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Background: CDK12 regulates the expression of multiple genes in the DNA damage response (DDR) pathway by phosphorylating the C-terminal domain of RNA polymerase II (CTD pSer2), and it is a potential therapeutic target to exacerbate DNA damage and cell death in DDR-addicted, HER2-amplified, MYC-overexpressing, or EWS-FLI1-positive cancers. PARP inhibitor resistance is often driven by dysregulation of DDR signaling, and CDK12 inhibition may induce a state of DDR deficiency and increase the effectiveness of PARP inhibition. Materials and methods: Biochemical activity was assayed by ADP-glo (Promega), cellular proliferation was quantified by CellTiter-Glo (Promega), and CTD pSer2 was quantified by MSD immunoassay (Meso Scale Discovery). Kinase panels and in vivo studies are described in figure legends. Results: OKI-1546 has a biochemical IC50 of 15 nM against CDK12 and is highly selective for CDK12 and CDK13 when compared to other closely related CDKs; it is 649-, 81-, 11-, and 64-fold selective over CDKs 1, 2, 7, and 9, respectively. This selectivity was further confirmed via testing OKI-1546 against an affinity-based kinase panel. OKI-1546 potently inhibited proliferation of multiple cancer cell lines, with IC50 values that correlated with 50% inhibition of CTD pSer2. In CD-1 mice, OKI-1546 showed 77% oral bioavailability (%F) with a half-life of 2.7 hrs, and SD rats had 81 %F. Furthermore, a single 10 mg/kg dose reached plasma concentrations that surpassed the plasma protein binding-adjusted IC50 of CTD pSer2, which was confirmed in tumor-bearing mice showing 60% CTD pSer2 inhibition in tumors 4 hours after a single oral dose. That same dose translated into 119% tumor growth inhibition following 21 days of daily administration in the BRCA1 wild-type MDA-MB-231 tumor model and 121% tumor growth inhibition following 21 days of daily administration in the BRCA1-mutant MDA-MB-436 tumor model in mice. Four different PARP inhibitors synergistically combined with OKI-1546 to inhibit cell growth in multiple in vitro cancer models. Conclusion: OKI-1546 is a novel, covalent inhibitor of CDK12 and CDK13 that is potent, specific, orally available, and shows in vivo target engagement and tumor growth inhibition in preclinical cancer models. OKI-1546 showed activity in vivo as a single agent and cellular data suggest that inhibition of CDK12 has the potential to enhance the effects of other DDR-targeting therapeutic agents. Citation Format: Alexander A. Strait, David A. Mareska, Jyothi Mahadevan, Christie A. Schulte, Yevgeniy Izrayelit, Mark L. Boys, Richard Woessner. OKI-1546 is a selective, orally bioavailable inhibitor of CDK12 that causes tumor regression in multiple preclinical cancer models [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 1635.

Summary/AbstractPARP1 contributes to genome architecture and DNA damage repair through its dynamic association with chromatin. PARP1 and PARP2 (PARP1/2) recognize damaged DNA and recruit the DNA repair machinery. Using single molecule microscopy in live cells, we monitored the movement of PARP1/2 on undamaged and damaged chromatin. We identify two classes of freely diffusing PARP1/2 and two classes of bound PARP1/2. The majority (> 60%) of PARP1/2 diffuse freely in both undamaged and damaged nuclei and in the presence of inhibitors of PARP1/2 used for cancer therapy (PARPi). Laser induced DNA damage results in a small fraction of slowly diffusing PARP1 and PARP2 to become transiently bound. Treatment of cells with PARPi in the presence of DNA damage causes subtle changes in the dynamics of bound PARP1/2, in contrast to bulk studies that suggest PARP trapping. Our results imply that next-generation PARPi could specifically target the small fraction of DNA-bound PARP1/2.

Poly(ADP-ribose) polymerase 1 (PARP1) is an important first responder in the mechanism of DNA repair in eukaryotic cells. It is also a validated drug target, with four different PARP inhibitors (PARPi) approved for the treatment of BRCA-negative cancers. Despite past efforts, many aspects of PARPi are poorly understood, in particular their ability to trap PARP1 on chromatin and the relationships between their potencies, cellular toxicities, and trapping efficiencies. Because PARP trapping is widely believed to originate in allosteric coupling between DNA binding and the catalytic site, we further investigated the binding properties of PARP1 to a model for DNA with a double-strand break in the presence and absence of PARPi. Specifically, we have used sequential mixing stopped-flow spectroscopy to identify a slow conformational change that follows rapid DNA binding. Using a range of DNA concentrations and different mutants of PARP1 we demonstrate that this conformational change is one of the steps of the "monkey bar mechanism" that promotes DNA-dependent dissociation of DNA. This conformational change also corresponds to the previously identified conformational change associated with DNA-dependent activation of PARP1. Despite linking the conformational change associated with DNA binding and release to DNA activation, we find no evidence for PARPi perturbing this allosteric coupling.

The repair of DNA damage requires the ordered recruitment of many different proteins that are responsible for signaling and subsequent repair. A powerful and widely used tool for studying the orchestrated accumulation of these proteins at damage sites is laser microirradiation in live cells, followed by monitoring the accumulation of the fluorescently labeled protein in question. Despite the widespread use of this approach, there exists no rigorous method for characterizing the recruitment process quantitatively. Here, we introduce a diffusion model that explicitly accounts for the unique sizes and shapes of individual nuclei and uses two variables: D(eff), the effective coefficient of diffusion, and F, the fraction of mobile protein that accumulates at sites of DNA damage. Our model quantitatively describes the accumulation of three test proteins, poly-ADP-ribose polymerases 1 and 2 (PARP1/2) and histone PARylation factor 1. D(eff) for PARP1, as derived by our approach, is 6× greater than for PARP2 and in agreement with previous literature reports using fluorescence correlation spectroscopy and fluorescence recovery after photobleaching. Our data indicate that histone PARylation factor 1 arrives at sites of DNA damage independently of either PARP. Importantly, our model, which can be applied to existing data, allows for the direct comparison of the coefficient of diffusion for any DNA repair protein between different cell types, obtained in different laboratories and by different methods, and also allows for the interrogation of cell-to-cell variability.

Cells are exposed to a plethora of influences that can cause damage to DNA and alter the genome, often with detrimental consequences for health. Cells mitigate this damage through a variety of repair protein pathways, and accurate measurement of the accumulation, action, and dissipation timescales of these repair proteins is required to fully understand the DNA damage response. Recently, we described the Q-FADD (Quantitation of Fluorescence Accumulation after DNA Damage) method, which enhances the analytical power of the widely used laser microirradiation technique. In that study, Q-FADD and its preprocessing operations required licensed software and a significant amount of user overhead to find the model of best fit. Here, we present “qFADD.py”, an open-source implementation of the Q-FADD algorithm that is available as both a stand-alone software package and on a publicly accessible webserver (https://qfadd.colorado.edu/). Furthermore, we describe significant improvements to the fitting and preprocessing methods that include corrections for nuclear drift and an automated grid-search for the model of best fit. To improve statistical rigor, the grid-search algorithm also includes automated simulation of replicates. As an example, we discuss the recruitment dynamics of the signaling protein PARP1 to DNA damage sites, and we show how to compare different populations of qFADD.py models.Statement of SignificanceCells are constantly bombarded by factors that can alter or damage their genome, and they have evolved a variety of proteins that can identify and fix this damage. To fully understand how these proteins interact in repair pathways, we need robust methods to quantify the timescales between the initial identification of the DNA damage event and the subsequent protein-protein interactions that lead to repair. Laser microirradiation is a popular method for studying these repair protein cascades in vivo, and methods for quantifying the timescales of recruitment in these experiments have historically been simple to implement but lacking in physical interpretation. Here, we present qFADD.py, the next iteration of the Q-FADD method, which uses Monte Carlo diffusion models to interpret repair protein recruitment timescales to sites of DNA damage. By moving towards automated fitting procedures with minimal bias from the user, qFADD.py provides a statistically robust but low-effort means to analyze laser microirradiation experiments through a biophysical framework.

Poly(ADP-ribose) polymerase 1 (PARP1) is a key player in the response to DNA damage and is the target of four different clinical inhibitors used for the treatment of cancers. Binding of PARP1 to damaged DNA leads to activation via destabilization of a subdomain of the catalytic domain, and activated PARP1 utilizes NAD+ to add chains of poly(ADP-ribose) onto itself and other nuclear proteins. PARP1 also binds abundantly to intact DNA and chromatin, where it remains enzymatically inactive. Here we show that intact DNA makes contacts with the BRCT domain, which was not previously recognized as a DNA-binding domain. This binding mode does not result in the concomitant reorganization and activation of the catalytic domain. We visualize the BRCT domain bound to nucleosomal DNA by cryogenic electron microscopy (cryoEM) and identify a key motif conserved from ancestral BRCT domains for binding phosphates on DNA and phospho-peptides. Finally, we demonstrate that the DNA-binding properties of the BRCT domain contribute to the ‘monkey-bar mechanism’ that mediates transfer of PARP1 from one DNA segment to another.

Mammalian CXXC finger protein 1 (Cfp1) is a DNA-binding protein that is a component of the Setd1 histone methyltransferase complexes and is a critical epigenetic regulator of both histone and cytosine methylation. Murine embryonic stem (ES) cells lacking Cfp1 exhibit a loss of histone H3-Lys4 tri-methylation (H3K4me3) at many CpG islands, and a mis-localization of this epigenetic mark to heterochromatic sub-nuclear domains. Furthermore, these cells fail to undergo cellular differentiation in vitro. These defects are rescued upon introduction of a Cfp1-expression vector. Cfp1 contains an N-terminal plant homeodomain (PHD), a motif frequently observed in chromatin associated proteins that functions as a reader module of histone marks. Here, we report that the Cfp1 PHD domain directly and specifically binds to histone H3K4me1/me2/me3 marks. Introduction of individual mutations at key Cfp1 PHD residues (Y28, D44, or W49) ablates this histone interaction both in vitro and in vivo. The W49A point mutation does not affect the ability of Cfp1 to rescue appropriate restriction of histone H3K4me3 to euchromatic sub-nuclear domains or in vitro cellular differentiation in Cfp1-null ES cells. Similarly, a mutated form of Cfp1 that lacks DNA-binding activity (C169A) rescues in vitro cellular differentiation. However, rescue of Cfp1-null ES cells with a double mutant form of Cfp1 (W49A, C169A) results in partially defective in vitro differentiation. These data define the Cfp1 PHD domain as a reader of histone H3K4me marks and provide evidence that this activity is involved in the regulation of lineage commitment in ES cells.

All organisms must protect their genome from constantly occurring DNA damage. To this end, cells have evolved complex pathways for repairing sites of DNA lesions, and multiple in vitro and in vivo techniques have been developed to study these processes. In this review, we discuss the commonly used laser microirradiation method for monitoring the accumulation of repair proteins at DNA damage sites in cells, and we outline several strategies for deriving kinetic models from such experimental data. We discuss an example of how in vitro measurements and in vivo microirradation experiments complement each other to provide insight into the mechanism of PARP1 recruitment to DNA lesions. We also discuss a strategy to combine data obtained for the recruitment of many different proteins in a move toward fully quantitating the spatiotemporal relationships between various damage responses, and we outline potential venues for future development in the field.

The repair of DNA damage requires the ordered recruitment of many different proteins that are responsible for signaling and subsequent repair. A powerful tool for studying the orchestrated accumulation of these proteins at damage sites is laser microirradiation in live cells, followed by monitoring of the accumulation of the fluorescently labeled protein in question. Despite the widespread use of this approach, there exists no rigorous method for characterizing this process quantitatively. Here we introduce a free diffusion model that explicitly accounts for the unique topology of individual nuclei and quantitatively describes the accumulation of two test proteins, poly-ADP-ribose polymerases 1 and 2. Application of our model to other proteins will yield novel insights into the timing and mechanism of DNA repair.