Isolation and characterization of the 5' region of the human mismatch repair gene hPMS1

Altered expression of mismatch repair proteins associated with acquisition of microsatellite instability in a clonal model of human T lymphocyte aging

The DNA mismatch repair system, the main postreplicative correction pathway in eukaryotic cells, has been shown to be involved in the acquisition of genetic damage during the aging of normal somatic cells, including those of the immune system. Previously, we showed that some but not all human T cell clones (TCC) in an in vitro culture aging model develop microsatellite instability (MSI), which is associated with altered expression of mismatch repair genes. Here, we analyzed levels of mismatch repair proteins as well as the corresponding mRNAs and related this to the development of microsatellite instability in TCC. Msh2, Msh3, Msh6, Pms1, and Pms2 protein expression was quantified by Western blotting. We found that clones not manifesting microsatellite instability in this in vitro model of T cell replicative aging, induced by persistent antigenic stimulation, maintain normal transcriptional control and coordination among the mismatch repair system genes, while clones which do manifest MSI display a general deregulation of gene expression, which is likely to contribute to its occurrence.

Myc down-regulation sensitizes melanoma cells to radiotherapy by inhibiting MLH1 and MSH2 mismatch repair proteins

PURPOSE

Melanoma patients have a very poor prognosis with a response rate of <1% due to advanced diagnosis. This type of tumor is particularly resistant to conventional chemotherapy and radiotherapy, and the surgery remains the principal treatment for patients with localized melanoma. For this reason, there is particular interest in the melanoma biological therapy.

EXPERIMENTAL DESIGN

Using two p53 mutant melanoma models stably expressing an inducible c-myc antisense RNA, we have investigated whether Myc protein down-regulation could render melanoma cells more susceptible to radiotherapy, reestablishing apoptotic p53-independent pathway. In addition to address the role of p53 in the activation of apoptosis, we studied the effect of Myc down-regulation on radiotherapy sensitivity also in a p53 wild-type melanoma cell line.

RESULTS

Myc down-regulation is able per se to induce apoptosis in a fraction of the cell population (approximately 40% at 72 hours) and in combination with gamma radiation efficiently enhances the death process. In fact, approximately 80% of apoptotic cells are evident in Myc down-regulated cells exposed to gamma radiation for 72 hours compared with approximately 13% observed after only gamma radiation treatment. Consistent with the enhanced apoptosis is the inhibition of the MLH1 and MSH2 mismatch repair proteins, which, preventing the correction of ionizing radiation mismatches occurring during DNA replication, renders the cells more prone to radiation-induced apoptosis.

CONCLUSIONS

Data herein reported show that Myc down-regulation lowers the apoptotic threshold in melanoma cells by inhibiting MLH1 and MSH2 proteins, thus increasing cell sensitivity to gamma radiation in a p53-independent fashion. Our results indicate the basis for developing new antitumoral therapeutic strategy, improving the management of melanoma patients.

Reduced expression of human mismatch repair genes in adult T-cell leukemia

In this study, we investigated the expression of six human DNA mismatch repair (MMR) genes, human MutS homologues 2 (hMSH2), 3 (hMSH3), and 6 (hMSH6), human MutL homologue 1 (hMLH1), human post-meiotic segregations 1 (hPMS1) and 2 (hPMS2), in primary leukemic cells obtained from 11 patients with acute-type adult T-cell leukemia (ATL) by using reverse transcription-polymerase chain reaction (RT-PCR). In contrast to normal peripheral lymphocytes, all primary ATL samples had reduced or loss of expression of two or more MMR genes, and the expression of several MMR genes was simultaneously suppressed in each ATL patient. Abnormal expression of hMSH2, hMSH3, hMSH6, hMLH1, and hPMS1 was observed more frequently than that of hPMS2. In particular, expression of hMSH2 and hPMS1 was reduced in all cases. Western blot analysis further showed reduced expression of both hMSH2 and hPMS1 proteins in all five cases examined. In three out of the 5 cases, both of the two proteins were undetectable. Interestingly, methylation-specific PCR indicated methylation of hPMS1 promoter in all of four ATL cases examined. hPMS1 expression, but not hMSH2 expression, was restored by treatment with a DNA demethylation agent, 5-aza-2'-deoxycytidine, suggesting that methylation plays a crucial role in inhibition of the hPMS1 gene expression in ATL. Our results demonstrate that defect of both human MutS and human MutL systems in primary ATL cells.

Regulation of the human MSH6 gene by the Sp1 transcription factor and alteration of promoter activity and expression by polymorphisms

Defects in human DNA mismatch repair have been reported to underlie a variety of hereditary and sporadic cancer cases. We characterized the structure of the MSH6 promoter region to examine the mechanisms of transcriptional regulation of the MSH6 gene. The 5'-flanking region of the MSH6 gene was found to contain seven functional Sp1 transcription factor binding sites that each bind Sp1 and Sp3 and contribute to promoter activity. Transcription did not appear to require a TATA box and resulted in multiple start sites, including two major start sites and at least nine minor start sites. Three common polymorphisms were identified in the promoter region (-557 T-->G, -448 G-->A, and -159 C-->T): the latter two were always associated, and each of these functionally inactivated a different Sp1 site. The polymorphic allele -448 A -159 T was demonstrated to be a common Caucasian polymorphism found in 16% of Caucasians and resulted in a five-Sp1-site promoter that had 50% less promoter activity and was more sensitive to inactivation by DNA methylation than the more common seven Sp1 site promoter allele, which was only partially inactivated by DNA methylation. In cell lines, this five-Sp1-site polymorphism resulted in reduced MSH6 expression at both the mRNA and protein level. An additional 2% of Caucasians contained another polymorphism, -210 C-->T, which inactivated a single Sp1 site that also contributes to promoter activity.

Steady-state regulation of the human DNA mismatch repair system

Steady-state levels of human DNA mismatch repair (MMR) transcripts and proteins were measured in MMR-proficient and -deficient cell lines by the newly developed competitive quantitative reverse transcription- polymerase chain reaction and Western analysis normalized with purified proteins. In MMR-proficient cells, hMSH2 is the most abundant MMR protein and is expressed 3 to 5 times more than hMLH1. The hMLH1 protein was expressed 1.5 to 2.5 times more than hPMS2. Steady-state levels of mRNA expression correlated well with protein expression. hMSH2-mutated LoVo cells did not express detectable hMSH3 or hMSH6 proteins. Similarly, hMLH1-mutated HCT116 cells did not express detectable hMLH1 or hPMS2 protein, whereas in hMLH1-restored HCT116+ch3 cells, hPMS2 protein was re-expressed. In hMSH6-mutated HCT15 cells, both hMSH3 protein and mRNA were increased. In SV40-transformed lung fibroblasts, all MMR mRNAs and proteins examined were expressed at levels 1.5-5-fold higher than in their nontransformed counterpart. The steady-state levels of MMR proteins indicate that substantially more hMutS proteins, which are involved in DNA mismatch recognition, are present in comparison with the hMutL proteins. Stability of hMSH3 and hMSH6 proteins appears to depend upon the presence of the hMSH2 protein, and, similarly, the stability of the hPMS2 protein depends upon hMLH1. When the hMSH6 is mutationally inactivated, hMSH3 increases by both transcriptional up-regulation and enhanced protein stability. A balanced up-regulation of all of the components was seen after viral transformation in a fibroblast model. Quantitative changes of the MMR components are a potential mechanism to modify the DNA MMR capabilities of a cell.

A core promoter and a frequent single-nucleotide polymorphism of the mismatch repair gene hMLH1

The hMLH1 gene encodes a protein that is involved in the DNA mismatch repair system. The coding region of the hMLH1 gene has been known to be mutated in a subset of patients with hereditary nonpolyposis colorectal cancer (HNPCC). Our current research characterized the promoter region of the hMLH1 gene and searched for mutations correlating to HNPCC. Utilizing the oligo-capping method, major transcription start sites of the hMLH1 gene were mapped at two locations. The core promoter region of about 180 bp was determined by the luciferase assay of serial deletion mutants. Although we did not find any pathogenic mutation in the hMLH1 promoter region by PCR-SSCP, we found a single-nucleotide polymorphism at position -93 nt from the adenine residue of the start codon. By PCR-RFLP analysis with Pvu II for this polymorphism, we detected LOH in four tumors from three patients. An easy detection of this polymorphism with PCR-RFLP and high incidence ( approximately 50%) of informative cases make this polymorphism a suitable marker for the detection of hMLH1 allelic losses.

Human DNA repair systems: an overview

DNA repair systems act to maintain genome integrity in the face of replication errors, environmental insults, and the cumulative effects of age. More than 70 human genes directly involved in the five major pathways of DNA repair have been described, including chromosomal location and cDNA sequence. However, a great deal of information as to the precise functions of these genes and their role in human health is still lacking. Hence, we summarize what is known about these genes and their contra part in bacterial, yeast, and rodent systems and discuss their involvement in human disease. While some associations are already well understood, it is clear that additional diseases will be found which are linked to DNA repair defects or deficiencies.

Promoter analysis of the human mismatch repair gene hMSH2

The human DNA mismatch repair gene homologue hMSH2 is involved in hereditary nonpolyposis colorectal cancer. We isolated and characterized the 5' upstream region, about 4.4kbp, of the hMSH2 gene. This region contains CpG islands and a number of elements involved in constitutive expression, but there is no TATA-box nearby the transcription start points. This is the typical structure for many promoters of housekeeping genes. Alu sequences and mononucleotide repeats are clustered in this region and there are two transcription start points. Deletion analysis revealed that less than 300bp was sufficient to initiate transcription. Although no mutation that influences promoter activity of this region was found, a polymorphism was detected by PCR-RFLP analysis. Because informative cases (C/T heterozygous) were relatively high ( approximately 30%), this polymorphism is suitable for a marker to examine allelic losses.