Deletions causing spinal muscular atrophy do not predispose to amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is a rapidly progressive, invariably lethal disease resulting from the premature death of motor neurons of the motor cortex, brainstem, and spinal cord. In approximately 15% of familial ALS cases, the copper/zinc superoxide dismutase gene is mutated; a juvenile form of familial ALS has been linked to chromosome 2. No cause has been identified in the remaining familial ALS cases or in sporadic cases and the selective neurodegenerative mechanism remains unknown. Deletions in 2 genes on chromosome 5q, SMN (survival motor neuron gene) and NAIP (neuronal apoptosis inhibitory protein gene), have been identified in spinal muscular atrophy, a disease also characterized by the loss of motor neurons. These genes are implicated in the regulation of apoptosis, a mechanism that may explain the cell loss found in the brains and spinal cords of patients with ALS.

To determine whether the mutations causing neurodegeneration in spinal muscular atrophy are present in patients with ALS in whom the copper/zinc superoxide dismutase gene is not mutated.

Patients in whom ALS was diagnosed were screened for mutations in the SMN and NAIP genes by single strand conformation analysis.

We found 1 patient with an exon 7 deletion in the SMN gene; review of clinical status confirmed the molecular diagnosis of spinal muscular atrophy. No mutations were found in the remaining patients.

The SMN and NAIP gene mutations are specific for spinal muscular atrophy and do not predispose individuals to ALS.