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Open Access Research

Toll-like receptor 9 (TLR9) polymorphism associated with symptomatic malaria: a cohort study

Ahmeddin H Omar1, Michio Yasunami1, Akiko Yamazaki1, Hiroki Shibata1, Michael F Ofori2, Bartholomew D Akanmori2, Mohammed Nasir Shuaibu1, Mihoko Kikuchi13 and Kenji Hirayama1*

Author Affiliations

1 Department of Immunogenetics, Institute of Tropical Medicine (NEKKEN) and Global COE Program, Nagasaki University, 1-12-4 Sakamoto, Nagasaki, 852-8523, Japan

2 Noguchi Memorial Institute for Medical Research, University of Ghana, Legon, Ghana

3 Center for International Collaborative Research (CICORN), Nagasaki University, 1-12-4 Sakamoto, Nagasaki, 852-8523, Japan

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Malaria Journal 2012, 11:168  doi:10.1186/1475-2875-11-168

Published: 17 May 2012

Abstract

Background

In areas mesoendemic for malaria transmission, symptomatic individuals play a significant role as reservoirs for malaria infection. Understanding the pathogenesis of symptomatic malaria is important in devising tools for augmenting malaria control. In this study, the effect of TLR9 polymorphisms on susceptibility to symptomatic malaria was investigated among Ghanaian children.

Methods

Four hundred and twenty nine (429) healthy Ghanaian children, aged three to eleven years (3–11 years), were enrolled into a cohort study and actively followed up for symptomatic malaria for one year. Four TLR9 single nucleotide polymorphisms (SNPs) namely: rs187084 (C-1486 T), rs5743836(C-1237 T), rs352139 (G + 1174A) and rs352140 (G + 2848A) were genotyped by direct sequencing, and their attributable and relative risks for symptomatic malaria determined. TLR9 haplotypes were inferred using the PHASE software and analysed for the risk of symptomatic malaria. A luciferase assay was performed to investigate whether the TLR9 haplotypes influence TLR9 promoter activity.

Results

The rs352139 GG genotype showed a significantly increased relative risk of 4.8 for symptomatic malaria (P = 0.0024) and a higher mean parasitaemia (P = 0.04). Conversely, the rs352140 GG genotype showed a significantly reduced relative risk of 0.34 (P = 0.048). TLR9 haplotypes analyses showed that TTAG haplotype was significantly associated with reduced relative risk of 0.2 for symptomatic malaria (P = 4×10-6) and a lower mean parasitaemia (0.007), while CTGA haplotype had an increased relative risk of 3.3 (P = 0.005). Functional luciferase reporter gene expression assay revealed that the TTA haplotype had a significantly higher promoter activity than the CCG, CTG and TCG haplotypes.

Conclusions

Taken together, these findings indicate a significant association of TLR9 gene polymorphisms with symptomatic malaria among Ghanaian children in Dangme-West district.

Keywords:
Cohort study; TLR9; Symptomatic malaria; Genetic susceptibility; Genetic polymorphism; Haplotype; Luciferase promoter assay