Background: Colorectal cancer (CRC) incidence is rising worldwide, as well as in the Republic of Kazakhstan, while its occurrence is also increasing in the younger population. Hereditary forms associated with the development of colon and rectal cancer and early-onset CRC have never been studied in the population of Kazakhstan. The aim of this research was to investigate the spectrum of CRC-related gene mutations to determine which mutations cause early onset of CRC in the Kazakhstan population. Methods: The study included 125 unrelated patients from Kazakhstan (range 17-50 years in age) with early onset CRC. Genomic DNA was obtained from peripheral blood of the patients. Next-generation sequencing was performed using the TruSightCancer Kit on the MiSeq platform. The Studio Variant was used to annotate and interpret genetic variants. Results: Bioinformatics analysis of Next-generation sequencing data revealed 11,152 variants from 85 genes, of them, 3,790 missense, 6,254 synonymous variants, 44 3'UTR variants, 10 frameshift variants, five stop-gain variants, four in-frame deletions, two splice donors, one splice acceptor variant, and 1,042 intron or non-coding variants. APC, BRCA2/1, ALK, BRIP1, EGFR, FANCA, FANCD2, FANCI, HNF1A, MEN1, NSD1, PMS2, RECQL4, RET, SLX4, WRN, and XPC genes mutated most often. According to the ACMG guidelines and LOVD/ClinVar databases, 24 variants were pathogenic (10 frameshifts, five missenses, five stop-gain, one in-frame deletion, and three splice-site mutations), and 289 were VUS with population frequency <1%, 131 of them were attributed as deleterious. In the study, 50% of all pathogenic mutations found in Kazakhstani patients with early CRC onset were identified in the subgroups with a family history of CRC and primary multiple tumors. In APC, pathogenic mutations were most often (21%). Conclusion: Pathogenic and likely pathogenic mutations were found in 20 (16%) out of 125 patients. Eight novel pathogenic mutations detected in FANCI, APC, BMPR1, ATM, and DICER1 genes have not been reported in previous literature. Given the high frequency and wide spectrum of mutations, NGS analysis must be carried out in families with a history of CRC/CRC-related cancers with the purpose to identify cause-effective mutations, clarify the clinical diagnosis, and prevent the development of the disease in other family members.

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