Subulade A. Olaniyan

Reappraisal of known malaria resistance loci in a large multicenter study

Many human genetic associations with resistance to malaria have been reported, but few have been reliably replicated. We collected data on 11,890 cases of severe malaria due to Plasmodium falciparum and 17,441 controls from 12 locations in Africa, Asia and Oceania. We tested 55 SNPs in 27 loci previously reported to associate with severe malaria. There was evidence of association at P < 1 × 10−4 with the HBB, ABO, ATP2B4, G6PD and CD40LG loci, but previously reported associations at 22 other loci did not replicate in the multicenter analysis. The large sample size made it possible to identify authentic genetic effects that are heterogeneous across populations or phenotypes, with a striking example being the main African form of G6PD deficiency, which reduced the risk of cerebral malaria but increased the risk of severe malarial anemia. The finding that G6PD deficiency has opposing effects on different fatal complications of P. falciparum infection indicates that the evolutionary origins of this common human genetic disorder are more complex than previously supposed.

Characterisation of the opposing effects of G6PD deficiency on cerebral malaria and severe malarial anaemia

Glucose-6-phosphate dehydrogenase (G6PD) deficiency is believed to confer protection against Plasmodium falciparum malaria, but the precise nature of the protective effect has proved difficult to define as G6PD deficiency has multiple allelic variants with different effects in males and females, and it has heterogeneous effects on the clinical outcome of P. falciparum infection. Here we report an analysis of multiple allelic forms of G6PD deficiency in a large multi-centre case-control study of severe malaria, using the WHO classification of G6PD mutations to estimate each individual’s level of enzyme activity from their genotype. Aggregated across all genotypes, we find that increasing levels of G6PD deficiency are associated with decreasing risk of cerebral malaria, but with increased risk of severe malarial anaemia. Models of balancing selection based on these findings indicate that an evolutionary trade-off between different clinical outcomes of P. falciparum infection could have been a major cause of the high levels of G6PD polymorphism seen in human populations. DOI: http://dx.doi.org/10.7554/eLife.15085.001

Tumour necrosis factor alpha promoter polymorphism, TNF-238 is associated with severe clinical outcome of falciparum malaria in Ibadan southwest Nigeria

Tumour necrosis factor (TNF) - α has been shown to play an important role in the pathogenesis of falciparum malaria. Two TNF promoter polymorphisms, TNF-308 and TNF-238 have been associated with differential activity and production of TNF. In order to investigate the association between TNF-308 and TNF-238 and the clinical outcome of malaria in a Nigerian population, the two TNF polymorphisms were analysed using Sequenom iPLEX Platform. A total of 782 children; 283 children with uncomplicated malaria, 255 children with severe malaria and 244 children with asymptomatic infection (controls) were studied. The distribution of TNF-308 and TNF-238 genotypes were consistent with the Hardy-Weinberg equilibrium. Distribution of both TNF polymorphisms differed significantly across all clinical groups (TNF-308: p=0.007; TNF-238: p=0.001). Further tests for association with severe malaria using genotype models controlling for age, parasitaemia and HbAS showed a significant association of the TNF-238 polymorphism with susceptibility to severe malaria (95% CI=1.43-6.02, OR=2.94, p=0.003237) The GG genotype of TNF-238 significantly increased the risk of developing cerebral malaria from asymptomatic malaria and uncomplicated malaria (95% CI=1.99-18.17, OR=6.02, p<0.001 and 95% CI=1.78-8.23, OR=3.84, p<0.001 respectively). No significant association was found between TNF-308 and malaria outcome. These results show thegenetic association of TNF-238 in the clinical outcome of malaria in Ibadan, southwest Nigeria. These findings add support to the role of TNF in the outcome of malaria infection. Further large scale studies across multiple malaria endemic populations will be required to determine the specific roles of TNF-308 and TNF-238 in the outcome of falciparum malaria infection.

Association of the sickle cell trait and the ABO blood group with clinical severity of malaria in southwest Nigeria

In regions of high Plasmodium falciparum malaria endemicity, certain erythrocyte polymorphisms confer resistance to severe disease. In this study, we evaluate the role of the sickle cell trait (HbS) and ABO blood groups in the clinical manifestations of childhood malaria in Southwest Nigeria. The subjects comprised 3100 children (53% males, median age 39 months), including 1400 children with uncomplicated malaria, 1000 children with asymptomatic malaria and 700 with severe malaria. Haemoglobin (Hb) types were determined using electrophoresis and serum agglutination techniques were used to determine ABO blood groups. Blood group O was the commonest ABO blood group (47.7%) in the study population, the others were A (22.5%), B (25.2%) and AB (4.6%). The frequencies of the HbAS and HbAC were 14.4% and 5.8%, respectively. In regression models adjusting for age, gender, parasite density and blood group, HbAS was associated with a reduced risk of severe malaria OR=0.46 (CI(95%): 0.273-0.773). Among severe malaria subjects, HbAS was associated with significantly lower parasite densities. The protective effect of blood group O was demonstrated with a decreased risk of severe malaria OR=0.743 (CI(95%): 0.566-0.976) after adjusting for age, gender and parasite density and Hb genotype. Blood group B was associated with increased risk of severe malaria OR=1.638 (CI(95%): 1.128-2.380) after adjusting for age, gender, packed cell volume, parasite density and Hb genotype. We have confirmed from this large study of Nigerian children the major protective effective of the sickle cell heterozygous state against both cerebral malaria and severe malarial anaemia. We also show that the B blood group is associated with an increased risk of severe malaria. In conclusion, the sickle cell haemoglobin type and ABO groups modulate the risk of severe malaria in Nigerian children.

Killer-cell immunoglobulin-like receptors and falciparum malaria in southwest Nigeria.

Killer-cell immunoglobulin-like receptors (KIRs) are a group of natural killer cell receptors (NKRs) that regulate NK-cell-mediated production of interferon gamma (IFN-γ) in response to infection. These receptors have recently been suggested to influence the severity of clinical Plasmodium falciparum malaria infection. We examined the KIR locus in relation to malaria in children from southwest Nigeria. Sequence specific priming (SSP)-PCR was used to detect the KIR genes. The presence or absence of fifteen different KIR genes was determined in each individual and the proportions compared across 3 clinical groups; asymptomatic malaria, uncomplicated clinical malaria and severe clinical malaria. The genes KIR2DL5, KIR2DS3 and KIR2DS5 were present in a significantly higher proportion of individuals in the asymptomatic control group than in the malaria cases. Furthermore, KIR2DS3 and KIR2DS5 were present in a higher proportion of uncomplicated malaria cases than severe malaria cases. Carriage c-AB2 genotype (which comprises all centromeric KIR genes including KIR2DL5, KIR2DS3 and KIR2DS5) decreases with severity of the disease suggesting that the KIR AB profile might be associated with protection from severe malaria infection in this population in Nigeria.

Factors Associated with a Poor Treatment Outcome among Children Treated for Malaria in Ibadan, Southwest Nigeria

We present data on factors associated with poor treatment outcome (death or recovery with a neurological complication) among children treated for malaria in Ibadan, Nigeria. A total of 2468 children (1532 with uncomplicated and 936 with severe malaria) were recruited from three government facilities. History was obtained from caregivers and malarial parasite test was carried out on each child. About 76.0% of caregivers had instituted home treatment. Following treatment, 2207 (89.5%) children recovered without complications, 9.1% recovered with neurological complications, and 1.4% died. The possibility of poor treatment outcome increased with decreasing child’s age (). A statistically significant proportion of children with pallor, jaundice, hepatomegaly, splenomegaly, respiratory distress, and severe anaemia had poor treatment outcome. Following logistic regression, child’s age < 12 months compared to older age groups (O.R = 5.99, 95% C.I = 1.15–31.15, and ) and loss of consciousness (O.R = 4.55, 95% CI = 1.72–12.08, and ) was significantly associated with poor treatment outcome. We recommend interventions to improve caregivers’ awareness on the importance of seeking medical care early. This will enhance early diagnosis and treatment and reduce the likelihood of complications that lead to poor treatment outcomes.