Spinal Muscular Atrophy: A Review

ABSTRACT Spinal muscular atrophy (SMA) is a dangerous autosomal recessive disorder in motoneurons associated with muscular weakness and breathing problems. There are numerous studies, metastasis, and reviews focused on the characteristics, pathogenesis, types, severity, diagnosis, and therapy of SMA. The findings of these reports are required to be linked to each other in some aspect. Therefore, this review mentioned the more important fact about SMA. Spinal muscular atrophy (SMA) is an inherited genetic rare neuromuscular disease that occurs due to a mutation in the survival motor neurons 1 (SMN1) gene. SMA1 normal gene responsible for the production of functional survival motor neuron protein with exon7. The mutation of SMN1 led to the production of nonfunctional SMA1protein lacking exon 7. The progeny from both SMA carrier parents yield 25% unaffected, 50% carrier, and 25% affected individuals. The motor neuron protein has crucial role in body bioprocess because this protein regulates muscle contraction by nervous stimuli from the central nervous system. SMA manifests in hypotonia, reduced muscular tone, scoliosis, and inadequate head control, and the more significant one is the weakening of the muscles of the lower limbs, which makes walking difficult. The severity, types, symptoms, and onset time of clinical manifestation depend on the amount and variant of protein produced by other genes called survival motor neurons2 (SMN2). The individual has many copies of SMN2 ranging from one to eight, an increased number of SMN2 gene copies is associated with low severity. In addition to severity, and the type of SMA depending on copies of SMN2, there are five common types of SMA. SMA type 0 associated with one copy of SMA2, is the most severe type lending to death of the embryo before birth; SMA type I associated with two copies of SMA2, is a sever type that occurs during 0-6 months; SMA type II has two to three copies of SMA2, is intermediate variant appears during 6-18 month; SMA type III correlated with three to four copies of SMA2, is mild form onset during 18 month to 18 year; and SMA type IV associated with four or more copies of SMA2, is less sever variant onset after18 year; moreover there is another type of SMA non-related to a mutation in SMN2. The success of SMA therapy, depending early diagnosis, which differs according to SMA severity, age of the affected patient, and availability of medical facilities, however, the diagnosis is determined by clinical evaluation, electromyography (EMG), nerve conduction studies (NCS), muscle biopsy, serum creatine kinase levels (CK), newborn screening, genetic testing. The treatment of SMA mostly depends on gene therapy for example onasemnogene abeparvovec, nusinersen, and risdiplam. More studies are needed to be understanding the basis of SMA pathogenesis and determine the best target of diagnosis and treatment before onset and during the course.