One of the most common mechanisms of immune evasion in MSI colorectal cancers (CRCs) is loss of HLA class I expression due to mutations in B2M gene which can become a negative predictor for checkpoint blockade therapy. The aim of this study was the determination of prevalence of B2M somatic mutations in MSI CRC patients and relationship between B2M mutations and lymphocytes infiltration and other clinicopathological features as well as detection of methylation changes in B2M promoter region which can be another mechanism of immune escape. In our study, 37 MSI-H and 5 MSI-L patients were selected for screening of B2M mutational and methylation status. The characterization of patients was based on standard histopathological diagnosis and TNM classification; BRAF, KRAS mutations, tumor-infiltrating lymphocytes and peritumoral lymphoid reaction were also determined. MSI analysis was performed using fragment analysis. B2M mutations were identified by Sanger sequencing, and methylation of CpG islands in promoter region was detected by methylation-specific PCR. Heterozygous mutations in the B2M gene were detected in five MSI-H patients (13.5%), while the mutation c.45_48delTTCT was determined in four patients and mutation c.276delC was found in two patients. One of these five patients was compound heterozygote harboring both mutations. Methylation of the promoter region of the B2M gene was observed in one patient with MSI-H colorectal cancer. Detection of genetic and epigenetic changes in B2M gene could be important in personalized therapy for CRC patients as these changes may be one of the mechanisms of secondary resistance of MSI positive tumors to immunotherapy.

1st Department of Internal Medicine Faculty of Medicine Comenius University and University Hospital Bratislava Slovakia

Clinic of Surgery and Transplant Center Jessenius Faculty of Medicine in Martin Comenius University in Bratislava Martin Slovakia

Department of Bioinformatics Biomedical Center Martin Jessenius Faculty of Medicine in Martin Comenius University in Bratislava Martin Slovakia

Department of Internal Medicine Brothers of Mercy Hospital Polní 553 3 63900 Brno Czech Republic 2nd Department of Surgery Faculty of Medicine Masaryk University and St Anne's University Hospital Brno Czech Republic

Department of Pathological Anatomy Jessenius Faculty of Medicine and University Hospital in Martin Comenius University in Bratislava Martin Slovakia

Division of Oncology Biomedical Center Martin Jessenius Faculty of Medicine in Martin Comenius University in Bratislava Mala Hora 4 036 01 Martin Slovakia

Division of Oncology Biomedical Center Martin Jessenius Faculty of Medicine in Martin Comenius University in Bratislava Mala Hora 4 036 01 Martin Slovakia Department of Molecular Biology and Genomics Jessenius Faculty of Medicine in Martin Comenius University in Bratislava Martin Slovakia

Faculty of Medicine University of Oviedo Central University Hospital of Asturias Oviedo Spain

Gastroenterology Unit Department of Medicine Azienda Ospedaliera Universitaria Integrata Policlinico GB Rossi University of Verona Verona Italy

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