646P - Mutation analysis of B2M gene in colorectal cancer patients with microsatellite instability

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BackgroundMutations in genes involved into antigen presentation such as B2M are enriched among colorectal cancer (CRC) patients with high microsatellite instability (MSI-H) and can become negative predictors for checkpoint inhibitor therapy. The aim of this study was the determination of prevalence of B2M somatic mutations in MSI-H CRC patients and relationship between B2M mutations and lymphocytes infiltration and other clinicopathological features. We focused also for methylation status of B2M promoter as another possible mechanism of gene expression downregulation. MethodsFrom our study group enrolling 420 CRC patients, 37 MSI-H and 5 MSI-L patients were selected for screening of B2M mutational status. The characterization of patients was based on standard histopathological diagnosis, TNM classification, tumor-infiltrating lymphocytes and peritumoral lymphoid reactions were determined. MSI analysis was performed using fragment analysis. BRAF, KRAS and B2M mutational status were identified by Sanger sequencing. For methylation analysis of B2M promoter region was used nested methylation specific PCR. ResultsB2M mutations were detected in 5 MSI-H patients (13.5%). In four cases, heterozygous mutation c.45_48delTTCT located in exon 1 was present and 2 patients exhibited heterozygous mutation c.276delC in exon 2. From group of 5 MSI-H patients, one case was compound heterozygote for both these mutations. Methylation of B2M promoter was observed only in 1 MSI-H case from our study group. ConclusionsThe presence of B2M mutations should be correlated with the response to the checkpoint inhibitor therapy in order to improve patient’s stratification and also development of new immunotherapeutic approaches. Our pilot study recommends detection of CpG methylation in B2M gene for the future studies in the area of colorectal cancer. Editorial acknowledgementBiomedical Center Martin (ITMS 26220220187), APVV-16-0066 and VEGA 1/0380/18. Legal entity responsible for the studyThe authors. DisclosureAll authors have declared no conflicts of interest.