PREX2 (phosphatidylinositol-3,4,5-triphosphate-dependent Rac-exchange factor 2) is a PTEN (phosphatase and tensin homolog deleted on chromosome 10) binding protein that is significantly mutated in cutaneous melanoma and pancreatic ductal adenocarcinoma. Here, genetic and biochemical analyses were conducted to elucidate the nature and mechanistic basis of PREX2 mutation in melanoma development. By generating an inducible transgenic mouse model we showed an oncogenic role for a truncating PREX2 mutation (PREX2^E824*) in vivo in the context of mutant NRAS. Using integrative cross-species gene expression analysis, we identified deregulated cell cycle and cytoskeleton organization as significantly perturbed biological pathways in PREX2 mutant tumors. Mechanistically, truncation of PREX2 activated its Rac1 guanine nucleotide exchange factor activity, abolished binding to PTEN and activated the PI3K (phosphatidyl inositol 3 kinase)/Akt signaling pathway. We further showed that PREX2 truncating mutations or PTEN deletion induces down-regulation of the tumor suppressor and cell cycle regulator CDKN1C (also known as p57^KIP2). This down-regulation occurs, at least partially, through DNA hypomethylation of a differentially methylated region in chromosome 11 that is a known regulatory region for expression of the CDKN1C gene. Together, these findings identify PREX2 as a mediator of NRAS-mutant melanoma development that acts through the PI3K/PTEN/Akt pathway to regulate gene expression of a cell cycle regulator. Significance Mutations in the PI3K/PTEN/Akt signaling pathway occur frequently across multiple tumor types. These mutations primarily serve to activate PI-3 and Akt kinases. PREX2 is a guanine nucleotide exchanger for Rac1 that is significantly mutated in melanoma and pancreatic ductal adenocarcinoma. Here we report that a mouse model of a truncating PREX2 mutation shows accelerated melanoma development in the context of mutant NRAS. Truncating PREX2 mutations have increased Rac1 guanine nucleotide exchange factor activity, and tumors harboring these mutations have elevated PI3K/Akt pathway activation and reduced expression of critical negative cell cycle regulators leading to increased cell proliferation. This work provides evidence for a previously unidentified mechanism of activating Rac1, the PI3K pathway, and regulation of cell cycle progression in melanoma.

Cancer genomic studies have identified frequent alterations in genes encoding components of the SWI/SNF chromatin remodeling complex, including SMARCA4 and ARID1A. Importantly, clinical reports indicate that SMARCA4-mutant lung cancers respond poorly to immunotherapy and have dismal prognosis. In this study, we corroborated the clinical findings by using immune-humanized, syngeneic, and genetically engineered mouse models of lung cancer harboring SMARCA4 deficiency. Specifically, models with SMARCA4 loss showed decreased response to anti-PD-1 immunotherapy associated with significantly reduced infiltration of dendritic cells and CD4+ T cells into the tumor microenvironment. SMARCA4 loss in tumor cells led to profound downregulation of STING1, IL1β, and other components of the innate immune system, as well as inflammatory cytokines that are required for efficient recruitment and activity of immune cells. The deregulation of gene expression was caused by cancer cell–intrinsic reprogramming of the enhancer landscape with marked loss of chromatin accessibility at enhancers of genes involved in innate immune response, such as STING1, IL1β, type I IFN, and inflammatory cytokines. Interestingly, the transcription factor NF-κB–binding motif was enriched in enhancers that lose accessibility upon SMARCA4 deficiency. Furthermore, SMARCA4 and NF-κB co-occupied the same genomic loci on enhancers associated with STING1 and IFNβ, indicating a functional interplay between SMARCA4 and NF-κB. Taken together, these findings provide the mechanistic basis for the poor response of SMARCA4-mutant tumors to immunotherapy and establish a functional link between SMARCA4 and NF-κB in innate immune and inflammatory gene expression regulation. Significance: Epigenetic reprogramming in SMARCA4-mutant cancer cells alters immune infiltration and limits immunotherapy efficacy by downregulating immunostimulatory gene expression, which could potentially be targeted to overcome immunotherapy resistance in SMARCA4-deficient tumors.

Genomic studies have identified frequent mutations in subunits of the SWI/SNF (switch/sucrose non-fermenting) chromatin remodeling complex including SMARCA4 and ARID1A in non-small cell lung cancer (NSCLC). Genetic evidence indicates that the paralog SMARCA2 is synthetic lethal to SMARCA4 suggesting SMARCA2 is a valuable therapeutic target. However, the discovery of selective inhibitors of SMARCA2 has been challenging. Here, we utilized structure-activity relationship (SAR) studies to develop YD23, a potent and selective proteolysis targeting chimera (PROTAC) targeting SMARCA2. Mechanistically, we show that SMARCA2 degradation induces reprogramming of the enhancer landscape in SMARCA4-mutant cells with loss of chromatin accessibility at enhancers of genes involved in cell proliferation. Furthermore, we identified YAP/TEADas key partners to SMARCA2 in driving growth of SMARCA4-mutant cells. Finally, we show that YD23 has potent tumor growth inhibitory activity in SMARCA4-mutant xenografts. These findings provide the mechanistic basis for development of SMARCA2 degraders as synthetic lethal therapeutics against SMARCA4-mutant lung cancers.

PREX2 is a PTEN interacting protein that is significantly mutated in melanoma and pancreatic ductal adenocarcinoma. Recently, we reported the mechanistic basis of melanomagenesis by PREX2 mutations. Truncating PREX2 mutations activate its guanine nucleotide exchange factor activity for its substrate RAC1. This leads to increased PI3K/AKT signaling associated with reduced DNA methylation and increased cell proliferation in NRAS-mutant melanoma. Here, we provide additional data that indicates a reciprocal regulation of PREX2 by PTEN whereby loss of PTEN results in a dramatic increase in expression of PREX2 at the protein level. Pharmacologic studies revealed destabilization of PREX2 by inhibition of PI3K/AKT signaling. Additionally, we provide data to show a selective decrease in a particular histone mark, H4 Lys20 trimethylation, in cells expressing PREX2 ^E824* truncating mutation globally and at the imprint control region of CDKN1C (also known as p57) and IGF2. The decrease in H4K20 trimethylation coupled with DNA hypomethylation at this particular locus is associated with genomic imprinting and regulation of expression of p57 and IGF2. Taken together, these results demonstrate the complex signaling mechanisms that involve PREX2, PI3K/AKT/PTEN and downstream epigenetic machinery to deregulate expression of key cell cycle regulators.