Mutations in the murine erythroid α-spectrin gene alter spectrin mRNA and protein levels and spectrin incorporation into the red blood cell membrane skeleton

Abstract

AbstractTetramers of α- and β-spectrin heterodimers, linked by intermediary proteins to transmembrane proteins, stabilize the red blood cell cytoskeleton. Deficiencies of either α- or β-spectrin can result in severe hereditary spherocytosis (HS) or hereditary elliptocytosis (HE) in mice and humans. Four mouse mutations,sph, sphDem,sph2BC, and sphJ, affect the erythroid α-spectrin gene, Spna1, on chromosome 1 and cause severe HS and HE. Here we describe the molecular alterations in α-spectrin and their consequences insph2BC/sph2BC andsphJ/sphJerythrocytes. A splicing mutation, sph2BC initiates the skipping of exon 41 and premature protein termination before the site required for dimerization of α-spectrin with β-spectrin. A nonsense mutation in exon 52, sphJ eliminates the COOH-terminal 13 amino acids. Both defects result in instability of the red cell membrane and loss of membrane surface area. Insph2BC/sph2BC, barely perceptible levels of messenger RNA and consequent decreased synthesis of α-spectrin protein are primarily responsible for the resultant hemolysis. By contrast, sphJ/sphJmice synthesize the truncated α-spectrin in which the 13-terminal amino acids are deleted at higher levels than normal, but they cannot retain this mutant protein in the cytoskeleton. ThesphJdeletion is near the 4.1/actin-binding region at the junctional complex providing new evidence that this 13-amino acid segment at the COOH-terminus of α-spectrin is crucial to the stability of the junctional complex.