Quantification of SMN protein in leucocytes from spinal muscular atrophy patients: effects of treatment with valproic acid

BACKGROUND: Spinal muscular atrophy (SMA) is caused by the homozygous deletion of the survival motor neuron (SMN)1 gene. The nearly identical SMN2 gene produces small amounts of full-length mRNA and functional SMN protein, due to a point mutation in a critical splicing site. Increasing SMN protein production by histone deacetylase inhibiting drugs such as valproic acid (VPA) is an experimental treatment strategy for SMA. OBJECTIVE: To investigate whether an SMN-specific ELISA could detect changes in SMN protein expression in peripheral blood mononuclear cells (PBMCs) after treatment with VPA. METHODS: The authors developed a sensitive SMN-specific ELISA. Six patients with SMA types 2 and 3 participated in the study. Recombinant SMN calibration curves were used to calculate SMN protein levels in PBMCs before and after 4 months of VPA treatment. RESULTS: The SMN ELISA was able to detect small differences in SMN protein concentrations, and differences in SMN protein levels in Epstein-Barr virus immortalised lymphocyte cell lines from SMA type 1 and 2 patients, carriers and healthy individuals (p<0.05). The mean SMN protein level in PBMCs from SMA patients was 22% (SD 15%) of the value in a healthy control. VPA treatment resulted in significantly increased SMN protein levels in five out of six SMA patients compared with baseline values (p<0.05), but did not restore SMN levels to normal values. CONCLUSIONS: SMN protein quantification by this SMN ELISA is a useful additional tool for evaluating the effects of experimental treatment in SMA.