The impact of single nucleotide polymorphisms in human genes that regulate hepcidin and iron on oral iron absorption and the risk of anaemia in Africans

Abstract

Background: Up to 60% of women and children living in low- and middle-income countries (LMICs) are anaemic. Food fortification and iron supplementation are the most common measures employed to combat anaemia. However, these are not effective treatments for anaemias caused by non-nutritional factors. Genome-wide association studies (GWAS) mainly in Europeans and Asians have identified single nucleotide polymorphisms (SNPs) within the hepcidin and iron regulatory genes that are associated with the risk of anaemia. Several of these SNPs are in the TMPRSS6 gene, which encodes matriptase-2, a protein that regulates the expression of hepcidin. This thesis examined the impact of SNPs in the iron regulatory genes previously reported in non-African populations, on the risk of anaemia and on impaired oral iron absorption in Africans. Methods: First, the literature was searched for genetic variants identified in the hepcidin and iron regulatory genes, that are associated with low iron status. Second, we investigated the effects of common TMPRSS6 and transferrin (TF) SNPs on iron status in a cohort of healthy individuals from rural Gambia (n=1315). Third, a recallby- genotype (RbG) study was conducted to investigate the impact of carrying single or multiple alleles at the common TMPRSS6 SNPs on oral iron absorption in healthy individuals from rural Gambia. Results: TMPRSS6 rs855791, rs4820268 and rs2235321, and TF rs3811647 are the most common SNPs that associated with low iron status. We did not find effects of any of the TMPRSS6 SNPs on the risk of anaemia. However, we found that TMPRSS6 rs2235321 was associated with serum hepcidin concentration, with a more substantial effect on individuals with low haemoglobin or ferritin. Also, TF rs3811647 had a significant influence on transferrin and its binding capacity, with a single allele effect of 8-12%. In the RbG study, we did not find any effect of the three TMPRSS6 SNPs on oral iron absorption. However, we found that each of the TMPRSS6 SNPs affects hepcidin, with carriers of major alleles having higher hepcidin compared to minor allele carriers. Also, we found that heterozygotes at both rs2235321 and rs855791 did not alter their hepcidin concentration after an oral iron dose, whereas, individuals in all the other genotype groups did. Conclusions: This thesis confirms the previously observed association between the TF rs3811647 and transferrin in other Africans and Europeans replicates in West Africans. However, we could not demonstrate that the previous associations between TMPRSS6 gene variants and iron status, exist in West Africans. This lack of replication might be due to the high genetic diversity that exists in African populations. We identified an effect of TMPRSS6 rs2235321 on serum hepcidin concentration. In the RbG study, the three TMPRSS6 SNPs studied influenced serum hepcidin levels but not oral iron absorption in healthy individuals. This finding suggests that there might be an alternate pathway of iron regulation independent of hepcidin at the enterocytes. These findings highlight the need to conduct more research on genetic determinants of iron status in African populations. Investigating more genetic markers and in different populations may provide a clearer insight into the role of genetic risk factors on iron deficiency and anaemia in African populations. Identifying the role of genetic risk factors of iron status may pave the way for the formulation of population specific anaemia control measures.