Decline of DNA damage response along with myogenic differentiation

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Abstract
DNA integrity is incessantly confronted to endogenous and exogenous agents inducing DNA lesions, which are harmful for cellular homeostasis. Luckily all organisms are equipped with a network of DNA damage response (DDR) mechanisms that will repair DNA lesions and restore the proper cellular activities. Despite DNA repair mechanisms have been revealed in vitro and in replicating cells, still little is known on how DNA lesions are repaired and consequently how cellular homeostasis is maintained in post-mitotic cells. Muscle fibers are highly specialised post-mitotic cells organized in syncytia and, they are vulnerable to age-related degeneration and atrophy following radiotherapy treatment. We have here studied in detail the DNA repair capacity of muscle fibers nuclei and compared it with the one measured in proliferative myoblasts. We focused on the DNA repair mechanisms that correct ionizing radiation (IR)-induced lesions, namely the base excision repair (BER), the non-homologous end joining (NHEJ) and the homologous recombination (HR). We found that in the most differentiated myogenic cells, myotubes, all of these DNA repair mechanisms present weakened kinetics of recruitment of DNA repair proteins to IR-damaged DNA. For BER and HR, this decline can be linked to reduced steady state levels of key proteins involved in these processes, probably since nuclei within muscle fibers no longer replicate their DNA. SIGNIFICANCE: Although skeletal muscle tissue has been considered as resistant to injury caused by ionizing radiations (IR), in the long term, muscle surrounding irradiated regions of the body following radiotherapy has physiological consequences such as atrophy and muscle wasting. To prevent and/or counteract the harmful effects of IR in muscle fibers, it is essential, to evaluate the DNA repair capacity of post-mitotic myonuclei. By using fluorescently tagged DNA repair proteins expressed in myoblasts and myotubes, we were able to assess the kinetics of the DNA damage response in these cells and determine that DNA damage response is weakened during the process of myofibrillogenesis.
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