Detection of the survival motor neuron (SMN) genes by FISH: further evidence for a role for SMN2 in the modulation of disease severity in SMA patients

Spinal muscular atrophy (SMA) is a common autosomal recessive neuromuscular disorder which presents with various clinical phenotypes ranging from seve re to very mild. All forms are caused by the homozygous absence of the surv ival motor neuron (SMN1) gene. SMN1 and a nearly identical copy (SMN2) are located in a duplicated region at 5q13 and encode identical proteins. The g enetic basis for the clinical variability of SMA remains unclear, but it ha s been suggested that the copy number of SMN2 could influence the disease s everity. We have assessed the number of SMN2 genes in patients with differe nt clinical phenotypes by fluorescence in situ hybridization (FISH) using a s SMN probe a mixture of small specific DNA fragments. Gene copy number was established by FISH on interphase nuclei, but the presence of two SMN2 gen es on the same chromosome could also be revealed by FISH on metaphase sprea ds. All patients had at least two SMN2 genes. We found two or three copies of SMN2 in severely affected type I patients, three copies in intermediatel y affected type II patients, generally four copies in mildly affected type III patients and four or eight copies in patients with very mild adult-onse t SMA, No alterations of the genes were detected by Southern blot and seque nce analysis, suggesting that all gene copies of SMN2 were intact. These da ta provide additional evidence that the SMN2 genes modulate the disease sev erity and suggest that knowledge of the gene copy number could be of some p rognostic value.