[Clinical characteristics and genetics functional analysis of two children with Spinal muscular atrophy]

To explore the characteristics of SMN1 gene variants and carry out functional verification for two children with Spinal muscular atrophy (SMA).

Two male children with complicated SMA diagnosed at the Children's Hospital Affiliated to Capital Institute of Pediatrics respectively in July 2021 and April 2022 due to delayed or retrograde motor development were selected as the study subjects. Clinical data of the children were collected. Primary culture of skin fibroblasts was carried out, and peripheral blood samples were collected from both children and their parents. Multiplex ligation-dependent probe amplification, combined long-range PCR and nested PCR, and Sanger sequencing were carried out to detect the copy number and variants of the SMN1 gene. Absolute quantitative real-time PCR, Western blotting and immunofluorescence were used to determine the transcriptional level of the SMN gene, expression of the SMN protein, and the number of functional SMN protein complexes (gems body), respectively. This study was approved by the Children's Hospital Affiliated to Capital Institute of Pediatrics (Ethics No. SHERLLM2021009).

Child 1, a 1-year-old boy, was clinically diagnosed with type 1 SMA. Child 2, a 2-and-a-half-year-old boy, was clinically diagnosed with type 3 SMA. Both children were found to harbor a paternally derived SMN1 deletion and a maternally derived SMN1 gene variant, namely c.824G>T (p.Gly275Val) and c.884A>T (p.*295Leu). Compared with the normal controls and carriers, the levels of full-length SMN1 transcripts in their peripheral blood and skin fibroblast cell lines were significantly decreased (P < 0.05), and the levels of SMN protein normalized to that of β-actin, and the numbers of gems bodies in the primary fibroblast cells were also significantly lower (P < 0.05). Based on the guidelines from the American College of Medical Genetics and Genomics, both variants were classified as likely pathogenic (PS3+PM3+PM5+PP3; PS3+PM3+PM4+PP3). Following the diagnosis, both children had received nusinersen treatment. Although their motor function was improved, child 1 still died at the age of 2 due to severe pulmonary infection. The walking ability of child 2 was significantly improved, and his prognosis appeared to be good.

Two cases of clinically complicated SMA have been confirmed by genetic testing and experimental studies, which has provided a reference for their accurate treatment.