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Case 6

A 71-year-old male was diagnosed with colorectal carcinoma of the ascending colon. Mutation testing revealed wild-type KRAS, NRAS and BRAF.

NTRK gene fusion testing

Subsequent IHC screening was positive for TRKA, TRKB, and TRKC.

 Figure 21: Pan-TRK IHC of Patient with Colon Adenocarcinoma

Figure 21:  Pan-TRK IHC of Patient with Colon Adenocarcinoma

Antibody for IHC: clone EPR17341 available from Abcam, Cambridge, MA, USA(www.abcam.com)

A DNA-only NGS (Foundation OneTM assay) was used to confirm the rearrangement. MTOR (A469T) and TP53 (R175H) mutations were detected, but no copy number variants were identified. The NTRK1-TPM3 fusion was detected. 

FISH was used as a final confirmatory approach.

Figure 22: FISH of Patient with Colon Adenocarcinoma

Figure 22: FISH of Patient with Colon Adenocarcinoma

Using a break-apart FISH probe (Vysis, top three panels), over 50% of the nuclei can be interpreted as positive (bottom right corner).

TRK inhibitor treatment

The patient was treated with a TRK inhibitor as first-line treatment. 

Clinical interpretation and impact of NTRK gene fusion testing

This case demonstrates that NTRK-rearranged colorectal carcinomas are usually right-sided and RAS-BRAF wild-type. The fusion can be detected by DNA-only NGS, but there is a greater risk of false-negative results. In addition, FISH could be useful for histologies with a very high rate of NTRK rearrangement, when the RNA part of the NGS fails, or if there is insufficient input material for NGS.

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