Common erythrocyte variants that affect susceptibility and resistance to P. falciparum malaria.
An estimated 300,000 babies are born each year with severe Inherited Disorders of Hemoglobin (IDH). Despite major advances in the understanding of the molecular pathology, control, and management of the IDH thousands of infants and children with these diseases are dying due to the accessibility to appropriate medical care. In addition, as malaria has been the principal cause of early mortality in several parts of the world for much of the last 5000 years, as a result, it is the strongest force for selective pressure on the human genome. That is why, in the world, there is an overlap of malaria endemicity and IDH. Over the past twenty years several studies have shown that IDH such us hemoglobin and/or red cell membrane abnormalities confer resistance to malaria reducing hence the mortality during the first years of life. This has led to the selection of populations with IDH in malaria-endemic areas. This may explain the overlap between these two pathologies. This chapter aims to present the relationship between IDH and malaria susceptibility, make an overview of the current state of knowledge and the burden of IDH, and highlight steps that require to be taken urgently to improve the situation.
- Inherited Disorders
- host genetics
Despite health better care and new strategies of disease control of, mortality remains high in many countries and worldwide . Malaria is the biggest cause of childhood mortality in Africa.
In 2018, malaria was the cause of about 405 000 deaths, More than 90% of these deaths occurred in sub-Saharan Africa .
Malaria parasites enter red blood cells during key stages of their life cycle so that there is no surprise that a change of red blood structures or make-up could affect malaria infection. Some changes of red blood cells make more resistant to malaria infection whereas others create the potential for a harmful reaction to certain antimalarial drugs .
Falciparum malaria has had a profound effect on human evolution, evidenced by the high frequencies of malaria protective mutations observed in populations from historically malarious regions. This big pressure has resulted in the selection of many genetic variants that confer protection against severe malaria and reducing death due to malaria in some populations [4, 5, 6].
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 .
The history of genetics and the study of malaria are much linked. Indeed Burden of disease due to malaria across much of the world has selected for a series of traits, including the alleles of genes encoding hemoglobin, red cell enzymes, and membrane proteins.
Each year more than 7000000 babies born with either a congenital abnormality and/or a genetic disease, mainly (up to 90%) in low or middle-income countries .
About 25% of these births consist of five disorders, two of which, the inherited disorders of hemoglobin and glucose-6-phosphate dehydrogenase (G6PD) deficiency, are monogenic diseases .
In recent years there has been a major revival in scientific studies interest in the study of interactions between the inherited hemoglobin disorders and P. falciparum malaria, work that has been the subject of several extensive reviews [5, 9, 10].
This chapter focuses on IDH that are common enough to be of public health concern particularly those significantly associated with malaria as summarized in Table 1.
|Gene (chromosome)||Mutation||Protein||Function||Reported Genetic Associations with Malaria||Mechanistic hypotheses|
Proposed protective mechanism
|G6PD (Xq28)||Asn126Asp A376||1||G6PD||Enzyme That protects against oxidative tress||G6PD deficiency protect against severe malaria and uncomplicated malaria||Increased phagocytosis of ring-parasitised variant RBCs due to enhanced oxidative stress [94, 103].|
Increased vulnerability of the G6PD deficient erythrocyte to oxidant stress causes its protection against parasitization and Reduced parasite replication in G6PD-deficient erythrocytes [94, 103].
|HBA (16p13.3)||α -thalassemia (α-thal)||α + or α one gene deleted||Many||α-Globin||Component of hemoglobin||Thalassemia protects against severe malaria and severe malaria aneamia but appears to enhance mild malaria episodes in some environments||Specific protection against malaria-induced anemia [94, 103].|
Reduced pathogenicity through reduced cytoadherence or resetting [86, 87].
Immunological priming through cross-species immunity between P.vivax and P.falciparum .
Increased phagocytosis of infected variant RBCs by monocytes and enhanced antibody binding and subsequent clearance of infected variant RBCs [88, 89].
Reduced resetting. Increased micro-erythrocyte count in homozygotes reduces the amount of Hb lost for given parasite density, thus protecting against SMA [43, 82, 88, 90].
|Mediterranean area, South /East Asia, Pacific||0.8|
|α 0 or - - both genes deleted||Many||0.03|
|HBB (11p15.5)||β-thalassemia (β-thal)||Many||Many||β-Globin||Component of hemoglobin||Thalassemia protects against severe malaria||Enhanced removal of parasite-infected red blood cells . Reduced invasion and growth of P. falciparum parasites [97, 98]. Reduced pathogenicity through reduced cytoadherence or resetting . Enhanced antibody binding and subsequent clearance of infected variant RBCs . Increased phagocytosis of ring-parasitised variant RBCs [80, 96, 100].||sub-Saharan Africa||0.1|
|S (HbS)||Glu6Val GAG/GTG||2–5||β-Globin||Component of hemoglobin||HbS alleles protect againstuncomplicated malaria and severe malaria.|
|Impairment of P. falciparum red cell invasion and growth under conditions of low oxygen tension [31, 43].|
Enhanced removal of parasite-infected HbAS red blood cell [31, 44].
Reduced pathogenicity of P. falciparum infected red blood cells because of reduced expression of PfEMP1 [33, 41].
Improved acquisition of malaria-specific immunity [33, 46, 48].
Selective sickling of infected sickle trait erythrocytes leading to enhanced clearance by the spleen .
Reduced erythrocyte invasion, early phagocytosis, and inhibited parasite growth under oxygen stress in venous micro vessels. .
Enhancement of innate and acquired immunity .
Increased clearance of sickled infected RBCs by the spleen Acquired host immunity and increased phagocytosis of ring-parasitised variant RBCs .
Reduced cytoadherence and rosetting Impaired trafficking of parasite proteins to RBC surface . And Inhibition of parasite growth due to oxygen-dependent polymerization of HbS .
|C (HbC)||Glu6Lys GAG/AAG||2||HbC alleles protect against uncomplicated malaria and severe malaria.||Increased immune clearance of infected erythrocytes [29, 35]|
Impairment of P. falciparum red cell invasion and growth under conditions of low oxygen tension [31, 43, 47]
Improved acquisition of malaria-specific immunity [33, 47, 58]
Reduced pathogenicity of P. falciparum infected red blood cells because of reduced expression of PfEMP1 [33, 64, 65] and Reduced cyto-adherence of infected erythrocyte 
increased immune clearance of infected erythrocytes [22, 29]
|E (HbE)||Glu26Lys GAG to AAG||1–3||HbE allele reduces parasite invasion and protects against severe malaria.||Impairment of P. falciparum red cell invasion and growth .|
AE heterozygotes appear to have protection from invasion into erythrocytes by P. falciparum malaria [53, 73].
Reduced erythrocyte invasion by merozoites, lower intra-erythrocytic parasite growth, and enhanced phagocytosis of infected erythrocytes [22, 75].
By presenting the relationship between IDH and malaria susceptibility, making an overview of the current state of knowledge and the burden of IDH, this chapter outline some of the more important protective genetic variants that have been identified as far as summarized in the Table 1. The knowledge of our understanding of the interaction between hemoglobin variants and malaria could give point to novel preventive and/or therapeutic approaches.
2. Inherited disorders of hemoglobin (IDH) and Plasmodium falciparum (P. falciparum) malaria
2.1 Brief review of malaria infection
Malaria is a severe infectious disease caused by parasites of the genus Plasmodium. Plasmodium is one of the longest-known parasites, which are transmitted to humans by a bite of an infected female mosquito of the species Anopheles.
Indeed, after inoculation into a human by a mosquito, the P. falciparum parasites enter the erythrocytic stage of their life cycle after a brief silent incubation in life (Figure 2). It is during this time that parasites sequentially invade and egress from their host RBCs and cause the signs and symptoms of malaria. Hemoglobin is the oxygen - carrying component and major protein of the RBC . Indeed, the RBC is essential for the spread of malaria parasites, as summarized in Figures 2 and 3.
Despite progress towards its control of malaria, it is still the most important parasitic disease and then, one of the world’s worst health problems. In 2018, about 228 million cases of malaria occurred worldwide. Most of these cases (93%) occurred in African Africa region In the same year malaria was responsible for 405 000 deaths made up to 67% (272000) of children under 5 years recognized as the most vulnerable group . However, early diagnosis and fast-acting treatment prevent unwanted outcomes. Until recently it was thought that only four species of malarial parasite (Plasmodium) especially Plasmodium. falciparum (P. falciparum), Plasmodium.vivax (P.vivax), Plasmodium. malariae (P.malariae), and Plasmoduim. ovale (P.ovale), have humans as their natural hosts. But, it has been found that many cases of malaria that were previously diagnosed as being due to P. malariae infection are in fact due to a fifth parasite, Plasmodium. knowlesi (P. knowlesi) mostly in Malaysia .
It has long been thought that P. falciparum was the only cause of severe malaria cases and deaths, until the equally destructive, if not worse, the role of P. vivax is gradually highlighted and established especially in South East Asia and in Latin America [2, 14, 15, 16].
Regarding the relationship between the severity of malaria and host genetics, it appears that P. falciparum malaria is one of the deadly forms of malaria with a life cycle including alternatives hosts: a sexual cycle in the insect vector, an Anopheles mosquito, and a human cycle in a liver stage and an erythrocyte stage. However, the resistance mechanisms have been described in the sporozoite entry to liver cells and in the erythrocyte invasion by merozoites (Figures 1 and 3) [17, 18]. Genetically based resistance is involved in either altering erythrocyte invasion by merozoites, in lowering parasite growth or in impairing merozoite viability after being released from schizonts [17, 19]. The genetic resistance in the blood stage step has been extensively documented .
There are multiple points in the parasite lifecycle that have impacted host genetic variation, but the majority of the malaria-protective variants described so far have various important impacts on the structure and function of the RBC .
P. falciparum malaria is a major cause of mortality and morbidity, particularly in endemic areas of sub-Saharan Africa [2, 20]. Indeed the disease etiology is variable and is attributable to environmental factors, parasite virulence and mostly host genetics . Variations in the severity of P. falciparum infections considered as different phenotypes include parasitaemia (hyperactive or asymptomatic), severe malaria anemia and cerebral malaria. Host genetic factors contribute to the variability of malaria phenotypes  and thus, should help to determine some of the mechanisms involved in susceptibility to P. falciparum infection. Some authors have summarized common mechanisms by which hemoglobinopathies may attenuate the pathogenesis of P. falciparum malaria (Figure 2) .
The knowledge gained with several studies has produced undisputed evidence about polymorphisms associated with malaria resistance. Indeed, several gene mutations and polymorphisms in the human hosts confer survival advantage and have increased in frequency through natural selection over generations. These include the classical polymorphisms that cause Sickle Cell Disorders (SCD) and haemoglobinopathies such as α-thalassaemias and G6PD deficiency and the major RCB group variants . However, with news technology and experimental design, other polymorphisms have been identified that include the Dantu blood group variant, polymorphisms in the red cell membrane protein ATP2B4, and some variants related to the immune response (Figure 3) .
2.2 Overview of inherited hemoglobin disorders
Inherited hemoglobin disorders include all disorders that are passed down through families and affect the normal properties of blood in humans. Figure 4 summarizes the general classification IDH.
Hemoglobin disorders can be broadly classified into two general categories .
Those in which there is a quantitative defect in the production of one of the globin subunits, either total absence or marked reduction. These are called the thalassemia syndromes (quantitative disorders of globin chain synthesis/accumulation: β-Thalassemia and α-Thalassemia
Those in which there is a qualitative disorders of globin structure defect in one of the globin subunits: Structural variant of hemoglobin
Sickle cell disorders: Sickle Cell trait, Sickle cell anemia disease, SC disease, sickle β-thalassemia disease
Hemoglobin with decreased stability (unstable hemoglobin variants): G6PD deficiency
Hemoglobin includes four globin chains:
fetal hemoglobin (HbF), the main hemoglobin in the fetal period which has two alpha (α) and two gamma (γ) chains (a2γ2),
adult hemoglobin (HbA), which increases after birth up to more than 96% of total hemoglobin, has two α and two β chains (α2β2).
Human Hemoglobin genes are located in the α-globin and β-globin gene clusters in chromosomes 16 and 11. Due to spontaneous mutation, hemoglobin gene variants are present at low prevalence in all sizeable populations .
They fall into two broad groups structural variants that change the amino acid sequence and produce an unusual hemoglobin,  and thalassaemias that lower or abolish production of globin chains .
Morbidity and mortality rates from SCD and β-thalassemia are still very high and represent an important challenge. Increased understanding of pathophysiological aspects has lead to significant improvements in the treatment and prevention of these diseases . However most hemoglobin gene variants are rare and many are harmless, but some are common because carriers are less likely than others to die from falciparum malaria.
We are interested in inherited hemoglobin disorders that are common enough to be of public health significance and particularly in those with a link to malaria.
2.2.1 Sickle cell disorders
SCD is a group of inherited RBC disorders; it is by far the most common IDH worldwide. SCD is caused by a variation in the gene that codes for hemoglobin, the protein in our red blood cells that helps carry oxygen to all parts of the body. The altered protein found in people with SCD is called hemoglobin S and occurs in people who have inherited the hemoglobin S (HbS), the red blood cells become hard and sticky .
Hemoglobin S results from an amino acid substitution at the sixth residue of the β-globin subunit: β6-Glu → Val. RBCs of persons with HbAS typically have 40% HbS and 56–58% HbA . The frequency of allele S is up to 0.2 in some parts of sub-Saharan Africa [26, 27, 28]. In equatorial Africa, where malaria is endemic, the prevalence of HbAS is much higher and can reach over 30% in some populations because of the survival advantage of HbAS heterozygotes from complications of P. falciparum malaria. Individuals with HbAS are typically asymptomatic; severe hypoxia is required for them to experience manifestations of SCD, called sickling.
Persons who have inherited the HbS gene from only one parent are Heterozygote for the Sickle gene (AS). They carry the gene certainly, but they usually do not have the disease and are more tolerant of malaria infection, making them more likely to survive the disease [12, 29].
SCD is most common in Africa where limited resources and these resources carefully targeted are often directed towards sectors other than health. However, it should be noted that the symptoms of SCD are often serious, substantially reducing life expectancy and often requiring intensive treatment throughout the patient’s life.
Hemoglobin C results from a variation in the gene that codes for hemoglobin (β6-Glu → Lys), the protein in our RBC that helps carry oxygen around the body, It causes hemolytic anemia, splenomegaly in homozygous state and provides a degree of protection against malaria infection [12, 26]. Persons with hemoglobin C trait (Hb AC) are phenotypically normal, with no clinical evident limitations or symptoms. However, their heterozygous status, gives them a degree of protection against developing severe malaria HbC is common in malarious areas of West Africa, especially in Burkina Faso, the prevalence of HbAC is much higher and can reach over 21% [26, 28, 30, 31].
HbS and HbC caused by point mutations in the beta-globin gene, offer both substantial malaria protection. Despite the fact that the blood disorder caused by homozygosity for HbC is much less severe than that caused by homozygosity for HbS [9, 12, 26, 32], it is the sickle mutation which has come to dominate many old-world malarious regions, whilst HbC is highly restricted in its geographical distribution . It is probable that this discrepancy (blood disorder between HbC and HbS) may be due to sickle cell heterozygotes enjoying a higher level of malaria protection than heterozygotes for HbC. A probable higher fitness of HbS heterozygotes relative to HbC heterozygotes could certainly have allowed the sickle cell allele to spread more rapidly. However, observations that carrying either HbC or HbS enhances an individual’s capacity to transmit malaria parasites to mosquitoes could also shed light on this hypothesis .
Hemoglobin E results from a glutamate to lysine substitution in codon 26 (β26 Glu-Lys and GAG-AAG). Besides being a structural variant, the E variant also causes the production of an abnormal mRNA with less b-globin being synthesized. It is synthesized at a slightly reduced rate and has a homozygous phenotype similar to heterozygous β thalassemia .
HbE is the second commonest abnormal hemoglobin after sickle cell hemoglobin (HbS). HbE is common in South-East Asia, where its prevalence can reach 30–40% in some parts of Thailand, Cambodia and in Laos .
2.2.2 Thalassemia syndromes
The thalassemia syndromes are inherited disorders characterized by absence or markedly decreased accumulation of one of the globin subunits of hemoglobin. Individuals with thalassemia disease are not able to make enough hemoglobin, which causes severe anemia .
There is two primary types of thalassemia disease: alpha (α) thalassemias and beta (β) thalassemia disease. In the α-thalassemias, there is absent or decreased production of α-globin subunits, whereas, in the β-thalassemias, there is absent or reduced production of β-globin subunits. Thalassemias affecting the production of delta (δ)- or gamma (γ)-globin subunits are also been described but are rare and not clinically significant disorder
The α-thalassemia syndromes are usually caused by the deletion of one or more α-globin genes and are sub classified according to the number of α-globin genes that are deleted or mutated .
There is two primary types of α-thalassemia:
α+ - thalassemia, in which one pair of the genes is deleted or inactivated by a point mutation,
α 0-thalassemia, in which both pairs of genes are deleted or inactivated.
The frequency of a α thalassemia is generally 41% in regions where malaria is prevalent and in some populations, such as in Nepal, parts of India, and Papua New Guinea, it is over 80% . However, in sub-Saharan African populations, a α-thalassemia frequencies do not exceed 50% despite intense malaria selection and some authors  suggested that this might occur because of negative epistasis with the S allele.
The β-thalassemias are characterized by a quantitative deficiency of β-globin chains, can be sub classified into those in which there is a total absence of normal b-globin subunit synthesis or accumulation. The β−thalassemias are divided into two main varieties (β0-thalassemia, there is no β-chain production and β+thalassemia, there is a partial deficiency of β-chain production) . The molecular basis of the β-thalassemias is very heterogeneous, with over 200 different mutations having been described . In general, the mutations causing β-thalassemia are point mutations affecting a single nucleotide, or a small number of nucleotides, in the b-globin gene. The frequency of carriers of β-thalassemia variants is from 5 to 20% in some areas, although not as high as the frequency of α-thalassemia variants .
2.2.3 Glucose-6-phosphate dehydrogenase deficiency and P. falciparum malaria
Glucose-6-phosphate dehydrogenase (G6PD) deficiency is a genetic disorder that results in impaired enzyme activity.
This X-linked genetic condition is characterized by reduced G6PD enzyme activity, which can remain asymptomatic. Red blood cells obtain reduced glutathione (GSH) solely from the G6PD/reduced nicotinamide adenine dinucleotide phosphate (NADH) pathway [7, 12, 30, 40, 41].
The deficiency makes red cells more susceptible to oxidative haemolysis, this disease that can cause jaundice in newborn babies and haemolytic anemia (when red blood cells break up) throughout life. This is, usually caused by an infection or exposure to certain foods or chemicals [41, 42]. One of the chemicals that can trigger severe symptoms in people with G6PD deficiency is Primaquine, the only drug currently available to clear the relapsing life stages of the Plasmodium vivax parasite from the liver.
G6PD and a number of other human genetic traits including sickle cell anemia and related haemoglobinopathies are predominantly found in populations living in malaria endemic countries and have been suggested to provide the host protection from severe forms of malaria [30, 43, 44, 45] and asymptomatic malaria .
G6PD deficiency can be common in populations with high levels of malaria infection, indeed the prevalence is even higher (8%) in malaria-endemic countries . Malaria control programs need to know this to inform their policies on using Primaquine as a treatment and as a malaria control measure.
2.3 Epidemiology of inherited hemoglobin disorders and P. falciparum malaria
For a very long time, human beings have interacted with malaria parasites and thus the parasite has had largely time to adapt and evolve with the human host . Immune processes and genetic traits have contributed to reducing the profligacy of the malaria parasite and a wide range of genetic polymorphisms has been developed to modify the individual response to this disease. Gene mutations involved in susceptibility and resistance to P. falciparum malaria. It has been shown that the severity of several malaria infections (such as asymptomatic, CM and SMA) varies significantly between individuals and between populations . Many of the protective variants identified thus far affect erythrocytes, where the malaria parasite spends a crucial stage of its life cycle. Several of the best studied mutations affect the globin genes encoding hemoglobin . Haemoglobinopathies and G6PD deficiency are among the most common single-gene disorders, which affect RBC stability and integrity .
More than 700 abnormal hemoglobin have been described worldwide and more than 200 million people worldwide have RBC enzyme abnormality .
These genetic mutations are major causes of morbidity and mortality around the world . Sickle hemoglobin and G6PD deficiency are genetically independent, their loci is located on chromosome 11 for sickle and chromosome X for G6PD deficiency genes.
Table 1 summarizes the Common Erythrocyte variant that affects malaria infection particularly P. falciparum malaria.
2.3.1 Sickle hemoglobin (HbS), hemoglobin C (HbC) and P. falciparum malaria
Sickle hemoglobin (HbS) and hemoglobin C (HbC) are both caused by point mutations in the beta globin gene, and both offer substantial malaria protection.
220.127.116.11 HbS and P. falciparum malaria
Heterozygosity for the sickle mutation (genotype AS) offers considerable protection against all forms of severe malaria, as well as protection against uncomplicated malaria [26, 32, 33] and parasitaemia [26, 32, 50, 51]. The different potential protective mechanisms that have been proposed and supported for sickle cell include, the growth of malarial parasites is suppressed in sickle cells , the abnormal display of PfEMP-1 , and the acceleration of acquired immunity . It has also been shown that the growth rate of P. falciparum is retarded in HbS containing erythrocytes under conditions of low oxygen tension in vitro ; inhibition parasite growth by , miRNAs found more commonly in sickle cell trait cells than in normal cells inhibit parasite growth [56, 57].
The mechanism of the most strongly protective variant (HbS) against P. falciparum , is very complex. However studies have been shown here are two plausible mechanisms, which are not mutually exclusive, in suppression of parasite growing in red cells  and enhanced splenic clearance of parasitized erythrocytes . Furthermore, a study summarized other possible protective effects of HbC. Indeed effects may result from :
Inhibition of parasite growth due to oxygen-dependent polymerization 
Selective sickling of infected sickle trait erythrocytes leading to enhanced clearance by the spleen. Reduced erythrocyte invasion, early phagocytosis, and inhibited parasite growth under oxygen stress in venous micro vessels .
Enhancement of innate and acquired immunity 
oxygen-dependent polymerization of HbS is responsible for P. falciparum growth inhibition 
A study with children (between the ages of 2 and 10 years) found that the protective effect of HbAS against malaria increased from 20% to 56%, which implies that it enhances or acts in synergy with the acquired immune response [54, 67].
The compromise between risks and benefits allows us to maintain the HbS polymorphism at allele frequencies of environ 10% in many parts of Africa, despite the lethal consequences for homozygotes, which provides the most striking known example of heterozygote advantage in human genetics .
Severe P. falciparum malaria
Uncomplicated P. falciparum malaria
A comparative studies [33, 44, 71, 72, 73] and several prospective studies [5, 26, 37, 74] have shown the reduction of risk in malaria attributable to HbS. In fact, the HbAS genotype protects against uncomplicated P. falciparum malaria by about 30% [33, 71]. In addition, this has been further confirmed by some genome-wide association studies [70, 71].
P. falciparum parasitaemia
Cross-sectional studies have reported conflicting data on the prevalence of P. falciparum parasitemia in asymptomatic HbAS children compared with HbAA children.
A lower prevalence of parasitaemia in HbAS children was reported in some studies, [6, 26, 75], when others studies found contrary results of similar prevalence [72, 76, 77] or of higher prevalence [78, 79].
In these surveys, parasite densities were reported in HbAS children as lower [6, 62, 76, 79, 80] or similar [27, 78, 81, 82] to those in HbAA children. We can conclude that HbAS does not consistently protect from P. falciparum parasitaemia.
18.104.22.168 HbC and P. falciparum malaria
A recent meta-analysis concluded that homozygotes for βC (Hb CC) were strongly protected against severe malaria, and heterozygotes (HbAC) were mildly protected [10, 33]. It has also been found that both both heterozygotes and homozygotes of HbC are protected against severe malaria [26, 30, 31, 44, 83] but the protective effect appears to be substantially greater in homozygotes .
Although a cohort study in Mali reports an increase in the incidence of clinical malaria in AC individuals relative to AA .
HbC genotypes are not fully elucidated , but several mechanisms have been proposed to explain the malaria protection offered by HbC , including abnormal intra-erythrocytic development of the parasite leading to lower P. falciparum replication rates in subsets of CC erythrocytes ; abnormal P. falciparum erythrocyte membrane protein 1 (PfEMP-1) display, leading to reduced cytoadherence and possibly reduced parasite sequestration , and accelerated acquisition of immunity against malaria . In addition, the protective effect of HbC may result from:
Besides, on observations of reduced parasite cytoadherence abnormal PfEMP1 expression, clustering of erythrocyte band3 protein, and altered surface topography of the erythrocyte membrane in the presence of HbC, it would appear that the protective effect of HbC works by increasing the immune clearance of infected erythrocytes [85, 87, 88].
22.214.171.124 Severe P. falciparum malaria
Compared to healthy children, HbC appears to protect against severe malaria to a lesser degree than HbS and in proportion to allele frequency [31, 33, 44, 73, 89]. Protection from specific severe malaria syndromes has not been fully investigated in HbCC; in one study  HbAC showed mild protection from cerebral malaria (CM) and severe malarial anemia (SMA). When compared to children with uncomplicated malaria, protection from severe malaria is inconsistent: non-significant protection is reported from severe malaria in some studies [30, 33, 44, 69] of HbCC and HbAC, and from SMA in other studies [30, 33, 69] that combined homozygotes and heterozygotes. Significant protection from CM was reported in one study of Malian children that combined homo- and heterozygotes [30, 33] Prospective studies have not reported the incidence of severe syndromes in HbC children. Thus, convincing evidence for protection from severe malaria owing to HbC derives largely from few case–control studies. Also, a further strong evidence for overall protection comes from a recent GWAS, which concluded that for each copy of the HbC allele, the risk for severe P. falciparum malaria was reduced by 29% [56, 70].
Uncomplicated P. falciparum malaria
Few studies have reported the risk of uncomplicated malaria associated with HbC. However some comparative studies [44, 72] and prospective studies have yielded conflicting results [26, 33, 74, 91]. Further studies are still needed to show the evidence of protection from uncomplicated malaria afforded by HbCC and HbAC.
P. falciparum parasitaemia
In most studies cross-sectional surveys with adults and children, HbC has not been associated with a reduced prevalence of P. falciparum parasitaemia [76, 78, 80, 92, 93] or P. falciparum density [76, 78, 80, 94]. The incidence of asymptomatic parasitaemia did not differ between HbAC and HbAA children in Mali study [26, 74]. However, in Burkina Study HbAC genotype was associated with a lower incidence of clinical malaria relative to AA among children. Thus, HbC does not appear to modify the risk of P. falciparum parasitaemia.
126.96.36.199 HbE and P. falciparum malaria
HbE is an extremely most common structural hemoglobin variant that occurs at high frequencies throughout Southeast Asia and has reached an allele frequency of up to 70% in some areas of northern Thailand and Cambodia . It is a β-hemoglobin variant, which is produced at a slightly reduced rate and hence has the phenotype of a mild form of β thalassemia .
Generally, none of HbS or C variants are present in Southeast Asia and HbE is in general also absent from populations in which HbS and HbC are present .
HbE is an extremely most common structural hemoglobin variant that occurs at high frequencies throughout Southeast Asia and has reached an allele frequency of up to 70% in some areas of northern Thailand and Cambodia .
AE heterozygotes appear to have protection from invasion into erythrocytes by P. falciparum malaria [4, 65, 72, 96]. Moreover, the protective effect of HbE may result from impairment of P. falciparum red cell invasion and growth , lower intra-erythrocytic parasite growth, and enhanced phagocytosis of infected erythrocytes [28, 96].
When the frequency of HbE is high, some other red cell disorders, such as a-thalassemia, can be also in high frequency. Although extensive sequence analysis has not been carried out. . However, the E allele found in China is on the same haplotype as that found in Thailand , suggesting that it does not have a different origin.
Few studies have been done to characterize the mechanisms of malaria protection. Three categories of effects are relevant: reduced parasite growth and development, altered adhesion of parasitized RBCs to endothelium, and impact on the immune system. In vitro studies of HbEE and HbAE RBCs have found reduced invasion and growth of HbE [96, 99]. Clearly, more work needs to be done to answer further questions about the protective impact of HbE.
Severe P. falciparum malaria
Meta-analysis of few studies [21, 33, 100] that compared the prevalence of HbE in severe and uncomplicated malaria cases demonstrated no evidence of protection, though this should be interpreted cautiously given the significant.
Considering heterogeneity of the findings and the highly selected settings of the studies, more investigations are necessary to conclude on possible protection of HbE.
Uncomplicated P. falciparum malaria
We have not identified studies that have quantified clearly susceptibility to malaria by HbE.
P. falciparum parasitaemia
A cross-sectional study conducted in India reported a significantly lower prevalence of P. falciparum parasitaemia in patients with HbE (AE or EE) compared with patients with HbAA .
188.8.131.52 Hemoglobins S/C and malaria transmission
Some studies suggest that human genetic variation at the β-globin locus can influence the transmission of malaria. Indeed the same genetic variants that are protective against infection also showed an association with the intensity of malaria transmission. Hemoglobin variants C and S protect against severe malaria but their influence probably on parameters not directly linked to disease severity such as gametocyte carriage and infection chronicity. Moreover, some studies provided evidence that hemoglobin variants selected for the protection against malaria might also have a broader impact on local epidemiology by influencing the frequency of parasite, including the carriage of gametocytes [3, 32].
2.3.2 Thalassemia (α and β) and P. falciparum malaria
The thalassemias are the most common Mendelian diseases of humans and constitute a major global health problem . This is a group of clinical disorders that result from defective production of α- or β-globin chains, which arise from deletions or other disruptions of the globin gene clusters on chromosomes 11 and 16 .
A study in Kenyan children found that both heterozygous and homozygous a1 thalassemia was protective against severe malaria , whereas a study in Ghanaian children found that heterozygotes were protected . However, a study conducted in Papua New Guinea, founded the risk of severe malaria (other childhood infections) was reduced by 60% in children who were homozygous for α1 thalassemia and to a lesser degree in heterozygotes . The protective mechanism of thalassemia is not well known. Flow-cytometry studies in vitro have shown that erythrocytes with the thalassemia phenotype show reduced parasite growth  and increased binding of antibodies from malaria-immune .
184.108.40.206 α - thalassemia
The distribution of both a and b thalassemia variants seems to correspond closely to the regions that have historically had high rates of malaria  and the local distribution of these variants also corresponds to endemic malaria [36, 107]. Several studies have shown protection from severe malaria for individuals with a α-thalassemia, compared with individuals without thalassemia [37, 103, 104]. In addition, some authors. In a case–control study have shown protection from severe malaria for a α heterozygotes and homozygotes compared to normal aa/aa genotype [7, 37]. Overall, it appears that.
many haplotypes that reduce the expression of α-globin provide a selective advantage in resistance to severe malaria. Indeed some mechanisms have been proposed to explain the malaria protection offered by α thalassemia:
Immunological priming through cross-species immunity between P. vivax and P. falciparum 
Severe P. falciparum malaria
Uncomplicated P. falciparum malaria
Several prospective studies have assessed the incidence of uncomplicated malaria in α-thalassaemic children, with conflicting results. Indeed some studies showed the incidence of falciparum malaria was higher in α-thalassemia homozygotes and heterozygotes ; in contrast, other studies, found a lower incidence [37, 113]. However, other studies have found no protection for both homozygotes and heterozygotes
P. falciparum parasitaemia
In cross-sectional studies, α-thalassaemia was not associated with the prevalence of parasitaemia [79, 103, 112, 114, 115]. In prospective study of children conducted in Papua New Guinea, both α- thalassaemia homozygotes and heterozygotes had fewer episodes of PCR-detectable parasitaemia than those without α thalassaemia, [115, 116] though this outcome has not been investigated in other studies.
Haldane has explain the very high level of β-thalassemia in some Mediterranean populations by the ‘malaria hypothesis’ of Haldane .
There is ordinarily only one copy of the HBB gene and βþ and β0 thalassemia showing the reduction and loss, respectively, of the production of functional protein. Individuals with α-thalassemia major, have profound anemia while Heterozygotes typically have mild anemia, however, symptoms can vary greatly in severity from having severe anemia to being a symptomless carrier. 
Generally, β-thalassemia is more of a public health problem because of this higher morbidity than α-thalassemia.
Several mechanisms have been also proposed to explain specific protection against malaria-induced
Enhanced removal of parasite-infected RBC 
Reduced pathogenicity through reduced cytoadherence or resetting 
Enhanced antibody binding and subsequent clearance of infected variant RBCs 
Severe P. falciparum malaria
To our knowledge, no studies have investigated the risk of severe malaria in patients with β-thalassaemia.
Uncomplicated P. falciparum malaria
A case–control study in Liberia, have showed a low prevalence of β-thalassaemia in participants with uncomplicated malaria compare to community controls .
P. falciparum parasitaemia
A other study that was done in Northern Liberia, found no differences, although they did report lower parasite densities in those with β-thalassaemia .
2.3.3 Deficiency Glucose-6-phosphate dehydrogenase (G6PD) and P. falciparum malaria
G6PD deficiency is a common X-linked recessive genetic disorder inherited from parents. Although, in most cases, G6PD-deficient individuals appear normal, it can lead to life-threatening anemia in severely G6PD-deficient individuals during oxidative stress-induced by the consumption of certain foodstuff (fava beans), legumes, and taking such as particular antimalarial (primaquine and pamaquine), sulfonamide, sulfamethoxazole, and other drugs and chemicals  and also probably infection with microorganisms . Additionally, some authors show that G6PD deficiency increases the risk of severe neonatal hyperbilirubinemia, which can lead to lifetime disability with kernicterus if inadequately treated [115, 125].
However, there is a big beneficial effect of G6PD deficiency. Some studies have reported that G6PD deficiency provides resistance against malaria as the malaria parasite cannot complete its life cycle in compromised G6PD deficient RBC which have a decrease in life span or because of early phagocytosis of deficient RBC [126, 127]. Deficient G6PD enzyme activity has been shown to correlate with protection against severe malaria [43, 89]. Reduced parasite replication in G6PD-deficient erythrocytes is thought to be the mechanism of protection , but the parasite appears to counter this by manufacturing G6PD itself .
The geographical distribution of G6PD deficiency is consistent with evolutionary selection by malaria , and a hypothesis of positive selection [131, 132, 133]. The results of studies examining the risk of malaria for various G6PD-deficient genotypes are not consistent. Some authors [43, 134] found in Gambia and Kenya that the reduction in risk of severe malaria in male hemizygotes was 58% and that the reduction in risk for heterozygous females was 46%. Other hand, other authors found that in two populations in Mali, the reduction in risk of severe malaria in male hemizygotes was also 58%, but no reduction in risk in the female heterozygotes . Some authors [27, 133] found no protective effect for either male hemizygotes or female heterozygotes. However, a protective effect has been reported on for females that were found to be G6PD deficient. This finding appears to be based on the incomplete correlation of genotype and phenotype for G6PD deficiency in female heterozygotes due to variable inactivation of the two X chromosomes .
Increased phagocytosis of ring-parasitized variant RBCs due to enhanced oxidative
Increased vulnerability of the G6PD deficient erythrocyte to oxidant stress causes its protection against parasitization
Reduced parasite replication in G6PD-deficient erythrocytes
2.4 Global burden of the hemoglobin disorders
Hemoglobinopathies are a group of IDH initially described in the subtropical regions, they are now spread all around the world. Their high frequency and clinical severity make them a global health burden mostly in Africa where there is a huge lack of resources. The measure of the yardstick of under-5 mortality has been used to assess the broad effect of hemoglobin disorders on health because most affected children can die in early childhood and most survivors can have a chronic disease. Some authors show that the disease may be cause of at least 3.4% of deaths in children aged under 5 years . However, it is very difficult to estimate the burden especially as inherited disorders affect families and then communities. Worldwide, over 1% of couples are at risk for IDH most have at least one affected child. Most affected children could die in early childhood although there are now better health facilities and medical care.
Although the West African death rate in children aged under 5 years is 18.4%, This rate is 16.5% for children born to couples, not at risk for sickle-cell disorders, and 40% for children born to couples who are at risk . Clearly, methods to assess the health burden of inherited disorders must include also a family perspective .
The burden of disease due to malaria across worldwide vary according to selected visible traits of major medical importance, including the alleles of genes encoding hemoglobin. There are several reasons for the extremely high frequency and uneven distribution of inherited hemoglobin disorders. Natural selection is by far the most important, because of the frequency of the heterozygote and the protection against malaria afforded to the homozygotes of thalassemia and HbC, followed by consanguineous marriages [9, 136]. The epidemiological transition whereby, owing to improvements of health care services, nutrition, and health positive social and behavioral factors, babies who would have probably died from the more severe hemoglobin disorders survive nowadays . Then, the migration from areas of high frequency of SCD into regions like Europe and the United States are also cited.
Currently, there are only limited data on the gene frequencies and the number of births of patients with common hemoglobin disorders, particularly in Africa. Micro mapping studies involving many different centers in these countries have recently found that there is remarkable diversity in the frequency of the hemoglobin disorders even over small geographical distances [137, 138, 139].
For the future more micro mapping data are then needed to provide an accurate picture global burden according selective factors distribution . Hemoglobinopathies are so common that they provide a convenient model for working out a genetic approach to the control of chronic childhood diseases. At present, about 250 million people (4.5% of the world population) carry a potentially pathological haemoglobinopathy gene. Haemoglobinopathy control programs, based on WHO approaches and recommendations, have been established in different countries in all WHO Regions and have been successful in the management of the problem by reducing the burden of the Hemoglobin Disorders .
Nowadays effective prevention programs have been carried out successfully in many developed countries concerning medical care for hemoglobinopathies. The programs should be extended and followed to African regions where hemoglobin disorders are frequency very high. Indeed this frequency accounts for more than 70% of total hemoglobinopathies in the world . Reducing the incidence of IDH, better prevention against IDH should remain the major priority of health services in order to reduce the burden of heamoglobinopathies.
2.5 Global burden of the Malaria
Despite the global awareness with the promulgation of malaria eradication goals, despite the colossal efforts deployed in many forms: international and bilateral cooperation, foundations and humanitarian agencies, philanthropic works. Malaria remains a crucial public health concern within the world in general and in tropical countries in particular. According to the latest Global Malaria Report, there were 228 million cases of malaria in 2018, of which there were 405,000 deaths. Almost all of the morbidity and associated lethality, respectively 93% and 94%, occur in Africa (WMR, 2019) more than 50% of the disease burden of malaria is borne by only 6 countries namely: Nigeria (25%), the Democratic Republic of Congo (12%), Uganda (5%) and Côte d’Ivoire, Mozambique and Niger (4% each). With 67% of malaria-related deaths, children under 5 remain the most vulnerable group . These health consequences of malaria bring with them very important economic and social tragedies. In some cases, the resulting disruption of family structures has consequences for the family itself and for the whole community over several generations . The incidence of malaria is inversely proportional to the level of development of the affected societies. The global distribution of gross domestic product per capita shows a strong correlation between malaria and poverty. Malaria generates direct costs (medical care) and indirect costs (loss of productivity linked to disabilities and deaths due to malaria) . The relief of all this sharp and stifled pain requires strengthening malaria prevention programs and promoting multidisciplinary research on effective and safe antimalarial drugs and vaccines.
This review confirms that the malaria parasite has co-evolved with its human host, each struggling for survival. The resulting stigmas appear as polymorphisms of the human genome. This process resulted in a symbiotic association, conferring to the host a relative protection against parasitic infection on one hand and allowing the parasite a greater longevity and proliferation due to host acquired resistance on other hand About polymorphisms of the proteins of red blood cells conferring an innate relative resistance to malaria, it is established that with an effect size >80%, the HbS variant confers the strongest protective effect against severe malaria, while the α- thalassemia confers a protective effect of about 40% in homozygotes [70, 71].
Other genes such as G6PD deficiency have also been shown to be present at high frequencies in endemic malaria populations [9, 10, 22, 142]. However, taking into account the limited mapping specific gene variants of early studies, recent studies have shown that the significant effects of known candidate genes would explain only a small fraction of the heritability of malaria [10, 22, 63, 143, 144]. This indicates that the genetic architecture of susceptibility to malaria is much more complex and that “missing heritability” could be explained by polygenetic or epigenetic effects or by gene–gene and gene–environment interactions [10, 145].
The MalariaGEN consortium has greatly contributed to understanding the correlation between the pathogenesis of malaria and polymorphisms in human RBC.
A better understanding of how changes in RBC physiology affects malaria pathogenesis may uncover new strategies to combat the disease. Understanding the molecular basis of these polymorphisms may shed additional light on the variation in human susceptibility to malaria and offer insight into mechanisms of malaria pathogenesis likewise, a better understanding of red cell membrane structure and function will offer opportunities for the discovery of new and urgently needed therapeutic targets for the treatment of malaria.
Hemoglobinopathies are among the most common monogenic diseases in populations. The complexity of their pathophysiological processes, the severity and diversity of their clinical manifestations reflect the relevance of their scientific interests. Genetic polymorphisms that affect the structure and production of the β- or α-chains of hemoglobin are variously associated with protection from a range of clinical manifestations of P. falciparum infection. The degree of protection conferred by hemoglobinopathies, in general, is greatest against severe malaria, moderate against uncomplicated malaria, and probably absent against asymptomatic P. falciparum parasitaemia. Therefore, there is a positive relationship between the frequency of either βC or βS and malaria selection intensity favors.
However, people with hemoglobin disorders could be high-risk groups. Indeed subjects admitted with malaria are twice more likely to die than those admitted for other pathologies. The screening and genetic counseling for hemoglobin disorders should be an intrinsic part of health care in most countries. Health facility’s services should be designed to provide a foundation for more comprehensive community genetics services because hemoglobin disorders are commonly a point of entry for genetic approaches into health systems.
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. Increased knowledge of the biological basis of these diseases would offer significant advances in their therapeutic management and in the prevention of the occurrence of new cases. Indeed the high frequency of IDH because of natural selection associated due to consanguineous marriages in some countries could be reduced through public awareness campaigns. Improving health care conditions in general and those related to pathologies associated with IDH would enhance affected children’s survival. Hemoglobin variants could shape the distribution of malaria parasites in human populations and their transmission potential. Therefore, the knowledge of our understanding of the interaction between hemoglobin variants and malaria parasites is still being incomplete even if it has improved these last years. Nonetheless, with the interest in malaria elimination, knowledge on how these prevalent genetic variants influence parasite distribution and probably cumulative host transmission potential would be particularly valuable and necessary.
The malaria parasite has co-evolved with its human host in a struggle for their survival. The scars of this war on the human genome are polymorphisms conferring an innate resistance to malaria. Regarding relationships between malaria and human genetic alterations of RBC proteins, it appears that the MalariaGEN studies have clearly opened new doors to understand the malaria burden on human RBC polymorphisms and thus malaria pathogenesis. These new pieces of knowledge will help to redefine or readjust malaria control strategies.
In fact, despite the complexity of these interactions, hemoglobin variants in general and hemoglobinopathies, in particular, show a good model and natural experiment identifying cellular and molecular mechanisms by which P. falciparum produces morbidity.
Vaccines are unquestionably the most cost-effective way for malaria control. The new generation of vaccine delivery systems is increasingly moving towards co-administration of certain immunostimulants and the use of more than one antigen in the same system. In any event, the best vaccine should be effective, safe, low cost, available, and easy to administer.
WHO has announced an ambitious goal of global malaria elimination by 2030. Malaria elimination is possible but will require adaptive and well-managed programs and the implementation of evidence-based surveillance strategies and strong national responses, with adequate funding and human resources.
Over the past 20 years, significant progress has been made in the fight against malaria worldwide, with impressive reductions in transmission in many endemic regions. However, the successful outcome of the global malaria eradication research program will depend on the development of new and more effective tools, including rapid diagnostic tests, drugs, vaccines, insecticides, and awareness raising. Nowadays, genetic and genomic knowledge of malaria parasites, vectors (mosquitoes), and human hosts are available. These bits of knowledge could and should be used for the development of the latest generation tools that are more efficient and secure. The situation is all the more urgent and worrying as there is the emergence of a resistance of Plasmodium falciparum to the first-line drug, namely artemisinin.
In any event, the development of an effective, safe and operational malaria vaccine remains the ultimate objective that we must achieve. In this perspective, we must watch and take into account the genetic variation of the parasite population, which threatens to undermine our efforts.
A big significant milestone in scientific advancements of the last twenty years was already the elucidation of the genomes, transcriptomes, and proteomes of many pathogens, including malaria parasites. These informations give a clearer and more detailed picture that provides the foundation for new approaches to refine existing targets as well as to identify new target antigens for the development of more efficient and effective vaccines, drugs and diagnostic tests. It is clear that further progress is needed for the development of a malaria vaccine, based on basic research, in order to identify new target antigens and better understand how different adjuvants will affect balance, sustainability, and the effectiveness of the responses. To hope for an effective vaccine, emphasis should be placed on mixtures of antigens combined with potent adjuvants, not only to induce the necessary effective responses, but also to increase the possibility of inducing at least partial cross-immunity by including a range of Plasmodium epitopes.
Innovative genome-based vaccination strategies have shown the potential of a number of pathogens, including malaria. A rational genome-based vaccine design, allowing the selection of the best possible targets by prioritizing antigens according to clinically relevant criteria (frequency and magnitude of the clinically relevant immune response and/or biological function), will overcome the problem of poor immunogenicity and poor vaccine protection that have undermined the development of malaria vaccines in the past 30 years.
The use of gene drive technology could revolutionize ecosystem management. These emerging technologies with potential global effects are being offered to researchers and now subject to public discussions regarding environmental and safety concerns. The relative protection against malaria that heamoglobinopathies would confer, justify justly that we are still investigating this correlation to determine all its nature and its power. Despite this relative advantage over the manifestations of malaria in subjects with sickle cell trait, gene therapy is a new approach to healing patients with hemoglobinopathies, which must be popularized. In this sense, clinical trials are underway with promising results. However, there are still frontiers to explore that could improve this approach: the stoichiometry between transgenic hemoglobin and endogenous hemoglobin in relation to the different genetic mutations of globins; the supply of donor cells, such as the use of induced pluripotent stem cells (iPSC); and the use of safer gene insertion methods to prevent oncogenesis.
The overall prospects for malaria elimination are clear, encouraging and the potential opportunities are endless. Technological discoveries and advances are happening at an incredibly fast pace. The context of discovery and use of new technologies makes the eradication of malaria within the reach of the test tube. However, regardless of the time it will take, we must increase our efforts; intensify networking, with the financial support and strong political will of our leaders, especially in Africa where the problem of malaria is the most important.
We would like to acknowledge all authors whose articles were cited in this chapter.
Conflicts of interest
The authors have declared no conflict of interest.
BEC wrote the manuscript. SBS read and approved the final manuscript and agree to be accountable for all aspects of the work.
Bougouma Edith Christiane, Email:
The authors received no specific funding for this work.
The authors declare no competing financial interests.
There is no ethical approval for this article.
Acronyms and Abbreviations
Institut National de Sante Publique
Groupe de Recherche Action en Santé
Institut National Sante Publique
Groupe de Recherche Action en Santé
Inherited Disorders of Hemoglobin
Sickle Cell Disorders
Red Blood Cells
normal genotype (hemoglobin AA genotype)
homozygotes for βC (hemoglobin CC genotype)
heterozygotes for βC (hemoglobin AC genotype)
homozygotes for βS (hemoglobin SS genotype)
heterozygotes for βS (hemoglobin AS genotype)
heterozygotes for βS and βC (hemoglobin AS genotype)
homozygotes for βE (hemoglobin EE genotype)
heterozygotes for βE (hemoglobin AE genotype)
Deficiency Glucose-6-phosphate dehydrogenase
severe malarial anemia
P. falciparum erythrocyte membrane protein 1
World Health Organization
Rapid diagnostic Tests
Artemisinin-based Combination Therapies
Indoor Residual insecticide Spray
genome-wide association study