SMN1 deletions among Singaporean patients with spinal muscular atrophy

INTRODUCTION: Spinal muscular atrophy (SMA) is an autosomal recessive neuromuscular disorder characterised by degeneration of spinal cord anterior horn cells, leading to muscular atrophy. It is the second most frequent autosomal recessive disease among Caucasian populations with a prevalence of between 1 in 6000 and 1 in 10,000 live births, and a carrier frequency of about 1 in 50. The International SMA Consortium classification defines several types of SMA depending on the age of onset and clinical severity. In the past, the diagnosis of SMA was confirmed by muscle biopsy and, sometimes, electromyography. In 1990, SMA was linked to the 5q13 region of chromosome 5. In 1995, it was found that >95% of patients with SMA have homozygous deletions of exons 7 and 8 of the survival motor neurone 1 (SMN1) gene, one of the candidate genes identified within 5q13. The purpose of our study was to determine the frequency of SMN1 deletions in patients with known SMA and the impact of this on the diagnosis of SMA. MATERIALS AND METHODS: Molecular analysis was performed on stored DNA and case notes were reviewed retrospectively. RESULTS: Twenty-two (91.7%) out of 24 patients with all types of SMA were homozygously deleted for exons 7 and/or 8 of SMN1. We also report our experience with prenatal diagnosis of SMA. CONCLUSIONS: Molecular studies can replace conventional investigations for SMA and have made the option of prenatal diagnosis possible for couples at risk.