The Curious Case of Sickle Cell C

Unusual observations in Medicine sometimes have very simple explanations. Take the case of sickle cell C disease, for example.

Evolutionary pressures have led to the existence of several mutants of the beta chain of haemoglobin. Thus Hb S, Hb C and Hb E all have mutations on the beta chain. For example, in Hb S, glutamic acid is replaced by valine in position 6, while in Hb C, lysine is substituted in the same position. Early on, epidemiologists noticed that these mutated haemoglobins were found in areas with high prevalence of falciparum malaria. For example, Hb C is found in Western Africa, Hb E is present in around 60% of subjects in the Indian subcontinent, and Hb S is widely prevalent in Africa. These variants have evolved because heterozygotes with Hb S, C or E are resistant to severe infestation with P.falciparum, and thus provide a survival advantage in these geographical locations.

In the normal adult, two beta chains combine with two alpha chains to form the complete globin chain (alpha2-beta2) and thus constitute the most abundant form of haemoglobin present in adults, known as Hb A. While the beta chain has only one gene, the alpha chain is coded by two genes. Thus, the alpha chains have 4 different alleles across the two chromosomes.

Heterozygotes with the sickle haemoglobin (sickle cell trait) have one normal allele producing the beta chain, and one mutant allele producing Hb S. Since each allele produces an equal amount of Hb A and Hb S, you'd expect an equal (50% each) proportion of Hb S and Hb A in subjects with sickle cell trait. Yet, this is not so. On haemoglobin electrophoresis, these subjects have 50-60% Hb A, and only 35-45% Hb S [the rest being contributed by Hb A2 (alpha2-delta2) and Hb F (alpha2-gamma2)]. Why does this happen?

As it happens, the reason beta chains and alpha chains join so harmoniously is because they carry an almost equal, and importantly, opposite electrical charge. Beta chains carry a negative charge of -2.5 coulomb (C), while alpha chains carry a positive charge of +2.4 C, thus ensuring electroneutrality (almost) when they combine.

However, the beta chain mutants are less negatively charged than the native beta chains. Thus, they combine less effectively with the alpha chain to form Hb S, C or E. This is why, in heterozygotes, instead of a 50-50 split, Hb A produced by the normal allele predominates over the variant haemoglobin. Thus, subjects with sickle cell trait have ~55% Hb A, and 40% Hb S, while heterozygotes for Hb E, have~ 70% Hb A and only 30% Hb E. This also explains why such heterozygotes are not anaemic. Subjects with sickle cell trait can only be picked up on electrophoresis, while heterozygotes with Hb E are only revealed by microcytosis with an absence of iron deficiency.

The principle is further illustrated in subjects with Hb SC disease. Here both alleles of the beta chain are mutant- one is producing Hb S, the other Hb C. As these two beta chain mutants have roughly equal charge (and thus affinity for the alpha chain), they are present in roughly equal concentration on electrophoresis~45-50% each. There is no normal beta chain to compete with.

A similar phenomenon occurs in sickle cell beta thalassaemia. As you may know, the defect in beta chain production in beta thalassaemia may be only partial (denoted as beta thal+) or severe (denoted as beta thal 0). Despite the deficit in production of normal beta chains, subjects with sickle cell beta(+) thalassaemia still have Hb A comprising around 30% of the total Hb in RBC, the other 70% being Hb S, as even in diminished quantities, the available normal beta chains combine more efficiently with alpha chains than the mutated beta chain found in Hb S. Thus, these subjects have a less severe phenotype than those with sickle cell beta (0) thalassaemia, who can't produce any Hb A.

This principle can be put to good use in the diagnosis of newborn subjects (with carrier parents) with one of the mutated beta chains. While Hb F is the predominant haemoglobin in newborns, the proportion of Hb A and Hb S will vary depending on homozygosity, heterozygosity and the co-existence of beta thal (+) trait. Thus, newborn with sickle cell disease will have a FS (F>S) pattern at birth, subjects with sickle cell trait will have a FAS (F>A>S)pattern, while a FSA (F>S>A)pattern at birth is diagnostic of sickle cell beta (+) thalassaemia.

Finally, a correction. In my post on hereditary spherocytosis, I had said that I did not know of any other condition that caused a high MCHC. This is incorrect. Subjects with Hb AC or Hb SC have RBC that are prone to dehydration due to a chloride channel defect, a condition known as xerocytosis. Due to loss of water, the RBC have a high MCHC, which might be the only clue to diagnosis in subjects with Hb AC.