TLR9 polymorphisms in African populations: no association with severe malaria, but evidence of cis-variants acting on gene expression.
BACKGROUND
During malaria infection the Toll-like receptor 9 (TLR9) is activated through induction with plasmodium DNA or another malaria motif not yet identified. Although TLR9 activation by malaria parasites is well reported, the implication to the susceptibility to severe malaria is not clear. The aim of this study was to assess the contribution of genetic variation at TLR9 to severe malaria.


METHODS
This study explores the contribution of TLR9 genetic variants to severe malaria using two approaches. First, an association study of four common single nucleotide polymorphisms was performed on both family- and population-based studies from Malawian and Gambian populations (n>6000 individual). Subsequently, it was assessed whether TLR9 expression is affected by cis-acting variants and if these variants could be mapped. For this work, an allele specific expression (ASE) assay on a panel of HapMap cell lines was carried out.


RESULTS
No convincing association was found with polymorphisms in TLR9 for malaria severity, in either Gambian or Malawian populations, using both case-control and family based study designs. Using an allele specific expression assay it was observed that TLR9 expression is affected by cis-acting variants, these results were replicated in a second experiment using biological replicates.


CONCLUSIONS
By using the largest cohorts analysed to date, as well as a standardized phenotype definition and study design, no association of TLR9 genetic variants with severe malaria was found. This analysis considered all common variants in the region, but it is remains possible that there are rare variants with association signals. This report also shows that TLR9 expression is potentially modulated through cis-regulatory variants, which may lead to differential inflammatory responses to infection between individuals.
Campino
Susana
S
Wellcome Trust Centre for Human Genetics, University of Oxford, Oxford, UK. sc11@sanger.ac.uk
Forton
Julian
J
Auburn
Sarah
S
Fry
Andrew
A
Diakite
Mahamadou
M
Richardson
Anna
A
Hull
Jeremy
J
Jallow
Muminatou
M
Sisay-Joof
Fatou
F
Pinder
Margaret
M
Molyneux
Malcolm E
ME
Taylor
Terrie E
TE
Rockett
Kirk
K
Clark
Taane G
TG
Kwiatkowski
Dominic P
DP
D000818:Animals; D016022:Case-Control Studies; D002874:Chromosome Mapping / Q000379:methods*; D005714:Gambia; D015870:Gene Expression / Q000235:genetics*; D020022:Genetic Predisposition to Disease; D005838:Genotype; D006239:Haplotypes; D006801:Humans; D016015:Logistic Models; D016778:Malaria, Falciparum / Q000235:genetics* / Q000469:parasitology; D008295:Malawi; D010641:Phenotype; D010963:Plasmodium falciparum / Q000235:genetics* / Q000302:isolation & purification; D020641:Polymorphism, Single Nucleotide / Q000235:genetics*; D051217:Toll-Like Receptor 9 / Q000235:genetics*
D051217:Toll-Like Receptor 9