NTRK oncogenic fusions are exclusively associated with the serrated neoplasia pathway in the colorectum and begin to occur in sessile serrated lesions
NTRK fusions are emerging tissue-agnostic drug targets in malignancies including colorectal cancers (CRCs), but their detailed landscape in the context of various colorectal carcinogenesis pathways remains to be investigated. Methods: Pan-TRK expression was assessed by immunohistochemistry in retrospectively collected colorectal epithelial tumor tissues, including 441 CRCs (133 microsatellite instability-high (MSI-high) and 308 microsatellite stable (MSS)) and 565 premalignant colorectal
... (300 serrated lesions and 265 conventional adenomas). TRK-positive cases were subjected to next-generation sequencing and/or fluorescence in situ hybridization to confirm NTRK rearrangements. Results: TRK positivity was not observed in any of the MSS CRCs, conventional adenomas, traditional serrated adenomas, or hyperplastic polyps, whereas TRK positivity was observed in 11 of 58 (19%) sporadic MSI-high CRCs, 4 of 23 (17%) sessile serrated lesions with dysplasia (SSLDs), and 5 of 132 (4%) SSLs. The 11 TRK-positive MSI-high CRCs commonly harbored CpG island methylator phenotype-high (CIMP-high), MLH1 methylation, KRAS/BRAF wild-type, and NTRK1 or NTRK3 fusion (TPM3-NTRK1, TPR-NTRK1, LMNA-NTRK1, SFPQ-NTRK1, ETV6-NTRK3, or EML4-NTRK3). Both NTRK1 or NTRK3 rearrangement and KRAS/BRAF wild-type were detected in all nine TRK-positive SSL(D)s, seven of which demonstrated MSS and/or CIMP-low. TRK overexpression and early dysplastic changes are occasionally co-localized in the crypt base area of SSLs. Age-related occurrence patterns suggest that the progression interval from NTRK-rearranged SSLs to CRCs may be shorter than from BRAF-mutated SSLs to CRCs. Conclusion: NTRK-rearranged colorectal tumors develop exclusively through the serrated neoplasia pathway and can be initiated from non-dysplastic SSLs without KRAS/BRAF mutations prior to full occurrence of MSI-high/CIMP-high.