Breast carcinomas with osteoclast-like giant cells: a comprehensive clinico-pathological and molecular portrait and evidence of RANK-L expression.

Breast carcinomas (BC) with osteoclast-like giant cells (OGC) are rare. Despite their distinct stromal features, their molecular characteristics remain unknown. Here, we report comprehensive clinico-pathological and molecular findings for 27 patients diagnosed with BC-OGC at Institut Curie between 2000 and 2021. Seventeen (63%) cases were invasive carcinomas of no special type (IC NST) with OGC (OGC-IC NST), four (15%) were mixed or multifocal cases with and without OGC (OGC-Mixed), and six (22%) were metaplastic carcinomas with OGC (OGC-MC). All OGC-IC NST and OGC-Mixed cases were ER+ HER2- tumors (most being luminal A based on transcriptomic subtyping, when available), while all OGC-MC were triple-negative. The median age at diagnosis was 46, 45 and 62 years for OGC-IC NST, OGC-Mixed and OGC-MC, respectively. Three patients developed distant metastases (one OGC-IC NST, two OGC-Mixed), one of whom died of metastatic disease (OGC-Mixed), and one other patient died of locally advanced disease (OGC-MC). Histopathological evaluation comparing 13 OGC-IC NST and 19 control IC NST without OGC confirmed that OGC-IC NST showed significantly higher density of vessels (by CD34 immunohistochemistry (IHC)), iron deposits (Perls stain), and CD68 and CD163-positive cell infiltrates. Genomic findings for nine OGC-IC NST and four OGC-MC were consistent with the underlying histologic subtype, including activating alterations of the PI3K/AKT/mTOR pathway in 7/13 cases. Using RNA-seq data, differential gene expression analysis between OGC-IC NST (n = 7) and control IC NST without OGC (n = 7) revealed significant overexpression of TNFSF11 (RANK-L), TNFRSF11A (RANK), CSF1 (M-CSF), CSF1R, and genes encoding osteoclastic enzymes (MMP9, ACP5, CTSK, CTSB) in OGC-IC NST, while OPG (osteoprotegerin) was underexpressed. We also confirmed for the first time RANK-L expression in BC with OGC by IHC (seen in 15 out of 16 cases, and only in 2 of 16 controls without OGC). These findings could offer a rationale for further investigating RANK-L as a therapeutic target in BC with OGC.

MED12 mutations in breast phyllodes tumors: evidence of temporal tumoral heterogeneity and identification of associated critical signaling pathways.

Exome sequencing has recently identified highly recurrent MED12 somatic mutations in fibroadenomas (FAs) and phyllodes tumors (PTs). In the present study, based on a large series, we confirmed the presence of MED12 exon 1 and 2 mutations in 49% (41/83) of PTs, 70% (7/10) of FAs and 9.1% (1/11) of fibromatoses. We show that MED12 mutations are associated with benign behavior of phyllodes tumors, as they are detected less frequently in malignant PTs (27.6%) compared to benign (58.3%) and borderline (63.3%) PTs, respectively (p = 0.0036). Phyllodes tumors presented marked temporal heterogeneity of MED12 mutation status, as 50% (3/6) of primary and recurrent phyllodes tumor pairs with MED12 mutation presented different MED12 mutations between the primary and recurrent tumors. There was no correlation between MED12 status and genomic profiles obtained by array-CGH. MED12 mutations are associated with altered expressions of the genes involved in the WNT (PAX3, WNT3A, AXIN2), TGFB (TAGLN, TGFBR2, CTGF) and THRA (RXRA, THRA) signaling pathways.In conclusion, this study confirmed that MED12 plays a central oncogenic role in breast fibroepithelial tumorigenesis and identified a limited number of altered signaling pathways that maybe associated with MED12 mutations. MED12 exon 1 and 2 mutation status and some of the altered genes identified in this study could constitute useful diagnostic or prognostic markers, and form the basis for novel therapeutic strategies for PTs.