The developmental pattern of myotubes in spinal muscular atrophy indicates prenatal delay of muscle maturation.

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-X(L) 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 Bcl-2/Bcl-X(L) expression in skeletal muscle from fetuses with type I SMA (at approximately 12 and 15 weeks' gestational ages, n = 4) and controls (at approximately 10-15 weeks' gestational ages, n = 7). Myotubes were smaller in the SMA than in control samples at all ages analyzed (p < 0.001) and were often arranged in clusters close to isolated 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-X(L) 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 undergo different pathogenetic processes in SMA during development; they imply that muscle as well as motor neurons may be targets for early therapeutic intervention in SMA.

Mutation update of spinal muscular atrophy in Spain: molecular characterization of 745 unrelated patients and identification of four novel mutations in the SMN1 gene.

Spinal muscular atrophy (SMA) is caused by mutations in the SMN1 gene. We have studied the molecular pathology of SMA in 745 unrelated Spanish patients using PCR-RFLP, SMN gene dosage analysis, linkage studies, long-range PCR and direct sequencing. Our systematic approach allowed us to complete genetic testing and risk assessment in 736 SMA patients (98.8%). Females were more frequently affected by the acute form of the disease (type I), whereas chronic forms (type II-III) predominated in males (p<0.008). Absence of the SMN1 gene was detected in 671 patients (90%), and hybrid SMN1-SMN2 genes were observed in 37 cases (5%). Furthermore, we detected 13 small mutations in 28 patients (3.8%), four of which were previously identified in other populations (c.91dupT; c.770_780dup11; p.Tyr272Cys and p.Thr274Ile), while five mutations were found to date only in Spanish patients (c.399_402delAGAG, p.Ile116Phe, p.Gln136Glu, c.740dupC and c.834+2T>G). The c.399_402delAGAG mutation accounted for 1.9% of all Spanish SMA patients. Finally, we discovered four novel mutations: c.312dupA, c.411delT, p.Trp190X and p.Met263Thr. Our results confirm that most SMA cases are due to large genetic rearrangements in the repetitive region of the SMA locus, resulting in absence-dysfunction of the SMN1 gene. By contrast, ancestrally inherited small mutations are responsible for only a small number of cases. Four prevalent changes in exons 3 and 6 (c.399_402delAGAG; c.770_780dup11; p.Tyr272Cys; p.Thr274Ile) accounted for almost 70% of our patients with these subtle mutations. An SMN-SMN dimer model featuring tight hydrophobic-aromatic interactions is proposed to explain the impact of mutations at the C-terminal end of the protein.

Investigation of the role of SMN1 and SMN2 haploinsufficiency as a risk factor for Hirayama's disease: clinical, neurophysiological and genetic characteristics in a Spanish series of 13 patients.

OBJECTIVE: The effect of the number of copies in the SMN1 and SMN2 genes - the most extensively studied susceptibility and modifying genetic factors in adult onset motor neuron diseases - as a genetic risk factor for Hirayama's disease (HirD) has never been studied. The purpose of this study was to investigate the influence of the number of copies of the SMN1/SMN2 genes on the resulting phenotype in 13 HirD Spanish patients. PATIENTS AND METHODS: We performed a qualitative and quantitative SMN1/SMN2 gene analysis in 13 unrelated HirD patients. The phenotype-genotype correlation was investigated, paying particular attention to the effect of the SMN1/SMN2 copy number on the disease's phenotype. RESULTS: No patient had a homozygous deletion of the SMN1 or SMN2. No differences were found when comparing the SMN1 and SMN2 copy number distributions of the healthy population and HirD patients, and they do not therefore appear to be a susceptibility factor. There was also no correlation found between the number of copies of the SMN1 and SMN2 and the severity of the resulting phenotype. CONCLUSION: Our results suggest that SMN1 and SMN2 are not predisposing factors for HirD and therefore support a lack of association between these genes and the resulting phenotype.