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Alexander Drilon, MD

Gregory J. Riely, MD, PhD


Screening for NTRK Fusions in Glioblastomas: FISH Versus IHC

By: Susan Reckling
Posted: Monday, April 11, 2022

With up to 5% of patients with glioblastomas having NTRK-rearranged tumors, detecting such genetic rearrangements has become a key component in therapeutic decision-making, especially with the approval of TRK-targeted therapies. Consequently, Arnaud Uguen, MD, PhD, of the University Hospital Morvan, Brest, France, and colleagues conducted a study to assess the diagnostic tools for identifying these genetic rearrangements in this patient population. In the RCPA (Royal College of Pathologists of Australasia) journal Pathology, they reported that as a screening tool prior to RNA sequencing, fluorescent in situ hybridization (FISH) appeared to be more useful than pan-TRK immunohistochemistry (IHC).

In this study, the investigators compared the results of NTRK1, NTRK2, and NTRK3 FISH assays with those of pan-TRK IHC using two EPR17341 and A7H6R clones in a set of 196 patients with glioblastomas. Further study using RNA sequencing was performed for the cases with at least 15% of positive nuclei using FISH analyses.

At the 15% threshold, seven positive glioblastomas (3.6%) were identified by FISH assays (four NTRK1 and three NTRK2). RNA-sequencing analyses confirmed these rearrangements in 2% of the 196 patients (n = 4; LMNA-NTRK1, PRKAR2A-NTRK2, SPECC1L-NTRK2, and NACC2-NTRK2 fusions). However, in three samples with less than 30% of positive tumor nuclei (as determined by NTRK1 FISH), no rearrangements were identified. “RNA-sequencing analyses are necessary in FISH-positive cases with less than 30% positive nuclei to avoid false positivity when scoring is close to the detection threshold,” the investigators indicated.

In comparison, with pan-TRK immunostaining, major discrepancies were seen when using either the EPR17341 or the A7H6R clones for certain criteria (main intensity, H Score–based scoring, and homogeneity/heterogeneity of staining). As a result, the investigators defined “adequate criteria to identify NTRK-rearranged gliomas exhibiting strong and diffuse immunostaining contrasting to the variable and heterogeneous staining in non-NTRK–rearranged gliomas (P < .0001).”

Disclosure: For full disclosures of study authors, visit

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