The lethal hemolytic mutation in beta I sigma 2 spectrin Providence yields a null phenotype in neonatal skeletal muscle

Abstract

Point mutations in beta I sigma 1 spectrin that impair the self-association of spectrin alpha beta heterodimers cause mild to severe hemolytic disease and erythrocyte shape abnormalities. Most such mutations act in a dominant negative fashion. One mutation that is particularly devastating is found in beta spectrin Providence. The Providence mutation replaces serine2019 with proline. Heterozygotes display microcytic and fragile erythrocytes; homozygotes die in the neonatal period. It has recently been determined that an alternative transcript of the same beta I sigma 1 spectrin gene expressed in erythroid lineage cells is the major spectrin in skeletal and cardiac muscle and in some neurons. Because the site of the Providence mutation is common to both beta I sigma 1 and beta I sigma 2 spectrin, defective protein must also be expressed in these tissues. Yet the impact of this or any other beta I spectrin mutation outside of the red cell is unexplored. To address this question with respect to skeletal muscle, we have examined the effects of the Providence mutation in cultured muscle cells, after adoptive gene transfer to adult mice, and in two infants homozygous for spectrin Providence. Transfection of the FLAG epitope tagged wild-type beta I sigma 2 or Providence beta I sigma 2 cDNA constructs into C2C12 myoblasts demonstrated by sedimentation velocity analysis that spectrin beta I sigma 2 Providence formed alpha II/-beta I sigma 2 heterodimers in muscle cells but not heterotetramers. Correspondingly, wild-type beta I sigma 2 spectrin formed both alpha II/beta I sigma 1 dimers and heterotetramers, although the proportion of dimers was surprisingly high, which suggested some limitation on self-association in the muscle environment. After adoptive gene transfer into adult mouse skeletal muscle in vivo, both the wild-type and mutant beta I sigma 2 spectrins assembled into a subsarcolemmal complex in a pattern indistinguishable from the native spectrin skeleton. Skeletal muscle taken at autopsy from two infants homozygous for spectrin Providence was normal histologically, as was the intracellular distribution of beta I sigma 2 spectrin as measured by immunoperoxidase staining. These patients also revealed no clinical evidence of myopathy or muscle wasting. It is unknown if they would have experienced dystrophic or myopathic changes if they had lived longer, although we believe that this is unlikely based on the absence of clinical myopathies in patients with other (albeit less severe) beta I spectrin self-association defects. Collectively, these observations indicate that the spectrin mutations that impact tetramer and oligomer formation, even those with a severe hemolytic phenotype, do not impact skeletal muscle function primarily because skeletal muscle does not use the oligomerizing feature of the spectrin skeleton to the same degree as erythrocytes.