The spinal muscular atrophy (SMA) is a relatively common neurodegenerative disorder and the leading cause of death for genetic disease in childhood. The prevalence is estimated at about 1/30.000 . Depending on the age of onset and the severity of disease is classified in four subtypes: type 1 (SMA1), the most severe form, with onset before 6 months of age, type 2 (SMA2), with onset between 6 and 18 months of age, type 3 (SMA3), with onset between childhood and adolescence and type 4 (SMA4), the less severe form, with onset in adulthood. All types are characterized by muscle weakness and atrophy of varying degree, which affects especially the lower limbs and respiratory muscles. SMA is an autosomal recessive genetic disorder caused by mutations in the SMN1 gene (Survival Motor Neuron 1, 5q12.2 – q13.3) that cause depletion of the SMN protein which results in the selective degeneration of motor neurons in the spinal cord. Patients with SMA are characterized clinically by muscle weakness, hypotonia and progressive paralysis. Currently for this disease, there are no effective treatments. In the human genome there are two copies of the SMN gene: SMN1 and SMN2. The SMN2 differs from SMN1 by a few nucleotides including a specific nucleotide in exon 7 that causes an alteration of splicing. Consequently the product is only 10% of the full-length protein and the remaining 90% is a not functional transcript missing exon 7 (SMNΔ7). Usually, the severity of the disease correlates inversely with the number of copies of the SMN2 gene. Patients with SMA1 have a low number of copies of SMN2 (1 or 2 copies); patients with three or four copies of SMN2 usually develop SMA3/4, rather than the SMA1.
Currently there is no effective therapy for this disease even if they are several ongoing pre-clinical studies and trials for the development of new therapeutic strategies pharmacological, molecular and cellular .