Evaluation of Mean Percentage of Full-Length SMN Transcripts as a Molecular Biomarker of Spinal Muscular Atrophy

The elevation of SMN transcript and protein level remains the principal aim of SMA therapy. Still, there is no standard molecular biomarker for the assessment of its efficacy. In the current study, we tested three methods of SMN transcript level measurement using real-time RT-PCR, quantitative fluorescent RT-PCR, and a semiquantitative RT-PCR gel densitometric assay. We examined several potential mRNA-based biomarkers and examined their sensitivity and reliability by comparing the obtained values in peripheral blood mononuclear cells of SMA patients, SMA carriers, and healthy individuals. We found that the mean percentage of full-length (FL-SMN) transcripts relative to the total sum of FL-SMN and exon 7-deleted (Delta 7 SMN) transcripts detected by semiquantitative and quantitative fluorescence RT-PCR differed significantly between the three analyzed groups. The relevance of this biomarker was proven in an SMN2-targeting therapeutic experiment. We showed that the values of the biomarker changed significantly in SMA fibroblast cell cultures after treatment with therapeutic antisense oligonucleotides targeting the ISS-N1 site in intron 7 of the SMN2 gene. The obtained results indicate the convenience of using the mean percentage of FL-SMN transcripts determined by semiquantitative and quantitative fluorescence RT-PCR as a putative biomarker for the assessment of SMA therapy efficacy in vitro.