MST2 kinase suppresses rDNA transcription in response to DNA damage by phosphorylating nucleolar histone H2B

Abstract

The heavily transcribed rDNA repeats that give rise to the ribosomal RNA are clustered in a unique chromatin structure, the nucleolus. Due to its highly repetitive nature and transcriptional activity, the nucleolus is considered a hotspot of genomic instability. Breaks in rDNA induce a transient transcriptional shut down to conserve energy and promote rDNA repair; however, how nucleolar chromatin is modified and impacts on rDNA repair is unknown. Here, we uncover that phosphorylation of serine 14 on histone H2B marks transcriptionally inactive nucleolar chromatin in response to DNA damage. We identified that the MST2 kinase localises at the nucleoli and targets phosphorylation of H2BS14p in an ATM‐dependent manner. We show that establishment of H2BS14p is necessary for damage‐induced rDNA transcriptional shut down and maintenance of genomic integrity. Ablation of MST2 kinase, or upstream activators, results in defective establishment of nucleolar H2BS14p, perturbed DNA damage repair, sensitisation to rDNA damage and increased cell lethality. We highlight the impact of chromatin regulation in the rDNA damage response and targeting of the nucleolus as an emerging cancer therapeutic approach.

Treacle controls the nucleolar response to rDNA breaks via TOPBP1 recruitment and ATR activation

“…ATM-dependent inhibition of RNA polymerase II transcription in response to DSBs has been shown to involve local histone H2A ubiquitylation and the recruitment of a specific chromatin remodeler as well as the cohesion complex 10,11,14 . Posttranslational histone modifications also seem to be key events in Pol I inhibition because ATM-dependent H2B S14 phosphorylation by MST2 kinase is required for transcriptional repression in the nucleoli 19 . Direct modification of the Pol I transcription machinery by ATM and ATR may also be involved as several key Pol I transcription factors appear to be targeted by PIKKs 49 .…”

“…We next aimed to establish the physiological significance of TOPBP1/ATRmediated transcriptional inhibition and nucleolar segregation. Previous studies suggested that these events are required for efficient repair of rDNA breaks [17][18][19] . We therefore studied recruitment of the DSB repair machinery in response to rDNA breaks in Treacle-and TOPBP1-deficient cells.…”

“…The induction of breaks in the rDNA repeats triggers rapid ATM-dependent shutdown of Pol I transcription in the nucleoli 9 . Transcriptional repression is accompanied by a large-scale structural alteration of the nucleoli whereby the rDNA repeats along with associated proteins translocate from inside the nucleoli into the nucleolar periphery, where they accumulate in focal structures that are called nucleolar caps [16][17][18][19] . These structural changes-often referred to as nucleolar segregation-are also induced by inhibition of Pol I transcription by small-molecule inhibitors, thus suggesting that transcriptional repression is the underlying cause of nucleolar segregation 20 .…”

“…These structural changes-often referred to as nucleolar segregation-are also induced by inhibition of Pol I transcription by small-molecule inhibitors, thus suggesting that transcriptional repression is the underlying cause of nucleolar segregation 20 . While nucleolar segregation has been proposed to facilitate the repair of rDNA breaks 16,17,19 , the mechanism by which ATM signaling suppresses rRNA transcription and the functional implication of transcriptional repression and nucleolar segregation have not yet been studied in detail.…”

FGFR4 phosphorylates MST1 to confer breast cancer cells resistance to MST1/2-dependent apoptosis

“…Upon apoptosis, caspase-3-mediated cleavage removes the C-terminal MST1/2 regulatory and nuclear export signals, triggering nuclear translocation of the active N-terminal kinase [20,21]. Although MST1/2 activity has been implicated in histone phosphorylation and chromatin condensation, functional contribution of endogenous MST1/2 to apoptosis, downstream of caspase-3, remains unclear [19,21,61,62]. Here, we show that in FGFR4 + /HER2 + breast cancer cells, particularly under tumor-mimicking 3D conditions, FGFR4 depletion-induced MST1/2 autophosphorylation coupled with MST1 cleavage and nuclear localization without additional stimuli.…”