Pan-Trk Immunohistochemistry Identifies NTRK Rearrangements in Pediatric Mesenchymal Tumors.

Activating neurotrophic receptor kinase (NTRK) fusions define certain pediatric mesenchymal tumors, including infantile fibrosarcoma and cellular mesoblastic nephroma. Traditionally, molecular confirmation of these fusions has included either fluorescent in situ hybridization for ETV6 rearrangements or reverse-transcriptase polymerase chain reaction for the classic ETV6-NTRK3 fusion. However, these methods overlook variant NTRK rearrangements, which are increasingly appreciated as recurrent events in a subset of pediatric mesenchymal tumors. New therapeutic agents successfully target these fusions and may prevent morbid surgeries in very young children, making recognition of tumors harboring NTRK rearrangements of increasing importance. We evaluated the performance of immunohistochemical (IHC) staining using pan-Trk and TrkA antibodies in 79 pediatric mesenchymal tumors. Negative controls included pediatric mesenchymal tumors not harboring (n=28) or not expected to harbor (n=22) NTRK fusions. NTRK rearrangements were detected predominantly by DNA-based next-generation sequencing assays, specifically UW OncoPlex and UCSF500 Cancer Gene Panel. Pan-Trk IHC (EPR17341) was 97% sensitive and 98% specific for the presence of an NTRK rearrangement, and TrkA IHC (EP1058Y) was 100% sensitive and 63% specific for the presence of an NTRK rearrangement. Tumors with NTRK1 or NTRK2 rearrangements showed cytoplasmic staining, whereas tumors with NTRK3 rearrangements showed nuclear +/- cytoplasmic staining. We conclude that pan-Trk IHC is a highly sensitive and specific marker for NTRK rearrangements in pediatric mesenchymal tumors.

BRAF exon 15 mutations in pediatric renal stromal tumors: prevalence in metanephric stromal tumors.

Metanephric stromal tumors (MSTs) are rare renal stromal tumors that predominantly affect children. They belong to the metanephric family of tumors, along with metanephric adenofibroma and metanephric adenoma. The previous documentation of BRAF exon 15 mutations in 88% of metanephric adenomas and in isolated cases of metanephric adenofibroma prompted us to investigate the prevalence of these mutations in MSTs and in other pediatric renal stromal tumors. In this study, 17 MSTs, 22 congenital mesoblastic nephromas, and 6 ossifying renal tumors of infancy were selected for BRAF exon 15 testing. Tumor genomic DNA was extracted from formalin-fixed paraffin-embedded tissue, followed by polymerase chain reaction amplification and Sanger dideoxy sequencing with primers flanking the BRAF exon 15 gene. BRAF exon 15 mutations were found in 11 (65%) of the 17 cases of MST, all corresponding to a thymidine-to-adenine substitution at codon 600 (BRAF V600E). All other renal stromal tumors tested were negative for BRAF exon 15 mutations. In conclusion, BRAF V600E mutations are encountered in most MSTs, supporting a link with other metanephric tumors and suggesting a clonal event possibly affecting primordial renal cells. In addition, BRAF V600E mutations have been associated with oncogene-induced senescence in other benign tumors, providing clues to the pathogenesis of metanephric neoplasms in keeping with their overall benign behavior. Our results also suggest a potential diagnostic use for BRAF exon 15 mutations in differentiating MSTs from other pediatric renal stromal tumors, particularly in limited samples.