Structural and functional heterogeneity of α spectrin mutations involving the spectrin heterodimer self-association site: Relationships to hematologic expression of homozygous hereditary elliptocytosis and hereditary pyropoikilocytosis

Abstract

Defects involving α spectrin (Sp) are found in patients with hereditary elliptocytosis and a related disorder, hereditary pyropoikilocytosis (HPP). We have previously found that the severity of hemolysis was related to the total spectrin content of the cells and the percentage of unassembled dimeric Sp (SpD) in the membranes, which, in turn, reflected the amount of mutant Sp in the cell. However, no data are available comparing differences in the function of various α Sp mutations to clinical severity. We now report studies of nine homozygotes or double heterozygotes for four a Sp mutations: α1/74, α1/46, α1/65, and α1/61, whose red blood cells (RBCs) contained only the mutant Sp and no normal Sp. Sp α1/74, Sp α1/46, and Sp α1/65 homozygotes differed strikingly in the severity of hemolysis that correlated with the severity of mutant Sp dysfunction, as reflected by the fraction of unassembled SpD in the membranes and the self-association of mutant Sp on inside-out vesicles. Homozygotes for Sp α1/74 had a very severe hemolytic anemia and their SpD were virtually incapable of self-association, whereas SpD α1/46 were not as severely affected. The Sp α1/65 homozygotes had a relatively mild hemolytic anemia and their SpD showed the least impairment of function. Ultrastructural examination of membrane skeletons from subjects whose SpD self-association was severely impaired showed gross skeletal disruption and loss of hexagonal structure. In striking contrast, the homozygote for the mildly dysfunctional Sp α1/65 had only a moderate disruption of the skeleton. Some of the homozygous or doubly heterozygous subjects also exhibited a partial deficiency of Sp that correlated with a RBC morphology characteristic of HPP, namely, marked microspherocytosis with virtual absence of elliptocytes. These data demonstrate striking differences in the function and structure of various α Sp mutants that underlie differences in clinical expression. © 199O by The American Society of Hematology.