Ovarian cancer (OC) is the fifth-most deadly cancer in women, worldwide, and the most lethal gynecological cancer [1]. The poor prognosis occurs due to asymptomatic or delayed symptom onset and lack of effective screening methods. As a result, the diagnosis is made at an advanced stage when treatment is less effective. Furthermore, due to frequent early diagnosis failure, the treatment cost OC patients is higher than of other types of cancer [2]. Accordingly, the need for new diagnostic tolls or novel targeted therapies are urgently needed. Epithelial ovarian cancer (EOC) is divided into different subtypes, based on histology, which is the appearance of the tumor cells and molecular characterizations to differentiate Type I or Type II EOC [3]. Clinically, type I tumors thought to arise as large, unilateral cystic neoplasms confined to the ovary. They are generally recognized as indolent or low grade with less aggressive clinical behavior and a more favorable prognosis [3]. In contrast, type II ovarian tumors are high-grade, can involve both ovaries, show aggressive biological behavior, tend to present at advanced stages, and have poorer outcomes. And type II EOC includes high-grade serous carcinoma (HGSOC), high-grade endometrioid carcinoma, carcinosarcoma, and undifferentiated carcinoma [3]. In particular, HGSOC is a subtype of epithelial ovarian cancer and accounts for about 90 % of ovarian cancer. In HGSOC, tissue or cells of the tumor are not properly differentiated and, therefore, do not have a clear structure or pattern [4]. Furthermore, HGSOC is a grade 3 ovarian carcinoma, which is abnormally organized and rapidly growing. Approximately 70 % of HGSOC patients are diagnosed at stage 3, where the tumor has spread to the abdomen or extraperitoneal cavity [5]. Moreover, HGSOC is one of the deadliest cancers in humans, and the prognosis for patients with HGSOC is very poor. Rapid acquisition of resistance to conventional chemotherapies contributes to poor patient outcome. Different histological subtypes of ovarian cancer are known to have different tumor genetic alterations, indicating that these patients may be more suitable for treatment with different targeted agents. Therefore, there are technologies which have impacted biomarker development for detection or therapy of OC [6]. Most of HGSOC cases have been attributed to germline mutations in BRCA1 and BRCA2 genes, which is frequent molecular event in women with the most lethal and prevalent type of HGSOC. More specifically, the prevalence of germline BRCA1/2 mutations in patients with this histology is up to 20 %, while and additional 5–10 % bear tumors with somatic mutations. This information has important clinical implications since individuals with deleterious BRCA mutations compared to non-carriers are known to have superior prognosis, exhibit better response to therapy [7]. Although targeted therapies for HGSOC are being developed, the efficacy of targeted therapies is limited due to the lack of biomarkers. Therefore, new biomarker-based treatments for HGSOC are urgently needed [8].

Roughly 50 % of HGSOC have defective DNA double-strand break (DSB) repair caused by mutations in BRCA1/2 [9]. There are six major DNA repair pathway can be used to address DNA damage including, namely base excision repair (BER), nucleotide excision repair (NER), single strand break repair (SSR, homologous recombination (HR), non-homologous end joining (NHEJ), mismatch repair (MMR). In cancer with deficiency in on DNA repair pathway, inhibition of the second DNA repair pathway often creates a synthetic lethality [10]. Synthetic lethality results from the combined effect of two genetic variations, which are not lethal when occurring alone. PARP inhibitors are synthetically lethal in HR deficient cells. Indeed, pharmacological PARP inhibition induces the development of DSBs at the replication fork and blocks alternative repair pathways like NER, BER, NEHJ, resulting in loss of genome integrity in cells with HR deficiency. Alterations in DNA damage response (DDR) genes are common in advanced ovarian tumors and are associated with unique genomic and clinical features. In an era focused on targeted strategies utilizing genetic modifications common in HGSOC, innovative clinical trials have led the FDA to approve PARP inhibitors (PARPis) as a maintenance therapy, in first-line and relapse settings [11]. In addition, although PARPis have moderate activity against BRCA1/2 mutated HGSOC, complete responses to PARPi monotherapy are rare, and partial responses to recurrent disease are more common [12,13]. Moreover, resistance to PARPis has been associated with multiple mechanisms, including secondary mutations in genes involved in HR, and stabilization of DNA replication forks [14]. Therefore, investigating the mechanisms of resistance to PARPis, and constructing strategies for subsequent combination therapy, has significant clinical value. Roughly partly of HGSOCs show defects in HR DNA repair due to defects in BRCA1/2 and other HR-related genes, making them ideal candidates for PARPi based therapy [15]. Finally, most patients discontinue PARPis because of progression, and the optimal management of PARPi-resistance HGSOC is a pressing clinical challenge [16]. According to the data, clinical studies have characterized several mechanisms of resistance to PARPis that include restoration of HR repair by several DDR target genes [17].

Vaccinia-related kinases (VRKs) are a family of serine/threonine (Ser/Thr) kinases, composed of VRKs 1–3 in mammals [18], sharing a similar Ser/Thr protein kinase domain. Additionally, VRK1 and VRK3 have nuclear localization signals in their C- and N-terminal, respectively, whereas VRK2 contains a transmembrane domain in its C-terminus [19,20]. Among VRK family proteins, VRK1 is the most extensively studied, and exhibits poor prognosis when overexpressed in several cancers [[21], [22], [23]]. Some proteins that interact with VRK1 are characterized, and reports suggest that VRK1 plays a crucial role in cancer progression by targeting several substrates involved in the cell cycle and DNA repair [24]. In resting cells, VRK1 localizes to chromatin, and in cycling cells, covers all DNA, except when chromosomes are already condensed in mitosis. When chromosomes segregate, VRK1 returns to chromatin in daughter cells. Because of the physical association of VRK1 with chromatin, VRK1 has also been implicated in the regulation of DDR proteins [[25], [26], [27]]. VRK1 also directly associates with different components of DDR pathways, especially in studies of the response to DSBs, in both resting and cycling cells, as well as in ATM null and p53 null cells [28].

According to published research, screening has identified several genes necessary for survival and proliferation of ovarian cancer cell lines growing as tumor masses in immunocompromised mice [29]. One study used a lentiviral library encoding almost 8000 shRNAs directed at all human protein kinases. The several genes identified through this screen included ataxia telangiectasia mutated (ATM), cyclin-dependent kinase 2 (CDK2), polo like kinase 1 (PLK1), protein kinase, DNA-activated, catalytic subunit (PRKDC), and VRK1 [29]. These candidate genes have previously been reported to be overexpressed in ovarian cancer, or to play a functional role in the disease [29]. In this study, we examined the role of VRK1 in regulating the growth of ovarian cancer cell lines. We demonstrate that VRK1 depletion activates DNA damage and increases genomic instability via dysregulation of DNA-PK stability and subsequent impairment of the NHEJ pathway. As a member of the phosphatidylinositol 3-kinase related kinase family, DNA-PK plays a key role in cellular DNA damage response. In response to DSBs, DNA repair occurs through NHEJ and HR. Unlike HR, which requires a homologous template, NHEJ can repair DNA without a homologous template, and DNA-PK plays a role in reconnecting double-strand breaks [30]. In addition, DNA-PK plays a multifaceted role in protecting against DNA damage and genome instability, including the replication stress response, cell cycle checkpoint, telomere length maintenance, and aging [31]. Thus, new drug development targeting DNA-PK is in progress, and combination efficacy studies with various therapies including radiation therapy are in progress.

Here, we hypothesized that VRK1 is responsible for DNA-PK-mediated impairment of the DDR pathway. We found that VRK1 depletion decreased DNA-PK expression, and increased olaparib sensitivity in ovarian cancer cell. These findings suggest that VRK1 is crucial for NHEJ impairment, via repression of DNA-PK stability.