Epistatic interactions between genetic disorders of hemoglobin can explain why the sickle-cell gene is uncommon in the Mediterranean

The erythrocyte membrane properties of beta thalassaemia heterozygotes and their consequences for Plasmodium falciparum invasion

Malaria parasites such as Plasmodium falciparum have exerted formidable selective pressures on the human genome. Of the human genetic variants associated with malaria protection, beta thalassaemia (a haemoglobinopathy) was the earliest to be associated with malaria prevalence. However, the malaria protective properties of beta thalassaemic erythrocytes remain unclear. Here we studied the mechanics and surface protein expression of beta thalassaemia heterozygous erythrocytes, measured their susceptibility to P. falciparum invasion, and calculated the energy required for merozoites to invade them. We found invasion-relevant differences in beta thalassaemic cells versus matched controls, specifically: elevated membrane tension, reduced bending modulus, and higher levels of expression of the major invasion receptor basigin. However, these differences acted in opposition to each other with respect to their likely impact on invasion, and overall we did not observe beta thalassaemic cells to have lower P. falciparum invasion efficiency for any of the strains tested.

Evolutionary history of sickle-cell mutation: implications for global genetic medicine

Resistance afforded by the sickle-cell trait against severe malaria has led to high frequencies of the sickle-cell mutation [HBB; c.20T>A, p.Glu6Val; OMIM: 141900 (HBB-βS)] in most parts of Africa. High-coverage sequencing and genotype data have now confirmed the single African origin of the sickle-cell gene variant [HBB; c.20T>A, p.Glu6Val; OMIM: 141900 (HBB-βS)]. Nevertheless, the classical HBB-like genes cluster haplotypes remain a rich source of HBB-βS evolutionary information. The overlapping distribution of HBB-βS and other disease-associated variants means that their evolutionary genetics must be investigated concurrently. In this review: (1) we explore the evolutionary history of HBB-βS and its implications in understanding human migration within and out of Africa: e.g. HBB haplotypes and recent migration paths of the Bantu expansion, occurrence of ~7% of the Senegal haplotype in Angola reflecting changes in population/SCD dynamics, and existence of all five classical HBB haplotype in Cameroon and Egypt suggesting a much longer presence of HBB-βS in these regions; (2) we discuss the time estimates of the emergence of HBB-βS in Africa and finally, (3) we discuss implications for genetic medicine in understanding complex epistatic interactions between HBB-βS and other gene variants selected under environmental pressure in Africa e.g. variants in HBB, HBA, G6PD, APOL1, APOE, OSBPL10 and RXRA.

Adaptive immunity selects against malaria infection blocking mutations

The mutation responsible for Duffy negativity, which impedes Plasmodium vivax infection, has reached high frequencies in certain human populations. Conversely, mutations capable of blocking the more lethal P. falciparum have not succeeded in malarious zones. Here we present an evolutionary-epidemiological model of malaria which demonstrates that if adaptive immunity against the most virulent effects of malaria is gained rapidly by the host, mutations which prevent infection per se are unlikely to succeed. Our results (i) explain the rarity of strain-transcending P. falciparum infection blocking adaptations in humans; (ii) make the surprising prediction that mutations which block P. falciparum infection are most likely to be found in populations experiencing low or infrequent malaria transmission, and (iii) predict that immunity against some of the virulent effects of P. vivax malaria may be built up over the course of many infections.

Hemoglobin E, malaria and natural selection

It is known that there has been positive natural selection for hemoglobin S and C in humans despite negative health effects, due to its role in malaria resistance. However, it is not well understood, if there has been natural selection for hemoglobin E (HbE), which is a common variant in Southeast Asia. Therefore, we reviewed previous studies and discussed the potential role of natural selection in the prevalence of HbE. Our review shows that in vitro studies, evolutionary genetics studies and epidemiologic studies largely support an involvement of natural selection in the evolution of HbE and a protective role of HbE against malaria infection. However, the evidence is inconsistent, provided from different regions, and insufficient to perform an aggregated analysis such as a meta-analysis. In addition, few candidate gene, genome-wide association or epistasis studies, which have been made possible with the use of big data in the post-genomic era, have investigated HbE. The biological pathways linking HbE and malaria infection have not yet been fully elucidated. Therefore, further research is necessary before it can be concluded that there was positive natural selection for HbE due to protection against malaria.

Lay summary: Our review shows that evidence largely supports an involvement of natural selection in the evolution of HbE and a protective role of HbE against malaria. However, the evidence is not consistent. Further research is necessary before it is concluded.

The Evolving Spectrum of the Epidemiology of Thalassemia

The thalassemias and other inherited disorders of hemoglobin are likely to remain a serious global health problem for the foreseeable future. Currently, they are most frequent in the tropical belt; an assessment of their true frequency and the likely cost of management for the governments of these countries will require a form of micromapping. Over recent years, there has been major progress toward better prevention and management of the thalassemias in richer countries; it is likely that, using the tools of molecular genetics, they will eventually be completely curable, although this is probably a long time in the future.

Two complement receptor one alleles have opposing associations with cerebral malaria and interact with α+thalassaemia

Malaria has been a major driving force in the evolution of the human genome. In sub-Saharan African populations, two neighbouring polymorphisms in the Complement Receptor One (CR1) gene, named Sl2 and McCb, occur at high frequencies, consistent with selection by malaria. Previous studies have been inconclusive. Using a large case-control study of severe malaria in Kenyan children and statistical models adjusted for confounders, we estimate the relationship between Sl2 and McCb and malaria phenotypes, and find they have opposing associations. The Sl2 polymorphism is associated with markedly reduced odds of cerebral malaria and death, while the McCb polymorphism is associated with increased odds of cerebral malaria. We also identify an apparent interaction between Sl2 and α+thalassaemia, with the protective association of Sl2 greatest in children with normal α-globin. The complex relationship between these three mutations may explain previous conflicting findings, highlighting the importance of considering genetic interactions in disease-association studies.

Detecting signatures of past pathogen selection on human HLA loci: are there needles in the haystack?

Human leucocyte antigens (HLAs) are responsible for the display of peptide fragments for recognition by T-cell receptors. The gene family encoding them is thus integral to human adaptive immunity, and likely to be under strong pathogen selection. Despite this, it has proved difficult to demonstrate specific examples of pathogen-HLA coevolution. Selection from multiple pathogens simultaneously could explain why the evolutionary signatures of particular pathogens on HLAs have proved elusive. Here, we present an individual-based model of HLA evolution in the presence of two mortality-causing pathogens. We demonstrate that it is likely that individual pathogen species causing high mortality have left recognizable signatures on the HLA genomic region, despite more than one pathogen being present. Such signatures are likely to exist at the whole-population level, and involve haplotypic combinations of HLA genes rather than single loci.

Rapid mortality transition of Pacific Islands in the 19th century

The depopulation of Pacific islands during the 16th to 19th centuries is a striking example of historical mass mortality due to infectious disease. Pacific Island populations have not been subject to such cataclysmic infectious disease mortality since. Here we explore the processes which could have given rise to this shift in infectious disease mortality patterns. We show, using mathematical models, that the population dynamics exhibited by Pacific Island populations are unlikely to be the result of Darwinian evolution. We propose that extreme mortality during first-contact epidemics is a function of epidemiological isolation, not a lack of previous selection. If, as pathogens become established in populations, extreme mortality is rapidly suppressed by herd immunity, Pacific Island population mortality patterns can be explained with no need to invoke genetic change. We discuss the mechanisms by which this could occur, including (i) a link between the proportion of the population transmitting infectious agents and case-fatality rates, and (ii) the course of infection with pathogens such as measles and smallpox being more severe in adults than in children. Overall, we consider the present-day risk of mass mortality from newly emerging infectious diseases is unlikely to be greater on Pacific islands than in other geographical areas.

Understanding the contrasting spatial haplotype patterns of malaria-protective β-globin polymorphisms

The malaria-protective β-globin polymorphisms, sickle-cell (β(S)) and β(0)-thalassaemia, are canonical examples of human adaptation to infectious disease. Occurring on distinct genetic backgrounds, they vary markedly in their patterns of linked genetic variation at the population level, suggesting different evolutionary histories. β(S) is associated with five classical restriction fragment length polymorphism haplotypes that exhibit remarkable specificity in their geographical distributions; by contrast, β(0)-thalassaemia mutations are found on haplotypes whose distributions overlap considerably. Here, we explore why these two polymorphisms display contrasting spatial haplotypic distributions, despite having malaria as a common selective pressure. We present a meta-population genetic model, incorporating individual-based processes, which tracks the evolution of β-globin polymorphisms on different haplotypic backgrounds. Our simulations reveal that, depending on the rate of mutation, a large population size and/or high population growth rate are required for both the β(S)- and the β(0)-thalassaemia-like patterns. However, whilst the β(S)-like pattern is more likely when population subdivision is high, migration low and long-distance migration absent, the opposite is true for β(0)-thalassaemia. Including gene conversion has little effect on the overall probability of each pattern; however, when inter-haplotype fitness variation exists, gene conversion is more likely to have contributed to the diversity of haplotypes actually present in the population. Our findings highlight how the contrasting spatial haplotype patterns exhibited by β(S) and β(0)-thalassaemia may provide important indications as to the evolution of these adaptive alleles and the demographic history of the populations in which they have evolved.

Negative Epistasis between Sickle and Foetal Haemoglobin Suggests a Reduction in Protection against Malaria

BACKGROUND

Haemoglobin variants, Sickle (HbS) and foetal (HbF) have been associated with malaria protection. This study explores epistatic interactions between HbS and HbF on malaria infection.

METHODS

The study was conducted between March 2004 and December 2013 within the sickle cell disease (SCD) programme at Muhimbili National Hospital, Tanzania. SCD status was categorized into HbAA, HbAS and HbSS using hemoglobin electrophoresis and High Performance Liquid Chromatography (HPLC). HbF levels were determined by HPLC. Malaria was diagnosed using rapid diagnostic test and/or blood film. Logistic regression and generalized estimating equations models were used to evaluate associations between SCD status, HbF and malaria.

FINDINGS

2,049 individuals with age range 0-70 years, HbAA 311(15.2%), HbAS 241(11.8%) and HbSS 1,497(73.1%) were analysed. At enrolment, malaria prevalence was significantly higher in HbAA 13.2% compared to HbAS 1.24% and HbSS 1.34% (p<0.001). Mean HbF was lower in those with malaria compared to those without malaria in HbAA (0.43% vs 0.82%) but was the reverse in HbSS (8.10% vs 5.59%). An increase in HbF was associated with a decrease in risk of malaria OR=0.50 (95%CI: 0.28, 0.90; p=0.021) in HbAA, whereas for HbSS the risk of malaria increased OR=2.94 (1.44, 5.98; p=0.003). A similar pattern was seen during multiple visits; HbAA OR=0.52 (0.34, 0.80; p=0.003) vs HbSS OR=2.01 (1.27, 3.23; p=0.003).

CONCLUSION

Higher prevalence of malaria in HbAA compared to HbAS and HbSS confirmed the protective effect of HbS. Lower prevalence of malaria in HbAA with high HbF supports a protective effect of HbF. However, in HbSS, the higher prevalence of malaria with high levels of HbF suggests loss of malaria protection. This is the first epidemiological study to suggest a negative epistasis between HbF and HbS on malaria.

The α-thalassemias

More than 100 varieties of α-thalassemia have been identified. Their geographic distribution and the challenges associated with screening, diagnosis, and management suggest that α-thalassemias should have a higher priority on global public health agendas.

A journey in science: early lessons from the hemoglobin field

Real innovations in medicine and science are historic and singular; the stories behind each occurrence are precious. At Molecular Medicine we have established the Anthony Cerami Award in Translational Medicine to document and preserve these histories. The monographs recount the seminal events as told in the voice of the original investigators who provided the crucial early insight. These essays capture the essence of discovery, chronicling the birth of ideas that created new fields of research; and launched trajectories that persisted and ultimately influenced how disease is prevented, diagnosed, and treated. In this volume, the Cerami Award Monograph is by David J Weatherall, Founder, Weatherall Institute of Molecular Medicine, Oxford University, John Radcliffe Hospital. A visionary in the field of hemoglobin, this is the story of Professor Weatherall's scientific journey.

Mechanistic Studies of the Negative Epistatic Malaria-protective Interaction Between Sickle Cell Trait and α+thalassemia

Background

Individually, the red blood cell (RBC) polymorphisms sickle cell trait (HbAS) and α⁺thalassemia protect against severe Plasmodium falciparum malaria. It has been shown through epidemiological studies that the co-inheritance of both conditions results in a loss of the protection afforded by each, but the biological mechanisms remain unknown.

Methods

We used RBCs from > 300 donors of various HbAS and α⁺thalassemia genotype combinations to study the individual and combinatorial effects of these polymorphisms on a range of putative P. falciparum virulence phenotypes in-vitro, using four well-characterized P. falciparum laboratory strains. We studied cytoadhesion of parasitized RBCs (pRBCs) to the endothelial receptors CD36 and ICAM1, rosetting of pRBCs with uninfected RBCs, and pRBC surface expression of the parasite-derived adhesion molecule P. falciparum erythrocyte membrane protein-1 (PfEMP1).

Findings

We confirmed previous reports that HbAS pRBCs show reduced cytoadhesion, rosetting and PfEMP1 expression levels compared to normal pRBC controls. Furthermore, we found that co-inheritance of HbAS with α⁺thalassemia consistently reversed these effects, such that pRBCs of mixed genotype showed levels of cytoadhesion, rosetting and PfEMP1 expression that were indistinguishable from those seen in normal pRBCs. However, pRBCs with α⁺thalassemia alone showed parasite strain-specific effects on adhesion, and no consistent reduction in PfEMP1 expression.

Interpretation

Our data support the hypothesis that the negative epistasis between HbAS and α⁺thalassemia observed in epidemiological studies might be explained by host genotype-specific changes in the pRBC-adhesion properties that contribute to parasite sequestration and disease pathogenesis in vivo. The mechanism by which α⁺thalassemia on its own protects against severe malaria remains unresolved.

An evolutionary perspective of how infection drives human genome diversity: the case of malaria

Infection with malaria parasites has imposed a strong selective pressure on the human genome, promoting the convergent evolution of a diverse range of genetic adaptations, many of which are harboured by the red blood cell, which hosts the pathogenic stage of the Plasmodium life cycle. Recent genome-wide and multi-centre association studies of severe malaria have consistently identified ATP2B4, encoding the major Ca(2+) pump of erythrocytes, as a novel resistance locus. Evidence is also accumulating that interaction occurs among resistance loci, the most recent example being negative epistasis among alpha-thalassemia and haptoglobin type 2. Finally, studies on the effect of haemoglobin S and C on parasite transmission to mosquitoes have suggested that protective variants could increase in frequency enhancing parasite fitness.

The distribution of haemoglobin C and its prevalence in newborns in Africa

Haemoglobin C (HbC) is one of the commonest structural haemoglobin variants in human populations. Although HbC causes mild clinical complications, its diagnosis and genetic counselling are important to prevent inheritance with other haemoglobinopathies. Little is known about its contemporary distribution and the number of newborns affected. We assembled a global database of population surveys. We then used a Bayesian geostatistical model to create maps of HbC frequency across Africa and paired our predictions with high-resolution demographics to calculate heterozygous (AC) and homozygous (CC) newborn estimates and their associated uncertainty. Data were too sparse outside Africa for this methodology to be applied. The highest frequencies were found in West Africa but HbC was commonly found in other parts of the continent. The expected annual numbers of AC and CC newborns in Africa were 672,117 (interquartile range (IQR): 642,116-705,163) and 28,703 (IQR: 26,027-31,958), respectively. These numbers are about two times previous estimates.

Global epidemiology of sickle haemoglobin in neonates: a contemporary geostatistical model-based map and population estimates

BACKGROUND

Reliable estimates of populations affected by diseases are necessary to guide efficient allocation of public health resources. Sickle haemoglobin (HbS) is the most common and clinically significant haemoglobin structural variant, but no contemporary estimates exist of the global populations affected. Moreover, the precision of available national estimates of heterozygous (AS) and homozygous (SS) neonates is unknown. We aimed to provide evidence-based estimates at various scales, with uncertainty measures.

METHODS

Using a database of sickle haemoglobin surveys, we created a contemporary global map of HbS allele frequency distribution within a Bayesian geostatistical model. The pairing of this map with demographic data enabled calculation of global, regional, and national estimates of the annual number of AS and SS neonates. Subnational estimates were also calculated in data-rich areas.

FINDINGS

Our map shows subnational spatial heterogeneities and high allele frequencies across most of sub-Saharan Africa, the Middle East, and India, as well as gene flow following migrations to western Europe and the eastern coast of the Americas. Accounting for local heterogeneities and demographic factors, we estimated that the global number of neonates affected by HbS in 2010 included 5,476,000 (IQR 5,291,000-5,679,000) AS neonates and 312,000 (294,000-330,000) SS neonates. These global estimates are higher than previous conservative estimates. Important differences predicted at the national level are discussed.

INTERPRETATION

HbS will have an increasing effect on public health systems. Our estimates can help countries and the international community gauge the need for appropriate diagnoses and genetic counselling to reduce the number of neonates affected. Similar mapping and modelling methods could be used for other inherited disorders.

FUNDING

The Wellcome Trust.

Resistance to malaria in humans: the impact of strong, recent selection

Malaria is one of the leading causes of death worldwide and has been suggested as the most potent type of selection in humans in recent millennia. As a result, genes involved in malaria resistance are excellent examples of recent, strong selection. In 1949, Haldane initially suggested that infectious disease could be a strong selective force in human populations. Evidence for the strong selective effect of malaria resistance includes the high frequency of a number of detrimental genetic diseases caused by the pleiotropic effects of these malaria resistance variants, many of which are "loss of function" mutants. Evidence that this selection is recent comes from the genetic dating of the age of a number of these malaria resistant alleles to less than 5,000 years before the present, generally much more recent than other human genetic variants. An approach to estimate selection coefficients from contemporary case-control data is presented. In the situations described here, selection is much greater than 1%, significantly higher than generally observed for other human genetic variation. With these selection coefficients, predictions are generated about the joint change of alleles S and C at the β-globin locus, and for α-thalassaemia haplotypes and S, variants that are unlinked but exhibit epistasis. Population genetics can be used to determine the amount and pattern of selection in the past and predict selection in the future for other malaria resistance variants as they are discovered.

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.

β-Globin chain abnormalities with coexisting α-thalassemia mutations

INTRODUCTION

The frequency of hemoglobinopathies is still high in Adana, the biggest city of the Cukurova Region that is located in the southern part of Turkey. Our aim was to identify the concomitant mutations in α- and β-globin genes which lead to complex hemoglobinopathies and to establish an appropriate plan of action for each subject, particularly when prenatal diagnosis is necessary.

MATERIAL AND METHODS

We studied the association between the β-globin gene and α-thalassemia genotypes. The reverse hybridization technique was employed to perform molecular analysis, and the results were confirmed by amplification refractory mutation system (ARMS) or restriction fragment length polymorphism (RFLP) technique.

RESULTS

We evaluated 36 adult subjects (28 female and 8 male; age range: 18-52 years) with concomitant mutations in their α- and β-globin genes. The -α(3.7)/αα deletion was the commonest defect in the α-chain as expected, followed by α(3.7)/-α(3.7) deletion. Twenty-five of 36 cases were sickle cell trait with coexisting α-thalassemia, while seven Hb S/S patients had concurrent mutations in their α-genes. The coexistence of α(PolyA-2)α/αα with Hb A/D and with Hb S/D, which is very uncommon, was also detected. There was a subject with compound heterozygosity for β-globin chain (-α(3.7)/αα with IVSI.110/S), and also a case who had -α(3.7)/αα deletion with IVSI.110/A.

CONCLUSIONS

Although limited, our data suggest that it would be valuable to study coexisting α-globin mutations in subjects with sickle cell disease or β-thalassemia trait during the screening programs for premarital couples, especially in populations with a high frequency of hemoglobinopathies.

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.

The emergence and maintenance of sickle cell hotspots in the Mediterranean

Genetic disorders of haemoglobin (haemoglobinopathies), including the thalassaemias and sickle cell anaemia, abound in historically malarious regions, due to the protection they provide against death from severe malaria. Despite the overall spatial correlation between malaria and these disorders, inter-population differences exist in the precise combinations of haemoglobinopathies observed. Greece and Italy present a particularly interesting case study: their high frequencies of beta thalassaemia speak to a history of intense malaria selection, yet they possess very little of the strongly malaria protective mutation responsible for sickle cell anaemia, despite historical migrational links with Africa where high frequencies of sickle cell occur. Twentieth century surveys of beta thalassaemia and sickle cell in Greece, Sicily and Sardinia have revealed striking sickle cell ‘hotspots’ – places where the frequency of sickle cell approaches that seen in Africa while neighbouring populations remain relatively sickle cell free. It remains unclear how these hotspots have been maintained over time without sickle cell spreading throughout the region. Here we use a metapopulation model to show that (i) epistasis between the alpha and beta forms of thalassaemia can restrict the spread of sickle cell through a network of linked subpopulations and (ii) the emergence of sickle cell hotspots requires relatively low levels of gene flow, but the aforementioned epistasis increases the chances of hotspots forming.

Sickle cell protection from malaria

A linkage between presence of Sickle Haemoglobin (HbS) and protection from malaria infection and clinical manifestations in certain areas was suspected from early observations and progressively elucidated by more recent studies. Research has confirmed the abovementioned connection, but also clarified how such protection may be abolished by coexistence of sickle cell trait (HbS trait) and alpha thalassemia, which may explain the relatively low incidence of HbS trait in the Mediterranean. The mechanisms of such protective effect are now being investigated: factors of genetic, molecular and immunological nature are prominent. As for genetic factors attention is given to the role of the red blood cell (RBC) membrane complement regulatory proteins as polymorphisms of these components seem to be associated with resistance to severe malaria; genetic ligands like the Duffy group blood antigen, necessary for erythrocytic invasion, and human protein CD36, a major receptor for P. falciparum-infected RBC's, are also under scrutiny: attention is focused also on plasmodium erythrocyte-binding antigens, which bind to RBC surface components. Genome-wide linkage and association studies are now carried out too, in order to identify genes associated with malaria resistance. Only a minor role is attributed to intravascular sickling, phagocytosis and haemolysis, while specific molecular mechanisms are the object of intensive research: among these a decisive role is played by a biochemical sequence, involving activation of haeme oxygenase (HMO-1), whose effect appears mediated by carbon monoxide (CO). A central role in protection from malaria is also played by immunological factors, which may stimulate antibody production to plasmodium antigens in the early years of life; the role of agents like pathogenic CD8 T-cells has been suggested while the effects of molecular actions on the immunity mechanism are presently investigated. It thus appears that protection from malaria can be explained by interaction of different factors: the elucidation of such mechanisms may prove valuable for the prevention and treatment strategy of a disease which still affects large parts of the world.

Genetic modifiers of sickle cell disease

Sickle cell disease is one of the best characterized human monogenic disorders. Complex genotype/phenotype correlations clearly demonstrate the interaction of multiple genetic and environmental factors. In the last 20 years, scientific research has applied genetic approaches to dissect some of these modifiers. This review highlights the more recent genetic association studies that have been applied to unravel the genetic modifiers of sickle cell disease including Hb F genetics, and the key genetic variants identified. Illumination of such modifying factors may guide future therapeutic interventions and improve prediction of disease severity, with implications for genetic counseling, prenatal diagnosis and implementation of high risk therapy.

The inherited disorders of haemoglobin: an increasingly neglected global health burden

An estimated 300,000 babies are born each year with a severe inherited disease of haemoglobin and that over 80 per cent of these births occur in low- or middle-income countries. As these countries go through the epidemiological transition, characterized by a reduction in childhood and infant mortality due to improved public health measures, infants who had previously died of these conditions before they were recognised are now surviving to present for diagnosis and treatment. For a variety of reasons, even in the rich countries there are limited data about the true frequency, natural history, and survival of patients with these disorders, information that is absolutely critical towards providing governments and international health agencies with accurate information about the true global health burden of these conditions. The situation can only be improved by major action on the part of the rich countries together with the formation of partnerships between rich and poor countries and input from the major international health agencies and funding organisations.

Genetic polymorphisms linked to susceptibility to malaria

The influence of host genetics on susceptibility to Plasmodium falciparum malaria has been extensively studied over the past twenty years. It is now clear that malaria parasites have imposed strong selective forces on the human genome in endemic regions. Different genes have been identified that are associated with different malaria related phenotypes. Factors that promote severity of malaria include parasitaemia, parasite induced inflammation, anaemia and sequestration of parasitized erythrocytes in brain microvasculature.

Recent advances in human genome research technologies such as genome-wide association studies (GWAS) and fine genotyping tools have enabled the discovery of several genetic polymorphisms and biomarkers that warrant further study in host-parasite interactions. This review describes and discusses human gene polymorphisms identified thus far that have been shown to be associated with susceptibility or resistance to P. falciparum malaria. Although some polymorphisms play significant roles in susceptibility to malaria, several findings are inconclusive and contradictory and must be considered with caution. The discovery of genetic markers associated with different malaria phenotypes will help elucidate the pathophysiology of malaria and enable development of interventions or cures. Diversity in human populations as well as environmental effects can influence the clinical heterogeneity of malaria, thus warranting further investigations with a goal of developing new interventions, therapies and better management against malaria.

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.

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.

Genomic polymorphisms in sickle cell disease: implications for clinical diversity and treatment

Sickle cell disease (SCD) is one of the best characterized human monogenic disorders. The development of molecular biology allowed the identification of several genomic polymorphisms responsible for its clinical diversity. Research on the first genetic modulators of SCD, such as coinheritance of α-thalassemia and haplotypes in the β-globin gene cluster, have been followed by studies associating single nucleotide polymorphisms (SNPs) with variable risks for stroke, leg ulceration, pulmonary hypertension, priapism and osteonecrosis, with differences in the response to hydroxyurea, and with variability in the management of pain. Furthermore, multigenic analyses based on genome-wide association studies have shed light on the importance of the TGF-β superfamily and oxidative stress to the pathogenesis of complex traits in SCD, and may guide future therapeutic interventions on a genetically oriented basis.

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.

Thalassemia as a global health problem: recent progress toward its control in the developing countries

The thalassemias, together with sickle cell anemia and its variants, are by far the most common monogenic diseases. They occur at their highest frequency in countries of the developing world where their control and management is hampered by a lack of knowledge of their true prevalence, adequate services for their management and control, and support by their governments and international health agencies. However, there has been some progress recently in addressing these problems and there are several ways in which the lot of children with thalassemia in poor countries could be improved in the future. Over the last 20 years there has been considerable improvement in the control and management of the thalassemias in the richer countries of the world. Unfortunately, however, this is not the case for many of the developing countries, where there have been few improvements in the control of the numbers of births of babies with thalassemia or in the care of thalassemic children since the frequency of the problem started to become evident in the 1960s. Here, I will try to assess the current situation in these countries and examine some of the potential approaches for improving the current situation.

Parallel adaptation: one or many waves of advance of an advantageous allele?

Models for detecting the effect of adaptation on population genomic diversity are often predicated on a single newly arisen mutation sweeping rapidly to fixation. However, a population can also adapt to a new environment by multiple mutations of similar phenotypic effect that arise in parallel, at the same locus or different loci. These mutations can each quickly reach intermediate frequency, preventing any single one from rapidly sweeping to fixation globally, leading to a "soft" sweep in the population. Here we study various models of parallel mutation in a continuous, geographically spread population adapting to a global selection pressure. The slow geographic spread of a selected allele due to limited dispersal can allow other selected alleles to arise and start to spread elsewhere in the species range. When these different selected alleles meet, their spread can slow dramatically and so initially form a geographic patchwork, a random tessellation, which could be mistaken for a signal of local adaptation. This spatial tessellation will dissipate over time due to mixing by migration, leaving a set of partial sweeps within the global population. We show that the spatial tessellation initially formed by mutational types is closely connected to Poisson process models of crystallization, which we extend. We find that the probability of parallel mutation and the spatial scale on which parallel mutation occurs are captured by a single compound parameter, a characteristic length, which reflects the expected distance a spreading allele travels before it encounters a different spreading allele. This characteristic length depends on the mutation rate, the dispersal parameter, the effective local density of individuals, and to a much lesser extent the strength of selection. While our knowledge of these parameters is poor, we argue that even in widely dispersing species, such parallel geographic sweeps may be surprisingly common. Thus, we predict that as more data become available, many more examples of intraspecies parallel adaptation will be uncovered.

The inherited diseases of hemoglobin are an emerging global health burden

It is estimated that in excess of 300 000 children are born each year with a severe inherited disorder of hemoglobin and that approximately 80% of these births occur in low- or middle-income countries. As these countries go through an epidemiologic transition, with a reduction in childhood and infant mortality due to improved public health measures, babies who would have previously died of these diseases before they were recognized are now surviving to present for diagnosis and treatment. Hence, they are presenting an increasing global health burden. Because of their uneven distribution in high-frequency populations, reflecting their complex population genetics, the true magnitude of this burden is still unknown. In many poor countries there are virtually no facilities for the diagnosis and management of these conditions, and even in richer countries there are limited data about their frequency, clinical course, or mortality. Without this information, it will be impossible to persuade governments about the increasing importance of these diseases. The situation will only be improved by concerted action on the part of the hematology community of the richer countries together with input from the major international health organizations and funding agencies.

The inherited diseases of hemoglobin are an emerging global health burden

It is estimated that in excess of 300 000 children are born each year with a severe inherited disorder of hemoglobin and that approximately 80% of these births occur in low- or middle-income countries. As these countries go through an epidemiologic transition, with a reduction in childhood and infant mortality due to improved public health measures, babies who would have previously died of these diseases before they were recognized are now surviving to present for diagnosis and treatment. Hence, they are presenting an increasing global health burden. Because of their uneven distribution in high-frequency populations, reflecting their complex population genetics, the true magnitude of this burden is still unknown. In many poor countries there are virtually no facilities for the diagnosis and management of these conditions, and even in richer countries there are limited data about their frequency, clinical course, or mortality. Without this information, it will be impossible to persuade governments about the increasing importance of these diseases. The situation will only be improved by concerted action on the part of the hematology community of the richer countries together with input from the major international health organizations and funding agencies.

The inherited diseases of hemoglobin are an emerging global health burden

It is estimated that in excess of 300 000 children are born each year with a severe inherited disorder of hemoglobin and that approximately 80% of these births occur in low- or middle-income countries. As these countries go through an epidemiologic transition, with a reduction in childhood and infant mortality due to improved public health measures, babies who would have previously died of these diseases before they were recognized are now surviving to present for diagnosis and treatment. Hence, they are presenting an increasing global health burden. Because of their uneven distribution in high-frequency populations, reflecting their complex population genetics, the true magnitude of this burden is still unknown. In many poor countries there are virtually no facilities for the diagnosis and management of these conditions, and even in richer countries there are limited data about their frequency, clinical course, or mortality. Without this information, it will be impossible to persuade governments about the increasing importance of these diseases. The situation will only be improved by concerted action on the part of the hematology community of the richer countries together with input from the major international health organizations and funding agencies.

The inherited diseases of hemoglobin are an emerging global health burden

It is estimated that in excess of 300 000 children are born each year with a severe inherited disorder of hemoglobin and that approximately 80% of these births occur in low- or middle-income countries. As these countries go through an epidemiologic transition, with a reduction in childhood and infant mortality due to improved public health measures, babies who would have previously died of these diseases before they were recognized are now surviving to present for diagnosis and treatment. Hence, they are presenting an increasing global health burden. Because of their uneven distribution in high-frequency populations, reflecting their complex population genetics, the true magnitude of this burden is still unknown. In many poor countries there are virtually no facilities for the diagnosis and management of these conditions, and even in richer countries there are limited data about their frequency, clinical course, or mortality. Without this information, it will be impossible to persuade governments about the increasing importance of these diseases. The situation will only be improved by concerted action on the part of the hematology community of the richer countries together with input from the major international health organizations and funding agencies.

The inherited diseases of hemoglobin are an emerging global health burden

It is estimated that in excess of 300 000 children are born each year with a severe inherited disorder of hemoglobin and that approximately 80% of these births occur in low- or middle-income countries. As these countries go through an epidemiologic transition, with a reduction in childhood and infant mortality due to improved public health measures, babies who would have previously died of these diseases before they were recognized are now surviving to present for diagnosis and treatment. Hence, they are presenting an increasing global health burden. Because of their uneven distribution in high-frequency populations, reflecting their complex population genetics, the true magnitude of this burden is still unknown. In many poor countries there are virtually no facilities for the diagnosis and management of these conditions, and even in richer countries there are limited data about their frequency, clinical course, or mortality. Without this information, it will be impossible to persuade governments about the increasing importance of these diseases. The situation will only be improved by concerted action on the part of the hematology community of the richer countries together with input from the major international health organizations and funding agencies.

The inherited diseases of hemoglobin are an emerging global health burden

It is estimated that in excess of 300 000 children are born each year with a severe inherited disorder of hemoglobin and that approximately 80% of these births occur in low- or middle-income countries. As these countries go through an epidemiologic transition, with a reduction in childhood and infant mortality due to improved public health measures, babies who would have previously died of these diseases before they were recognized are now surviving to present for diagnosis and treatment. Hence, they are presenting an increasing global health burden. Because of their uneven distribution in high-frequency populations, reflecting their complex population genetics, the true magnitude of this burden is still unknown. In many poor countries there are virtually no facilities for the diagnosis and management of these conditions, and even in richer countries there are limited data about their frequency, clinical course, or mortality. Without this information, it will be impossible to persuade governments about the increasing importance of these diseases. The situation will only be improved by concerted action on the part of the hematology community of the richer countries together with input from the major international health organizations and funding agencies.

The inherited diseases of hemoglobin are an emerging global health burden

It is estimated that in excess of 300 000 children are born each year with a severe inherited disorder of hemoglobin and that approximately 80% of these births occur in low- or middle-income countries. As these countries go through an epidemiologic transition, with a reduction in childhood and infant mortality due to improved public health measures, babies who would have previously died of these diseases before they were recognized are now surviving to present for diagnosis and treatment. Hence, they are presenting an increasing global health burden. Because of their uneven distribution in high-frequency populations, reflecting their complex population genetics, the true magnitude of this burden is still unknown. In many poor countries there are virtually no facilities for the diagnosis and management of these conditions, and even in richer countries there are limited data about their frequency, clinical course, or mortality. Without this information, it will be impossible to persuade governments about the increasing importance of these diseases. The situation will only be improved by concerted action on the part of the hematology community of the richer countries together with input from the major international health organizations and funding agencies.

The inherited diseases of hemoglobin are an emerging global health burden

It is estimated that in excess of 300 000 children are born each year with a severe inherited disorder of hemoglobin and that approximately 80% of these births occur in low- or middle-income countries. As these countries go through an epidemiologic transition, with a reduction in childhood and infant mortality due to improved public health measures, babies who would have previously died of these diseases before they were recognized are now surviving to present for diagnosis and treatment. Hence, they are presenting an increasing global health burden. Because of their uneven distribution in high-frequency populations, reflecting their complex population genetics, the true magnitude of this burden is still unknown. In many poor countries there are virtually no facilities for the diagnosis and management of these conditions, and even in richer countries there are limited data about their frequency, clinical course, or mortality. Without this information, it will be impossible to persuade governments about the increasing importance of these diseases. The situation will only be improved by concerted action on the part of the hematology community of the richer countries together with input from the major international health organizations and funding agencies.

The inherited diseases of hemoglobin are an emerging global health burden

It is estimated that in excess of 300 000 children are born each year with a severe inherited disorder of hemoglobin and that approximately 80% of these births occur in low- or middle-income countries. As these countries go through an epidemiologic transition, with a reduction in childhood and infant mortality due to improved public health measures, babies who would have previously died of these diseases before they were recognized are now surviving to present for diagnosis and treatment. Hence, they are presenting an increasing global health burden. Because of their uneven distribution in high-frequency populations, reflecting their complex population genetics, the true magnitude of this burden is still unknown. In many poor countries there are virtually no facilities for the diagnosis and management of these conditions, and even in richer countries there are limited data about their frequency, clinical course, or mortality. Without this information, it will be impossible to persuade governments about the increasing importance of these diseases. The situation will only be improved by concerted action on the part of the hematology community of the richer countries together with input from the major international health organizations and funding agencies.