Immunological consequences of LINE-1 expression in cancer

Project Summary Approximately 200,000 new cases of ovarian cancer (OC) occur per year, but overall survival has not significantly changed for decades and novel therapies are urgently needed. High grade serous OC makes up about 70% of cases. It is most commonly derived from the fallopian tube epithelium and is characterized by

nearly 100% mutant TP53 and high expression of transposable elements (TEs). Nearly half of our genome is composed of TEs, and their transcription is tightly regulated during development. Select TEs are expressed at high levels in embryonic stem cells, but most are silenced by DNA methylation and repressive histone

modifications in differentiated cells. As part of the global epigenetic dysregulation that cells undergo during transformation from normal to cancer, TE silencing is compromised and TEs become transcribed. Recent studies from our group and others have implicated TEs in control of innate immune signaling in OC. Our

studies in a cancer progression model show that LINE-1 elements lose methylation and are transcribed specifically during cellular transformation rather than immortalization. LINE-1 is the only protein-coding TE known to be able to retrotranspose, and LINE-1 is significantly overexpressed in OC. We showed that LINE-1

is expressed after P53 mutation in OC precursor lesions and that wild type and mutant P53 transcriptionally regulate LINE-1 in OC cell lines. Finally, LINE-1 activation in the absence of P53 interferes with DNA replication to activate the DNA damage response in cancer cells. We know that LINE-1 and other TEs become

activated during cancer progression but we do not understand how this occurs or the immune effects of this activation. The objective of this application is thus to uncover the biological mechanisms by which epigenetic dysregulation of LINE-1 changes the OC tumor microenvironment. We hypothesize that by loss of epigenetic

regulation, LINE-1 contributes to OC immune evasion. We will test this hypothesis with the following Aims: 1) Determine effects of LINE-1 expression on innate immune signaling, 2) Determine effects of LINE-1 retrotransposition on innate immune signaling, and 3) Determine how LINE-1 expression affects the OC

immune microenvironment. Using existing immortalized fallopian tube lines with wild type, mutant, and null P53, we will transform the lines with the C-Myc oncogene. We will perform genome-wide characterization of LINE elements during transformation and will assess effects of LINE-1 expression on the DNA damage

response and the tumor immune microenvironment. Taken together, the experiments described in this proposal will add to our knowledge of epigenetic regulation and immune activity of an understudied 20% of our genome, LINE-1 elements. These experiments will enable us to identify potential OC driving alterations and

vulnerabilities to novel immunotherapies.