Polymorphisms and expression ofTLR4and9in malaria in two ethnic groups of Assam, northeast India

Abstract

Infectious diseases have been postulated to play an important role in exerting pressure and in selection of TLR polymorphisms. Single nucelotide polymorphisms (SNPs) of TLR4 have been reported to show unique distributions in populations from Africa, Asia and Europe, and malaria is suggested to influence these patterns. In this context, we examined association of TLR polymorphisms with the risk of malaria in two ethnic groups-the Austro-Asiatics and Tibeto-Burmans-from malaria endemic districts of Assam to understand the influence of malaria in selection of TLRs in these genetically-distinct populations. TLR9 (T-1237C) mutation was positively associated with complicated (P = 0.001) and frequent (P = 0.035) malaria in Austro-Asiatics (relative risk = 0.595 95% CI: 0.479-0.836), but not in Tibeto-Burmans. Nonetheless, these alleles were not in Hardy-Weinberg Equilibrium in Tibeto-Burmans (P < 0.001). In contrast, the TLR9 1486T/C genotype was favourable where it was negatively associated with complicated malaria (Fishers exact P = 0.014). Sequencing data revealed that the two populations differed in nucleotide diversity of the TLR9 promoter region. Enhanced expression of TLR4 (P = 0.05), but not of TLR9, was associated with complicated malaria. Austro-Asiatics appeared to have accumulated favourable genotypes of TLR9, perhaps because of their longer exposure to malaria.

A meta-analysis of TLR4 and TLR9 SNPs implicated in severe malaria

“…Although some studies have reported no association between T-1486C and malaria susceptibility or severity 7,20,22,24,27,30 , we observed that the -1486C allele was a risk factor for severe malaria and high parasite load, which is in line with previous findings 21,24 . Moreover, -1486C has been associated with an increased risk of low birth weight in term infants born to P. falciparum-infected pregnant women 25 .Consistent with these findings, the meta-analysis of the T-1486C variant revealed a significant association with SM under the allele contrast (T vs. C) and homozygous (TT vs. CC) genetic models.…”

“…Although, the -1237C allele of TLR9 has been associated with disease susceptibility 7 and risk of severe malaria 2,20,21 , our meta-analysis results did not reveal a significant association. …”

“…The HWE analysis of genotype distributions showed significant deviations of the studied SNPs from HWE only in the Indian studies. While TLR4 D299G deviated from HWE in Sawain et al in both ethnic populations (Tibeto-Burman and Austro-Asiatic), the TLR9 promoter variants showed deviations in both the Sawain et al 21 and Kar et al 2 studies. The African studies did not show any deviation from HWE in the genotype distributions of any of the studied SNPs ( Table 1 …”

“…Interestingly, in each of these included studies, all three SNPs were investigated for their association with malaria. However, the study by Sawain et al 21 from Assam, India had two sets of data for two different ethnic groups (Tibeto-Burman and Austro-Asiatic), which were included as two separate studies in the meta-analysis ( Table 2).…”

“…Our meta-analysis results for the TLR4D299G variant were consistent with those of previous studies on diverse ethnicities from different malarial endemic regions 2,20,22 , with no difference in the genotype or allele frequency distributions of the D299G polymorphism between UM and SM subjects. Although the contribution of the 299G minor allele has been controversial in children 7,20,22,23 , adult studies show no association 2,21,24 with the exception of maternal anemia in pregnant women 25 . This could be due to a lack of adequate studies, ethnic differences, or the fact that the effect of 299G may vary with age groups.…”