Identification of sumoylation sites in CCDC6, the first identified RET partner gene in papillary thyroid carcinoma, uncovers a mode of regulating CCDC6 function on CREB1 transcriptional activity

MEX3A induces the development of thyroid cancer via targeting CREB1

Thyroid cancer is a prevalent form of endocrine cancer, and its global incidence has been steadily increasing. MEX3A is a protein that is known to be highly expressed in various human malignant tumors, including thyroid cancer, and it has been linked to patient prognosis. However, the molecular mechanisms underlying MEX3A's tumorigenic capabilities in thyroid cancer are not fully understood. In this study, we aimed to investigate the role of MEX3A in thyroid cancer. We confirmed that MEX3A was overexpressed in both thyroid cancer tissues and cell lines. Additionally, we found a positive correlation between high levels of MEX3A and the AJCC stage. To further understand the functional significance of MEX3A in thyroid cancer, we depleted MEX3A expression in B-CPAP and TPC-1 cells. Interestingly, we observed a significant reduction in thyroid cancer cell proliferation and migration, as well as ameliorated cell apoptosis and arrested tumor growth upon MEX3A depletion. These findings strongly suggested that MEX3A played a critical role in the development of thyroid cancer. Furthermore, our study uncovered an important interaction between MEX3A and CREB1 (cAMP response element-binding protein 1). The interaction between MEX3A and CREB1 appeared to contribute to the tumor-promoting effects of MEX3A in thyroid cancer by directly targeting CREB1. Silencing CREB1 was observed to alleviate the malignant phenotypes promoted by MEX3A in thyroid cancer cells. Together, this study highlighted the importance of the MEX3A-CREB1 interaction in thyroid cancer development and suggested the therapeutic potential of targeting MEX3A for the treatment of this disease.

Posttranslational Modifications in Thyroid Cancer: Implications for Pathogenesis, Diagnosis, Classification, and Treatment

There is evidence that posttranslational modifications, including phosphorylation, acetylation, methylation, ubiquitination, sumoylation, glycosylation, and succinylation, may be involved in thyroid cancer. We review recent reports supporting a role of posttranslational modifications in the tumorigenesis of thyroid cancer, sensitivity to radioiodine and other types of treatment, the identification of molecular treatment targets, and the development of molecular markers that may become useful as diagnostic tools. An increased understanding of posttranslational modifications may be an important supplement to the determination of alterations in gene expression that has gained increasing prominence in recent years.

The tumour suppressor CCDC6 is involved in ROS tolerance and neoplastic transformation by evading ferroptosis

Coiled-coil domain containing 6 (CCDC6) is a tumour suppressor gene involved in apoptosis and DNA damage response. CCDC6 is known to be functionally impaired upon gene fusions, somatic mutations, and altered protein turnover in several tumours. Testicular germ cell tumours are among the most common malignancies in young males. Despite the high cure rate, achieved through chemotherapy and/or surgery, drug resistance can still occur. In a human cellular model of testis Embryonal Carcinoma, the deficiency of CCDC6 was associated with defects in DNA repair via homologous recombination and sensitivity to PARP1/2 inhibitors. Same data were obtained in a panel of murine testicular cell lines, including Sertoli, Spermatogonia and Spermatocytes. In these cells, upon oxidative damage exposure, the absence of CCDC6 conferred tolerance to reactive oxygen species affecting regulated cell death pathways by apoptosis and ferroptosis. At molecular level, the loss of CCDC6 was associated with an enhancement of the xCT/SLC7A11 cystine antiporter expression which, by promoting the accumulation of ROS, interfered with the activation of ferroptosis pathway. In conclusion, our data suggest that the CCDC6 downregulation could aid the testis germ cells to be part of a pro-survival pathway that helps to evade the toxic effects of endogenous oxidants contributing to testicular neoplastic growth. Novel therapeutic options will be discussed.

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

CCDC6 is implicated in cell cycle checkpoints and DNA damage repair by homologous recombination (HR). In NSCLC, CCDC6 is barely expressed in about 30% of patients and CCDC6 gene rearrangements with RET and ROS kinases are detected in about 1% of patients. Recently, CCDC6 point-mutations naming E227K, S351Y, N394Y, and T462A have been identified in primary NSCLC. In this work, we analyze the effects exerted by the CCDC6 mutated isoforms on lung cancer cells. By pull-down experiments and immunofluorescence, we evaluated the biochemical and morphological effects of CCDC6 lung-mutants on the CCDC6 wild type protein. By using two HR-reporter assays, we analyzed the effect of CCDC6 lung-mutants in perturbing CCDC6 physiology in the HR process. Finally, by cell-titer assay, we evaluated the response to the treatment with different drugs in lung cancer cells expressing CCDC6 mutants. This work shows that the CCDC6 mutated and truncated isoforms, identified so far in NSCLC, affected the intracellular distribution of the wild type protein and impaired the CCDC6 function in the HR process, ultimately inducing cisplatinum resistance and PARP-inhibitors sensitivity in lung cancer cells. The identification of selected molecular alterations involving CCDC6 gene product might define predictive biomarkers for personalized treatment in NSCLC.

The rationale for druggability of CCDC6-tyrosine kinase fusions in lung cancer

Gene fusions occur in up to 17% of solid tumours. Oncogenic kinases are often involved in such fusions. In lung cancer, almost 30% of patients carrying an activated oncogene show the fusion of a tyrosine kinase to an heterologous gene. Several genes are partner in the fusion with the three kinases ALK, ROS1 and RET in lung. The impaired function of the partner gene, in combination with the activation of the kinase, may alter the cell signaling and promote the cancer cell addiction to the oncogene. Moreover, the gene that is partner in the fusion to the kinase may affect the response to therapeutics and/or promote resistance in the cancer cells. Few genes are recurrent partners in tyrosine kinase fusions in lung cancer, including CCDC6, a recurrent partner in ROS1 and RET fusions, that can be selected as possible target for new strategies of combined therapy including TKi.

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

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.

The between Now and Then of Lung Cancer Chemotherapy and Immunotherapy

Lung cancer is the most common cancer worldwide. Disappointingly, despite great effort in encouraging screening or, at least, a close surveillance of high-risk individuals, most of lung cancers are diagnosed when already surgically unresectable because of local advancement or metastasis. In these cases, the treatment of choice is chemotherapy, alone or in combination with radiotherapy. Here, we will briefly review the most successful and recent advances in the identification of novel lung cancer genetic lesions and in the development of new drugs specifically targeting them. However, lung cancer is still the leading cause of cancer-related mortality also because, despite impressive initial responses, the patients often develop resistance to novel target therapies after a few months of treatment. Thus, it is literally vital to continue the search for new therapeutic options. So, here, on the basis of our recent findings on the role of the tumor suppressor CCDC6 protein in lung tumorigenesis, we will also discuss novel therapeutic approaches we envision for lung cancer.

USP7 inhibitors, downregulating CCDC6, sensitize lung neuroendocrine cancer cells to PARP-inhibitor drugs

OBJECTIVES

CCDC6 gene product is a tumor-suppressor pro-apoptotic protein, substrate of ATM, involved in DNA damage response and repair. Altered levels of CCDC6 expression are dependent on post-translational modifications, being the de-ubiquitinating enzyme USP7 responsible of the fine tuning of the CCDC6 stability. Thus, our aim was to investigate CCDC6 and USP7 expression levels in Lung-Neuroendocrine Tumors (L-NETs) to verify if they correlate and may be exploited as novel predictive therapeutic markers.

MATERIALS AND METHODS

Tumor tissues from 29 L-NET patients were investigated on tissue microarrays. CCDC6 levels were scored and correlated with immunoreactivity for USP7. Next generation sequencing (NGS) of a homogenous group of Large Cell Neuroendocrine Carcinoma (LCNEC) (N=8) was performed by Ion AmpliSeq NGS platform and the Ion AmpliSeq Cancer Hotspot Panel v2. The inhibition of USP7, using P5091, was assayed in vitro to accelerate CCDC6 turnover in order to sensitize the neuroendocrine cancer cells to PARP-inhibitors, alone or in association with cisplatinum.

RESULTS

The immunostaining of 29 primary L-NETs showed that the intensity of CCDC6 staining correlated with the levels of USP7 expression (p≤0.05). The NGS analysis of 8 LCNEC revealed mutations in the hot spot regions of the p53 gene (in 6 out of 8). Moreover, gene polymorphisms were identified in the druggable STK11, MET and ALK genes. High intensity of p53 immunostaining was reported in the 6 tissues carrying the TP53 mutations. The inhibition of USP7 by P5091 accelerated the degradation of CCDC6 versus control in cycloheximide treated L-NET cells in vitro and sensitized the cells to PARP-inhibitors alone and in combination with cisplatinum.

CONCLUSION

Our data suggest that CCDC6 and USP7 have a predictive value for the clinical usage of USP7 inhibitors in combination with the PARP-inhibitors in L-NET in addition to standard therapy.

FBXW7 and USP7 regulate CCDC6 turnover during the cell cycle and affect cancer drugs susceptibility in NSCLC

CCDC6 gene product is a pro-apoptotic protein substrate of ATM, whose loss or inactivation enhances tumour progression. In primary tumours, the impaired function of CCDC6 protein has been ascribed to CCDC6 rearrangements and to somatic mutations in several neoplasia. Recently, low levels of CCDC6 protein, in NSCLC, have been correlated with tumor prognosis. However, the mechanisms responsible for the variable levels of CCDC6 in primary tumors have not been described yet.

We show that CCDC6 turnover is regulated in a cell cycle dependent manner. CCDC6 undergoes a cyclic variation in the phosphorylated status and in protein levels that peak at G2 and decrease in mitosis. The reduced stability of CCDC6 in the M phase is dependent on mitotic kinases and on degron motifs that are present in CCDC6 and direct the recruitment of CCDC6 to the FBXW7 E3 Ubl. The de-ubiquitinase enzyme USP7 appears responsible of the fine tuning of the CCDC6 stability, affecting cells behaviour and drug response.

Thus, we propose that the amount of CCDC6 protein in primary tumors, as reported in lung, may depend on the impairment of the CCDC6 turnover due to altered protein-protein interaction and post-translational modifications and may be critical in optimizing personalized therapy.