Abstract 3490: Molecular profiling of MSI-high colorectal cancer (CRC) in Indian population

Abstract

Abstract Background: CRC is the 3rd most commonly diagnosed malignancy and 4th leading cause of cancer related deaths worldwide. Over two decades of extensive research from the West has revealed three major tumorigenesis pathways responsible for CRC viz. a) Chromosomal Instability (CIN) arising from aberrant activation of Wnt signalling, b) Microsatellite Instability (MSI) caused by inactivation of MisMatch Repair (MMR) genes and c) CpG Island Methylator Phenotype resulting in promoter DNA methylation induced inactivation of tumour suppressor genes. Previous results from our laboratory showed a significant proportion of Early Onset Sporadic Rectal Cancer (EOSRC) to be driven neither by aberrant Wnt signalling nor MSI. We also identified a significant proportion of MSI+ familial CRC not exhibiting loss either of the four major MMR genes indicating involvement of addition MMR or non-MMR genes. We therefore endeavoured to determine status of MSI in sporadic CRC samples from the Indian population. We are performing analysis of the status of other molecular features including CIMP, BRAF/KRAS/p53 mutation status, Wnt, etc. to obtain a wholesome molecular profile of sporadic CRC from the Indian population. Methodology: The microsatellite pattern in CRC tumours was determined using the five microsatellite marker panel as per the Bethesda guidelines. Expression status of MMR proteins (MLH1, MSH2, MSH6 & PMS2) was evaluated using immunohistochemistry. Mutations in BRAF (V600E) and KRAS (Codon 12 and 13) were identified by Sanger sequencing. CIMP status is being determined using the MethyLight technology as well as methylation specific PCR. Results: A comprehensive screen of close to 200 CRC samples revealed a significantly higher proportion of sporadic CRC tumors (41%; 50% of colonic and 31% of rectal cancer samples) exhibiting MSI as compared to 14% reported in The Cancer Genome Atlas (TCGA) data. Among MSI-high tumours, only 27% and 15% showed loss of MLH1 or MSH2 respectively, a significantly lower frequency compared to Caucasian data. Instability in the BAT25 microsatellite was significantly high in tumors harboring MLH1 loss. More importantly, a significant proportion of MSI+ CRC tumors were devoid of loss of either of the four major MMR genes. In addition, the BRAF V600E mutation, a known cause of MLH1 silencing, is absent in all but one MSI+ tumors. Work on determining status of CIMP and Wnt signalling is currently underway. Conclusions: Significantly higher proportion of sporadic CRCs from Indian population exhibit MSI but the underlying mechanism appears to be novel and distinct from those identified from the Caucasian population. Deep sequencing based studies are currently underway to determine molecular aberrations driving MSI in the Indian sporadic CRC. Note: This abstract was not presented at the meeting. Citation Format: Ashmala Naz, Vijaya Tourani, Satish Rao, Murali Dharan Bashyam. Molecular profiling of MSI-high colorectal cancer (CRC) in Indian population [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 3490.