Double strand DNA breaks in the genome lead to the activation of the
ataxia-telangiectasia mutated (ATM)
kinase in a process that requires ATM autophosphorylation at serine-1981. ATM autophosphorylation only occurs if ATM is previously acetylated by Tip60. The activated ATM
kinase phosphorylates
proteins involved in arresting the cell cycle, including p53, and in repairing the DNA breaks.
Chloroquine treatment and other manipulations that produce
chromatin defects in the absence of detectable double strand breaks also trigger ATM phosphorylation and the phosphorylation of p53 in primary human fibroblasts, while other downstream substrates of ATM that are involved in the repair of
DNA double strand breaks remain unphosphorylated. This raises the issue of whether ATM is constitutively activated in patients with
genetic diseases that display
chromatin defects. We examined lymphoblastoid cell lines (LCLs) generated from patients with different types of
chromatin disorders: Immunodeficiency, Centromeric instability, Facial anomalies (
ICF) syndrome,
Coffin Lowry syndrome,
Rubinstein Taybi syndrome and Fascioscapulohumeral
Muscular Dystrophy. We show that ATM is phosphorylated on serine-1981 in LCLs derived from ICF patients but not from the other syndromes. The phosphorylated ATM in ICF cells did not phosphorylate the downstream targets NBS1, SMC1 and H2AX, all of which require the presence of double strand breaks. We demonstrate that ICF cells respond normally to ionizing radiation, ruling out the possibility that genetic deficiency in ICF cells renders activated ATM incapable of phosphorylating its downstream substrates. Surprisingly, p53 was also not phosphorylated in ICF cells or in
chloroquine-treated wild type LCLs. In this regard the response to
chromatin-altering agents differs between primary fibroblasts and LCLs. Our findings indicate that although phosphorylation at serine-1981 is essential in the activation of the ATM
kinase, serine-1981 phosphorylation is insufficient to render ATM an active
kinase towards downstream substrates, including p53.