QUANTIFICATION, BY SOLID-PHASE MINISEQUENCING, OF THE TELOMERIC AND CENTROMERIC COPIES OF THE SURVIVAL MOTOR-NEURON GENE IN FAMILIES WITH SPINAL MUSCULAR-ATROPHY

In an analysis of 30 families affected by spinal muscular atrophy (SMA ) we have used the solid-phase minisequencing method to determine the ratio between the number of telomeric and centromeric copies of the su rvival motor neuron gene (SMN and (c)BCD541 respectively) on normal an d SMA chromosomes, This has enabled us to establish haplotypes with re gard to SMN and (c)BCD541, and estimate their frequencies, on both typ es of chromosomes, Six predominant haplotypes were identified, three f or normal chromosomes and three for SMA chromosomes, characterized by having 0, 1, or 2 copies, respectively, of (c)BCD541, We found evidenc e for the presence of patients homozygous for a deletion of SMN and wi th only one copy of (c)BCD541, but found none deleted for all copies o f this gene. Several asymptomatic carriers of SMA with only a single c opy of SMN and no copy of (c)BCD541 were identified, We could not conf irm the hypothesis that the presence of more copies of (c)BCD541 is co rrelated to a less severe course of the disease, The frequencies of ha plotypes characterized by having 0, 1, or 2 copies, respectively, of ( c)BCD541 were found to differ significantly between normal and SMA chr omosomes, This distribution can be explained by an underrepresentation of the haplotype completely lacking SMN genes, which is expected to c ause early embryonic death in homozygotes. This first report of a dire ct haplotype analysis of SMN and (c)BCD541 should help clarify the rol e of (c)BCD541 in the pathogenesis of SMA.