Abstract
Undifferentiated round cell sarcomas (URCS) of soft tissue and bone and tumours of uncertain differentiation (TUD) are commonly ascribed to a subset of neoplasms with low frequency of NTRK gene fusions. However, more recently NTRK-rearranged round and spindle cell tumours have been noted in case reports and in limited or heterogeneous cohorts. The aim of our study was to investigate the presence of NTRK gene fusions in a large retrospective cohort of paediatric URCS and TUD after a systematic review of the diagnosis, according to the recently updated WHO classification scheme. One-hundred and five patients with diagnosis of URCS or TUD, involving the bone or soft tissue, were retrospectively evaluated. After the case selection and the histopathological review of the case cohort, pan-Trk immunohistochemistry (IHC) testing was performed on formalin-fixed paraffin-embedded (FFPE) tissues. Tumour RNA was extracted from FFPE tissue and subjected to next-generation sequencing (NGS) library preparation, using a 10-gene NGS fusion panel, sequenced on an Illumina MiSeq. The NGS-positive cases were further confirmed by real-time PCR. On immunohistochemical screening, 12/105 (11.4%) cases were positive using the pan-Trk antibody, showing three different staining patterns with the cytoplasmic distribution being most common. Molecular analysis using NGS and confirmed by the real-rime PCR detected two positive cases for the ETV6-NTRK3 fusion. The histological pattern of the two positive cases, together with the demonstration of the NTRK rearrangement, leaded to re-classify these previously not otherwise specified sarcomas with uncertain differentiation into the emerging category of NTRK-rearranged neoplasms. In addition, we found the two NTRK fused neoplasms showing a clinical indolent course, in contrast with literature.
Highlights
According to the 2020 World Health Organization (WHO) Classification of Soft Tissue and Bone Tumours [1], the category of undifferentiated round cell sarcomas (URCS) includes a new section containing the paradigmatic round cell sarcoma, well known as Ewing sarcoma, and three distinct entities with different clinical, pathological, and molecular features: round cell sarcomas with EWSR1 gene fusion to non-ETS family members, CIC-rearranged sarcomas, and BCOR-rearranged sarcomas
This study evaluated the presence of neurotrophic tyrosine kinase receptor (NTRK) gene fusions in a large paediatric cohort entirely composed of URCS and undifferentiated soft tissue sarcomas (USTS) of soft tissue and bone, where the cases were categorized, according to the 2020 WHO classification
The immunohistochemistry performed with pan-tropomyosin receptor kinase (Trk) antibody showed reactivity in 12/105 (11.4%) analyzed samples, but the molecular analysis showed positivity for ETV6-NTRK3 fusion in 2 cases only; we can confirm that NTRK gene fusions emerge in a only a small fraction in contradistinction to other rare paediatric neoplasms, including secretory carcinoma of the breast [15], congenital mesoblastic nephroma [16] and infantile fibrosarcoma [17] where NTRK fusion is characteristic and often disease defining
Summary
According to the 2020 WHO Classification of Soft Tissue and Bone Tumours [1], the category of undifferentiated round cell sarcomas (URCS) includes a new section containing the paradigmatic round cell sarcoma, well known as Ewing sarcoma, and three distinct entities with different clinical, pathological, and molecular features: round cell sarcomas with EWSR1 gene fusion to non-ETS family members, CIC-rearranged sarcomas, and BCOR-rearranged sarcomas. The inclusion of NTRK-rearranged spindle cell neoplasms (excluding infantile fibrosarcoma that represent a distinct clinicopathologic entity molecularly characterized by the presence of NTRK3-ETV6 fusion gene), within the category of tumours of uncertain differentiation represents one of the most relevant innovations. The aim of our retrospective study was to detect NTRK fusions in a cohort of more than 100 paediatric cases of undifferentiated round cell sarcoma and tumours with uncertain differentiation, following the published ESMO guidelines on methods and diagnostic algorithm to target NTRK fusion [8]. Prior to the immunohistochemical and molecular testing, we re-examined the entire case collection to assess the diagnosis in the light of the diagnostic criteria of the 2020 WHO Classification of Soft Tissue and Bone Tumours
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