Methylation-independent silencing of the p73 gene in neuroblastoma

Deciphering the Role of p53 and TAp73 in Neuroblastoma: From Pathogenesis to Treatment

Neuroblastoma (NB) is an embryonic cancer that develops from neural crest stem cells, being one of the most common malignancies in children. The clinical manifestation of this disease is highly variable, ranging from spontaneous regression to increased aggressiveness, which makes it a major therapeutic challenge in pediatric oncology. The p53 family proteins p53 and TAp73 play a key role in protecting cells against genomic instability and malignant transformation. However, in NB, their activities are commonly inhibited by interacting proteins such as murine double minute (MDM)2 and MDMX, mutant p53, ΔNp73, Itch, and Aurora kinase A. The interplay between the p53/TAp73 pathway and N-MYC, a known biomarker of poor prognosis and drug resistance in NB, also proves to be decisive in the pathogenesis of this tumor. More recently, a strong crosstalk between microRNAs (miRNAs) and p53/TAp73 has been established, which has been the focused of great attention because of its potential for developing new therapeutic strategies. Collectively, this review provides an updated overview about the critical role of the p53/TAp73 pathway in the pathogenesis of NB, highlighting encouraging clues for the advance of alternative NB targeted therapies.

Neuroblastoma: oncogenic mechanisms and therapeutic exploitation of necroptosis

Neuroblastoma (NB) is the most common extracranial childhood tumor classified in five stages (1, 2, 3, 4 and 4S), two of which (3 and 4) identify chemotherapy-resistant, highly aggressive disease. High-risk NB frequently displays MYCN amplification, mutations in ALK and ATRX, and genomic rearrangements in TERT genes. These NB subtypes are also characterized by reduced susceptibility to programmed cell death induced by chemotherapeutic drugs. The latter feature is a major cause of failure in the treatment of advanced NB patients. Thus, proper reactivation of apoptosis or of other types of programmed cell death pathways in response to treatment is relevant for the clinical management of aggressive forms of NB. In this short review, we will discuss the most relevant genomic rearrangements that define high-risk NB and the role that destabilization of p53 and p73 can have in NB aggressiveness. In addition, we will propose a strategy to stabilize p53 and p73 by using specific inhibitors of their ubiquitin-dependent degradation. Finally, we will introduce necroptosis as an alternative strategy to kill NB cells and increase tumor immunogenicity.

Transcriptional regulation of the p73 gene by Nrf-2 and promoter CpG methylation in human breast cancer

To understand the transcriptional regulation of p73 by promoter methylation and Nrf-2 in breast carcinogenesis, ChIP assay indicated that Nrf-2 can bind to both promoters and can activate the transcription of TAp73 and ΔΝp73 in MCF-7 cell line, knockdown of Nrf-2 gene resulted in an abrogation of TAp73 and ΔΝp73 expression in the cells transfected with sh-Nrf-2 as well as Nrf-2 knock out mouse model. However, we found Nrf-2 induced ΔΝp73 expression was abolished with 5-aza-dC treatment, thus lead to a down-regulated ΔΝp73 and an up-regulated TAp73 expression in breast cancer cells lines. Consistent with this model, we detected decreased TAp73 and increased ΔNp73 expression in breast cancer tissue, along with increased TAp73 but decreased ΔNp73 expression in corresponding surrounding noncancerous tissues (NCTs) in a breast cancer tissue assay. A significant inverse correlation was found between TAp73 and ΔNp73 expression in the above tissue-array (P = 0.047) and validated in another set consisting of 128 breast cancer tumor tissue (P = 0.034). Taken together, our findings suggest that Nrf-2 and promoter methylation cooperatively govern the transcriptional regulation of p73, and unbalanced expression of TAp73 and ΔNp73 expression plays a critical role in breast cancer development.

Hypoxia-induced epigenetic regulation and silencing of the BRCA1 promoter

Disruption of the BRCA1 tumor suppressor can be caused not only by inherited mutations in familial cancers but also by BRCA1 gene silencing in sporadic cancers. Hypoxia, a key feature of the tumor microenvironment, has been shown to downregulate BRCA1 at the transcriptional level via repressive E2F4/p130 complexes. Here we showed that hypoxia also drives epigenetic modification of the BRCA1 promoter, with decreased H3K4 methylation as a key repressive modification produced by the lysine-specific histone demethylase LSD1. We also observed increased H3K9 methylation coupled with decreased H3K9 acetylation. Similar modifications were seen in the RAD51 promoter, which is also downregulated by hypoxia, whereas exactly opposite changes were seen in the promoter of the hypoxia-inducible gene VEGF. In cells containing the BRCA1 promoter driving a selectable HPRT gene, long-term silencing of the promoter was observed following exposure to hypoxic stress. Clones with silenced BRCA1 promoters were detected at frequencies of 2% or more following hypoxia, but at less than 6 × 10(-5) without hypoxia. The silenced clones showed decreased H3K4 methylation and decreased H3K9 acetylation in the BRCA1 promoters, consistent with the acute effects of hypoxic stress. Hypoxia-induced BRCA1 promoter silencing persisted in subsequent normoxic conditions but could be reversed by treatment with a histone deacetylase (HDAC) inhibitor but not with a DNA methylation inhibitor. Interestingly, treatment of cells with inhibitors of poly(ADP-ribose) polymerase (PARP) can cause short-term repression of BRCA1 expression, but such treatment does not produce H3K4 or H3K9 histone modification or BRCA1 promoter silencing. These results suggest that hypoxia is a driving force for long-term silencing of BRCA1, thereby promoting genome instability and tumor progression.

p53 family: Therapeutic targets in neuroblastoma

Survival rates for metastatic neuroblastoma remain poor, despite significant increase in the intensity of therapy. Although it represents approximately 7% of pediatric cancer, neuroblastoma accounts for approximately 15% of childhood cancer deaths. Thus, novel approaches to enhance neuroblastoma chemotherapy sensitivity and prevent or bypass chemoresistance are required. Disruption of the p53 pathway is a common mechanism leading to defects in apoptosis in cancer cells. Increasing evidence suggests that the p53 pathway may be inactivated in neuroblastoma. Inactivation of the p53 pathway occurs most commonly at the time of relapse, and probably contributes to chemoresistance. The p53 family proteins, p73 and p63, can also induce apoptosis, and early studies suggest that p73 may be important in neuroblastoma pathogenesis and response to treatment. This article focuses on current therapies and novel drugs targeting p53 and p73 signaling pathways in neuroblastoma. Understanding the balance between the p53 family proteins in neuroblastoma and how their expression and activity are regulated will hopefully lead to the discovery of agents that target these pathways to induce neuroblastoma cell death, alone or in combination with chemotherapies.

Aberrant epigenetic silencing is triggered by a transient reduction in gene expression

Background

Aberrant epigenetic silencing plays a major role in cancer formation by inactivating tumor suppressor genes. While the endpoints of aberrant silencing are known, i.e., promoter region DNA methylation and altered histone modifications, the triggers of silencing are not known. We used the tet-off system to test the hypothesis that a transient reduction in gene expression will sensitize a promoter to undergo epigenetic silencing.

Methodology/Principal Findings

The tet responsive promoter (P_TRE) was used to drive expression of the selectable human HPRT cDNA in independent transfectants of an Hprt deficient mouse cell line. In this system, high basal HPRT expression is greatly reduced when doxycycline (Dox) is added to the culture medium. Exposure of the P_TRE-HPRT transfectants to Dox induced HPRT deficient clones in a time dependent manner. A molecular analysis demonstrated promoter region DNA methylation, loss of histone modifications associated with expression (i.e., H3 lysine 9 and 14 acetylation and lysine 4 methylation), and acquisition of the repressive histone modification H3 lysine 9 methylation. These changes, which are consistent with aberrant epigenetic silencing, were not present in the Dox-treated cultures, with the exception of reduced H3 lysine 14 acetylation. Silenced alleles readily reactivated spontaneously or after treatment of cells with inhibitors of histone deacetylation and/or DNA methylation, but re-silencing of reactivated alleles did not require a new round of Dox exposure. Inhibition of histone deacetylation inhibited both the induction of silencing and re-silencing, whereas inhibition of DNA methylation had no such effect.

Conclusions/Significance

This study demonstrates that a transient reduction in gene expression triggers a pathway for aberrant silencing in mammalian cells and identifies histone deacetylation as a critical early step in this process. DNA methylation, in contrast, is a secondary step in the silencing pathway under study. A model to explain these observations is offered.

Aberrant methylation of candidate tumor suppressor genes in neuroblastoma

CpG island hypermethylation has been recognized as an alternative mechanism for tumor suppressor gene inactivation. In this study, we performed methylation-specific PCR (MSP) to investigate the methylation status of 10 selected tumor suppressor genes in neuroblastoma. Seven of the investigated genes (CD44, RASSF1A, CASP8, PTEN, ZMYND10, CDH1, PRDM2) showed high frequencies (> or =30%) of methylation in 33 neuroblastoma cell lines. In 42 primary neuroblastoma tumors, the frequencies of methylation were 69%, CD44; 71%, RASSF1A; 56%, CASP8; 25%, PTEN; 15%, ZMYND10; 8%, CDH1; and 0%, PRDM2. Furthermore, CASP8 and CDH1 hypermethylation was significantly associated with poor event-free survival. Meta-analysis of 115 neuroblastoma tumors demonstrated a significant correlation between CASP8 methylation and MYCN amplification. In addition, there was a correlation between ZMYND10 methylation and MYCN amplification. The MSP data, together with optimized mRNA re-expression experiments (in terms of concentration and time of treatment and use of proper reference genes) further strengthen the notion that epigenetic alterations could play a significant role in NB oncogenesis. This study thus warrants the need for a global profiling of gene promoter hypermethylation to identify genome-wide aberrantly methylated genes in order to further understand neuroblastoma pathogenesis and to identify prognostic methylation markers.

Malignant transformation-related genes in meningiomas: allelic loss on 1p36 and methylation status of p73 and RASSF1A

OBJECT

Analysis of meningiomas supports the suggestion that loss of heterozygosity (LOH) of chromosome arm 1p plays an important role in malignancy. The aim of this study was to identify genes related to meningioma progression from the benign state to the atypical and anaplastic states by examining 1p LOH and the promoter methylation of RASSF1A and p73.

METHODS

The authors studied 40 surgical specimens (22 WHO Grade I, 11 Grade II, and seven Grade III) obtained in 37 patients with meningioma. The LOH at 1p36 was analyzed using microsatellite markers, and promoter methylation of p73 and RASSFIA was analyzed using methylation-specific polymerase chain reaction.

RESULTS

No 1p LOH was detected in the Grade I tumors, whereas it was detected in more than 80% of the Grade II and III tumors. Methylation of the p73 promoter was observed in 81.8 and 71.4% of the Grade II and III tumors, respectively, but it was not observed in any of the Grade I tumors; methylation of the RASSF1A promoter was observed in 18.2, 63.6, and 42.9% of the Grade I, II, and III tumors, respectively. Interestingly, 1p LOH and p73 promoter hypermethylation were detected in the malignantly transformed tumors but not in the lower-grade primary ones.

CONCLUSIONS

Based on the hypothesis that meningiomas cumulatively acquire genetic alterations and thus progress from the benign to the atypical and anaplastic states, genetic alterations in the methylation status of p73 or RASSF1A along with 1p LOH may result in the malignant transformation of a meningioma. This type of genetic fingerprint may play both diagnostic and therapeutic roles.

Meth-DOP-PCR: an assay for the methylation profiling of trace amounts of DNA extracted from bodily fluids

Cancer cells release their DNA into the patient's bodily fluids and cancer-specific signatures can be recognized in the circulating DNA. The aberrant methylation of CpG-rich regions in gene promoter sequences is an early marker of cell transformation whose specificity and optimal sensitivity can be achieved by assessing the methylation status of multiple genes ('methylation profiling'). Most of the current technologies for methylation analysis rely upon the combination of chemical conversion of the DNA and PCR analysis for the detection of methylated and unmethylated alleles. However, the small amount of circulating DNA, and its fragmentation, dramatically reduces the template DNA molecules making difficult the methylation profiling. To overcome this limitation, we have developed the Meth-DOP-PCR assay, a combination between a modified degenerate oligonucleotide primed PCR (DOP-PCR) and methylation-specific PCR (MSP), for the high-throughput methylation analysis of trace-amount of circulating DNA. We have demonstrated the concordance between Meth-DOP-PCR and MSP and shown the application of this technique for the methylation analysis of DNA extracted from the serum of lung cancer patients. We have estimated that through this procedure it is possible to obtain at least a 25-fold increase of the number of determinations allowing the methylation profiling from less than 1 ml of serum. Thus, Meth-DOP-PCR appears as a simple, cost-effective and efficient technique, for the development of novel methylation-based diagnostic assays.

Allelic expression and quantitative RT-PCR study of TAp73 and ΔNp73 in non-Hodgkin's lymphomas

p73 shares structural and functional homology to p53. p73 generates different proteins using alternative promoters and splicing which have different biological characteristics. We investigated the pattern (monoallelic or biallelic) of expression of TAp73 and DeltaNp73 in normal lymphocytes and lymphomas using two p73 polymorphisms. We found monoallelic expression of TAp73 in normal lymphocytes and tumors, and a selective expression of AT allele in all cases. Moreover, the quantitative expression analysis revealed DeltaNp73 over-expression in both B- and T-cell lymphomas comparing with normal lymphoid cells, suggesting a role in tumorigenesis. Finally, we have confirmed that although DeltaNp73 over-expression could be an alternative mechanism of p53 inactivation, both alterations may appear together.

p73, a sophisticated p53 family member in the cancer world

p73 belongs to a family of p53-related nuclear transcription factors that includes p53, p73 and p63. The overall structure and sequence homology indicates that a p63/p73-like protogene is the ancestral gene, whereas p53 evolved later in higher organisms. In accordance with their structural similarity, p73 functions in a manner analogous to p53 by inducing tumor cell apoptosis and participating in the cell cycle checkpoint control through transactivating an overlapping set of p53/p73-target genes. In sharp contrast to p53, however, p73 is expressed as two NH(2)-terminally distinct isoforms including transcriptionally active (TA) and transcriptionally inactive (DeltaN) forms. DeltaNp73, which has oncogenic potential, acts in a dominant negative manner against TAp73 as well as p53. p73 is induced to be stabilized in response to a subset of DNA-damaging agents in a way that is distinct from that of p53, and exerts its pro-apoptotic activity. Several lines of evidence suggest that p73 can induce tumor cell apoptosis in a p53-dependent and p53-independent manner. Some tumors exhibit resistance to the p53-dependent apoptotic program, therefore p73, which can induce apoptotic cell death by p53-independent mechanisms, is particularly useful. In this review, we discuss the regulatory mechanisms of p73 activity, and also the functional significance of p73 in the regulation of cellular processes including tumorigenesis, apoptosis and neurogenesis.

Epigenetics of cervical cancer. An overview and therapeutic perspectives

Cervical cancer remains one of the greatest killers of women worldwide. It is difficult to foresee a dramatic increase in cure rate even with the most optimal combination of cytotoxic drugs, surgery, and radiation; therefore, testing of molecular targeted therapies against this malignancy is highly desirable. A number of epigenetic alterations occur during all stages of cervical carcinogenesis in both human papillomavirus and host cellular genomes, which include global DNA hypomethylation, hypermetylation of key tumor suppressor genes, and histone modifications. The reversible nature of epigenetic changes constitutes a target for transcriptional therapies, namely DNA methylation and histone deacetylase inhibitors. To date, studies in patients with cervical cancer have demonstrated the feasibility of reactivating the expression of hypermethylated and silenced tumor suppressor genes as well as the hyperacetylating and inhibitory effect upon histone deacetylase activity in tumor tissues after treatment with demethylating and histone deacetylase inhibitors. In addition, detection of epigenetic changes in cytological smears, serum DNA, and peripheral blood are of potential interest for development of novel biomolecular markers for early detection, prediction of response, and prognosis.

Distinct CpG methylation profiles characterize different clinical groups of neuroblastic tumors

The hypermethylation of CpG islands within gene promoter regions is an epigenetic phenomenon that is often, but not always, associated with the transcriptional silencing of downstream genes and contributes to carcinogenesis. We have determined the pattern of methylation of several genes involved in distinct biological pathways, including cell proliferation and apoptosis, in neuroblastoma and in the nonmalignant ganglioneuroma. The purpose of this work was to search for epigenetic signatures that could be associated with defined clinical and biological parameters and that, in prospective, could identify specific risk categories among the patients. We have analysed 31 malignant neuroblastoma with or without MYCN amplification and 13 benign ganglioneuroma and we have observed dramatic differences in the methylation pattern of five genes (CASP8, 14.3.3sigma, DeltaN-p73, RASSF1A and DCR2) between these tumors indicating that this phenomenon is not tissue-specific and can be considered as cancer-dependent. Furthermore, the methylation pattern of 14.3.3sigma, RASSF1A and of an intragenic segment of CASP8 was significantly different between MYCN amplified and single copy neuroblastoma suggesting a specific role of epigenetic alterations in aggressive neuroblastoma.

p16(INK4a) promoter methylation and protein expression in breast fibroadenoma and carcinoma

The potential role of p16(INK4a) methylation in breast cancer is controversial whereas there are no data on fibroadenoma. To assess if inactivation of p16(INK4a) by promoter hypermethylation occurs in this hyperproliferative benign breast lesion or, on the contrary, it is strictly related to the carcinogenic process, we have tested the different histological components of 15 cases of fibroadenoma and the intraductal and infiltrating components of 15 cases of carcinoma and their adjacent non-tumoral epithelium. All samples were obtained by laser-assisted microdissection. The relationship between promoter methylation status, immunohistochemical protein expression and ki67 proliferative activity was evaluated for each lesion. Our data demonstrate that hypermethylation of p16(INK4a) promoter is a common event occurring at similar frequency in all the different histological areas of the benign and malignant breast lesions taken into exam. Conversely, protein p16 expression, although heterogeneously distributed within the section, is considerably higher in breast carcinoma as compared to fibroadenoma in both tumoral and non-tumoral epithelia and stroma. The protein localization was almost exclusively nuclear in fibroadenoma and non-tumoral epithelia whereas, in carcinoma, the staining was both nuclear and cytoplasmic or cytoplasmic alone. Furthermore, in a subset of fibroadenoma with higher proliferative activity, p16 protein expression was substantially decreased as compared to those showing lower proliferation. We did not observe this association in carcinomas. Our data demonstrate that the hypermethylation of the p16(INK4a) promoter is not specifically associated with malignancy and that, on the contrary, the overexpression of p16 and its cytoplasmic sequestration is a feature of breast carcinoma.

p73 expression is associated with the cellular radiosensitivity in cervical cancer after radiotherapy

Apoptosis is one of the causes of cell death in cervical cancer following radiotherapy. By studying the gene expression profile with cDNA apoptotic array, the p73 gene was found overexpressed in radiosensitive cervical cancers when compared with radioresistant ones. To investigate the role of the p73 gene in relation to clinical assessment of radiosensitivity in cervical cancer based on the findings of residual tumor cells in cervical biopsies after completion of radiotherapy, we studied the protein expression of p73 in 59 cervical cancers after radiotherapy and 68 normal cervices using immunohistochemistry. The expression of p73 was found to be significantly increased in cancer samples and, more importantly, in those samples sensitive to radiotherapy (P < 0.001). The overexpression of p73 actually predicted a better prognosis in cervical cancer patients (P < 0.001). To investigate the possible involvement of p73 downstream genes, the protein expressions of p21 and Bax were studied. The expression of p21, but not Bax, was found to be positively correlated with the expression of p73 (P = 0.001). Furthermore, the epigenetic regulation of p73 expression via DNA methylation was also investigated in 103 cervical cancers and 124 normals. Hypermethylation of p73 gene was observed in 38.8% of cervical cancers, and it was significantly associated with reduced or absent p73 expression (P < 0.001). Reactivation of p73 expression in two cervical cancer cell lines by demethylation treatment supported the role of methylation in the regulation of p73 expression. Our findings suggested that p73 expression was related to the radiosensitivity of cervical cancer cells and may play an important role in the regulation of cellular radiosensitivity.

Biological and clinical role of p73 in neuroblastoma

The p73 gene is a p53 homologue localized at 1p36.3, a chromosomal region frequently deleted in neuroblastoma. p73 was originally considered an oncosuppressor gene. However, it was soon realized that its mode of action did not resemble that of a classic anti-oncogene. The recent discovery of N-terminal truncated isoforms, with oncogenic properties, showed that p73 has a 'two in one' structure. Indeed, the full-length variants are strong inducers of apoptosis while the truncated isoforms inhibit the pro-apoptotic activity of p53 and of the full-length p73. This review summarizes some aspects of p73 biology with particular reference to its possible role in neuroblastoma.

Aberrant methylation of multiple genes in neuroblastic tumours. relationship with MYCN amplification and allelic status at 1p

Aberrant hypermethylation occurs in tumour cell CpG islands and is an important pathway for the repression of gene transcription in cancers. We investigated aberrant hypermethylation of 11 genes by methylation-specific polymerase chain reaction (PCR), after treatment of the DNA with bisulphite, and correlated the findings with MYCN amplification and allelic status at 1p in a series of 44 neuroblastic tumours. This tumour series includes five ganglioneuromas (G), one ganglioneuroblastoma (GN) and 38 neuroblastomas (six stage 1 tumours; five stage 2 tumours; six stage 3 cases; 19 stage 4 tumours, and two stage 4S cases). Aberrant methylation of at least one of the 11 genes studied was detected in 95% (42 of 44) of the cases. The frequencies of aberrant methylation were: 64% for thrombospondin-1 (THBS1); 30% for tissue inhibitor of metalloproteinase 3 (TIMP-3); 27% for O6-methylguanine-DNA methyltransferase (MGMT); 25% for p73; 18% for RB1; 14% for death-associated protein kinase (DAPK), p14ARF, p16INK4a and caspase 8, and 0% for TP53 and glutathione S-transferase P1 (GSTP1). No aberrant methylation was observed in four control normal tissue samples (brain and adrenal medulla). MYCN amplification was found in 11 cases (all stage 4 neuroblastomas), whereas allelic loss at 1p was identified in 16 samples (13 stage 4 and two stage 3 neuroblastomas, and one ganglioneuroma). All but one case with caspase 8 methylation also displayed MYCN amplification. Our results suggest that promoter hypermethylation is a frequent epigenetic event in the tumorigenesis of neuroblastic tumours, but no specific pattern of hypermethylated genes could be demonstrated.

Novel cell-specific and dominant negative anti-apoptotic roles of p73 in transformed leukemia cells

Although extensive homology exists between related genes p53 and p73, recent data suggest that the family members have divergent roles. We demonstrate that the differential regulatory roles of p53 family member p73 are highly cell-context and promoter-specific. Full-length p73 expressed in the transformed leukemia cell line Jurkat behaves as a specific dominant negative transcriptional repressor of the cell cycle inhibitor gene p21 and blocks p53-mediated apoptosis. These findings provide evidence for a new mechanism in oncogenesis through which the functional properties of p73 can be altered in an inheritable and cell-specific fashion independent of transcriptional coding.

Transcriptional inactivation of TP73 expression in oligodendroglial tumors

The TP73 gene, located on chromosome 1p36.3, encodes a product that shares significant structural homology with the tumor suppressor TP53. The aim of this study was to investigate whether TP73 is involved in the development of oligodendroglial tumors, which frequently carry deletions involving 1p36.3. Semi-quantitative reverse transcription-polymerase chain reaction was used to determine TP73 transcript levels. Ten of 24 (42%) tumors showed negligible to more than 5-fold reduction in TP73 expression when compared to normal brain level. To identify potential mechanisms that may modulate TP73 transcription in oligodendroglial tumors, we performed mutation analysis on the TP73 gene. No somatic mutations were however detected in the gene sequence. We then evaluated the possible involvement of epigenetic change in TP73 expression. Bisulfite genomic sequencing detected aberrant hypermethylation at the 5' region upstream and including the first exon of the TP73 gene in 17 of 44 (39%) oligodendroglial tumors, whereas normal brain tissues showed no methylation in the same region examined. Moreover, 6 of 10 (60%) tumors with negligible or decreased levels of TP73 transcripts were methylation-positive. In conclusion, our results showed that inactivation of TP73 occurs at the transcriptional level and is associated with promotor hypermethylation. Loss of or reduced TP73 transcript expression may contribute to the tumorigenesis of oligodendroglial tumors.

Concomitant inactivation of p53 and Chk2 in breast cancer

The structure and expression of the human Rad53 homologue Chk2 was analysed in breast cancer. The previously described silent polymorphism at nucleotide 252 in codon 84 (GAA>GAG) was observed in 5/141 cases. Somatic Chk2 coding mutations were detected in 7/141 cases, these occurring in 4/18 BRCA1-associated breast cancers, 1/78 sporadic breast cancers and 2/25 typical medullary carcinomas. Each of the BRCA1-associated cancers with Chk2 mutations also contained mutations in p53, whereas the single sporadic cancer with Chk2 mutation was wild-type for p53. Expression of Chk2 was ubiquitously detected in normal ductal epithelium of the breast, but there was loss of expression in a significant proportion of breast carcinomas, and this occurred in cancers both with and without p53 mutation. A CpG island was identified 5' of the Chk2 transcriptional start site, but there was no evidence of cytosine methylation in any of the cancers with down-regulated Chk2 expression. Analysis of the germ-line of 45 individuals with hereditary or early onset breast cancer revealed wild-type Chk2 sequence in all cases. Thus, despite the rarity of somatic mutations in Chk2 in sporadic breast carcinomas, our results nevertheless reveal that concomitant loss of function in Chk2 (via down-regulation of expression) and p53 (via mutation) occurs in a proportion of sporadic cases. However, consistent with other studies, we show that germ-line mutations in Chk2 are unlikely to account for a significant proportion of non BRCA1-, non BRCA2-associated hereditary breast cancers.

DNA methylation, imprinting and cancer

It is well known that a variety of genetic changes influence the development and progression of cancer. These changes may result from inherited or spontaneous mutations that are not corrected by repair mechanisms prior to DNA replication. It is increasingly clear that so called epigenetic effects that do not affect the primary sequence of the genome also play an important role in tumorigenesis. This was supported initially by observations that cancer genomes undergo changes in their methylation state and that control of parental allele-specific methylation and expression of imprinted loci is lost in several cancers. Many loci acquiring aberrant methylation in cancers have since been identified and shown to be silenced by DNA methylation. In many cases, this mechanism of silencing inactivates tumour suppressors as effectively as frank mutation and is one of the cancer-predisposing hits described in Knudson's two hit hypothesis. In contrast to mutations which are essentially irreversible, methylation changes are reversible, raising the possibility of developing therapeutics based on restoring the normal methylation state to cancer-associated genes. Development of such therapeutics will require identifying loci undergoing methylation changes in cancer, understanding how their methylation influences tumorigenesis and identifying the mechanisms regulating the methylation state of the genome. The purpose of this review is to summarise what is known about these issues.