TP53loss initiates chromosomal instability in high-grade serous ovarian cancer

Abstract

High-grade serous ovarian cancer (HGSOC) originates in the fallopian tube epithelium and is characterized by ubiquitous TP53 mutation and extensive chromosomal instability (CIN). While the direct causes of CIN are errors during DNA replication and/or chromosome segregation, mutations in genes encoding DNA replication and mitotic factors are rare in HGSOC. Thus, the drivers of CIN remain undefined. We therefore asked whether the oncogenic lesions that are frequently observed in HGSOC are capable of driving CIN via indirect mechanisms. To address this question, we genetically manipulated non-transformed hTERT-immortalized human fallopian tube epithelial cells to model homologous recombination deficiency (HRD) and oncogenic signalling in HGSOC. Using CRISPR/Cas9-mediated gene editing, we sequentially mutagenized the tumour suppressors TP53 and BRCA1, followed by overexpression of the MYC oncogene. Single-cell shallow-depth whole-genome sequencing revealed that loss of p53 function was sufficient to lead to the emergence of heterogenous karyotypes harbouring whole chromosome and chromosome arm aneuploidies, a phenomenon exacerbated by subsequent loss of BRCA1 function. In addition, whole-genome doubling events were observed in independent p53/BRCA1-deficient subclones. Global transcriptomics showed that TP53 mutation was also sufficient to deregulate gene expression modules involved in cell cycle commitment, DNA replication, G2/M checkpoint control and mitotic spindle function, suggesting that p53-deficiency induces cell cycle distortions that could precipitate CIN. Again, loss of BRCA1 function and MYC overexpression exacerbated these patterns of transcriptional deregulation. Thus, our observations support a model whereby the initial loss of the key tumour suppressor TP53 is sufficient to deregulate gene expression networks governing multiple cell cycle controls, and that this in turn is sufficient to drive CIN in pre-malignant fallopian tube epithelial cells.

The Genomic Landscape of Early-Stage Ovarian High-Grade Serous Carcinoma

“…Although the rates of CCNE1 amplification here did not differ significantly between our early-stage and late-stage cohorts, our data support the idea that high ploidy is associated with advanced HGSC and poorer prognosis, a concept first explored over 20 years ago (37). HGSC has profound levels of segregation error during cell division, an essential precursor of aneuploidy and WGD (38), and recent data suggest that WGD can emerge in hTERT-immortalized human fallopian tube epithelial cells with loss of wild-type p53 function in the presence of BRCA1 mutation and MYC overexpression, but not with TP53 mutation alone (39).…”

The genomic landscape of early stage ovarian high grade serous carcinoma

“…Although the rates of CCNE1 amplification did not differ significantly between our early and late stage cohorts, our data support the idea that WGD is associated with advanced HGSC and poorer prognosis. HGSC generally has profound levels of segregation error during cell division, an essential precursor of aneuploidy and WGD (38), and recent data suggest that WGD can emerge in hTERT -immortalised human fallopian tube epithelial cells with loss of wild-type p53 function in the presence of BRCA1 mutation and MYC over-expression, but not with TP53 mutation alone (39). However, one outstanding question is whether WGD per se promotes rapid dissemination in HGSC, or whether it is a time-related consequence and is thus more likely to be observed in late stage than early stage disease: detailed multi-site analysis of disseminated HGSC and in vitro models will be required as well as further analyses of early stage samples to address this question.…”