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 PSNs, the World Health Organization classification introduced a new entity named atypical placental site nodule (APSN), corresponding to an ETT precursor, for which diagnostic criteria remain unclear, leading to a risk of overdiagnosis and difficulties in patient management. We retrospectively studied 8 PSNs, 7 APSNs, and 8 ETTs to better characterize this new entity and performed immunohistochemical analysis (p63, human placental lactogen, Cyclin E, and Ki67), transcriptional analysis using the NanoString method to quantify the expression of 760 genes involved in the main tumorigenesis pathways, and RNA sequencing to identify fusion transcripts. The immunohistochemical analysis did not reveal any significant difference in Cyclin E expression among the 3 groups (P = .476), whereas the Ki67 index was significantly (P < .001) higher in ETT samples than in APSN and PSN samples. None of the APSN samples harbored the LPCAT1::TERT fusion transcripts, in contrast to 1 of 6 ETT samples, as previously described in 2 of 3 ETT samples. The transcriptomic analysis allowed robust clustering of ETTs distinct from the APSN/PSN group but failed to differentiate APSNs from PSNs. Indeed, only 7 genes were differentially expressed between PSN and APSN samples; CCL19 upregulation and EPCAM downregulation were the most distinguishing features of APSNs. In contrast, 80 genes differentiated ETTs from APSNs, 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 ETTs and APSNs. These results suggested that APSN might not represent a distinct entity but rather 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 cases that need further clinical investigations.