Epigenetic regulation: PRC1 is involved in tumor initiation and progression

Epigenetic regulation modulates DNA functions without the alteration of nucleotide sequence and influences multiple processes of DNA, including replication and transcription. The disruption of epigenetic changes such as DNA methylation, chromatin modifications, nucleosome positioning and alterations in noncoding RNA profiles, cause the alteration of gene function and cellular neoplastic transformation ¹. Polycomb repressive complexes 1 (PRC1) modifies chromatin to regulate multiple physiological processes including cell senescence, cell apoptosis, cell proliferation, and cell differentiation ². There are reliable evidences for functions of PRC1 in regulating genome-related alterations, which recognizes DNA sequence through sequence-specific DNA-binding factors motifs at target sites. What’s more, PRC1 plays a dichotomous role in different situations.

Chromobox protein homolog 4 (CBX4), as a component of PRC1, has been proved to participate in several processes including growth, senescence, immunity, and tissue repair. Through the combination of Polycomb- and SUMO E3 ligase–dependent functions via N-terminal chromodomain and two SUMO-interacting motifs (SIM), CBX4 can regulate some cellular processes ³. Previous study shows that CBX4 can increase hypoxia-induced VEGF expression through stimulating sumoylation of HIF-1α via its SIM domain, and promote tumor progression in hepatocellular carcinoma. CBX4 is also proved to activate Wnt/β-catenin pathway to induce lung adenocarcinoma. β-catenin in nuclear is a main effector of Wnt signaling, regulating the transcription of Wnt target genes by binding to transcription factors. YAP and TAZ, known as the transcriptional regulators and effectors of the Hippo signaling cascade, play an essential role for phospho-β-catenin degradation by recruiting β-TrCP. Other chromatin accessibility studies have proved that C-terminal fusion partners of TAZ and YAP fusion proteins can alter the chromatin landscape, evading Hippo regulation. The above studies suggest the interactive relationships between Hippo, Wnt/β-catenin pathways and epigenetic regulations ^4-6. Promoting the study of relevant mechanisms provides potential clues for research on relevant targeted therapies in the following future.

Clinical phenomenon: low level of CBX4 is found in some lung adenocarcinoma (LUAD) patients

Lung cancer is one of the leading causes of cancer-related death worldwide, and is especially prevalent in China. Initiated by various factors, like smoking and air pollution, lung cancer has multiple subtypes. Generally, lung cancer can be classified as small cell lung cancer (SCLC) and non-small cell lung cancer (NSCLC, a heterogeneous class of tumors, represents approximately 85% of all new lung cancer diagnoses) ⁷. And adenocarcinoma accounts for nearly 40% of NSCLS and is the most common among all subtypes of lung cancer. lung cancer can be triggered by genetic mutations and effected by epigenetic regulation. Several epigenetic modifications have been applied in appraisal of the detection, progression and prognosis of lung cancer. Thus, the research on epigenetics is emphasized in the field of cancer biology. 

Previous study has shown that CBX4 is highly expressed in a part of LUAD clinical samples and promotes LUAD in P53^-/-, Kras^G12D background through Wnt/β-catenin pathway ⁴. In our study, we found that there were also a significant number of clinical samples with low CBX4 level. Furthermore, patients with low CBX4 expression in certain stage have a worse prognosis under the background of Kras^G12D mutation. RAS is a membrane protein with GTPase activity and oncogenic mutations including Kras^G12D are frequently observed in approximately 20% of all types of human cancers including carcinomas of the lung, colon, and pancreas ⁸. Kras^G12D mutation is a common mutation type in variety kinds of tumors, therefore it is often used as a tumor model in the studies of lung cancer. Strategies targeting Kras^G12D were nonspecific and inefficient based on the previous studies, but it still suggests a promising future because a variety of new strategies in targeting Kras^G12D have been developed in recent years.

Interesting discovery: CBX4 deletion in Kras^G12D background induces genomic instability and promotes LUAD progression

We firstly proved that loss of CBX4 promoted tumorigenesis in Kras^G12D mouse model. However, our result presented cell apoptotic in the initial stage after CBX4 knockout. Then in the soft agar assay which lasted for 4 weeks, we found that abilities of colony formation and cell transformation increased dramatically in Cbx4^-/-, Kras^G12D group. So, we further extended the 2D culture of the Cbx4^-/-, Kras^G12D for 4 weeks and found that a small portion of Cbx4^-/-, Kras^G12D MEFs survived in long-term (28 days) culture, called the selected cells.We also confirmed the stronger proliferation and invasion ability through following research. This is consistent with the trend in mouse model. Our result also implied CBX4 deletion induced genomic instability. RNA-seq data showed multiple genes were altered after CBX4 loss, including the Hippo signaling pathway. These data prove that CBX4 loss induces genomic instability and promotes LUAD progression in Kras^G12D background.

As components of PRC1, CBX2 knockdown has been proved to increase the phosphorylation of YAP and inhibit the tumor proliferation of hepatocellular carcinoma, and CBX7 acts as an inhibitor of glioma cell migration through influence YAP/TAZ-dependent transcription. PRC1 members play different roles in Hippo-related regulation and tumor progression in the context of different tumor types or genetic mutations. PRC1 is also proved to influence Wnt/β-catenin pathway in hepatocellular carcinoma, lung adenocarcinoma and colorectal cancer, regulating chemical resistance and tumor progression ⁴. Though, previous studies have only predicted the association between CBX4 function and Hippo signaling through functional enrichment analysis. Our study shows that most cells with Kras^G12D mutation undergo apoptosis for chromosome instability in the initial stage when CBX4 is deficient, but a small population of cells survive from genomic instability for acquiring stronger abilities in proliferation and transformation, due to changes of multiple pathways including Hippo. Our study provides a possible mechanism of lung cancers with low CBX4 level and suggests new clues for targeted therapies in cancer treatment with different mutations.

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