PP2A, CDK9 transcription checkpoint interplay? Check! Targetable tumorigenesis inhibitor? Check!

RNA polymerase II (RNAPII)-driven transcription is a highly conserved process with distinct checkpoints, each controlled by dedicated transcriptional cyclin-dependent kinases (CDKs). CDK9 controls the pause-release checkpoint following transcription initiation. CDK action is counterbalanced by phosphatases, which control phosphorylation turnover. However, the mechanism remains poorly understood.

In this paper, Vervoort et al establish that protein phosphatase 2A (PP2A) action functionally opposes CDK9-mediated transcription. PP2A is recruited to active transcription sites by the RNAPII-associated Integrator complex subunit INTS6. Employing CRISPR-Cas9 screens and LC-MS/MS (equipped with IonOpticks Aurora Series columns) the authors detail how PP2A, INTS6, and INTS8 together form an auxiliary module (Int-PP2A) of the Integrator complex.

Int-PP2A functional loss affects phosphorylation dynamics, with only limited CDK9 activity then required for effective RNAPII-driven transcription. Loss effectively confers molecular and biological resistance to CDK9 inhibition, allowing unopposed activation of acute transcriptional responses to cellular stress, growth signalling, or inflammatory cues. Reduction of PP2A activity via epigenetic silencing and/or somatic mutation is a feature of several human cancers. Conversely, pharmacological PP2A activation synergizes with CDK9 inhibition to kill both leukemic and solid tumour cells, providing therapeutic benefits in vivo.

Mounting evidence suggests loss or downregulation of INTS6 is also significant – it is frequently deleted in human cancers and correlated with poor prognosis. Acute depletion of INTS6 (or INTS8) has multiple effects: resistance to tumour cell death mediated by CDK9 inhibition, decreased CDK9 phospho-substrate turnover, and amplification of acute oncogenic transcriptional responses.

Antagonism of CDK9 kinase activity by Int-PP2A at the pause-release checkpoint is thus critical to the fine-tuning of gene expression, relying as it does on the balance between kinase and phosphatase activity throughout the transcription cycle. Because this process is dysregulated in cancer, Int-PP2A can be exploited as therapeutically targetable.

Read the full paper
The PP2A-Integrator-CDK9 axis fine-tunes transcription and can be targeted therapeutically in cancer.
Cell 184, 3143–3162; 17 May 2021. doi: https://doi.org/10.1016/j.cell.2021.04.022
Stephin J. Vervoort, Sarah A. Welsh, Jennifer R. Devlin, Elisa Barbieri, Deborah A. Knight, Sarah Offley, Stefan Bjelosevic, Matteo Costacurta, Izabela Todorovski, Conor J. Kearney, Jarrod J. Sandow, Zheng Fan, Benjamin Blyth, Victoria McLeod, Joseph H.A. Vissers, Karolina Pavic, Ben P. Martin, Gareth Gregory, Elena Demosthenous, Magnus Zethoven, Isabella Y. Kong, Edwin D. Hawkins, Simon J. Hogg, Madison J. Kelly, Andrea Newbold, Kaylene J. Simpson, Otto Kauko, Kieran F. Harvey, Michael Ohlmeyer, Jukka Westermarck, Nathanael Gray, Alessandro Gardini, and Ricky W. Johnstone

Commentary by Jarrod Sandow, PhD.

About the author
Jarrod has a background in biotechnology and completed his PhD at the Institute of Medical and Veterinary Science in Adelaide. He is a co-inventor of IonOpticks’ core technology and is driven towards developing innovative solutions for the global proteomics research community that will enable scientists and clinicians to discover more from their samples to accelerate advances in biological and medical research.