Abstract: Objective To investigate the clinical phenotype and genotype characteristics of spinal muscular atrophy (SMA). Methods The clinical data of 31 children with SMA diagnosed from February 2014 to May 2019 were retrospectively analyzed. Results Among the 31 SMA children, the male to female ratio was 1 . 8 : 1 . The age of onset was as followed: 18 months in 2 cases ( 6 . 5 %). The initial symptoms were as followed: muscle hypotonia in 13 cases ( 41 . 9 %), muscle strength decline in 9 cases ( 29 . 0 %), abnormal gait in 5 cases ( 16 .1 %), and growth retardation in 4 cases ( 12 .9 %). Myasthenia was mainly found in the proximal body, the lower limb was more serious than the upper limb, and the tendon reflex weakened or disappeared. There was no change in sensation or cognition. The multiplex ligation-dependent probe amplification (MLPA) was used for gene testing. Among the 31 children, 29 ( 93 . 5 %) had homozygous deletion of exon 7 and exon 8 in SMN 1 gene, and only deletion of exon 7 was found in 2 children ( 6 . 5 %), all of whom were type 2 . The results showed that there was no significant difference between different phenotypes of SMA and SMN 1 gene deletion types (P>0 . 05 ). The SMN 2 gene copy number of SMA type 2 and 3 was higher than that of SMA type 1 , and SMN 2 gene copy number of SMA type 3 was significantly higher than that of SMA type 2 . The distribution of SMN 2 copy number among different SMA phenotypes was statistically significant (P< 0 . 05 ). The parents of 30 children ( 96 . 8 %) were found to have loss of heterozygosity of SMN1 gene. In the remaining cases, the father has SMN1 heterozygous deletion, which was not detected by the mother. Conclusions The detection and analysis of SMN 1 gene is of diagnostic significance in children with SMA. The clinical phenotypic severity of SMA is inversely proportional to the increase of copy number of SMN 2 gene.

Key words: spinal muscular atrophy; phenotype; gene; child