Hereditary elliptocytosis, spherocytosis and related disorders: Consequences of a deficiency or a mutation of membrane skeletal proteins

Abstract

The membrane skeleton, a protein lattice that laminates the internal side of the red cell membrane, contains four major proteins: spectrin, actin, protein 4.1 and ankyrin. By mass, the most abundant of these proteins is spectrin, a fibre-like protein composed of two chains, α and β, which are twisted along each other into a heterodimer. At their head region, spectrin heterodimers are assembled into tetramers. At their distal end, these tetramers are interconnected into a two dimensional network by their linkage to oligomers of actin. This interaction is greatly strengthened by protein 4.1. The skeleton is attached to the membrane by ankyrin, a protein that connects the spectrin β chain to the major transmembrane protein band 3, the anion channel protein. Additional attachment sites are those of protein 4.1 with several glycoproteins, namely glycophorin A and C, as well as direct interactions between spectrin, protein 4.1 and the negatively charged lipids of the inner membrane lipid bilayer. Hereditary spherocytosis, elliptocytosis and pyropoikilocytosis represent a group of disorders that are due to deficiency or dysfunction of one of the membrane skeletal proteins (Fig. 1). Known deficiency states include that of spectrin, ankyrin and protein 4.1. Severe spectrin and ankyrin deficiencies (with decrease in spectrin and ankyrin contents to about 50% of the normal amount) are both rare disorders associated with severe autosomal recessive hereditary spherocytosis. On the other hand, mild spectrin deficiency is found in the majority of patients with autosomal dominant spherocytosis in which the degree of spectrin deficiency correlates with the clinical severity of the disease. Protein 4.1 deficiency, in contrast, is associated with hereditary elliptocytosis, which in certain populations constitutes about 20% of all such patients. Known skeletal protein dysfunctions include mutants of both α and β spectrin that involve the spectrin heterodimer self-association site. These are clinically expressed as hereditary elliptocytosis (HE) and a closely related disorder, hereditary pyropoikilocytosis (HPP). At the level of protein function, this defect can be detected by analysis of the content of spectrin dimers and tetramers in 0°C low ionic strength extracts of red cell membranes. Their structural identification is accomplished by limited proteolytic digestion of spectrin followed by two-dimensional tryptic peptide mapping. This approach consistently reveals the presence of abnormal peptides, which are derived from the αI domain, (the self-association site of the α spectrin). The severity of HE and HPP depends on the amounts of mutant spectrin in the cells and the severity of the spectrin self-association defect. In addition, subjects with HPP, but not HE, are also partially deficient (up to 30%) in spectrin. In this paper, an hypothesis is advanced that the loss of surface area in hereditary spherocytosis is a consequence of partial deficiency of spectrin. In contrast, poikilocytosis, which is characteristically present in more severe forms of HE or HPP, is principally due to a loss of two dimensional integrity of membrane skeleton.