A rare syndrome characterized by lower motor neuron disease associated with progressive myoclonic epilepsy, referred to as "spinal muscular atrophy associated with progressive myoclonic epilepsy" (SMA-PME), has been described in childhood and is inherited as an autosomal recessive trait.
We searched for deletions of SMN1 and SMN2 in a group of 11 patients with sporadic adult-onset lower motor neuron disease (also referred to as "progressive muscular atrophy") and found an excess of patients carrying homozygous deletions of SMN2 exon 7 (36% versus 5% in the normal population).
A rare variant with associated myoclonic epilepsy and lower motor neuron disease had been previously described in three families before the SMN gene, responsible for the common form of SMA, was isolated.
Homozygous SMN1 (survival motor neuron) gene deletion causes spinal muscular atrophy, and SMN2 gene deletions are possible risk factors in lower motor neuron disease.