Novel De Novo BRCA2 Variant in an Early-Onset Ovarian Cancer Reveals a Unique Tumor Evolution Pathway

Ovarian cancer (OC) is a highly heterogeneous malignancy, often characterized by complex genomic alterations that drive tumor progression and therapy resistance. In this paper, we report a novel de novo <i>BRCA2</i> germline variant NM_000059.3:c.(8693_8695delinsGT) associated with early-onset OC that featured two regions with differential MMR (Mismatch Repair) gene expression. To date, only six cases of de novo <i>BRCA2</i> variants have been reported, none of which were associated with early-onset high-grade serous OC. The immunohistochemical analysis of MMR genes revealed two distinct tumor areas, separated by a clear topographic boundary, with the heterogeneous expression of MLH1 and PMS2 proteins. Seventy-five percent of the tumor tissue showed positivity, while the remaining 25% exhibited a complete absence of expression, underscoring the spatial variability in MMR gene expression within the tumor. Integrated comparative spatial genomic profiling identified several tumor features associated with the genetic variant as regions of loss of heterozygosity (LOH) that involved <i>BRCA2</i> and <i>MLH1</i> genes, along with a significantly higher mutational tumor burden in the tumor area that lacked MLH1 and PMS2 expression, indicating its further molecular evolution. The following variants were acquired: c.6572C>T in <i>NOTCH2</i>, c.1852C>T in <i>BCL6</i>, c.191A>T in <i>INHBA</i>, c.749C>T in <i>CUX1</i>, c.898C>A in <i>FANCG</i>, and c.1712G>C in <i>KDM6A</i>. Integrated comparative spatial proteomic profiles revealed defects in the DNA repair pathways, as well as significant alterations in the extracellular matrix (ECM). The differential expression of proteins involved in DNA repair, particularly those associated with MMR and Base Excision Repair (BER), highlights the critical role of defective repair mechanisms in driving genomic instability. Furthermore, ECM components, such as collagen isoforms, Fibrillin-1, EMILIN-1, Prolargin, and Lumican, were found to be highly expressed in the MLH1/PMS2-deficient tumor area, suggesting a connection between DNA repair deficiencies, ECM remodeling, and tumor progression. Thus, the identification of the <i>BRCA2</i> variant sheds light on the poorly understood interplay between DNA repair deficiencies and ECM remodeling in OC, providing new insights into their dual role in shaping tumor evolution and suggesting potential targets for novel therapeutic strategies.