SMN2 gene copy number affects the incidence and prognosis of motor neuron diseases in Japan

Background: The copy number status (CNS) of the survival motor neuron (SMN) gene may influence the risk and prognosis of amyotrophic lateral sclerosis (ALS) and lower motor neuron diseases (LMND) other than spinal muscular atrophy (SMA). However, previous studies of this association, mainly from Europe, have yielded controversial results, suggesting possible regional differences. Here, we investigated the effect of the SMN gene in Japanese patients with ALS and LMND.

Methods: We examined the SMN copy numbers and clinical histories of 487 Japanese patients with sporadic ALS (281 men; mean age at onset 61.5 years), 50 with adult LMND (50 men; mean age at onset 58.4 years) and 399 Japanese controls (171 men; mean age 62.2 years). Patients with pathogenic mutations in ALS-causing genes were excluded. SMN1 and SMN2 copy numbers were determined using the droplet digital polymerase chain reaction.

Results: The frequency of a copy number of one for the SMN2 gene was higher in patients with ALS (38.0%) than in healthy controls (30.8%) (odds ratio (OR) = 1.37, 95% confidence interval (CI) = 1.04-1.82, p < 0.05). The SMN2 copy number affected the survival time of patients with ALS (median time: 0 copies, 34 months; 1 copy, 39 months; 2 copies, 44 months; 3 copies, 54 months; log-rank test, p < 0.05). Cox regression analysis revealed that the SMN2 copy number was associated with increased mortality (hazard ratio = 0.84, 95% CI = 0.72-0.98, p < 0.05). Also, null SMN2 cases were significantly more frequent in the LMND group (12.0%) than in the control group (4.8%) (OR = 2.73, 95% CI = 1.06-6.98, p < 0.05).

Conclusions: Our findings suggest that SMN2 copy number reduction may adversely affect the onset and prognosis of MND, including ALS and LMND, in Japanese.