Recent advantages in DNA sequencing facilitate analyzing genomes of human tumors. Instead of considering each tumor as a result of a series of point mutations in oncogenes or tumor suppressor genes researches regard the malignancies as completely different tissue type (23). Cancer cells pass through rigorous natural selection accompanied by genome reorganization (24, 25). In any given cancer cell, a majority of mutations are the passengers, or innocent bystanders rather than drivers. T They do not improve ability of the tumors to survive, and, therefore, are a subject of either a negative selection or a selective sweep (26). Nevertheless, a variety of true driver mutations were discovered. The most common way of searching for driver mutations is to compare genomes of cancer samples and adjusting healthy tissues following by identification of genes repeatedly mutated in many samples of human tumors cancer (10). This approach may be complemented by an approach which is exactly opposite of one described above, namely, by looking for genes which never get any mutation in the tumors of a given type, with an assumptions that these mutations would render respective cells non-viable or non-competitive [12]. Execution of this approach in the model of human cutaneous melanoma revealed 91 genes which could be essential for melanoma surviving. Therefore, knockdown experiments in melanoma cell lines were performed to evaluate this theory.
According to Fantom5 dataset, just 44 of these genes) were, in fact, expressed in melanoma cell lines. This discrepancy may be explained by the differences could be explained by a difference between expression profiles of real tumors collected from patient and the immortalized cell lines explored in Fantom5. Further whittling down the gene list was performed by exclusion the genes previously associated with any human disease. No consensus trends were observed in subsequent analysis of 18 remaining genes, with some of them behaving in accordance with initial hypothesis, and some in exactly opposite fashion. For instance, previously described knockdowns of UNC45A in ovarian cancer and in myoprogenitor cells led to a decrease in their ability to proliferate (27). Similarly, shRNA inhibition of PTK2B resulted in the reduction of the growth of multiple myeloma tumors in vivo and a decrease in cell proliferation, cell-cycle progression, and adhesion ability in vitro (28). On the other hand, some genes behaved as cancer drivers. For example, downregulation of MYCT1 was observed in a majority of studied gastric carcinoma samples, in accordance of its ability to promote apoptosis of gastric carcinoma cell lines when overexpressed (29). In a similar fashion, overexpression of TGM5 was reported as inducing cell death (30).
We have taken onto account that for some genes effects on cell survival may be tissue-specific, and some other genes may play a role in survival of the cells only in the contexts of multicellular 3D structures, or when cancer cell would interact with immune cells. To minimize effects of the interaction taking place at the cell-to-cell interfaces, we have reduced the list to seven genes, by excluding all genes encoding for the proteins expressed on a surface of human cells.
After analyzing expression levels for these 7 genes in available melanoma cell lines sk-mel-1, sk-mel-5, G361 and A375, four genes with the highest expression levels all over, and in the A375 cell line in particular, were selected for knockdowns. Notably, for all four selected genes, namely, UNC45A, STK11IP, RHPN2 and ZNFX1, detected expression levels were extremely low. Nevertheless, we assumed that these genes may be important, for example, they may serve as transcription factors, or other regulatory molecules, and proceeded with wet-lab experiments.
The results on knockdown experiments show that the selected genes are not essential for melanoma cells viability or proliferation, but evidently exert their influence at the speed of the cell migration, and, possibly, other processes, not yet explored experimentally. None of these genes were extensively studied by other research groups, hence, current study does add to the current body of evidence for their roles in the functioning of the human body.
Gene UNC45A encodes co-chaperone of heat shock protein 90 (Hsp90). This gene is essential for certain types of cancers cells, including breast carcinoma, but is dispensable for normal cells (31), and for some other cancer types. Its connection to cell motility phenotypes are numerous as its function as a mitotic spindle-associated protein that destabilizes microtubules (MT) activity (32). In ovarian tumor cells, its depletion restores sensitivity to pactitaxel (32). Notably, paclitaxel is one of the prominent suppressors of chronic inflammation and supporters of immunosurveillance, and is currently in trials as anti-melanoma adjuvant. It is tempting to speculate that the maintenance of UNC45A is essential for masking of highly immunogenic melanoma cells from the restrictive actions of immune system.
Gene STK11IP encodes a leucine-rich repeat containing cytoplasmatic protein functionally linked to the Peutz-Jeghers syndrome kinase LKB1, and capable of interacting with the TGFbeta-regulated transcription factor SMAD4, and a formation of a ternary complex of LKB1- STK11IP-SMAD4 (33). LKB1 is a bioenergetic sensor that controls cell metabolism and growth by phosphorylating and activating AMP-activated Kinase (AMPK) in starving cells, but also is a key player in regulation of the immune system, which dampens proinflammatory responses in macrophages and maintenance of immunosurveillance (34). Accordingly, LKB1 deficient mice are less capable of controlling melanoma tumor growth due do an inherent defect in their DC-driven immunity and tolerance (34). If the effects of a product of STK11IP antagonize that of LKB1, than the lack of STK11IP mutations in melanoma cells may be expected. On the other hand, in melanoma cells, loss of LKB1 promotes cell invasion and migration through upregulation of MMP-2 (35). Loss of another component of LKB1-STK11IP-SMAD4 complex may lead to enhancement of the migration cells, if the effects of this component are synergistic with that of LKB1. While the function of STK11IP at the level of the whole body may not be relevant to the melanoma cell model in vitro, the observed pro-migratory effects of STK11IP knockdown align with a tumor suppressor and anti-metastatic function of its major protein partner, LKB1.
Rhophilin Rho GTPase binding protein 2 (RHPN2) drives mesenchymal transformation of malignant gliomas (36) and is likely to participate in actin skeleton organization (37). With that, its involvement in preventing an increase in the motility of the cells and in their migration is not surprising. More intriguing, similarly to gliomas, melanomas rely on phenotype switching between differentiated/proliferative and stem-cell/invasive transformation states as a key to their intra-tumor heterogeneity and resistance to treatment (38). While the promotion of the migration observed in the RHPN2 deficient cells was certainly not expected as it directly contradicts the data obtained in neuroprogenitor cells (36), protecting the central EMT driver from unwanted mutational events may certainly be expected.
Zinc finger NFX1-type containing 1 (ZNFX1) encodes an interferon (IFN)-stimulated, mitochondrial-localized dsRNA sensor capable of restricting the replication of RNA viruses (39). While its antiviral functions may not be relevant to the progression of human tumors, its antisense lncRNA ZFAS1 (zinc finger antisense 1) is well known for promoting growth of melanoma and other tumors (40). Specifically, in melanoma, knockdown of ZFAS1 was shown to reduces migration, invasion, and the markers of epithelial-mesenchymal transition (40). It is very likely that the knockdown of ZNFX1 resulted in the proportional increase in the levels of its antisense, thus, explaining the respective increase in melanoma cell migration.