Mutation analysis of 419 family and prenatal diagnosis of 339 cases of spinal muscular atrophy in China

Abstract

Background: Spinal muscular atrophy (SMA) is a common and lethal autosomal recessive neurodegenerative disease caused by mutations in the survival motor neuron 1 (SMN1) gene. At present, gene therapy medicine for SMA, i.e., Spinraza (Nusinersen), has been approved by the FDA, bringing hope to SMA patients and families. Accurate diagnosis is essential for treatment. Our goal was to detect genetic mutations in SMA patients in China and to show the results of the prenatal diagnosis of SMA. Methods: In this study, we examined 419 patients in our hospital from January 2010 to September 2019. Multiplex ligation-dependent probe amplification analysis was used to determine the copy numbers of SMN1 and SMN2. Long-range PCR combined with nested PCR was used to detect point mutations in SMN1. In addition to the above detection methods, we also used QF-PCR in prenatal diagnosis to reduce the impact of maternal contamination. We conducted a total of 339 prenatal diagnoses from January 2010 to September 2019. Results: Homozygous deletion of SMN1 exon 7 was detected in 96.40% (404/419) of patients. Homozygous deletion of SMN1 exon 7 alone was detected in 15 patients (3.60%). In total, 10 point mutations were detected in the 15 pedigrees. Most patients with SMA Type I have 1~2 copies of the SMN2 gene. Patients with SMA Type II have 2 or 3 copies of the SMN2 gene. The results of prenatal diagnoses showed that 118 fetuses were normal, 149 fetuses were carriers of heterozygous variants, and the remaining 72 fetuses harbored compound heterozygous variants or homozygous variants. Conclusions: Our study found that the most common mutation in SMA was homozygous deletion of SMN1 exon 7 in our study. We suggest that detecting only the deletion of exon 7 of SMN1 can meet most of the screening needs. We also believe that SMN2 copy numbers can help infer the disease classification and provide some reference for future treatment options.

Long-Read Sequencing Revealed Extragenic and Intragenic Duplications of Exons 56–61 in DMD in an Asymptomatic Male and a DMD Patient

“…Intragenic mutation screening in SMN1 (exon 1–8) was performed by long-range PCR (LR-PCR) and nested PCR as described previously in our laboratory ( Kubo et al, 2015 ; Sun et al, 2020 ). LR-PCR and nested PCR were amplified using KOD FX Neo Polymerase (TOYOBO, Osaka, Japan) and 2× Taq PCR Mastermix (TIANGEN, Beijing, China), respectively.…”

The Importance of Digging into the Genetics of SMN Genes in the Therapeutic Scenario of Spinal Muscular Atrophy

“…Interestingly, the proportion of type I patients with 3 SMN2 genes was increased in some cohorts, reaching 57% among SMA I patients, while Calucho's compilation reported only 23% [33,34]. In contrast, the cohort described by Sun et al (2020) seems to have, in global numbers, a better-than-expected phenotype, as the majority of SMA type II patients presented 2 SMN2 genes, type III patients had 2 or 3 SMN2 copies and more than half of SMA type IV had only 3 SMN2 genes [35]. Despite these differences in the literature, all studies agree that the higher the SMN2_CN, the less severe the SMA phenotype.…”

“…This fact is reflected in the work of Schorling et al (2019), in which 20 SMA patients were retested for their SMN2 dosage using new DNA samples and 45% of the results were discrepant in comparison with the initial ones [55]. Retesting of cohorts with discrepancies (i.e., [33][34][35]) would be interesting to confirm if some of the results are due to problems with SMN2_CN determination or clinical misclassification of patients. In fact, a guideline is proposed to manage the discordant situations that are present in SMA patients [42].…”

The Frequency of SMN1, SMN2 Copy Numbers in 246 Turkish Cases Analyzed with MLPA Method

“…SMA is at a 25% risk of recurrence for carrier couples and a disease can be diagnosed with prenatal diagnosis; it is essential to establish the molecular genetic diagnosis of affected patients. 26 Parents should be informed about carrier screening, newborn screening, prenatal diagnosis, and preimplantation genetic diagnosis.…”