Spinal Muscular Atrophies: An Ongoing Diagnostic Dilemma?

Background: Spinal muscular atrophies (SMAs) are a group of autosomal recessive disorders of anterior horn cell degeneration. Three genes-survival motor neuron (SMN), neuronal apoptosis inhibitory protein (NAIP), and, more recently, p44 (subunit of basal transcription factor II)-have been considered as candidate genes. The region spanning these genes has a complex organization, which makes molecular analysis difficult. Methods and Results: Molecular genetic testing of deletions of exons 7 and 8 of the SMN(T) (telomeric copy) gene and exon 5 of the NAIP(T) (telomeric copy) gene was performed in 39 diagnosed SMA patients, 31 cases referred as possible SMA, and 24 cases of prenatal diagnosis of SMA. Linkage analysis using markers flanking the SMA region was also performed. In general, the findings of involvement of SMN and NAIP gene deletions in patients diagnosed with SMA are in agreement with those previously published. One possible SMA case was found to be homozygously deleted only for exon 7 of SMN(T) and one deleted only for exon 5 of the NAIP(T) gene. Conclusions: SMAs exemplify human inherited disorders in which application of a variety of different techniques and a search for mutations in multiple genes are involved. Deletion testing of candidate genes (SMN, NAIP) is a powerful approach in patients affected or suspected of being affected with SMA. It is proposed that the direct SMN gene deletion test can be offered as the only test for prenatal diagnosis of SMA in families in which the clinically affected sibling has also been shown to have the homozygous deletion.