The use of these systems, primarily in suicide gene therapy and mutation compensation targeted approaches against ovarian cancer, has also yielded significant results. Specifically, Bai et al. transferred gelonin toxin in ovarian cancer cells using cationic heparin PEI (HPEI) nanogels and managed to reduce cancer cell growth and induce apoptosis
[35]. Furthermore, Huang et al.
[36] delivered diphtheria toxin subunit-A (DT-A) DNA in mice with ovarian tumours by means of cationic polymer administered IP, placing it under the control of a human epididymis protein 4 (HE4) promoter, whose activity is increased in ovarian cancer cells. The latter promoter was also used to drive the
TK gene, leading to the inhibition of tumour growth and increased survival in mice upon delivery
[36].
miRNAs
Large-scale microarray analysis has highlighted the role of many microRNAs [miR(s)] in different types of cancer, including ovarian and cervical cancer
[37]. miRNAs in ovarian cancer have shown to have either a tumour-promoting or tumour-suppressing role, depending on whether their expression is up-regulated or down-regulated, respectively, and hence can be employed both as therapeutic targets, as well as biomarkers for diagnosis and/or prognosis
[38]. Regarding ovarian cancer, Iorio et al.
[39] performed an initial and comprehensive miRNAs comparison between normal and ovarian cancer tissues and showed that miR-14, mir-199a, miR-200a, miR-200b, and miR-200c were up-regulated in ovarian cancer and thus acted as oncogenic miRs (oncomiRs), while miR-15, miR-16, mir-140, mir-145, mir-199a, and miR-125b1 were down-regulated, pointing toward a tumour-suppressing role (tumour suppressor miRs). Further studies highlighted novel players, such as miR-214, miR-150, miR-140-5P, miR-21, miR-29a, and let-7a and provided more insights into the role of different miRNAs in ovarian cancer.
2.2. Cervical Cancer
Cervical cancer remains the fourth most common and most lethal cancer type in women, despite regular screening and prevention strategies
[40]. Targeted gene therapy presents a promising approach for the treatment of the specific malignancy and focuses primarily on mutation compensation strategies, suicide gene therapy, oncolytic virotherapy, antiangiogenic strategies, immunopotentiation, and drug resistance therapies
[41], employing both viral and non-viral systems.
2.2.1. Viral Vectors
Lentiviruses
Cervical cancer gene therapy approaches employ LVs primarily for mutation compensation, antiangiogenic, and suicide gene therapy strategies. The restoration of important tumour suppressor genes’ expression, such as tyrosine phosphatase receptor J
(PTPRJ), asparaginase and isoaspartyl peptidase 1
(ASRGL1), and homeobox-containing 1
(HMBOX1), has been successfully achieved with LVs. Specifically, Yan et al.
[42] used pSicoR-PTPRJ LV to overexpress
PTPRJ, a tumour suppressor gene whose expression is down-regulated in human cervical cancer tissues and demonstrated significant suppression of cell viability, migration, and growth in HPV-negative C33A cells. On the contrary, the knock-down of PTPRJ expression in the above cervical cancer cell line led to increased resistance to 5-FU-mediated apoptosis, verifying the importance of elevated PTPRJ expression for cervical cancer prevention. Moreover, Zhou et al. used a lentiviral shRNA to knock-down HMBOX1 expression in HeLa and C33A cancer cells and demonstrated increased radiosensitivity as a result of telomere shortening
[43]. Lastly, the knock-down of ASRGL1 expression in SiHa cells by means of shRNA led to decreased proliferation, possibly through the reduced expression of CDK2 and cyclin A2 and the induction of apoptosis
[44], which was characterised by the increased expression of the pro-apoptotic Bax and the decreased expression of anti-apoptotic Bcl-2. Regarding the antiangiogenic approach, Qi et al. used a lentiviral shRNA-VEGF construct to knock down VEGF expression in vitro and in vivo in nude mice and was able to inhibit tumour growth and tumour radiosensitivity
[45]. One of the most promising approaches for cervical cancer gene therapy using lentiviruses is the genetic modification of T-cell receptors (TCRs) to target tumour-specific antigens. Based on the above, Jin et al. developed E7-specific T cells and achieved regression of HPV-positive mouse tumours
[46]. The specific approach is currently under clinical trial NCT02379520 and is being used on patients with metastatic cervical cancer
[47].
Adenoviruses (Ad) and Adeno-Associated Viruses (AAV)
They comprise the majority of viral vectors used for cervical cancer gene therapy, often utilised for tumour suppressor gene restoration or blocking of oncogenic expression. The
p53 gene, a key regulator of cell proliferation, apoptosis, and genetic stability
[48], plays a fundamental role in most gynaecological cancers and has therefore been a major candidate for most targeted gene therapy approaches
[49]. More specifically, since the
p53 gene is often inactivated by HPV E6 protein in most cervical cancers
[50]; either its delivery or inhibition of the E6 protein could result in significant antitumour effects. Indeed, Su et al. employing Genidicine
®, a gene therapy product approved in 2003 by the China Food and Drug Administration (CFDA) for head and neck cancer gene therapy
[51], showed that the injection of a recombinant human adenovirus engineered to express wild-type
p53 gene (rArd-p53) in cervical cancer patients, may lead to an increased 5-year overall survival rate
[52]. Moreover, Kajitani et al., using siRNA for E6 protein in HeLa cells, demonstrated successful
p53 transduction following adenovirus treatment
[53]. When the above transduction in HeLa cells was combined with PTX, it led to enhanced growth inhibition and apoptosis, as demonstrated by Liu et al.
[54].
2.2.2. Non-Viral Systems
Nanoparticles (NPs)
Gene therapy strategies by means of nanoparticles focus primarily on mutation compensation and immunopotentiation strategies. Regarding the former approach, Liu et al.
[55] developed polyethylene glycol-polylactic acid (PEG-PLA) NPs linked to folate and targeted cancer cells through the folate receptor α (FRα), a membrane-bound protein mediating folate uptake. Although the scholars observed enhanced gene transfection efficiency, higher compared to naked DNA, and reduced cytotoxicity, the strategies targeting FRα receptors are hampered by its heterogeneous expression among cervical cancer patients
[56]. A few years later, Yang et al. overexpressed FRα, previously shown to be highly expressed in cervical cancer, using an FRα-targeted liposome (FLP) to deliver a pigment epithelium-derived factor
(PEDF) gene into HeLa cells and observed significant anti-tumour activity, as demonstrated by significant growth inhibition, the suppression of adhesion and invasion, and cancer cell migration in vitro
[57].
Plasmids
The use of different plasmids in cervical cancer gene therapy approaches focuses on mutational compensation, antiangiogenesis, immunopotentiation and chemoresistance. With regards to blocking oncogenic expression, Hu et al. employed the genome editing approach using both the CRISPR/Cas9 and transcription-activator-like nucleases
(TALEN) systems to disrupt the HPV16
E7 oncoprotein coding gene
[58]. The data documented induction of apoptosis and growth inhibition in HPV16-positive human cancer cells. The latter approach is being studied in Phase I clinical trials for the treatment of CIN-1 patients with HPV16 and HPV18 infection
[47]. Recently, two groups demonstrated the therapeutic effect of targeting HPV
E6 and
E7 genes. Ling et al., using the CRISPR/Cas9 approach to delete the HPV18
E6 and
E7 genes, achieved robust knock-out of these proteins and increased apoptosis and tumour size reduction
[59]. In an attempt to compare the efficiency of CRISPR/Cas9 with the established TALEN approach, Gao et al. employed the former system against the HPV16
E7 gene and succeeded in reverting cervical carcinogenesis, both in vitro and in vivo
[60]. Another tumour suppressor gene with clinical importance in cervical cancer gene therapy is the retinoblastoma protein zinc finger gene 1
(RIZ1) since it can induce apoptosis and cell cycle arrest. Cheng et al. demonstrated that overexpression of RIZ1 expression in HPV16-positive cervical cancer cells could lead to impaired cell proliferation and increased apoptosis
[61].
miRNAs
miRNAs involved in cervical cancer induction and development are classified into oncogenic miRNAs (oncomiRs) and tumour suppressor miRNAs (tumour suppressor miRs)
[62][63], and therefore, their down-regulation or overexpression, respectively, can be curative, while their presence in patients’ serum is considered as an important tool for cancer diagnosis and prognosis. More specifically and regarding oncomiRs, the most important ones include miR-10a, miR-19, miR-20, miR-21, miR-133b, and miR-886-5p
[62]. Long et al. showed that miR-10a suppresses cell adhesion molecule L1, such as
(CHL1), and thus leads to enhancement of tumour growth, metastasis, and invasion
[64], while miR-19 is overexpressed in cervical cancer, and its silencing in SiHa cells led to a reduction in the proliferation and induction of apoptosis, through the up-regulation of Bax and down-regulation of Bcl-2 expression
[65]. MiR-20 is a positive regulator of tyrosine kinase non-receptor 2
(TNKS2), an oncogene involved in metastasis and invasion
[66]. In contrast, miR-21, which is the most well-known oncogenic miRNA, acts as a negative regulator of the tumour suppression gene programmed cell death 4
(PDCD4), which normally inhibits cell proliferation and induces apoptosis
[67], thus promoting tumour growth. Silencing by means of siRNA in cervical cancer cell lines led to inhibition of cell proliferation and induction of cell death by autophagy and caspase 3/7-mediated apoptosis
[68]. Moreover, through targeting and regulating
CCL20, a gene involved in tumour differentiation and metastasis, miR-20 was implicated in cervical squamous carcinogenesis
[67].
2.3. Endometrial Cancer
Despite being usually curable following surgery, endometrial cancer still presents as one of the most common female reproductive tract cancer types
[69]. Occasionally aggressive tumours, such as uterine papillary serous carcinomas (UPSC), are observed, with most of them demonstrating aberrant expression of p53
[70]. Gene therapy strategies toward endometrial cancer involve both viral and non-viral systems, with the former employing primarily adenoviral and retroviral vectors and the latter plasmid DNA/RNA.
analysis has highlighted the role of many microRNAs [miR(s)] in different types of cancer, including ovarian and cervical cancer
[37]. miRNAs in ovarian cancer have shown to have either a tumour-promoting or tumour-suppressing role, depending on whether their expression is up-regulated or down-regulated, respectively, and hence can be employed both as therapeutic targets, as well as biomarkers for diagnosis and/or prognosis
[38]. Regarding ovarian cancer, Iorio et al.
[39] performed an initial and comprehensive miR
2.3.1. Viral Vectors
Ramondetta et al. performed an adenovirus-mediated expression of
p53 or
p21 in a papillary serous endometrial carcinoma cell line and demonstrated growth inhibition and apoptotic cell death
[71]. Similarly, Ural et al. performed an in vitro suicide gene therapy approach using
HSV-TK and documented inhibition of endometrial cancer cell growth
[72]. A similar approach, but with the use of the pNF-κB plasmid, along with the gonadotropin-releasing hormone receptor (GnRH-R) agonist triptorelin and the prodrug GCV, resulted in reduced cancer cell growth, both in vitro and in vivo in mice
[73]. Recently, Xia et al.
[74] employed next-generation sequencing (NGS) in four out of the twelve patients enrolled in the clinical trial using the Ad-p53 vector, Genidicine
® [52] and observed reduced p53 expression in the tumours of three patients, all carrying mutations in tumour protein
p53 CREB binding protein
(CREBBP), cyclin-dependent kinase inhibitor 2A
(CDKN2A), LYN proto-oncogene
(LYN) and Janus kinase 2
(JAK2) genes. These data provide strong evidence that NGS can aid in the recruitment of suitable patients for Ad-53 uterine gene therapy.
2.3.2. Non-Viral Systems
Effective gene transfer with a significant inhibition of cancer cells in vitro was also achieved with non-viral approaches. Specifically, Maurice-Duelli et al. demonstrated that the transfer of
PTEN, a frequently mutated gene in endometrial cancer
[75], using PEI-photochemical irradiation could lead to a 44% inhibition in cancer cell growth
[76]. Moreover, the role of certain miRNAs in endometrial cancer has gained a lot of ground, highlighting their potential diagnostic and therapeutic value. Banno et al.
[77] highlighted the role of several miRNAs differentially expressed in endometrial cancer. Specifically, miR-185, miR-106a, miR-181a, miR-210, miR-423, miR-107, miR-let7c, miR-205, miR-449, and miR-429 were found up-regulated, while miR-let7e, miR-221, miR-30c, miR-152, miR-193, miR-204, miR-99b and miR-193b were significantly down-regulated, suggesting a tumorigenic or tumour-suppressing activity, respectively
[77]. miR-129-2 and miR-152 are involved in the development of endometrial cancer via DNA methylation; miR-125b, mir-30c, miR-200b/c, and miR-429 are related to cisplatin resistance, while miR-125b, miR-30c, miR-194 and miR-34b regulate proliferation, metastasis and invasion of endometrial cancer cells
[77]. Similarly, Donkers et al.
[78] highlighted the importance of miR-205, miR-200 family (miR-200a, miR-200b, and miR-200c), miR-135b, miR-182, miR-183, and miR-223 in endometrial cancer prognosis
[78]. As the above miRNAs are found to be up-regulated in most endometrial cancers, the down-regulation of their expression may have therapeutic outcomes. Regarding miRNAs, such as miR-137, miR-129-3p, and miR-410, whose expression is down-regulated in endometrial cancer, the scholars observed little to no consensus
[78], and therefore, their role in both predicting and treating endometrial cancer remains vague.