Abstract
Autosomal recessive proximal spinal muscular atrophy (SMA) is a neurodegenerative disorder resulting from functional loss of survival motor neuron 1 (SMN1). Homozygous absence of SMN1 due to deletion or gene conversion accounts for about 96% of SMA cases. In the remaining 4%, subtle SMN1 mutations are commonly identified. Here, we describe two novel intragenic SMN1 mutations in three type I SMA individuals: a point mutation in exon 3 (c.469C > T) and a substitution in intron 4 (c.628-140A > G). In-vivo splicing assays demonstrated that the intronic substitution creates a novel splice donor site, culminating in aberrant splicing and insertion of 65 bp from intron 4 between exons 4 and 5 in SMN1 transcripts (c.627_628ins65). Both mutations render SMN1 transcripts susceptible to nonsense-mediated mRNA decay (NMD), resulting in mRNA degradation, insufficient SMN protein levels and development of an SMA phenotype. Treatment of patient cell lines with the translation inhibitors puromycin and emetine markedly increased the levels of mutant SMN1 transcripts. A similar effect was observed after siRNA-mediated knockdown of UPF1, a factor essential for NMD. This study provides first evidence that NMD of SMN1 transcripts is responsible for the molecular basis of disease in a subset of SMA patients.
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Acknowledgments
We are grateful to all SMA patients who contributed to this project. This work was supported by grants from the Deutsche Forschungsgemeinschaft (Wi 945/12-1), Families of SMA, Center for Molecular Medicine Cologne, Initiative “Forschung und Therapie für SMA”, and Köln Fortune to B.W.
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The authors Lars Brichta and Lutz Garbes contributed equally to this work.
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Brichta, L., Garbes, L., Jedrzejowska, M. et al. Nonsense-mediated messenger RNA decay of survival motor neuron 1 causes spinal muscular atrophy. Hum Genet 123, 141–153 (2008). https://doi.org/10.1007/s00439-007-0455-7
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DOI: https://doi.org/10.1007/s00439-007-0455-7