Enhanced expression of human cyclin G1 ( CCNG1 ) in tumors, a novel biomarker in development for cancer therapy/gene therapy

Background: Metastatic cancer is associated with an invariably fatal outcome. However, DeltaRex-G, a tumor- targeted retrovector encoding a CCNG1 inhibitor gene, has induced long term (>10 years) survival of patients with chemo-resistant metastatic pancreatic adenocarcinoma, MPNST, osteosarcoma, B-cell lymphoma, and breast carcinoma. Objective: To evaluate the level of CCNG1 expression in tumors as a potential biomarker for CCNG1 inhibitor therapy. Methods: CCNG1 RNA expression levels were measured in tumors (TCGA, N=9161), adjacent “tissues” (TCGA, N=678) and GTEx normal tissues (N=7187) across 22 organ sites. Differential expression of CCNG1 and Ki-67 in primary (N= 9161) vs metastatic (N= 393) tumors were also compared and particularly in primary (N=103) vs. metastatic (N=367) skin cancer (i.e., melanoma). Results: Enhanced CCNG1 RNA and protein expression were noted in tumors compared to normal analogous counterparts, and CCNG1 expression correlated significantly with that of Ki-67. Further, CCNG1 expression tended to be higher than that of Ki-67 in metastatic vs primary tumors, particularly in skin cancer (melanoma). Conclusions: Taken together, these data indicate that (1) CCNG1 expression is frequently enhanced in tumors when compared to their normal analogous counterparts, (2) CCNG1 and Ki-67 expressions are higher in metastatic vs primary tumors, (3) CCNG1 expression is significantly correlated with that of Ki-67, and (4) CCNG1 may be a stronger marker of metastasis than Ki-67. Phase 2 studies are planned to identify patients who are likely to respond favorably to CCNG1 inhibitor therapy by correlating treatment outcome parameters with CCNG1 expression levels in various cancer types.


INTRODUCTION
Cyclins are a family of regulatory proteins that exert control over cell cycle progression through interaction with cyclin-dependent kinases (CDKs) 1 . In particular, the cyclin G1 gene, CCNG1, is known to have a negative effect on stabilization of p53 through Mdm2-and ATMdependent mechanisms, thus assisting in progression through multiple p53-regulated cell cycle checkpoints, namely the G1/S and G2 checkpoints. [2][3][4][5] Cyclin G1 dysregulation is thus believed to have a role in aberrant cell division and tumorigenesis, and overexpression of cyclin G1 has been noted in breast and colorectal cancers, and is important in hepatocellular carcinoma metastasis. [6][7][8] As such, CCNG1 has been suggested as a possible target for antineoplastic gene therapy.
DeltaRex-G, previously known as Rexin-G, is an amphotropic MLV-based retroviral vector displaying (1) a Signature (SIG)-binding peptide and (2) encoding a cytocidal CCNG1 inhibitor gene. When administered intravenously, the DeltaRex-G nanoparticles track down and accumulate in the tumor microenvironment (TME) through binding to abnormal SIG proteins exposed during tumor invasion and angiogenesis [9][10][11]. Treatment with DeltaRex-G has demonstrated clinical benefit and prolonged survival in metastatic, chemotherapy-resistant sarcoma, pancreatic ductal adenocarcinoma, and ductal carcinoma of the breast 9,12,13 . Nonetheless, due to the nature of its mechanism, DeltaRex-G may be most efficacious in tumors that overexpress CCNG1. Herein, we report augmented CCNG1 expression in many solid tumors and provide the basis for investigating CCNG1 expression levels in tumors to identify patients who are likely to benefit from CCNG1 inhibitor therapy.

METHODS AND MATERIALS
Archived tumor samples and post-mortem samples are de-identified tissue samples previously used for other histopathologic examination and molecular profiling studies at NantOmics Bioinformatics Department of Nant and at the Cancer Center of Southern California. Therefore, we believe that FDA, IRB and patient informed consent were not needed.

Qualification of IHC assay for Cyclin G1 expression
An anti-cyclin G1 IgG1 kappa light chain murine monoclonal antibody (Santa Cruz Biotechnology, Santa Cruz, CA, cat# sc-8016), coupled with MACH4 anti-mouse secondary antibody (BioCare Medical, Pacheco, CA) were used for immunohistochemical analysis. The mouse monoclonal antibody was optimized on cell line tissue microarray and normal tissue microarray samples sectioned at approximately 4 μm and adhered to positively charged glass. Multiple antigen retrieval methods and a series of titrations of the cyclin G1 antibody were performed using the tissue microarray slides to determine the optimal antigen retrieval method and antibody concentration. Once the antibody concentration was chosen, accuracy, specificity, sensitivity, and the range of staining intensity was assessed using de-identified samples of hepatocellular carcinoma and breast carcinoma samples as positive controls, and normal tissues as negative controls. Additionally, three neoplastic tissues representing various degrees of cyclin G1 expression were chosen for an assessment of reproducibility and precision. Then, immunohistochemical analysis was performed on 27 de-identified formalin-fixed, deparaffinized samples, representing 13 types of cancer and 4 types of normal tissue. The optimized method is as follows: First, the samples were deparaffinized and rehydrated, and endogenous peroxidase activity was blocked. Then, antigen retrieval was performed in a decloaking chamber with Tris-EDTA for 20 minutes at 95° C. The anti-cyclin G1 antibody was applied at a dilution of 1:2000, and incubated for 30 minutes at room temperature, followed by application of the MACH4 mouse secondary antibody for 15 minutes at room temperature. A MACH4 HRP Polymer (BioCare Medical, Pacheco, CA) was then applied as the detection system, and incubated for 30 minutes at room temperature. The resulting product was incubated in chromogen DAB-beta (BioCare Medical, Pacheco, CA) for 10 minutes at room temperature, and was then counterstained with hematoxylin, dehydrated, cleared in xylene, and finally a coverslip was applied. The samples were then analyzed by a board-certified pathologist who scored the staining intensities of the samples on a scale of 0 to 3+.
Reproducibility was tested in several ways: (1) intra-run reproducibility, with identical slides run in triplicate in one run on one day; (2) inter-run reproducibility with identical slides tested on three separate runs in three separate days; (3) inter-tech reproducibility, with identical slides run by two different technologists in two separate runs; and (4) inter-instrument reproducibility with identical slides run by the same technologist on two different instruments. Concordance factors were calculated based on the pathologist's scoring of the reproducibility slides. As an indirect confirmation of cyclin G1 assay specificity, additional immunohistochemical experiments were performed to compare expression of cyclin G1 to that of cellular proliferation marker Ki-67.
Intra-run, inter-run, inter-tech, and inter-instrument reliability were all deemed as acceptable and CCNG1 expression observed by IHC was in line with that predicted in the literature.

RESULTS
Points to Consider #1: CCNG1 is overexpressed throughout the TME, including adjacent non-tumorous areas. Figure 1 shows the differential expression of CCNG1 (x-axis) vs Ki-67 (y-axis) in tumor (green) and normal (blue and red) settings. There was no significant difference in CCNG1 and Ki-67 expression across organ sites in TCGA tumor and adjacent non-tumor tissue within the TME (TCGA normal). In contrast, as shown in Table 1, CCNG1 expression was significantly enhanced in TCGA tumor compared to normal tissue obtained post-mortem from patients with no tumors (GTEX normal). This is an important observation since proliferative tumor-associated microvasculature (TAM) and tumor-associated fibroblasts (TAFs) would also show enhanced expression of CCNG1 and Ki-67 in the adjacent non-tumor tissue within the TME.

Figure 1
CCNG1 expression across organ sites in normal and tumor tissues, contrasted with Ki-67. Scatter plots of CCNG1 (x-axis) vs. Ki-67 (y-axis) for tumor (green) and normal (blue and red) settings. Plots are ordered by differential expression between TCGA tumor and GTEx normal. TCGA normal tissue has altered molecular expression status closer to the TCGA tumor and GTEX normal is distinctly different from both TCGA normal and TGCA tumor. Points to Consider #2: CCNG1 expression is higher than that of Ki-67 in metastatic skin cancers (i.e., melanoma). As shown in Figure 2 left panel, CCNG1 and Ki-67 both showed enhanced expression in metastatic settings (orange) compared to primary tumors (green), but CCNG1 outperformed Ki-67 specifically in metastatic skin cancer (melanoma; right panel). Further, CCNG1 expression was relatively higher in many metastatic samples than that of Ki-67 and did not track with Ki-67 ( Figure 3). These data indicate that CCNG1 gene expression may be a better indicator of metastasis than primary tumors.  Points to Consider #3: CCNG1 expression is enhanced in specific tumors by RNA sequence analysis.

DISCUSSION
As cancer treatment continues to trend toward more highly directed molecular therapies, efforts have been made to determine how to best assess which patients may benefit from specific targeted therapies. Cyclin G1 dysregulation is an important down regulator of p53 activity, allowing for disinhibited cell cycle progression. [14][15][16] In the present study, we found that cyclin G1 is overexpressed in many tumor samples, and that in metastatic tumors, cyclin G1 expression is higher than that of Ki-67. Currently, DeltaRex-G, a retrovector encoding a CCNG1 inhibitor gene is in development as therapy for various cancer types, and should be included as part of molecular profiling of tumors at diagnosis and in post-treatment tumor specimens DeltaRex-G is the first, and so far, only, tumor-targeted retrovector encoding a CCNG1 inhibitor gene. 10,11,13,17 DeltaRex-G has been found to be of clinical benefit in an array of metastatic cancers, 9,18 and the ability to identify patients with specific overexpression of CCNG1 can help inform clinicians as to which patients may benefit the most from this gene therapy. Additionally, our finding that CCNG1 expression has varied over time in some tumors and suggests that genomic data from multiple biopsies or from circulating cell-free tumor DNA over time may be most informative relating to the optimal timing of DeltaRex-G and other future CCNG1 inhibitor therapies.
Our findings that in some tumors, cyclin G1 is overexpressed in metastatic sites when compared to primary tumors piques our interest into the role of cyclin G1 and CCNG1 inhibitors in the treatment of metastatic disease. One important pathway in the epithelial-mesenchymal transition (EMT), a crucial process in tumor metastasis and dissemination, is downregulation of E-cadherin. 19,20 Physiologically, GSK-3β phosphorylates Snail (the most important transcription factor in negative regulation of E-cadherin), causing its degradation, and thus disallowing it from downregulating E-cadherin. 21,22 Cyclin G1 is thought to incite EMT by activating PI3K/Akt signaling to downregulate GSK-3β, eventually leading to loss of E-cadherin. 8 Conceptually, DeltaRex-G would inhibit EMT and downregulate the PI3K/Akt signaling pathway as well. The data showing overexpression of cyclin G1 at primary and metastatic tumors solidifies the case for CCNG1 inhibition for cancer therapy/gene therapy and the results of an advanced Phase 1/2 study of DeltaRex-G in the metastatic pancreatic adenocarcinoma have been successful in gaining orphan drug and fast track designation from the USFDA 23 .
We also found cyclin G1 to be overexpressed not just in tumors, but within the TME as well. The angiogenic switch is the term used to describe the time in tumor progression when proangiogenic factors begin to predominate over anti-angiogenic factors, leading to vascular proliferation in the TME, allowing the tumor to receive nutrients required to progress and to maintain its high energy demand. 24,25 Additionally, TME vasculature encourages evasion of antitumor immune surveillance, in part by impeding lymphocyte-endothelial cell interaction and upregulating immunosuppressive leukocytes. 25 The specific cytocidal activity of DeltaRex-G in TAM and TAFs suggests that DeltaRex-G breaks anergy by enhancing immune cell trafficking in the TME. 11 Future studies should investigate possible synergy between DeltaRex-G administered with other anti-angiogenic therapies like nab-sirolimus, an mTOR inhibitor, and immune checkpoint inhibitors such as nivolumab and ipilimumab. 26 Phase 2 studies are planned to identify patients who are likely to respond favorably and/or benefit most to DeltaRex-G therapy by correlating CCNG1 expression levels and treatment outcome parameters in cancer patients treated with DeltaRex-G.