TRK wild-type and fusion protein expression in solid tumors: Characterization by immunohistochemistry and in situ hybridization

Abstract

Background: Neurotrophic tyrosine receptor kinases (NTRK1-3) are a gene family encoding kinases involved in development and maturation of the central and peripheral nervous system. In cancer, fusion of one of these genes with various upstream partners leads to aberrant protein expression and unchecked proliferation. Identification of tumors driven by TRK fusions is clinically relevant because they can be targeted by highly selective small molecule inhibitors. Pan-TRK immunohistochemical (IHC) staining for aberrant expression of TRK proteins may be an important approach to identify tumors with TRK fusions when followed by confirmation. We describe the expression, both wild-type (WT) and fusion, of TRK proteins in 19 solid tumors. Methods: An IHC staining protocol was developed using the pan-TRK (EPR17341) antibody in combination with VENTANA OptiView DAB IHC Detection kit, on the VENTANA BenchMark ULTRA platform. This protocol was tested on 3900 tissue samples, across 19 tumor types. A subset of IHC cases was designated as eitherWT or fused at the NTRK loci using in-situ hybridization (ISH). Staining patterns and percentages are reported, as is the rate of ISH positivity by tumor type. Results: The pan-TRK (EPR17341) IHC assay detected TRK protein expression across 271 samples in 16 solid tumor types. Tumors with-1% staining represented-7% of all tissues evaluated by IHC. TRK protein expression was negative in most solid tumors. However, in tumors of neuroendocrine/neural origin and in GIST,WTTRK expression ranged from-50-75%. The percentage of IHC positive tumors that went on to be ISH positive ranged from 0% in GIST to 60% in colorectal carcinoma, highlighting the differential utility of this stain by tumor histology. Conclusions: In solid tumors with low WTTRK expression, this assay effectively identifies potential TRK fusions for confirmation. However, pan-TRK IHC is hampered in identifying potential fusions in neuroendocrine/neural tumors and GIST. Additional assay development and pathologist training is warranted to guide the interpretation of pan-TRK IHC staining.