MRTFB suppresses colorectal cancer development through regulating SPDL1 and MCAM.

Myocardin-related transcription factor B (MRTFB) is a candidate tumor-suppressor gene identified in transposon mutagenesis screens of the intestine, liver, and pancreas. Using a combination of cell-based assays, in vivo tumor xenograft assays, and Mrtfb knockout mice, we demonstrate here that MRTFB is a human and mouse colorectal cancer (CRC) tumor suppressor that functions in part by inhibiting cell invasion and migration. To identify possible MRTFB transcriptional targets, we performed whole transcriptome RNA sequencing in MRTFB siRNA knockdown primary human colon cells and identified 15 differentially expressed genes. Among the top candidate tumor-suppressor targets were melanoma cell adhesion molecule (MCAM), a known tumor suppressor, and spindle apparatus coiled-coil protein 1 (SPDL1), which has no confirmed role in cancer. To determine whether these genes play a role in CRC, we knocked down the expression of MCAM and SPDL1 in human CRC cells and showed significantly increased invasion and migration of tumor cells. We also showed that Spdl1 expression is significantly down-regulated in Mrtfb knockout mouse intestine, while lower SPDL1 expression levels are significantly associated with reduced survival in CRC patients. Finally, we show that depletion of MCAM and SPDL1 in human CRC cells significantly increases tumor development in xenograft assays, further confirming their tumor-suppressive roles in CRC. Collectively, our findings demonstrate the tumor-suppressive role of MRTFB in CRC and identify several genes, including 2 tumor suppressors, that act downstream of MRTFB to regulate tumor growth and survival in CRC patients.

ZFR promotes cell proliferation and tumor development in colorectal and liver cancers.

Colorectal cancer (CRC) and liver cancer are the second and fourth leading causes of cancer-related deaths in the whole world, respectively, and each year over 1.6 million people die from these diseases. To identify driver genes in CRC and liver cancer, we have performed Sleeping Beauty transposon mutagenesis screens in mouse models. Zinc finger RNA binding protein, ZFR, was one of the novel candidate cancer genes identified in these forward genetic screens. Consistent with this discovery, a pan-cancer analysis of sequencing results of thousands of human cancer genomes demonstrated that ZFR is a potential potent oncogene. In this study, we aimed to investigate ZFR's roles in both types of cancer and found that overexpression of ZFR in CRC and liver cancer cells led to accelerated tumor development. Consistently, knockdown of ZFR resulted in significantly decelerated tumor development. ZFR overexpression also promoted tumor development of immortalized mouse liver cells. ZFR overexpression and shRNA knockdown led to accelerated and decelerated cell proliferation, respectively, indicating that ZFR promotes tumor development mainly by regulating cell proliferation. To identify ZFR's targets in transcription, we performed whole transcriptome sequencing using ZFR small interfering RNAs in a primary human colon cell line. All potential target genes were validated by real time PCR. FAM49B was a tumor suppressor candidate for ZFR targets. When we knocked down the expression of FAM49B in CRC and liver cancer cells, we observed significantly accelerated cell proliferation, consistent with the results with ZFR overexpression. The results presented here demonstrate the oncogenic role of ZFR in both CRC and liver cancer, providing a potential drug target for both cancers' treatment. We also identified ZFR's potential transcriptional targets, and further investigations on those targets, especially FAM49B, will help us understand more about the important role of ZFR in digestive system cancers.