Molecular diagnosis of non-deletion SMA patients using quantitative PCR of SMN exon 7

ABSTRACT

The telomeric survival motor neuron (SMN^T gene is a valuable molecular diagnostic tool for childhood-onset spinal muscular atrophy (SMA) as homozygous deletions of SMN^T exon 7 (Δ7SMN^T) are present in ∼94% of patients. In this report, we provide the first comprehensive study of 32 unrelated non-deletion SMA patients. Quantitative polymerase chain reaction (PCR) studies established that 90% had two intact copies of SMN^T exon 7 suggesting that that these patients do not have 5q SMA. Once 5q SMA is confirmed, the SMN^T gene can be screened for subtle mutations. Using single strand conformation analysis, we identified two missense mutations (P245L and Y272C) in exon 6 of the SMN^T gene of two SMA patients shown to have a single copy of SMN^T exon 7. Y272 is most likely critical for SMN^T function as it is target for recurring mutations and is associated with type I SMA. These results emphasize the need for dosage analysis in the differential diagnosis of 5q SMA in non-deletion patients, consistent with extensive clinical heterogeneity and some genetic heterogeneity in this disease. Homozygosity or heterozygosity for a Δ7SMN^T allele confirms the diagnosis of 5q SMA with greater precision than clinical examination alone.