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Transient loss of Polycomb components induces an epigenetic cancer fate

ABSTRACT: Although cancer initiation and progression are generally associated with the accumulation of somatic mutations, substantial epigenomic alterations underlie many aspects of tumorigenesis and cancer susceptibility, suggesting that genetic mechanisms might not be the only drivers of malignant transformation. However, whether purely non-genetic mechanisms are sufficient to initiate tumorigenesis irrespective of mutations is unknown. Here, we show that a transient perturbation of transcriptional silencing mediated by Polycomb Group proteins is sufficient to induce an irreversible switch to a cancer cell fate in Drosophila. This is linked to the irreversible derepression of genes that can drive tumorigenesis, including members of the JAK-STAT signaling pathway and zfh1, the fly homolog of the ZEB1 oncogene, whose aberrant activation is required for Polycomb-induced tumorigenesis. These data show that a reversible perturbation of Polycomb Group protein levels can induce cancer in the absence of driver mutations, suggesting that tumors can emerge through epigenetic dysregulation leading to inheritance of altered cell fates.

ORGANISM(S): Drosophila melanogaster

PROVIDER: GSE222193 | GEO | 2024/02/28

REPOSITORIES: GEO

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Project description:Kaposi's sarcoma (KS), is an AIDS-associated neoplasm caused by the KS herpesvirus (KSHV/ HHV-8). KSHV-sarcomagenesis is the consequence of oncogenic viral gene expression as well as host genetic and epigenetic alterations. Although KSHV is invariably found in all KS lesions, the percentage of KSHV-infected (LANA+) spindle-cells of the lesion is variable, suggesting the existence of paracrine oncogenic mechanisms and/or spindle cells that have lost KSHV. A mouse model of KSHVBac36-driven tumorigenesis allowed us to induce KSHV-episome loss before and after tumor development. Although cells that lose the KSHV episome prior to tumor formation lose their tumorigenicity, explanted KSHV positive tumor cells that lost the KSHV episome remained tumorigenic. This pointed to the existence of virally-induced irreversible oncogenic alterations occurring during KSHV-tumor formation that support the possibility of hit and run viral sarcomagenesis. RNA sequencing and CpG-methylation analysis were performed on KSHV positive and negative cells, KSHV positive and KSHV negative tumors that developed following KSHV-episome loss from explanted tumor cells. When KSHV positive cells form KSHV-driven tumors, along with viral-gene upregulation there is a tendency for hypo-methylation in genes from oncogenic and differentiation pathways. In contrast, KSHV-negative tumors formed after KSHV-episome loss, show a tendency towards gene hyper-methylation when compared to KSHV-positive tumors. Regarding occurrence of host-mutations, we found the same set of innate-immunity related mutations undetected in KSHV-infected cells but present in all KSHV positive tumors, indicating that pre-existing host mutations that provide an in vivo growth advantage are clonally-selected and contribute to KSHV-tumorigenesis. In addition, KSHV negative tumors display de novo mutations related to cell proliferation that, together with the PDGFRAD842V, were responsible for driving tumorigenesis in absence of the KSHV-episomes. KSHV-induced irreversible genetic and epigenetic oncogenic alterations support the possibility of “hit and run” KSHV-sarcomagenesis consistent with the presence of LANA-negative spindle-cells in KS lesions.

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Transient loss of Polycomb components induces an epigenetic cancer fate

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