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

Abstract

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

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

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

Figure 1. Parasite Density in Community Children and Those with Acute Malaria According to α⁺-Thalassaemia Genotype

Parasite density is represented by (A) the number of P. falciparum-infected erythrocytes per microlitre of blood and (B) the proportion of P. falciparum-infected erythrocytes. Values are median (interquartile range). There was no statistically significant difference in parasite density or percent parasitaemia among α⁺-thalassaemia genotypes, in either children living in the community or those with acute malaria (p ≥ 0.3).

Figure 2. Consequence of Reduction in Erythrocyte Count on Total Haemoglobin Concentration According to α⁺-Thalassaemia Genotype

The linear relationship between Hb concentrations and reduction in erythrocyte count can be described by the following linear equation: y _i = b − m _i x, where y _i refers to predicted Hb concentration in the ith child, b is the observed Hb value in the community children prior to acute malaria infection taken from Table 1, m _i represents observed MCH in the ith child, and x represents the reduction in erythrocyte count. (A and B) Predicted total Hb concentration of children of normal genotype together with (A) α⁺-thalassaemia heterozygotes and (B) α⁺-thalassaemia homozygotes during reductions in erythrocyte count. Thick lines represent median values and thin lines represent the interquartile range. The equations are y = 104 − 24.3x [y =104 − 25.5, y = 104 − 23.1x] for normal individuals; y = 103 − 22.5x [y = 103 − 23.9, y =103 − 21.0x] for heterozygous; and y = 99 − 19.8x [y = 99 − 18.8x, y = 99 − 21.1x] for children homozygous for α⁺-thalassaemia. (C) Difference in total predicted Hb (y) between those of normal genotype and heterozygous children (y = 1.8x − 1, red line, data from [A]), and between those of normal genotype and homozygous children (y = 4.5x − 5, green line, data from [B]). The crossover point where heterozygous individuals have a greater Hb concentration relative to those of normal genotype is an erythrocyte reduction of 0.56 x10¹²/l (red arrow). The crossover point where homozygous individuals have a greater Hb concentration relative to those of normal genotype is an erythrocyte reduction of 1.1 x10¹²/l (green arrow). These crossovers are also seen in (A) and (B) but are better visualised here.

Figure 3. Genotype-Specific Erythrocyte Cutoffs for Severe Malarial Anaemia

Data points represent haemoglobin concentration and erythrocyte count values for all children with acute malaria. Genotype-specific lines of best fit have been generated for the association of haemoglobin with erythrocyte count. The horizontal black line represents the cutoff for severe malarial anaemia (haemoglobin = 50 g/l). The coloured vertical lines represent the genotype-specific erythrocyte cutoffs for severe malarial anaemia based on a haemoglobin concentration of 50 g/l.