SP-ALPHA-V/41 - A COMMON SPECTRIN POLYMORPHISM AT THE ALPHA-IV-ALPHA-V DOMAIN JUNCTION - RELEVANCE TO THE EXPRESSION LEVEL OF HEREDITARY ELLIPTOCYTOSIS DUE TO ALPHA-SPECTRIN VARIANTS LOCATED IN TRANS

Spectrin alpha-chain mutants associated with hereditary elliptocytosis are highly variable in their level of expression. It has been assumed that the degree of elliptocytosis can be increased when the spectrin alpha-chain, encoded by the alpha-gene in trans to the variant, is expressed at a low level. We now provide strong evidence for the existence of low-level expression of spectrin alpha-chains. This condition is referred to as the alpha-V/41 polymorphism. It has been observed in 15 different families or individuals of French, North African, and African ancestry in which seven distinct elliptocytogenic alpha-spectrin variants were co-inherited. Whenever the alpha-V/41 polymorphism was present, the severity of the biochemical, morphological, and, sometimes, the clinical phenotype of elliptocytosis was increased. The alpha-V/41 polymorphism was also frequently encountered among 36 unrelated control subjects in the heterozygous or homozygous states, and was entirely asymptomatic in both cases. The main biochemical feature was an increased susceptibility to proteolysis of the alpha-IV-alpha-V domain junction. Alteration of the facing beta-IV domain of spectrin was demonstrated by in vitro spectrin dimer reconstitution experiments. It appears that the alpha-V/41 polymorphism is often required for alpha-spectrin elliptocytogenic variants to become manifest in the heterozygous state. Thus, alpha-spectrin-related elliptocytosis may be viewed as a bifactorial condition.