Ribozyme as an approach for growth suppression of human pancreatic cancer

Ribozymes (catalytic RNAs, RNA enzymes) are effective modulators of gene expression because of their simple structure, site-specific cleavage activity, and catalytic potential, and have potentially important implications for cancer gene therapy. Point mutations in the K-ras oncogene are found in approx 90% of human pancreatic carcinomas, and can be used as potential targets for specific ribozyme-mediated reversal of the malignant phenotype. In this study, we focused on in vitro manipulation of ribozyme targeting of the mutated K-ras oncogene in a human pancreatic carcinoma cell line. We evaluated the efficacy of an anti-K-ras hammerhead ribozyme targeted against GUU-mutated codon 12 of the K-ras gene in cultured pancreatic carcinoma cell lines. The anti-K-ras ribozyme significantly reduced cellular K-ras mRNA level (GUU-mutated codon 12) when the ribozyme was transfected into the Capan-1 pancreatic carcinoma cells. The ribozyme inhibited proliferation of the transfected Capan-1 cells. These results suggested that this ribozyme is capable of reversing the malignant phenotype in human pancreatic carcinoma cells.     

Development of a simple and sensitive technique for detection of point mutations in the K-ras oncogene

We sought to develop a simple and sensitive method based on mutant allele-specific amplification (MASA) for the detection of point mutations in the k-ras oncogene in blood samples. We used MASA and three nested MASA methods to detect a point mutation (GGT→GAT) in rat DHD cells at codon 12 of exon 1 of the k-ras gene. MASA allowed us to detect one k-ras mutated cell on a background of 10⁷ normal cells. The third nested-MASA (nested-MASA.c) method that we developed allowed us to detect one mutated cell among 10¹⁰ normal cells. Our methods should allow the detection of small amounts of mutant k-ras DNA in tissue, serum, and plasma, combining speed with efficiency and specificity.     

K-ras Gene Mutation Related to Histological Atypias in Human Colorectal Adenomas

To investigate the relationship of oncogene analysis to morphology, we analyzed K-ras gene mutations by dot-blot hybridization with and without consideration of histological atypias in individual colorectal adenomas. Each of 54 colon polyps were divided into two parts after fixation. One part was used as a mass to assess point mutations; the remaining portion of each polyp was paraffin-embedded, stained with hematoxylin and eosin, and examined for point mutations related to histological atypias. In the first part of our study, K-ras gene mutations at codon 12 were detected in 13 cases (24%). In the second part of our study, 12 cases had distinctly different histological atypias. From each of these 12 cases, two areas, one with higher or one with lower grade atypia in the same polyp were excised to analyze for K-ras gene mutation. Two of these 12 cases (17%) had the mutation in different areas of the same tumor. These two cases contained the mutation only in the areas with higher grade atypia, and only one case added information regarding ras mutation upon microdissection when compared to the entire biopsy. These results suggest that oligonucleotide hybridization can identify the majority of cases containing ras mutations despite regional morphologic variation. Individual cases, however, may contain clonal subpopulations within adenomas with different ras sequences from other regions within the same adenoma.     

pcDNA3.1(−)-mediated ribozyme targeting of <Emphasis Type="Italic">HER-2</Emphasis> suppresses breast cancer tumor growth

The HER-2 proto-oncogene (also called c-erbB-2/neu) encodes the protein, p185, which is closely related to the growth and metastasis of adenocarcinoma, and is overexpressed in 25–30% of human breast cancers. In this study, we attempt to reverse the malignant phenotype of the breast cancer cell line, MCF-7, using a HER-2-specific hammerhead ribozyme. Two anti-HER-2 hammerhead ribozymes, RZ1 and RZ2, were synthesized, inserted separately into the nonviral eukaryotic expression vector, pcDNA3.1(−), and transfected into MCF-7 cells. Analyses showed that the HER-2 mRNA and p185, as well as oncogene k-ras were down-regulated remarkably in the ribozyme-transfected cells, while the onco-suppressor gene, p53, was up-regulated. Furthermore, the tumorigenicity of the RZ1-stably transfected MCF-7 cells was decreased dramatically in nude mice. These results demonstrate that the use of anti-HER-2 ribozymes may be a beneficial strategy for gene therapy of breast cancer.     

Fluorescent oligonucleotide ligation technology for identification of ras oncogene mutations

A mutation detection strategy based on multiplex PCR followed by multiplex allele-specific oligonucleotide probe ligation was developed to detect single nucleotide substitutions in ras oncogenes, a common genetic abnormality in many human cancers. Mutation-specific probes are synthesized for each possible single-base, nonsilent mutation in codons 12, 13, and 61 of H-, K-, and N-ras oncogenes. Mutations are identified by competitive oligonucleotide probe ligation to detect normal and /or mutant genotypes in one reaction. Three probes (one common and two allelic probes) are needed for analysis of each mutation. Probes hybridized to target ras oncogene DNA are joined by a thermostable ligase if there are no mismatches at their junctions; temperature cycling results in a linear increase in product. Common probes are labeled with fluorochromes, and allelic probes each have different lengths. Ligation products are analyzed by denaturing polyacrylamide gel electrophoresis on a fluorescent DNA sequencer. We have applied this technology to identify ras mutations in pancreatic cancers and lung cancers and in patients with myelodysplastic syndromes and leukemias.     

Enzymatic and antisense effects of a specific anti-Ki-ras ribozyme in vitro and in cell culture.

Due to their mode of action, ribozymes show antisense effects in addition to their specific cleavage activity. In the present study we investigated whether a hammerhead ribozyme is capable of cleaving mutated Ki-ras mRNA in a pancreatic carcinoma cell line and whether antisense effects contribute to the activity of the ribozyme. A 2[prime]-O-allyl modified hammerhead ribozyme was designed to cleave specifically the mutated form of the Ki- ras mRNA (GUU motif in codon 12). The activity was monitored by RT-PCR on Ki- ras RNA expression by determination of the relative amount of wild type to mutant Ki-ras mRNA, by 5-bromo-2[prime]-deoxy-uridine incorporation on cell proliferation and by colony formation in soft agar on malignancy in the human pancreatic adenocarcinoma cell line CFPAC-1, which is heterozygous for the Ki-ras mutation. A catalytically inactive ribozyme was used as control to differentiate between antisense and cleavage activity and a ribozyme with random guide sequences as negative control. The catalytically active anti-Ki-ras ribozyme was at least 2-fold more potent in decreasing cellular Ki-ras mRNA levels, inhibiting cell proliferation and colony formation in soft agar than the catalytically inactive ribozyme. The catalytically active anti-Ki-ras ribozyme, but not the catalytically inactive or random ribozyme, increased the ratio of wild type to mutated Ki-ras mRNA in CFPAC-1 cells. In conclusion, both cleavage activity and antisense effects contribute to the activity of the catalytically active anti-Ki-ras hammerhead ribozyme. Specific ribozymes might be useful in the treatment of pancreatic carcinomas containing an oncogenic GTT mutation in codon 12 of the Ki-ras gene.     

Mutation analysis of <Emphasis Type="Italic">K-ras</Emphasis> protooncogene in colorectal adenocarcinomas and polyps in Russian patients

To estimate diagnostic value of K-ras mutations during cancer risk group formation, they were studied in the samples of sporadic carcinomas (n = 58), benign (n = 33), and malignant (n = 13) polyps of large intestine obtained during surgery or polypectomy. Using PCR analysis, restriction analysis, SSCP analysis and automated sequencing, eight various point mutations were revealed. Six of them were located in codon 12 and two, in codon 13 of the K-ras gene. Mutation frequency in carcinomas, benign and malignant polyps was 43, 49, and 69%, respectively. In the normal tissue samples of colorectum, no changes in codons 12 and 13 in the K-ras gene were observed. Mutations in the groups of Russian patients examined partially overlapped. In patients with colorectal carcinoma the mutation frequency in the K-ras gene was not associated with disease onset age, location, and the extent of tumor differentiation while it was associated with the stage of tumor process. In polyps, the maximum mutation frequency was revealed among patients over 70 years of age as well as in the adenomas of villous histology and large size (≥1cm). No correlation between the K-ras mutation frequency and the extent of polyp dysplasia was observed.     

Investigation of the mechanism underlying the inhibitory effect of heterologous ras genes in plant cells

The ras genes from yeast and mammalian cells were fused to plant expression promoters, and introduced into plant cells via Agrobacterium, to study their effect on cell growth and development. All introduced ras genes had a strong inhibitory effect on callus and shoot regeneration from plant tissues. This is consistent with earlier findings that heterologous ras genes were highly lethal to protoplasts following direct DNA uptake. These effects could not be reversed by increasing exogenous or endogenous cytokinin levels. These effects were also independent of the v-Ha-ras mutations in functionally important regions of Ras proteins such as effector-binding and membrane-binding sites. Similarly, co-transformation with the genes encoding the Ras-negative regulators, GTPase-activating protein and neurofibromin did not affect the ras inhibitory effect, indicating that the mechanism of ras inhibition of plant cells is not related to normal ras cellular functions. This conclusion was supported by further studies in which ras gene expression was modified using various promoters and antisense constructs. The introduced ras sequences remained fully inhibitory regardless of which promoters (inducible or tissue-specific) or which orientations (sense or antisense) were tested. This strongly suggests that the ras DNA sequence itself, rather than the Ras protein or ras mRNA, is directly involved in the inhibitory effect. The mechanism underlying this novel phenomenon remains unknown. Introduced ras genes may inhibit plant cell growth by inducing co-suppression of unknown endogenous ras or ras-related genes, thereby leading to the arrest of cell growth.     

Mutational analysis of BRAF and K-ras in gastric cancers: absence of BRAF mutations in gastric cancers

Recently, BRAF mutations were found in a variety of human cancers. Interestingly, the most common of BRAF mutation (V599E) has not been identified in tumors with K-ras mutations. Whereas the majority of human cancer types has been screened for BRAF mutations, no detailed studies on gastric cancers have been investigated. Thus, we decided to investigate the incidence of BRAF mutations in gastric cancers, and the relationship between BRAF and K-ras mutations in such cancers. Three non-pathogenic BRAF polymorphisms and seven K-ras missense mutations were found in 66 gastric cancers and 16 gastric cancer cell lines. Although only 9% of our gastric cancer panels had K-ras mutations, the incidence of BRAF mutations was not high. Thus, BRAF mutations, which are present in a variety of other human cancers, do not seem to be involved in gastric cancer development.     

Comparison of K-ras gene mutations in tumour and sputum DNA of patients with lung cancer

Mutations in the K-ras gene are frequently found in lung tumours and are implicated in the development of lung cancer. In order to investigate the clinical usefulness of these mutations in lung cancer, we applied a sensitive method to compare mutations in codon 12 of the K-ras gene in DNA extracted from lung tumours and the matched sputum samples obtained from 22 lung cancer patients. K-ras mutations were identified in the lung tumours of 12 patients (54.5%) and in the sputum samples of 10 patients (45.5%). Nine patients showed an identical mutation in both the tumour and the matched sputum samples. There was a significant association between the presence of a K-ras mutation in a lung tumour and the detection of an identical mutation in the matched sputum sample of the lung cancer patient (κ = 0.64, 95% confidence interval 0.32-0.95, p <0.01). K-ras mutations were detected in sputum samples from cancer patients with all lung tumour grades, and both in the presence and the absence of lymph node metastasis. Therefore, K-ras mutations may provide useful diagnostic markers for lung cancer.     

Dose effects of transfected c-Ha-ras oncogene in transformed cell clones

We have examined the expression of the transformed phenotype in a series of clonal lines of NIH/3T3 cells transfected with the human c-Ha-ras^{Val 12} oncogene and the neomycin phosphotransferase gene. Cells from individual transformed foci were cloned and subjected to detailed analyses of the ras sequences. Three clones were found that expressed approximately one, 2–4, or 4–8 copies of the human c-ras oncogene, respectively. A fourth clone had multiple copies of the transfected sequences, and expressed abundant c-Ha-ras RNA. Analysis of the tranformed phenotype of various clones indicated that cells expressing low levels of mutant c-Ha-ras had lost some of their extracellular fibronectin network, and were barely altered in their cytoskeleton. In contrast, cells expressing abundant c-Ha-ras had lost both their actin and fibronectin networks and showed an increase in plasminogen activator activity. Cells with amplified c-Ha-ras^{Val 12} grew better in low serum, formed large colonies in soft agar and showed enhanced activity of ornithine decarboxylase, the rate-controlling enzyme in polyamine biosynthesis. These results show that the dosage level of the mutant oncogene makes a significant contribution to the transformed phenotype of c-Ha-ras oncogene-transformed cells.     

Genetic analysis of the catalytic domain of the GAP gene in human lung cancer cell lines

Cell lines of non-small cell lung cancer (nonSCLC) have been shown to contain activating mutation of the K-ras oncogene in about 30% of cases, whereas no small cell lung cancer (SCLC) cell lines displayed these mutations. Biochemically, these mutations result in the ras gene product (p21) being constitutively activated in its GTP-bound form and insensitive to the hydrolytic action of the ras-specific GTPase-activating protein (ras GAP). We hypothesized that, if tumor development is related to the p21 ras being in the active GTP-bound state, then a similar malignant phenotype may result from an inactivating mutation in the ras GAP gene in the region that interacts with ras p21 (so-called catalytic domain). To test this hypothesis, we screened a panel of SCLC and non-SCLC cell lines for major genetic alterations in the catalytic domain of the GAP gene with the Sothern blot technique, and for minor genetic abnormalities (e.g., point mutations) with denaturing gradient gel electrophoresis and single-strand conformation polymorphism. Mutations in the catalytic domain of the GAP gene could not be demonstrated by any technique in any cell line examined. We conclude that mutational inactivation of the catalytic domain of the GAP gene probably does not contribute to the development of lung cancer.     

基于微流控芯片的温度梯度毛细管电泳检测大肠癌石蜡标本中K-ras基因突变

K-ras基因突变检测可用于大肠癌的早期筛查与诊断,并有利于筛选出抗表皮生长因子受体靶向药物治疗有效的大肠癌患者,以实现肿瘤的个体化治疗．采用以倾斜式热辐射原理建立的微流控温度梯度毛细管电泳(temperature gradient capillary electrophoresis,TGCE)基因突变检测系统,实现了对98例石蜡包埋大肠癌组织中K-ras基因突变的高灵敏度筛查,突变阳性检出率为47.96%,显著高于PCR产物直接测序的23.47%．克隆测序显示该方法至少能检测到2.08%的K-ras基因突变体．K-ras基因突变与临床病理学参数的关系分析显示,直肠癌中K-ras基因突变率明显高于结肠癌(P < 0.05),而与年龄、性别、组织学类型和肿瘤分期等无显著相关性．该检测方法为肿瘤早期诊断和指导临床用药提供了一种灵敏度高、检测速度快、便于大规模筛查的有效手段.     

Detection of K-ras Mutations in Predicting Efficacy of Epidermal Growth Factor Receptor Tyrosine Kinase (EGFR-TK) Inhibitor in Patients with Metastatic Colorectal Cancer

Epidermal growth factor receptor tyrosine kinase (EGFR-TK) inhibitors are useful in treating different advanced human cancers; however, their clinical efficacy varies. This study detected K-ras mutations to predict the efficacy of EGFR-TK inhibitor cetuximab treatment on Chinese patients with metastatic colorectal cancer (mCRC). A total of 87 patients with metastatic colorectal cancer were treated with cetuximab for 2-16 months, in combination with chemotherapy between August 2008 and July 2012, and tissue samples were used to detect K-ras mutations. The data showed that K-ras mutation occurred in 27/87 (31%). The objective response rates and disease control rate in K-ras wild type and mutant patients were 42% (25/60) versus 11% (3/27) (p<0.05) and 60% (36/60) versus 26% (7/27) (p<0.05), respectively. Patients with the wild-type K-ras had significantly higher median survival times and progression-free survival, than patients with mutated K-ras (21 months versus 17 months, p=0.017; 10 months versus 6 months, p=0.6). These findings suggest that a high frequency of K-ras mutations occurs in Chinese mCRC patients and that K-ras mutation is required to select patients for eligibility for cetuximab therapy. Further prospective studies using a large sample size are needed to confirm these preliminary findings.     

Molecular cloning of the intronless EJ ras oncogene using a murine retrovirus shuttle vector

We have inserted a genomic clone of the human EJ bladder oncogene into a murine retrovirus shuttle vector. Cotransfection of this shuttle vector DNA containing the activated ras oncogene with molecularly cloned Moloney murine leukemia virus into NIH/3T3 cells was able to rescue a replicating and transforming retrovirus. The viral DNA from infected cells was excised by fusion to mouse COP-5 cells and recloned into Escherichia coli as plasmid DNA. Analysis of the recloned plasmids by size, restriction enzyme mapping, and DNA sequence indicated that approximately 5% of the recloned plasmids contained the intronless EJ ras oncogene.     

Modified Hammerhead Ribozymes as Potential Therapeutics

Abstract

Hammerhead ribozymes were modified in the 2′-position by flouro-, amino-, deoxy-, O-methyl- and methoxyethoxy-groups to stabilize against degradations. They were tested for their ability to cut specifically oncogenic N-ras RNA in vitro. Exogenous delivery by lipofection as well as viral vector mediated transfection showed comparable results in reducing N-ras mRNA.     

Mutagenicity of bisphenol A and its suppression by interferon-α in human RSa cells

Bisphenol A is used as a monomer in the production of polycarbonate plastic products. The widespread use of bisphenol A has raised concerns about its effects in humans. Since there is little information on the mutagenic potential of the chemical, the mutagenicity of bisphenol A was tested using human RSa cells, which has been utilized for identification of novel mutagens. In genomic DNA from cells treated with bisphenol A at concentrations ranging from 1×10⁻⁷ to 1×10⁻⁵ M, base substitution mutations at K-ras codon 12 were detected using PCR and differential dot-blot hybridization with mutant probes. Mutations were also detected using the method of peptide nucleic acid (PNA)-mediated PCR clamping. The latter method enabled us to detect the mutation in bisphenol A-treated cells at a dose (1×10⁻⁸ M) equivalent to that typically found in the environment. Induction of ouabain-resistant (Oua^R) phenotypic mutation was also found in cells treated with 1×10⁻⁷ and 1×10⁻⁵ M of bisphenol A. The induction of K-ras codon 12 mutations and Oua^R mutations was suppressed by pretreating RSa cells with human interferon (HuIFN)-α prior to bisphenol A treatment. The cells treated with bisphenol A at the concentration of 1×10⁻⁶ M elicited unscheduled DNA synthesis (UDS). These findings suggested that bisphenol A has mutagenicity in RSa cells as well as mutagens that have been tested in these cells, and furthermore, that a combination of the PNA-mediated PCR clamping method with the human RSa cell line may be used as an assay system for screening the mutagenic chemicals at very low doses.