The Adenoma-Carcinoma Sequence in Colorectal Cancer: Scratching the Surface

Colorectal cancer represents not only the second leading cause of cancer-related death in Western world. The sequence of events that describes colorectal cancer development, the so called “adenoma-carcinoma sequence” is characterized by the progressive increase in signal transduction activity and chromosomal instability levels resulting from the accumulation of somatic mutations in specific tumor suppressor genes and oncogenes. This thesis aims to describe and illustrate the multiple interactions and mutual influences of defective signal transduction pathways like Wnt, TGF-ß and Receptor Tyrosine Kinase (RTK) in CRC and the mechanisms that are responsible for ‘just-right’ level of genetic instability. Through the generation and molecular analysis of the compounds Apc+/1638N/KRASV12G, Apc+/1638N/Smad4+/E6sad and Apc+/1638N/Tp53-/- mouse models for colorectal cancer, we explored and elucidated the effect of the different signaling transduction cross talk and measured the level of chromosomal instability. First of all we demonstrated by in vitro and in vivo analysis that activated KRAS, via tyrosine phosphorylation of ß-catenin, leads to its release from E-cadherin at the adherens junctions, and to its consequent nuclear translocation and increased transcription of Wnt downstream target genes in an Apc-mutant background. Second we characterized Smad4–haploinsufficiency that underlies tumor initiation in the GI tract. Reporter assay analysis showed the presence of TGF-ß and BMP signaling defects already in heterozygous cells, and the expression profiling analysis of the Smad4 mutant ES cell lines allowed the identification of a shortlist of Smad4 dosage-related target genes involved in signaling, metabolism and regulation of the transcription. Finally, by Comparative Genomic Hybridization (CGH) Array we observed that loss of the Tp53 tumor suppressor gene, but not KRAS oncogenic activation, results in an increase of chromosomal instability in the Apc-mutant genetic background. Through the comparative analysis of the overall genomic alterations found in human and mouse intestinal adenomas, we identified a subset of syntenic regions likely to encompass rate-limiting genes for intestinal tumor initiation and progression. All these elements clearly contribute to the establishment of the multiple cellular defects necessary to promote tumor progression and malignancy. These important clues relative to the cellular processes underlying tumorigenesis in the gastro-intestinal tract will open new avenues for tailor-made therapeutic approaches.