Abstract B33: Alterations in the binding of Meis proteins to HoxB13 or to DNA promote prostate cancer progression

Introduction: One of the first germline mutations shown to confer an increased risk of early-onset prostate cancer (PrCa) was recently found in the Meis-binding domain of Homeobox B13 (HoxB13). Neither the function of this mutation nor the role of Meis proteins in PrCa has been elucidated. There are three Meis genes, each with variously spliced isoforms, some of which lack a DNA binding domain. Meis proteins normally function as Hox gene cofactors, and regulate proliferation and cell fate. In normal and other disease contexts, Meis proteins with an abrogated homeodomain have been shown to act as a dominant negative and squelch normal Hox gene transcription. The androgen receptor (AR), the major oncogene in PrCa, has been shown to interact with both Meis and Hox proteins. We hypothesize that Meis proteins function as AR-regulated tumor suppressors in advanced prostate cancer, and isoforms lacking the homeodomain prevent HoxB13 DNA binding to promote uncontrolled cell proliferation . Methods: Meis1/2 expression stratified outcomes of men with Gleason 6/7 tumors in a retrospective bioinformatics analysis of microarray data obtained from the Swedish Watchful Waiting cohort. Meis1/2/3 mRNA expression in normal and cancerous prostate cell lines as well as adjacent normal stromal and epithelial patient-derive cell lines was compared by qRT-PCR. Cell number over time was determined by SRB assay for lines stably overexpressing full length or homeodomains-less Meis2 as compared to GFP in castration-sensitive LNCaP and castration-resistant CWR-22Rv1 PrCa cells. Meis2 expression was analyzed by qPCR in androgen-dependent LAPC4 cells. LAPC4 cells were incubated in distinct androgen conditions, including incubation with an AR agonist (R1881) or potent antagonist (enzalutamide). Publicly available microarray data from 19 patients were obtained for analysis from the Gene Expression Omnibus. Results: Through retrospective bioinformatics analysis, we found that men diagnosed with intermediate Gleason scores and low Meis expression had a worse prognosis than men with high Meis levels. qPCR data indicated lower Meis expression in PrCa cell lines as compared to normal prostate epithelial cells. Stromal patient-derived cell lines had significantly higher levels of Meis1 and 2 than their patient-matched epithelial counterparts, while Meis3 expression was consistent across all cell types. Proliferation assays indicated CWR-22Rv1 cells that overexpress full length Meis2 showed significantly lower cell number over five days than GFP- or abrogated homeodomain Meis-expressing cells. qPCR data indicates that PrCa cells exposed to R1881 had significantly lower Meis2 expression as compared to cells treated with enzalutamide. Analysis of publicly available microarrays showed that patients with metastatic prostate cancer were more likely to have lower Meis2 expression than patients with localized or benign disease. Conclusions: Our data indicate that full length Meis protein levels are consistently lower in cancer as compared to normal tissues, and are decreased to a greater extent in more aggressive cancers. Full length Meis proteins may act as tumor suppressors while the homeodomain-less isoforms may act as dominant negatives. Further, we demonstrate a regulatory interaction between Meis expression and AR signaling, providing evidence for a downstream role of Meis proteins in the most important pathway for PrCa progression. We are the first to investigate the functional differences of these isoforms in the context of PrCa. An understanding of how the functional domains of Meis and HoxB13 interact with each other, the AR and the genome are crucial steps towards evaluating the utility of Meis expression as a biomarker to stratify aggressive versus indolent PrCa patients, as well as aid our general understanding of prostate tumor initiation and progression. Citation Format: Hannah Brechka, Raj Bhanvadia, Donald Vander Griend. Alterations in the binding of Meis proteins to HoxB13 or to DNA promote prostate cancer progression. [abstract]. In: Proceedings of the AACR Special Conference: Developmental Biology and Cancer; Nov 30-Dec 3, 2015; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Res 2016;14(4_Suppl):Abstract nr B33.