The findings of the study presented herein provide evidence that the DEAD/H (Asp-Glu-Ala-Asp/His) box helicase, DDX11, is upregulated with progression from early to advanced melanoma, and that this gene plays a pivotal role in shielding advanced melanomas from chromosome segregation defects and apoptosis.
Overshadowed for many years by efforts to find efficacious immunotherapies for advanced melanoma, and more recently by the focus on molecular therapy to target BRAF-mutated melanomas, considerably less attention has been paid to genes that are not only upregulated to high levels in advanced melanoma, but also play a pertinent role in protecting VGP and MGP melanomas from apoptosis. In a recent study, we documented by way of molecular targeting of the cell cycle regulator, CDK2, that VGP and MGP melanoma cells are highly vulnerable to interference with their progression through S phase. The data, summarized herein, provide not only further and strong support for this observation, but also demonstrate that inhibiting expression of a helicase such as DDX11, which has a vital function in sister chromatid cohesion[7, 9, 11], is deleterious for melanoma cells.
Thus far, little is known regarding the involvement, function, and importance of helicases in the progression from early to advanced melanoma, and their role in locally advanced and/or stage IV melanoma. Melanoma differentiation antigen 5 (MDA5), which comprises a caspase activation and recruitment domain (CARD) and an RNA helicase domain with ATP-dependent RNA-unwinding activity, was first isolated from a melanoma cell line. However, induced by Interferon-β (IFN-β), MDA5 is not expressed in cells representing advanced melanoma unless the cells are treated with the cytokine. A second helicase, recently shown to be expressed in a subpopulation of melanoma cells, referred to as ABCB5+ malignant melanoma-initiating cells, is HAGE (alias Cancer/testis antigen 13 (CT13); DDX43). HAGE was shown to promote proliferation and tumor growth of this subpopulation of melanoma cells, and to regulate AKT and ERK phosphorylation through NRAS.
The novel and important findings regarding the herein described pivotal role of DDX11 in advanced melanoma is that following inhibition of DDX11 expression, the cells not only exhibited a significantly higher number of chromosomes with partially closed as well as open/separated arms, but also that compared with the control, the average length of their chromosomes was shorter. To date, little if anything is known about how VGP and MGP melanoma cells guard against DNA damage, control and maintain their genome stability, and related to these survival processes, retain telomere length. In the context of a study published a few years ago, a hypothesis was put forward but not tested that DDX11 might be involved in telomere length determination. Recently, however, it has been documented that loss of DDX11 leads to changes in telomeric chromatin formation, and that DDX11 interacts with the flap structure-specific endonuclease 1 (FEN-1) gene, which has a vital role in telomere stability. Thus, it is possible that like DDX39, which when overexpressed leads to progressive telomere elongation and to telomere shortening when depleted, DDX11 has an important function in maintaining telomere length and stability in a malignancy such as advanced melanoma. The second and even more pertinent finding described herein is that inhibition of DDX11 expression leads to rapid and massive melanoma cell apoptosis. In the context of mouse mutant studies, it has been shown that loss of Ddx11 causes apoptosis[11, 19], but this is the first study which shows that inhibiting DDX11 expression in a malignancy that is refractory to virtually all apoptosis-inducing agents/therapies, leads to rapid and massive programmed cell death.
The biochemical functions of DDX11 have been established, but hitherto, a DDX11-specific small-molecule inhibitor is not available that would make it possible to systemically treat human melanoma xenografts and establish in vivo, therapeutic efficacy of blocking the function of DDX11. Given the possibility that like in the case of the Werner syndrome helicase, a human DDX11-specific small-molecule inhibitor will be isolated in the near future, it will be of importance to determine whether systemic therapy with such an inhibitor, alone or in combination with an inhibitor that blocks the function of FGFR1, which together with bFGF (FGF2) is a key regulator of melanoma proliferation, will have efficacy for advanced melanomas, and in particular for melanomas that are BRAF wild-type, which are the most aggressive type of advanced melanoma. Another interesting and important aspect that begs exploration is whether there is a link between DDX11 and pigmentation. Genetic disorders such as Bloom syndrome, Fanconi anemia, and the recently identified cohesinopathy, Warsaw breakage syndrome, are associated with abnormal skin pigmentation. Thus, it is a possibility that there is link between helicases such as DDX11 and the microphthalmia-associated transcription factor (MITF), which is a master regulator of melanocyte development.
In addition to the findings of our study presented herein, novel and compelling evidence that some members of the family of DEAD box helicases have pertinent functions in certain types of cancer has recently also been obtained in two other cases – one being breast cancer and the other medulloblastoma, which like melanoma is a neural crest-derived malignancy. In the case of breast cancer, the DEAD box helicase, DDX5, was found to be amplified and often co-amplified along with ERBB2, and breast cancer cell lines with amplification of the DDX5 locus were considerably more sensitive to its knockdown than breast cancer cell lines lacking this amplification. In the case of WNT-subgroup medulloblastomas that unlike the other three medulloblastoma subtypes have a good long-term prognosis, exome sequencing revealed somatic missense mutations in the gene DDX3X[23–25], which is a paralogue of the DEAD box helicase, DDX3.