A review on the role of DANCR in the carcinogenesis

DANCR is an RNA gene located on chr4. This gene has several splice variants. Up-regulation of DANCR has been reported in many types of cancers. This lncRNA is mainly located in the cytoplasm and regulates genes expression at post-transcriptional level. In fact, it acts as a molecular sponge for a variety of miRNAs, including miR-874-3P, miR-335, miR-149, miR-4319, miR-758-3p, miR-216a-5p, miR-874-3p, miR-33a-5p, miR-335-5p, miR-145-3p, miR-665, miR-345-5p and miR-125b-5p. DANCR also regulates activity of PI3K/AKT/NF-κB, Wnt/β-catenin, ERK/SMAD, MAPK, IL-6/JAK1/STAT3, Smad2/3, p53, FAK/PI3K/AKT/GSK3β/Snail pathways. In the current narrative review article, we summarize the roles of DANCR in the carcinogenesis, with an especial emphasis on its role in the development of osteosarcoma and lung, liver, pancreatic and colorectal cancers.


Introduction
DANCR (Differentiation Antagonizing Non-Protein Coding RNA) is an RNA gene located on chr4: 52,712,257-52,723,623, plus strand. It has a size of 11,367 bases. This gene has 14 splice variants with sizes ranging from 272 bp (DANCR-207) to 6065 bp (DANCR-203), all of them being categorized as long non-coding RNA (lncRNA). This lncRNA has been regarded as a cancer-associated lncRNA, since its up-regulation has been reported in several cancer types in association with enhancement of cell proliferation and malignant properties [1]. DANCR regulates gene expression at post-transcriptional level [1]. Based on the findings obtained from RNA fluorescence in situ hybridization and expression assays in the cellular fractions, DANCR has been found to be primarily located in the cytoplasm [2]. In the current narrative review article, we summarize the roles of DANCR in the carcinogenesis, with an especial emphasis on its role in the development of osteosarcoma and lung, liver, pancreatic and colorectal cancers.

Cell line studies
Up-regulation of DANCR has been shown to upsurge proliferation, migratory propensity, and invasiveness of osteosarcoma cells. From a functional aspect, DANCR promotes progression of osteosarcoma through induction of cancer stem cells properties. DANCR up-regulates expression of AXL through sequestering miR-33a-5p. Further, DANCR enhances activity of AXL/Akt pathway. Cumulatively, DANCR is an important regulator of osteosarcoma progression [2]. Another study in osteosarcoma cells has indicated that inhibition of DANCR leads to decrease in ROCK1-mediated proliferation and metastasis. Mechanistically, DANCR regulates expression of ROCK1 through sequestering miR-335-5p and miR-1972 [3]. Other studies have revealed the impacts of DANCR/ miR-149/MSI2 axis [4] and DANCR/miR-216a-5p/SOX5 [5] axes in the pathoetiology of osteosarcoma. Moreover, METTL3 has been shown to contribute in this type of cancer through enhancement of stability of DANCR transcripts through m6A modification [6].
In bladder cancer cells, DANCR silencing has inhibited proliferation, migratory potential and invasion. DANCR has been shown to target miR-335/VEGF-C. miR-335 mimics could promote proliferation and invasive properties bladder cancer cells. In contrast, up-regulation of DANCR removes the effect of miR-335 mimics on these cells [7]. In addition, DANCR enhances metastatic and proliferative abilities of bladder cancer cells through increasing IL-11-STAT3 signals and CCND1 levels [8]. Finally, miR-149/MSI2 has been identified as another route of participation of DANCR in progression of bladder cancer [9].
In lung cancer cells, DANCR expression levels have been negatively correlated with levels of miR-216a [10]. Another study has identified the impact of DANCR/miR-1225-3p/ErbB2 axis in the regulation of metastasis of lung cancer cells [11]. Moreover, DANCR participates in the progression of this type of cancer through sequestering miR-496 and further modulating expression of mTOR [12]. DANCR can also regulate miR-214-5p/CIZ1 axis [13]. Moreover, invasive properties of lung cancer cells are regulated by DANCR through suppression of miR-216 and subsequent activation of Wnt/β-Catenin signals [14]. Figure 1 shows roles of DANCR in osteosarcoma, lung cancer, liver cancer, colorectal cancer, bladder cancer, and pancreatic cancer. Fig. 1 Oncogenic role of DANCR in osteosarcoma, lung cancer, liver cancer, colorectal cancer, bladder cancer, and pancreatic cancer Hepatocellular carcinoma is another type of cancer in which DANCR has an important effect. Up-regulation of DANCR in these cells has been associated with downregulation of miR-125b-5p. DANCR silencing or miR-125b-5p mimics could reduce cell cycle progression in HepG2 or Huh-7 cells, while promoting cell apoptosis. Both interventions could also inhibit migratory potential and invasiveness of these cells. Mechanistically, DANCR facilitates progression of this cancer through sponging miR-125b-5p and activating MAPK pathway [15]. DANCR could also contribute in the liver carcinogenesis through sponging miR-216a-5p and surging expression of KLF12 [16]. Another study in hepatocellular carcinoma cells has shown over-expression of DANCR and ATG7, and down-regulation of miR-222-3p. Besides, DANCR silencing has intimidated proliferation and autophagy of these cells. Mechanistically, DANCR induces proliferation, colony construction and autophagy of these cells through enhancing expression of ATG7 and decreasing expression of miR-222-3p [17]. Notably, DANCR can also affect response of hepatocellular carcinoma cells to sorafenib through enhancing activity of IL-6/STAT3 signals [18]. This lncRNA can also affect stemness and epithelial-mesenchymal transition (EMT) through modulating expression of CTNNB1 [19] and regulation of activity of ROCK1/LIMK1/COFILIN1 pathway [20], respectively.
In colorectal cancer cells, DANCR has been shown to affect activity of miR-125b-5p/HK2 axis to induce resistance to cisplatin through induction of anaerobic glycolysis [21]. In addition, DANCR/miR-518a-3p/MDMA axis has been identified as an imperative regulator of growth and malignant behavior of these malignant cells [22]. Most notably, the interaction between DANCR and the important oncogenic lncRNA MALAT1 has been found to induce resistance to doxorubicin-associated apoptosis in colorectal cancer cells [23].
In pancreatic cancer cells, DANCR regulates expression of miR-33b to promote proliferation and metastatic abilities [24]. Moreover, the invasive properties of these cells are regulated by DANCR/miR-214-5p/E2F2 [25] and DANCR/miR-135a/NLRP37 [26] axes. Figure 1 shows oncogenic roles of DANCR in osteosarcoma, lung cancer, liver cancer, colorectal cancer, bladder cancer, and pancreatic cancer. Expression of DANCR has been found to be increased in triple negative breast cancer cell lines. Notably, DANCR silencing has led to suppression of proliferation of these cells. Functional studies have detected that DANCR binding with RXRA enhances phosphorylation of this protein on its serine 49/78 via GSK3β, which subsequently leads to activation of PIK3CA transcription, and induction of PI3K/AKT signals [27]. Another study has shown over-expression of DANCR and VAPB in breast cancer cells, parallel with down-regulation of miR-4319. DANCR silencing not only has stalled proliferation, migratory potential, and invasiveness of breast cancer cells, but also has induced their apoptosis. These effects have been found to be mediated through regulation of miR-4319. This study has revealed the importance of DANCR/miR-4319/VAPB axis in development of this cancer [28]. Another mechanism of involvement of DANCR in the pathogenesis of breast cancer is mediated through enhancement of the EZH2 binding to the promoter of SOCS3, which results in suppression of expression of SOCS3. Up-regulation of SOCS3 or suppression of EZH2 has led to reversion of malignant features stimulated by DANCR [29].
Expression of DANCR has been found to be high in cisplatin-resistant gastric cancer cells. However, siRNAmediated silencing of this lncRNA in SGC7901/DDP and BGC823/DDP cells has led to significant decrease in their survival and induction of apoptosis. Furthermore, DANCR up-regulation could up-regulate expression levels of MDR1 and MRP1 in cisplatin resistant gastric cancer cells [30]. Another study in gastric cancer cells has shown that KLF5 activates DANCR transcription. DANCR could act as a molecular sponge for miR-194 to suppress its expression and increase expression of AKT2, thus promoting gastric carcinogenesis through inhibition of autophagy [31]. Moreover, expression of DANCR in gastric cancer can be induced by SALL4 [32]. Table 1 summarizes the molecular axes mediating the effects of DANCR in the carcinogenesis, based on the results of in vitro studies.

Animal studies
Up-regulation of DANCR in osteosarcoma cells has been shown to promote xenograft tumor growth and lung metastases [2]. Critical roles of this lncRNA in induction of metastatic pathways have also been confirmed in animal models of colon cancer [22], nasopharyngeal carcinoma [73] and prostate cancer [85]. Moreover, results of experiments in animal models of cancer have suggested the impact of DANCR in resistance to sorafenib and cisplatin in hepatocellular carcinoma [18] and colon cancer [21], respectively. Moreover, bulk of evidence from investigations in xenograft models of cancer firmly supports the role of DANCR in induction of tumor growth ( Table 2).

Clinical studies
Expression of DANCR has been constantly enhanced in osteosarcoma samples, and its up-regulation has been positively associated with size of tumors and their metastatic ability. In fact, it is regarded as an independent poor prognostic factor for osteosarcoma. Besides,

Discussion
DANCR is regarded as an oncogene in almost all types of cancers. All conducted studies have indicated upregulation of DANCR in cancer tissues/cell lines except for a single study in renal cell carcinoma [86]. Moreover, two studies in papillary thyroid cancer [91] and hepatocellular carcinoma [92] reported down-regulation of this lncRNA, in spite of the bulk of evidence regarding upregulation of DANCR in these two types of cancers. In support of the oncogenic role of DANCR, several studies have indicated association between up-regulation of DANCR and poor clinical outcomes. Moreover, overexpression of DANCR has been more frequently detected in patients having advanced clinical stages and distant metastases.
Over-expression of DANCR has also been associated with resistance to anti-cancer agents such as cytarabine, sorafenib, cisplatin and docetaxel. These findings indicate that DANCR-targeting therapies might affect response of cancer cells to a wide array of drugs, possibly conquering multidrug resistance.
DANCR has also been shown to possess appropriate diagnostic power to differentiate patients with liver cancer from healthy persons or those with non-malignant liver disorders [62]. Since this expression assay has been conducted in plasma samples, it potentiates DANCR as a non-invasive marker for cancer detection.
Tens of tumor suppressor miRNAs have been shown to be sponged by DANCR, leading to release of miRNA targets from their inhibitory effects. DANCR can also regulate activity of several important cancer-related pathways such as PI3K/AKT/NF-κB, Wnt/β-catenin, ERK/SMAD, MAPK, IL-6/JAK1/STAT3, Smad2/3, p53, FAK/PI3K/ AKT/GSK3β/Snail pathways. Since several signaling pathways are influenced by DANCR, drugs targeting this lncRNA are expected to affect numerous aspects of carcinogenesis, thus being effective in treatment of a wide range of cancers with different biological behaviors.
In addition, DANCR has interactions with a number of proteins including CTNNB1, RXRA, EZH2 and PRC2. Most importantly, interaction of DANCR with proteins that influence epigenetic marks shows the importance of DANCR in the regulation of gene expression.

Conclusion
Although several expression assays have assessed expression levels of DANCR in biological samples obtained from different types of cancers, the underlying cause of dysregulation of DANCR in cancer has not been identified. In addition, the impacts of genomic variants on expression of this lncRNA and possible associations between single nucleotide polymorphisms within DANCR gene and susceptibility to cancer have not been appraised yet. Thus, future investigations should focus on these aspects. High throughput sequencing techniques could facilitate answering to these questions in near future.