homeostasis under normal or high‐iron dietary conditions.

Iron as a Therapeutic Target in HFE-Related Hemochromatosis: Usual and Novel Aspects

Genetic hemochromatosis is an iron overload disease that is mainly related to the C282Y mutation in the HFE gene. This gene controls the expression of hepcidin, a peptide secreted in plasma by the liver and regulates systemic iron distribution. Homozygous C282Y mutation induces hepcidin deficiency, leading to increased circulating transferrin saturation, and ultimately, iron accumulation in organs such as the liver, pancreas, heart, and bone. Iron in excess may induce or favor the development of complications such as cirrhosis, liver cancer, diabetes, heart failure, hypogonadism, but also complaints such as asthenia and disabling arthritis. Iron depletive treatment mainly consists of venesections that permit the removal of iron contained in red blood cells and the subsequent mobilization of stored iron in order to synthesize hemoglobin for new erythrocytes. It is highly efficient in removing excess iron and preventing most of the complications associated with excess iron in the body. However, this treatment does not target the biological mechanisms involved in the iron metabolism disturbance. New treatments based on the increase of hepcidin levels, by using hepcidin mimetics or inducers, or inhibitors of the iron export activity of ferroportin protein that is the target of hepcidin, if devoid of significant secondary effects, should be useful to better control iron parameters and symptoms, such as arthritis.