Comparative Morphologic and Molecular Classification of Neuroendocrine Tumors of the Thymus in Humans

Neuroendocrine tumors (NET) of the thymus are rare neoplasms that share many similarities with their pulmonary counterpart. The WHO 2015 classification of thymic and pulmonary NET divides tumors into typical and atypical carcinoids (TC, AC) and large cell and small cell neuroendocrine carcinoma (LCNEC, SCC). The classification is based on histomorphology: mitotic rate and necrosis. In comparison, the ENETS classification of gastro-intestinal and pancreatic NET includes a fifth subgroup NET G3 and is additionally based on the ki67 labeling index. Little is known about prognostic markers and the molecular background of TNET. With low coverage whole genome sequencing and immunohistochemistry, our goal was to achieve a better understanding of subgroup specific characteristics and their prognostic importance. The study revealed that TNET fall into three molecular clusters and that there is evidence of NET G3 in the thymus. Low- and intermediate-grade molecular categories are comprised of TC, AC and even LCNEC. High-grade molecular categories contain AC, LCNEC and SCC. Morphology alone is not sufficient in recreating these categories.  EZH2 and chromogranin A immunohistochemistry can help differentiate not only NET G3 from the classic LCNEC, but also function as a marker for genomic instability for all subgroups. Additionally, this study found that morphologic and molecular progression in TNET is possible. Further investigations will be crucial to discover if there is evidence of NET G3 in the pulmonary NET and use these findings for treatment planning and risk stratification. 2021-05-0

Molecular Classification of Neuroendocrine Tumors of the Thymus

INTRODUCTION: The WHO classification of pulmonary neuroendocrine tumors (PNETs) is also used to classify thymic NETs (TNETs) into typical and atypical carcinoid (TC and AC), large cell neuroendocrine carcinoma (LCNEC), and small cell carcinoma (SCC), but little is known about the usability of alternative classification systems. METHODS: One hundred seven TNET (22 TC, 51 AC, 28 LCNEC, and 6 SCC) from 103 patients were classified according to the WHO, the European Neuroendocrine Tumor Society, and a grading-related PNET classification. Low coverage whole-genome sequencing and immunohistochemical studies were performed in 63 cases. A copy number instability (CNI) score was applied to compare tumors. Eleven LCNEC were further analyzed using targeted next-generation sequencing. Morphologic classifications were tested against molecular features. RESULTS: Whole-genome sequencing data fell into three clusters: CNIlow, CNIint, and CNIhigh. CNIlow and CNIint comprised not only TC and AC, but also six LCNECs. CNIhigh contained all SCC and nine LCNEC, but also three AC. No morphologic classification was able to predict the CNI cluster. Cases where primary tumors and metastases were available showed progression from low-grade to higher-grade histologies. Analysis of LCNEC revealed a subgroup of intermediate NET G3 tumors that differed from LCNEC by carcinoid morphology, expression of chromogranin, and negativity for enhancer of zeste 2 polycomb repressive complex 2 subunit (EZH2). CONCLUSIONS: TNETs fall into three molecular subgroups that are not reflected by the current WHO classification. Given the large overlap between TC and AC on the one hand, and AC and LCNEC on the other, we propose a morphomolecular grading system, Thy-NET G1-G3, instead of histologic classification for patient stratification and prognostication. peerReviewe

Enhancer of Zeste Homolog 2 (EZH2) Is a Marker of High-Grade Neuroendocrine Neoplasia in Gastroenteropancreatic and Pulmonary Tract and Predicts Poor Prognosis

Tumor grading is a robust prognostic predictor in patients with neuroendocrine neoplasms (NEN) and guides therapy, especially in tumors with high proliferation. NEN can be separated into well-differentiated and poorly differentiated types. The more aggressive NEN have been further separated into neuroendocrine tumors (NET G3) with a better prognosis and neuroendocrine carcinomas (NEC) with a worse prognosis. Despite this distinction’s tremendous clinical and therapeutic relevance, optimal diagnostic biomarkers are still lacking. In this study, we analyzed the protein expression and prognostic impact of Enhancer of Zeste Homolog 2 (EZH2) by immunohistochemistry in 219 tissue samples of gastroenteropancreatic (GEP-NEN) and pulmonary NEN (P-NEN). EZH2 was almost exclusively expressed in NEN with a proliferation rate above 20% (G3), while all low-grade tumors were nearly negative. Among high-grade NEN, 65% showed high and 35% low expression of EZH2. In this group, the high expression of EZH2 was significantly associated with poor overall survival and NEC histology. Interestingly, EZH2 seems to act independently of Polycomb Repressive Complex 2 (PRC2) in NEN. In conclusion, we propose EZH2 as a robust biomarker for distinguishing between NET G3 and NEC among gastroenteropancreatic and pulmonary NEN

Recent advances in the molecular landscape of lung neuroendocrine tumors

Neuroendocrine tumors of the lung (Lung-NETs) make up a heterogenous family of neoplasms showing neuroendocrine differentiation and encompasses carcinoids and neuroendocrine carcinomas. On molecular grounds, they are considered two completely distinct and separate tumor groups with no overlap of molecular alterations nor common developmental mechanisms. Areas covered: Two perspectives were evaluated based on an extensive review and rethinking of literature: (i) the current classification as an instrument to obtaining clinical and molecular insights into the context of Lung-NETs; and (ii) an alternative and innovative interpretation of these tumors, proposing a tripartite separation into early aggressive primary high-grade neuroendocrine tumors (HGNET), differentiating or secondary HGNET, and indolent NET. Expert opinion: We herein provide an alternative outlook on Lung-NETs, which is a paradigm shift to current pathogenesis models and expands the understanding of these tumors