Proteolysis-targeting chimeras with reduced off-targets.

Proteolysis-targeting chimeras (PROTACs) are molecules that induce proximity between target proteins and E3 ligases triggering target protein degradation. Pomalidomide, a widely used E3 ligase recruiter in PROTACs, can independently degrade other proteins, including zinc-finger (ZF) proteins, with vital roles in health and disease. This off-target degradation hampers the therapeutic applicability of pomalidomide-based PROTACs, requiring development of PROTAC design rules that minimize off-target degradation. Here we developed a high-throughput platform that interrogates off-target degradation and found that reported pomalidomide-based PROTACs induce degradation of several ZF proteins. We generated a library of pomalidomide analogues to understand how functionalizing different positions of the phthalimide ring, hydrogen bonding, and steric and hydrophobic effects impact ZF protein degradation. Modifications of appropriate size on the C5 position reduced off-target ZF degradation, which we validated through target engagement and proteomics studies. By applying these design principles, we developed anaplastic lymphoma kinase oncoprotein-targeting PROTACs with enhanced potency and minimal off-target degradation.

Development and Applications of Chimera Platforms for Tyrosine Phosphorylation.

Chimeric small molecules that induce post-translational modification (PTM) on a target protein by bringing it into proximity to a PTM-inducing enzyme are furnishing novel modalities to perturb protein function. Despite recent advances, such molecules are unavailable for a critical PTM, tyrosine phosphorylation. Furthermore, the contemporary design paradigm of chimeric molecules, formed by joining a noninhibitory binder of the PTM-inducing enzyme with the binder of the target protein, prohibits the recruitment of most PTM-inducing enzymes as their noninhibitory binders are unavailable. Here, we report two platforms to generate phosphorylation-inducing chimeric small molecules (PHICS) for tyrosine phosphorylation. We generate PHICS from both noninhibitory binders (scantily available, platform 1) and kinase inhibitors (abundantly available, platform 2) using cysteine-based group transfer chemistry. PHICS triggered phosphorylation on tyrosine residues in diverse sequence contexts and target proteins (e.g., membrane-associated, cytosolic) and displayed multiple bioactivities, including the initiation of a growth receptor signaling cascade and the death of drug-resistant cancer cells. These studies provide an approach to induce biologically relevant PTM and lay the foundation for pharmacologic PTM editing (i.e., induction or removal) of target proteins using abundantly available inhibitors of PTM-inducing or -erasing enzymes.

Proximity-inducing modalities: the past, present, and future.

Living systems use proximity to regulate biochemical processes. Inspired by this phenomenon, bifunctional modalities that induce proximity have been developed to redirect cellular processes. An emerging example of this class is molecules that induce ubiquitin-dependent proteasomal degradation of a protein of interest, and their initial development sparked a flurry of discovery for other bifunctional modalities. Recent advances in this area include modalities that can change protein phosphorylation, glycosylation, and acetylation states, modulate gene expression, and recruit components of the immune system. In this review, we highlight bifunctional modalities that perform functions other than degradation and have great potential to revolutionize disease treatment, while also serving as important tools in basic research to explore new aspects of biology.

A general approach to identify cell-permeable and synthetic anti-CRISPR small molecules.

The need to control the activity and fidelity of CRISPR-associated nucleases has resulted in a demand for inhibitory anti-CRISPR molecules. The small-molecule inhibitor discovery platforms available at present are not generalizable to multiple nuclease classes, only target the initial step in the catalytic activity and require high concentrations of nuclease, resulting in inhibitors with suboptimal attributes, including poor potency. Here we report a high-throughput discovery pipeline consisting of a fluorescence resonance energy transfer-based assay that is generalizable to contemporary and emerging nucleases, operates at low nuclease concentrations and targets all catalytic steps. We applied this pipeline to identify BRD7586, a cell-permeable small-molecule inhibitor of SpCas9 that is twofold more potent than other inhibitors identified to date. Furthermore, unlike the reported inhibitors, BRD7586 enhanced SpCas9 specificity and its activity was independent of the genomic loci, DNA-repair pathway or mode of nuclease delivery. Overall, these studies describe a general pipeline to identify inhibitors of contemporary and emerging CRISPR-associated nucleases.

Synthetic Reprogramming of Kinases Expands Cellular Activities of Proteins.

Phosphorylation-inducing chimeric small molecules (PHICS) can enable a kinase to act at a new cellular location or phosphorylate non-native substrates (neo-substrates)/ sites (neo-phosphorylations).[1, 2] We report a modular design and high-yielding synthesis of such PHICS that endowed multiple new activities to protein kinase C (PKC). For example, while PKC is unable to downregulate the activity of a gain-of-function variant (S180A) of Bruton's tyrosine kinase that evokes B cell malignancy phenotype, PHICS enabled PKC to induce inhibitory neo-phosphorylations on this variant. Furthermore, while PKC typically phosphorylates its membrane-associated substrates, PKC with PHICS phosphorylated multiple cytosol-based neo-substrates (e.g., BCR-ABL). Finally, a PHICS for BCR-ABL induced death of chronic myeloid leukemia cell lines. These studies show the power of synthetic chemistry to expand the chemical and functional diversity of proteins in cells using bifunctional molecules.

Chemogenetic System Demonstrates That Cas9 Longevity Impacts Genome Editing Outcomes.

Prolonged Cas9 activity can hinder genome engineering as it causes off-target effects, genotoxicity, heterogeneous genome-editing outcomes, immunogenicity, and mosaicism in embryonic editing-issues which could be addressed by controlling the longevity of Cas9. Though some temporal controls of Cas9 activity have been developed, only cumbersome systems exist for modifying the lifetime. Here, we have developed a chemogenetic system that brings Cas9 in proximity to a ubiquitin ligase, enabling rapid ubiquitination and degradation of Cas9 by the proteasome. Despite the large size of Cas9, we were able to demonstrate efficient degradation in cells from multiple species. Furthermore, by controlling the Cas9 lifetime, we were able to bias the DNA repair pathways and the genotypic outcome for both templated and nontemplated genome editing. Finally, we were able to dosably control the Cas9 activity and specificity to ameliorate the off-target effects. The ability of this system to change the Cas9 lifetime and, therefore, bias repair pathways and specificity in the desired direction allows precision control of the genome editing outcome.

Phosphorylation-Inducing Chimeric Small Molecules.

Small molecules have been classically developed to inhibit enzyme activity; however, new classes of small molecules that endow new functions to enzymes via proximity-mediated effect are emerging. Phosphorylation (native or neo) of any given protein-of-interest can alter its structure and function, and we hypothesized that such modifications can be accomplished by small molecules that bring a kinase in proximity to the protein-of-interest. Herein, we describe phosphorylation-inducing chimeric small molecules (PHICS), which enable two example kinases-AMPK and PKC-to phosphorylate target proteins that are not otherwise substrates for these kinases. PHICS are formed by linking small-molecule binders of the kinase and the target protein, and exhibit several features of a bifunctional molecule, including the hook-effect, turnover, isoform specificity, dose and temporal control of phosphorylation, and activity dependent on proximity (i.e., linker length). Using PHICS, we were able to induce native and neo-phosphorylations of BRD4 by AMPK or PKC. Furthermore, PHICS induced a signaling-relevant phosphorylation of the target protein Bruton's tyrosine kinase in cells. We envision that PHICS-mediated native or neo-phosphorylations will find utility in basic research and medicine.