Intragenic telSMN mutations: Frequency, distribution, evidence of a founder effect, and modification of the spinal muscular atrophy phenotype by cenSMN copy number

The autosomal recessive neuromuscular disorder proximal spinal muscular atr ophy (SMA) is caused by the loss or mutation of the survival motor neuron ( SMN) gene, which exists in two nearly identical copies, telomeric SMN (telS MN) and centromeric SMN (cenSMN). Exon 7 of the telSMN gene is homozygously absent in similar to 95% of SMA patients, whereas loss of cenSMN does not cause SMA. We searched for other telSMN mutations among 23 SMA compound het erozygotes, using heteroduplex analysis. We identified telSMN mutations in 11 of these unrelated SMA-like individuals who carry a single copy of telSM N: these include two frameshift mutations (800ins11 and 542delGT) and three missense mutations (A2G, S262I, and T2741). The telSMN mutations identifie d to date cluster at the 3' end, in a region containing sites for SMN oligo merization and binding of Sm proteins. Interestingly, the novel A2G missens e mutation occurs outside this conserved carboxy-terminal domain, closely u pstream of an SIP1 (SMN-interacting protein 1) binding site. In three patie nts, the A2G mutation was found to be on the same allele as a rare polymorp hism in the 5' UTR, providing evidence for a founder chromosome; Ag1-CA mar ker data also support evidence of an ancestral origin for the 800ins11 and 542delGT mutations. We note that telSMN missense mutations are associated w ith milder disease in our patients and that the severe type I SMA phenotype caused by frameshift mutations can be ameliorated by an increase in cenSMN gene copy number.