On the nature of the Duchenne muscular dystrophy locus: a portion of a complex of related gene clusters of recent pseudoautosomal origin?

The current hypothesis that the Duchenne/Becker muscular dystrophy locus encodes a single 2,000 kb gene is analyzed. The apparent encoding efficiency, the individual and total exon/intron ratios, and the heterogeneity of deletions associated with the disease, which are currently interpreted as supporting the single gene hypothesis, are also consistent with the alternative hypothesis that this locus is a portion of a complex of related gene clusters which include synthenic transcriptional units of enzymes and ligand transport proteins of one or more convergent metabolic pathways. The high recombination frequency and high rate of deletions are consistent with a locus that has recently evolved from pseudoautosomal origin. The propositions that nebulin or dystrophin is the product of the DMD locus, and that the mdx locus in the mouse is homologous to that of DMD, are critically evaluated. Several lines of evidence support the contention that developmental and tissue-specific enzymes of acyl-specific phospholipid synthesis are encoded in these clusters. Phenotypic variability not accountable for by deletion heterogeneity is postulated to arise from epistatic interactions with other loci within or outside these putative clusters. Some testable predictions of these hypotheses are suggested.