Characterization of loss of HLTF function in the development of colon cancer
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Helicase-like Transcription Factor (HLTF) is a DNA helicase protein which is homologous to SNF/SWI family. It has been demonstrated to be a functional homolog of yeast Rad5, required for the maintenance of genomic stability. Although the physiologic role of HLTF is largely unknown,inactivation of HLTF by promoter hypermethylation has been found in more than 40% human colon cancers. In this study, we have applied mouse transgenic approaches to determine whether loss of HLTF function could be important for colorectal carcinogenesis. HLTF knockout mice were generated by the deletion of first 5 exons of the HLTF gene. The complete loss of HLTF expression in HLTF -/- mice was confirmed by northern blot and real time RT-PCR assays. HLTF -/- mice did not show any developmental defects within a 2-year observation indicating that HLTF is dispensable for mouse development. Furthermore, HLTF -/- mice were free of intestinal or colorectal tumors or other types of tumors, suggesting that loss of HLTF function alone is not sufficient to drive oncogenic transformation in intestinal track and other tissues. To determine whether loss of HLTF function could cooperate with other tumor suppressors in the formation of colorectal cancers, we have bred HLTF knockout mice with the mutant mice for APC (adenomtous polyposis coli) and P53. In HLTF -/-APC Min/+ mice, a significantly increased formation of intestinal adenocarcinoma and colorectal cancers were observed. Although very few HLTF -/-P53 -/- mice developed colorectal cancers, these mice had increased incidence of the formation of metastatic lymphomas. Cytogenetic analysis of colorectal cancer cells derived from HLTF -/-APC Min/+ mice demonstrated a high incidence of gross chromosomal instabilities, including Robertsonian fusions, fragments and aneuploidy. All these genetic alterations were not observed in the intestinal tumor cells from APC Min/+, implicating that loss of HLTF function could induce genomic instability which contributes to intestinal carcinogenesis. To further investigate the role of HLTF in colorectal carcinogenesis, we have also applied a shRNA knockdown approach to down-regulate HLTF expression in human HCT-116 colon cancer cells. HCT-116 cells highly express HLTF and show less chromosomal instability, making these cells as a very useful model to investigate the loss of function of HLTF in human colorectal carcinogenesis. Using Western blot approach, we confirmed that HLTF knockdown HCT-116 cells had less than 5% of HLTF expression as compared to the scramble controls. By inoculating HLTF knockdown HCT-116 cells to Rag1 -/-IL2 -/- immunocompromised mice, we further demonstrated that HLTF knockdown promote tumor growth and invasion. Moreover, spectral karyotyping analysis revealed that HLTF knockdown human colon cancer cells had significantly increased chromosomal instability, including both aneuploidy and chromosomal translocation. Taken together, our work strongly indicates that loss of HLTF function can promote the malignant transformation of intestinal or colonic adenomas to carcinomas by inducing genomic instability. Given the high frequency of epigenetic inactivation by hypermethylation of HLTF in human colon cancers, our studies strongly suggest that this epigenetic alteration could be directly involved in the development of colorectal cancer rather than a consequence of this carcinogenesis.