Poster abstracts

Poster number 43 submitted by Thomas Bebee

Development of mouse models to study SMN splicing and replacement therapy

Thomas W. Bebee (Molecular Cellular and Developmental Biology, The Ohio State University), Jordan T. Gladman (Integrated Biomedical Graduate Program, The Ohio State University), Dawn S. Chandler (Pediatrics, The Ohio State University)

Abstract:
Proximal spinal muscular atrophy (SMA) is a neuromuscular disease caused by low levels of SMN protein due to loss of the Survival Motor Neuron-1 (SMN1) gene. Humans have a duplicate gene, SMN2, that generates low levels of SMN protein due to a C>T point mutation that skips exon 7. We generated a mouse model that recapitulates SMN2 altered splicing by engineering the exon 7 C>T mutation into the homologous mouse Smn gene. The C>T mutation in Smn induces skipping of exon 7 in multiple tissues in the mouse, reduces SMN protein levels, and leads to a mild form of SMA exhibited by reduced hind limb grip strength, rearing, and activity. Increasing SMN expression by correcting splicing or increasing transcription of SMN2 are attractive therapeutic options in SMA patients. However, the timing of SMN replacement will be crucial in the treatment of SMA. Studies in zebrafish predict SMN function during embryogenesis may be important for axonal pathfinding, while mouse models and SMA disease progression in humans suggest that post-natal treatment may be sufficient to protect motor neurons. AAV9 encoding SMN has shown that early postnatal SMN expression in spinal cord motor neurons is sufficient to rescue survival though complete rescue of normal development was not observed, indicating that earlier treatment may be required for full restoration of SMN function and disease correction. To address the question of optimal therapeutic timing for SMA we have developed a temporally inducible transgenic mouse in which the expression of human SMN cDNA is under the control of tamoxifen inducible Cre-recombinase. In our inducible mouse, SMN expression after tamoxifen treatment was validated in adult mice and PND1.5 treated neonatal mice. The temporal induction of SMN expression was further validated in these mice in various tissues including the brain and spinal cord. When crossed to SMA mouse models our inducible SMN mouse model will allow for the temporal induction of SMN at varying time-points during development, both in utero and postnatally, to evaluate the time-point in which SMN replacement is required for SMA correction. The therapeutic window determined from these experiments can then be used in our Smn C>T mouse model to validate the use of drug therapies targeting splicing correction in the treatment of SMA mice, and in turn SMA patients.

Keywords: Splicing, Spinal Muscular Atrophy, Mouse Model