Human Molecular Genetics Advance Access originally published online on July 28, 2008
Human Molecular Genetics 2008 17(20):3247-3253; doi:10.1093/hmg/ddn220
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Replication independent ATR signalling leads to G2/M arrest requiring Nbs1, 53BP1 and MDC1
1 Genome Damage and Stability Centre, University of Sussex, Brighton, East Sussex BN1 9RQ, UK 2 Molecular Genetics, Benaroya Research Institute, Seattle, WA 98101, USA 3 Department of Biochemistry & Molecular Genetics and Center for Public Health Genomics, University of Virginia, Charlottesville, VA 22908, USA
* To whom correspondence should be addressed. Tel: +44 1273678482; Fax: +44 1273678121; Email: p.a.jeggo{at}sussex.ac.uk
Received July 11, 2008; Revised July 11, 2008; Accepted July 22, 2008
Ataxia telangiectasia and Rad3-related (ATR) is a phosphoinositol-3-kinase like kinase (PIKK) that initiates a signal transduction response to replication fork stalling. Defects in ATR signalling have been reported in several disorders characterized by microcephaly and growth delay. Here, we gain insight into factors influencing the ATR signalling pathway and consider how they can be exploited for diagnostic purposes. Activation of ATR at stalled replication forks leads to intra-S and G2/M phase checkpoint arrest. ATR also phosphorylates 
-H2AX at single-stranded (ss) DNA regions generated during nucleotide excision repair (NER) in non-replicating cells, but the critical analysis of any functional consequence has not been reported. Here, we show that UV irradiation of G2 phase cells causes ATR-dependent but replication-independent G2/M checkpoint arrest. This process requires the Nbs1 N-terminus encompassing the FHA and BRCT domains but not the Nbs1 C-terminus in contrast to ATM-dependent activation of G2/M arrest in response to ionizing radiation. Thus, Nbs1 has a function in ATR signalling in a manner distinct to any role at stalled replication forks. Replication-independent ATR signalling also requires the mediator proteins, 53BP1 and MDC1, providing direct evidence for their role in ATR signalling, but not H2AX. Finally, the process is activated in Cockayne's syndrome but not Xeroderma pigmentosum group A cells providing evidence that ssDNA regions generated during NER are the ATR-pathway-specific activating lesion. Replication-independent G2/M checkpoint arrest represents a suitable assay to specifically identify patients with defective ATR signalling, including Seckel syndrome, Nijmegen breakage syndrome and MCPH-1-dependent primary microcephaly.
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