hMLH1 promoter hypermethylation is an early event in human endometrial tumorigenesis

Epigenetic lesions causing genetic lesions in human cancer: promoter hypermethylation of DNA repair genes

The existence of genetic alterations affecting genes involved in cellular proliferation and death, such as TP53 and K-ras, is one of the most common features of tumour cells. Recently, gene inactivation by promoter hypermethylation has been demonstrated. Methylation is the main epigenetic modification in mammals and abnormal methylation of the CpG islands located in the promoter region of the genes leads to transcriptional silencing. Examples include the p16INK4a, p15INK4B, p14ARF, Von Hippel-Lindau (VHL), the oestrogen and progesterone receptors, E-cadherin, death associated protein (DAP) kinase and the first tumour suppressor gene described, retinoblastoma (Rb) gene. In most cases, methylation involves loss of expression, absence of a coding mutation and restoration of transcription by the use of demethylating agents. However, is there a linkage between genetic and epigenetic alterations? Our results show one side of this puzzle demonstrating that epigenetic lesions drive genetic lesions in cancer. Four specific epigenetic lesions, promoter hypermethylation of the DNA mismatch repair gene hMLH1, the DNA alkyl-repair gene O(6)-methylguanine-DNA methyltransferase (MGMT), the detoxifier glutathione S-transferase P1 (GSTP1) and the familial breast cancer gene BRCA1 may lead to four specific genetic lesions, microsatellite instability, G to A transitions, steroid-related adducts and double-strand breaks in DNA. This is probably only the beginning of an extensive list of epigenetic events that change and make the genetic environment of the transformed cell unstable.

Genetic instability is associated with histological transformation of follicle center lymphoma

Follicle center lymphoma (FCL) is an indolent B cell non-Hodgkin's lymphoma (NHL) characterized genetically by the t(14;18) translocation. Histological transformation and clinical progression of FCLs are frequently associated with secondary genetic alterations at both nucleic acid and chromosomal levels. To determine the type and pattern of genomic instability occurring in histological transformation of FCLs and the role of DNA mismatch repair defects in this procedure, we have performed microsatellite analysis, comparative genomic hybridization (CGH) and mutational analysis of hMLH1 and hMSH2 genes on serial biopsy specimens from patients with FCL transformed to diffuse large cell lymphoma (DLCL). Paired biopsy samples of eight patients were analyzed for microsatellite instability and structural alterations for hMLH1 and hMSH2 genes, and tumor samples of five patients were subjected to CGH analysis. A high level of microsatellite instability was associated with histological transformation of two cases of FCL, but no mutations of the hMLH1 and hMSH2 genes were detected in any of the lymphoma samples. In the five cases subjected to CGH analysis, the histological transformation of FCLs was associated with genomic imbalances at 21 chromosomal regions. The genomic abnormalities found were rather heterogeneous and none of the genetic changes were overrepresented in the transformed DLCLs. These data suggest that histological transformation of FCLs to DLCL is frequently associated with genome wide instability at both nucleic acid and chromosomal levels, although mutations of the hMSH1 and hMLH2 genes are not involved in this process.

MSI-L gastric carcinomas share the hMLH1 methylation status of MSI-H carcinomas but not their clinicopathological profile

Sporadic gastric carcinomas (SGC) with microsatellite instability (MSI) exhibit mutations in target genes and display a particular clinicopathological profile. In SGC the MSI phenotype has been associated with hMLH1 promoter hypermethylation. Fifty-seven SGC, classified as high-frequency MSI (MSI-H), low-frequency MSI (MSI-L), and microsatellite stable (MSS), were analyzed for hMLH1 promoter methylation status and clinicopathological features. hMLH1 mutations and hMLH1 expression, as well as target gene mutations, were also evaluated. Our aims were to characterize the molecular and clinicopathological features of SGC, with and without hMLH1 promoter hypermethylation, and to compare the molecular and clinicopathological features of MSI-L, MSI-H, and MSS tumors in an attempt to clarify the place of MSI-L tumors in the mismatch repair (MMR) pathway. Hypermethylation of hMLH1 promoter occurred in 27 of 57 SGC (47.3%) and was significantly associated with MSI status, target gene mutations, and expansive pattern of growth of the tumors. Seventy-five percent of the MSI-H and 50% of MSI-L carcinomas showed hypermethylation (Met+) of hMLH1 in contrast to 0% in MSS carcinomas. No hMLH1 expression was observed in MSI-L/Met+ and MSI-H/Met+ cases. MSS and MSI-L tumors share the same clinicopathological profile regardless of the methylation status of the latter and are distinct from MSI-H tumors. We conclude that mutations in target genes, more than hypermethylation or absence of expression of hMLH1, are the link between MSI status and most of the clinicopathological features of SGC.

Histopathology of genetically defined endometrial precancers

Endometrial precancers are monoclonal, benign neoplasms prone to malignant transformation. A type collection (deposited at www.endometrium.org) of confirmed precancers has been identified by their monoclonal growth and continuity of acquired genetic markers that occur between premalignant and malignant phases of tumorigenesis. Computerized morphometry of these premalignant lesions, designated endometrial intraepithelial neoplasia (EIN), has disclosed new architectural criteria and revised cytologic criteria for their diagnosis. EIN lesions originate focally and expand in size over time, in keeping with a proliferative monoclonal origin. They are characterized by closely packed glands (volume percentage stroma < 55%) with cytology that is clearly demarcated from that of the adjacent field. A minimum homogeneous field of cytologically demarcated glands is required to accurately assess the architecture diagnostic of EIN, and morphometry-diagnosed lesions with a largest diameter of at least 1 to 2 mm have previously been shown to predict the relevant clinical outcome of concurrent or future endometrial adenocarcinoma. Nonphysiologic loss of the PTEN protein, a tumor suppressor gene mutated in many endometrioid adenocarcinomas, is seen in individual glands of endometrium exposed to unopposed estrogens and in packed clusters of EIN glands. The isolated PTEN-free glands in anovulatory endometria may be the earliest stage of endometrial tumorigenesis, but they are not readily distinguishable by routine histology, nor do they have a defined natural history.

Mismatch repair defects in cancer

Post-replicative mismatch repair in humans utilises the hMSH2, hMSH6, hMSH3, hMLH1 and hPMS2 genes and possibly the newly identified hMLH3 gene. Recently, a link has been established between hMSH6 mutations and 'atypical' hereditary non-polyposis colon cancer (HNPCC) with an increased incidence of endometrial cancers. To satisfy the need for a diagnostic test capable of differentiating between pathogenic mutations and polymorphisms, several functional assays that fulfil these criteria have been described. These should allow for better diagnosis of HNPCC.

DNA hypermethylation in tumorigenesis: epigenetics joins genetics

Recently, the concept that epigenetic, as well as genetic, events might be central to the evolution of human cancer is re-emerging. Cancers often exhibit an aberrant methylation of gene promoter regions that is associated with loss of gene function. This DNA change constitutes a heritable state, not mediated by altered nucleotide sequence, that appears to be tightly linked to the formation of transcriptionally repressive chromatin. This epigenetic process acts as an alternative to mutations to disrupt tumor-suppressor gene function and can predispose to genetic alterations through inactivating DNA-repair genes. Dissecting the molecular processes that mediate these methylation changes will enhance our understanding of chromatin modeling and gene regulation and might present novel possibilities for cancer therapy. Methylation changes constitute potentially sensitive molecular markers to define risk states, monitor prevention strategies, achieve early diagnosis, and track the prognosis of cancer.