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Background: Investigating the manner in which genetic and environmental factors interact to increase susceptibility to iron deficiency, has the potential to impact on strategies to overcome iron deficiency as well as the development of biomarkers to monitor iron status in populations. Single nucleotide polymorphisms or genetic variants that may affect the composition and hence the functionality of proteins involved in iron metabolism have been the subject of recent genetic association studies. However, these investigations have not yet been carried out in African populations that differ genetically from populations of European ancestry and which bear the highest burden of iron deficiency. The overall aim of this thesis was to investigate the genetics of iron status in African populations using a candidate gene approach.
Methods: In order to evaluate the association between identified TMPRSS6 gene variants and iron status we conducted a systematic review with meta-analyses. We primarily searched the literature using the HuGE Navigator, Pubmed and Scopus databases for primarily genome wide association studies. Fixed effects meta-analysis was used to obtain summary estimates. Associations between reported variants and iron status as well as gene-gene and variant interactions that influence iron status were investigated in a female black South African cohort (n=686; range 32–86 years) which were part of the Prospective Urban and Rural Epidemiology (PURE) study. Concentrations of haemoglobin, serum ferritin, serum transferrin receptor and body iron stores were determined. Thirty SNPs were genotyped and passed all quality criteria. To investigate whether previously identified associations in populations of European ancestry are replicated in populations of African ancestry, we conducted candidate gene association studies. Twenty iron status-associated variants in 628 Kenyans, 609 Tanzanians, 608 South Africans and 228 African Americans were genotyped and associations investigated using haemoglobin and serum ferritin as outcome measures. Finally, we assessed the effect of TNF-α allele variants (TNF‒1031, TNF‒308) on malaria rates, the severity of malaria as indicated by haemoglobin concentrations at the time of presentation with febrile episodes and the association between Plasmodium infection and haemoglobin concentration in symptomless parasite carriers. We used data from a placebo-controlled trial which consisted of 612 Tanzanian children aged 6–60 months. Cox regression models were used in the primary analysis to account for multiple episodes per child.
Results: In our systematic review we included eleven studies on Caucasian populations, four on Asian populations and one study on an African-American population. Differences in minor allele frequencies (MAF) of 8 TMPRSS6 SNPs (rs855791, rs4820268, rs2111833, rs1421312, rs228921, rs228918, rs228919 and rs575620) across ethnic groups were observed; with the MAF of rs855791 being significantly higher in Asian populations than in Caucasians (0.55 vs 0.42). In the meta-analysis, the A allele of rs855791 was associated with lower haemoglobin and ferritin concentrations in all populations. This allele was also associated with increased serum transferrin receptor and transferrin concentrations. We observed similar associations for the G allele in rs4820268. In general, minor allele frequencies (MAF) from females in the PURE population were lower compared to those of males and females of European ancestry populations in the 1000 Genomes Project. In the TF gene, the SNP rs1799852 was associated with decreased serum ferritin (p=0.01) and body iron concentrations (p=0.03) and increased serum transferrin receptor (sTfR) concentrations (P=0.004), while rs3811647 was associated with transferrin receptor and body iron (both P=0.03) in a U-shaped manner. The chromosome 6 SNP allele combination (AAA) consisting of rs1799964 and rs1800629 both in TNF α and rs2071592 in NFKBIL1 was associated with higher odds for low serum ferritin concentrations (serum ferritin<15µg/L; OR:1.86 (95%-CI, 1.23-2.79)). The chromosome 22 SNP allele combination (GG) consisting of rs228918 and rs228921 in the TMPRSS6 gene was associated with lower odds for increased sTfR concentrations (sTfR>8.3mg/L; OR:0.79 (95%-CI,0.63-0.98). We successfully replicated reported significant associations with lowered haemoglobin concentrations for two loci in TMPRSS6 namely rs2413450 and rs4820268 and with increased haemoglobin concentrations for one locus in TF (rs3811658) when analysing the four populations of African ancestry. When ferritin was considered as an outcome measure, we replicated associations with increased ferritin concentrations in two loci namely, rs228918 in TMPRSS6 and rs1525892 in TF. No other significant associations were determined. Malaria rates were higher in Tanzanian children with the TNF‒1031CC genotype (rs1799964) compared to the AA genotype (crude hazard ratio (HR), 95%CI: 1.41 [1.01‒1.97], adjusted HR 1.31 [0.97‒1.76]y) but were lower in those with the TNF‒308AA genotype (rs1800629) (adjusted HR 0.13 [0.02‒0.63]) compared to those harbouring the wild type homozygous genotype.
Conclusions: This thesis demonstrates previously observed associations between TMPRSS6 gene variants and haemoglobin concentrations in European ancestry populations are replicated in African populations. Replication of results in other loci previously associated with iron status in European ancestry populations was not achieved. Additionally, minor allele frequencies of single nucleotide polymorphisms associated with iron status are generally higher in European ancestry cohorts compared to those of African ancestry populations. The lack of association of reported variants may indicate that novel loci are responsible for the heritability of iron status in African populations. We have additionally observed that TNF α variants increase malaria severity. Malaria is a major cause of iron deficiency in malaria endemic areas. Our finding emphasizes that to alleviate iron deficiency in malaria endemic areas prevention and treatment of malaria is necessary. This thesis highlights the need to conduct genetic association studies in African populations where iron deficiency is of utmost public health significance. In addition, investigations into the genetics of iron status are bound to contribute towards the development of biomarkers that are useful in the determination of iron status in areas of high inflammation burden.
|Qualification||Doctor of Philosophy|
|Award date||11 Mar 2015|
|Place of Publication||Wageningen|
|Publication status||Published - 2015|
- iron deficiency anaemia
- genetic factors
- public health
- nutrition and health