PROJECT SUMMARY
Repetitive elements (REs) compose ~45% of the human genome and are normally transcriptionally silenced in
somatic cells, although the mechanism had remained elusive. Through a high-content RNAi screen, we identified
the largely uncharacterized protein FBXO44 as an essential repressor of REs in breast cancer cells. FBXO44
bound repressive histone H3 lysine 9 trimethylated (H3K9me3) nucleosomes at the replication fork and recruited
H3K9me3 methyltransferase SUV39H1, ubiquitin ligase CRL4RBBP4/7, and histone deacetylase and chromatin-
remodeling complex Mi-2/NuRD to transcriptionally silence REs post-DNA replication. FBXO44/SUV39H1
inhibition transcriptionally reactivated endogenous retroviruses (ERVs) and retrotransposons (e.g. Alu, LINE-1)
in breast cancer cells, leading to extensive DNA replication stress and stimulation of RIG-I/MDA5-MAVS and
cGAS-STING intracellular antiviral pathways to promote enhanced immunogenicity and decreased
tumorigenicity. In silico analysis revealed the FBXO44/SUV39H1 pathway inversely correlated with DNA
replication stress, antiviral pathways, and cytotoxic T and natural killer (NK) cell infiltration in human breast
tumors. Importantly, FBXO44/SUV39H1 were found dispensable for RE silencing in normal cells and their
inhibition had no effect on H3K9me3 levels, DNA replication stress, or viability, suggesting a therapeutic window.
Our hypothesis is that FBXO44/SUV39H1-mediated RE silencing is an epigenetic vulnerability of breast
cancer cells that could be targeted to inhibit tumor growth/progression and enhance the efficacy of
certain antitumor therapies through the unique mechanism of induced viral mimicry. In this proposal, we
will evaluate 3 potential therapeutic applications of FBXO44/SUV39H1 pathway targeting in the treatment of
estrogen receptor (ER)+ breast cancers based on our preliminary data: 1) prevention of bone metastasis relapse
through stimulation of NK cell recognition and killing of dormant breast cancer cells; 2) enhancement of immune
checkpoint blockade (ICB) therapy through stimulation of IFN signaling and intratumoral infiltration of cytotoxic
T cells; and 3) synergy with PARP inhibitors through induction of DNA replication stress and double-strand
breaks (DSBs) at REs. These studies could lead to the development of a safe and effective therapeutic approach
that selectively induces viral mimicry in ER+ breast cancer cells to prevent bone metastasis relapse and enhance
the efficacy of ICB and PARP inhibitor therapies, undoubtedly leading to a significant reduction in disease
mortality.