Pan-TRK Immunohistochemistry and NTRK Gene Fusions in Primary Carcinomas of the Liver.

Gene fusions involving NTRK are not common in solid tumors. The aim of this study was to investigate the TRK protein expression and molecular characteristics of gene fusions in primary liver carcinomas. A total of 110 hepatocellular carcinomas (HCC) and 69 intrahepatic cholangiocarcinomas were retrieved for tissue microarray (TMA) construction and clinicopathologic characterization. Immunohistochemistry (IHC) for pan-TRK was initially performed on TMA slides and evaluated for staining intensity. Twelve (10.9%) of 110 HCC showed weak cytoplasmic TRK expression by IHC on TMA, while all others, including 69 intrahepatic cholangiocarcinomas, were negative for TRK. The TRK expression did not correlate with patient's age, sex, tumor differentiation, or tumor stage. The 12 cases were then validated by IHC on whole sections but all turned out to be negative. Further, RNA sequencing analysis did not detect any NTRK fusions in all 12 HCC cases; however, it did identify many fusions frequently involving genes that encode mitochondrial and ribosomal proteins, microRNAs, and some transcription factors. A few fusions were recurrent, including MT-ATP6/MT-ATP8 fusion (n=9, 75%), Ig κ light chain gene IGKV/IGKJ fusion (n=5, 41.7%), and histocompatibility complex gene HLA-C/HLA-B fusion (n=4, 33.3%). In summary, NTRK fusion is very rare in primary liver carcinomas. IHC on TMA for TRK expression yields high false positive results, which should be validated on whole sections and confirmed by molecular genetic studies such as RNA sequencing. Many fusions involving genes other than NTRK are detected in HCC, the significance of which warrants further studies.