Microsatellite instability and mutation of DNA mismatch repair genes in gliomas

Germline, somatic and epigenetic events underlying mismatch repair deficiency in colorectal and HNPCC-related cancers

High-frequency microsatellite instability (MSI-H) results from deficiency in nucleotide mismatch repair. It contributes significantly to carcinogenesis in the human colorectal mucosa. Here we study 41 colorectal and three other HNPCC-related cancers with MSI-H to provide comprehensive information on the mechanisms of inactivation of the two major proteins involved, hMLH1 and hMSH2. Seventeen of the patients had family histories meeting the criteria for Bethesda grades 1, 2 or 3. Of these familial cases, 14 (83%) had early-onset disease, defined on the basis of diagnosis prior to the age of 50, but in three the disease was of late onset (>50 years). A second subset of 20 patients had early onset disease without family history. The remaining seven patients were selected to allow comparisons with sporadic, late-onset disease, the molecular basis of which has been extensively reported elsewhere. We stratified the tumours initially on the basis of hMLH1 or hMSH2 protein deficiency, detected by immunohistochemistry, and then by analysis of germline and somatic mutation, mRNA transcription, loss of heterozygosity (LOH) at the hMLH1 and hMSH2 loci, and methylation status in two regions of the hMLH1 promoter. The functional significance of several of these changes in the MSI-H tumours was confirmed by comparisons with 16 tumours with low-frequency microsatellite instability and 56 tumours with stable microsatellites. As anticipated, patients with family histories usually showed germline mutation of hMSH2 or hMLH1. In many cases the residual normal allele was silenced in their tumours by loss of heterozygosity (LOH). The small subset of late-onset, sporadic cases confirmed the preponderance in this group of biallelic hMLH1 promoter methylation. In the early-onset, apparently sporadic subset there were 11 tumours with hMLH1 deficiency, five with hMSH2 deficiency and four with no detectable abnormality in expression of either protein. These showed a complex mixture of lesions, including germline and somatic mutations, promoter methylation, LOH, suppression of wild-type RNA by as yet undiscovered mechanisms, or no detectable abnormality in any of these parameters. Evidence is presented to indicate that methylation in proximal region of the hMLH1 promoter is a more reliable correlate of transcriptional silencing in colorectal cancers than methylation in upstream region. These observations have significant implications for management of patients with MSI-H tumours.

Poly(ADP-ribose) polymerase inhibitor increases growth inhibition and reduces G(2)/M cell accumulation induced by temozolomide in malignant glioma cells

Temozolomide (TZM) is a novel methylating agent currently under investigation for treatment of recurrent high-grade gliomas. Although TZM generates a wide spectrum of methyl adducts, its cytotoxicity has been attributed to mismatch repair (MR)-mediated processing of O(6)-methylguanine:T mispairs. N3-methyladenine and N7-methylguanine adducts are promptly repaired by the base excision repair system, unless a poly(ADP-ribose) polymerase (PARP) inhibitor is combined to TZM. In this case, the repair process of N-methylpurines cannot be completed and the deriving DNA strand breaks contribute to cytotoxicity. In this study, we investigated the influence on cell growth and cell cycle of treatment with TZM + PARP inhibitor in glioma cells characterized by different susceptibility to TZM. The results indicated that PARP inhibitor increases growth inhibition induced by TZM in either p53-wild-type or p53-mutant glioblastoma cells, as early as 24 h after drug exposure. The enhancing effect exerted by PARP inhibitor was particularly evident in glioma cells characterized by a defective expression of MR, since these cells are tolerant to O(6)-methylguanine damage and show low sensitivity to TZM. In O(6)-alkylguanine-DNA alkyltransferase (OGAT)-deficient and MR-proficient tumor cells bearing wild-type p53, the drug combination markedly reduced cell accumulation in the G(2)/M phase of cell cycle and induction of the G(2) checkpoint regulator Chk1 kinase. In short-term cultures of glioma cells derived from surgical specimens, PARP inhibitor enhanced chemosensitivity to TZM and this effect was especially evident in OGAT-proficient tumors. Thus, a pharmacological strategy based on the interruption of N-methylpurine repair might represent a novel strategy to restore or increase glioma sensitivity to TZM.

High-resolution genome-wide allelotype analysis identifies loss of chromosome 14q as a recurrent genetic alteration in astrocytic tumours

Diffusely infiltrative astrocytic tumours are the most common neoplasms in the human brain. To localise putative tumour suppressor loci that are involved in low-grade astrocytomas, we performed high-resolution genome-wide allelotype analysis on 17 fibrillary astrocytomas. Non-random allelic losses were identified on chromosomal arms 10p (29%), 10q (29%), 14q (35%), 17p (53%), and 19q (29%), with their respective common regions of deletions delineated at 10p14-15.1, 10q25.1-qter, 14q212.2-qer, 17p11.2-pter and 19q12-13.4. These results suggest that alterations of these chromosomal regions play important roles in the development of astrocytoma. We also allelotyped 21 de novo glioblastoma multiforme with an aim to unveil genetic changes that are common to both types of astrocytic tumours. Non-random allelic losses were identified on 9p (67%), 10p (62%), 10q (76%), 13q (60%), 14q (50%), and 17p (65%). Allelic losses of 10p, 10q, 14q and 17p were common genetic alterations detectable in both fibrillary astrocytomas and glioblastoma multiforme. In addition, two common regions of deletions on chromosome 14 were mapped to 14q22.3-32.1 and 14q32.1-qter, suggesting the presence of two putative tumour suppressor genes. In conclusion, our comprehensive allelotype analysis has unveiled several critical tumour suppressor loci that are involved in the development of fibrillary astrocytomas and glioblastoma multiforme. Although these two types of brain tumours are believed to evolve from different genetic pathways, they do share some common genetic changes. Our results indicate that deletions of chromosome 14q is a recurrent genetic event in the development of astrocytoma and highlight the subchromosomal regions on this chromosome that are likely to contain putative tumour suppressor genes involved in the oncogenesis of astrocytic tumours.

Building bridges in cancer: mismatch repair and microsatellite instability

Mismatch repair (MMR) defects and microsatellite instability (MSI) are two genetic alterations that have been documented in a wide variety of human cancers, including some that involve the skin. Available evidence indicates that these two features are sometimes directly related, although their connection seems to be indirect or nonexistent in other instances. The purposes of this review are to summarize the variable relations between MMR and MSI as deduced from analysis of a diverse array of human neoplasms and to give brief insights as to the other molecular mechanisms potentially involved in the maintenance of genomic stability.

Microsatellite instability, PTEN and p53 germline mutations in glioma families

Rare inherited syndromes that to some extent explain familial glioma include Turcot's syndrome, Li-Fraumeni syndrome and neurofibromatosis types I and II. The majority of families with glioma do not meet the clinical criteria for any of these syndromes. In order to study the genetic origin of familial glioma, tumour DNA (n = 35) or blood samples (n = 8) were collected from 25 families. The glioma tumours were tested for microsatellite instability (MSI) with two markers, BAT25 and BAT26, since glioma is associated with hereditary non-polyposis colon cancer (HNPCC) in Turcot's syndrome. Furthermore, p53 was screened from blood DNA (exons 2-11) with temporal temperature gradient electrophoresis (TTGE) since germline mutations in p53 are seen in Li-Fraumeni syndrome. In gliomas, there is a wide variety of somatic mutations, such as, for instance, in p53, the epidermal growth factor receptor (EGFR) and p16. The tumour suppressor gene PTEN is also often somatically mutated in glioma, therefore it is attractive as a candidate gene for germline mutations in familial glioma. Blood DNA was directly sequenced for mutations in PTEN exons 1-9. The analysis showed that no mutations were found in either of the studied tumour suppressor genes, and no MSI-positive tumours were found. A common polymorphism in p53 at codon 72 (arginine/proline) was found in 6/8 of the patients. Apparently, mutation in the tested tumour suppressor genes or DNA mismatch repair genes does not explain the familial glioma observed in these families.

Targeting DNA mismatch repair for radiosensitization

Postreplicational mismatch repair (MMR) proteins are capable of recognizing and processing not only single base-pair mismatches and insertion-deletion loops (IDLs) that occur during DNA replication, but also adducts in DNA resulting from treatment with cancer chemotherapy agents. These include widely varying types of DNA adducts resulting from methylating agents such as MNNG, MNU, temozolomide, and procarbazine; CpG crosslinks resulting from cisplatin and carboplatin; and S(6)-thioguanine and S(6)-methylthioguanine residues in DNA. Although MMR proteins can recognize both replicational errors and chemotherapy-induced adducts in DNA, the end results of this recognition are very different. Base-base mismatches and IDLs can be repaired by MMR, restoring genomic integrity, whereas MMR-mediated recognition and processing of chemotherapy-induced adducts in DNA results in apoptosis. After the loss of MMR, the inability of cells to recognize and correct single base-pair mismatches and insertion-deletion loops can lead to secondary mutations in proto-oncogenes and tumor-suppressor genes, thereby contributing to the development of cancer. In addition, the inability of MMR-deficient cells to recognize chemotherapy-induced adducts in DNA can result in a damage-tolerant phenotype that translates to clinically significant resistance by allowing for selection of MMR-deficient cancer cells. We have shown recently that these MMR-deficient, drug-resistant cells can be targeted for radiosensitization by the halogenated thymidine analogs iododeoxyuridine (IdUrd) and bromodeoxyuridine (BrdUrd). These thymidine (dThd) analogs become incorporated into DNA and form reactive uracil radicals after ionizing radiation (IR), increasing strand breaks. IdUrd and BrdUrd appear to be removed from DNA in MMR-proficient cells with limited toxicity or disruption of the cell cycle, while accumulating at much higher levels in MMR-deficient cells. As a result, it is possible to effectively increase the radiosensitization of MMR-deficient cells at levels of halogenated dThd analog that demonstrate limited toxicity to MMR-proficient cells. This indicates that a combined approach of IdUrd or BrdUrd with IR may be effective in killing MMR-deficient tumors in patients, which are resistant to many cancer chemotherapy agents commonly used in the clinic.

A novel germline 1.8-kb deletion of hMLH1 mimicking alternative splicing: a founder mutation in the Chinese population

We have previously reported that there is a high incidence of microsatellite instability (MSI) and germline mismatch repair gene mutation in colorectal cancer arising from young Hong Kong Chinese. Most of the germline mutations involve hMSH2, which is different from the mutation spectrum in the Western population. It is well known that alternative splicing is common in hMLH1, which complicates RNA based mutation detection methods. In contrast, large deletions in hMLH1, commonly observed in some ethnic groups, tend to escape detection by exon-by-exon direct DNA sequencing. Here we report the detection of a novel germline 1.8 kb deletion involving exon 11 of hMLH1 in a local hereditary non-polyposis colorectal cancer family. This mutation generates a mRNA transcript with deletion of exons 10-11, which is indistinguishable from one of the most common and predominant hMLH1 splice variants. A diagnostic test based on PCR of the breakpoint region led to the identification of an additional young colorectal cancer patient with this mutation. Haplotype analysis suggests that they may share a common ancestral mutation. Our results caution investigators in the interpretation of alternative splicing and have important implications for the design of hMLH1 mutation detection strategy in the Chinese population.

Chromosomal instability and p53 inactivation are required for genesis of glioblastoma but not for colorectal cancer in patients with germline mismatch repair gene mutation

We have previously reported high-frequency microsatellite instability (MSI-H) and germ-line mismatch repair gene mutation in patients with unusually young onset of high-grade glioma. Some of these patients developed metachronous MSI-H colorectal cancer and conformed to the diagnosis of Turcot's syndrome. Frameshift mutation of TGFbetaRII was present in all the colorectal carcinomas but not in brain tumours. We further characterized the genetic pathways of tumour evolution in these metachronous gliomas and colorectal carcinomas. All MSI-H glioblastomas had inactivation of both alleles of the p53 gene and showed over-expression of the p53 protein while none of the colorectal carcinomas had p53 mutation or protein over-expression. Flow cytometry and comparative genomic hybridization revealed that all glioblastomas were chromosomal unstable with aneuploid DNA content, and with a variable number of chromosomal arm aberrations. In contrast, the colorectal carcinomas had diploid or near-diploid DNA content with few chromosomal arm aberrations. The pattern of chromosomal aberrations in the two organs was different. Loss of 9p was consistently observed in all glioblastomas but not in colorectal carcinomas. Epidermal growth factor receptor amplification was absent in all glioblastomas and colorectal carcinomas. Our results suggest that both the frequency of p53 mutation and its effects differ greatly in the two organs. Following loss of mismatch repair function, p53 inactivation and chromosomal instability are not necessary for development of colorectal carcinoma, but are required for genesis of glioblastoma. Oncogene (2000) 19, 4079 - 4083.

Distinct clinical features associated with microsatellite instability in colorectal cancers of young patients

The Hong Kong Chinese population has an unusually high incidence of colorectal cancer in the young, suggestive of hereditary susceptibility. To search for a genetic basis for this predisposition, we studied the incidence of microsatellite instability (MSI) in paraffin-embedded colectomy specimens of 124 young (<50 years old) Chinese colorectal cancer patients referred to the Hong Kong Hereditary Gastrointestinal Cancer Registry from 1995 to 1998. By medical record review and personal interview, we searched for distinct clinical features associated with the manifestation of MSI in this group of patients. For patients with MSI tumours, blood was taken for detection of germline mutation in 2 mismatch repair (MMR) genes. MSI was present in 33 tumours from 23 males and 10 females (26.6%). Ongoing mutation analysis has so far identified MMR gene mutations in 8 patients with MSI tumours. The incidence of MSI increased significantly with decreasing age at cancer diagnosis. For patients aged 30 to 49, MSI tumours were located mainly at the proximal colon. However, for exceptionally young patients (<30 years), MSI tumours tended to be at the distal large bowel. This observation suggested a differential activity of the MMR pathway in colorectal carcinogenesis in different age groups. On multivariate analysis, young age at cancer diagnosis, proximal tumour location, a strong family history of colorectal cancer, and a personal history of metachronous cancer were independent predictors for MSI status. This knowledge may have an impact on the management of young colorectal cancer patients and their families.

M6P/IGF2R is mutated in squamous cell carcinoma of the lung

In addition to the intracellular sorting of lysosomal enzymes, the mannose 6-phosphate/insulin-like growth factor II receptor (M6P/IGF2R) plays a critical role in regulating the bioavailability of extracellular proteolytic enzymes and growth factors. It has also been shown to be mutated in a number of human cancers, and to suppress cancer cell growth. The purpose of this study was to determine if the M6P/IGF2R is mutated in lung cancer, a leading cause of cancer death worldwide. Archival pathology specimens were obtained on 22 patients with newly diagnosed, untreated squamous cell carcinoma of the lung. Two polymorphisms in the 3'-untranslated region of the M6P/IGF2R were used to screen lung tumors for loss of heterozygosity (LOH) by PCR amplification of DNA. Nineteen of 22 (86%) patients were informative (heterozygous), and 11/19 (58%) squamous cell carcinomas of the lung had LOH at the M6P/IGF2R locus. The remaining allele in 6/11 (55%) LOH patients contained mutations in either the mannose 6-phosphate or the IGF2 binding domain of the M6P/IGF2R. Thus, the M6P/IGF2R is mutated frequently in squamous cell carcinoma of the lung, providing further support for its function as a tumor suppressor.

Microsatellite instability and the PTEN1 gene mutation in a subset of early onset gliomas carrying germline mutation or promoter methylation of the hMLH1 gene

High-frequent microsatellite instability (MSI-H) was detected in two of the 80 gliomas examined, whlie the other 78 gliomas showed microsatellite stable (MSS) phenotype. Both of the two MSI-H tumors were glioblastomas which developed in teenage patients. One of the patient was diagnosed as having Turcot's syndrome and had a germline mutation in the hMLH1 gene. Loss of expression due to promoter methylation was selectively observed in the wild type allele of the hMLH1 gene in the tumor of this patient. The other patient had neither a family history nor a past personal history of malignancy. Although no mutation in the mismatch repair genes was detected in the tumor of this patient, the level of expression of the hMLH1 gene was markedly decreased and the promoter sequence of the gene was highly methylated. In the tumor of this patient, the PTEN1 gene, one of the genes carrying microsatellite sequences in their coding regions, was altered by a slippage mutation within five adenine repeat sequences. These findings indicate that the genetic or epigenentic inactivation of the hMLH1 gene is involved in a subset of early-onset gliomas and the PTEN1 gene could be a downstream target for mutation as observed in glioblastoma without MSI.

Imprinted genes as potential genetic and epigenetic toxicologic targets

Genomic imprinting is an epigenetic phenomenon in eutherian mammals that results in the differential expression of the paternally and maternally inherited alleles of a gene. Imprinted genes are necessary for normal mammalian development. This requirement has been proposed to have evolved because of an interparental genetic battle for the utilization of maternal resources during gestation and postnatally. The nonrandom requisite for monoallelic expression of a subset of genes has also resulted in the formation of susceptibility loci for neurobehavioral disorders, developmental disorders, and cancer. Since imprinting involves both cytosine methylation within CpG islands and changes in chromatin structure, imprinted genes are potential targets for dysregulation by epigenetic toxicants that modify DNA methylation and histone acetylation.

Mannose 6-phosphate/insulin-like growth factor 2 receptor, a bona fide tumor suppressor gene or just a promising candidate?

The mannose 6-phosphate/insulin-like growth factor 2 receptor (M6P/IGF2R) is considered a "candidate" tumor suppressor gene. This hypothesis has been provoked by the identification of loss of heterozygosity (LOH) at the M6P/IGF2R locus on chromosome 6q26 in breast and liver cancer, accompanied by point mutations in the remaining allele. Somatic mutations in coding region microsatellites have also been described in replication error positive (RER+) tumors of the gastrointestinal tract, endometrium and brain. These genetic data are compelling, but a tumor suppressor gene candidate has to meet functional as well as genetic criteria. This review weighs the evidence and discusses the observations that are necessary to promote M6P/IGF2R from candidate to bona fide tumor suppressor gene.

Genetic alterations in pediatric high-grade astrocytomas

High-grade astrocytomas are tumors that are uncommon in children. Relatively few studies have been performed on their molecular properties and so it is not certain whether they follow different genetic pathways from those described in adult diffuse astrocytomas. In this study, we evaluated 24 pediatric high-grade astrocytomas (11 anaplastic astrocytomas and 13 glioblastomas) all of which were sporadic and primary. We studied mutations of p53, phosphatase and tensin homolog (PTEN), loss of heterozygosity (LOH) of chromosomes 17p13, 9p21 and 10q23-25, amplification of epidermal growth factor receptor (EGFR), and overexpression of EGFR and p53 protein. In addition, we searched for microsatellite instability (MSI) by using MSI sensitive and specific microsatellite markers. p53 mutations were found in 38% (9/24) of the high-grade astrocytomas and all brain stem tumors except 2 (71%, 5/7) had p53 mutations. PTEN mutations were found in 8% (2/24) of high-grade astrocytomas. However, no EGFR amplification was found in any of them. LOH was found at 17p13.1 in 50% (3/6 informative tumors), 9p21 in 83% (5/6 informative tumors), and 10q23-25 in 78% (7/9 informative tumors). Four tumors showed MSI, and 2 of them that showed widespread MSI were regarded as tumors with replication error (RER+) phenotype. All 4 tumors with MSI showed concurrent LOH of 9p21 and 10q23-25. Combining gene alterations, LOH, MSI, and gene mutations, inactivation of both alleles of PTEN and p53 was found in 57% (4/7 informative tumors) and 50% (3/6 informative tumors) of the cases respectively. We conclude that development of pediatric high-grade astrocytomas may follow pathways different from the primary or secondary paradigm of adult glioblastomas. In a subset of these tumors, genomic instability was also implicated.

Molecular genetic studies of chromosome 11 and chromosome 22q DNA sequences in pediatric medulloblastomas

Medulloblastomas are primitive neuroectodermal tumors (PNETs) of the cerebellum with poorly understood pathogenesis. Previous molecular studies suggested a role for loci on chromosome 11 in the development of medulloblastomas-PNETs. In order to identify the frequency of loss and eventually the extent of allelic loss on chromosome 11, we have examined 23 pediatric medulloblastomas for loss of heterozygosity (LOH) with 16 polymorphic microsatellites. Our data reveal that LOH on 11p or 11q occurs rarely (13%) suggesting the unlikely involvement of chromosome 11 in most cases of medulloblastomas. The same frequency of LOH in medulloblastomas was detected using 8 microsatellites on 22q. Alterations of microsatellite length were found in only 4/594 PCR analyses using 28 markers located on chromosomes 2, 9, 11, 18, and 22, demonstrating that genomic instability is uncommon in medulloblastomas.

Frequent microsatellite instability and mismatch repair gene mutations in young Chinese patients with colorectal cancer

BACKGROUND

The incidence of colorectal cancer in persons under 46 years of age is substantially higher in Hong Kong than in Scotland and many other countries. Consequently, we examined whether there is a hereditary predisposition for colorectal cancer in this Southern Chinese population.

METHODS

We investigated the incidence of microsatellite instability (MSI) at 10 DNA sites in 117 colorectal cancer specimens from Chinese patients of various ages. Those tumors with new alleles at 40% or more of the sites investigated were identified as highly unstable MSI (MSI-H). In young patients, we also searched for germline mutations in three mismatch repair genes (hMSH2, hMLH1, and hMSH6).

RESULTS

The incidence of MSI-H varied statistically significantly with age, being observed in more than 60% of those younger than age 31 years at diagnosis and in fewer than 15% of those age 46 years or older. In 15 patients (<46 years old) whose colorectal cancers showed MSI-H, eight possessed germline mutations in either hMSH2 or hMLH1. When mutations in hMSH6 were included, more than 80% of Chinese colorectal cancer patients younger than 31 years had germline mutations in mismatch repair genes. We found a novel germline missense mutation in hMSH6 in a 29-year-old man whose tumor showed no MSI. Two patients had a 4-base-pair insertion in exon 10 causing a truncated protein; this insertion is a common polymorphism with a population allele frequency in Chinese of 5.6%.

CONCLUSIONS

Our results indicate that germline mutations in mismatch repair genes contribute substantially to the pathogenesis and high incidence of colorectal cancer in young Hong Kong Chinese. However, because young Chinese and Caucasians show similar proportions of colorectal cancers with MSI-H, despite the higher incidence in the former, additional factors may underlie the high susceptibility of young Chinese to colorectal cancer.

Genomic imprinting and cancer

Although we inherit two copies of all genes, except those that reside on the sex chromosomes, there is a subset of these genes in which only the paternal or maternal copy is functional. This phenomenon of monoallelic, parent-of-origin expression of genes is termed genomic imprinting. Imprinted genes are normally involved in embryonic growth and behavioral development, but occasionally they also function inappropriately as oncogenes and tumor suppressor genes. The evidence that imprinted genes play a role in carcinogenesis will be discussed in this review. Additional information about imprinted genes can be found on the Genomic Imprinting Website at: (http://www.geneimprint.com).

Genomic imprinting: implications for human disease

Genomic imprinting refers to an epigenetic marking of genes that results in monoallelic expression. This parent-of-origin dependent phenomenon is a notable exception to the laws of Mendelian genetics. Imprinted genes are intricately involved in fetal and behavioral development. Consequently, abnormal expression of these genes results in numerous human genetic disorders including carcinogenesis. This paper reviews genomic imprinting and its role in human disease. Additional information about imprinted genes can be found on the Genomic Imprinting Website at http://www.geneimprint.com.