Spinal motor neuron loss occurs through a p53-and-p21-independent mechanism in the Smn2B/- mouse model of spinal muscular atrophy

Exp Neurol. 2021 Mar;337:113587. doi: 10.1016/j.expneurol.2020.113587. Epub 2020 Dec 28.


Spinal muscular atrophy (SMA) is a pediatric neuromuscular disease caused by genetic deficiency of the survival motor neuron (SMN) protein. Pathological hallmarks of SMA are spinal motor neuron loss and skeletal muscle atrophy. The molecular mechanisms that elicit and drive preferential motor neuron degeneration and death in SMA remain unclear. Transcriptomic studies consistently report p53 pathway activation in motor neurons and spinal cord tissue of SMA mice. Recent work has identified p53 as an inducer of spinal motor neuron loss in severe Δ7 SMA mice. Additionally, the cyclin-dependent kinase inhibitor P21 (Cdkn1a), an inducer of cell cycle arrest and mediator of skeletal muscle atrophy, is consistently increased in motor neurons, spinal cords, and other tissues of various SMA models. p21 is a p53 transcriptional target but can be independently induced by cellular stressors. To ascertain whether p53 and p21 signaling pathways mediate spinal motor neuron death in milder SMA mice, and how they affect the overall SMA phenotype, we introduced Trp53 and P21 null alleles onto the Smn2B/- background. We found that p53 and p21 depletion did not modulate the timing or degree of Smn2B/- motor neuron loss as evaluated using electrophysiological and immunohistochemical methods. Moreover, we determined that Trp53 and P21 knockout differentially affected Smn2B/- mouse lifespan: p53 ablation impaired survival while p21 ablation extended survival through Smn-independent mechanisms. These results demonstrate that p53 and p21 are not primary drivers of spinal motor neuron death in Smn2B/- mice, a milder SMA mouse model, as motor neuron loss is not alleviated by their ablation.

Keywords: Motor neuron; Mouse; Smn(2B/−); Spinal muscular atrophy (SMA); Survival motor neuron (SMN); p21 (cdkn1a); p53.

Publication types

  • Research Support, N.I.H., Extramural
  • Research Support, Non-U.S. Gov't

MeSH terms

  • Animals
  • Cell Death
  • Cell Survival
  • Cyclin-Dependent Kinase Inhibitor p21 / genetics*
  • Disease Models, Animal
  • Female
  • Immunohistochemistry
  • Life Expectancy
  • Male
  • Mice
  • Mice, Knockout
  • Motor Neurons / pathology*
  • Muscular Atrophy, Spinal / genetics*
  • Muscular Atrophy, Spinal / pathology*
  • Signal Transduction
  • Spinal Cord / pathology*
  • Survival Analysis
  • Survival of Motor Neuron 2 Protein / genetics*
  • Tumor Suppressor Protein p53 / genetics*


  • Cdkn1a protein, mouse
  • Cyclin-Dependent Kinase Inhibitor p21
  • SMN2 protein, mouse
  • Survival of Motor Neuron 2 Protein
  • Trp53 protein, mouse
  • Tumor Suppressor Protein p53