Ataxia telangiectasia mutated-dependent apoptosis after genotoxic stress in the developing nervous system is determined by cellular differentiation status

Ataxia-telangiectasia (A-T) is a neurodegenerative syndrome resulting from dysfunction of ATM (ataxia telangiectasia mutated). The molecular details of ATM function in the nervous system are unclear, although the neurological lesions in A-T are probably developmental because they appear during childhood. The nervous systems of Atm-null mice show a pronounced defect in apoptosis that is induced by DNA damage, suggesting that ATM may function to eliminate DNA-damaged neurons. Here we show that Atm-dependent apoptosis occurs at discrete stages of neurogenesis. Analysis of gamma-irradiated mouse embryos showed that Atm-dependent apoptosis occurred only in the postmitotic populations that were present in the neuroepithelial subventricular zone of the developing nervous system. Notably, Atm deficiency did not prevent radiation-induced apoptosis in multipotent precursor cells residing in the proliferating ventricular zone. Atm-dependent apoptosis required p53 and coincided with the specific phosphorylation of p53 and caspase-3 activation. Thus, these data show that Atm functions early in neurogenesis and underscore the selective requirement for Atm in eliminating damaged postmitotic neural cells. Furthermore, these data demonstrate that the differentiation status of neural cells is a critical determinant in the activation of certain apoptotic pathways.

Atm and Bax cooperate in ionizing radiation-induced apoptosis in the central nervous system

Ataxia-telangiectasia is a hereditary multisystemic disease resulting from mutations of ataxia telangiectasia, mutated (ATM) and is characterized by neurodegeneration, cancer, immune defects, and hypersensitivity to ionizing radiation. The molecular details of ATM function in the nervous system are unclear, although the neurological lesion in ataxia-telangiectasia becomes apparent early in life, suggesting a developmental origin. The central nervous system (CNS) of Atm-null mice shows a pronounced defect in apoptosis induced by genotoxic stress, suggesting ATM functions to eliminate neurons with excessive genomic damage. Here, we report that the death effector Bax is required for a large proportion of Atm-dependent apoptosis in the developing CNS after ionizing radiation (IR). Although many of the same regions of the CNS in both Bax-/- and Atm-/- mice were radioresistant, mice nullizygous for both Bax and Atm showed additional reduction in IR-induced apoptosis in the CNS. Therefore, although the major IR-induced apoptotic pathway in the CNS requires Atm and Bax, a p53-dependent collateral pathway exists that has both Atm- and Bax-independent branches. Further, Atm- and Bax-dependent apoptosis in the CNS also required caspase-3 activation. These data implicate Bax and caspase-3 as death effectors in neurodegenerative pathways.

Ataxia telangiectasia mutated deficiency affects astrocyte growth but not radiosensitivity

The cancer-prone neurodegenerative disorder, ataxia telangiectasia (A-T), results from mutations of ATM (ataxia telangiectasia mutated). Individuals with A-T are also hypersensitive to ionizing radiation (IR). Cultured cells from A-T individuals or Atm-/- mice have cell cycle and growth defects and are generally considered radiosensitive. However, it has been shown recently that cell populations in the Atm-/- central nervous system are radioresistant. To define specific IR sensitivities of neural populations, we analyzed Atm-/- astrocytes. Here we show that Atm-/- astrocytes exhibit premature senescence, express constitutively high levels of p21, and have impaired p53 stabilization. However, in contrast to radiosensitive Atm-/- fibroblasts and radioresistant Atm-/- neurons, survival of Atm-/- astrocytes after IR was similar to wild-type astrocytes. Additionally, p53-null astrocytes, but not fibroblasts, were moderately more radioresistant than their wild-type counterparts, suggesting that the deficit in p53 stabilization observed in Atm-null cells is not a measure of radiation susceptibility. Thus, in astrocytes, the function of Atm in cellular growth and radiosensitivity is distinct. These data may have implications for ATM disruption strategies as a radiosensitizing treatment for brain tumors.

Loss of the ARF tumor suppressor reverses premature replicative arrest but not radiation hypersensitivity arising from disabled atm function

The alternative reading frame product (p19ARF) of the mouse INK4a/ARF locus is induced by oncoproteins such as Myc and E1A as part of a checkpoint response that limits cell cycle progression in response to hyperproliferative signals. ARF binds directly to Mdm2 to prevent down-regulation of p53 and thereby promotes p53-dependent transcription and cell cycle arrest. However, ARF is not required for p53 induction in response to ionizing radiation or other forms of DNA damage. Animals lacking a functional ataxia telangiectasia (Atm) gene are exquisitely sensitive to ionizing radiation; Atm-null mouse embryo fibroblasts (MEFs) undergo premature replicative arrest, which is relieved by the loss of p53. Here we show that the loss of ARF expands the life expectancy of Atm-null MEFs, but alters neither the sensitivity of Atm-null mice to ionizing radiation nor their propensity to develop lymphomas early in life. Therefore, whereas ARF and Atm signal to p53 through distinct pathways, the loss of ARF can modify p53-dependent features of the Atm-null phenotype.

Requirement for Atm in ionizing radiation-induced cell death in the developing central nervous system

Ataxia telangiectasia (AT) is characterized by progressive neurodegeneration that results from mutation of the ATM gene. However, neither the normal function of ATM in the nervous system nor the biological basis of the degeneration in AT is known. Resistance to apoptosis in the developing central nervous system (CNS) of Atm-/- mice was observed after ionizing radiation. This lack of death occurred in diverse regions of the CNS, including the cerebellum, which is markedly affected in AT. In wild-type, but not Atm-/- mice, up-regulation of p53 coincided with cell death, suggesting that Atm-dependent apoptosis in the CNS is mediated by p53. Further, p53 null mice showed a similar lack of radiation-induced cell death in the developing nervous system. Atm may function at a developmental survival checkpoint that serves to eliminate neurons with excessive DNA damage.