CCDC6: the identity of a protein known to be partner in fusion

Abstract

Coiled Coil Domain Containing 6 gene, CCDC6, was initially isolated as part of a tumorigenic DNA originated by the fusion of CCDC6 with the tyrosine kinase of RET receptor, following a paracentric inversion of chromosome 10. For a long time, CCDC6 has been considered as an accidental partner of the RET protooncogene, providing the promoter and the first 101 aa necessary for the constitutive activation of the oncogenic Tyrosine Kinase (TK) RET in thyroid cells. With the advent of more refined diagnostic tools and bioinformatic algorithms, an exponential growth in fusion genes discoveries has allowed the identification of CCDC6 as partner of genes other than RET in different tumor types. CCDC6 gene product has a proper role in sustaining the DNA damage checkpoints in response to DNA damage. The inactivation of CCDC6 secondary to chromosomal rearrangements or gene mutations could enhance tumor progression by impairing the apoptotic response upon the DNA damage exposure, contributing to the generation of radio- and chemoresistance. Preclinical studies indicate that the attenuation of CCDC6 in cancer, while conferring a resistance to cisplatinum, sensitizes the cancer cells to the small molecule inhibitors of Poly (ADP-ribose) polymerase (PARP1/2) with a synthetic lethal effect. Several CCDC6 mutations and gene rearrangements have been described so far in different types of cancer and CCDC6 may represent a possible predictive biomarker of tumor resistance to the conventional anticancer treatments. Nevertheless, the detection of a CCDC6 impairment in cancer patients may help to select, in future clinical trials, those patients who could benefit of PARP-inhibitors treatment alone or in combination with other treatments.

NSCLC Mutated Isoforms of CCDC6 Affect the Intracellular Distribution of the Wild Type Protein Promoting Cisplatinum Resistance and PARP Inhibitors Sensitivity in Lung Cancer Cells

“…The overall importance of the cell cycle checkpoints and DNA damage repair (DDR) proteins in maintaining genomic integrity is highlighted by the observation that the genes involved in the DDR process are often lost, mutated, or silenced in cancer cells [45][46][47][48][49]. Because of its role in the surveillance of DNA integrity, CCDC6 has been proposed as a tumor suppressor gene [4,50].…”

“…Coiled Coil Domain Containing 6 (CCDC6) gene product is ubiquitously expressed and target substrate of several serine/treonine kinases named ERK1/2, ATM, CDK1, and GSK3β, which, depending on the intracellular signaling, modulate the stability and the intracellular distribution of the protein [1][2][3][4][5][6][7]. In the nucleus CCDC6 exerts a main action in the process of DNA homologous recombination (HR) for the repair of DNA double strand breaks (DSB) and for the cell mitotic entry.…”

“…As the NHEJ process is prone to errors, the cancer cells defective for CCDC6 show tolerance to the DNA damage induced by several mutagens, including cis-platinum. Furthermore, the switch to the NHEJ repair process that involves the poly (ADP-ribose) polymerase enzymes (PARP1/2) makes the CCDC6 defective cells sensitive to PARP inhibition because of a synthetic lethal effect [11].In several in vitro cancer cell systems CCDC6 protein levels have been correlated with cancer cell tolerance to DNA damage, resistance to cisplatinum and sensitivity to poly (ADP-ribose) polymerase inhibitors (PARPi) [4,[11][12][13][14][15][16].To date, besides the first CCDC6 fusion with the tyrosine kinase RET identified in papillary thyroid cancer [17,18], additional CCDC6 fusions have been reported with PDGFRb [19,20], PTEN [21], FGFR2 [22,23] ANK3 [24], and UBE2D1 [25] and with other genes in different tumor types [26].Following the fusions, the recombinant oncogenes always include, at least in part, the CCDC6 coiled-coil region that is able to induce protein dimers [27,28]. Interestingly, in the case of CCDC6/RET, the first 101 amminoacid (aa) of CCDC6 involved in the fusion can form heterodimers with wild type protein of CCDC6 and impair its physiological function [1].In some cases, cancer cells carrying the rearranged allele of CCDC6 also show the loss of the unrearranged allele which is suggestive of a suppressive role of CCDC6 in tumor progression [29].In NSCLC, the CCDC6 involvement can be ascribed both to the low levels of the protein expression, as reported in 30% of patients, and correlated with tumor progression and bad prognosis (OS and DFS) [16], and to the fusion of the first CCDC6 101 aa with the RET and ROS1 kinases, as detected in about 1% of NSCLC patients.…”

“…">CCDC6 Lung-Mutants Form Heterodimers with the Wild Type Protein Shaping Its Intracellular DistributionBeside the reduced levels of CCDC6 protein expression and the CCDC6 fusions with different kinases, four missense mutations of CCDC6 have been identified so far in NSCLC ( Figure 1A).The mutated isoforms E227K, S351Y, N394Y, and T462A of CCDC6 tagged with a myc-epitope were expressed in Hela Kyoto S-tag-GFP-CCDC6 cells in order to characterize their function. The missense mutations inserted in the CCDC6 sequence, with the exception of the E227K, are not predicted to affect the coiled-coil structure of the CCDC6 protein (aa 53-237 and aa 253-332), that is necessary for the homodimerization of the CCDC6 wild type protein [4]. Then, by pooling down the S-tag-GFP-CCDC6 protein with the S-agarose beads, we isolated the hetero-complex of each of the CCDC6 mutants with Cancers 2020, 12, 44 3 of 18 the CCDC6 wild type protein, as the myc-tagged CCDC6 mutants were able to interact and form heterodimers with the S-tagged GFP-CCDC6 wild type protein stably expressed in Hela Kyoto cells ( Figure 1B,C; Figures S1 and S4).…”

“…In several in vitro cancer cell systems CCDC6 protein levels have been correlated with cancer cell tolerance to DNA damage, resistance to cisplatinum and sensitivity to poly (ADP-ribose) polymerase inhibitors (PARPi) [4,[11][12][13][14][15][16].…”