Mutation frequency in coding and non-coding repeat sequences in mismatch repair deficient cells derived from normal human tissue

MicroRNA-125b may function as an oncogene in lung cancer cells

The present study aimed to investigate the biofunctions of microRNA (miR)‑125b on lung cancer cells. A miR genechip array was used to examine the differential expression of miRs between 95D lung cancer cells and 16 human bronchial epithelial (HBE) cells. Overexpression of miR‑125b was observed in the cell lines and in the lung carcinoma tissues compared with the adjacent tissues, confirmed using reverse transcription quantitative polymerase chain reaction. Bioinformatic analysis of miR‑125b was also performed, including target prediction, gene ontology and pathway analysis. MTT, flow cytometry and Transwell assays were also used to examine the effect of downregulated miR‑125b on the proliferation, apoptosis, invasive ability and cell cycle of 95D cells. Significant differences were observed in the expression of 45 miRs in the 95D cells compared with those in 16HBE cells and the expression of miR‑125b was significantly higher in 95D cells compared with that in 16HBE cells as well as in lung tumor tissues compared with that in adjacent tissues. In addition, inhibition of the expression of miR‑125b in 95D cells induced apoptosis, G1/S phase arrest and reduction of their invasive ability. In addition, bioinformatics software predicted that miR‑125b was involved in the regulation of several pathways associated with cancer, including the transforming growth factor‑β, Wnt and mitogen‑activated protein kinase signaling pathways. These data indicated for the first time, to the best of our knowledge, that miR‑125b may function as an oncogene in lung cancer.

In vitro stability of APC gene sequences and the influence of DNA repair status

APC is a key 'gatekeeper' gene in colorectal tumorigenesis. The high frequency of APC defects observed in colorectal cancer tissue is the result of selective growth advantage of cells with loss-of-function mutations at that locus. However, mutations may also arise due to inherent sequence instability. Defective DNA mismatch repair (MMR) and base excision repair (BER) also contribute to colorectal carcinogenesis and may compound such instability. To avoid the effect of clonal selective advantage imparted by APC mutation in cancer cells, we assessed in vitro APC mutation frequency in cell lines of lymphoid lineage to investigate the influence of defective MMR and BER. In DNA repair proficient cells, we observed substantially greater inherent sequence instability in APC gene coding sequences compared to reference sequences. Surprisingly, however, this difference was abrogated in MMR defective lines. We also found greater mutation frequency at exonic DNA sequences outwith the APC region in cells defective for either MMR or BER defects. The underlying propensity for mutation at the APC gene is intriguing, while the greater frequency of mutation in cells defective for DNA repair has relevance to understanding events leading to colorectal cancer and other malignancies.

Analysis of microsatellite mutations in buccal cells from a case-control study for lung cancer

Exposure to tobacco carcinogens is the major cause of human lung cancer, but even heavy smokers have only about a 10% life-time risk of developing lung cancer. Currently used screening processes, based largely on age and exposure status, have proven to be of limited clinical utility in predicting cancer risk. More precise methods of assessing an individual's risk of developing lung cancer are needed. Because of their sensitivity to DNA damage, microsatellites are potentially useful for the assessment of somatic mutational load in normal cells. We assessed mutational load using hypermutable microsatellites in buccal cells obtained from lung carcinoma cases and controls to test if such a measure could be used to estimate lung cancer risk. There was no significant association between smoking status and mutation frequency with any of the markers tested. No significant association between case status and mutation frequency was observed. Age was significantly related to mutation frequency in the microsatellite marker D7S1482. These observations indicate that somatic mutational load, as measured using mutation frequency of microsatellites in buccal cells, increases with increasing age but that subjects who develop lung cancer have a similar mutational load as those who remain cancer free. This finding suggests that mutation frequency of microsatellite mutations in buccal cells may not be a promising biomarker for lung cancer risk.

Current research in perineural invasion of cholangiocarcinoma

Background

Perineural invasion is a common path for cholangiocarcinoma (CCA) metastasis, and it is highly correlated with postoperative recurrence and poor prognosis. It is often an early event in a disease that is commonly diagnosed in advanced stages, and thus it could offer a timely therapeutic and diagnostic target if better understood. This article systematically reviews the progress of CCA neural invasion-related molecules.

Methods

Studies were identified by searching MEDLINE and PubMed databases for articles from January 1990 to December 2009, using the keywords "cholangiocarcinoma," "perineural invasion," "nerve growth factor"(NGF), "neural cell adhesion molecule" (NCAM), "matrix metalloproteinase"(MMP), "neurotransmitter," "acetylcholine" (Ach), and "transforming growth factor" (TGF)." Additional papers and book chapters were identified by a manual search of references from the key articles.

Results

From above we found that the molecules NGF, NCAM, MMP, Ach and TGF may have prognostic significance in, and offer clues to the mechanism of CCA neural invasion.

Conclusions

Cholangiocarcinoma's increasing worldwide incidence is especially poignant in view of both the lacking effective therapies, and the fact that it is commonly diagnosed in advanced stages. As CCA neural invasion often appears early, more complete characterization of its molecular pathology could lead to the identification of targets for the diagnosis and therapy of this devastating malignancy.

MLH1 mediates PARP-dependent cell death in response to the methylating agent N-methyl-N-nitrosourea

Background:

Methylating agents such as N-methyl-N-nitrosourea (MNU) can cause cell cycle arrest and death either via caspase-dependent apoptosis or via a poly(ADP-ribose) polymerase (PARP)-dependent form of apoptosis. We wished to investigate the possible role of MLH1 in signalling cell death through PARP.

Methods:

Fibroblasts are particularly dependent on a PARP-mediated cell death response to methylating agents. We used hTERT-immortalised normal human fibroblasts (WT) to generate isogenic MLH1-depleted cells, confirmed by quantitative PCR and western blotting. Drug resistance was measured by clonogenic and cell viability assays and effects on the cell cycle by cell sorting. Damage signalling was additionally investigated using immunostaining.

Results:

MLH1-depleted cells were more resistant to MNU, as expected. Despite having an intact G₂/M checkpoint, the WT cells did not initially undergo cell cycle arrest but instead triggered cell death directly by PARP overactivation and nuclear translocation of apoptosis-inducing factor (AIF). The MLH1-depleted cells showed defects in this pathway, with decreased staining for phosphorylated H2AX, altered PARP activity and reduced AIF translocation. Inhibitors of PARP, but not of caspases, blocked AIF translocation and greatly decreased short-term cell death in both WT and MLH1-depleted cells. This MLH1-dependent response to MNU was not blocked by inhibitors of ATM/ATR or p53.

Conclusion:

These novel data indicate an important role for MLH1 in signalling PARP-dependent cell death in response to the methylating agent MNU.

Human postmeiotic segregation 2 exhibits biased repair at tetranucleotide microsatellite sequences

The mismatch repair (MMR) system plays a major role in removing DNA polymerization errors, and loss of this pathway results in hereditary cancers characterized by microsatellite instability. We investigated microsatellite stability during DNA replication within human postmeiotic segregation 2 (hPMS2)-deficient and proficient human lymphoblastoid cell lines. Using a shuttle vector assay, we measured mutation rates at reporter cassettes containing defined mononucleotide, dinucleotide, and tetranucleotide microsatellite sequences. A mutator phenotype was observed in the hPMS2-deficient cell line. The mutation rate of vectors containing [G/C](10) or [GT/CA](10) alleles was elevated 20-fold to 40-fold in hPMS2-deficient cells, relative to an hPMS2-expressing cell line. We observed a 6-fold and 12-fold relative increase in mutation rate of [TTTC/AAAG](9) and [TTCC/AAGG](9) sequences, respectively, in hPMS2-deficient cells. Mutational specificity analyses suggested that repair by hPMS2 is biased. In the absence of hPMS2, a greater number of microsatellite expansion versus deletion mutations was observed, and expansion rates of the tetranucleotide alleles were similar. In the presence of hPMS2, we observed a 29-fold decrease in the [TTCC/AAGG](9) expansion rate but only a 6-fold decrease for the [TTTC/AAAG](9) allele. Our data indicate that hPMS2 is more protective of tetranucleotide expansions than deletions and that hPMS2 displays a sequence bias, wherein [TTCC/AAGG] sequences are stabilized to a greater extent than [TTTC/AAAG]. Our results allow for greater accuracy during identification of MMR defects by providing a mutational signature characteristic of hPMS2 defect. This study also provides clues to possible mechanisms of repair by hPMS2 in the context of the MMR system.

Gene Expression Profiling of Rewarding Effect in Methamphetamine Treated Bax-deficient Mouse

Methamphetamine is an illicit drug that is often abused and can cause neuropsychiatric and neurotoxic damage. Repeated administration of psychostimulants such as methamphetamine induces a behavioral sensitization. According to a previous study, Bax was involved in neurotoxicity by methamphetamine, but the function of Bax in rewarding effect has not yet been elucidated. Therefore, we have studied the function of Bax in a rewarding effect model. In the present study, we treated chronic methamphetamine exposure in a Bax-deficient mouse model and examined behavioral change using a conditioned place preference (CPP) test. The CPP score in Bax knockout mice was decreased compared to that of wild-type mice. Therefore, we screened for Bax-related genes that are involved in rewarding effect using microarray technology. In order to confirm microarray data, we applied the RT-PCR method to observe relative changes of Bcl2, a pro-apoptotic family gene. As a result, using our experiment microarray, we selected genes that were associated with Bax in microarray data, and eventually selected the Tgfbr2 gene. Expression of the Tgfbr2 gene was decreased by methamphetamine in Bax knockout mice, and the gene was overexpressed in Bax wild-type mice. Additionally, we confirmed that Creb, FosB, and c-Fos were related to rewarding effect and Bax using immunohistochemistry.

Tumour microenvironment: TGFbeta: the molecular Jekyll and Hyde of cancer

Transforming growth factor-beta (TGFbeta) signalling regulates cancer through mechanisms that function either within the tumour cell itself or through host-tumour cell interactions. Studies of tumour-cell-autonomous TGFbeta effects show clearly that TGFbeta signalling has a mechanistic role in tumour suppression and tumour promotion. In addition, factors in the tumour microenvironment, such as fibroblasts, immune cells and the extracellular matrix, influence the ability of TGFbeta to promote or suppress carcinoma progression and metastasis. The complex nature of TGFbeta signalling and crosstalk in the tumour microenvironment presents a unique challenge, and an opportunity to develop therapeutic intervention strategies for targeting cancer.

Frequent mutations of hepatocyte nuclear factor 1 in colorectal cancer with microsatellite instability

BACKGROUND & AIMS

The TCF1 gene encoding hepatocyte nuclear factor 1 alpha (HNF1), a transcription factor germline mutated in patients with maturity-onset diabetes of the young type 3, was recently found to be frequently inactivated by biallelic alterations in liver adenoma and in rare hepatocellular carcinomas. The impact of HNF1 in colorectal carcinogenesis has not been studied until now. Colorectal cancer is characterized by the existence of different molecular mechanisms known as microsatellite stable or unstable tumors.

METHODS

At first, a series of 10 adenomas and 29 colon cancers regardless of microsatellite instability status were screened for TCF1 mutations on the entire coding sequence.

RESULTS

Three mutations in microsatellite instability high (MSI-H) tumors were found in the exon 4 polymorphic poly-cytosin (C)(8) or (C)(9) tract and consisted of a cytosin deletion at position 291. To further characterize the prevalence of TCF1 mutations in the subgroup of MSI-H tumors, 52 additional MSI-H samples were screened for exon 4 alterations; 23% of MSI-H tumors (95% confidence interval, 14%-36%) were found to harbor frameshift at the poly-cytosin tract. The (C)(9) allele was significantly more frequently mutated than the (C)(8) allele (22% vs. 8%; P = 0.03), showing a higher instability of the longer repetition.

CONCLUSIONS

These results show a role for HNF1 in MSI-H colorectal carcinogenesis.

Hypermutability at a poly(A/T) tract in the human germline

Poly(A/T) tracts are abundant simple sequence repeats (SSRs) within the human genome. They constitute part of the coding sequence of a variety of genes, encoding polylysine stretches that are important for protein function. Assessment of poly(A/T) tract stability is also used to identify microsatellite unstable colorectal cancers, which are characteristic of tumours defective in DNA mismatch repair. Despite their importance, little is known about the stability of poly(A/T) SSRs in the human germline. We have determined the stability of a paradigm poly(A/T) tract, BAT-40, by study of population allele frequencies, mutation frequency in families and mutation frequency in sperm DNA. We show that the locus is polymorphic, with a level of heterozygosity of 59.7%. Germline mutation was observed in 13 of 187 germline transmissions (7.0%) in 10 families suggesting BAT-40 is unstable in the germline. Further evidence for germline instability at BAT-40 was provided by small pool PCR analysis of matched blood and sperm DNA templates, revealing a significantly elevated frequency of mutation in the germline (P < 0.001). These findings provide insight into poly(A/T) tract stability in the germline. They also have relevance to the study of gene expression and to determination of microsatellite instability in tumours.