Abstract 4464: The role of KMT5A in prostate cancer

Castrate resistant prostate cancer (CRPC) remains a significant clinical problem with no currently available cure. The Androgen receptor (AR) continues to function in CRPC through a plethora of mechanisms and the AR remains the primary target for therapeutic intervention in CRPC. Targeting AR co-regulating proteins to indirectly target AR signaling may prove beneficial. Recently, our group identified KMT5A as a potential regulator of AR from selective siRNA library screening. KMT5A is a methyltransferase which monomethylates histone 4 on lysine 20. The establishment of this epigenetic mark directly impacts chromatin structure, genome stability and cell cycle progression, all hallmarks of cancer. In addition, it monomethylates non-histone proteins such as p53. It is therefore important that KMT5A activity and its cellular localization are under tight regulation. Using a relevant in vitro CRPC model we have shown that KMT5A acts as an AR co-activator while in androgen sensitive models KMT5A restricts AR activity. This highlights the importance of studying how KMT5A itself is regulated. KMT5A is primarily regulated through post-translational modifications in a cell cycle dependent manner. KMT5A polyubiquitination by E3 ligases CUL4-Cdt2 and APC-Cdh1 causes its proteasomal mediated degradation in S and late mitosis cell cycle phases, respectively. Moreover, the Skp2 E3 ligase has been suggested to play a role in KMT5A ubiquitination and degradation; but there is no direct evidence to show this to date. Therefore, the role of Skp2 in regulating KMT5A is understudied and remains elusive. It is also not known whether KMT5A could be modified directly by ubiquitination without leading to its degradation. As such, we aimed to investigate KMT5A monoubiquitination and the role of Skp2 in regulating KMT5A. We demonstrated monoubiquitinated KMT5A is present in a panel of PC cell lines. This was further confirmed by performing ubiquitination assays in COS7 cells. We determined that the KMT5A C-terminal SET domain is targeted for monoubiquitination and identification of the site(s) is ongoing. Furthermore, monoubiquintinated KMT5A was found to be exclusively cytoplasmic. In addition, Skp2 markedly enhances KMT5A monoubiquitination in a concentration dependent manner. We were also able to show that KMT5A and Skp2 interact. Skp2 mediated monoubiquitination of KMT5A however is not associated with protein turn over as addition of MG132 (proteasome inhibitor) does not further stabilize KMT5A. Interestingly, Skp2 mediated monoubiquitination of KMT5A downregulates MG132 mediated stabilization of KMT5A, possibly indicating this monoubiquitination impacts the accessibility of KMT5A for other ligases which promote KMT5A polyubiquitination and degradation. The newly characterized modification of KMT5A is a prevalent form of total KMT5A which can affect its function. Thus, insight into its physiological significance may provide a novel therapeutic target to indirectly target the AR. Citation Format: Mahsa Azizyan, Kelly Coffey, Craig N. Robson. The role of KMT5A in prostate cancer. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 4464.