Both heterozygous and homozygous alpha+ thalassemias protect against severe and fatal Plasmodium falciparum malaria on the coast of Kenya

Biochemical and immunological mechanisms by which sickle cell trait protects against malaria

Sickle cell trait (HbAS) is the best-characterized genetic polymorphism known to protect against falciparum malaria. Although the protective effect of HbAS against malaria is well known, the mechanism(s) of protection remain unclear. A number of biochemical and immune-mediated mechanisms have been proposed, and it is likely that multiple complex mechanisms are responsible for the observed protection. Increased evidence for an immune component of protection as well as novel mechanisms, such as enhanced tolerance to disease mediated by HO-1 and reduced parasitic growth due to translocation of host micro-RNA into the parasite, have recently been described. A better understanding of relevant mechanisms will provide valuable insight into the host-parasite relationship, including the role of the host immune system in protection against malaria.

Patient-driven discontinuation of tyrosine kinase inhibitors: single institution experience

Abstract With improved outcome for patients with chronic myeloid leukemia (CML) treated with tyrosine kinase inhibitors (TKIs), treatment discontinuation has become increasingly attractive to patients. We analyzed the outcomes of patients who chose to discontinue TKI therapy regardless of their ongoing response. Thirty-five patients with chronic phase CML discontinued TKI in complete cytogenetic response. Of them, 51% discontinued due to adverse effects, 23% due to long complete molecular response (CMR) (> 5 years), 9% due to pregnancy and 17% due to financial problems. After TKI discontinuation, patients were followed for a median of 16 months. Among 27 patients (77%) who discontinued TKIs in CMR, 11 (41%) had a molecular relapse after a median of 3.5 months. In univariate analysis we observed that patients with ≥ 64 months of CMR before TKI discontinuation had superior cumulative proportions of sustained CMR and major molecular response (MMR) at 12 months after discontinuation: 88.9% vs. 45.5% (p = 0.02) and 100% vs. 75% (p = 0.05), respectively. Patients treated with high dose imatinib or second generation TKIs had a higher cumulative proportion of sustained MMR at 12 months after discontinuation than patients treated with standard dose imatinib: 100% vs. 72.2% (p = 0.03), respectively. Of the five patients who stopped TKI in MR(4.5) (molecular response of 4.5-log reduction) one lost cytogenetic response. All three patients who discontinued TKIs in MMR lost cytogenetic response; one progressed to accelerated phase. Thirteen patients (37%) restarted TKIs after loss of response: 11 improved their response, and for two it is too early to assess. Treatment discontinuation can lead to sustained CMR in some patients, but risk of relapse is higher if patients discontinue TKIs when not in CMR.

An investigation of the protective effect of alpha+-thalassaemia against severe Plasmodium falciparum amongst children in Kumasi, Ghana

INTRODUCTION

Several factors influence the severity of Plasmodium falciparum; here, we investigate the impact of alpha+-thalassaemia genotype on P. falciparum parasitemia and prevalence of severe anaemia amongst microcytic children from Kumasi, Ghana.

METHODS

Seven hundred and thirty-two children (≤10 years) with P. falciparum were categorised into normocytic and microcytic (mean cell volume ≤76 fL). Microcytic individuals were genotyped for the -α(3.7) deletional thalassaemia mutation and parasite densities determined.

RESULTS

Amongst microcytic patients both parasite densities and prevalence of severe malaria parasitemia (≥100 000/μL) were significantly lower (P < 0.001) in the presence of an alpha+-thalassaemia genotype compared with non-alpha+-thalassaemia genotype. There was no evidence that alpha+-thalassaemia protected against severe anaemia. The protection conferred by alpha-thalassaemia genotype against severe P. falciparum parasitemia did not change with increasing age.

CONCLUSION

The severity of P. falciparum parasitemia was significantly lower in both the homozygous and heterozygous alpha+-thalassaemia groups compared with microcytic individuals with non-alpha+-thalassaemia genotype. The protective effect, from severe malaria, of the alpha+-thalassaemia allele does not alter with age.

Genetic variants that confer resistance to malaria are associated with red blood cell traits in African-Americans: an electronic medical record-based genome-wide association study

To identify novel genetic loci influencing interindividual variation in red blood cell (RBC) traits in African-Americans, we conducted a genome-wide association study (GWAS) in 2315 individuals, divided into discovery (n = 1904) and replication (n = 411) cohorts. The traits included hemoglobin concentration (HGB), hematocrit (HCT), RBC count, mean corpuscular volume (MCV), mean corpuscular hemoglobin (MCH), and mean corpuscular hemoglobin concentration (MCHC). Patients were participants in the electronic MEdical Records and GEnomics (eMERGE) network and underwent genotyping of ~1.2 million single-nucleotide polymorphisms on the Illumina Human1M-Duo array. Association analyses were performed adjusting for age, sex, site, and population stratification. Three loci previously associated with resistance to malaria—HBB (11p15.4), HBA1/HBA2 (16p13.3), and G6PD (Xq28)—were associated (P ≤ 1 × 10⁻⁶) with RBC traits in the discovery cohort. The loci replicated in the replication cohort (P ≤ 0.02), and were significant at a genome-wide significance level (P < 5 × 10⁻⁸) in the combined cohort. The proportions of variance in RBC traits explained by significant variants at these loci were as follows: rs7120391 (near HBB) 1.3% of MCHC, rs9924561 (near HBA1/A2) 5.5% of MCV, 6.9% of MCH and 2.9% of MCHC, and rs1050828 (in G6PD) 2.4% of RBC count, 2.9% of MCV, and 1.4% of MCH, respectively. We were not able to replicate loci identified by a previous GWAS of RBC traits in a European ancestry cohort of similar sample size, suggesting that the genetic architecture of RBC traits differs by race. In conclusion, genetic variants that confer resistance to malaria are associated with RBC traits in African-Americans.

Determinants of anemia among preschool children in rural, western Kenya

Although anemia in preschool children is most often attributed to iron deficiency, other nutritional, infectious, and genetic contributors are rarely concurrently measured. In a population-based, cross-sectional survey of 858 children 6-35 months of age in western Kenya, we measured hemoglobin, malaria, inflammation, sickle cell, α-thalassemia, iron deficiency, vitamin A deficiency, anthropometry, and socio-demographic characteristics. Anemia (Hb < 11 g/dL) and severe anemia (Hb < 7 g/dL) prevalence ratios (PRs) for each exposure were determined using multivariable modeling. Anemia (71.8%) and severe anemia (8.4%) were common. Characteristics most strongly associated with anemia were malaria (PR: 1.7; 95% confidence interval [CI] = 1.5-1.9), iron deficiency (1.3; 1.2-1.4), and homozygous α-thalassemia (1.3; 1.1-1.4). Characteristics associated with severe anemia were malaria (10.2; 3.5-29.3), inflammation (6.7; 2.3-19.4), and stunting (1.6; 1.0-2.4). Overall 16.8% of anemia cases were associated with malaria, 8.3% with iron deficiency, and 6.1% with inflammation. Interventions should address malaria, iron deficiency, and non-malarial infections to decrease the burden of anemia in this population.

The host genetic diversity in malaria infection

Populations exposed to Plasmodium infection develop genetic mechanisms of protection against severe disease. The clinical manifestation of malaria results primarily from the lysis of infected erythrocytes and subsequent immune and inflammatory responses. Herein, we review the genetic alterations associated with erythrocytes or mediators of the immune system, which might influence malaria outcome. Moreover, polymorphisms in genes related to molecules involved in mechanisms of cytoadherence and their influence on malaria pathology are also discussed. The results of some studies have suggested that the combinatorial effects of a set of genetic factors in the erythrocyte-immunology pathway might be relevant to host resistance or susceptibility against Plasmodium infection. However, these results must be interpreted with caution because of the differences observed in the functionality and frequency of polymorphisms within different populations. With the recent advances in molecular biology techniques, more robust studies with reliable data have been reported, and the results of these studies have identified individual genetic factors for consideration in preventing severe disease and the individual response to treatment.

Lack of associations of α(+)-thalassemia with the risk of Plasmodium falciparum and Plasmodium vivax infection and disease in a cohort of children aged 3-21 months from Papua New Guinea

Despite consistent evidence of a protective effect of α(+)-thalassemia against severe Plasmodium falciparum disease, the mechanisms underlying this protection remain unknown. An increase in risk of Plasmodium vivax malaria in early childhood resulting in a cross-species protection against severe P. falciparum malaria has been proposed as a possible mechanism in Melanesian children. The association of α(+)-thalassemia genotypes with a risk of P. falciparum and P. vivax infection and uncomplicated illness was reassessed in a cohort of 1,112 Papua New Guinean children, followed from 3 to 21 months of age. Three hundred and eighty-nine (35.0%) children were homozygous for α(+)-thalassemia (-α/-α), 506 (45.5%) heterozygous (αα/-α) and 217 (19.5%) homozygous for the wild-type allele. No significant differences in the incidence of P. falciparum (Pf) or P. vivax (Pv) malaria were observed between α(+)-thalassemia homozygote (Pf: incidence rate ratio (IRR)=1.13, CI(95) (0.82, 1.56), P=0.45, Pv: IRR=1.15, CI(95) (0.88, 1.50), P=0.31), heterozygote (Pf: IRR=0.98, CI(95) (0.71, 1.34), P=0.93, Pv: IRR=1.14, CI(95) (0.88, 1.48), P=0.33) and wild-type children. The prevalence of infection with either species did not differ between α(+)-thalassemia genotypes, although densities of P. vivax (but not of P. falciparum) infections were significantly higher in α(+)-thalassemia homozygote and heterozygote children. An excessive risk of moderate-to-severe anemia (Hb<8 g/dl) was observed in α(+)-thalassemia homozygote children (IRR=1.54, CI(95) (1.12, 2.11), P=0.008). This study therefore failed to confirm an increased risk of P. vivax or P. falciparum malaria in very young, α(+)-thalassemic children without significant levels of acquired immunity. This confirms the lack of protection by α(+)-thalassemia against uncomplicated P. falciparum and challenges the hypothesis of immunological cross-protection between P. falciparum and P. vivax as a mechanism underlying α(+)-thalassemia protection against severe P. falciparum disease in Melanesian children.

The burden and consequences of inherited blood disorders among young children in western Kenya

Although inherited blood disorders are common among children in many parts of Africa, limited data are available about their prevalence or contribution to childhood anaemia. We conducted a cross‐sectional survey of 858 children aged 6–35 months who were randomly selected from 60 villages in western Kenya. Haemoglobin (Hb), ferritin, malaria, C‐reactive protein (CRP) and retinol binding protein (RBP) were measured from capillary blood. Using polymerase chain reaction (PCR), Hb type, −3.7 kb alpha‐globin chain deletion, glucose‐6‐phosphate dehydrogenase (G6PD) genotype and haptoglobin (Hp) genotype were determined. More than 2 out of 3 children had at least one measured blood disorder. Sickle cell trait (HbAS) and disease (HbSS) were found in 17.1% and 1.6% of children, respectively; 38.5% were heterozygotes and 9.6% were homozygotes for α⁺‐thalassaemia. The Hp 2‐2 genotype was found in 20.4% of children, whereas 8.2% of males and 6.8% of children overall had G6PD deficiency. There were no significant differences in the distribution of malaria by the measured blood disorders, except among males with G6PD deficiency who had a lower prevalence of clinical malaria than males of normal G6PD genotype (P = 0.005). After excluding children with malaria parasitaemia, inflammation (CRP > 5 mg L⁻¹), iron deficiency (ferritin < 12 μg L⁻¹) or vitamin A deficiency (RBP < 0.7 μg L⁻¹), the prevalence of anaemia among those without α⁺‐thalassaemia (43.0%) remained significantly lower than that among children who were either heterozygotes (53.5%) or homozygotes (67.7%, P = 0.03). Inherited blood disorders are common among pre‐school children in western Kenya and are important contributors to anaemia.

World distribution, population genetics, and health burden of the hemoglobinopathies

Although information about the precise world distribution and frequency of the inherited hemoglobin disorders is still limited, there is no doubt that they are going to pose an increasing burden on global health resources in the future. Their high frequency is a reflection of natural selection combined with a high frequency of consanguineous marriages in many countries, together with an epidemiological transition; whereby, as public health measures improve in the poorer countries of the world, more babies with these disorders are surviving to present for treatment.

Abnormal PfEMP1/knob display on Plasmodium falciparum-infected erythrocytes containing hemoglobin variants: fresh insights into malaria pathogenesis and protection

Hemoglobin (Hb) variants are associated with reduced risk of life-threatening Plasmodium falciparum malaria syndromes, including cerebral malaria and severe malarial anemia. Despite decades of research, the mechanisms by which common Hb variants - sickle HbS, HbC, α-thalassemia, fetal HbF - protect African children against severe and fatal malaria have not been fully elucidated. In vitro experimental and epidemiological data have long suggested that Hb variants do not confer malaria protection by restricting the growth of parasites in red blood cells (RBCs). Recently, four Hb variants were found to impair cytoadherence, the binding of P. falciparum-infected RBCs (PfRBCs) to microvascular endothelial cells (MVECs), a centrally important event in both parasite survival and malaria pathogenesis in humans. Impaired cytoadherence is associated with abnormal display of P. falciparum erythrocyte membrane protein 1 (PfEMP1), the parasite's major cytoadherence ligand and virulence factor, on the surface of host RBCs. We propose a model in which Hb variants allow parasites to display relatively low levels of PfEMP1, sufficient for sequestering PfRBCs in microvessels and avoiding their clearance from the bloodstream by the spleen. By preventing the display of high levels of PfEMP1, Hb variants may weaken the binding of PfRBCs to MVECs, compromising their ability to activate endothelium and initiate the downstream microvascular events that drive the pathogenesis of malaria.

α-Thalassemia impairs the cytoadherence of Plasmodium falciparum-infected erythrocytes

BACKGROUND

α-Thalassemia results from decreased production of α-globin chains that make up part of hemoglobin tetramers (Hb; α(2)β(2)) and affects up to 50% of individuals in some regions of sub-Saharan Africa. Heterozygous (-α/αα) and homozygous (-α/-α) genotypes are associated with reduced risk of severe Plasmodium falciparum malaria, but the mechanism of this protection remains obscure. We hypothesized that α-thalassemia impairs the adherence of parasitized red blood cells (RBCs) to microvascular endothelial cells (MVECs) and monocytes--two interactions that are centrally involved in the pathogenesis of severe disease.

METHODS AND FINDINGS

We obtained P. falciparum isolates directly from Malian children with malaria and used them to infect αα/αα (normal), -α/αα and -α/-α RBCs. We also used laboratory-adapted P. falciparum clones to infect -/-α RBCs obtained from patients with HbH disease. Following a single cycle of parasite invasion and maturation to the trophozoite stage, we tested the ability of parasitized RBCs to bind MVECs and monocytes. Compared to parasitized αα/αα RBCs, we found that parasitized -α/αα, -α/-α and -/-α RBCs showed, respectively, 22%, 43% and 63% reductions in binding to MVECs and 13%, 33% and 63% reductions in binding to monocytes. α-Thalassemia was associated with abnormal display of P. falciparum erythrocyte membrane protein 1 (PfEMP1), the parasite's main cytoadherence ligand and virulence factor, on the surface of parasitized RBCs.

CONCLUSIONS

Parasitized α-thalassemic RBCs show PfEMP1 display abnormalities that are reminiscent of those on the surface of parasitized sickle HbS and HbC RBCs. Our data suggest a model of malaria protection in which α-thalassemia ameliorates the pro-inflammatory effects of cytoadherence. Our findings also raise the possibility that other unstable hemoglobins such as HbE and unpaired α-globin chains (in the case of β-thalassemia) protect against life-threatening malaria by a similar mechanism.

Haemoglobinopathies and the clinical epidemiology of malaria: a systematic review and meta-analysis

BACKGROUND

Haemoglobinopathies can reduce the risk of malaria syndromes. We aimed to quantify the relation between different haemoglobin mutations and malaria protection to strengthen the foundation for translational studies of malaria pathogenesis and immunity.

METHODS

We systematically searched the Medline and Embase databases for studies that estimated the risk of malaria in patients with and without haemoglobinopathies up to Sept 9, 2011, and identified additional studies from reference lists. We included studies that enrolled mainly children or pregnant women and had the following outcomes: Plasmodium falciparum severe malaria, uncomplicated malaria, asymptomatic parasitaemia, or pregnancy-associated malaria, and Plasmodium vivax malaria. Two reviewers identified studies independently, assessed quality of the studies, and extracted data. We produced odds ratios (ORs; 95% CIs) for case-control studies and incidence rate ratios (IRRs; 95% CIs) for prospective studies. We did the meta-analysis with a random-effects model when equivalent outcomes were reported in more than one study.

FINDINGS

Of 62 identified studies, 44 reported data for haemoglobin AS, 19 for haemoglobin AC and CC, and 18 for α-thalassaemia. Meta-analysis of case-control studies showed a decreased risk of severe P. falciparum malaria in individuals with haemoglobin AS (OR 0·09, 95% CI 0·06-0·12), haemoglobin CC (0·27, 0·11-0·63), haemoglobin AC (0·83, 0·67-0·96), homozygous α-thalassaemia (0·63, 0·48-0·83), and heterozygous α-thalassaemia (0·83, 0·74-0·92). In meta-analysis of prospective trials only haemoglobin AS was consistently associated with protection from uncomplicated malaria (IRR 0·69, 95% CI 0·61-0·79); no haemoglobinopathies led to consistent protection from asymptomatic parasitaemia. Few clinical studies have investigated β-thalassaemia, haemoglobin E, P. vivax malaria, or pregnancy-associated malaria.

INTERPRETATION

Haemoglobin AS, CC, and AC genotypes and homozygous and heterozygous α-thalassaemia provide significant protection from severe malaria syndromes, but these haemoglobinopathies differ substantially in the degree of protection provided and confer mild or no protection against uncomplicated malaria and asymptomatic parasitaemia. Through attenuation of severity of malaria, haemoglobinopathies could serve as a model for investigation of the mechanisms of malaria pathogenesis and immunity.

FUNDING

US National Institute of Allergy and Infectious Diseases.

Effect of α(+)-thalassaemia on episodes of fever due to malaria and other causes: a community-based cohort study in Tanzania

Background

It is controversial to what degree α⁺-thalassaemia protects against episodes of uncomplicated malaria and febrile disease due to infections other than Plasmodium.

Methods

In Tanzania, in children aged 6-60 months and height-for-age z-score < -1.5 SD (n = 612), rates of fevers due to malaria and other causes were compared between those with heterozygous or homozygotes α⁺-thalassaemia and those with a normal genotype, using Cox regression models that accounted for multiple events per child.

Results

The overall incidence of malaria was 3.0/child-year (1, 572/526 child-years); no differences were found in malaria rates between genotypes (hazard ratios, 95% CI: 0.93, 0.82-1.06 and 0.91, 0.73-1.14 for heterozygotes and homozygotes respectively, adjusted for baseline factors that were predictive for outcome). However, this association strongly depended on age: among children aged 6-17 months, those with α⁺-thalassaemia experienced episodes more frequently than those with a normal genotype (1.30, 1.02-1.65 and 1.15, 0.80-1.65 for heterozygotes and homozygotes respectively), whereas among their peers aged 18-60 months, α⁺-thalassaemia protected against malaria (0.80, 0.68-0.95 and 0.78, 0.60-1.03; p-value for interaction 0.001 and 0.10 for hetero- and homozygotes respectively). No effect was observed on non-malarial febrile episodes.

Conclusions

In this population, the association between α⁺-thalassaemia and malaria depends on age. Our data suggest that protection by α⁺-thalassaemia is conferred by more efficient acquisition of malaria-specific immunity.

Negative epistasis between α+ thalassaemia and sickle cell trait can explain interpopulation variation in South Asia

Recent studies in Kenya and Ghana have shown that individuals who inherit two malaria-protective genetic disorders of haemoglobin—α⁺ thalassaemia and sickle cell trait—experience a much lower level of malaria protection than those who inherit sickle cell trait alone. We have previously demonstrated that this can limit the frequency of α⁺ thalassaemia in a population in which sickle cell is present, which may account for the frequency of α⁺ thalassaemia in sub-Saharan Africa not exceeding 50%. Here we consider the relationship between α⁺ thalassaemia and sickle cell in South Asian populations, and show that very high levels of α⁺ thalassaemia combined with varying levels of malaria selection can explain why sickle cell has penetrated certain South Asian populations but not others.

Complement polymorphisms: geographical distribution and relevance to disease

The evolution of man has been characterised by recurrent episodes of migration and settlement with infectious disease a constant threat. This long history of demographic change, together with the action of evolutionary forces such as natural selection and genetic drift, has shaped human genetic diversity. In particular, the interaction between humans, pathogens and the environment has played a crucial role in generating patterns of human genetic variation. The complement system plays a crucial role in the early protective immune response after exposure to a pathogen. Pathogens, over time, have developed mechanisms to circumvent the effects of complement which in turn has led to development of a more complex complement system. During the evolution of the complement system genes coding complement proteins have evolved polymorphisms, some of which have a functional effect, and this may reflect human-pathogen interaction and geographical origin. An example is the polymorphism Ile62Val (rs800292 (A>G)) in the complement regulator Factor H gene which alters the susceptibility to age-related macular degeneration (AMD), with the Ile62 polymorphism protecting against AMD. When sub-Saharan African and European populations are compared, the frequency of this polymorphism shows a very marked geographical distribution. Polymorphisms in other complement genes such as complement factor B show similar trends. This paper describes the geographical variation present in complement genes and discusses the implications of these observations. The analysis of genetic variation in complement genes is a promising tool to unravel mechanisms of host-pathogen interaction and can provide new insights into the evolution of the human immune system.

Selection and mutation for α Thalassemia in nonmalarial and malarial environments

α thalassemia is the result of the loss of one or both copies of the two human α globin genes. α thalassemia appears to be the most common monogenic disease in the world and is in high frequency where malaria is, or has been, endemic. In nonmalarial environments, α thalassemia is rare and its frequency can be explained by a balance of deletional mutation and purifying selection. In malarial environments, the loss of one or two copies of the four α globin genes in normal diploid genotypes confers resistance (lower mortality) to malaria. Fitness estimates from data from Kenyan and Papua New Guinea populations are used to predict the increase in the --α haplotype (with one deleted gene). The frequency of double deletions (-- haplotypes) is higher in some Asian populations than that of single deletions. In this case, heterozygotes with normal αα haplotypes are expected to have the highest fitness. Overall, this population genetic examination provides an evolutionary framework for understanding the worldwide frequency of α thalassemia and the deletions that cause it in both nonmalarial and malarial environments.

Population genetics of malaria resistance in humans

The high mortality and widespread impact of malaria have resulted in this disease being the strongest evolutionary selective force in recent human history, and genes that confer resistance to malaria provide some of the best-known case studies of strong positive selection in modern humans. I begin by reviewing JBS Haldane's initial contribution to the potential of malaria genetic resistance in humans. Further, I discuss the population genetics aspects of many of the variants, including globin, G6PD deficiency, Duffy, ovalocytosis, ABO and human leukocyte antigen variants. Many of the variants conferring resistance to malaria are 'loss-of-function' mutants and appear to be recent polymorphisms from the last 5000-10 000 years or less. I discuss estimation of selection coefficients from case-control data and make predictions about the change for S, C and G6PD-deficiency variants. In addition, I consider the predicted joint changes when the two β-globin alleles S and C are both variable in the same population and when there is a variation for α-thalassemia and S, two unlinked, but epistatic variants. As more becomes known about genes conferring genetic resistance to malaria in humans, population genetics approaches can contribute both to investigating past selection and predicting the consequences in future generations for these variants.

Global distribution of the sickle cell gene and geographical confirmation of the malaria hypothesis

It has been 100 years since the first report of sickle haemoglobin (HbS). More than 50 years ago, it was suggested that the gene responsible for this disorder could reach high frequencies because of resistance conferred against malaria by the heterozygous carrier state. This traditional example of balancing selection is known as the 'malaria hypothesis'. However, the geographical relationship between the transmission intensity of malaria and associated HbS burden has never been formally investigated on a global scale. Here, we use a comprehensive data assembly of HbS allele frequencies to generate the first evidence-based map of the worldwide distribution of the gene in a Bayesian geostatistical framework. We compare this map with the pre-intervention distribution of malaria endemicity, using a novel geostatistical area-mean comparison. We find geographical support for the malaria hypothesis globally; the relationship is relatively strong in Africa but cannot be resolved in the Americas or in Asia.

Genetic variation, Fcγ receptors, KIRs and infection: the evolution of autoimmunity

Recent work has emphasised the marked genetic variability that exists in the Fc receptor locus. This variation can contribute to the risk of autoimmune disease in both mice and humans, but can also have a profound impact on defence against infection. Using FcγRIIB and FcγRIIIB as examples, we demonstrate that variations associated with increased susceptibility to autoimmunity may be maintained in populations for their beneficial effect against infection. We examine the KIR locus from the same perspective and highlight similarities between the two loci. Intense selection pressure by pathogens presumably accounts for the marked variability within both regions and leads to susceptibility to autoimmunity for some alleles.

Evaluation of recurrent parasitemia after artemether-lumefantrine treatment for uncomplicated malaria in children in western Kenya

From April 2005 to April 2006, a phase 2 malaria vaccine trial in Kenya enrolled 400 children aged 12-47 months. Each received mixed supervised and unsupervised artemether-lumefantrine for uncomplicated malaria, using a standard six-dose regimen, by weight. Children were followed for detection of parasitemia and clinical malaria. A median of two negative malaria blood films occurred during every recurrent parasitemia (RP) episode, suggesting reinfection over late recrudescence. Median time to RP after starting artemether-lumefantrine was 37 days (36-38). Of 2,020 evaluable artemether-lumefantrine treatments, there were no RPs in 99% by day 14, 71% by day 28, and 41% by day 42. By World Health Organization standards, 71% of treatment courses had adequate responses. Although recrudescence in some cannot be ruled out, our cohort had a shorter median time to RP compared with other artemether-lumefantrine treatment studies. This underscores patient counseling on completing all treatment doses for optimal protection from RP.

Transforming growth factor beta 2 and heme oxygenase 1 genes are risk factors for the cerebral malaria syndrome in Angolan children

Background

Cerebral malaria (CM) represents a severe outcome of the Plasmodium falciparum infection. Recent genetic studies have correlated human genes with severe malaria susceptibility, but there is little data on genetic variants that increase the risk of developing specific malaria clinical complications. Nevertheless, susceptibility to experimental CM in the mouse has been linked to host genes including Transforming Growth Factor Beta 2 (TGFB2) and Heme oxygenase-1 (HMOX1). Here, we tested whether those genes were governing the risk of progressing to CM in patients with severe malaria syndromes.

Methodology/Principal Findings

We report that the clinical outcome of P. falciparum infection in a cohort of Angolan children (n = 430) correlated with nine TGFB2 SNPs that modify the risk of progression to CM as compared to other severe forms of malaria. This genetic effect was explained by two haplotypes harboring the CM-associated SNPs (Pcorrec. = 0.035 and 0.036). In addition, one HMOX1 haplotype composed of five CM-associated SNPs increased the risk of developing the CM syndrome (Pcorrec. = 0.002) and was under-transmitted to children with uncomplicated malaria (P = 0.036). Notably, the HMOX1-associated haplotype conferred increased HMOX1 mRNA expression in peripheral blood cells of CM patients (P = 0.012).

Conclusions/Significance

These results represent the first report on CM genetic risk factors in Angolan children and suggest the novel hypothesis that genetic variants of the TGFB2 and HMOX1 genes may contribute to confer a specific risk of developing the CM syndrome in patients with severe P. falciparum malaria. This work may provide motivation for future studies aiming to replicate our findings in larger populations and to confirm a role for these genes in determining the clinical course of malaria.

Copy number, linkage disequilibrium and disease association in the FCGR locus

The response of a leukocyte to immune complexes (ICs) is modulated by receptors for the Fc region of IgG (FcγRs), and alterations in their affinity or function have been associated with risk of autoimmune diseases, including systemic lupus erythematosus (SLE). The low-affinity FcγR genomic locus is complex, containing regions of copy number variation (CNV) which can alter receptor expression and leukocyte responses to IgG. Combined paralogue ratio tests (PRTs) were used to distinguish three intervals within the FCGR locus which undergo CNV, and to determine FCGR gene copy number (CN). There were significant differences in FCGR3B and FCGR3A CNV profiles between Caucasian, East Asian and Kenyan populations. A previously noted association of low FCGR3B CN with SLE in Caucasians was supported [OR = 1.57 (1.08–2.27), P = 0.018], and replicated in Chinese [OR = 1.65 (1.25–2.18), P = 4 × 10⁻⁴]. There was no association of FCGR3B CNV with vasculitis, nor with malarial or bacterial infection. Linkage disequilibrium (LD) between multi-allelic FCGR3B CNV and SLE-associated SNPs in the FCGR locus was defined for the first time. Despite LD between FCGR3B CNV and a variant in FcγRIIB (I232T) which abolishes inhibitory function, both reduced CN of FCGR3B and homozygosity of the FcγRIIB-232T allele were individually strongly associated with SLE risk. Thus CN of FCGR3B, which controls IC responses and uptake by neutrophils, and variations in FCGR2B, which controls factors such as antibody production and macrophage activation, are important in SLE pathogenesis. Further interpretations of contributions to pathogenesis by FcγRs must be made in the context of LD involving CNV regions.

FcgammaRIIB in autoimmunity and infection: evolutionary and therapeutic implications

FcgammaRIIB is the only inhibitory Fc receptor. It controls many aspects of immune and inflammatory responses, and variation in the gene encoding this protein has long been associated with susceptibility to autoimmune disease, particularly systemic lupus erythematosus (SLE). FcgammaRIIB is also involved in the complex regulation of defence against infection. A loss-of-function polymorphism in FcgammaRIIB protects against severe malaria, the investigation of which is beginning to clarify the evolutionary pressures that drive ethnic variation in autoimmunity. Our increased understanding of the function of FcgammaRIIB also has potentially far-reaching therapeutic implications, being involved in the mechanism of action of intravenous immunoglobulin, controlling the efficacy of monoclonal antibody therapy and providing a direct therapeutic target.

A defunctioning polymorphism in FCGR2B is associated with protection against malaria but susceptibility to systemic lupus erythematosus

Systemic lupus erythematosus (SLE) is a multisystem autoimmune disease more prevalent in people of African and Asian origin than Caucasian origin. FcgammaRIIb is an inhibitory Fc receptor with a critical role in immune regulation. Mouse data suggest that FcgammaRIIb deficiency increases susceptibility to autoimmune disease but protects against infection. We show that a SNP in human FCGR2B that abrogates receptor function is strongly associated with susceptibility to SLE in both Caucasians and Southeast Asians. The minor allele of this SNP is more common in Southeast Asians and Africans, populations from areas where malaria is endemic, than in Caucasians. We show that homozygosity for the minor allele is associated with substantial protection against severe malaria in an East African population (odds ratio = 0.56; P = 7.1 x 10(-5)). This protective effect against malaria may contribute to the higher frequency of this SNP and hence, SLE in Africans and Southeast Asians.

HO-1 polymorphism as a genetic determinant behind the malaria resistance afforded by haemolytic disorders

Malaria affects thousands of people around the world representing a critical issue regarding health policies in tropical countries. Similarly, also haemolytic diseases such as sickle cell disease and thalassemias are a concern in different parts of the globe. It is well established that haemolytic diseases, such as sickle cell disease (SCD) and thalassemias, represent a resistance factor to malaria, which explains the high frequencies of such genetic variants in malaria endemic areas. In this context, it has been shown that the rate limiting enzyme heme oxygenase I (HO-1), responsible for the catabolism of the free heme in the body, is an important resistance factor in malaria and is also important in the physiopathology of haemolytic diseases. Here, we suggest that allelic variants of HO-1, which display significant differences in terms of protein expression, have been selected in endemic malaria areas since the HO-1 enzyme can enhance the protection against malaria conferred by haemolytic diseases This protection apply mainly in what concerns protection against severe malaria forms. Therefore, HO-1 genotyping would be fundamental to determine resistance of a given individual to lethal forms of malaria as well as to common clinical complications typical to haemolytic diseases and would be helpful in the establishment of public health politics.

Polymorphisms of HLA-A and HLA-B genes in genetic susceptibility to esophageal carcinoma in Chaoshan Han Chinese

Esophageal carcinoma (EC) occurs at high rate in Chaoshan region of southern China. Human leukocyte antigen (HLA) polymorphism has been implicated in risk for various cancers. To investigate the impact of HLA-A and HLA-B polymorphisms on susceptibility to EC, a case-control study was conducted among 206 patients with esophageal squamous cell carcinoma and 524 controls from Chaoshan Han population. HLA-A and HLA-B polymorphisms were genotyped by polymerase chain reaction-sequence-specific primers. Genotypic association tests for dominant, recessive, and additive models, and haplotypic association were calculated using unconditional logistic regression. A*11 was identified in a recessive model as an only allele strongly associated with EC risk (odds ratios [OR]=2.10, 95% confidence interval [CI]=1.33-3.31) even after correction for multiple test. The haplotypes A*02-B*46 (OR=1.53, 95% CI=1.04-2.24) and A*11-B*51 (OR=2.29, 95% CI=1.20-4.40) showed association with increased risk for EC, whereas A*11-B*58 (OR=0.00, 95% CI=0.00-0.82) was associated with decreased risk, though the significance of these haplotypes was lost after correction. This is a first association study at genetic level identifying HLA-A and HLA-B-related variations in genetic susceptibility to EC among Chaoshan population. The variation pattern is likely to be EC-specific because it is different from that observed for nasopharyngeal carcinoma in the same study population and might, at least in part, explain the high rate of EC in this ethnic group.

Recent insights into the population genetics and dynamics of the inherited disorders of hemoglobin

The inherited disorders of hemoglobin are by far the commonest monogenic diseases and there is considerable evidence that they have reached their very high frequencies due to heterozygote advantage against malaria. Recent studies have begun to clarify the effect of interactions between malaria and some of the more severe inherited hemoglobin disorders and demonstrated how complex epistatic interactions between different hemoglobin variants with respect to malaria resistance and modification of their phenotypic severity may explain the remarkable heterogeneity of distribution and the frequency of these conditions both between and within individual populations.

Genetic and genomic analyses of host-pathogen interactions in malaria

The Plasmodium parasite successfully infects and replicates in both human and insect vectors. Population studies in humans have long detected the enormous selective pressure placed by the parasite on its human host, revealing the footprints of co-evolution. Available complete genomic sequences for the human and insect hosts, and additional sequences from multiple field isolates of Plasmodiumfalciparum have identified a wide array of protein and gene families that play a crucial role at the interface of host-parasite interaction. Selected examples of such interactions will be reviewed herein.

Genome-wide and fine-resolution association analysis of malaria in West Africa

We report a genome-wide association (GWA) study of severe malaria in The Gambia. The initial GWA scan included 2,500 children genotyped on the Affymetrix 500K GeneChip, and a replication study included 3,400 children. We used this to examine the performance of GWA methods in Africa. We found considerable population stratification, and also that signals of association at known malaria resistance loci were greatly attenuated owing to weak linkage disequilibrium (LD). To investigate possible solutions to the problem of low LD, we focused on the HbS locus, sequencing this region of the genome in 62 Gambian individuals and then using these data to conduct multipoint imputation in the GWA samples. This increased the signal of association, from P = 4 × 10(-7) to P = 4 × 10(-14), with the peak of the signal located precisely at the HbS causal variant. Our findings provide proof of principle that fine-resolution multipoint imputation, based on population-specific sequencing data, can substantially boost authentic GWA signals and enable fine mapping of causal variants in African populations.

Genetics of susceptibility to Plasmodium falciparum: from classical malaria resistance genes towards genome-wide association studies

Plasmodium falciparum represents one of the strongest selective forces on the human genome. This stable and perennial pressure has contributed to the progressive accumulation in the exposed populations of genetic adaptations to malaria. Descriptive genetic epidemiology provides the initial step of a logical procedure of consequential phases spanning from the identification of genes involved in the resistance/susceptibility to diseases, to the determination of the underlying mechanisms and finally to the possible translation of the acquired knowledge in new control tools. In malaria, the rational development of this strategy is traditionally based on complementary interactions of heterogeneous disciplines going from epidemiology to vaccinology passing through genetics, pathogenesis and immunology. New tools including expression profile analysis and genome-wide association studies are recently available to explore the complex interactions of host-parasite co-evolution. Particularly, the combination of genome-wide association studies with large multi-centre initiatives can overcome the limits of previous results due to local population dynamics. Thus, we anticipate substantial advances in the interpretation and validation of the effects of genetic variation on malaria susceptibility, and thereby on molecular mechanisms of protective immune responses and pathogenesis.

The acute phase response in children with mild and severe malaria in Papua New Guinea

The production of acute phase proteins during infection is an important part of innate immunity and limits inflammation. However, little is known of the acute phase response in malaria. We measured acute phase proteins in plasma in children attending clinics and admitted to hospital with acute malaria in Papua New Guinea. Plasma ferritin concentration increased progressively with disease severity with markedly elevated levels in the most severely ill children. Plasma ferritin was >500 ng/ml in 7/99 (7.1%) outpatients with uncomplicated malaria, 22/100 (22.0%) hospital non-severe cases, 64/175 (36.6%) severe malaria cases who survived and 7/9 (77.8%) severe malaria deaths (P<0.001). The greatest concentration of ferritin (3561 ng/ml) was observed in a child who died. By contrast, C-reactive protein concentration was markedly increased in 153 children with uncomplicated malaria [median 203 (interquartile range 51-365) microg/ml] but, surprisingly, was only moderately increased in 135 children with one or more severe manifestations of malaria [47 (17-97) microg/ml; P<0.001] and in 6 children who died [41 (22-280) microg/ml]. Excessive free-radical damage resulting from a combination of iron-induced oxidant stress and reduced levels of C-reactive protein may be an important pathological mechanism in severe malaria and amenable to therapeutic intervention.

Decomposing health: tolerance and resistance to parasites in animals

Plant biologists have long recognized that host defence against parasites and pathogens can be divided into two conceptually different components: the ability to limit parasite burden (resistance) and the ability to limit the harm caused by a given burden (tolerance). Together these two components determine how well a host is protected against the effects of parasitism. This distinction is useful because it recognizes that hosts that are best at controlling parasite burdens are not necessarily the healthiest. Moreover, resistance and tolerance can be expected to have different effects on the epidemiology of infectious diseases and host-parasite coevolution. However, studies of defence in animals have to date focused on resistance, whereas the possibility of tolerance and its implications have been largely overlooked. The aim of our review is to (i) describe the statistical framework for analysis of tolerance developed in plant science and how this can be applied to animals, (ii) review evidence of genetic and environmental variation for tolerance in animals, and studies indicating which mechanisms could contribute to this variation, and (iii) outline avenues for future research on this topic.

Survival and haematological recovery of children with severe malaria transfused in accordance to WHO guidelines in Kilifi, Kenya

Background

Severe anaemia requiring emergency blood transfusion is a common complication of malaria in children. To ensure access for urgent blood transfusion, the World Health Organization has developed clear guidelines with haemoglobin thresholds prevent unwarranted transfusion,. Few studies have reported outcome and haematological recovery of children with severe malaria where transfusion practice complies with WHO recommendations.

Methods

A prospective observational study of survivors of severe and complicated malaria transfused in accordance with WHO guidelines. Children were invited for review at one month post-discharge. Non-attendees were traced in the community to ascertain survival.

Results

Outcome was assessed in 213 survivors. Those transfused were younger, had a higher base deficit, mean lactate levels and a higher prevalence of respiratory distress. As expected mean admission haemoglobin (Hb) was significantly lower amongst transfused [5.0 g/dL SD: 1.9] compared to non-transfused children [8.3 g/dL SD: 1.7] (p < 0.001). At discharge mean Hb was similar 6.4 g/dL [SD: 1.5] and 6.8 g/dL [SD: 1.6] respectively (p = 0.08), most children remained moderately to severely anaemic. At one month follow up 166 children (78%) returned, in whom we found no differences in mean Hb between the transfused (10.2 g/dL [SD: 1.7]) and non-transfused (10.0 g/dL [SD: 1.3]) survivors (p = 0.25). The major factors affecting haematological recovery were young age (<24 months) and concomitant malaria parasitaemia; Hb being 8.8 g/dL [SD: 1.5] in parasitaemic individuals compared with 10.5 g/dL [SD: 1.3] in those without (p < 0.001).

Conclusion

This data supports the policy of rational use of blood transfusion, as proposed in the WHO guidelines, for children with anaemia in areas where access to emergency transfusion is not guaranteed. We have provided empirical data indicating that transfusion does not influence superior recovery in haemoglobin concentrations and therefore cannot be justified on this basis alone. This may help resolve the disparity between international policy and current clinical practice. Effective anti-malarial treatment at discharge may prevent reoccurrence of anaemia.

Sickle cell trait is associated with a delayed onset of malaria: implications for time-to-event analysis in clinical studies of malaria

BACKGROUND

The World Health Organization (WHO) recently recommended that the time to first malaria episode serve as the primary end point in phase III malaria vaccine trials--the first of which will be held in Africa. Although common red blood cell (RBC) polymorphisms such as sickle hemoglobin (HbS) are known to protect against malaria in Africa, their impact on this end point has not been investigated.

METHODS

A longitudinal study of 225 individuals aged 2-25 years was conducted in Mali. The association between common RBC polymorphisms and the time to first malaria episode was evaluated.

RESULTS

Among children aged 2-10 years, sickle cell trait (HbAS) was associated with a 34-day delay in the median time to first malaria episode (P= .017) Cox regression analysis showed that greater age (hazard ratio [HR], 0.87 [95% CI, 0.80-0.94]; (P= .001), HbAS (HR, 0.48 [95% CI, 0.26-0.91]; (P= .024), and asymptomatic parasitemia at enrollment (HR, 0.35 [95% CI, 0.14-0.85]; (P= .021) were associated with decreased malaria risk.

CONCLUSION

Given the delay in the time to first malaria episode associated with HbAS, it would be advisable for clinical trials and observational studies that use this end point to include Hb typing in the design of studies conducted in areas where HbAS is prevalent.

Two ways to survive infection: what resistance and tolerance can teach us about treating infectious diseases

A host can evolve two types of defence mechanism to increase its fitness when challenged with a pathogen: resistance and tolerance. Immunology is a well-defined field in which the mechanisms behind resistance to infection are dissected. By contrast, the mechanisms behind the ability to tolerate infections are studied in a less methodical manner. In this Opinion, we provide evidence that animals have specific tolerance mechanisms and discuss their potential clinical impact. It is important to distinguish between these two defence mechanisms because they have different pathological and epidemiological effects. An increased understanding of tolerance to pathogen infection could lead to more efficient treatments for infectious diseases and a better description of host-pathogen interactions.

Alpha(+)-thalassaemia and malarial anaemia

The mechanisms by which alpha(+)-thalassaemia protects against severe malaria, and severe malarial anaemia in particular, are poorly understood. A recent report proposes that the increased count of microcytic and hypochromic erythrocytes in alpha(+)-thalassaemia reduces the haemoglobin decline during acute malaria and, thus, reduces the risk of anaemia. This mechanism might add to further alpha(+)-thalassaemic attributes that are involved in the attenuation of anaemia caused by both acute and chronic Plasmodium infections.

Malaria's indirect contribution to all-cause mortality in the Andaman Islands during the colonial era

Malaria has a substantial secondary effect on other causes of mortality. From the 19th century, malaria epidemics in the Andaman Islands' penal colony were initiated by the brackish swamp-breeding malaria vector Anopheles sundaicus and fuelled by the importation of new prisoners. Malaria was a major determinant of the highly variable all-cause mortality rate (correlation coefficient r(2)=0.60, n=68, p<0.0001) from 1872 to 1939. Directly attributed malaria mortality based on post-mortem examinations rarely exceeded one-fifth of total mortality. Infectious diseases such as pneumonia, tuberculosis, dysentery, and diarrhoea, which combined with malaria made up the majority of all-cause mortality, were positively correlated with malaria incidence over several decades. Deaths secondary to malaria (indirect malaria mortality) were at least as great as mortality directly attributed to malaria infections.

Alpha+ -thalassemia protects against anemia associated with asymptomatic malaria: evidence from community-based surveys in Tanzania and Kenya

BACKGROUND

In hospital-based studies, alpha(+)-thalassemia has been found to protect against severe, life-threatening falciparum malaria. alpha(+)-Thalassemia does not seem to prevent infection or high parasite densities but rather limits progression to severe disease--in particular, severe malarial anemia. We assessed to what extent alpha(+)-thalassemia influences the association between mild, asymptomatic Plasmodium falciparum infection and hemoglobin concentration.

METHODS

The study was based on 2 community-based surveys conducted among afebrile children (0.5-8 years old; n=801) in Kenya and Tanzania.

RESULTS

Among children without inflammation (whole-blood C-reactive protein concentration <or=10 mg/L), P. falciparum infection was associated with only small reductions in hemoglobin concentration, and effects were similar across alpha-globin genotypes. By contrast, the reduction in hemoglobin concentration associated with P. falciparum infection accompanied by inflammation was larger and strongly depended on genotype (normal, -21.8 g/L; heterozygous, -16.7 g/L; and homozygous, -4.6 g/L). Relative to children with a normal genotype, this difference in effect was 5.1 g/L (95% confidence interval [CI], -1.0 to 11.1 g/L) for heterozygotes and 17.2 g/L (95% CI, 8.3 to 26.2 g/L) for homozygotes (estimates are adjusted for study site, age, height-for-age z score, and iron deficiency).

CONCLUSIONS

alpha(+)-Thalassemia limits the decline in hemoglobin concentration that is associated with afebrile infections, particularly those that are accompanied by inflammation.

A large deletion in the human alpha-globin cluster caused by a replication error is associated with an unexpectedly mild phenotype

We have characterized a newly identified 16.6 kb deletion which removes a significant proportion of the human alpha-globin cluster including the psizeta1, alpha(D), psialpha1 and alpha2-globin genes but leaves the duplicated alpha1 gene intact. This complicated rearrangement results from a combination of slippage and strand switching at sites of microhomology during replication. Functional analysis shows that expression of the remaining alpha1 gene is increased, rather than down-regulated by this deletion. This could be related to its proximity to the remote upstream alpha-globin regulatory elements or reduced competition for these elements in the absence of the dominant alpha2-globin gene. The finding of a very mild phenotype associated with such an extensive deletion in the alpha-globin cluster implies that much of the DNA removed by the deletion is likely to be functionally unimportant. These findings suggest that other than the upstream regulatory elements and promoter proximal elements there are unlikely to be additional positive cis-acting sequences in the alpha-globin cluster.

Haptoglobin HP2-2 genotype, alpha-thalassaemia and acute seizures in children living in a malaria-endemic area

Polymorphisms of the haptoglobin (HP) gene and deletions in alpha-globin gene (alpha-thalassaemia) are common in malaria-endemic Africa. The same region also has high incidence rates for childhood acute seizures. The haptoglobin HP2-2 genotype has been associated with idiopathic generalized epilepsies and altered iron metabolism in children with alpha-thalassaemia can potentially interfere with neurotransmission and increase the risk of seizures. We investigated the hypothesis that the HP2-2 genotype and the common African alpha-globin gene deletions are associated with the increased risk of seizures. 288 children aged 3-156 months admitted with acute seizures to Kilifi District Hospital (Kenya), were matched for ethnicity to an equal number of community controls. The proportion of cases (72/288 [25.0%]) and controls (80/288 [27.8%]) with HP2-2 genotype was similar, p=0.499. The allele frequency of HP2 gene in cases (49.3%) and controls (48.6%) was also similar, p=0.814. Similarly, we found no significant difference between the proportion of cases (177/267 [66.3%]) and controls (186/267 [69.7%]) with deletions in alpha-globin gene (p=0.403). Among cases, HP2-2 polymorphism and deletions in alpha-globin gene were neither associated with changes in the type, number or duration of seizures nor did they affect outcome. We conclude that the HP2-2 polymorphism and deletions in alpha-globin gene are not risk factors for acute seizures in children. Future studies should examine other susceptibility genes.

Genetic studies of African populations: an overview on disease susceptibility and response to vaccines and therapeutics

Africa is the ultimate source of modern humans and as such harbors more genetic variation than any other continent. For this reason, studies of the patterns of genetic variation in African populations are crucial to understanding how genes affect phenotypic variation, including disease predisposition. In addition, the patterns of extant genetic variation in Africa are important for understanding how genetic variation affects infectious diseases that are a major problem in Africa, such as malaria, tuberculosis, schistosomiasis, and HIV/AIDS. Therefore, elucidating the role that genetic susceptibility to infectious diseases plays is critical to improving the health of people in Africa. It is also of note that recent and ongoing social and cultural changes in sub-Saharan Africa have increased the prevalence of non-communicable diseases that will also require genetic analyses to improve disease prevention and treatment. In this review we give special attention to many of the past and ongoing studies, emphasizing those in Sub-Saharan Africans that address the role of genetic variation in human disease.