An intronic element contributes to splicing repression in spinal muscular atrophy

Area of Science:

• Genetics
• Molecular Biology
• Neurodegenerative Diseases

Background:

• Spinal muscular atrophy (SMA) is a severe neurodegenerative disease caused by mutations in the survival motor neuron 1 (SMN1) gene.
• The SMN2 gene, a near-identical copy of SMN1, produces insufficient functional protein, leading to SMA, primarily due to exon 7 skipping in its transcripts.

Purpose of the Study:

• To investigate the molecular mechanisms underlying SMN2 exon 7 skipping.
• To identify additional factors contributing to the inefficient splicing of SMN2 transcripts.
• To explore the role of heterogeneous nuclear ribonucleoprotein (hnRNP) A1 in SMN2 splicing regulation.

Main Methods:

• Comparative analysis of SMN1 and SMN2 sequences, focusing on exon 7 and intron 7.
• In vitro binding assays to assess hnRNP A1 interaction with SMN2 sequences.
• In vivo studies using base substitutions to evaluate the impact on exon 7 inclusion.

Main Results:

• A single nucleotide difference in SMN2 intron 7 creates a high-affinity hnRNP A1 binding site.
• Disrupting this intronic binding site restores SMN2 exon 7 inclusion in vivo.
• hnRNP A1 binding to both exonic and intronic sites appears to cooperate in excluding exon 7.

Conclusions:

• Two distinct hnRNP A1 binding sites, one exonic and one intronic, contribute to SMN2 exon 7 exclusion.
• These findings provide a deeper understanding of SMN splicing regulation and potential therapeutic targets for SMA.