Investigations of Potential Modifier Genes in Hereditary Haemochromatosis
Abstract
Hereditary haemochromatosis (OMIM #235200) is a late onset disorder resulting from excess iron absorption. It causes symptoms such as bronze pigmentation of the skin, joint pain, abdominal pain, weight loss, impotence, cardiomyopathy, diabetes and may lead to cirrhosis of the liver. The most common cause of this disease in Caucasians is homozygosity for the C282Y mutation in the HFE gene on chromosome 6p21.3. The penetrance of the C282Y mutation is not complete with many environmental and genetic factors influencing the phenotypic expression of the disease.
A number of SNPs in genes in the iron metabolic pathway previously associated with altered serum iron indices were tested in the current study. The results suggested that two of the SNPs, namely rs1799852 in Transferrin and rs884409 in CYBRD1 may contribute to altered SF levels in HFE-associated haemochromatosis.
The first major finding of this study was a significantly higher serum ferritin (SF) level observed in iron-loaded male C282Y homozygotes who were symptomatic compared to those that were asymptomatic. This was not associated with age and suggests there is a threshold level of SF above which symptoms occur. This discordance between the symptomatic and asymptomatic C282Y homozygotes formed the basis of subsequent analyses.
A second major finding was the association of the TNFalpha 21112 promoter haplotype with increased SF levels in symptomatic male C282Y homozygotes. To explore the mechanism underlying this association, luciferase assays were performed in the presence and absence of extracellular iron. The results revealed there was no definitive change in TNFalpha expression associated with the 21112 haplotype compared to the other haplotypes.
The expression levels of a number of genes in the iron pathway (HFE, HAMP, TFRC, TFR2 and TNFalpha) were measured in lymphoblastoid cell lines derived from males of different TNFalpha haplotypes and HFE genotypes. The C282Y homozygotes analysed in this study included both symptomatic and asymptomatic samples. The major finding of the expression studies was that the TNFalpha 21112 haplotype was associated with altered TFRC expression levels specifically in the symptomatic C282Y homozygotes. This provides an explanation of the increased SF levels seen in these patients. While a mechanism for the modulation of TFRC expression by the TNFalpha haplotype remains unclear, this study has confirmed TNFalpha as a significant modifier gene in HFE-associated haemochromatosis.
The discovery of modifier genes in both conventional and unconventional pathways of iron metabolism may allow for the development of novel preventative and treatment strategies in a new era for hereditary haemochromatosis.