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Synergistic PIM kinase and proteasome inhibition in MYC-overexpressing triple-negative breast cancer

ABSTRACT: Triple-negative breast cancer (TNBC) is the breast cancer (BC) subtype with the poorest outcome. The PIM family of kinases has recently emerged as a factor that is both overexpressed in TNBC samples and associated with poor outcomes. Preclinical data suggest that TNBC that exhibits an elevated MYC oncoprotein expression (MYC+TNBC), accounting for 50% of TNBC cases, is sensitive to PIM inhibition. However, ongoing clinical observations indicate that the efficacy of PIM inhibitors as single agents may be limited in solid tumors, suggesting the need for combination therapies. We conducted drug combination screens to identify a combination that targets PIM and the 20S proteasome (the PIMi/20Si combination) as the most synergistic combination. Following the screening, we used a chemical genetic approach to reveal that the mechanisms of drug synergy involve disruption of protein homeostasis and obstruction of an adaptive resistance mechanism associated with proteasome inhibition. Thus, the PIMi/20Si combination could represent a rational combination therapy against MYC+ TNBC that is readily translatable to early-stage clinical investigations.

INSTRUMENT(S): Q Exactive HF

ORGANISM(S): Homo Sapiens (ncbitaxon:9606)

SUBMITTER: Young Ah Goo

PROVIDER: MSV000087521 | MassIVE | Wed May 26 07:43:00 BST 2021

SECONDARY ACCESSION(S): PXD026291

REPOSITORIES: MassIVE

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Project description:Clinical responses to kinase inhibitor therapy in acute myeloid leukemia (AML) are limited by the inevitable development of resistance. A major contributor to resistance is early epigenetic adaptation, leading to persistence of a small number of leukemia cells. Here we show that inhibition of the epigenetic regulator lysine-specific deme-thylase 1 (LSD1) augments the response to inhibitors of the FLT3 kinase in AML. We demonstrate that combined FLT3 and LSD1 inhibition results in synergistic cell death of FLT3-mutant AML cells via proliferative arrest and apoptosis. The drug combination synergistically activates a pro-differentiative epigenetic and transcriptional program while simultaneously suppressing the activity of MYC target genes. High resolution multi-modal epigenetic analyses revealed that combined FLT3 and LSD1 resulted in the suppression of MYC-bound promoters and the activation of PU.1-bound enhanc-ers. Forced expression of MYC partially abrogated the drug effect, and regulon en-richment analysis in primary AML samples nominated STAT5 as a putative regulator of MYC gene expression. STAT5 is highly bound to the MYC blood super-enhancer and inhibition of FLT3 results in a loss of STAT5 binding and a loss of super enhancer acti-vation. Furthermore, knockdown of STAT5 augments LSD1-inhibitor induced cell death. LSD1 inhibition also directly represses MYC target genes which show specific accumulation of the repressive LSD1 substrate H3K9me1. We validated these findings in 67 primary AML samples including 19 FLT3-ITD positive AML samples, with the vast majority demonstrating improved responses with the drug combination. High MYC regulon activity was a predictor of response to the drug combination and RNA-seq on drug treated AML samples revealed suppression of MYC target genes. Finally, single cell ATAC seq on primary AML blasts treated ex-vivo with combined FLT3 and LSD1 inhibition results in a shift from MYC super enhancer-high to a MYC super enhancer-low cell state. Collectively, these studies provide preclinical rationale for the investiga-tion of dual FLT3 and LSD1 inhibition in clinical trial.

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Synergistic PIM kinase and proteasome inhibition in MYC-overexpressing triple-negative breast cancer

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