Transcriptional slippage of p53 gene enhanced by cellular damage in rat liver: monitoring the slippage by a yeast functional assay

Transforming Growth Factor β Signaling in Colorectal Cancer Cells With Microsatellite Instability Despite Biallelic Mutations in TGFBR2

BACKGROUND & AIMS

Most colorectal cancer (CRC) cells with high levels of microsatellite instability (MSI-H) accumulate mutations at a microsatellite sequence in the gene encoding transforming growth factor β receptor II (TGFBR2). TGFβ signaling therefore is believed to be defective in these tumors, although CRC cells with TGFBR2 mutations have been reported to remain sensitive to TGFβ. We investigated how TGFβ signaling might continue in MSI-H CRC cells.

METHODS

We sequenced the 10-adenines microsatellite sequence in the TGFBR2 gene of 32 MSI-H colon cancer tissues and 6 cell lines (HCT116, LS180, LS411N, RKO, SW48, and SW837). Activation of TGFβ signaling was detected by SMAD2 phosphorylation and through use of a TGFβ-responsive reporter construct in all CRC cell lines. Transcripts of TGFBR2 were knocked-down in CRC cells using short hairpin RNA. Full-length and mutant forms of TGFBR2 were expressed in LS411N cells, which do not respond to TGFβ, and their activities were measured.

RESULTS

SMAD2 was phosphorylated in most MSI-H CRC tissues (strong detection in 44% and weak detection in 34% of MSI-H tumors). Phosphorylation of SMAD2 in MSI-H cells required TGFBR2—even the form encoding a frameshift mutation. Transcription and translation of TGFBR2 with a 1-nucleotide deletion at its microsatellite sequence still produced a full-length TGFBR2 protein. However, protein expression required preservation of the TGFBR2 microsatellite sequence; cells in which this sequence was replaced with a synonymous nonmicrosatellite sequence did not produce functional TGFBR2 protein.

CONCLUSION

TGFβ signaling remains active in some MSI-H CRC cells despite the presence of frameshift mutations in the TGFBR2 gene because the mutated gene still expresses a functional protein. Strategies to reactivate TGFβ signaling in colorectal tumors might not be warranted, and the functional effects of mutations at other regions of microsatellite instability should be evaluated.

Isolation and characterization of RNA polymerase rpoB mutations that alter transcription slippage during elongation in Escherichia coli

Transcription fidelity is critical for maintaining the accurate flow of genetic information. The study of transcription fidelity has been limited because the intrinsic error rate of transcription is obscured by the higher error rate of translation, making identification of phenotypes associated with transcription infidelity challenging. Slippage of elongating RNA polymerase (RNAP) on homopolymeric A/T tracts in DNA represents a special type of transcription error leading to disruption of open reading frames in Escherichia coli mRNA. However, the regions in RNAP involved in elongation slippage and its molecular mechanism are unknown. We constructed an A/T tract that is out of frame relative to a downstream lacZ gene on the chromosome to examine transcriptional slippage during elongation. Further, we developed a genetic system that enabled us for the first time to isolate and characterize E. coli RNAP mutants with altered transcriptional slippage in vivo. We identified several amino acid residues in the β subunit of RNAP that affect slippage in vivo and in vitro. Interestingly, these highly clustered residues are located near the RNA strand of the RNA-DNA hybrid in the elongation complex. Our E. coli study complements an accompanying study of slippage by yeast RNAP II and provides the basis for future studies on the mechanism of transcription fidelity.

LEF-1 and TCF4 expression correlate inversely with survival in colorectal cancer

BACKGROUND

Most colorectal carcinomas are driven by an activation of the canonical Wnt signalling pathway, which promotes the expression of multiple target genes mediating proliferation inavasion and invasion. Upon activation of the Wnt signalling pathway its key player β-catenin translocates from the cytoplasm to the nucleus and binds to members of the T-cell factor (TCF)/lymphoid enhancer factor (LEF-1) family namely LEF-1 and TCF4 which are central mediators of transcription. In this study we investigated the expression of β-Catenin, LEF1 and TCF4 in colorectal carcinomas and their prognostic significance.

METHODS

Immunohistochemical analyses of LEF-1, TCF4 and nuclear β-Catenin were done using a tissue microarray with 214 colorectal cancer specimens. The expression patterns were compared with each other and the results were correlated with clinicopathologic variables and overall survival in univariate and multivariate analysis.

RESULTS

LEF-1 expression was found in 56 (26%) and TCF4 expression in 99 (46%) of colorectal carcinomas and both were heterogeneously distributed throughout the tumours. Comparing LEF-1, TCF4 and β-catenin expression patterns we found no correlation. In univariate analysis, TCF4 expression turned out to be a negative prognostic factor being associated with shorter overall survival (p = 0.020), whereas LEF-1 expression as well as a LEF-1/TCF4 ratio were positive prognostic factors and correlated with longer overall survival (p = 0.015 respectively p = 0.001). In multivariate analysis, LEF-1 and TCF4 expression were confirmed to be independent predictors of longer respectively shorter overall survival, when considered together with tumour stage, gender and age (risk ratio for LEF-1: 2.66; p = 0.027 risk ratio for TCF4: 2.18; p = 0.014).

CONCLUSIONS

This study demonstrates different prognostic values of LEF-1 and TCF4 expression in colorectal cancer patients indicating different regulation of these transcription mediators during tumour progression. Moreover both factors may serve as new potential predictive markers in low stage colon cancer cases in advance.

Characterization of cupric glutamate extinguishing mechanism of Alexandrium sp. LC3 with two-dimensional electrophoresis and MALDI-TOF MS

Mechanisms by which cupric glutamate, a novel algicide, extinguishes Alexandrium sp. LC3 are shown in this study. We show that cupric glutamate not only stimulated the production of malonaldehyde (MDA) and dramatically promoted cell plasma membrane permeability (p < 0.01) but also remarkably reduced sulfhydryl (SH) group content (p < 0.01). Analysis of protein expression profiles by two-dimensional electrophoresis (2-DE) indicated that only 47 protein spots were detected in both control and cupric glutamate treated cells. Three reliable spots were identified by matrix-assisted laser desorption/ionization-time-of-flight mass spectrometry (MALDI-TOF MS) as ribulose-bisphosphate carboxylase large subunit precursor, RNA polymerase beta chain, and hypothetical protein, which can be well correlated with cupric glutamate stress. Based on above results, we hypothesize that the extinguishing mechanisms include (1) the cell membrane being damaged by cupric glutamate; (2) cupric glutamate probably induced denaturation and disintegration of intracellular protein, which led to inhibition of cell growth.

Nonrandom variations in human cancer ESTs indicate that mRNA heterogeneity increases during carcinogenesis

Virtually all cancer biological attributes are heterogeneous. Because of this, it is currently difficult to reconcile results of cancer transcriptome and proteome experiments. It is also established that cancer somatic mutations arise at rates higher than suspected, but yet are insufficient to explain all cancer cell heterogeneity. We have analyzed sequence variations of 17 abundantly expressed genes in a large set of human ESTs originating from either normal or cancer samples. We show that cancer ESTs have greater variations than normal ESTs for >70% of the tested genes. These variations cannot be explained by known and putative SNPs. Furthermore, cancer EST variations were not random, but were determined by the composition of the substituted base (b0) as well as that of the bases located upstream (up to b - 4) and downstream (up to b + 3) of the substitution event. The replacement base was also not randomly selected but corresponded in most cases (73%) to a repetition of b - 1 or of b + 1. Base substitutions follow a specific pattern of affected bases: A and T substitutions were preferentially observed in cancer ESTs. In contrast, cancer somatic mutations [Sjoblom T, et al. (2006) Science 314:268-274] and SNPs identified in the genes of the current study occurred preferentially with C and G. On the basis of these observations, we developed a working hypothesis that cancer EST heterogeneity results primarily from increased transcription infidelity.

The LEC rat: a useful model for studying liver carcinogenesis related to oxidative stress and inflammation

Growing evidence indicates oxidative stress as a mechanism of several diseases including cancer. Oxidative stress can be defined as the imbalance between cellular oxidant species production and antioxidant capability shifted towards the former. Lipid peroxidation is one of the processes that takes place during oxidative stress. Lipid peroxidation products, such as malondialdehyde (MDA) and 4-hydroxy-2-nonenal (HNE), are closely related to carcinogenesis as they are potent mutagens and they have been suggested as modulators of signal pathways related to proliferation and apoptosis, two processes implicated in cancer development. Mechanisms by which oxidative stress leads to tumor formation are still under investigation. The need of suitable in vivo models that could reflect that inflammation-related human carcinogenesis is evident. In this regard, the mutant strain Long Evans Cinnamon-like (LEC) rat provides a promising model for investigation of the relationship between hepatitis induced by oxidative stress and hepatocarcinogenesis because it has been demonstrated to develop spontaneous liver tumor formation related to copper accumulation and oxidative stress. In this review, the findings regarding oxidative stress and its relation with liver pathologies in LEC rats are discussed; we focus on the mechanisms proposed for HNE carcinogenesis.

The gateway pDEST17 expression vector encodes a -1 ribosomal frameshifting sequence

The attB1 site in the Gateway (Invitrogen) bacterial expression vector pDEST17, necessary for in vitro site-specific recombination, contains the sequence AAA-AAA. The sequence A-AAA-AAG within the Escherichia coli dnaX gene is recognized as ‘slippery’ and promotes −1 translational frameshifting. We show here, by expressing in E. coli several plant cDNAs with and without single nucleotide deletions close to the translation initiation codons, that pDEST17 is intrinsically susceptible to −1 ribosomal frameshifting at the sequence C-AAA-AAA. The deletion mutants produce correct-sized, active enzymes with a good correlation between enzyme amount and activity. We demonstrate unambiguously the frameshift through a combination of Edman degradation, MALDI-ToF mass fingerprint analysis of tryptic peptides and MALDI-ToF reflectron in-source decay (rISD) sequencing. The degree of frameshifting depends on the nature of the sequence being expressed and ranged from 25 to 60%. These findings suggest that caution should be exercised when employing pDEST17 for high-level protein expression and that the attB1 site has some potential as a tool for studying −1 frameshifting.

Flow cytometric detection of tandem repeat mutations induced by various chemical classes

To facilitate detection of genotoxicity from environmental mutagen exposure, we generated an in vitro enhanced green fluorescence protein (EGFP) reactivation assay that quickly and effectively detects frameshift mutations in tandem repeat sequences (TRS). Two murine cell lines, C3H10T1/2 and mismatch repair deficient MC2a, were stably transfected with EGFP reporter plasmids in which the EGFP constructs contain TRS that put the EGFP sequence out of frame. These included several 2, 3, 4, 5 and 6 bp repeat sequences, a control non-repetitive sequence and a human gene sequence containing a 4 bp repeat motif. Transfected cultures were exposed to five model mutagens and carcinogens: hydrogen peroxide (H(2)O(2)), 12-O-tetradecanoyl-phorbol-13-acetate (TPA), benzo-a-pyrene-diol-epoxide (BPDE), ethyl nitrosourea (ENU), 9-aminoacridine (9AA) and two controls: acetone and ethanol. Frameshift mutations resulted in green fluorescent revertants, as determined by flow cytometry, and were confirmed, for 9AA treatments, by sequencing. All five treatments with model agents induced statistically significant sequence- and exposure-dependent responses in MC2a cells and a negative response with the two negative control treatments, acetone and ethanol. Similar responses were seen in a smaller panel of treatments and plasmids in C3H10T1/2 cells. The mutation frequencies were higher in cells transfected with the plasmids containing TRS than those harbouring the control construct lacking repeats. The highest mutation frequencies were observed with H(2)O(2) and 9AA treatments, yielding up to a 50-fold difference between vehicle and highest concentration treatment. ENU, BPDE, and to a lesser extent TPA treatments, also showed a statistically significant exposure response. Results from these experiments reveal that the assay responds robustly to various classes of mutagenic substances, as well as to rodent carcinogens that are inactive in conventional mutation assays, and that responses are not linked to cytotoxicity. This assay is a promising approach for detecting chemically induced frameshifts within certain DNA sequences of interest, but further characterization and validation are required prior to general use in genotoxicity screening.

Molecular misreading: the frequency of dinucleotide deletions in neuronal mRNAs for beta-amyloid precursor protein and ubiquitin B

Human neuronal cells contain mutant beta-amyloid precursor protein (APP) and ubiquitin B (UBB) mRNAs, in which dinucleotide deletions ('Delta') are generated in/around GAGAG-motifs by an unknown mechanism referred to as 'Molecular Misreading.' The encoded frameshifted (+1) proteins accumulate in the neuropathological hallmarks of Alzheimer's disease (AD) and in other neurodegenerative and age-related diseases. To measure the concentration of Delta mRNAs, we developed a highly sensitive and specific assay, utilizing peptide nucleic acid-mediated PCR clamping, followed by cloning and colony hybridization with sequence-specific oligonucleotide probes. We found only a few molecules of Delta mRNA/microg of cellular RNA, at levels <10(-5) to 10(-6) x the concentration of WT mRNA, in RNA extracted from: (i) cultured human neuroblastoma cells grown under a variety of conditions, (ii) the frontal half of brains from wild type and XPA(-/-) DNA repair-deficient mice, and (iii) post-mortem temporal cortices from humans. Importantly, in RNA from the temporal cortices of AD and Down Syndrome patients that contain betaAPP+1 and UBB+1 immunoreactive cells, we found the same low levels of Delta mRNA. We infer that the accumulation of +1 proteins in neurons of these patients is not caused by an increase in the concentration of Delta mRNAs.

Paradoxical homozygous expression from heterozygotes and heterozygous expression from homozygotes as a consequence of transcriptional infidelity through a polyadenine tract in the AP3B1 gene responsible for canine cyclic neutropenia

Canine cyclic neutropenia is an autosomal recessive disease in which the number of neutrophils, the primary blood phagocyte, oscillates between almost zero and normal values with two week frequency. We previously found that the causative mutation is an insertion of an extra adenine residue within a tract of nine A's in exon 21 of the 27 exon canine AP3B1 gene. In the course of identifying the mutation, however, we observed an unusual phenomenon: heterozygous carrier dogs, who have one normal allele and one mutant allele, produce a homogeneous population of normal AP3B1 transcripts (containing nine A's), but homozygous affected dogs, who have two mutant alleles, produce a heterogeneous population of AP3B1 mRNA containing mutant transcripts with ten A's and, unexpectedly, wild-type transcripts with nine A's. By RT-PCR subclone analysis and use of an in vitro reporter assay, we show that there is a high frequency of errors made during the transcription of homopolymeric adenine sequences, such that the A tract in the mRNA is frequently shortened or lengthened by an extra residue. Out of frame transcripts are degraded, accounting for this paradox through the preferential accumulation of normal message from mutant alleles.

Host RNA polymerase II makes minimal contributions to retroviral frame-shift mutations

The rate of mutation during retrovirus replication is high. Mutations can occur during transcription of the viral genomic RNA from the integrated provirus or during reverse transcription from viral RNA to form viral DNA or during replication of the proviral DNA as the host cell is dividing. Therefore, three polymerases may all contribute to retroviral evolution: host RNA polymerase II, viral reverse transcriptases and host DNA polymerases, respectively. Since the rate of mutation for host DNA polymerase is very low, mutations are more likely to be caused by the host RNA polymerase II and/or the viral reverse transcriptase. A system was established to detect the frequency of frame-shift mutations caused by cellular RNA polymerase II, as well as the rate of retroviral mutation during a single cycle of replication in vivo. In this study, it was determined that RNA polymerase II contributes less than 3 % to frame-shift mutations that occur during retrovirus replication. Therefore, the majority of frame-shift mutations detected within the viral genome are the result of errors during reverse transcription.

Targeted expression of oncogenic K-ras in intestinal epithelium causes spontaneous tumorigenesis in mice

BACKGROUND & AIMS

Ras oncoproteins are mutated in about 50% of human colorectal cancers, but their precise role in tumor initiation or progression is still unclear.

METHODS

This study presents transgenic mice that express K-ras(V12G), the most frequent oncogenic mutation in human tumors, under control of the murine villin promoter in epithelial cells of the large and small intestine.

RESULTS

More than 80% of the transgenic animals displayed single or multiple intestinal lesions, ranging from aberrant crypt foci (ACF) to invasive adenocarcinomas. Expression of K-ras(V12G) caused activation of the MAP kinase cascade, and the tumors were frequently characterized by deregulated cellular proliferation. Unexpectedly, we obtained no evidence of inactivating mutations of the tumor suppressor gene Apc, the "gatekeeper" in colonic epithelial proliferation. However, spontaneous mutation of the tumor-suppressor gene p53, a frequent feature in the human disease, was found in 3 of 7 tumors that were tested.

CONCLUSIONS

This animal model recapitulates the stages of tumor progression as well as a part of the genetic alterations found in human colorectal cancer. Furthermore, it indicates that activation of K-ras in concert with mutations in p53 may constitute a route to digestive tumor formation and growth, underlining the fact that the pathway to intestinal cancer is not necessarily a single road.

Isolation of differentiated squamous and undifferentiated spindle carcinoma cell lines with differing metastatic potential from a 4-nitroquinoline N-Oxide-induced tongue carcinoma in a F344 rat

One differentiated squamous cell carcinoma (SCC) cell line (RSC3-E2) and two undifferentiated tumor cell lines (RSC3-LM and RSC3-E2R) with different metastatic potential were established from a 4-nitroquinoline N-oxide (4NQO)-induced differentiated SCC in F344 rat tongue. The RSC3-E2 subline was isolated from a parental cell line (RSC3-P) by single cell cloning in vitro, whereas the RSC3-LM subline was isolated from a lung metastatic focus after subcutaneous (s.c.) injection of RSC3-P cells. The RSC3-E2R cell line was isolated from a lung metastatic focus following s.c. injection of RSC3-E2 cells after X-irradiation in vitro. The RSC3-E2 cell line is keratin-positive and grows as a keratinizing tumor in nude mice, whereas RSC3-LM and RSC3-E2R cells are keratin-negative, vimentin-positive and form undifferentiated tumors. When s.c. injected into nude mice, the RSC3-E2 cell line proved to be non-metastatic, while the RSC3-LM cell line was metastatic by both hematogenous and lymphogenous routes, and the RSC3-E2R cell line was metastatic only hematogenously. In vitro relative growth rates and in vitro invasion activity of these cell lines were in the order RSC3-LM > RSC3-E2R > RSC3-E2. Chromosome analysis revealed two peaks with modal chromosome numbers of 83 and 78 for RSC3-P cells and single peaks at 83, 78 and 56 for RSC3-LM, RSC3-E2 and RSC3-E2R cell lines, respectively. Common structural abnormalities on chromosome 11 were shared by all cell lines. Mutation analysis of the p53 gene using a yeast functional assay demonstrated RSC3-LM cell line to have a point mutation at codon 269, whereas RSC3-E2 and RSC3-E2R had double mutations at codons 106 and 170 on each allele. These results suggest that the two undifferentiated RSC3-LM and RSC3-E2R tumor cell lines with different metastatic potential were generated from differentiated SCC cells via different genetic pathways as a consequence of tumor progression in vivo and in vitro, respectively. These cell lines should provide a useful model for understanding mechanisms of hematogenous and lymphogenous metastasis, as well as tumor progression of oral SCCs.