PP039 Inhibition of CAF-1 histone chaperone complex triggers cytosolic DNA and dsRNA sensing pathways and induces intrinsic immunity of hepatocellular carcinoma

Hepatocellular carcinoma (HCC) is the third most common cause of cancer mortality worldwide but only a few FDA-approved drugs are available for advanced HCC patients. Immune checkpoint inhibitor (ICI) has emerged as a new and effective treatment for HCC, yet only a quarter of HCC patients are responsive to ICI treatment. Epigenetic deregulation plays a critical role in HCC initiation and progression whereas the contribution of histone chaperones and histone variants in liver carcinogenesis remains largely unexplored. Histone Chaperone complex Chromatin assembly factor 1 (CAF-1) knockout cell lines were established using the CRISPR/Cas9 system. The effects of CAF-1 in HCC were studied in HCC cell lines, nude mice, and immunocompetent mice. RNA-Seq was applied to measure the expression of genes and repetitive elements. ChIP-Seq and ATAC-Seq were used to explore the changes in the epigenome. CAF-1 complex was significantly upregulated in human and mouse HCCs and was associated with poor prognosis of HCC patients. Knockout of CAF-1 remarkably suppressed HCC growth in both in vitro and in vivo models. Mechanistically, depletion of CAF-1 induced replicative stress and chromatin instability, which eventually led to cytoplasmic DNA leakage as micronuclei. Also, ChIP-Seq analyses revealed a massive H3.3 histone variant replacement upon CAF-1 knockout. Enrichment of euchromatic H3.3 increased chromatin accessibility and activated the expression of endogenous retrovirus elements (ERVs), a phenomenon known as viral mimicry. Altogether, cytosolic micronuclei and ERVs are recognized as ectopic elements by the STING and dsRNA viral sensing pathways, respectively. As a result, knockout of CAF-1 activated inflammatory response and anti-tumor immune surveillance and thereby significantly enhanced the anti-cancer effect of immune checkpoint therapy in HCC. Our findings suggest that CAF-1 is essential for HCC development, targeting CAF-1 may awaken the anti-cancer immune response and may work cooperatively with ICI treatment in cancer therapy.