Treacher Collins syndrome TCOF1 protein cooperates with NBS1 in the DNA damage response

Abstract

The signal transduction pathway of the DNA damage response (DDR) is activated to maintain genomic integrity following DNA damage. The DDR promotes genomic integrity by regulating a large network of cellular activities that range from DNA replication and repair to transcription, RNA splicing, and metabolism. In this study we define an interaction between the DDR factor NBS1 and TCOF1, a nucleolar protein that regulates ribosomal DNA (rDNA) transcription and is mutated in Treacher Collins syndrome. We show that NBS1 relocalizes to nucleoli after DNA damage in a manner dependent on TCOF1 and on casein kinase II and ATM, which are known to modify TCOF1 by phosphorylation. Moreover, we identify a putative ATM phosphorylation site that is required for NBS1 relocalization to nucleoli in response to DNA damage. Last, we report that TCOF1 promotes cellular resistance to DNA damaging agents. Collectively, our findings identify TCOF1 as a DDR factor that could cooperate with ATM and NBS1 to suppress inappropriate rDNA transcription and maintain genomic integrity after DNA damage.T he faithful conservation of genomic information is an essential process for cell survival and for preventing malignant transformation (1). To maintain genomic integrity, DNA has to be protected from damage either spontaneously induced or generated by environmental sources, including ionizing radiation or chemical agents. The DNA damage response (DDR) is a signal transduction network that is activated to maintain genomic integrity after DNA damage (1, 2). A principal component of the DDR is the ATM kinase, which is primarily activated by the presence of DNA double-strand breaks (DSBs).DSBs are deleterious DNA lesions that can lead to cell death if unresolved. DSBs are fixed either by joining the two DNA ends together by nonhomologous end joining (NHEJ) or by homologydirected repair mediated by homologous recombination (HR) (3). The regulation of DSB end-processing represents a key step in the choice between NHEJ and HR. Whereas NHEJ occurs with minimal end-processing, extensive resection of DNA ends and formation of single-stranded DNA regions is required for the initiation of HR (4, 5).NBS1 is a critical component of the heterotrimeric MRE11-RAD50-NBS1 (MRN) complex, which plays a central role in the repair of DSBs through the activation of the DDR and the initiation of HR. After binding and stabilizing DSB ends, the MRN complex recruits ATM and the mediator protein MDC1 to the break site through their direct interaction with NBS1. MDC1 subsequently associates with the phosphorylated histone variant H2AX (γH2AX) locally to amplify the ATM signaling cascade at DSBs (6-9). Direct interaction with NBS1 also promotes the recruitment of the DNA repair factor CtIP to DSB ends by the MRN complex, where it promotes end resection to initiate HR (10). The importance of NBS1 to the maintenance of genomic integrity is further highlighted by the predisposition to growth defects, craniofacial abnormalities, and B-cell lymphomas of patients with Nijmegen breaka...

A localized nucleolar DNA damage response facilitates recruitment of the homology-directed repair machinery independent of cell cycle stage

“…Other identified nucleolar targets include the Pol I transcription termination factor TTF1 and a component of the early pre-rRNA processing machinery, UTP14A. Finally, Treacle-a factor implicated in transcription and rRNA modification and an interaction partner of Nijmegen breakage syndrome protein 1 (NBS1)-is also an ATM target (Ciccia et al 2014;Larsen et al 2014).…”

“…It should be pointed out, however, that similar experiments performed in human cells failed to show any inhibition of Pol I transcription following γ-irradiation (Moore et al 2011). More surprisingly, it has been demonstrated that laser microirradiation-induced DSBs outside of nucleoli lead to an ATM-dependent pan-nuclear silencing of Pol I transcription (Ciccia et al 2014;Larsen et al 2014). The difficulty in interpreting such experiments is that we have no knowledge of the number or distribution of DSBs.…”

BMI1–UBR5 axis regulates transcriptional repression at damaged chromatin

“…ATM is a key regulator for DSB-induced gene silencing in a mechanism involving H2A ubiquitination (15). ATM also represses Pol I-meditated transcription in the nucleolus (16,26,27), by a mechanism involving the interaction between NBS1 and Treacle protein (28,29). DNA-PK also induces gene silencing near DSB lesions (17), and a bromodomain protein ZMYND8 and the NuRD complex induce gene silencing at DSB sites (30).…”

Nucleolar organizer regions: genomic ‘dark matter’ requiring illumination

“…It should be pointed out, however, that similar experiments performed in human cells failed to show any inhibition of Pol I transcription following γ-irradiation (Moore et al 2011). More surprisingly, it has been demonstrated that laser microirradiation-induced DSBs outside of nucleoli lead to an ATMdependent pan-nuclear silencing of Pol I transcription (Ciccia et al 2014;Larsen et al 2014). The difficulty in interpreting such experiments is that we have no knowledge of the number or distribution of irradiation-induced DSBs.…”