On the feasibility of malaria hypothesis

In 1954, Allison proposed that hemoglobin S (HbS) gene causes protection against fatal malaria. This would explain the high HbS gene frequency observed in certain regions hyperendemic for malaria, so-called "malaria hypothesis". This in silico study was conducted to examine the feasibility of the hypothesis under more realistic initial conditions, where a mutant gene with heterozygous advantage against malaria (e.g., HbS) was introduced in a group of Neolithic hunter-gatherers who decided to start agriculture nearby water where malaria killed a proportion of population. The tribe population size, number of children born to each woman in each generation, mortality from malaria and sickle cell disease, the protection factor provided by the gene carriers against malaria, the probability of mating between the members of the parent and offspring populations, population growth, and increased fertility in women heterozygous for HbS, were also considered. For effectively confer protection against malaria within the shortest possible period, the mutation needs to be happened in a small population. For a large population, the process would take around 100 generations (~ 2500 years) or more to provide an effective protection. Even then, the probability that the new gene could survive and propagate to future generations is about 35%. Conventional population genetics equations with differential or difference equations, give totally incorrect estimates of the gene frequency in small populations; discrete mathematics should be used, instead. After introduction of the advantageous mutation, the gene frequency increased until a steady state value. This value is far less than the gene frequency reported in certain tribes of Africa. It seems that the malaria hypothesis, per se, could not explain such a high observed gene frequency, unless HbS is associated with lower mortality from other causes too.

The effect of the coinheritance of Glucose-6-phosphate dehydrogenase deficiency on the severity of sickle cell disease

Background

Sickle cell disease (SCD) and glucose-6-phosphate dehydrogenase (G6PD) deficiency are inherited disorders associated with chronic haemolysis. Therefore, coinheritance of both disorders could worsen haemolysis in the former and compound a haemolytic crisis. This study compared clinical and laboratory features of deficient and non-deficient SCD patients and the G6PD activities of SCD patients and apparently healthy controls.

Materials and Methods

This is a case-control study of 175 SCD patients and 166 non-SCD controls. G6PD assay was carried out on haemolysate from washed red cells. The G6PD activity was measured by spectrophotometry.

Results

The mean age of patients and controls was 27.3 ± 9.4 and 35.9 ± 9.7 years, respectively, with 75 (46.2%) and 87 (52.4%) being males, respectively. G6PD activity was similar in cases and controls (6.7 ± 3.3 vs. 6.9 ± 3.0 IU/gHb), respectively (P = 0.6). The prevalence of G6PD deficiency was higher in patients than controls (28.6% vs. 22.3%, P = 0.18), and SCD patients were twice more likely to have enzyme activities below 3.0 IU/gHb. No significant difference was observed in the clinical parameters between deficient and non-deficient patients. Deficient patients were more likely to have lower haematocrit (22.8 ± 3.9% vs. 24.5 ± 5%, P = 0.04) and non-significantly higher bilirubin and reticulocyte counts. Furthermore, in patients, severe deficiency resulted in higher bilirubin than in those with mild deficiency (60.5 vs. 21.7 IU/L, P < 0.001). G6PD activity correlated positively with haematocrit (r = 0.91, P = 0.01) and mean corpuscular haemoglobin concentration (r = 0.17, P = 0.02).

Conclusions

Coinheritance of both disorders could worsen haemolysis in SCD patients, and care should, therefore, be taken in the choice of drugs in deficient SCD patients.

Falciparum malaria infection in a case of sickle cell trait; unbalancing the balanced polymorphism

Suffers of Sickle cell trait are protected against plasmodium falciparum malaria because of the law of balanced polymorphism. We present a case of sickle cell trait infected with severe falciparum malaria unbalancing of balanced polymorphism.