Main Finding
In this study ctDNA was measured in plasma samples from early-stage ovarian cancer patients and borderline ovarian tumor patients using the SAGAsafe dPCR technology. The plasma levels of ctDNA mutations were increased in patients with higher stage of disease. In patients with ovarian cancer, higher levels of ctDNA in plasma were associated with worse OS. These results indicate that ctDNA analyzed in plasma can be useful pre-operatively both as a diagnostic and as a prognostic biomarker in ovarian cancer.
Value of ctDNA in ovarian cancer clinical practice
Specific genomic alterations in ctDNA can guide cancer treatment
TP53:
The most frequently mutated gene in these patients with ovarian tumors was TP53, especially in advanced stage OvCa. Several studies have shown that TP53 mutations are significantly associated with advanced OvCa since they are found in more than 90% of the high-grade serous ovarian carcinomas [28]. Loss of p53 function makes cells unable to induce apoptosis and therefore primes these cells for transformation into malignancy. Most mutations are single-base substitutions distributed throughout the coding sequence. TP53 mutations have been shown to be prognostic and targets for pharmacological intervention. In the current Swedish national guidelines (RCC), PARP inhibitors are recommended for use in treatment of BRCA-mutated epithelial ovarian cancer [29]. PARP inhibitors are effective in BRCA-mutated OvCa [29] and in other tumors with homologous recombination deficiency by inhibiting PARP1 DNA repair in cancers with a 8 mutation. Only a few studies have been performed to evaluate the efficacy of PARP inhibitors in terms of TP53 status [30, 31] and it has been presumed that targeting mutant p53, which destabilizes PARP repair function, may impede the distant spread of cancer cells [32].
KRAS:
KRAS gene mutations were found in seven tissue specimens, and one was found in plasma ctDNA. KRAS is a member of the Ras family of oncogenes, which also includes two other genes: HRAS and NRAS. The proteins these genes encode play important roles in cell division, cell differentiation, and the self-destruction of cells (apoptosis). Six mutations in KRAS were found in borderline tumors. Low-grade serous carcinomas (LGSOC) may progress from borderline tumors with frequent mutations of the KRAS, BRAF, or ERBB2 genes and lack of TP53 mutations [33]. Two large multicenter trials studying MEK inhibitors in LGSOC demonstrated activity in LGSOC, especially in KRAS-mutated disease. Accordingly, MEK inhibitors could be an alternative treatment in LGSOC [34]. In patients with advanced malignant melanoma with BRAF mutations, KRAS inhibition (KRASi) has shown very good results [35]. In lung adenocarcinoma patients with co-occurring TP53 mutations, clinical benefit has been demonstrated from PD-1 inhibitors and mutation status may guide anti-PD-1/PD-L1 immunotherapy [36].
Concordance between tumor DNA and ctDNA
Various plasma ctDNA technologies are being tested in academic and commercial laboratories [37]. Plasma ctDNA testing has a promising role in follow up as a tumor marker, especially when treatment is based or monitored on a gene mutation profile [38]. In this study, 15 out of 24 (62,5%) patients with tumor DNA mutations also had mutations detected in plasma ctDNA, in line with previous reports [39]. However, the possibility to detect ctDNA mutations in plasma depends on the morphology of the tumor, the stage of the disease, tumor burden, as well as DNA degradation [39, 40].
ctDNA in relation to stage
The plasma ctDNA mutant concentration increased with higher stage, which is in accordance with previous studies [41, 42]. In late-stage cancer, especially in abdominal organs such as colon, pancreas, or ovaries, ctDNA has been commonly detected in more than 60% of patients [43]. In this study ctDNA was detected in two patients in stage I OvCa. Interestingly, our study also detected ctDNA mutation in plasma in one patient with a borderline tumor, which has not previously been reported. Other studies observed that tumor volume assessed by CT imaging correlated with ctDNA levels in patients with relapsed high-grade serous ovarian cancer [44, 45]. Patients with advanced ovarian cancer have had median concentrations of 100–1,000 mutated gene copies per 5 ml of plasma [45]. The analyses in this study were able to detect less than 10 copies/mL in plasma in early stage of disease, indicating very high sensitivity. Our and previous observations indicate that the amount of ctDNA in plasma relates to tumor volume and stage of disease.
ctDNA mutations/mL plasma as prognostic factor
Our results showed that patients in advanced stage of ovarian cancer had more mutated gene copies than in early stage of disease. The prognosis for ovarian cancer patients is highly related to stage [46]. Similar studies with gynecologic cancers have found that higher percentage of ctDNA correlated with worse survival [47]. In addition, higher ctDNA concentration significantly correlated with worse progression-free survival (PFS) [48]. Our results showed that patients with more than 10 ctDNA mutation copie/mL plasma had significantly worse overall survival. This indicates that the amount of ctDNA in plasma may be used as a prognostic marker.
Limitations of the study
This study needs to be validated in larger patient cohorts to confirm and reproduce our results before clinical implementation. Only genetic variants in the coding regions of genes were investigated. Previous studies have found that variants located in the intronic, and non-coding regions of genomes may play a role in identifying individuals at genetic risk of developing ovarian cancer [49, 50] . Thus, this and other sequencing studies have limitations in identifying genetic variants associated with OvCa in non-coding regions.
In this study tumor material was available to sequence candidate somatic mutations for analyses of ctDNA in plasma. If no tumor material can be sequenced, and tumor-specific mutations analyzed, broad genomic panels must be designed for analyses of ctDNA in plasma.