Activation of Ki-ras2 gene in human colon and lung carcinomas by two different point mutations

p53-independent inhibition of proliferation and p21(WAF1/Cip1)-modulated induction of cell death by the antioxidants N-acetylcysteine and vitamin E

Epidemiological evidence has suggested an association between diets rich in antioxidants and diminished risks of various types of cancer. Proposed mechanisms for protective effects of antioxidants have involved inhibition of free radical-mediated DNA damage. Recent data suggest that antioxidants may prevent or eliminate cancerous cells through their ability to inhibit proliferation or to induce programmed cell death (PCD). To begin to identify cell cycle and cell death regulatory factors involved in antioxidant-induced growth arrest and PCD, we have studied colorectal carcinoma cells (CRCs) that differ in expression of the tumor suppressor protein p53, and of the cyclin-dependent kinase (CDK) inhibitor p21(Waf1/Cip1). The antioxidants, N-acetylcysteine (NAC) and vitamin E either inhibited proliferation in a p53-independent manner without affecting cell viability or induced cell death. Growth arrest was not associated with upregulation of the CDK inhibitors p21(Waf1/Cip1), p18(ink4c) or p16(ink4a), but was associated with a decrease in reactive oxygen species (ROS). In contrast to previous observations, the absence of p21(Waf1/Cip1) increased susceptibility of CRCs to antioxidant-induced PCD. NAC decreased levels of retinoblastoma protein (Rb) phosphorylation in all cells tested, but Rb was cleaved only in cells which underwent NAC-induced death. Although NAC decreased ROS in all cells studied, cell lines in which PCD occurred had higher baseline levels of ROS than cell lines in which proliferation was blocked. These observations suggest that expression of p21(Waf1/Cip1) and basal levels of ROS are important determinants of outcome after antioxidant treatment.

Intramolecular quadruplex formation of the G-rich strand of the mouse hypervariable minisatellite Pc-1

The minisatellite Pc-1, isolated from the mouse genome consisting of a tandem repeat of d(GGCAG), is hypervariable with a mutation rate of 0.15/generation. Here we describe a structural characterization of the G-rich strand of Pc-1 by biochemical and physicochemical methods. It was found to be comparatively resistant to both single-stranded DNA-binding protein binding and digestion by single-stranded DNA-specific nuclease and to cause arrest of DNA synthesis. The guanine imino proton NMR signals observed on the Pc-1 G-rich strand and their slow (1)H/(2)H exchange profiles pointed to a quadruplex structure with guanine quartets. The melting temperature of the quadruplex determined by CD was not dependent on DNA concentration. These results indicate that the G-rich strand of Pc-1 forms an intramolecular folded-back quadruplex structure under physiological conditions. Possible mechanisms of the Pc-1 mutations implicated with the formation of the quadruplex structure are discussed.

Four human ras homologs differ in their abilities to activate Raf-1, induce transformation, and stimulate cell motility

Human cells contain four homologous Ras proteins, but it is unknown whether each of these Ras proteins participates in distinct signal transduction cascades or has different biological functions. To directly address these issues, we assessed the relative ability of constitutively active (G12V) versions of each of the four Ras homologs to activate the effector protein Raf-1 in vivo. In addition, we compared their relative abilities to induce transformed foci, enable anchorage-independent growth, and stimulate cell migration. We found a distinct hierarchy between the four Ras homologs in each of the parameters studied. The hierarchies were as follows: for Raf-1 activation, Ki-Ras 4B > Ki-Ras 4A >>> N-Ras > Ha-Ras; for focus formation, Ha-Ras >/= Ki-Ras 4A >>> N-Ras = Ki-Ras 4B; for anchorage-independent growth, Ki-Ras 4A >/= N-Ras >>> Ki-Ras 4B = Ha-Ras = no growth; and for cell migration, Ki-Ras 4B >>> Ha-Ras > N-Ras = Ki-Ras 4A = no migration. Our results indicate that the four Ras homologs significantly differ in their abilities to activate Raf-1 and induce distinctly different biological responses. These studies, in conjunction with our previous report that demonstrated that the Ras homologs can be differentially activated by upstream guanine nucleotide exchange factors (Jones, M. K., and Jackson, J. H. (1998) J. Biol. Chem. 273, 1782-1787), indicate that each of the four Ras proteins may qualitatively or quantitatively participate in distinct signaling cascades and have significantly different biological roles in vivo. Importantly, these studies also suggest for the first time that the distinct and likely cooperative biological functions of the Ki-ras-encoded Ki-Ras 4A and Ki-Ras 4B proteins may help explain why constitutively activating mutations of Ki-ras, but not N-ras or Ha-ras, are frequently detected in human carcinomas.

Establishment and characterization of human laryngeal squamous cell carcinoma cell lines

OBJECTIVES

Six human laryngeal squamous cell carcinoma cell lines (SNU-46, -585, -899, -1066, -1076, -1214) established from Korean patients are reported.

STUDY DESIGN

In vitro culture of six squamous cell carcinoma cell lines derived from primary tumors of the larynx. Description of the cell line phenotypes and determination of molecular characteristics.

METHODS

Six laryngeal squamous cell carcinoma cell lines were cultured. The cell phenotypes, including the histopathology of the primary tumors and in vitro growth characteristics, were determined. Molecular characterization was also performed, including DNA fingerprinting analysis and abnormalities of p15, p16, p53, and TGF-betaRII genes by polymerase chain reaction-based single strand conformation polymorphism and sequencing analysis.

RESULTS

All cell lines grew as adherent cells; five lines grew as monolayers and one other line grew as stratifying colonies. All lines showed 1) high viability (75%-92%) with various doubling times (36-96 h); 2) absence of Mycoplasma and other bacteria; and 3) genetic heterogeneity by DNA profile analysis. p53 Mutations were found in three lines and p16 mutations were observed in five cell lines. TGF-betaRII mutations were found in two lines: one line had frameshift mutation and another line had a missense mutation at the kinase domain.

CONCLUSIONS

These newly established and characterized laryngeal squamous cell carcinoma cell lines will be useful for investigating the biologic characteristics of laryngeal cancer.

Analysis of K-ras, N-ras, H-ras, and p53 in lung neuroendocrine neoplasms

This study screened 11 samples of typical carcinoid (TC), 4 samples of atypical carcinoid (AC), 1 sample of large cell neuroendocrine carcinoma (LCNEC), and four metastases for point mutations in exons 5 to 8 of the p53 gene, and exons 1 and 2 of the K-ras. H-ras, and N-ras genes using polymerase chain reaction (PCR)-single-strand conformation polymorphism (SSCP) and direct sequencing and by immunohistochemistry for p53. Exon 1 of K-ras was mutated in two samples of low-grade AC and a metastasis from one of these tumors (GAT12 and AGT12, respectively). No mutations in N-ras or H-ras were found. Mutations in exons 5 and 8 of the p53 gene were identified in a high-grade AC and a LCNEC. Positive immunostaining for p53 was present in three samples, with only one genotypic mutation shown (LCNEC). In conclusion, point mutations of the p53 gene were infrequent in these pulmonary neuroendocrine tumors, did not correlate in all samples with immunostaining, and were associated with the higher-grade tumors. Second, the presence of K-ras mutations seems to be associated with the higher-grade carcinomas. Third, N-ras and H-ras mutations were not found with these pulmonary neuroendocrine tumors.

DPC4/SMAD4 mediated tumor suppression of colon carcinoma cells is associated with reduced urokinase expression

We recently identified DPC4/Smad4 as a candidate tumor suppressor gene mutated or lost in one half of pancreatic carcinomas and in a subset of colon and biliary tract carcinomas. DPC4 plays a key role in signal transduction of the TGF-beta superfamily of molecules and inactivation of TGF-beta mediated growth inhibition is supposed to be the driving force for DPC4 inactivation in human tumors. However, DPC4 mediated tumor suppression by reconstitution of defective cells has not yet been reported. Here we show suppression of tumorigenicity in nude mice by stable reexpression of DPC4 in SW480 colon carcinoma cells. In vitro growth of DPC4-transfected cells was not affected and resistance towards TGF-beta mediated growth inhibition was retained. Instead, cells exhibited morphological alterations and adhesion and spreading were accelerated. These phenotypic changes were associated with reduced expression levels of the endogenous urokinase-type plasminogen activator (uPA) and plasminogen-activator-inhibitor-1 (PAI-1) genes, the products of which are implicated in the control of cell adhesion and invasion. In patients, high expression levels of uPA and PAI-1 correlate with poor prognosis. Thus, reduced expression of uPA and PAI-1 is consistent with suppression of tumorigenicity in DPC4 reconstituted cells. These results demonstrate DPC4's tumor suppressive function and suggest a potential role for DPC4 as a modulator of cell adhesion and invasion.

Role of far upstream repressor elements controlling proto-Ha-ras gene transcription

The far upstream region of the rat Ha-ras gene has been characterized to determine whether possible repressor sequences may control the low level of Ha-ras gene transcription from its TATA-less, GC-rich strong promoter. The chloramphenicol acetyl transferase (CAT) gene under the control of the 3.8-kb Ha-ras upstream promoter was minimally expressed in HeLa cells. Surprisingly, CAT gene expression was increased by the deletion of a 0.7-kb BglII fragment containing non-coding exon minus 2 and TATA box promoter elements located 1.7 kb upstream of the GC-rich strong promoter. Far upstream (CA)25 repeats also appeared to repress Ha-ras gene activity. Sequences within the 0.7-kb BglII fragment suppressed CAT gene expression when placed upstream of a heterologous thymidine kinase (tk) gene promoter. Repressor activity was further localized to a 160-bp AvrII-BglII sub-fragment. Gel shift assays identified two sequence-specific DNA binding proteins. The results demonstrated for the first time that far upstream repressor sequences control normal transcription of the Ha-ras proto-oncogene.

Non-radioactive detection of K-ras mutations by nested allele specific PCR and oligonucleotide hybridization

The development of methods to detect point mutations has been rapid over the recent years. In human colon tumours, a significant percentage of mutations are found in the K-ras gene. Faster and more sensitive methods for detection of these mutations are important for patient management and treatment. The author has extended the specificity of allele amplification of point mutations by using mismatch oligonucleotide primers in a polymerase chain reaction (PCR). It is shown that in colon tumours, the sensitivity of detecting a mutation is significantly higher when K-ras exon 1 is amplified prior to PCR with allele specific primers (mismatch PCR). It is shown that allele specific oligonucleotide probes which are non-radioactively labelled could be used to detect these point mutations. By utilizing this two-step PCR, K-ras codon 12 mutations were studied in 10 colon carcinoma cell lines and 25 colon tumours. By combining the two-step PCR together with non-radioactively labelled oligonucleotide probes, the detection of point mutations is both accurate and rapid.

Presence of activated ras correlates with increased cysteine proteinase activities in human colorectal carcinomas

The metastatic potential of ras-transfected cells has been attributed in part to significant ras induction of proteinase expression. To determine whether primary cancers also demonstrate higher cysteine proteinase activities in the presence of activated ras genes or altered ras protein expression, we have analyzed 60 primary human colorectal carcinomas for ras gene or protein changes together with the expression of cathepsins B and L. Cancers containing K-ras mutations (47% of 60 carcinomas) demonstrated greater increases in cathepsin L activity than cancers without K-ras mutations (p = 0.029), with particularly significant correlations for earlier stage cancers (Dukes' A and B carcinomas, p = 0.006). Western blots used to characterize ras protein patterns in the same cancer/normal pairs have demonstrated that N-ras protein is more highly expressed in colon tissues than H- or K-ras proteins and that N-ras overexpression occurs in almost 70% of colorectal cancers, with or without a concurrent change in electrophoretic mobility of N-ras protein. Our current study has now shown that N-ras protein overexpression alone does not significantly induce cathepsin B or L activity levels in colon cancers. However, carcinomas demonstrating altered N-ras protein forms, in the absence of any K- or N-ras mutations, expressed significantly higher levels of cathepsin B and L activities compared with carcinomas with normal N-ras protein banding patterns. Our data suggest that colorectal carcinomas with either K-ras mutations or altered forms of N-ras protein may increase their tumorigenic potential via the induction of cathepsin L or B expression levels. Our results also confirm that ras oncogene up-regulation of cathepsin B and L activities, previously reported in cultured cells, is a frequent event in primary human colorectal carcinomas.

Identification of a human CD8+ T lymphocyte neo-epitope created by a ras codon 12 mutation which is restricted by the HLA-A2 allele

Point mutations in the ras proto-oncogenes, notably at codon 12, are found in high frequency of human malignancies and, thus, may be appropriate targets for the induction of tumor-specific T cell responses in cancer immunotherapy. In this study, we examined the mutant ras protein sequence reflecting the substitution of Gly to Val at position 12 as a putative point-mutated determinant for potential induction of an HLA-A2-reactive, CD8+ cytotoxic T lymphocyte (CTL) response. We identified the ras 4-12(Val12) sequence as a minimal 9-mer peptide, which displayed specific binding to HLA-A2 by T2 bioassays. Peptide binding to HLA-A2 on T2 cells was weak and required coincubation with exogenous beta(2)-microglobulin to facilitate and enhance complex formation. In contrast, the wild-type ras 4-12(Gly12) peptide failed to bind to HLA-A2 even in the presence of beta(2)-microglobulin, consistent with the hypothesis that the point mutation creates a C-terminus anchor residue. A CD8+ CTL line against the ras 4-12(Val12) peptide was derived in vitro from a normal HLA-A2+ donor using a model culture system consisting of T2 cells as antigen presenting cells pulsed with exogenous mutant ras peptide and beta(2)-microglobulin plus cytokines (interleukin-2 and 12). Functional characterization of CD8+ CTL line revealed (1) peptide-specific and HLA-A2-restricted cytotoxicity against a panel of peptide-pulsed targets; (2) no specific lysis using the normal ras peptide sequence; (3) half-maximal lysis with exogenous peptide of approximately 0.3 microM; (4) lysis of HLA-A2+ B cell lines infected with a recombinant vaccinia virus construct encoding the point-mutated human K-ras gene; and (5) specific lysis of the HLA-A2+ SW480 colon carcinoma cell line expressing the naturally occurring K-ras Val12 mutation. Maximal lysis of SW480 cells occurred following interferon (IFN)-gamma pretreatment, which correlated with enhanced HLA-A2 and ICAM-1 (CD54) expression. Specificity of lysis was revealed by the absence of lysis against a HLA-A2+ melanoma cell line (+/- IFN-gamma), which lacked the mutant Val12 mutation, and the inability of an irrelevant CD8+ CTL line to lyse SW480 (+/- IFN-gamma) unless the appropriate exogenous peptide was added. These findings demonstrated that tumor cells may endogenously process and express mutant ras epitopes, such as the 4-12(Val12) sequence, albeit in limiting amounts that may be potentiated by IFN-gamma treatment. These data support the biological relevance of this sequence and, thus, may have important implications for the generation of ras oncogene-specific CTL responses in clinical situations.

Validation of the Point-EXACCT method in non-small cell lung carcinomas

K-ras point mutations are often detected in part of the lung carcinomas. For the validation of a highly sensitive and rapid assay for known point mutations, Point-EXACCT (Biochim Biophys Acta 1998; 1379:42–52), we analyzed 89 non-small cell lung carcinomas and compared the results with two sequencing methods. No point mutations were found with double-stranded sequencing. Single-stranded sequencing detected six patients positive for K-ras codon 12. When Point-EXACCT was used, K-ras codon 12 mutations were detected in 8 of 52 patients with squamous cell carcinomas, 10 of 29 patients with adenocarcinomas, and 3 of 8 patients with large cell carcinomas. The finding of K-ras mutations in squamous cell carcinomas is explained by the high sensitivity of the method. Therefore, Point-EXACCT may be applicable to detection of those alterations occurring at a low frequency among an excess of cells with wild-type DNA.

Mutations of K-ras oncogene in human adrenal tumours in Taiwan

Recently, we have found a high frequency of p53 gene mutations in human functional adrenal tumours. As the tumorigenesis is a multigene defect, we believe that other oncogenes may also be involved in the initiation or progression of adrenal tumours. Using the single-strand conformational polymorphism (SSCP) method, we chose the ras oncogenes as the target in this screening procedure because their high mutation rates were detected in thyroid tumours. For the ras oncogenes analysed, exon 1 to exon 2 of H-ras and K-ras genes in the tumour tissues of 13 Conn's syndrome, two adrenal Cushing's syndrome, two non-functional adrenal tumours, one adrenocortical hyperplasia and eight phaeochromocytomas and its paired adjacent normal adrenal tissues were amplified and sequenced. No mutations were detected in the H-ras gene. But mutations of the K-ras gene were detected in 46% (6 of 13) of Conn's syndrome; the hot spots were located at codon 15, 16, 18 and 31, which were different from those previously found in other tumours (codon 12, 13 and 61). Northern blot analysis with 1.1 kb K-ras cDNA revealed that K-ras mRNA was more than tenfold over-expressed in four of Conn's syndrome, one case of Cushing's syndrome and one case of adrenocortical hyperplasia. The mutation sites and mutation type were not found in other tissues, which conferred that this was highly related to adrenocortical tumours. Yet, the correlation between K-ras oncogene and adrenocortical tumours needs to be clarified by further studies.

Genetic alterations in a patient with Turcot's syndrome

Turcot's syndrome (TS) is a rare disorder associated with the development of both brain and colon neoplasms. Because of the very low incidence of the disease, its molecular basis remains unclear. Presented is a TS case of a 30-year-old Japanese male with a histopathologically confirmed diagnosis of both brain tumor (glioblastoma multiforme) and colon tumor (well-differentiated adenocarcinoma). Germline mutations of the p53 gene, somatic mutations of the Ki-ras, p53 and APC genes, and microsatellite instability (MSI) was examined using polymerase chain reaction (PCR)-single strand conformation polymorphism analysis, followed by PCR-direct sequencing, and sequencing after subcloning. No germline mutations of the p53 gene were found. Somatic mutations of Ki-ras and APC genes were found in the colon adenocarcinoma but not in the brain tumor. No somatic mutation of the p53 gene was present in either colon or brain tumors. Microsatellite instability of both colon and brain tumors was positive in two of four loci. These results indicate that the colon tumor of the TS patient carries the Ki-ras and APC gene mutations. The finding of MSI in both the brain and the colon tumors may support the hypothesis that alterations of DNA repair genes are involved in the tumor development of the TS patient.

Generation of stable CD4+ and CD8+ T cell lines from patients immunized with ras oncogene-derived peptides reflecting codon 12 mutations

Previous studies have identified and characterized both murine in vivo and human in vitro T cell responses reflecting specific mutations in the ras proto-oncogenes at codon 12, 13, or 61. In an attempt to determine whether peptide epitopes reflecting point mutations in the ras oncogenes are immunogenic in humans for the production of CD4+ and/or CD8+ T cell responses, a phase I clinical trial was initiated in metastatic carcinoma patients whose primary tumors harbor mutations in the K-ras proto-oncogenes at codon 12. The peptides used here as immunogens, which were administered in Detox adjuvant, spanned the ras sequence 5-17 and reflected the amino acid substitution of glycine (Gly) at position 12 to aspartic acid (Asp), cysteine (Cys), or valine (Val). Three of eight evaluable patients have demonstrated peptide-specific cell-mediated immunity, as determined by the production of T cell lines resulting from the vaccination. First, an antigen (Ag)-specific, major histocompatibility complex (MHC) class II (DP)-restricted CD4+ T cell line was established in vitro from postvaccination lymphocytes of a non-small cell lung carcinoma patient whose primary tumor contained a Cys12 mutation when cultured on the immunizing peptide. Moreover, CD4+ proliferation was inducible against the corresponding mutant K-ras protein, suggesting productive T cell receptor recognition of exogenously processed Ag. Second, an Ag-specific, MHC class I (HLA-A2)-restricted CD8+ cytotoxic T lymphocyte (CTL) line was established in vitro from postvaccination lymphocytes of a colon carcinoma patient whose primary tumor contained an Asp12 mutation. To that end, a 10-mer peptide, nested within the 13-mer immunizing peptide, was identified [i.e., ras5-14(Asp12)], which was shown to bind to HLA-A2 and display specific functional capacity for expansion of the in vivo primed CD8+ CTL precursors. Third, both Ag-specific, MHC class II (DQ)-restricted CD4+ and MHC class I-restricted (HLA-A2) CD8+ T cell lines were generated from a single patient with duodenal carcinoma whose primary tumor contained a Val12 mutation when cultured on the immunizing 13-mer peptide or a nested 10-mer peptide [i.e., ras5-14(Val12)], respectively. Evidence is thus provided that vaccination with mutant ras oncogene peptides in adjuvant may induce specific anti-ras cellular immune responses, with no detectable cross-reactivity toward normal proto-ras sequences. Moreover, we have identified for the first time human HLA-A2-restricted, CD8+ CTL epitopes reflecting specific point mutations in the K-ras oncogenes at codon 12 which, in concert with the activation of the CD4+ T cell response, may have important implications for both active and passive immunotherapies in selected cancer patients.

K-RAS-2 gene mutations as predictors of metachronous colorectal adenomas

BACKGROUND

Mutations of K-RAS-2 gene and tumour suppressor genes have been found in both colorectal adenomas and carcinomas. The aim of this study was to investigate the prognostic value of K-RAS-2 gene mutations found in initial colorectal adenomas for predicting the risk of metachronous adenomas.

METHODS

Genomic DNA was extracted from formalin-fixed and paraffin-embedded adenomas larger than 5 mm in diameter removed at the initial total colonoscopy between 1980 and 1982. All patients underwent colonoscopic follow-up for at least 10 years. The sequence of exon 1 of the K-RAS-2 oncogene was amplified with the polymerase chain reaction technique and screened for mutation by single-strand conformation polymorphism analysis. All suspected mutations were confirmed by direct DNA sequencing. The predictive value of K-RAS-2 gene mutations for the risk of metachronous adenomas was assessed by chi-square testing and logistic regression analysis.

RESULTS

Of 54 patients 39 (72%) were male and 15 (28%) female. At the time the initial adenoma was removed, 31 (57%) patients were younger than 60, whereas 23 (43%) were 60 years or older. Point mutations of the K-RAS-2 oncogene were found in the index adenomas of 15 (27.7%) patients. Mutations were found more frequently in large (> or = 20 mm) adenomas and in adenomas with severe dysplasia (P = 0.0011 and P = 0.0310, respectively). There were no significant associations between K-RAS-2 mutations and anatomic location, histologic type, or number of synchronous initial lesions. Mutations were found predominantly at codon 12 with transversions from GGT to GTT (57%), from GGT to GAT (36%), and from GGT to TTT (one patient). The single mutation found at codon 13 showed a transversion from GGC to GAC. There were significant associations between size (> or = 20 mm) and K-RAS-2 mutation of the initial adenomas and the size (> 5 mm) of metachronous adenomas (P = 0.0259 and P = 0.0265, respectively). However, multivariate analysis showed that K-RAS-2 mutations did not provide a significant additional contribution to the prognostic value of the size of the initial adenoma (odds ratio, 7.62; 95% confidence interval (CI), 1.68-34.48) and the amount of villous structure (odds ratio, 0.22; 95% CI, 0.05-0.90) it contained.

CONCLUSIONS

Patients with large (> or = 20 mm) adenomas and adenomas with K-RAS-2 mutations found at the initial examination have a significantly higher risk of developing large (> 5 mm) metachronous adenomas during surveillance. Multivariate analysis of initial adenoma characteristics showed that the risk of metachronous colorectal adenomas can be adequately estimated by the size and the histologic type of the largest initial adenoma and that K-RAS-2 mutations are of secondary importance only. Further studies based on a larger series will have to identify the adenoma characteristics that will help to improve follow-up strategies.

K-ras is an essential gene in the mouse with partial functional overlap with N-ras

Mammalian ras genes are thought to be critical in the regulation of cellular proliferation and differentiation and are mutated in approximately 30% of all human tumors. However, N-ras and H-ras are nonessential for mouse development. To characterize the normal role of K-ras in growth and development, we have mutated it by gene targeting in the mouse. On an inbred genetic background, embryos homozygous for this mutation die between 12 and 14 days of gestation, with fetal liver defects and evidence of anemia. Thus, K-ras is the only member of the ras gene family essential for mouse embryogenesis. We have also investigated the effect of multiple mutations within the ras gene family. Most animals lacking N-ras function and heterozygous for the K-ras mutation exhibit abnormal hematopoietic development and die between days 10 and 12 of embryogenesis. Thus, partial functional overlap appears to occur within the ras gene family, but K-ras provides a unique and essential function.

Induction of minisatellite mutation in NIH 3T3 cells by treatment with the tumor promoter okadaic acid

Okadaic acid (OA) is a strong tumor promoter of mouse skin carcinogenesis and also a potent inhibitor of serine/threonine protein phosphatases. OA induces various genetic alterations in cultured cells, such as diphtheria-toxin-resistance mutations, sister chromatid exchange, exclusion of exogenous transforming oncogenes, and gene amplification. The present study revealed that it caused minisatellite mutation (MSM) at a high frequency in NIH 3T3 cells, although no microsatellite mutation was found. Nine of 31 clones (29%) exhibited MSM after 6 days of OA treatment, as opposed to only 1 of 30 clones (3%) without OA exposure. Moreover, NIH 3T3 cells treated with OA acquired tumorigenicity in nude mice, giving rise to 7 tumors within 25 weeks in 20 sites where 3 x 10(6) cells were injected. In contrast, the same numbers of untreated cells gave rise to only one tumor, and the tumor grew much slower. All of three OA-induced tumors examined manifested the MSM. The findings thus point to a molecular mechanism by which OA could function as a tumor promoter, and also the biological relevance of the induction of MSM in the tumorigenic process by OA.

H-ras oncogene point mutations in arthritic synovium

OBJECTIVE

To examine mutational activation of ras proto-oncogenes in synovial tissue from patients with rheumatoid arthritis (RA) compared with synovial specimens from patients with osteoarthritis (OA) or other arthropathies. Synovial samples from cadavers, without any signs of joint disease, were used as control material.

METHODS

Using a combination of polymerase chain reaction (PCR) and automated sequencing of the amplified PCR product, regions around codons 12, 13, and 61 of the H-, K-, and N-ras proto-oncogenes were analyzed. Confirmation of mutations was based on restriction fragment length polymorphism analysis and/or oligonucleotide hybridization.

RESULTS

Four (6%) of 72 patients with RA, 2 (13%) of 16 with OA, and 1 (8%) of 12 with other arthropathies harbored mutant H-ras proto-oncogenes, and were heterozygous at codon 13 for the GGT-->GAT (Gly-->Asp) change. An unexpected mutation was found in the H-ras gene, in which a heterozygous GTG-->ATG (Val-->Met) mutation was observed over codon 14. The incidence for this mutation was 39% (28 of 72) in RA patients, 94% (15 of 16) in OA patients, and 42% (5 of 12) in patients with other arthropathies. All samples carrying the codon 13 mutation of H-ras were also codon 14-mutated, i.e., double mutations existed. Identical point mutations were also detected in a few synovial specimens obtained from cadavers (n = 8), including a single case of double mutation. All specimens showed normal K- and N-ras loci.

CONCLUSION

Activation of proto-oncogene H-ras by point mutation in codons 13 and 14 occurred in the synovial tissue of patients with RA, OA, or other arthropathies, as well as, to some extent, in the control synovia, indicating that the phenomenon is not specific for RA. In codon 14, incidence of the H-ras point mutation was highest in OA tissue. The possible significance of this codon 14-mutated H-ras gene needs to be clarified.

Ras2 and Ras1 protein phosphorylation in Saccharomyces cerevisiae

This work describes the phosphorylation of Saccharomyces cerevisiae Ras proteins and explores the physiological role of the phosphorylation of Ras2 protein. Proteins expressed from activated alleles of RAS were less stable and less phosphorylated than proteins from cells expressing wild-type alleles of RAS. This difference in phosphorylation level did not result from increased signaling through the Ras-cAMP pathway or reflect the primarily GTP-bound nature of activated forms of Ras protein per se. In addition, phosphorylation of Ras protein was not dependent on proper localization of the Ras2 protein to the plasma membrane nor on the interaction of Ras2p with its exchange factor, Cdc25p. The preferred phosphorylation site on Ras2 protein was identified as serine 214. This site, when mutated to alanine, led to promiscuous phosphorylation of Ras2 protein on nearby serine residues. A decrease in phosphorylation may lead to a decrease in signaling through the Ras-cAMP pathway.

Response to radiotherapy of human uterine cervix carcinoma is not correlated with rearrangements of the Ha-ras-1 and/or c-myc genes

An association between the presence of the activated form of Ha-ras-1 and c-myc genes and increased cellular radioresistance has been shown in several cell lines. The aim of this study was to determine whether such an association could be observed in clinical tumour biopsies. We examined 70 tumour specimens and 51 samples of peripheral blood obtained from untreated patients with carcinoma of the uterine cervix mainly stage II and III. In addition to initial clinical tumour response to radiotherapy, radiosensitivity was also analysed by the subrenal capsule assay (SRCA). Mutations in exons 1 and 2 of the Ha-ras-1 gene were examined using PCR single-strand conformation polymorphism (PCR-SSCP) and direct sequencing; and restriction fragment length polymorphism of the Ha-ras-1 gene was analysed using Southern hybridisation. Eight (11%) out of 70 tumours contained mutations in exons 1 and 2 of the Ha-ras-1 gene. Eleven (22%) out of the 51 tumours displayed rearrangements of the Ha-ras-1 gene (six tumours (12%) showed alterations of allele length, one amplification and four (8%) loss of one Ha-ras-1 allele). In addition, 12 (17%) out of 70 patients demonstrated the presence of rare alleles. Only one of the 70 tumours contained an amplified c-myc gene. There was no significant correlation between either rearrangements of the structure of the Ha-ras-1 and/or c-myc genes or presence of rare alleles in tumours and tumour response to radiotherapy.

Detection of K-ras point mutation by enriched PCR-colorimetric plate assay

Point mutations in the K-ras gene are frequently observed in a variety of human malignancies, including colorectal and pancreatic cancers. In this paper, we describe a sensitive procedure for the detection of point mutations of codon 12 of the K-ras gene. The assay employs a single-tube enriched PCR procedure, coupled to colorimetric detection. In the enriched PCR procedure, the first round of amplification introduces a restriction enzyme site in the wild type, but not in mutant K-ras PCR product. The wild type products are then digested and the second round of PCR enriches for the mutant sequences by amplifying the resistant products. The second round of amplification allows the incorporation of biotin and a substrate binding tag at opposite ends of the mutant product, thus allowing detection of the product by a simple colorimetric assay. The assay has been validated using DNA from a variety of cell lines known to contain either mutant or wild type K-ras. Under these conditions, the assay has proved both reproducible and sensitive, with the ability to detect one mutant molecule in a background of 1000 wild type molecules. The assay allowed discrimination of mutant from wild type K-ras in samples from colonic adenocarcinomas and normal colonic mucosa. The use of a colorimetric detection system reduces observer bias and facilitates analysis of large numbers of samples. As such, the assay may have specific application in the sensitive detection of K-ras mutations in a variety of clinical samples.

A detailed analysis of the role of K-ras gene mutation in the progression of colorectal adenoma

To elucidate the role of ras gene mutations during the early stage of colorectal tumour progression, K-ras gene mutations were analysed in 32 benign adenomas and 36 adenomas with focal carcinoma in the colorectum by microscraping of histologically pure regions from tissue sections, polymerase chain reaction-restriction fragment length polymorphism and in part by direct sequencing. Several regions were scraped out and analysed when an adenoma contained areas with different grades of dysplasia. The frequencies of K-ras gene mutation in mild dysplasia, moderate dysplasia and focal carcinoma were 19% (7/36), 51% (25/49) and 39% (14/36) respectively. The K-ras gene status was heterogeneous in 4 of the 11 benign adenomas from which multiple samples were obtained, and mutations were always found in the regions with more advanced dysplasia in these adenomas. Thirteen of the 36 adenomas with focal carcinoma showed heterogeneity of mutations between the adenoma region and the focal carcinoma. Seven of which had mutations only in the adenoma region. These findings indicated that the K-ras gene mutations occur during the late stage of adenoma progression and may confer a more advanced morphological phenotype of adenoma, but these mutations are not mainly involved in malignant transformation from adenoma to carcinoma.

Codon 12 Ki-ras mutation in non-small-cell lung cancer: comparative evaluation in tumoural and non-tumoural lung

Ki-ras activation by point mutation on codon 12 has been reported in non-small-cell lung carcinomas and in various models of experimental lung tumours induced by chemical carcinogens. The cellular targets for carcinogenic compounds of tobacco smoke are usually considered to be the cells of the bronchial mucosa or alveolar epithelium. However, little is known about preneoplastic events in bronchopulmonary carcinogenesis. The hypothesis of the presence of widespread target cells containing Ki-ras mutation was investigated by evaluating concurrent neoplastic and non-neoplastic bronchial and alveolar samples from 51 patients with non-small-cell lung carcinomas. The polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) method used can detect one cell with a mutation on codon 12 among 10(2) normal cells. In tumour samples, a mutation was detected in 20% of adenocarcinomas, but in none of the adenosquamous or squamous cell carcinomas. No mutation was detected in the non-neoplastic bronchial or parenchymal samples. When using an enriched PCR-RFLP method detecting one mutated allele among 10(3) normal alleles a mutation was detected in 23% of adenocarcinomas. In conclusion, Ki-ras activation by mutation on codon 12 was not observed in non-neoplastic bronchial or parenchymal tissues in patients with bronchopulmonary cancers and does not appear to be a genetic event present in non-malignant epithelial target cells exposed to tobacco smoke.

Point mutations in the Ki-ras2 gene of codon 12 in the Dunning R-3327 Prostatic Adenocarcinoma system

Detection of point mutations in the Dunning System with the Ki-ras2 gene in the first and second positions of codon 12 exon I, has been performed using allele-specific oligonucleotide (ASO) primers. PCR generated 94bp templates and genomic DNA extracted from the Dunning cell lines AT-1, AT-3, Mat-Lu, and Mat-Lylu were tested. Our investigation revealed that the first and second positions of codon 12 have undergone either transitions or transversions from the wild type sequence (GG). Dorsal prostate (DP) solid tissues used as controls revealed the wild type configuration as well as transversions at both positions, in addition to a transition in the first position for Mat-Lu. The most aberrant of the Dunning lines (AT-3 and Mat-LyLu) collectively displayed sequence changes (transitions and transversions) with no evidence of the wild type configuration. These findings are supportive of biometric studies (area, shape factor, and motility) along with adhesion and invasion assays done in our laboratory in correlating genotypic and phenotypic properties to metastatic potential.

Detection of hepatic micrometastasis in pancreatic adenocarcinoma patients by two-stage polymerase chain reaction/restriction fragment length polymorphism analysis

Hepatic metastasis and retroperitoneal recurrence generally are considered to be the two primary modes of recurrence in pancreatic cancer. The goal of this study was to determine if patients with pancreatic adenocarcinoma have hepatic and peritoneal micrometastasis at operation. Pancreatic adenocarcinomas are known to have a high incidence of K-ras gene mutations. Liver tissue specimens were obtained from 30 patients (17 with pancreatic adenocarcinoma and 13 with other diseases) with a biopsy needle at operation. Peritoneal washings were obtained during operation from 20 patients with pancreatic adenocarcinoma. Two-stage polymerase chain reaction (PCR) and restriction fragment length polymorphism (RFLP) analysis were used to detect K-ras oncogene mutation at codon 12. Thirteen of 17 pancreatic adenocarcinoma patients had K-ras gene mutations in the liver, whereas all 13 patients with other diseases did not. However, only two of 20 pancreatic adenocarcinoma patients revealed K-ras gene mutation in peritoneal lavage fluids. These results indicate the feasibility of detecting hepatic micrometastasis in patients with pancreatic adenocarcinoma, and imply that PCR/RFLP analysis may be of value in the diagnosis, treatment and follow-up of hepatic metastasis of pancreatic adenocarcinoma.

Mutant K-ras in apparently normal mucosa of colorectal cancer patients. Its potential as a biomarker of colorectal tumorigenesis

BACKGROUND

The best way to reduce the incidence of colorectal cancer mortality would be to prevent this cancer. However, none of the biomarkers proposed can accurately identify persons at increased risk of colorectal cancer or those at low risk. As a possible genetic biomarker, K-ras mutations, which are frequently found in colorectal cancers, were analyzed in apparently normal colorectal mucosa.

METHODS

Nonneoplastic mucosa and tumor tissues were collected at surgery from 70 patients with colorectal cancer: one sample each from 50 patients (group A) and multiple samples from the other 20 patients (group B). Mutant K-ras codon 12 was analyzed by the enriched polymerase chain reaction (EPCR), by which one mutant can be detected among 10(3) to 10(4) normal alleles.

RESULTS

Only with the aid of EPCR was mutant K-ras detected in nonneoplastic mucosa of nine patients (18%) in Group A and five patients (25%) in Group B. This increased incidence could be attributed to the multiple tissue sampling. The presence of mutant K-ras in nonneoplastic mucosae was not consistently correlated with that in the tumors. These findings suggest that the mutant K-ras identified in nonneoplastic mucosa actually represents de novo mutations, which may be initiated by different etiologic factors and at different times.

CONCLUSION

Mutant K-ras detected in apparently normal mucosa should be a useful biomarker for identifying persons at higher risk of colorectal cancer. Our study also emphasizes the need for improving the method for sample collection to achieve true representation of the colorectal mucosa.

Low frequency and late occurrence of p53 and dcc aberrations in colorectal tumours

Whilst p53 aberrations have been documented in numerous malignancies, reports of alterations to the deleted in colorectal cancer (dcc) gene are infrequent, and studies investigating the status of both genes in the same colon tumour are rare. In this study we have analysed a panel of 35 pairs of normal and neoplastic human colorectal tissues for abnormalities in these tumour-suppressor genes. In contrast to previous studies we have found only a low incidence of mutations and deletions. p53 point mutations were identified in 8/35 tumours (22%). All were G.C to A.T transitions, with 7/8 occurring at CpG dinucleotides. p53 allelic loss was detected in 4/11 informative cases (36%). Although not quite attaining statistical significance, p53 alteration correlated with the adenoma/carcinoma transition. Gross dcc alterations were identified by Southern blotting in 7/35 (20%) tumours. Microsatellite analysis using two markers, one within and one proximal to the dcc gene, detected a low frequency of deletion overall (41% informative cases). 18q/dcc aberrations were associated with the progression of early to late carcinoma, rather than with increasing adenoma size, as has been previously reported. Both p53 alterations and dcc deletions were detected at a higher frequency in distal tumours than in proximal malignancies. Two tumours exhibiting microsatellite instability in both markers were each of proximal origin.

Lung tumor induction in A/J mice and clastogenic effects in CD-1 mice of the sequence-selective DNA alkylating agents (+)-CC-1065 and (-)-CC-1065

The in vivo genotoxic effects of the antitumor antibiotic, (+)-CC-1065, and its unnatural enantiomer, (-)-CC-1065, were investigated in two mouse models. These two compounds alkylate AT-rich regions of double stranded DNA with distinct sequence selectivities. (+)-CC-1065 dose-dependently increased the chromosomal aberration frequency in bone marrow cells of CD-1 mice from 1.2 +/- 0.8% in vehicle control animals to 5.0 +/- 1.2%, 11.4 +/- 3.9%, and 20.6 +/- 2.3% 24 hours following single intravenous doses of 2, 4, and 8 micrograms/kg, respectively. (-)-CC-1065 was significantly less potent with a maximal response at 8 micrograms/kg approximately one-third of that observed for (+)-CC-1065. (+)-CC-1065 induced a significant (P < or = 0.05), three-fold increase in the number of lung tumors/mouse in strain A/J mice from 0.27 +/- 0.15 for vehicle control animals to 0.83 +/- 0.15 24 weeks following a single intravenous dose of 8 micrograms/kg. This effect was paralleled by corresponding threefold increases in the percentage of mice with tumors and the percentage of mice with multiple tumors, compared to vehicle controls. (-)-CC-1065 at 8 micrograms/kg induced 0.67 +/- 0.15 tumors/mouse and resulted in slightly smaller increases in the tumor incidence and multiple tumor incidence, compared to (+)-CC-1065. The above results demonstrate that single intravenous doses of (+)- CC-1065 and (-)-CC-1065 which cause chromosomal damage in CD-1 mice also induce an increased incidence of lung tumors in A/J mice.(ABSTRACT TRUNCATED AT 250 WORDS)

Polylysine domain of K-ras 4B protein is crucial for malignant transformation

Previous studies have shown that posttranslational modifications are required for both oncogenic K-ras 4B protein membrane binding and transforming activity. In addition, Hancock et al. [Hancock, J. F., Patterson, H. & Marshall, C. J. (1990) Cell 63, 133-139] found that a polylysine domain contained at the C terminus of K-ras 4B was also absolutely essential for K-ras 4B membrane binding but, surprisingly, neither the polylysine domain nor membrane binding was required for transforming activity. We have performed similar studies, but our results are distinctly different. Our studies indicate that the polylysine domain is crucial for K-ras 4B transforming activity. Moreover, we demonstrate that although the polylysine domain increases K-ras 4B membrane binding, significant amounts of membrane binding can occur in the absence of this domain. Finally, while our studies are consistent with the notion that membrane binding is required for K-ras 4B transforming activity, we show that membrane binding, in and of itself, is not sufficient for efficient K-ras 4B transforming activity.

Cationic modulation of rho 1-type gamma-aminobutyrate receptors expressed in Xenopus oocytes

A study was made of the effects of di- and trivalent cations on homomeric rho 1-type gamma-aminobutyrate (GABA rho 1) receptors expressed in Xenopus oocytes after injection of mRNA coding for the GABA rho 1 subunit. GABA elicited large currents with a Kd approximately 1 microM. The properties of these GABA rho 1 receptors were similar to those of native bicuculline-resistant GABA receptors expressed by retinal mRNA. GABA rho 1 currents showed very little desensitization, were blocked by picrotoxin but not by bicuculline, and were not modulated by barbiturates, benzodiazepines, or beta-carbolines. Zn2+ reversibly decreased GABA rho 1 responses (IC50 = 22 microM). Other divalent cations were also tested and their rank order of potency was: Zn2+ approximately Ni2+ approximately Cu2+ >> Cd2+, whereas Ba2+, Co2+, Sr2+, Mn2+, Mg2+, and Ca2+ showed little or no effect. In contrast, La3+ reversibly potentiated the GABA currents mediated by homomeric GABA rho 1 receptors, with an EC50 = 135 microM and a maximal potentiation of about 100% (GABA, 1 microM; La3+, 1 mM). Other lanthanides showed similar effects (Lu3+ > Eu3+ > Tb3+ > Gd3+ > Er3% > Nd3+ > La3+ > Ce3+). Thus, GABA rho 1 receptors contain sites for cationic recognition, and in particular, Zn2+ may play a role during synaptic transmission in the retina.

Non-involvement of ras mutations in flat colorectal adenomas and carcinomas

Ras gene mutations occur relatively early during colorectal tumor development and have been observed in 40-50% of malignant colorectal tumors. Advances in endoscopic techniques have made it possible to detect small, flat colorectal tumors that could not be detected by standard examinations. To determine whether ras gene mutations are also involved in the genesis of small, flat colorectal tumors, we examined ras point mutations in 34 cases of small polypoid or flat elevated colorectal tumors (32 adenomas, 2 carcinomas) and in 26 cases of small, flat colorectal tumors (13 adenomas, 13 carcinomas) by means of the polymerase chain reaction (PCR) and dot-blot hybridization. Ras gene point mutations were observed in 16 of the 34 tumors of the former type (47%), but in none of the 26 tumors of the latter type, even though the grade of dysplasia was severe in the flat tumors. Our results suggest that different genetic pathways for tumor progression may exist for polypoid and for flat colorectal carcinomas.

Neuroendocrine neoplasms of the lung are not associated with point mutations at codon 12 of the Ki-ras gene

The most prominent abnormality of ras proto-oncogenes in human lung tumours has involved point mutations at codon 12 of the Ki-ras gene. We have analysed 35 tumour samples of neuroendocrine lung neoplasms (ten carcinoid tumours, ten well-differentiated neuroendocrine carcinomas, and 15 intermediate/small cell neuroendocrine carcinomas) for a point mutation at this site. For this purpose, formalin-fixed and paraffin-embedded tissue sections were microdissected to remove non-tumours areas. DNA in the remaining tumour tissue was amplified in vitro by the polymerase chain reaction (PCR) and double-stranded PCR products were subjected to sequence analysis. Neither point mutations at codon 12 nor additional structural alterations at codons 1-32 were detected in Ki-ras gene. Our results suggest that point mutations at codon 12 of the Ki-ras gene do not seem to be involved in the pathogenesis of pulmonary neuroendocrine neoplasms.

Genotypic classification of colorectal adenocarcinoma. Biologic behavior correlates with K-ras-2 mutation type

BACKGROUND

New measures enabling better prediction of biologic behavior of large bowel cancer are highly desirable. One hundred ninety-four consecutive primary, recurrent, and metastatic colorectal adenocarcinomas, accessioned during 1991 at Rhode Island Hospital, were classified according to the presence and specific type of K-ras-2 point mutation.

METHODS

An integrated histopathologic-genetic approach was used to detect mutations starting with minute, topographically selected, tissue samples from formaldehyde-fixed, paraffin-embedded specimens.

RESULTS

Each colorectal adenocarcinoma exhibited either no or only one of seven specific types of K-ras-2 mutation. The mutation type of each primary tumor was present consistently in its metastatic deposits. Thirty-five percent of primary colorectal adenocarcinomas were found to be mutated (42 of 119). A significantly higher mutation rate (65%) was seen in lymphogenous-hematogenous metastases as a group (35 of 54; P < 0.005). By contrast, 22% of anastomotic recurrences and transcoelomic metastasis were mutated (4 of 18). Twenty-eight percent of adenocarcinomas with invasion limited to muscularis propria (Tis, T1, T2) were mutated (16 of 57), compared to 41% for more deeply invasive tumors (T3, T4; 26 of 63). When colorectal adenocarcinomas were analyzed by specific K-ras-2 mutation type, it was found that codon 13 mutated tumors did not progress to local or distant metastasis (P < 0.01). Tumors having a codon 12 valine substitution did not metastasize beyond pericolonic-perirectal lymph nodes. In contrast, colorectal cancers with codon 12 aspartic acid substitutions accounted for most of the distant hematogenous deposits (P < 0.01). Tumors with normal K-ras-2 accounted for most intraperitoneal deposits.

CONCLUSIONS

Genotyping of colorectal adenocarcinoma by K-ras-2 status can identify subsets of patients likely to pursue indolent and aggressive forms of disease. The integrated histopathologic-genetic approach outlined is feasible for use in diagnostic pathology, providing information that together with clinicopathologic staging may individualize and optimize treatment.

Predisposition to neoplastic transformation caused by gene replacement of H-ras1

Homologous recombination was used to introduce a nominally transforming mutation into an endogenous H-ras1 gene in Rat1 fibroblasts. Although both the mutant and the remaining normal allele were expressed equally, the heterozygous cells were not neoplastically transformed. Instead, spontaneously transformed cells arose from the heterozygotes at a low frequency, and the majority of these cells had amplified the mutant allele. Thus, the activated H-ras1 allele was not by itself dominant over the normal allele but predisposed cells to transformation by independent events, such as amplification of the mutant allele.

Identification of two cytotoxic T lymphocyte-recognized epitopes in the Ras protein

We have investigated the possibility of inducing cytotoxic T lymphocytes (CTL) to Ras containing a mutation at position 61 or to normal Ras, using recombinant vaccinia viruses expressing these proteins. CTL from C57Bl/10 mice immunized with vaccinia expressing mutant Ras showed specificity for the mutant Ras protein and recognition of normal Ras was inefficient. The opposite specificity was observed after immunization with vaccinia expressing normal Ras, since CTL isolated from these mice recognized normal Ras well and mutant Ras inefficiently. Levels of endogenous Ras expression were insufficient for lysis by these CTL. One CTL epitope mapped to amino acids 60-67 and residue 61 was critical for T cell recognition. CTL generated against mutant Ras protein recognized peptide 60-67 containing mutant residue 61, while anti-normal Ras CTL recognized the wild-type 60-67 sequence. A second epitope mapped to residues 152-159 of Ras and was recognized equally well by CTL raised to normal or mutant Ras. The murine data raise the possibility of exploiting Ras-specific CTL for targeted immunotherapy of certain human cancers.

Altered growth of human colon cancer cell lines disrupted at activated Ki-ras

Point mutations that activate the Ki-ras proto-oncogene are presented in about 50 percent of human colorectal tumors. To study the functional significance of these mutations, the activated Ki-ras genes in two human colon carcinoma cell lines, DLD-1 and HCT 116, were disrupted by homologous recombination. Compared with parental cells, cells disrupted at the activated Ki-ras gene were morphologically altered, lost the capacity for anchorage-independent growth, grew more slowly both in vitro and in nude mice, and showed reduced expression of c-myc. Thus, the activated Ki-ras gene plays a key role in colorectal tumorigenesis through altered cell differentiation and cell growth.

Oncogene activation: c-raf-1 gene mutations in experimental and naturally occurring tumors

We demonstrate here consistent point mutations of the c-raf-1 proto-oncogene, within a small region of the kinase domain, in a mouse model for chemical tumor induction. This is the first demonstration of point mutated raf genes in vivo, and the first isolation of activating in vivo point mutations in the kinase domain of a proto-oncogene. The specific region where these mutations are clustered also has biological significance. This is precisely the region where 5/5 independently generated monoclonal antibodies raised against Raf-1 map to [29], and predictions based upon the crystal structure of A kinase identify this as the substrate pocket. The tumors examined show a selective specificity for Raf-1 mutations in that another family of genes, the ras proto-oncogenes which are frequently activated by point mutation in both animal and human tumors [15-21,26], is not involved. Our consistent finding of Raf-1 mutations in a mouse tumor model also has consequences for further evaluation of the role of Raf-1 in human tumor development, as it emphasizes the need to examine c-raf-1 at the sequence level. In fact preliminary screening of human lung tumors indicates point mutations at amino acid 533 (John Lyons, personal communication). Finally, the cumulative data on the critical role of Raf-1 in signal transduction and the occurrence of oncogenic Raf-1 in tumors [32-41] highlight this enzyme as an attractive target for development of novel anticancer regimens.

Characterization of the K-ras gene of the marsupial Monodelphis domestica

A cDNA clone isolated from a plasmid library contains the complete coding sequence for the K-ras gene of the marsupial Monodelphis domestica, a South American opossum. The nucleotide sequence of the coding region of the opossum K-ras gene is very similar to the K-ras coding sequences of placental mammals. The coding region of the opossum gene is 95% identical to the human gene at the nucleotide level; the human and opossum genes are 99% identical at the level of encoded amino acids. Transcribed but untranslated regions of the opossum gene 3' and 5' to the coding region are similar to corresponding regions of the human and mouse genes, but are less highly conserved than translated sequences. Based on the nucleotide sequence of the opossum K-ras cDNA clone, primers were designed that allowed amplification of exons 1 and 2 of the gene from opossum genomic DNA by the polymerase chain reaction. When exons 1 and 2 of K-ras were amplified from DNA isolated from an ultraviolet radiation-induced eye tumor of M. domestica and the nucleotide sequence of amplified material was determined, a heterozygous mutation in codon 61 of the gene was detected. This T to A transversion resulted in a change in the amino acid encoded by the codon. The tumor from which DNA was isolated had previously been shown to contain a transforming K-ras oncogene. Thus, the opossum K-ras gene can be mutationally activated in a manner similar to the K-ras genes of placental mammals.

The gene structure of Xenopus nuclear lamin A: a model for the evolution of A-type from B-type lamins by exon shuffling

Nuclear lamins are intermediate filament (IF) type proteins that form a fibrillar network underlying the inner nuclear membrane. The existence of multiple subtypes of lamins in vertebrates has been interpreted in terms of functional specialization during cell division and differentiation. The structure of a gene encoding an A-type lamin of Xenopus laevis was analysed. Comparison with that of a B-type lamin of the same species shows remarkable conservation of the exon/intron pattern. In both genes the last exon, only 9-12 amino acids in length, encodes the complete information necessary for membrane targeting of lamins, i.e. a ras-related CaaX motif. The lamin A specific extension of the tail domain is encoded by a single additional exon. The 5' boundary of this exon coincides with the sequence divergence between human lamins A and C, for which an alternative splice mechanism had previously been suggested. Arguments are presented suggesting that B-type lamins represent the ancestral type of lamins and that A-type lamins derived there from by exon shuffling. The acquisition of the new exon might explain the different fates of A- and B-types lamins during cell division.