The DREAM complex assembles during G0/G1 when RB-like protein p130 recruits E2F4, DP1, and a core complex of five MuvB proteins to repress genes involved in cell cycle progression. In S-phase, the MuvB core dissociates from p130 and binds to BMYB transcription factor. Binding of the MuvB core to p130 requires phosphorylation of its subunit LIN52 at S28 residue by DYRK1A protein kinase. However, little is known about how the MuvB core interacts with p130 to form the DREAM complex, and how these interactions are manipulated throughout the cell cycle. In collaboration with Dr. Seth Rubin, we characterized the structural basis for DREAM assembly, and found that the LxSxExL sequence in LIN52 directly interacts with the LxCxE binding cleft within the pocket domain of p130. Furthermore, immunoprecipitation and proliferation assays revealed that mutating the LIN52 LxSxExL sequence to mimic the canonical LxCxE motif found in viral oncoproteins reduces cellular proliferation and stabilizes the DREAM complex in the presence of viral proteins. We addressed how the DREAM complex is disassembled upon cell cycle entry and found that CDK phosphorylation of p130 inactivates the DREAM complex by displacing p130 from the MuvB core. Under certain conditions, we found that BMYB and p130 simultaneously bind the MuvB core, while overexpression of BMYB disrupts DREAM assembly. Together, our study provides insight into the structural mechanisms of DREAM assembly and function, which can help identify novel approaches to halt tumor cell proliferation or dormancy.

According to the American Cancer Society, it has been estimated that in 2019 alone, there will be approximately 53,000 new cases of oropharyngeal cancers. Oropharyngeal cancers are the largest subset of head and neck squamous cell carcinomas (HNSCCs), which are the sixth most common cancer across worldwide populations. They, along with other HNSCCs, fall under a category of cancers known as Human papillomavirus (HPV)-associated cancers, and it has been found that upwards of 70% of these cancers can be attributed to high-risk HPV infections. Specifically, the high-risk HPV gene, E7, plays a key role in relieving cell cycle repression by disrupting the DREAM complex via competitive binding with p130, driving the cell cycle and cell proliferation. In order to combat this interaction, a LIN52-S20C mutation was developed, in hopes of reducing E7 binding of p130 and stabilizing the DREAM complex. We utilized human cervical cell lines, immortalized keratinocytes, and mouse fibroblasts, all of which contained the HPV16 genome, as models to observe the effects of the LIN52-S20C mutation on HPV-mediated hijacking of the cell cycle. Not only were we able to replicate the increased proliferation and upregulated DREAM gene expression in infected cells, but we were also able to observe some reversal of these effects in many of our cell models through the expression of the LIN52-S20C variant. The findings of these studies have been promising and provide a basis for future works, and we hope that the effects of the LIN52-S20C mutation can be translated into studies in in vivo models.