Molecular diagnosis of spinal muscular atrophy

The frequency of deletions within the survival motor neurone (SMN) and neuronal apoptosis inhibitory protein (NAIP) genes in patients with spinal muscular atrophy (SMA), and the impact of this on the diagnosis and prenatal diagnosis of SMA, were investigated by molecular analysis of stored DNA and retrospective review of case notes. In type I SMA, 16 of 17 cases were homozygously deleted for exons 7 and 8 of SMN, 14 of 17 were homozygously deleted for exon 5 of NAIP, and 13 of 17 were deleted for both. In types II and III SMA, seven of nine cases were deleted for exons 7 and 8 of SMN. Deletions of SMN and NAIP occurred in four of nine cases. With one exception, the deletion genotypes of probands, affected siblings, and terminated fetuses were identical. Molecular studies are replacing conventional investigations for SMA and have a high uptake prenatally.