The importance of the SMN genes in the genetics of sporadic ALS.

The human genome contains two SMN (survival motor neuron) genes: SMN1, the telomeric gene whose homozygous deletion causes spinal muscular atrophy (SMA), and SMN2, the centromeric version whose copy number modulates the phenotype of SMA. We performed a Medline search and reviewed all of the publications that focus on SMN1 and SMN2 in amyotrophic lateral sclerosis (ALS) to analyse whether these genes also act as risk factors or phenotypic modulators in ALS. While homozygous deletion of SMN1 was not associated in ALS, abnormal SMN1 copy numbers significantly increased the risk of ALS. The role of the SMN2 gene in ALS needs further clarification. The existence of abnormal SMN1 copy numbers in ALS provides additional evidence that gene copy number variants may contribute to neurodegeneration and might open new approaches to treatment.