A change in microsatellite instability caused by cisplatin-based chemotherapy of ovarian cancer.

To clarify the mechanism of acquired CDDP resistance in ovarian cancer, we compared the microsatellite instability (MSI) by the amplification of 10 microsatellite loci and immunohistochemical detection of hMSH2 and hMLH1 expression between the primary resected tumours and the secondary resected residual tumours after 5 or 6 courses of CDDP-based chemotherapy in the 24 cases of ovarian cancer. Of the 24 primary resected tumours, 9 (37.5%) showed MSI (7 cases of MSI-L, 2 cases of MSI-H), while 15 (72.5%) were microsatellite stable tumours (MSS). The primary tumours also had MSI in the residual tumours after CDDP-based chemotherapy. However, all of the cases with MSS in the primary resected tumours exhibited MSI (2 cases were MSI-L, and 13 cases were MSI-H) in the residual tumours after CDDP-based chemotherapy (P< 0.001). Furthermore, 11 (73.3%) of these cases which changed from MSS to MSI also had a change in the expression of hMLH1 from positive to undetectable (P< 0.001). Our data suggest that tumour MSI changes during CDDP-based chemotherapy, and that the loss of hMLH1 expression is one of the factors that has the greatest effect on this transformation.

Growth inhibition due to complementation of transforming growth factor-beta receptor type II-defect by human chromosome 3 transfer in human colorectal carcinoma cells.

The transforming growth-beta receptor type II (TGF-beta RII) gene is one of the target genes of the DNA mismatch repair (MMR) defect. The human colorectal carcinoma cell line HCT116 has mutations in the hMLH1 gene and in the microsatellite region of the TGF-beta RII gene, both located on the short arm of chromosome 3. Introduction of the wild-type hMLH1 gene on transferred human chromosome 3 restores many characteristics of MMR-deficiency in HCT116. In this study, we determined whether transfer of chromosome 3 into HCT116 also complements the TGF-beta RII gene defect. We compared in vitro growth characteristics between HCT116 and HCT116 with a transferred chromosome 3 (HCT116 + ch3). The growth was suppressed in HCT116 + ch3 compared with parental HCT116. This suppression was abolished by frequent replacement with fresh medium, suggesting that the autocrine TGF-beta-TGF-beta RII system may be responsible for growth suppression. To explore this possibility, we determined several characteristics essential for the autocrine system. We found that HCT116 + ch3 expresses wild-type as well as mutated TGF-beta RII mRNA. In addition, phosphorylation of TGF-beta RI and growth inhibition were observed in HCT116 + ch3 but not in HCT116 by exposure to exogenous TGF-beta. The amount of TGF-beta1 in HCT116 + ch3 cultures was remarkably less than that in the HCT116, suggesting that TGF-beta produced by HCT116 + ch3 cells may be consumed by the cells. The conditioned medium from HCT116 cultures inhibits HCT116 + ch3 growth. This inhibition was neutralized by the anti-TGF-beta antibody. Taken together, these results strongly suggest that the TGF-beta RII gene defect in HCT116 is complemented by a wild-type gene on the transferred chromosome 3 and that HCT116 + ch3 gained the ability to respond to TGF-beta. Simultaneous complementation of defects of a responsible gene and a major target gene by the chromosome transfer is useful to prove the inactivated phenotypes acquired during colorectal tumorigenesis.

Complementation of an hMSH2 defect in human colorectal carcinoma cells by human chromosome 2 transfer.

The human colorectal tumor cell line LoVo has a homozygous deletion in the hMSH2 gene from exon 3 to exon 8, is deficient in mismatch repair (MMR) activity, and exhibits microsatellite instability. To determine whether the introduction of a wild type hMSH2 into LoVo restores MMR activity and stabilizes microsatellite loci, we transferred a chromosome 2 fragment containing hMSH2 into a homologous recombination-proficient chicken DT40/human hybrid (DT40 2C) and a human chromosome 2 in a mouse A9 hybrid to LoVo. Transfers of these chromosomes into LoVo resulted in LoVo both with and without a wild-type hMSH2. Complete correlation was found between the presence of the wild-type hMSH2 and hMSH2 expression, an increased stability in microsatellite loci, and competency in MMR. The hMSH2-positive LoVo hybrids also showed an increased sensitivity to N-methyl-N'-nitro-N-nitrosoguanidine. This enhanced toxicity is associated with G(2) cell-cycle arrest followed by premature mitosis and cell death. These results suggest that hMSH2 may be responsible for complementing mutator and drug-resistant phenotypes in chromosome 2-transferred LoVo cells. To test whether the hMSH2 in DT40 2C cells can be modified by homologous recombination, we transfected DT40 2C with a targeting vector containing an hMSH2 exon 4 disrupted by the zeocin-resistant gene. The results showed that the hMSH2 locus in DT40 2C was efficiently targeted by an exogeneously transfected homologous sequence, suggesting that transfer of a modified hMSH2 from DT40 2C to LoVo via chromosome transfer could be used to determine the function of hMSH2.

Complementation of mismatch repair gene defects by chromosome transfer.

The study of the multiple functions of mismatch repair genes in humans is being facilitated by the use of human tumor cell lines carrying defined MMR gene mutations. Such cell lines have elevated spontaneous mutation rates and may accumulate mutations in other genes, some of which could be causally related to the phenotypes of these cells. One approach to establish a cause-effect relationship between a MMR gene defect and a phenotype is to determine if that phenotype is reversed when a normal chromosome carrying a wild-type MMR gene is introduced by microcell fusion. This approach has the advantage of presenting the gene in its natural chromosomal environment with normal regulatory controls and at a reasonable dosage. The approach also limits candidate genes to only those encoded by the introduced chromosome and not elsewhere in the genome. Here we review studies demonstrating that hMSH2, hMSH3, hMSH6 and hMLH1 gene defects can each be complemented by transferring human chromosome 2, 5, 2 or 3, respectively. These transfers restore MMR activity, sensitivity to killing by MNNG, stability to microsatellite sequences and low spontaneous HPRT gene mutation rates.

Rescue of targeted regions of mammalian chromosomes by in vivo recombination in yeast.

In contrast to other animal cell lines, the chicken pre-B cell lymphoma line, DT40, exhibits a high level of homologous recombination, which can be exploited to generate site-specific alterations in defined target genes or regions. In addition, the ability to generate human/chicken monochromosomal hybrids in the DT40 cell line opens a way for specific targeting of human genes. Here we describe a new strategy for direct isolation of a human chromosomal region that is based on targeting of the chromosome with a vector containing a yeast selectable marker, centromere, and an ARS element. This procedure allows rescue of the targeted region by transfection of total genomic DNA into yeast spheroplasts. Selection for the yeast marker results in isolation of chromosome sequences in the form of large circular yeast artificial chromosomes (YACs) up to 170 kb in size containing the targeted region. These YACs are generated by homologous recombination in yeast between common repeated sequences in the targeted chromosomal fragment. Alternatively, the targeted region can be rescued as a linear YACs when a YAC fragmentation vector is included in the yeast transformation mixture. Because the entire isolation procedure of the chromosomal region, once a target insertion is obtained, can be accomplished in approximately 1 week, the new method greatly expands the utility of the homologous recombinationproficient DT40 chicken cell system.

Loss of heterozygosity at chromosome segment Xq25-26.1 in advanced human ovarian carcinomas.

To determine whether a tumor suppressor gene of importance to epithelial ovarian cancer resides on the X chromosome, we examined loss of heterozygosity (LOH) in 123 epithelial ovarian cancer cases. In 54 such cases, we examined LOH at 26 loci on the human X chromosome. In eight cases, we examined LOH in 14 loci and in 61 cases we examined LOH in 13 loci. Matched DNA samples from tumors and corresponding normal tissues were analyzed by polymerase chain reaction (PCR) amplification of microsatellite markers. Frequent losses were found in epithelial carcinomas at the Xq25-26.l region, including DXS1206 (34.5% loss in informative cases), DXS1047 (27.7%), HPRT (24.1%), and DXS1062 (33.3%). The minimum overlapping region of LOH was approximately 5 megabases (Mb), flanked by DXS1206 (Xq25) and HPRT (Xq26.1). The methylation status of the remaining allele of the androgen receptor gene in the tumors exhibiting LOH at the Xq25-26.1 region suggested that the loss was exclusively in the inactive X chromosome. We next determined whether a significant relationship exists between Xq LOH and other parameters, including histologic grade and/or clinical stage of the tumors and LOH at TP53. The Xq LOH had a significant association with grade 2 to 3 tumors at stages II to IV. Sixteen of 18 cases that showed Xq LOH revealed LOH at the TP53 locus, and 45% of tumors exhibiting LOH at TP53 showed Xq LOH. These results suggest that there may be a tumor suppressor gene or genes which escape inactivation of the X chromosome at Xq25-26.1, and that the loss of the gene(s) at Xq25-26.1 is frequently accompanied by loss of the TP53 or loss of another gene on chromosome 17. These losses may contribute to the progression from a well-differentiated to a more poorly differentiated state or to metastatic aggressiveness.

Correction of hypermutability, N-methyl-N'-nitro-N-nitrosoguanidine resistance, and defective DNA mismatch repair by introducing chromosome 2 into human tumor cells with mutations in MSH2 and MSH6.

The human DNA mismatch repair genes hMSH2 and hMSH6 encode the proteins that, together, bind to mismatches to initiate repair of replication errors. Human tumor cells containing mutations in these genes have strongly elevated mutation rates in selectable genes and at microsatellite loci, although mutations in these genes cause somewhat different mutator phenotypes. These cells are also resistant to killing by certain drugs and are defective in mismatch repair. Because the elevated mutation rates in these cells may lead to mutations in additional genes that are causally related to the other defects, here we attempt to establish a cause-effect relationship between the hMSH2 and hMSH6 gene mutations and the observed phenotypes. The endometrial tumor cell line HEC59 contains mutations in both alleles of hMSH2. The colon tumor cell line HCT15 contains mutations in hMSH6 and also has a sequence change in a conserved region of the coding sequence for DNA polymerase delta, a replicative DNA polymerase. We introduced human chromosome 2 containing the wild-type hMSH2 and hMSH6 genes into HEC59 and HCT15 cells. Introduction of chromosome 2 to HEC59 cells restored microsatellite stability, sensitivity to N-methyl-N'-nitro-N-nitrosoguanidine treatment, and mismatch repair activity. Transfer of chromosome 2 to HCT15 cells also reduced the mutation rate at the HPRT locus and restored sensitivity to N-methyl-N'-nitro-N-nitrosoguanidine treatment and mismatch repair activity. The results demonstrate that the observed defects are causally related to mutations in genes on chromosome 2, probably hMSH2 or hMSH6, but are not related to sequence changes in other genes, including the gene encoding DNA polymerase delta.

Undectable expression of hMLH1 protein in sporadic colorectal cancer with replication error phenotype.

PURPOSE: Four DNA mismatch repair genes have been identified as being susceptible genes for hereditary nonpolyposis colorectal cancer. Deficiency of one of the mismatch repair genes causes the replication error phenotype in more than 80 percent of patients with hereditary nonpolyposis colorectal cancer and in 10 to 30 percent of patients with sporadic colorectal cancer. To determine which mismatch repair gene is lacking the function in patients with replication error-positive colorectal cancer, several approaches have been used at the nucleic acid and protein levels. We studied replication error in 40 samples of randomly selected colorectal cancers and expression of hMSH2 and hMLH1 proteins analyzed by immunoblot in the tumor and normal tissues of the replication error-positive and replication error-negative samples. MATERIALS AND METHODS: Frozen tumor and normal tissues were obtained from 40 Japanese patients who had colorectal cancer. According to the Amsterdam criteria, those patients were classified as having 39 sporadic and 1 unknown colorectal cancers. Genomic DNA was extracted from tumor and normal tissues for determining replication error with eight microsatellite markers. Expression of hMSH2 and hMLH1 proteins in cell lysates of tumor and normal tissues of 16 patients was analyzed by immunoblot. RESULTS: The replication error phenotype was found in 6 (15 percent) of the 39 sporadic cases. hMLH1 protein was not detected in two of the six replication error-positive tumor tissues and not in the normal tissues, indicating that the tumor cells of the two patients had severe mutations in both alleles of the hMLH1 gene. Another four replication error-positive and ten replication error-negative tumors and normal tissues expressed hMLH1 protein. hMSH2 protein was detected in all samples. CONCLUSION: hMLH1 protein was undetectable in the two tumor tissues of the six replication error-positive samples of sporadic colorectal cancer. The detection procedure used here may have potential use for determining a dysfunctional mismatch repair gene product.

Construction of chicken x human microcell hybrids for human gene targeting.

Human chromosomes 1, 2, 3, and 11 were tagged with pSV2 neo and transferred via microcell fusion from mouse A9 human monochromosomal hybrids to a chicken pre-B cell line, DT40, proficient for homologous recombination. Hybrids containing two copies of human chromosome 11 were transfected with targeting vectors containing a mammalian selectable gene with either the D11S16 or HRAS genomic sequences corresponding to two different chromosome 11 loci. Analysis of stable transfectants showed a high frequency (approximately 80%) of targeted integration of these constructs into each of the homologous loci of human chromosome 11 in DT40 hybrids. The results suggest that any human genomic sequences on human chromosomes transferred into DT40 cells could be targeted at high frequency, thereby allowing for subsequent modification of human genes and chromosomes.

Competency in mismatch repair prohibits clonal expansion of cancer cells treated with N-methyl-N'-nitro-N-nitrosoguanidine.

The phenomenon of alkylation tolerance has been observed in cells that are deficient in some component of the DNA mismatch repair (MMR) system. An alkylation-induced cell cycle arrest had been reported previously in one MMR-proficient cell line, whereas a MMR-defective clone derived from this line escapes from this arrest. We examined human cancer cell lines to determine if the cell cycle arrest were dependent upon the MMR system. Growth characteristics and cell cycle analysis after MNNG treatment were ascertained in seven MMR-deficient and proficient cell lines, with and without confirmed mutations in hMLH1 or hMSH2 by an in vitro transcription/translation assay. MMR-proficient cells underwent growth arrest in the G2 phase of the cell cycle after the first S phase, whereas MMR-deficient cells escaped an initial G2 delay and resumed a normal growth pattern. In the HCT116 line corrected for defective MMR by chromosome 3 transfer, the G2 phase arrest lasted more than five days. In another MMR-proficient colon cancer cell line, SW480, cell death occurred five days after MNNG treatment. A competent MMR system appears to be necessary for G2 arrest or cell death after alkylation damage, and this cell cycle checkpoint may allow the cell to repair damaged DNA, or prevent the replication of mutated DNA by prohibiting clonal expansion.

Loss of DNA mismatch repair in acquired resistance to cisplatin.

Selection of cells for resistance to cisplatin, a well-recognized mutagen, could result in mutations in genes involved in DNA mismatch repair and thereby to resistance to DNA-alkylating agents. Parental cells of the human ovarian adenocarcinoma cell line 2008 expressed hMLH1 when analyzed with immunoblot. One subline selected for resistance to cisplatin (2008/A) expressed no hMLH1, whereas another (2008/C13*5.25) expressed parental levels. Microsatellite instability was readily demonstrated in 2008/A cells but not in 2008 and in 2008/C13*5.25 cells. In addition, the 2008/A cells were 2-fold resistant to methyl-nitro-nitrosoguanidine and had a 65-fold elevated mutation rate at the HPRT locus as compared to 2008 cells, both of which are consistent with the loss of DNA mismatch repair in these cells. To determine whether the loss of DNA mismatch repair itself contributes to cisplatin resistance, studies were carried out in isogenic pairs of cell lines proficient or defective in this function. HCT116, a human colon cancer cell line deficient in hMLH1 function, was 2-fold resistant to cisplatin when compared to a subline complemented with chromosome 3 and expressing hMLH1. Similarly, the human endometrial cancer cell line HEC59, which expresses no hMSH2, was 2-fold resistant to cisplatin when compared to a subline complemented with chromosome 2 that expresses hMSH2. Therefore, the selection of cells for resistance to cisplatin can result in the loss of DNA mismatch repair, and loss of DNA mismatch repair in turn contributes to resistance to cisplatin.

Transcription-coupled repair deficiency and mutations in human mismatch repair genes.

Deficiencies in mismatch repair have been linked to a common cancer predisposition syndrome in humans, hereditary nonpolyposis colorectal cancer (HNPCC), and a subset of sporadic cancers. Here, several mismatch repair-deficient tumor cell lines and HNPCC-derived lymphoblastoid cell lines were found to be deficient in an additional DNA repair process termed transcription-coupled repair (TCR). The TCR defect was corrected in a mutant cell line whose mismatch repair deficiency had been corrected by chromosome transfer. Thus, the connection between excision repair and mismatch repair previously described in Escherichia coli extends to humans. These results imply that deficiencies in TCR and exposure to carcinogens present in the environment may contribute to the etiology of tumors associated with genetic defects in mismatch repair.

Evidence for a connection between the mismatch repair system and the G2 cell cycle checkpoint.

The human colon tumor cell line HCT116 is deficient in wild-type hMLH1, is defective in mismatch repair (MMR), exhibits microsatellite instability, and is tolerant to N-methyl-N'-nitro-N-nitrosoguanidine (MNNG). Transferring a normal copy of hMLH1 on chromosome 3 into the cell line restores MMR activity, stabilizes microsatellite loci, and increases the sensitivity of the cell to MNNG. Previous studies in other cell lines tolerant to alkylating agents such as MNNG or N-methylnitrosourea have shown cross-tolerance to 6-thioguanine (6TG), leading to a hypothesis that tolerance to MNNG or 6TG may be the result of MMR deficiency. To test this hypothesis, we studied the effects of 6TG on the MNNG-tolerant, MMR-deficient HCT116 cell line and its MNNG-sensitive, MMR-proficient, MNNG-tolerant, and MMR-deficient derivatives. Continuous exposure to low doses of 6TG (0.31-1.25 micrograms/ml) had no apparent effect on colony-forming ability (CFA) in MNNG-tolerant, MMR-deficient cells, whereas MNNG-sensitive, MMR-proficient cells exhibited a dose-dependent decrease in CFA. Growth kinetics and cell cycle analysis revealed that the growth of 6TG-treated HCT116 + chr3 cells was arrested at G2 after exposure to low dose of 6TG. In contrast, the same exposure to 6TG did not induce G2 arrest but rather a G1 delay in HCT116 and HCT116 + chr2. To obtain further evidence for the role of MMR on 6TG and MNNG toxicity, we isolated an MNNG-resistant revertant clone, M2, from the MNNG-sensitive, MMR-proficient HCT116 + chr3 cell line and characterized the MMR activity, hMLH1 status, and 6TG response. The results showed that M2 cells lost MMR activity as well as the previously introduced normal hMLH1 gene. Restoration of the CFA of M2 and an absence of G2 arrest were observed after treatment with low doses of 6TG. These results suggest that the mismatch repair system interacts with the G2 checkpoint in response to 6TG or MNNG-induced DNA lesions. The results further suggest that any agent that induces DNA mispairs will cause G2 arrest in MMR-proficient cells but not in MMR-deficient cells.

Human chromosome 3 corrects mismatch repair deficiency and microsatellite instability and reduces N-methyl-N'-nitro-N-nitrosoguanidine tolerance in colon tumor cells with homozygous hMLH1 mutation.

The human colon tumor cell line HCT 116 is known to have a homozygous mutation in the mismatch repair gene hMLH1 on human chromosome 3, to exhibit microsatellite instability, and to be defective in mismatch repair. In order to determine whether the introduction of a normal copy of hMLH1 gene restores mismatch repair activity and corrects microsatellite instability, a single human chromosome 3 from normal fibroblasts was transferred to HCT 116 cells via microcell fusion. As a control, human chromosome 2 was also transferred to HCT 116 cells. Two HCT 116 microcell hybrid clones that received a single copy of chromosome 2 (HCT 116 + ch2) and two that received a single copy of chromosome 3 (HCT 116 + ch3) were isolated and characterized. A G-G mismatch in M13-derived heteroduplex DNA was efficiently repaired in cell extracts from HCT 116 + ch3 cells, but not in those of parent HCT 116 cells or HCT 116 + ch2 cells. Microsatellite alterations at the D5S107 locus containing CA repeats were seen in 8 of 80 subclones from HCT 116 cells, and in 13 of 150 subclones from HCT 116 + ch2 cells. In contrast, none of the 225 subclones derived from mismatch repair-proficient HCT 116 + ch3 cells showed alterations in the microsatellite at the same locus. The effect of introducing chromosome 3 on the sensitivity of HCT 116 cells to N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) was examined, since enhanced tolerance to MNNG is accompanied by loss of mismatch repair activity in several cell lines. Within 3 days after treatment with 5 microM MNNG, HCT 116 + ch3 cells became morphologically flat and stopped growing. Their colony-forming ability, determined 10 days after treatment, was reduced 200-fold when compared to MNNG-treated parental HCT 116 and HCT 116 + ch2 cells. These results support the hypothesis that mutations in both alleles of the hMLH1 gene are necessary for the manifestation of defective mismatch repair and microsatellite instability and for enhanced MNNG tolerance. The results also suggest that the mismatch repair system contributes to the process that causes growth arrest in response to DNA damage by alkylating agents.

Simple purification of human chromosomes to homogeneity using muntjac hybrid cells.

Chromosome sorting from hybrid cells offers enormous advantages for gene mapping and cloning, but purification of most chromosomes has been largely hindered by their similarity in size to other chromosomes. We have developed a novel cell line and strategy that allows simple, mass purification of mammalian chromosomes, permitting significant target genome enrichment. This strategy takes advantage of the small number of giant chromosomes (1,2,X) of the female Indian muntjac, a barking deer, avoiding the problem of size similarity. We introduced human chromosomes into a cell line derived from a muntjac and purified them to homogeneity using a relatively simple technique. This strategy should facilitate the isolation of chromosomes from species other than human for which hybrid cells are not available currently.

Tumor cell growth arrest caused by subchromosomal transferable DNA fragments from chromosome 11.

A fundamental problem in the identification and isolation of tumor suppressor and other growth-inhibiting genes is the loss of power of genetic complementation at the subchromosomal level. A direct genetic strategy was developed to isolate subchromosomal transferable fragments (STFs) from any chromosome, each containing a selectable marker within the human DNA, that could be transferred to any mammalian cell. As a test of the method, several overlapping STFs from 11p15 were shown to cause in vitro growth arrest of rhabdomyosarcoma cells. This activity mapped between the beta-globin and insulin genes.

Multiple tumor suppressor genes in multistep carcinogenesis.

One of the most exciting areas of molecular oncology is the convergence of two independent lines of evidence suggesting involvement of multiple tumor suppressor genes in a given type of cancer. First, epidemiology and somatic cell genetics indicate the presence of multiple tumor suppressor genes in each of several malignancies. Second, cancers often lose multiple chromosomal regions during tumor progression. We will use two tumors, colorectal cancer and Wilms tumor, to illustrate the questions that multiple tumor suppressor genes raise.

A novel general strategy for cloning tumor suppressor genes using radiation-reduced chromosomal superfragments.

To identify the tumor suppressor gene on human chromosome 11p15, we generated mouse microcell hybrids containing small transferable chromosome 11p15 fragments, which we have termed "DNA superfragments". These hybrids will be used to identify which fragments contain a tumor suppressor gene by direct transfer of the fragments to tumor cells via microcell fusion.

Wilms tumor locus on 11p13 defined by multiple CpG island-associated transcripts.

Wilms tumor is an embryonal kidney tumor involving complex pathology and genetics. The Wilms tumor locus on chromosome 11p13 is defined by the region of overlap of constitutional and tumor-associated deletions. Chromosome walking and yeast artificial chromosome (YAC) cloning were used to clone and map 850 kilobases of DNA. Nine CpG islands, constituting a "CpG island archipelago," were identified, including three islands that were not apparent by conventional pulsed-field mapping, and thus were at least partially methylated. Three distinct transcriptional units were found closely associated with a CpG island within the boundaries of a homozygous DNA deletion in a Wilms tumor.