Abstract 1198: Presence of non-canonical tumorigenesis pathways in early-onset sporadic rectal cancer.
Authors: Ratheesh Raman, Viswakalyan Kotapalli, Raju SR Adduri, Vasantha Kumar Bhaskara, Swarnalata Gowrishankar, Leena Bashyam, Ajay Chaudhary, Mohana Vamsy Chigurupati, Sujith Patnaik, Mukta Srinivasulu, Regulagadda Sastry, Subramanyeshwar Rao, Anjayneyulu Vasala, NarasimhaRaju Kalidindi, Jonathan Pollack, Sudha Murthy, Murali D. Bashyam
Journal: Cancer Research (2013)

Abstract Chromosomal instability (CIN) caused by β-Catenin dependent aberrant activation of canonical Wnt signaling or Microsatellite instability (MSI) triggered by inactivation of mismatch repair (MMR) pathway, are the two major genetic instability pathways that drive classical age-related sporadic colorectal cancer (CRC). Activation of canonical Wnt signaling and MSI are primary tumor initiating events in about 80% and 15% of late-onset CRC cases, respectively. Canonical Wnt signaling is also reported as a secondary event in tumors primarily driven by MSI. These inferences are based on seminal studies that included a disproportionately greater number of colon tumors (as compared to rectal tumors). Recent genome-wide studies have suggested colon and rectal cancer to be a single entity, though several other studies appear to indicate otherwise. Exclusive studies on rectal cancer (RC), especially the early-onset sporadic subtype, have been fewer. Despite a recent trend of increased worldwide incidence, early-onset sporadic rectal cancer (EOSRC) is not well understood. We profiled canonical Wnt, KRAS and p53 (components of the classical colorectal carcinoma progression model) and MSI status in a panel of 298 colorectal cancer samples. 41% of EOSRC samples did not harbor Wnt or MSI pathways; the high proportion of a ‘double negative’ entity was neither identified in late-onset RC samples nor in colon cancer samples. KRAS mutation frequency was also significantly lower in EOSRC (24%). Since CIN is a hallmark of canonical Wnt activation driven CRC, we profiled genome-wide DNA copy number alterations (CNAs) in microsatellite stable EOSRC samples and surprisingly identified extensive chromosomal aberrations in both Wnt active and Wnt inactive subtypes suggesting the interesting possibility of presence of CIN in the absence of canonical Wnt activation. Several CNAs were detected exclusively in Wnt inactive samples (being absent in Wnt active samples) and were validated by quantitative PCR. As expected, a few CNAs, such as an amplification detected at 17q12 (ERBB2/GRB7), were present in both subtypes. Genome-wide transcript profiling performed in parallel revealed the elevated expression of genes located within the amplifications, validated by quantitative reverse transcription PCR (Q-RT-PCR). More importantly, aberrant activation of non-canonical signaling pathways was identified in a subset of the ‘double negative’ EOSRC samples, based on unsupervised (hierarchical clustering) and supervised (significance analysis of microarrays and gene set enrichment analysis) analyses of the transcriptome data. Elevation of non-canonical pathway gene transcripts was confirmed by Q-RT-PCR. Our study has therefore revealed presence of unique tumorigenesis pathways in EOSRC samples distinct from canonical pathways that drive late-onset CRC. Citation Format: Ratheesh Raman, Viswakalyan Kotapalli, Raju SR Adduri, Vasantha Kumar Bhaskara, Swarnalata Gowrishankar, Leena Bashyam, Ajay Chaudhary, Mohana Vamsy Chigurupati, Sujith Patnaik, Mukta Srinivasulu, Regulagadda Sastry, Subramanyeshwar Rao, Anjayneyulu Vasala, NarasimhaRaju Kalidindi, Jonathan Pollack, Sudha Murthy, Murali D. Bashyam. Presence of non-canonical tumorigenesis pathways in early-onset sporadic rectal cancer. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 1198. doi:10.1158/1538-7445.AM2013-1198