Increased expression of a subset of genes within reduced copy number regions across multiple cancer types

The TGIF1 transcription factor gene is present on chromosome 18, which is subject to whole chromosome copy number reduction in colon cancer. Despite this, TGIF1 expression is significantly higher in cancer than in normal. In mice complete deletion of Tgif1 reduced tumor burden in an Apc mutant model of intestinal cancer. Here we show that reducing TGIF1 expression in a human colon cancer cell line slows proliferation and reduces growth of orthotopic xenografts. To ask if additional genes with copy number loss are more highly expressed in tumors we identified chromosomal regions subject to copy number reductions from ten TCGA cancer datasets. Within these regions a small proportion of genes, generally less than 10%, are expressed at higher levels in the tumor than in corresponding normal samples. Enrichment analysis using a set of 435 genes that have copy number reduction and increased expression identified mitosis as the most enriched gene set and FOXM1 and E2F family transcription factors as potential regulators. For mitotic genes, the average expression increase in tumor compared to normal is independent of copy number. In contrast, while DepMap common essential genes are generally more highly expressed in cancer than normal tissue, the relative increase in expression tracks well with copy number. Similarly, expression differences for gene sets such as S-phase, rRNA processing and DNA repair show increased expression in cancer versus normal, but changes also track with copy number. Thus, genes with increased expression despite copy number reduction may represent the output of key pro-tumorigenic transcriptional programs and could be potential therapeutic targets.