The ATR (ATM (ataxia telangiectasia mutated) and rad3-related) checkpoint kinase is considered critical for signalling DNA replication stress and its dysfunction can lead to the neurodevelopmental disorder, ATR-Seckel syndrome. To understand how ATR functions during neurogenesis, we conditionally deleted Atr broadly throughout the murine nervous system, or in a restricted manner in the dorsal telencephalon. Unexpectedly, in both scenarios, Atr loss impacted neurogenesis relatively late during neural development involving only certain progenitor populations. Whereas the Atr-deficient embryonic cerebellar external germinal layer underwent p53-(and p16 Ink4a/Arf )-independent proliferation arrest, other brain regions suffered apoptosis that was partially p53 dependent. In contrast to other organs, in the nervous system, p53 loss did not worsen the outcome of Atr inactivation. Coincident inactivation of Atm also did not affect the phenotype after Atr deletion, supporting non-overlapping physiological roles for these related DNA damage-response kinases in the brain. Rather than an essential general role in preventing replication stress, our data indicate that ATR functions to monitor genomic integrity in a selective spatiotemporal manner during neurogenesis.

The MRN complex (Mre11/RAD50/NBS1) and ATM (ataxia telangiectasia, mutated) are critical for the cellular response to DNA damage. ATM disruption causes ataxia telangiectasia (A-T), while MRN dysfunction can lead to A-T-like disease (ATLD) or Nijmegen breakage syndrome (NBS). Neuropathology is a hallmark of these diseases, whereby neurodegeneration occurs in A-T and ATLD while microcephaly characterizes NBS. To understand the contrasting neuropathology resulting from Mre11 or Nbs1 hypomorphic mutations, we analyzed neural tissue from Mre11 ATLD1/ATLD1 and Nbs1 DB/DB mice after genotoxic stress. We found a pronounced resistance to DNA damage-induced apoptosis after ionizing radiation or DNA ligase IV (Lig4) loss in the Mre11 ATLD1/ATLD1 nervous system that was associated with defective Atm activation and phosphorylation of its substrates Chk2 and p53. Conversely, DNA damage-induced Atm phosphorylation was defective in Nbs1 DB/DB neural tissue, although apoptosis occurred normally. We also conditionally disrupted Lig4 throughout the nervous system using Nestin-cre (Lig4Nes-Cre ), and while viable, these mice showed pronounced microcephaly and a prominent age-related accumulation of DNA damage throughout the brain. Either Atm À/À or Mre11 ATLD1/ATLD1 genetic backgrounds, but not Nbs1 DB/DB , rescued Lig4 Nes-Cre microcephaly. Thus, DNA damage signaling in the nervous system is different between ATLD and NBS and likely explains their respective neuropathology.[Keywords: NBS; ATLD; A-T; DNA damage; neurodegeneration] Supplemental material is available at http://www.genesdev.org.

DEDICATIONTo my Herbert, whose sturdy character and unwavering dedication to me made it possible to tolerate the sacrifice that the completion of this work required and which will help sustain us both in pursuing whatever endeavors follow.I also dedicate this work to my Mom, who endowed me with will and perseverance, and to my brothers James and Brian who keep me grounded.iv ACKNOWLEDGMENTS