The VTI1A-TCF4 colon cancer fusion protein is a dominant negative regulator of Wnt signaling and is transcriptionally regulated by intestinal homeodomain factor CDX2

<div><p>Sequencing of primary colorectal tumors has identified a gene fusion in approximately 3% of colorectal cancer patients of the <i>VTI1A</i> and <i>TCF7L2</i> genes, encoding a VTI1A-TCF4 fusion protein containing a truncated TCF4. As dysregulation of the Wnt signaling pathway is associated with colorectal cancer development and progression, the functional properties and transcriptional regulation of the VTI1A-TCF4 fusion protein may also play a role in these processes. Functional characteristics of the VTI1A-TCF4 fusion protein in Wnt signaling were analyzed in NCI-H508 and LS174T colon cancer cell lines. The NCI-H508 cell line, containing the <i>VTI1A</i>-<i>TCF7L2</i> fusion gene, showed no active Wnt signaling, and overexpression of the VTI1A-TCF4 fusion protein in LS174T cells along with a Wnt signaling luciferase reporter plasmid showed inhibition of activity. The transcriptional regulation of the <i>VTI1A-TCF4</i> fusion gene was investigated in LS174T cells where the activity of the <i>VTI1A</i> promoter was compared to that of the <i>TCF7L2</i> promoter, and the transcription factor CDX2 was analyzed for gene regulatory activity of the <i>VTI1A</i> promoter through luciferase reporter gene assay using colon cancer cell lines as a model. Transfection of LS174T cells showed that the <i>VTI1A</i> promoter is highly active compared to the <i>TCF7L2</i> promoter, and that CDX2 activates transcription of <i>VTI1A</i>. These results suggest that the VTI1A-TCF4 fusion protein is a dominant negative regulator of the Wnt signaling pathway, and that transcription of <i>VTI1A</i> is activated by CDX2.</p></div

Colorectal cancer-associated SNP rs17042479 is involved in the regulation of NAF1 promoter activity

A novel risk locus at 4q32.2, located between the Nuclear Assembly Factor 1 (NAF1) and Follistatin Like 5 (FSTL5) genes, was associated with increased risk of developing colorectal cancer (CRC), with SNP rs17042479 being the most associated. However, the link between CRC development and the risk locus at 4q32.2 is unknown. We investigated the promoter activity of NAF1 and FSTL5 and analyzed the risk locus at 4q32.2 as gene regulatory region. Our results showed that the activity of the FSTL5 promoter was low compared to the NAF1 promoter. Analyses of the NAF1 promoter in conjunction with the region containing the risk locus at 4q32.2 showed that the region functions as gene regulatory region with repressor activity on NAF1 promoter activity. The SNP rs17042479(G) increased the repressor effect of the region. CRC patients’ biopsies were genotyped for SNP rs17042479(A/G), and NAF1 expression profiles were examined. We found an association between SNP rs17042479(G), cancer stage and tumor location. Additionally, patients with SNP rs17042479(G) showed lower NAF1 expression in comparison to patients with SNP rs17042479(A) in tumor tissue and the NAF1 expression in tumor tissue was lower compared to healthy tissue. The results in the study imply that reduced NAF1 expression in the tumor contribute to a more aggressive phenotype. Furthermore, this study suggests that the SNP rs17042479(G) change the expression of NAF1 and thereby increases the risk of developing CRC