The loss and degeneration of spinal cord motor neurons result in muscle denervation in spinal muscular atrophy (SMA), but whether there are primary pathogenetic abnormalities of muscle in SMA is not known. We previously detected increased DNA fragmentation and downregulation of Bcl-2 and Bcl-XL expression but no morphological changes in spinal motor neurons of SMA fetuses. Here, we performed histological and morphometric analysis of myotubes and assessed DNA fragmentation and Bcl2/Bcl-XL expression in skeletal muscle from fetuses with type I SMA (at ∼12 and 15 weeks' gestational ages, n = 4) and controls (at ∼10-15 weeks' gestational ages, n = 7). Myotubes were smaller in the SMA than in control samples at all agesanalyzed (p< 0.001) and were often arranged in clusters close toisolated and larger myotubes. Numbers of terminal deoxynucleotidyl transferase dUTP nick end labeling-positive cells in control and SMA fetuses were similar, and no differences in Bcl-2 or Bcl-XL immunostaining between control and SMA muscle were identified. Areas with smaller myotubes and the morphometric analysis suggested a delay in growth and maturation in SMA muscle. These results suggest that spinal motor neurons and skeletal muscle undergodifferent pathogenetic processes in SMA during development; they imply that muscle as well as motor neurons may be targets for early therapeutic intervention in SMA.

Homozygous mutations of the telomeric SMN1 gene lead to degeneration of motor neurons causing spinal muscular atrophy (SMA). A highly similar centromeric gene (SMN2) can only partially compensate for SMN1 deficiency. The c.859G>C variant in SMN2 has been recently reported as a positive disease modifier. We identified the variant in 10 unrelated chronic SMA patients with a wide spectrum of phenotypes ranging from type II patients who can only sit to adult walkers. Haplotype analysis strongly suggests that the variant originated from a common ancestor. Our results confirm that the c.859G>C variant is a milder SMN2 allele and predict a direct correlation between SMN activity and phenotypic severity.