Case Report

Rapid Response to Larotrectinib (LOXO-101) in an Adult Chemotherapy-Naive Patients With Advanced Triple-Negative Secretory Breast Cancer Expressing ETV6-NTRK3 Fusion

Secretory carcinoma of the breast (SCB) is a rare low-grade often triple-negative breast carcinoma. We aim to analyze the pathological and molecular features of 21 SCBs, especially the SCBs with axillary lymph node metastasis.

The clinicopathological characteristics of 21 SCBs were reviewed. Breast biomarkers, Pan-TRK and ETV6 break, and ETV6-NTRK3 fusion were performed on all cases. Next-Generation Sequencing (NGS) was performed on two cases with lymph node metastasis.

21 SCBs consisted of 2 men and 19 women aged 5～73 years (median 43 years), with a mean 2.1 cm (range 0.5～3.5 cm) tumor size. 90.1% (19/21) cases had mixed microcystic, solid, tubular, and papillary patterns. Pan-TRK and S100 are positive in 95% (20/21) and 90% (19/21) of cases, respectively. Tumor markers ER, PR, and HER2 expressions were 62% (13/21), 33% (7/21), and 0% (0/21). All cases showed ETV6 (21/21) rearrangement and ETV6-NTRK3 (11/11) fusion. 57% (12/21) of the cases had a balanced translocation and 38% (8/21) with unbalanced signals of ETV6. There was no clinical difference between balanced and unbalanced translocations in histological morphology and other prognosis factors. Furthermore, one case (#4) had a duplication of the ETV6 gene and presented axillary lymph node metastasis. NGS analysis revealed simple genomes, low tumor mutation burden, stable microsatellite sites, and single nucleotide polymorphism (SNP) heterozygous mutation in both SCBs with nodal metastasis.

SCB is an indolent invasive carcinoma, even the cases with axillary lymph node metastasis, presenting simple genomes. Duplication of ETV6 cases may indicate lymph node metastasis.

Chromosomal rearrangements involving the NTRK1, NTRK2, and NTRK3 genes (NTRK genes), which encode the high-affinity nerve growth factor receptor (TRKA), brain-derived neurotrophic factor/neurotrophin-3 (BDNF/NT-3) growth factor receptor (TRKB), and neurotrophin-3 (NT-3) growth factor receptor (TRKC) tyrosine kinases (TRK proteins), act as oncogenic drivers in a broad range of pediatric and adult tumor types. NTRK gene fusions have been shown to be actionable genomic events that are predictive of response to TRK kinase inhibitors, making their routine detection an evolving clinical priority. In certain exceedingly rare tumor types, NTRK gene fusions may be seen in the overwhelming majority of cases, whereas in a range of common cancers, reported incidences are in the range of 0.1% to 2%. Herein, we review the structure of the three NTRK genes and the nature and incidence of NTRK gene fusions in different solid tumor types, and we summarize the clinical data showing the importance of identifying tumors harboring such genomic events. We also outline the laboratory techniques that can be used to diagnose NTRK gene fusions in clinical samples. Finally, we propose a diagnostic algorithm for solid tumors to facilitate the identification of patients with TRK fusion cancer. This algorithm accounts for the widely varying frequencies by tumor histology and the underlying prevalence of TRK expression in the absence of NTRK gene fusions and is based on a combination of fluorescence in situ hybridization, next-generation sequencing, and immunohistochemistry assays.

Targeted inhibitors of neurotropic tyrosine kinases are highly effective in selected patients with gene fusions involving NTRK1, NTRK2, or NTRK3. These fusions are consistently detected in rare cancer types (e.g., secretory breast carcinoma and congenital infantile fibrosarcoma), but the occurrence of NTRK fusions in common cancers and their relationship to other therapy biomarkers are largely unexplored. Tissue samples from 11,502 patients were analyzed for 53 gene fusions and sequencing of 592 genes, along with an immunohistochemical evaluation of TrkA/B/C and PD-L1. Thirty-one cases (0.27% of the entire cohort) had NTRK fusions. The most common fusions were ETV6:NTRK3 (n = 10) and TPM3:NTRK1 (n = 6). Gliomas had the highest number of NTRK fusions (14/982, 1.4%), most commonly involving NTRK2 (n = 9). Seventeen non-glioma cases with NTRK fusions included carcinomas of the lungs, thyroid, breast, cervix, colon, nasal cavity, cancer of unknown primary and soft tissue sarcomas. Strong and uniform Trk expression detected with a pan-Trk immunohistochemistry characterized 7/8 NTRK1 fusion cases and 8/9 NTRK2 fusion cases, while NTRK3 fused cases were positive in 6/11 (55%) of cases. 29% of NTRK fusion cases had no other pathogenic genomic alteration. PD-L1 expression was observed in 23% of NTRK fused cases while high tumor DNA microsatellite instability was detected in two cases. We confirm the rarity of NTRK genes fusions outside the brain malignancies. NTRK inhibitors alone or combined with immune checkpoint inhibitors may be a therapeutic option for a substantial proportion of these patients. Strategies for detection of the NTRK fusion-driven cancers may include immunohistochemistry, but gene fusion detection remains the most reliable tool.