Expression of ATM is associated with both good and poor responses to DNA-damaging chemotherapy, so Hai Jiang, H. Christian Reinhardt and colleagues used short hairpin RNAs to knock down the expression of ATM in HrasV12-immortalized mouse embryonic fibroblasts (MEFs) that had an intact or a defective p53 pathway to assess sensitivity to chemotherapy. MEFs that lacked both an intact p53 pathway and ATM had a higher rate of apoptosis in response to cisplatin or doxorubicin than MEFs that lacked ATM but had a functional p53 pathway. Similar results were obtained when the ATM substrate checkpoint kinase 2 (
Chek2
) was knocked down in MEFs and when Atm was knocked down in E μ –Myc;Trp53−/− and E μ –Myc;Trp53+/+ mouse models of lymphoma. What is the molecular basis for this differential response? The authors found that in cells with an intact p53 pathway the loss of ATM prevented p53-mediated transcription of the pro-apoptotic genes
Puma
and
Noxa
but did not affect the transcription of
Cdkn1a
, which encodes the cyclin-dependent kinase inhibitor p21. MEFs that lacked Trp53 but expressed ATM went into cell cycle arrest in response to DNA damage, but underwent 'mitotic catastrophe' if both ATM and p53 were absent. The authors suggest that, in cells with intact p53, ATM is required for an effective induction of apoptosis in response to DNA damage but ATM is not required for p53-mediated cell cycle arrest. In the absence of p53, ATM can induce a G2 arrest in response to DNA damage, but this checkpoint is lost in p53- and ATM-deficient cells, allowing the cells to enter mitosis with damaged DNA, resulting in cell death.
Previous publications have indicated that the loss of ATM affects homologous recombination-mediated double-strand break repair but has less of an effect on the non-homologous end joining (NHEJ) repair pathway, indicating that cells without ATM are reliant on NHEJ. DNA protein kinase catalytic subunit (DNA-PKcs) is required for efficient NHEJ, and the authors were able to show that inhibition of DNA-PKcs in ATM-deficient but p53-wild-type MEFs induced sensitivity to doxorubicin.
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