Molecular analyses of chorionic-type intermediate trophoblastic lesions: Atypical placental site nodules are closer to placental site nodules than epithelioid trophoblastic tumors

Gestational trophoblastic diseases derived from the chorionic-type intermediate trophoblast include benign placental site nodule (PSN) and malignant epithelioid trophoblastic tumor (ETT). Among PSN, the WHO classification introduced a new entity named atypical placental site nodule (APSN), corresponding to an ETT precursor, for which the diagnostic criteria remain unclear, leading to a risk of over-diagnosis and difficulties in patient management. We retrospectively studied 8 PSN, 7 APSN and 8 ETT to better characterize this new entity. We performed an immunohistochemical analysis (p63, hPL, Cyclin E, and Ki67), a transcriptional analysis using the Nanostring method to quantify the expression of 760 genes involved in the main tumorigenesis pathways, and a RNA sequencing to identify fusion transcripts. The immunohistochemical analysis did not reveal any significant difference in Cyclin E expression between the three groups (p = 0.476), whereas the Ki67 index was significantly (p LPCAT1-TERT fusion transcripts previously reported in ETT. The transcriptomic analysis allowed robust clustering of ETT distinct from the APSN/PSN group but failed to distinguish APSN from PSN. Indeed, only seven genes were differentially-expressed between PSN and APSN samples, CCL19 upregulation and EPCAM downregulation were the most discriminating features of APSN. In contrast, 80 genes discriminated ETT from APSN, establishing a molecular signature for ETT. Gene set analysis identified significant enrichments in the DNA damage repair, immortality and stemness, and cell cycle signaling pathways when comparing ETT and APSN. These results suggested that APSN might not represent a distinct entity but rather a variant of PSN or a transitional stage between PSN and ETT. RNA sequencing and the transcriptional signature of ETT described herein could serve as triage for APSN from curettage or biopsy material, enabling the identification of the cases that need further clinical investigations.