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

Increased microerythrocyte count in homozygous alpha(+)-thalassaemia contributes to protection against severe malarial anaemia

BACKGROUND

The heritable haemoglobinopathy alpha(+)-thalassaemia is caused by the reduced synthesis of alpha-globin chains that form part of normal adult haemoglobin (Hb). Individuals homozygous for alpha(+)-thalassaemia have microcytosis and an increased erythrocyte count. Alpha(+)-thalassaemia homozygosity confers considerable protection against severe malaria, including severe malarial anaemia (SMA) (Hb concentration < 50 g/l), but does not influence parasite count. We tested the hypothesis that the erythrocyte indices associated with alpha(+)-thalassaemia homozygosity provide a haematological benefit during acute malaria.

METHODS AND FINDINGS

Data from children living on the north coast of Papua New Guinea who had participated in a case-control study of the protection afforded by alpha(+)-thalassaemia against severe malaria were reanalysed to assess the genotype-specific reduction in erythrocyte count and Hb levels associated with acute malarial disease. We observed a reduction in median erythrocyte count of approximately 1.5 x 10(12)/l in all children with acute falciparum malaria relative to values in community children (p < 0.001). We developed a simple mathematical model of the linear relationship between Hb concentration and erythrocyte count. This model predicted that children homozygous for alpha(+)-thalassaemia lose less Hb than children of normal genotype for a reduction in erythrocyte count of >1.1 x 10(12)/l as a result of the reduced mean cell Hb in homozygous alpha(+)-thalassaemia. In addition, children homozygous for alpha(+)-thalassaemia require a 10% greater reduction in erythrocyte count than children of normal genotype (p = 0.02) for Hb concentration to fall to 50 g/l, the cutoff for SMA. We estimated that the haematological profile in children homozygous for alpha(+)-thalassaemia reduces the risk of SMA during acute malaria compared to children of normal genotype (relative risk 0.52; 95% confidence interval [CI] 0.24-1.12, p = 0.09).

CONCLUSIONS

The increased erythrocyte count and microcytosis in children homozygous for alpha(+)-thalassaemia may contribute substantially to their protection against SMA. A lower concentration of Hb per erythrocyte and a larger population of erythrocytes may be a biologically advantageous strategy against the significant reduction in erythrocyte count that occurs during acute infection with the malaria parasite Plasmodium falciparum. This haematological profile may reduce the risk of anaemia by other Plasmodium species, as well as other causes of anaemia. Other host polymorphisms that induce an increased erythrocyte count and microcytosis may confer a similar advantage.

Polymorphisms of TNF-enhancer and gene for FcgammaRIIa correlate with the severity of falciparum malaria in the ethnically diverse Indian population

BACKGROUND

Susceptibility/resistance to Plasmodium falciparum malaria has been correlated with polymorphisms in more than 30 human genes with most association analyses having been carried out on patients from Africa and south-east Asia. The aim of this study was to examine the possible contribution of genetic variants in the TNF and FCGR2A genes in determining severity/resistance to P. falciparum malaria in Indian subjects.

METHODS

Allelic frequency distribution in populations across India was first determined by typing genetic variants of the TNF enhancer and the FCGR2A G/A SNP in 1871 individuals from 55 populations. Genotyping was carried out by DNA sequencing, single base extension (SNaPshot), and DNA mass array (Sequenom). Plasma TNF was determined by ELISA. Comparison of datasets was carried out by Kruskal-Wallis and Mann-Whitney tests. Haplotypes and LD plots were generated by PHASE and Haploview, respectively. Odds ratio (OR) for risk assessment was calculated using EpiInfotrade mark version 3.4.

RESULTS

A novel single nucleotide polymorphism (SNP) at position -76 was identified in the TNF enhancer along with other reported variants. Five TNF enhancer SNPs and the FCGR2A R131H (G/A) SNP were analyzed for association with severity of P. falciparum malaria in a malaria-endemic and a non-endemic region of India in a case-control study with ethnically-matched controls enrolled from both regions. TNF -1031C and -863A alleles as well as homozygotes for the TNF enhancer haplotype CACGG (-1031T>C, -863C>A, -857C>T, -308G>A, -238G>A) correlated with enhanced plasma TNF levels in both patients and controls. Significantly higher TNF levels were observed in patients with severe malaria. Minor alleles of -1031 and -863 SNPs were associated with increased susceptibility to severe malaria. The high-affinity IgG2 binding FcgammaRIIa AA (131H) genotype was significantly associated with protection from disease manifestation, with stronger association observed in the malaria non-endemic region. These results represent the first genetic analysis of the two immune regulatory molecules in the context of P. falciparum severity/resistance in the Indian population.

CONCLUSION

Association of specific TNF and FCGR2A SNPs with cytokine levels and disease severity/resistance was indicated in patients from areas with differential disease endemicity. The data emphasizes the need for addressing the contribution of human genetic factors in malaria in the context of disease epidemiology and population genetic substructure within India.

Host erythrocyte polymorphisms and exposure to Plasmodium falciparum in Papua New Guinea

Background

The protection afforded by human erythrocyte polymorphisms against the malaria parasite, Plasmodium falciparum, has been proposed to be due to reduced ability of the parasite to invade or develop in erythrocytes. If this were the case, variable levels of parasitaemia and rates of seroconversion to infected-erythrocyte variant surface antigens (VSA) should be seen in different host genotypes.

Methods

To test this hypothesis, P. falciparum parasitaemia and anti-VSA antibody levels were measured in a cohort of 555 asymptomatic children from an area of intense malaria transmission in Papua New Guinea. Linear mixed models were used to investigate the effect of α⁺-thalassaemia, complement receptor-1 and south-east Asian ovalocytosis, as well as glucose-6-phosphate dehydrogenase deficiency and ABO blood group on parasitaemia and age-specific seroconversion to VSA.

Results

No host polymorphism showed a significant association with both parasite prevalence/density and age-specific seroconversion to VSA.

Conclusion

Host erythrocyte polymorphisms commonly found in Papua New Guinea do not effect exposure to blood stage P. falciparum infection. This contrasts with data for sickle cell trait and highlights that the above-mentioned polymorphisms may confer protection against malaria via distinct mechanisms.

Experimental validation of Haldane's hypothesis on the role of infection as an evolutionary force for Metazoans

A common drawback in evolutionary science is the fact that the evolution of organisms occurs in geological timing, completely out of the time scale of laboratory experimental work. For this reason, some relevant hypotheses on evolution of Metazoans are based on correlations more than on experimental data obtained for testing the robustness of those hypotheses. In the current work, we implement an experimental methodology to analyze the role of infections as a driving force in the evolution of Metazoans (Haldane's hypothesis). To that goal, we have used simple models of virulence with short reproduction times, large populations, and that are easily testable in the laboratory. Using the bacteriovirus nematode Caenorhabditis elegans as a model organism under evolution and their infection by the environmental opportunistic bacterial pathogen Pseudomonas aeruginosa as the selective force, we have demonstrated that bacterial infection selects an evolved nematode lineage resistant to infection, with changes in its respiration and capability of consuming novel food resources. Using an experimental approach, we show that infection is a selective force in the evolution of Metazoans as proposed earlier by Haldane.

Racial variation in the relationship of anemia with mortality and mobility disability among older adults

Anemia is more common among older blacks than older whites. However, it is unclear whether anemia predicts adverse events similarly in both races. Data on 1018 black and 1583 white adults aged 71 to 82 years were analyzed. Anemia, as defined by World Health Organization (WHO) criteria, was used to predict mortality over 6 years and incidence of mobility disability over 4 years. In proportional hazards models of mortality in whites, the age-adjusted hazard ratio (HR) for anemia in men was 1.96 (95% confidence interval [CI]: 1.35, 2.83) and in women was 2.86 (95% CI: 1.69, 4.82). In contrast, anemia was not associated with mortality in black men (HR = 1.15 [95% CI: 0.77, 1.72]) or women (HR = 1.39 [95% CI: 0.91, 2.14]). Higher mortality rate was observed only in black men with hemoglobin values more than 20 g/L (2.0 g/dL) below the WHO cutoff, whereas mortality rates were elevated in white men with hemoglobin values 1 to 10, 11 to 20, and more than 20 g/L below the WHO cutoff. In conclusion, anemia was significantly associated with increased risk of death and mobility disability in community-dwelling older whites. Conversely, older blacks classified as anemic by WHO criteria were not at risk for adverse events, indicating that alternative criteria are warranted.

Systemic lupus erythematosus-associated defects in the inhibitory receptor FcgammaRIIb reduce susceptibility to malaria

Polygenic autoimmune diseases, such as systemic lupus erythematosus (SLE), are a significant cause of morbidity and mortality worldwide. In recent years, functionally important genetic polymorphisms conferring susceptibility to SLE have been identified, but the evolutionary pressures driving their retention in the gene pool remain elusive. A defunctioning, SLE-associated polymorphism of the inhibitory receptor FcgammaRIIb is found at an increased frequency in African and Asian populations, broadly corresponding to areas where malaria is endemic. Here, we show that FcgammaRIIb-deficient mice have increased clearance of malarial parasites (Plasmodium chabaudi chabaudi) and develop less severe disease. In vitro, the human lupus associated FcgammaRIIb polymorphism enhances phagocytosis of Plasmodium falciparum-infected erythrocytes. These results demonstrate that FcgammaRIIb is important in controlling the immune response to malarial parasites and suggests that the higher frequency of human FcgammaRIIb polymorphisms predisposing to SLE in Asians and Africans may be maintained because these variants reduce susceptibility to malaria.

Genetic link between Chaoshan and other Chinese Han populations: Evidence from HLA-A and HLA-B allele frequency distribution

The genetic polymorphism of HLA-A and HLA-B loci was investigated in 505 Chaoshanese using PCR-SSP method. Among the HLA-A alleles detected, A*11 (35.64%) was most frequent, followed by A*02 (31.78%). Of 34 HLA-B alleles tested, 30 were observed, in which B*60 (21.68%), B*46 (14.46%), and B*58 (10.69%) were highly predominant. Comparison was made with other nine Chinese Han ethnic groups covering the Mainland China, Taiwan, Hong Kong, and Singapore. The high frequent alleles found in Chaoshanese were also common in other Chinese groups compared though the frequency levels varied from group to group. The phylogenic tree analysis based on the HLA-A and -B allele frequencies of all the 10 Chinese ethnic groups revealed that Chaoshanese, while clustering in general with the southern China-related Han Chinese, had the highest affinity to the Mainland Minnanese, but separated distinctively from the northern Han Chinese. The study, however, was yet to confirm the hypothesis of the Central Plains Han origin of Chaoshanese. Interestingly, the alleles (B*46, B*38, and B*58) and the related haplotypes (A*02-B*46 and A*33-B*58) that are positively associated with nasopharyngeal carcinoma (NPC), a disease prevailing predominantly among southern Chinese, were always at much higher frequencies among southern Chinese than among northern Chinese, whereas A*31 and B*13, the two alleles with highly protective effects for NPC, and the associated haplotype A*30-B*13 were predominantly high in northern Chinese. The different genetic background between northern and southern China may explain, at least partially, the prevalence of NPC among southern Chinese.

Mendelian and complex genetics of susceptibility and resistance to parasitic infections

Uncovering the complex genetic basis of susceptibility and resistance to parasitic infectious diseases is an enormous challenge. It probably involves many different host genes, interacting with multiple parasite genetic and environmental factors. Several genes of interest have been identified by family and association studies in humans and by using mouse models, but more robust epidemiological studies and functional data are needed to authenticate these findings. With new technologies and statistical tools for whole-genome association analysis, the next few years are likely to see acceleration in the rate of gene discovery, which has the potential to greatly assist drug and vaccine development.

Alpha+ -thalassaemia and pregnancy in a malaria endemic region of Papua New Guinea

The effect of maternal alpha+ -thalassaemia on pregnancy was assessed in the north coastal region of Papua New Guinea (PNG), where malaria is hyperendemic and alpha+ -thalassaemia is extremely common. In a prospective study of 987 singleton hospital deliveries, we correlated maternal alpha-globin genotype with markers of reproductive fitness (age in primigravidae, gravidity, pregnancy interval and the number of miscarriages and stillbirths), Plasmodium falciparum(P. falciparum) infection of the mother and placenta, maternal haemoglobin, preterm delivery and birthweight. The frequency of the -alpha genotype in mothers was 0.61. Markers of reproductive fitness were similar in women with and without alpha+ -thalassaemia. Median haemoglobin concentration during pregnancy and after delivery was about 1.0 g/dl lower in homozygous alpha+ -thalassaemia than in women with a normal alpha- globin genotype (P < or = 0.001). The frequency of placental P. falciparum infection and systemic malaria infection after delivery showed no consistent relationship to alpha-globin genotype. The frequency of preterm delivery and low birthweight did not vary significantly according to maternal alpha-globin genotype. Maternal alpha+ -thalassaemia does not affect reproductive fitness or susceptibility to malaria during pregnancy. Although median haemoglobin concentration was significantly lower in mothers homozygous for alpha+ -thalassaemia than those with a normal alpha-globin genotype, this did not result in an adverse outcome of pregnancy.

Human red blood cell polymorphisms and malaria

Genetic factors are a major determinant of child survival in malaria endemic countries. Identifying which genes are involved and how they affect the malaria disease risk potentially offers a powerful mechanism through which to learn more about the host-parasite relationship. The past few years have seen significant progress towards achieving this goal for some of the best-known malaria resistance genes that determine the structure or function of red blood cells: Gerbich blood group antigen negativity; polymorphisms of the complement receptor genes (most notably CR1); Southeast Asian ovalocytosis; pyruvate kinase deficiency; haemoglobin E; the sickle cell trait; and alpha-thalassaemia are all examples. The challenge for the future must be to translate such advances into fresh approaches to the prevention and treatment of malaria.

Red blood cell defects and malaria

Malaria is a major cause of childhood death throughout much of the tropical world. As a result, it has exerted a powerful force for the evolutionary selection of genes that confer a survival advantage. Identifying which genes are involved, and how they affect malaria risk, is a potentially useful way of exploring the host-parasite relationship. To date, some of the best-described malaria-protective polymorphisms relate to genes that affect the structure or function of red blood cells (RBC). Recent years have seen significant advances in our understanding of the importance of some of these genes, including glycophorin C (GYPC); complement receptor 1 (CR1); band 3 (SLC4A1); pyruvate kinase (Pklr); and the genes for alpha-(HBA) and beta-globin (HBB). The challenge for the future must be to convert these advances into fresh approaches to the prevention and treatment of malaria.

The effect of alpha+-thalassaemia on the incidence of malaria and other diseases in children living on the coast of Kenya

BACKGROUND

The alpha-thalassaemias are the commonest genetic disorders of humans. It is generally believed that this high frequency reflects selection through a survival advantage against death from malaria; nevertheless, the epidemiological description of the relationships between alpha-thalassaemia, malaria, and other common causes of child mortality remains incomplete.

METHODS AND FINDINGS

We studied the alpha+-thalassaemia-specific incidence of malaria and other common childhood diseases in two cohorts of children living on the coast of Kenya. We found no associations between alpha+-thalassaemia and the prevalence of symptomless Plasmodium falciparum parasitaemia, the incidence of uncomplicated P. falciparum disease, or parasite densities during mild or severe malaria episodes. However, we found significant negative associations between alpha+-thalassaemia and the incidence rates of severe malaria and severe anaemia (haemoglobin concentration < 50 g/l). The strongest associations were for severe malaria anaemia (> 10,000 P. falciparum parasites/mul) and severe nonmalaria anaemia; the incidence rate ratios and 95% confidence intervals (CIs) for alpha+-thalassaemia heterozygotes and homozygotes combined compared to normal children were, for severe malaria anaemia, 0.33 (95% CI, 0.15,0.73; p = 0.006), and for severe nonmalaria anaemia, 0.26 (95% CI, 0.09,0.77; p = 0.015).

CONCLUSIONS

Our observations suggest, first that selection for alpha+-thalassaemia might be mediated by a specific effect against severe anaemia, an observation that may lead to fresh insights into the aetiology of this important condition. Second, although alpha+-thalassaemia is strongly protective against severe and fatal malaria, its effects are not detectable at the level of any other malaria outcome; this result provides a cautionary example for studies aimed at testing malaria interventions or identifying new malaria-protective genes.

Infections and thalassaemia

Infections are major complications and constitute the second most common cause of mortality and a main cause of morbidity in patients with thalassaemia, a group of genetic disorders of haemoglobin synthesis characterised by a disturbance of globin chain production. Thalassaemias are among the most common genetic disorders in the world. Predisposing factors for infections in thalassaemic patients include severe anaemia, iron overload, splenectomy, and a range of immune abnormalities. Major causative organisms of bacterial infections in thalassaemic patients are Klebsiella spp in Asia and Yersinia enterocolitica in western countries. Transfusion-associated viral infections (especially hepatitis C) can lead to liver cirrhosis and hepatocellular carcinoma. A unique and challenging infection detected in Asian patients is pythiosis, caused by a fungus-like organism, the mortality rate of which is very high. Because the prognosis for thalassaemia has much improved, with many patients surviving to the fifth decade of life in developed countries, it is mandatory to reduce mortality by recognising and presumptively treating infections in these patients as quickly as possible.

Heritability of malaria in Africa

BACKGROUND

While many individual genes have been identified that confer protection against malaria, the overall impact of host genetics on malarial risk remains unknown.

METHODS AND FINDINGS

We have used pedigree-based genetic variance component analysis to determine the relative contributions of genetic and other factors to the variability in incidence of malaria and other infectious diseases in two cohorts of children living on the coast of Kenya. In the first, we monitored the incidence of mild clinical malaria and other febrile diseases through active surveillance of 640 children 10 y old or younger, living in 77 different households for an average of 2.7 y. In the second, we recorded hospital admissions with malaria and other infectious diseases in a birth cohort of 2,914 children for an average of 4.1 y. Mean annual incidence rates for mild and hospital-admitted malaria were 1.6 and 0.054 episodes per person per year, respectively. Twenty-four percent and 25% of the total variation in these outcomes was explained by additively acting host genes, and household explained a further 29% and 14%, respectively. The haemoglobin S gene explained only 2% of the total variation. For nonmalarial infections, additive genetics explained 39% and 13% of the variability in fevers and hospital-admitted infections, while household explained a further 9% and 30%, respectively.

CONCLUSION

Genetic and unidentified household factors each accounted for around one quarter of the total variability in malaria incidence in our study population. The genetic effect was well beyond that explained by the anticipated effects of the haemoglobinopathies alone, suggesting the existence of many protective genes, each individually resulting in small population effects. While studying these genes may well provide insights into pathogenesis and resistance in human malaria, identifying and tackling the household effects must be the more efficient route to reducing the burden of disease in malaria-endemic areas.

Negative epistasis between the malaria-protective effects of alpha+-thalassemia and the sickle cell trait

The hemoglobinopathies, disorders of hemoglobin structure and production, protect against death from malaria. In sub-Saharan Africa, two such conditions occur at particularly high frequencies: presence of the structural variant hemoglobin S and alpha(+)-thalassemia, a condition characterized by reduced production of the normal alpha-globin component of hemoglobin. Individually, each is protective against severe Plasmodium falciparum malaria, but little is known about their malaria-protective effects when inherited in combination. We investigated this question by studying a population on the coast of Kenya and found that the protection afforded by each condition inherited alone was lost when the two conditions were inherited together, to such a degree that the incidence of both uncomplicated and severe P. falciparum malaria was close to baseline in children heterozygous with respect to the mutation underlying the hemoglobin S variant and homozygous with respect to the mutation underlying alpha(+)-thalassemia. Negative epistasis could explain the failure of alpha(+)-thalassemia to reach fixation in any population in sub-Saharan Africa.

How malaria has affected the human genome and what human genetics can teach us about malaria

Malaria is a major killer of children worldwide and the strongest known force for evolutionary selection in the recent history of the human genome. The past decade has seen growing evidence of ethnic differences in susceptibility to malaria and of the diverse genetic adaptations to malaria that have arisen in different populations: epidemiological confirmation of the hypotheses that G6PD deficiency, alpha+ thalassemia, and hemoglobin C protect against malaria mortality; the application of novel haplotype-based techniques demonstrating that malaria-protective genes have been subject to recent positive selection; the first genetic linkage maps of resistance to malaria in experimental murine models; and a growing number of reported associations with resistance and susceptibility to human malaria, particularly in genes involved in immunity, inflammation, and cell adhesion. The challenge for the next decade is to build the global epidemiological infrastructure required for statistically robust genomewide association analysis, as a way of discovering novel mechanisms of protective immunity that can be used in the development of an effective malaria vaccine.