RASSF1A is a target tumor suppressor from 3p21.3 in nasopharyngeal carcinoma

Activator protein-1 involved in growth inhibition by RASSF1A gene in the human gastric carcinoma cell line SGC7901

AIM: To investigate the role of Ras association domain family protein 1 isoform A (RASSF1A) in gastric tumorigenesis.

METHODS: Through over-expression of RASSF1A gene in the SGC7901 cell line which was induced by a lipofectamine-mediated gene transfer approach. Activator protein-1 (AP-1) DNA binding activity was measured by electrophoretic mobility shift assay (EMSA).

RESULTS: Compared with the control clones, cells over-expressing RASSF1A exhibited significant inhibition of cell growth with G₁ cell cycle arrest in vitro and in vivo. The over-expression of RASSF1A significantly inhibited AP-1 activity in SGC7901 cells (0.981 ± 0.011 vs 0.354 ± 0.053, P < 0.001). In addition, both Western blot analysis and immunocytochemistry demonstrated that RASSF1A down-regulated the expression of c-Fos (0.975 ± 0.02 vs 0.095 ± 0.024, P < 0.001) but not c-Jun.

CONCLUSION: Over-expression of RASSF1A inhibits the growth of SGC7901 cells by negatively regulating the AP-1 activity, the latter in turn negatively signals cell proliferation.

Epigenetic inactivation of the deleted in lung and esophageal cancer 1 gene in nasopharyngeal carcinoma

Deletion of the short arm of chromosome 3 is a common event in nasopharyngeal carcinoma (NPC), suggesting that one or more tumor suppressor genes at 3p are involved in this cancer. DLEC1, Deleted in Lung and Esophageal Cancer 1, located at 3p22.2, was recently identified as a candidate tumor suppressor gene in lung, esophageal, and renal cancers. In this study, we investigated the involvement of DLEC1 in the development of NPC. Down-regulation of DLEC1 and promoter hypermethylation were observed in all NPC cell lines and xenografts but not in normal nasopharyngeal epithelial cells. Promoter hypermethylation of DLEC1 was also detected in 30 of 42 (71%) NPC primary tumors. Treatment of NPC cell lines with demethylating agent or histone deacetylase inhibitor resulted in restoration of DLEC1 expression. Overexpression of DLEC suppressed growth and reduced invasiveness of NPC cells. Furthermore, the tumorigenic potential of DLEC1 expressing NPC cells was highly reduced in nude mice. Taken together, our results strongly suggest that silencing of DLEC1 expression by promoter hypermethylation and histone deacetylation may be important in NPC tumorigenesis.

Reduced expression of RASSF1A in esophageal and nasopharyngeal carcinomas significantly correlates with tumor stage

Reduced expression or loss of tumor suppressor genes play a key role in many cancers. In this study, we investigated the role of RASSF1A in the pathogenesis of esophageal squamous cell carcinoma (ESCC) and nasopharyngeal carcinoma (NPC). We detected the down-regulated expression of both RASSF1A transcripts and protein in tumor tissues using RT-PCR and tissue microarray immunohistochemical staining analyses. Down-regulated expression of RASSF1A showed a significant association with WHO grade, tumor status, and lymph node metastasis, showing its possible utility as a biomarker for clinical specimens.

Mahanine reverses an epigenetically silenced tumor suppressor gene RASSF1A in human prostate cancer cells

It is becoming clear that frequent epigenetic silencing of tumor suppressor genes could be responsible for the development of cancer in various organs. Several recent reports suggest that suppression of RASSF1A is associated with the advanced grade and stage of prostate cancer and many other cancers. In this investigation, we demonstrated that, mahanine, a plant derived carbazole alkaloid, induced RASSF1A expression in both androgen-responsive (LNCaP) and androgen-negative (PC3) prostate cancer cells by inhibiting DNA methyltransferase (DNMT) activity. Mahanine-induced expression of RASSF1A in turn significantly reduced cyclin D1 but not other cyclins. To understand the inverse relationship between RASSF1A and cyclin D1, we observed that mahanine treatment down-regulates cyclin D1 and addition of RASSF1A siRNA prevented this inhibition. This study show for the first time that mahanine can reverse an epigenetically silenced gene, RASSF1A in prostate cancer cells by inhibiting DNMT activity that in turn down-regulates a key cell cycle regulator, cyclin D1. Mahanine therefore, promises an encouraging therapeutic choice for advanced prostatic cancer.

Analysis of phenotype-genotype connection: the story of dissecting disease pathogenesis in genomic era in China, and beyond

DNA is the ultimate depository of biological complexity. Thus, in order to understand life and gain insights into disease pathogenesis, genetic information embedded in the sequence of DNA base pairs comprising chromosomes should be deciphered. The stories of investigating the association between phenotype and genotype in China and other countries further demonstrate that genomics can serve as a probe for disease biology. We now know that in Mendelian disorders, one gene is not only a dictator of one phenotype but also a dictator of two or more distinct disorders. Dissecting genetic abnormalities of complex diseases, including diabetes, hypertension, mental diseases, coronary heart disease and cancer, may unravel the complicated networks and crosstalks, and help to simplify the complexity of the disease. The transcriptome and proteomic analysis for medicine not only deepen our understanding of disease pathogenesis, but also provide novel diagnostic and therapeutic strategies. Taken together, genomic research offers a new opportunity for determining how diseases occur, by taking advantage of experiments of nature and a growing array of sophisticated research tools to identify the molecular abnormalities underlying disease processes. We should be ready for the advent of genomic medicine, and put the genome into the doctors' bag, so that we can help patients to conquer diseases.

The Ras-association domain family (RASSF) members and their role in human tumourigenesis

Ras proteins play a direct causal role in human cancer with activating mutations in Ras occurring in approximately 30% of tumours. Ras effectors also contribute to cancer, as mutations occur in Ras effectors, notably B-Raf and PI3-K, and drugs blocking elements of these pathways are in clinical development. In 2000, a new Ras effector was identified, RAS-association domain family 1 (RASSF1), and expression of the RASSF1A isoform of this gene is silenced in tumours by methylation of its promoter. Since methylation is reversible and demethylating agents are currently being used in clinical trials, detection of RASSF1A silencing by promoter hypermethylation has potential clinical uses in cancer diagnosis, prognosis and treatment. RASSF1A belongs to a new family of RAS effectors, of which there are currently 8 members (RASSF1-8). RASSF1-6 each contain a variable N-terminal segment followed by a Ras-association (RA) domain of the Ral-GDS/AF6 type, and a specialised coiled-coil structure known as a SARAH domain extending to the C-terminus. RASSF7-8 contain an N-terminal RA domain and a variable C-terminus. Members of the RASSF family are thought to function as tumour suppressors by regulating the cell cycle and apoptosis. This review will summarise our current knowledge of each member of the RASSF family and in particular what role they play in tumourigenesis, with a special focus on RASSF1A, whose promoter methylation is one of the most frequent alterations found in human tumours.

Evaluation of the 3p21.3 tumour-suppressor gene cluster

Deletions of the 3p21.3 region are a frequent and early event in the formation of lung, breast, kidney and other cancers. Intense investigation of allelic losses and the discovery of overlapping homozygous deletions in lung and breast tumour-cell lines have defined a minimal critical 120 kb deletion region containing eight genes and likely to harbor one or more tumour-suppressor genes (TSGs). The candidate genes are HYAL2, FUS1, Ras-associated factor 1 (RASSF1), BLU/ZMYND10, NPR2L, 101F6, PL6 and CACNA2D2. Recent research indicates that several of these genes can suppress the growth of lung and other tumour cells. Furthermore, some genes (RASSF1A and BLU/ZMYND10) are very frequently inactivated by non-classical mechanisms such as promoter hypermethylation resulting in loss of expression. These data indicate that the 120 kb critical deletion region at 3p21.3 may represent a TSG cluster with preferential inactivation of particular genes depending on tumour type. The eight genes within this region and their potential role in cancer will be the focus of this review.

Nasopharyngeal carcinoma: molecular pathogenesis and therapeutic developments

Nasopharyngeal carcinoma (NPC) is a prevalent tumour in southern China and southeast Asia, particularly in the Cantonese population, where its incidence has remained high for decades. Recent studies have demonstrated that the aetiology of NPC is complex, involving multiple factors including genetic susceptibility, infection with the Epstein-Barr virus (EBV) and exposure to chemical carcinogens. During development of the disease, viral infection and multiple somatic genetic and epigenetic changes synergistically disrupt normal cell function, thus contributing to NPC pathogenesis. NPC is highly radiosensitive and chemosensitive, but treatment of patients with locoregionally advanced disease remains problematic. New biomarkers for NPC, including EBV DNA copy number or methylation of multiple tumour suppressor genes, which can be detected in serum and nasopharyngeal brushings, have been developed for the molecular diagnosis of this tumour. Meanwhile, new therapeutic strategies such as intensity-modulated radiation therapy and immuno- and epigenetic therapies might lead to more specific and effective treatments.

Positional expression profiling indicates candidate genes in deletion hotspots of hepatocellular carcinoma

Molecular characterizations of hepatocellular carcinoma have indicated frequent allelic losses on chromosomes 4q, 8p, 16q and 17p, where the minimal deleted regions have been further defined on 4q12-q23, 4q31-q35, 8p21-p22, 16q12.1-q23.1 and 17p13. Despite these regions are now well-recognized in early liver carcinogenesis, few underlying candidate genes have been identified. In an effort to define affected genes within common deleted loci of hepatocellular carcinoma, we conducted transcriptional mapping by high-resolution cDNA microarray analysis. In 20 hepatocellular carcinoma cell lines and 20 primary tumors studied, consistent downregulations of novel transcripts were highlighted throughout the entire genome and within sites of frequent losses. The array-derived candidates including fibrinogen gamma peptide (FGG, at 4q31.3), vitamin D binding protein (at 4q13.3), fibrinogen-like 1 (FGL1, at 8p22), metallothionein 1G (MT1G, at 16q12.2) and alpha-2-plasmin inhibitor (SERPINF2, at 17p13) were confirmed by quantitative reverse transcription-polymerase chain reaction, which also indicated a more profound downregulation of FGL1, MT1G and SERPINF2 relative to reported tumor-suppressor genes, such as DLC1 (8p22), E-cadherin (16q22.1) and TP53 (17p13.1). In primary hepatocellular carcinoma examined, a significant repression of MT1G by more than 100-fold was indicated in 63% of tumors compared to the adjacent nonmalignant liver (P = 0.0001). Significant downregulations of FGG, FGL1 and SERPINF2 were also suggested in 30, 23 and 33% of cases, respectively, compared to their nonmalignant counterparts (P < 0.016). In summary, transcriptional mapping by microarray indicated a number of previously undescribed downregulated genes in hepatocellular carcinoma, and highlighted potential candidates within common deleted regions.

Functional analysis of lung tumor suppressor activity at 3p21.3

The early and frequent occurrence of deletions at 3p21.3 in lung cancer has led to the consideration of this chromosomal region as a lung cancer (LUCA) critical region with tumor suppressor activity. We covered this 19 genes-containing region with overlapping P1 artificial chromosomes (PACs), in which genes are likely accompanied by their own promoters or other regulatory sequences. With these PACs we transfected cells from a small cell lung cancer (SCLC) cell line which readily caused tumors in nude mice. Per PAC we selected two cell clones with a low number of PAC copies integrated at a single genomic site. The selected clones were s.c. injected into nude mice to investigate whether the integrated genes suppressed the tumor-inducing capacity of the original SCLC cell line. We could demonstrate PAC-specific gene expression in the transfected cells. All of the PAC integration sites were different. It appeared that introduction of a PAC or even an empty PAC vector causes some chromosomal instability, which in principle may either promote or inhibit cell growth. However, both cell clones with integration of the same PAC from the centromeric part of the LUCA region in different genomic sites were the sole pair of clones that caused smaller tumors than did the original SCLC cell line. This suggests that rather than the induced chromosomal instability, the DNA sequence of that PAC, which in addition to two protein-encoding genes contains at least one potential miRNA gene, is responsible for the tumor suppressor activity.

Latent membrane protein 1 suppresses RASSF1A expression, disrupts microtubule structures and induces chromosomal aberrations in human epithelial cells

Epstein-Barr virus (EBV) infection is closely associated with nasopharyngeal carcinoma (NPC) and can be detected in early premalignant lesions of nasopharyngeal epithelium. The latent membrane protein 1 (LMP1) is an oncoprotein encoded by the EBV and is believed to play a role in transforming premalignant nasopharyngeal epithelial cells into cancer cells. RASSF1A is a tumor-suppressor gene commonly inactivated in many types of human cancer including NPC. In this study, we report a novel function of LMP1, in down-regulating RASSF1A expression in human epithelial cells. Downregulation of RASSF1A expression by LMP1 is dependent on the activation of intracellular signaling of NF-kappaB involving the C-terminal activating regions (CTARs) of LMP1. LMP1 expression also suppresses the transcriptional activity of the RASSF1A core promoter. RASSF1A stabilizes microtubules and regulates mitotic events. Aberrant mitotic spindles and chromosome aberrations are reported phenotypes in RASSF1A inactivated cells. In this study, we observed that LMP1 expression in human epithelial cells could induce aberrant mitotic spindles, disorganized interphase microtubules and aneuploidy. LMP1 expression could also suppress microtubule dynamics as exemplified by tracking movements of the growing tips of microtubules in live cells by transfecting EGFP-tagged EB1 into cells. The aberrant mitotic spindles and interphase microtubule organization induced by LMP1 could be rescued by transfecting RASSF1A expression plasmid into cells. Downregulation of RASSF1A expression by LMP1 may facilitate its role in transformation of premalignant nasopharyngeal epithelial cells into cancer cells.

Molecular and cytogenetic changes involved in the immortalization of nasopharyngeal epithelial cells by telomerase

Nasopharyngeal carcinoma (NPC) is a common disease in Hong Kong and southern provinces of China. EBV infection is believed to play a critical role in the development of NPC. Previous studies on the transformation mechanism of EBV genes were mostly performed in either NPC or nonnasopharyngeal epithelial cells which may not be representative of premalignant nasopharyngeal epithelial cells. Establishment of a representative cell system would greatly facilitate the elucidation of the role of EBV infection in the development of NPC. Using telomerase alone, we were able to establish an immortalized nasopharyngeal epithelial cell line from primary nonmalignant nasopharyngeal biopsies. The telomerase-immortalized nasopharyngeal epithelial cells are largely diploid in karyotype. Interestingly, this newly immortalized nasopharyngeal epithelial cell line, referred as NP460hTert, harbors genetic alterations previously identified in premalignant and malignant nasopharyngeal epithelial cells. These include inactivation of p16 by homozygous deletion of the p16(INK4A) locus and downregulation of RASSF1A expression. The deletion of the p16(INK4A) locus appears to be the most crucial event for the immortalization of nasopharyngeal epithelial cells by telomerase and precedes RASSF1A downregulation. In addition, detailed analysis of the cytogenetic changes by conventional cytogenetics, spectral karyotyping (SKY) and array-based CGH revealed a gain of a 17q21-q25 fragment on 11p15 chromosome in all NP460hTert cells which occurred before deletion of the p16(INK4A) locus. Gain of 17q has been previously reported in NPC. In addition, activation of NF-kappaB was observed in immortalized NP460hTert cells at the later population doublings, and may play a role in the survival of immortalized NP epithelial cells. Id1 which is commonly expressed in various human cancers, including NPC, was also upregulated in the immortalized NP460hTert cells. Thus, the establishment of an immortalized nasopharyngeal epithelial cell line harboring common genetic alterations present in premalignant and cancerous nasopharyngeal epithelial cells may provide a valuable cell system to examine for early events involved in NPC carcinogenesis, particularly in elucidating the role of EBV infection in NPC development.

Expression of candidate chromosome 3p21.3 tumor suppressor genes and down-regulation of BLU in some esophageal squamous cell carcinomas

The expression of six chromosome 3p21.3 candidate tumor suppressor genes (BLU, FUS2, HYAL2, NPRL2, RASSF1A, and SEMA3B) in esophageal squamous cell carcinoma (ESCC) has been investigated. Reduced expression of BLU was detected in some ESCC cell lines and tumor tissues and the difference was quantitated by real-time quantitative polymerase chain reaction. Methylation specific-PCR revealed the down-regulation of BLU by epigenetic inactivation. However, exogenous expression of BLU did not functionally suppress tumorigenicity in nude mice. These results suggest that over-expression of BLU alone is not sufficient to inhibit tumorigenicity. Further studies on BLU interacting proteins are required to elucidate the possible role of BLU in the development of ESCC.

14-3-3sigma, a p53 regulator, suppresses tumor growth of nasopharyngeal carcinoma

The 14-3-3sigma gene product, up-regulated by p53 in response to DNA damage, is involved in cell-cycle checkpoint control and is a human cancer epithelial marker down-regulated in various tumors. However, its role and function have not been established in nasopharyngeal carcinoma (NPC), a tumor of epithelial origin. Recently, we found that 14-3-3sigma interacts with p53 in response to DNA damage and stabilizes the expression of p53. In addition, we also showed that overexpression of 14-3-3sigma inhibits oncogene-activated tumorigenicity. In the present study, we investigated the tumor-suppressive role of 14-3-3sigma in NPC cells. We found that there is a failure to up-regulate 14-3-3sigma in response to DNA damage in two NPC cell lines that have p53 mutation. We also found that 14-3-3sigma interacted with protein kinase B/Akt and negatively regulated the activity of Akt. Overexpression of 14-3-3sigma inhibited NPC cell growth and blocks DNA synthesis. Overexpression of 14-3-3sigma also led to inhibition of anchorage-independent growth of NPC cells. In addition, we found that 14-3-3sigma sensitized NPC cells to apoptosis induced by the chemotherapeutic agent 2-methoxyestradiol. Overexpression of 14-3-3sigma in both NPC cell lines reduced the tumor volume in nude mice, which could have significance for clinical application. These findings provide an insight into the roles of 14-3-3sigma in NPC and suggest that approaches that modulate 14-3-3sigma activity may be useful in the treatment of NPC.

Frequent hypermethylation of RASSF1A and TSLC1, and high viral load of Epstein-Barr Virus DNA in nasopharyngeal carcinoma and matched tumor-adjacent tissues

We examined the promoter hypermethylation of tumor-suppressor genes RASSF1A and TSLC1, quantitated EBV DNA load in nasopharyngeal carcinoma (NPC) tissues (T tissues), and matched tumor-adjacent tissues outside 0.5 cm (P tissues) and outside 1.0 cm (Z tissues) to evaluate the role of promoter hypermethylation of RASSF1A and TSLC1 as well as viral load in the pathogenesis of NPC. Methylation-specific polymerase chain reaction (PCR) for RASSF1A and TSLC1 and quantitative real-time PCR analysis of EBV DNA were performed on matched T, P, and Z tissues (n = 28) as well as chronic nasopharyngitis tissues (n = 8). Hypermethylated RASSF1A was frequently detected in the T (82%) and P tissues (75%), but less frequently in Z tissues (46%). he average quantities of EBV DNA (copies/microg DNA) in matched T, P, and Z tissues were 673,000, 90,000, and 7000. The differences of promoter hypermethylation of RASSF1A and EBV viral load among T, P, and Z tissues were statistically significant, with more frequent methylation and higher viral load detected when tissues examined were nearer to the NPC tissues. Our results suggest that aberrant hypermethylation of RASSF1A and high EBV load might be important events in NPC pathogenesis, and they may be useful molecular diagnostic markers for this cancer.

Chromosomal imbalances in nasopharyngeal carcinoma: a meta-analysis of comparative genomic hybridization results

Nasopharyngeal carcinoma (NPC) is a highly prevalent disease in Southeast Asia and its prevalence is clearly affected by genetic background. Various theories have been suggested for its high incidence in this geographical region but to these days no conclusive explanation has been identified. Chromosomal imbalances identifiable through comparative genomic hybridization may shed some light on common genetic alterations that may be of relevance to the onset and progression of NPC. Review of the literature, however, reveals contradictory results among reported findings possibly related to factors associated with patient selection, stage of disease, differences in methodological details etc. To increase the power of the analysis and attempt to identify commonalities among the reported findings, we performed a meta-analysis of results described in NPC tissues based on chromosomal comparative genomic hybridization (CGH). This meta-analysis revealed consistent patters in chromosomal abnormalities that appeared to cluster in specific "hot spots" along the genome following a stage-dependent progression.

Functional studies of the chromosome 3p21.3 candidate tumor suppressor geneBLU/ZMYND10 in nasopharyngeal carcinoma

Chromosome 3p plays an important role in tumorigenesis in many cancers, including nasopharyngeal carcinoma (NPC). We have previously shown chromosome 3p can suppress tumor growth in vivo by using the monochromosome transfer approach, which indicated the chromosome 3p21.3 region was critical for tumor suppression. BLU/ZMYND10 is one of the candidate tumor suppressor genes mapping in the 3p21.3 critical region and is a candidate TSG for NPC. By quantitative RT-PCR, it is frequently downregulated in NPC cell lines (83%) and NPC biopsies (80%). However, no functional studies have yet verified the functional role of BLU/ZMYND10 as a tumor suppressor gene. In the current study, a gene inactivation test (GIT) utilizing a tetracycline regulation system was used to study the functional role of BLU/ZMYND10. When BLU/ZMYND10 is expressed in the absence of doxycycline, the stable transfectants were able to induce tumor suppression in nude mice. In contrast, downregulation of BLU/ZMYND10 in these tumor suppressive clones by doxycycline treatment restored the tumor formation ability. This study provides the first significant evidence to demonstrate BLU/ZMYND10 can functionally suppress tumor formation in vivo and is, therefore, likely to be one of the candidate tumor suppressor genes involved in NPC.

Effect of demethylating agent 5-Aza-2'-deoxycytidine on human colonic carcinoma cell line SW48

AIM: To explore the effect of demethylating agent 5-Aza-2'-deoxycytidine (5-Aza-CdR) on the growth of human colonic carcinoma cell line SW48, and to investigate the possibility of its application in clinical treatment.

METHODS: Human colonic carcinoma SW48 cells were treated with 0.4, 1.6, 6.4, 25.8, and 102.4 µmol/L 5-Aza-CdR, respectively. Then the growth of the cells was observed by MTT assay. The cell cycle and apoptosis were analyzed by flow cytometry. The expression of the tumor suppressor gene RASSF1A mRNA was detected by reverse transcription polymerase chain reaction (RT-PCR).

RESULTS: 5-Aza-CdR inhibited the proliferation of SW48 cells in a time- and concentration-dependent manner (1-5 d, 0.4-102.4 µmol/L). After 5-Aza-CdR treatment, the number of G0/G1 cells was increased, and 5-Aza-CdR blocked the cell cycle at G1 phase. The apoptotic rate was also increased significantly. RASSF1A gene was reactivated by 5-Aza-CdR in SW48 cells not expressing RASSF1A.

CONCLUSION: 5-Aza-CdR can inhibit the growth, and promote the apoptosis of SW48 cells by eliminating the methylation status of RASSF1A promoter.

Identification of RASSF1A modulated genes in nasopharyngeal carcinoma

RASSF1A is a tumor suppressor gene on 3p21.3 frequently inactivated by promoter hypermethylation in nasopharyngeal carcinoma (NPC). To identify RASSF1A target genes in NPC, we have investigated the expression profile of the stable RASSF1A transfectants and controls by high-density oligonucleotide array. A total of 57 genes showed differential expression in the RASSF1A-expressing cells. These RASSF1A target genes were involved in multiple cellular regulatory processes such as transcription, signal transduction, cell adhesion and RNA processing. The RASSF1A-modulated expression of eight selected genes with the highest fold changes (ATF5, TCRB, RGS1, activin betaE, HNRPH1, HNRPD, Id2 and CKS2) by RASSF1A was confirmed in both stable and transient transfectants. Compared with the RASSF1A transfectants, an inverse expression pattern of activin betaE, Id2 and ATF5 was shown in the immortalized nasopharyngeal epithelial cells treated with siRNA against RASSF1A. The findings imply that the expression of activin betaE, Id2 and ATF5 was tightly regulated by RASSF1A and may associate with its tumor suppressor function. Strikingly, overexpression of Id2 is common in NPC and RASSF1A-induced repression of Id2 was mediated by the overexpression of activin betaE. The results suggest a novel RASSF1A pathway in which both activin betaE and Id2 are involved.

Role of the Ras-association domain family 1 tumor suppressor gene in human cancers

In recent years, the list of tumor suppressor genes (or candidate TSG) that are inactivated frequently by epigenetic events rather than classic mutation/deletion events has been growing. Unlike mutational inactivation, methylation is reversible and demethylating agents and inhibitors of histone deacetylases are being used in clinical trails. Highly sensitive and quantitative assays have been developed to assess methylation in tumor samples, early lesions, and bodily fluids. Hence, gene silencing by promoter hypermethylation has potential clinical benefits in early cancer diagnosis, prognosis, treatment, and prevention. The hunt for a TSG located at 3p21.3 resulted in the identification of the RAS-association domain family 1, isoform A gene (RASSF1A). RASSF1A falls into the category of genes frequently inactivated by methylation rather than mutational events. This gene is silenced and frequently inactivated by promoter region hypermethylation in many adult and childhood cancers, including lung, breast, kidney, gastric, bladder, neuroblastoma, medulloblastoma, gliomas and it has homology to a mammalian Ras effector (i.e., Nore1). RASSF1A inhibits tumor growth in both in vitro and in vivo systems, further supporting its role as a TSG. We and others identified the gene in 2000, but already there are over a 150 publications demonstrating RASSF1A methylation in a large number of human cancers. Many laboratories including ours are actively investigating the biology of this novel protein family. Thus far, it has been shown to play important roles in cell cycle regulation, apoptosis, and microtubule stability. This review summarizes our current knowledge on genetic, epigenetic, and functional analysis of RASSF1A tumor suppressor gene and its homologues.

Methylation of the Tumor Suppressor Gene RASSF1A in Human Tumors

Loss of heterozygosity of a segment at 3p21.3 is frequently observed in lung cancer and several other carcinomas. We have identified the Ras-association domain family 1A gene (RASSF1A), which is localized at 3p21.3 in a minimum deletion sequence. De novo methylation of the RASSF1A promoter is one of the most frequent epigenetic inactivation events detected in human cancer and leads to silencing of RASSF1A expression. Hypermethylation of RASSF1A was frequently found in most major types of human tumors including lung, breast, prostate, pancreas, kidney, liver, cervical, thyroid and many other cancers. The detection of RASSF1A methylation in body fluids such as serum, urine, and sputum promises to be a useful marker for early cancer detection. The functional analysis of RASSF1A reveals a potential involvement of this protein in apoptotic signaling, microtubule stabilization, and cell cycle progression.

Loss of heterozygosity in multistage carcinogenesis of esophageal carcinoma at high-incidence area in Henan Province, China

AIM: Microsatellites are the repeated DNA sequences scattered widely within the genomes and closely linked with many important genes. This study was designed to characterize the changes of microsatellite DNA loss of heterozygosity (LOH) in esophageal carcinogenesis.

METHODS: Allelic deletions in 32 cases of matched precancerous, cancerous and normal tissues were examined by syringe microdissection under an anatomic microscope and microsatellite polymorphism analysis using 15 polymorphic markers on chromosomes 3p, 5q, 6p, 9p, 13q, 17p, 17q and 18q.

RESULTS: Microsatellite DNA LOH was observed in precancerous and cancerous tissues, except D9S1752. The rate of LOH increased remarkably with the lesions progressed from basal cell hyperplasia (BCH) to squamous cell carcinoma (SCC) (P<0.05). Three markers, D9S171, D13S260 and TP53, showed the highest incidence of LOH (>60%). LOH loci were different in precancerous and cancerous tissues. LOH in D3S1234 and TP53 was the common event in different lesions from the same patients.

CONCLUSION: Microsatellite DNA LOH occurs in early stage of human esophageal carcinogenesis, even in BCH. With the lesion progressed, gene instability increases, the accumulation of this change may be one of the important mechanisms driving precancerous lesions to cancer.

Tumor suppressive role of a 2.4 Mb 9q33-q34 critical region and DEC1 in esophageal squamous cell carcinoma

The key genes involved in the development of esophageal squamous cell carcinoma (ESCC) remain to be elucidated. Previous studies indicate extensive genomic alterations occur on chromosome 9 in ESCC. Using a monochromosome transfer approach, this study provides functional evidence and narrows down the critical region (CR) responsible for chromosome 9 tumor suppressing activity to a 2.4 Mb region mapping to 9q33-q34 between markers D9S1798 and D9S61. Interestingly, a high prevalence of allelic loss in this CR is also observed in primary ESCC tumors by microsatellite typing. Allelic loss is found in 30/34 (88%) tumors and the loss of heterozygosity (LOH) frequency ranges from 67 to 86%. Absent to low expression of a 9q32 candidate tumor suppressor gene (TSG), DEC1 (deleted in esophageal cancer 1), is detected in four Asian ESCC cell lines. Stably expressing DEC1 transfectants provide functional evidence for inhibition of tumor growth in nude mice and DEC1 expression is decreased in tumor segregants arising after long-term selection in vivo. There is 74% LOH in the DEC1 region of ESCC primary tumors. This study provides the first functional evidence for the presence of critical tumor suppressive regions on 9q33-q34. DEC1 is a candidate TSG that may be involved in ESCC development.

3p21.3 tumor suppressor cluster: prospects for translational applications

Chromosomal abnormalities at the 3p21.3 region, including homozygous deletions and loss of heterozygosity and expressional deficiencies in 3p21.3 genes including transcriptional silences by promoter hypermethylation, altered mRNA splicing and aberrant transcripts, and lost or defect protein translation and post-translational modifications, are frequently found in most human cancers. Inactivation of 3p21.3 genes in primary tumors affects a wide spectrum of key biological processes such as cell proliferation, cell cycle kinetics, signaling transduction, ion exchange and transportation, apoptosis and cell death, and demonstrates the molecular signatures of carcinogenesis. Restoration of defective 3p21.3 genes with several wild-type 3p21.3 genes suppresses tumor cell growth both in vitro and in vivo. These findings suggest several 3p21.3 genes as potential tumor suppressors and implicates these 3p21.3 genes for future development as biomarkers for the early detection and diagnosis of cancer, and as prognostic and therapeutic tools for cancer prevention and molecular cancer therapy.

Les carcinomes du nasopharynx : de la biologie à la clinique

Nasopharyngeal carcinomas (NPC) are very different from other head and neck cancers because of their specific multifactorial etiology and their geographic distribution. Epstein-Barr Virus (EBV) is implicated in oncogenesis of NPC in association with genetic alterations such as inactivation of the p16/Ink4, p19/ARF, RASSF1 or Blu genes. Tumoral tissues include a very abundant characteristic lymphoid infiltrate. Inflammatory cytokines are produced by both malignant and infiltrating cells. There is no efficient immune response against the tumor. On the opposite, infiltrating lymphocytes might play a role in tumor development. Serological methods and detection of circulating viral DNA are expected to become useful for early detection of relapse and on a longer term for primary screening. NPC are often diagnosed at a late stage because patients may remain asymptomatic for a long time. Computed tomography (CT scan) and magnetic resonance imaging (MRI) are complementary for the initial evaluation. Positron emission tomography (PET) is efficient for the evaluation of treatment efficiency and detection of relapses. Treatment is based on radiotherapy and chemotherapy. Their optimal use needs to be evaluated by phase III trials but positive results have been obtained by concomitant association of radiotherapy and chemotherapy. Targeted therapies are being studied with strategies based on disruption of viral latency, use of replicative adenoviruses or anti-tumor vaccination.