2. Current Molecular Approaches for the Diagnosis and Prognosis of HGSOC
Difficulties in the early detection of HGSOC, before the disease develops to advanced stages, can be attributed to the lack of specific symptoms, which are usually missed or attributed to other pathologies
[21]. In clinical practice, the diagnosis of EOC is based on four main techniques: pelvic palpation examination (PPE), imaging (which includes transvaginal ultrasound or sonography, magnetic resonance imaging, computed tomography, and positron-emission computed tomography
[22,23,24][22][23][24]), serum levels of specific proteins, and surgery (either laparoscopic or laparotomic)
[25]; however, there is an urgent need to develop alternative techniques for early-stage HGSOC identification. Accordingly, new diagnostic methods based on cellular techniques or molecular approaches—such as gene expression profiling via NGS—are under development.
2.1. Molecular Markers and Algorithm Decisions for the Diagnosis of HGSOC: Carbohydrate Antigen 125 (CA125), Alone or in Combination with Other Imaging Techniques or Biomarkers
Initial studies found elevated levels of CA125—a transmembrane cell-surface protein encoded by the
MUC16 gene—in HGSOC compared to healthy ovarian tissue
[26,27][26][27]. CA125 remains the most-used molecular marker; however, the use of CA125 suffers from several drawbacks. Elevated CA125 levels occur in only 23–50% of stage I-II cases, and CA125 cannot be detected in all advanced HGSOC cases
[28,29,30,31,32][28][29][30][31][32]. Moreover, female patients who smoke or develop inflammatory processes, as well as those with physiological or benign conditions (e.g., menstruation, pregnancy, uterine fibroids), might display altered serum CA125 levels, thereby increasing false positive rates and HGSOC misdiagnoses
[33,34,35,36,37,38,39,40,41,42,43,44][33][34][35][36][37][38][39][40][41][42][43][44]. This fact highlights the need to conduct further studies to elucidate the relationship between these variables and CA125 levels. Therefore, CA125 levels alone cannot discriminate early HGSOC cases with sufficient sensitivity and specificity. To solve this problem, clinical trials and triage algorithms have explored the power of combining CA125 with imaging techniques for the early diagnosis of HGSOC. Additionally, distinct predictive indices based on biomarkers and ultrasound have been developed to differentiate the nature of adnexal masses, which have the advantage of eliminating inter- and intra-observer variability, estimating the probability of mass malignancy, and increasing efficacy and efficiency
[45].
The Risk of Malignancy Index (RMI) algorithm combines CA125 levels, ultrasound results, and menopausal status
[46]; however, despite the proposal of several combinations of the formula
[47], the RMI possesses lower sensitivity than transvaginal sonography (TVS) alone
[32,48][32][48]. For this reason, clinical trials such as the Prostate, Lung, Colorectal, and Ovarian (PLCO) Cancer Screening Trial evaluated CA125 levels combined with TVS; however, the results failed to provide evidence of improvements in mortality rates or early-stage HGSOC detection rates
[30,31,49,50][30][31][49][50].
Other strategies based on the Risk of Ovarian Cancer Algorithm (ROCA)
[51] aimed to stratify risk according to CA125 levels; however, the use of ROCA in the Normal Risk Ovarian Screening Study (NROSS)
[52] or the United Kingdom Collaborative Trial of Ovarian Cancer Screening (UKCTOCS)
[53] clinical trials failed to suffice for the early detection of OC due to a high number of false positives and the associated failure to reduce HGSOC-associated mortality rates
[14,54][14][54]. Conversely, a study by Srivastava et al. that combined CA125 with additional protein biomarkers suggested some clinical impact
[20]. The glycoprotein HE4 (human epididymis 4) displays elevated levels in HGSOC and endometrioid EOCs, presumably at advanced tumor stages
[55,56][55][56]. Meanwhile, other biomarkers examined in combination with CA125 and HE4 include mesothelin
[57], CEA and VCAM-1
[58], glycodelin
[59], IL-6 and E-cadherin
[60], and transthyretin
[61]. The Assessment of Different Neoplasias in the Adnexa (ADNEX) model represents an alternative strategy that considers six ultrasound and three clinical predictors (including CA125 levels) to discriminate between benign, borderline, invasive, and metastatic ovarian tumors
[62].
Alternative strategies such as the Risk of Ovarian Malignancy Algorithm (ROMA), OVA1™, or OVERA™ deserve special consideration given their U.S. Food and Drug Administration (FDA) approval and their relevance to routine clinical practice. ROMA, which displays high performance in menopausal patients, evaluates HE4 and CA125 levels and stratifies patients with adnexal masses into low or high risk of malignancy
[63,64][63][64]. HE4 levels are not usually modified by benign pathologies or external factors, and they display greater specificity in differentiating between malignant and benign tumors; furthermore, HE4 combined with CA125 outperforms the specificity and sensitivity in detecting cases missed when using CA125 alone
[32,65,66,67,68][32][65][66][67][68]. OVA1™ comprises a multivariate index assay (MIA) test that examines five biomarkers (i.e., transthyretin, apolipoprotein A-1, 2-microglobulin, transferrin, and CA125) and scores the likelihood of malignancy of a pre-detected adnexal mass prior to surgical intervention
[69,70,71][69][70][71]. OVERA™ examines levels of CA125, HE4, apolipoprotein A-1, FSH (follicle-stimulating hormone), and transferrin, achieving a sensitivity of 91% and specificity of 61% for HGSOC screening
[72]. Other recently proposed protein panels have included CA125, vitamin-K-dependent protein Z, C-reactive protein, and LCAT
[73]; CA125, HE4, CA72-4, and MMP-7
[74]; or CA125, HE4, FOLR1, KLK11, WISP1, MDK, CXCL13, MSLN, and ADAM8
[75]. The most recent strategies rely on the detection of autoantibody levels in combination with CA125
[76,77][76][77] and involve the detection of anti-TP53
[78], anti-HSF1, or anti-CCDC155
[79].
2.2. Gene Expression Profiling and Gene Panels
While ongoing gene expression profiling studies have suggested that HGSOC represents a highly heterogeneous pathology
[6], expression analyses can differentiate between HGSCO and other types of EOC. For example, Sallum et al. reported a differential
WT1,
TP53, and
P16 expression profile that distinguishes HGSCO from LGSOC
[80], while Li et al. found that 11 differentially expressed genes (DEGs) could discriminate between borderline cases and HGSOC tumors
[81].
Different gene expression profiles reported by studies such as the Cancer Genome Atlas (TCGA) project
[82,83][82][83] can be associated with pathological outcomes
[84]. For instance, high expression of
HOX,
FAP (myofibroblast markers), and
ANGPTL1/2 (markers of microvascular pericytes) or high expression of
HMGA2,
SOX11 (transcription factors),
MCM2, and
PCNA (proliferation markers) is correlated with worse prognosis
[85,86,87][85][86][87]. Additionally, those studies linked improved survival rates and patient prognoses to
MUC gene (
MUC16) expression or
CXCL11,
CXCL10 (chemokine ligands), and
CXCR3 (receptor) expression
[85,86,87][85][86][87].
Gene expression panels such as
NR5A1,
GATA4,
FOXL2,
TP53, and
BMP7 possess different profiles when comparing primary OC tumors and their metastases, and could eventually be used to predict patient survival
[88]. Additionally, the expression of homologous recombination repair (HRR) genes (e.g.,
BRCA1,
ATR,
FANCD2,
BRIP1,
BARD1, and
RAD51) is associated with a better prognosis in HGSOC cases, whereas the expression of epithelial-to-mesenchymal genes (e.g.,
GATA4,
GATA6,
FOXC2,
KLF6, and
TWIST2) is associated with a worse prognosis
[89].
Studies have also assessed differential gene expression in HGSOC to guide the optimal therapeutic choice and measure expected responses
[84]. A meta-analysis by Matondo et al. that included 1020 patients identified a prognostic signature regulated by
HIF1α and
TP53 in therapy-unresponsive patients as an indicator of a worse overall prognosis
[90]. Lee et al. also identified several DEGs in patients who underwent complete gross resection or neoadjuvant chemotherapy with either positive or negative responses
[91]. Most recently, Buttarelli et al. identified a 10-gene signature (including genes such as
CTNNBL1,
CKB,
GNG11,
IGFBP7, and
PLCG2) for classifying wild-type BRCA HGSOC patients into sensitive- and resistant-to-therapy groups
[92].
Novel bioinformatic approaches are currently attempting to refine gene expression signatures that predict therapeutic responses
[93] and differentiate between HGSOC and other cancer types
[94]. For example, co-expression network analyses have identified
UBE2Q1 as a prognostic biomarker associated with poor relapse-free overall survival in HGSOC patients
[95].
Overall, using DEG analysis in early diagnosis has the potential to improve the management and survival of HGSOC patients.