Differential expression of the actin-binding proteins, alpha-actinin-2 and -3, in different species: implications for the evolution of functional redundancy.

The alpha-actinins are a multigene family of four actin-binding proteins related to dystrophin. The two skeletal muscle isoforms of alpha-actinin (ACTN2 and ACTN3) are major structural components of the Z-line involved in anchoring the actin-containing thin filaments. In humans, ACTN2 is expressed in all muscle fibres, while ACTN3 expression is restricted to a subset of type 2 fibres. We have recently demonstrated that alpha-actinin-3 is absent in approximately 18% of individuals in a range of human populations, and that homozygosity for a premature stop codon (577X) accounts for most cases of true alpha-actinin-3 deficiency. Absence of alpha-actinin-3 is not associated with an obvious disease phenotype, raising the possibility that ACTN3 is functionally redundant in humans, and that alpha-actinin-2 is able to compensate for alpha-actinin-3 deficiency. We now present data concerning the expression of ACTN3 in other species. Genotyping of non-human primates indicates that the 577X null mutation has likely arisen in humans. The mouse genome contains four orthologues which all map to evolutionarily conserved syntenic regions for the four human genes. Murine Actn2 and Actn3 are differentially expressed, spatially and temporally, during embryonic development and, in contrast to humans, alpha-actinin-2 expression does not completely overlap alpha-actinin-3 in postnatal skeletal muscle, suggesting independent function. Furthermore, sequence comparison of human, mouse and chicken alpha-actinin genes demonstrates that ACTN3 has been conserved over a long period of evolutionary time, implying a constraint on evolutionary rate imposed by continued function of the gene. These observations provide a real framework in which to test theoretical models of genetic redundancy as they apply to human populations. In addition we highlight the need for caution in making conclusions about gene function from the phenotypic consequences of loss-of-function mutations in animal knockout models.