Homozygous SMN2 deletion is a protective factor in the Swedish ALS population

Abnormal survival motor neuron 1 (SMN1)-copy number has been associated with an increased risk of amyotrophic lateral sclerosis (ALS) in French and Dutch population studies. The aim of this study was to determine whether SMN gene copy number increases the risk of ALS or modulates its phenotype in a cohort of Swedish sporadic ALS (SALS) patients. In all, 502 Swedes with SALS and 502 Swedish controls matched for gender and age were enrolled. SMN1 and SMN2 gene copy numbers were studied by a semi-quantitative PCR method. A genotype-phenotype comparison was performed in order to determine whether SMN genes modulate the phenotype of ALS. The results were also compared with our previously reported French cohort of ALS patients. There was no difference between Swedish patients and controls in the frequency of SMN1 and SMN2 copy numbers. The frequency of SMN1 gene copies differed significantly between the French and Swedish ALS populations. The duration of the disease was significantly longer in the Swedish cohort with homozygous deletions of SMN2 when compared with the French cohort. Abnormal SMN1 gene copy number cannot be considered as a universal genetic susceptibility factor for SALS and this result underlines the importance of reproducing association gene studies in groups from different origins. We also suggest that SMN2 gene copy number might have different effects on ALS progression in disparate human populations.