Hereditary elliptocytosis due to both qualitative and quantitative defects in membrane skeletal protein 4.1

Protein 4.1 is an important structural component of the membrane skeleton that helps determine erythrocyte morphology and membrane mechanical properties. In a previous study we identified a case of human hereditary elliptocytosis (HE) in which decreased membrane mechanical stability was due to deletion of 80 amino acids encompassing the entire 10-Kd spectrin-actin binding domain. A portion of this domain (21 amino acids) is encoded by an alternatively spliced exon that is expressed in late but not early erythroid cells. We now report a case of canine HE in which the abnormal phenotype is caused by failure to express this alternative peptide in the mature red blood cell (RBC) membrane skeleton, in conjunction with quantitative deficiency of protein 4.1. Western blotting of RBC membranes from a dog with HE showed a truncated protein 4.1 that did not react with antibodies directed against the alternative peptide. In addition, sequencing of cloned reticulocyte protein 4.1 cDNA showed a precise deletion of 63 nucleotides comprising this exon. Normal dog reticulocytes did express this exon. Expression of this 21 amino acid peptide during erythroid maturation is therefore essential for proper assembly of a mechanically competent membrane skeleton, because RBCs lacking this peptide have unstable membranes.