KMT2D deficiency alters CDK-cyclin regulation of Rb in endometrial cancer

Endometrial cancer (EC) is the most common gynecological cancer and ranks fourth overall among all cancers affecting women in the United States. Its incidence, especially among premenopausal women, continues to increase in contrast to many other malignancies. Recent genomic investigations, including our lab's preliminary findings, have identified KMT2D, a histone methyltransferase that modulates enhancer activity, as being highly mutated in early onset EC. Our initial observations suggest that KMT2D deficiency triggers aberrant histone methylation at the Rb/E2F pathway genes, leading to upregulation of the pathway. These findings motivated the development of a KMT2D CRISPR knockout (KO) cell line to explore the mechanistic consequences of KMT2D deficiency. We are employing 12Z endometrial epithelial cell lines, derived from endometriosis, with and without CRISPR-mediated KMT2D KO to examine the mechanistic basis of Rb/E2F pathway dysregulation at both the gene expression and post-translational levels. Preliminary data indicate that KMT2D deficiency enriches retinoblastoma (Rb) phosphorylation levels, consistent with upregulation of the Rb/E2F pathway. Additionally, cyclin E, cyclin D, p21, and CDK1 were more highly expressed in KMT2D KO cells compared to the control. To further investigate upstream regulators, we examined CDK1/2 pathway dynamics. In control and KMT2D KO cells, we found pharmacologic inhibition of CDK1/2 activity led to increased expression of cyclin A2 and cyclin E1. There was also a divergent effect on Rb phosphorylation following CDK1/2 inhibition, with phospho-Rb increasing expression in control cells but reducing in KMT2D KO cells. This indicates KMT2D deficiency alters CDK-Rb signaling pathways. To explore additional mechanisms contributing to altered Rb regulation, we performed Rb immunoprecipitation followed by mass spectroscopy to identify changes in Rb protein interactions in the absence of KMT2D. Ongoing analysis of these datasets aims to determine whether KMT2D deficiency exposes novel regulatory interactions affecting Rb phosphorylation. This study builds on preliminary findings implicating KMT2D in Rb pathway dysfunction in early-onset EC. By investigating the contribution of CDK-mediated phosphorylation, we have sought to clarify the mechanistic basis of elevated Rb activity in KMT2D-deficient cancer. These findings may reveal novel therapeutic vulnerabilities that can be exploited for targeted treatment strategies in patients with early-onset EC.