Response to DNA damage in terminally differentiated muscle cells

Mouse post-mitotic muscle cells accumulate single strand breaks (SSB) after oxidative and alkylation DNA damage but they are resistant to their killing effects. We demonstrate that, upon SSB induction, H2AX phosphorylation occurs in myotubes and is largely ATM-dependent. However, DNA damage signalling cascade downstream of ATM is defective as shown by lack of p53 increase and phosphorylation. The stabilization of p53 by nutlin-3 was ineffective in activating the cell death pathway indicating that the resistance to SSB inducers is due to defective p53 downstream signalling. Conversely, doxorubicin and menadione were able to activate p53 and to kill myotubes. We show that cell killing is p53-dependent although a restriction of p53 activated genes was observed. To gain insights into the cell death pathways active in post-mitotic muscle cells experiments were carried out to characterise autophagy as a cellular stress response. Autophagy was activated during muscle differentiation both in wild type and p53 null cells and it was independent of mTOR inactivation. Whether autophagy is implicated as a pro-survival mechanism in the resistance of post-mitotic cells to different genotoxic insults is currently under investigation.