Alteracions de la VIA RAS-RAF en càncer gastrointestinal amb defectes de reparació genòmica
SUMMARY: The Ras-Raf-MAPK pathway regulates functions such as cell proliferation, transformation and apoptosis. In cancer activating mutations are present in the three Ras genes (KRAS, HRAS and NRAS) and in one Raf gene (BRAF). Moreover, Ras and Raf mutations hardly ever are found together in the same tumor suggesting that they activate the same pathway. In colorectal cancer (CRC) the BRAF hotspot mutation V600E is associated to microsatellite instability (MSI), which is characterised by insertion/deletion mutations in repetitive sequences. Here we have determined possible molecular or clinico-pathological associations of V600E to understand its higher tumorogenic capabilities in this subset of tumors. We found that BRAF was not mutated in gastric cancer with MSI or without (MSS), although gastric and colorectal MSI tumors presented the same mutational patterns. Interestingly, gastric cancer only presented KRAS mutations in MSI tumors. Furthermore, in sporadic MSI CRC (which is usually caused by MLH1 hypermethylation) BRAF-V600E was not associated to KRAS, APC or p53 mutations but it was to MLH1 hypermethylation and to genomic hypermethylation after analysing p16, p14, RASSF1A, APC, MGMT and THBS1. In addition, V600E was associated significantly to tumors located in the proximal colon. Moreover, tumors from HNPCC (a hereditary form of MSI cancer caused by germline mutations in mismatch repair genes) never had V600E mutations. We have also analysed KRAS in CRC showing MSS, sporadic MSI with or without MLH1 hypermethylation and HNPCC. We found that KRAS mutation frequency is higher if MLH1 is not hypermethylated, independently of its sporadic or hereditary origin. And although in sporadic tumors these mutations are located in codon 12, in the hereditary ones they are located in codons 12 or 13 equally. Furthermore, HNPCC tumors carrying germline MLH1 mutations have less KRAS mutations than tumors with germline mutations in MSH2 or MSH6. Finally, we concluded that KRAS and BRAF differences in sporadic and hereditary CRC suggest a different Ras-Raf-MAPK modulation and possible alternative activation pathways depending on the genetic and epigenetic background of the tumor. Moreover, V600E is a reliable, fast and cheap tool for HNPCC diagnosis to exclude a hereditary origin.
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Advisor:Schwartz Navarro, Simó; Cormand Rifa, Bru
School:Universitat de Barcelona
Source Type:Master's Thesis
Date of Publication:02/27/2007