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Spinal muscular atrophy (SMA) is caused by mutations or deletions in the  Survival Motor Neuron 1  ( SMN1 ) gene in humans. Modifiers of the SMA symptoms have been identified and genetic background has a substantial effect in the phenotype and survival of the severe mouse model of SMA. Previously, we generated the less severe  Smn 2B/-   mice on a mixed genetic background. To assess the phenotype of Smn deficiency on a pure genetic background, we produced  Smn 2B/2B   congenic mice on either the C57BL/6 (BL6) or FVB strain background and characterized them at the 6 th  generation by breeding to  Smn +/-   mice.  Smn 2B/-   mice from these crosses were evaluated for growth, survival, muscle atrophy, motor neuron loss, motor behaviour, and neuromuscular junction pathology. FVB  Smn 2B/-   mice had a shorter life span than BL6  Smn 2B/-   mice (median of 19 days  vs.  25 days). Similarly, all other defects assessed occurred at earlier stages in FVB  Smn 2B/-  mice when compared to BL6  Smn 2B/-  mice. However, there were no differences in Smn protein levels in the spinal cords of these mice. Interestingly, levels of Plastin 3, a putative modifier of SMA, were significantly induced in spinal cords of BL6  Smn 2B/-   mice but not of FVB  Smn 2B/-  mice. Our studies demonstrate that the phenotype in  Smn 2B/-  mice is more severe in the FVB background than in the BL6 background, which could potentially be explained by the differential induction of genetic modifiers.

Effect of genetic background on the phenotype of the Smn 2B/- mouse model of spinal muscular atrophy