SMN2-Deletion in childhood-onset spinal muscular atrophy

The human genome has two homologous survival motor neuron genes, SMN1 and S MN2. Although deletions of SMN1 are frequently reported in childhood-onset spinal muscular atrophy (SMA), SMN2 have been found to be intact in patient s with the disorder. We report on a 5-year-old boy with childhood-onset SMA who has a homozygous deletion of SMN2. He had wasting, weakness, and hypor eflexia, predominantly in the distal muscles. The muscles involved showed c hronic neurogenic changes on electromyogram. There was no sensory involveme nt. A nerve conduction study showed near normal conduction velocity with re duction in the amplitude of the compound muscle action potential. Analysis of polymerase chain reaction-restriction fragment length polymorphism as we ll as single-strand conformation polymorphism on exons 7 and 8 of the SMN g enes revealed the SMN2-deletion. Base sequencing and densitometric analysis of the critical region (exon 7) did not show any microdeletion or duplicat ion of SMN1, but confirmed the deletion of SMN2. We conclude that a deletio n of SMN2 may also result in the SMA phenotype. (C) 2001 Wiley-Liss,Inc.