Functional evidence for involvement of multiple putative tumor suppressor genes on the short arm of chromosome 3 in human oral squamous cell carcinogenesis

Modeling specific aneuploidies: from karyotype manipulations to biological insights

An abnormal chromosome number, or aneuploidy, underlies developmental disorders and is a common feature of cancer, with different cancer types exhibiting distinct patterns of chromosomal gains and losses. To understand how specific aneuploidies emerge in certain tissues and how they contribute to disease development, various methods have been developed to alter the karyotype of mammalian cells and mice. In this review, we provide an overview of both classic and novel strategies for inducing or selecting specific chromosomal gains and losses in human and murine cell systems. We highlight how these customized aneuploidy models helped expanding our knowledge of the consequences of specific aneuploidies to (cancer) cell physiology.

Studies of Tumor Suppressor Genes via Chromosome Engineering

The development and progression of malignant tumors likely result from consecutive accumulation of genetic alterations, including dysfunctional tumor suppressor genes. However, the signaling mechanisms that underlie the development of tumors have not yet been completely elucidated. Discovery of novel tumor-related genes plays a crucial role in our understanding of the development and progression of malignant tumors. Chromosome engineering technology based on microcell-mediated chromosome transfer (MMCT) is an effective approach for identification of tumor suppressor genes. The studies have revealed at least five tumor suppression effects. The discovery of novel tumor suppressor genes provide greater understanding of the complex signaling pathways that underlie the development and progression of malignant tumors. These advances are being exploited to develop targeted drugs and new biological therapies for cancer.

Repression of hTERT transcription by the introduction of chromosome 3 into human oral squamous cell carcinoma

Telomerase is a ribonucleoprotein enzyme that maintains telomere length. Telomerase activity is primarily attributed to the expression of telomerase reverse transcriptase (TERT). It has been reported that introduction of an intact human chromosome 3 into the human oral squamous cell carcinoma cell line HSC3 suppresses the tumorigenicity of these cells. However, the mechanisms that regulate tumorigenicity have not been elucidated. To determine whether this reduction in tumorigenicity was accompanied by a reduction in telomerase activity, we investigated the transcriptional activation of TERT in HSC3 microcell hybrid clones with an introduced human chromosome 3 (HSC3#3). HSC#3 cells showed inhibition of hTERT transcription compared to that of the parental HSC3 cells. Furthermore, cell fusion experiments showed that hybrids of HSC3 cells and cells of the RCC23 renal carcinoma cell line, which also exhibits suppression of TERT transcription by the introduction of human chromosome 3, also displayed suppressed TERT transcription. These results suggested that human chromosome 3 may carry functionally distinct, additional TERT repressor genes.

Genome stability pathways in head and neck cancers

Genomic instability underlies the transformation of host cells toward malignancy, promotes development of invasion and metastasis and shapes the response of established cancer to treatment. In this review, we discuss recent advances in our understanding of genomic stability in squamous cell carcinoma of the head and neck (HNSCC), with an emphasis on DNA repair pathways. HNSCC is characterized by distinct profiles in genome stability between similarly staged cancers that are reflected in risk, treatment response and outcomes. Defective DNA repair generates chromosomal derangement that can cause subsequent alterations in gene expression, and is a hallmark of progression toward carcinoma. Variable functionality of an increasing spectrum of repair gene polymorphisms is associated with increased cancer risk, while aetiological factors such as human papillomavirus, tobacco and alcohol induce significantly different behaviour in induced malignancy, underpinned by differences in genomic stability. Targeted inhibition of signalling receptors has proven to be a clinically-validated therapy, and protein expression of other DNA repair and signalling molecules associated with cancer behaviour could potentially provide a more refined clinical model for prognosis and treatment prediction. Development and expansion of current genomic stability models is furthering our understanding of HNSCC pathophysiology and uncovering new, promising treatment strategies.

DNA repair deficiency and BPDE-induced chromosomal alterations in CHO cells

The induction of chromosomal aberrations and sister chromatid exchanges by BPDE was evaluated in parental and different DNA repair deficient Chinese hamster ovary cell lines in order to elucidate the mechanisms involved in their induction. These included the parental line (AA8), nucleotide excision repair (UV4, UV5, UV61), base excision repair (EM9), homologous recombination repair (Irs1SF) and non-homologous end joining (V3-3) deficient ones. The ranking of different cell lines for BPDE-induced chromosome aberrations was: UV4, Irs1SF, UV5, UV 61, EM9, V3-3, and AA8 in a descending order. Cells deficient in NER and HRR were found to be very sensitive, indicating the importance of these pathways in the repair of lesions induced by BPDE. For induction of SCEs, HRR and BER deficient cells were refractory, whereas the other cell lines responded with a dose-dependent increase. The possible mechanisms involved in BPDE-induced chromosomal alterations are discussed.

Chromosomal imbalances in oral squamous cell carcinoma: examination of 31 cell lines and review of the literature

Classical and molecular cytogenetic analysis, including fluorescence in situ hybridization (FISH) and chromosomal comparative genomic hybridization (CGH), were used to examine genetic changes involved in the development and/or progression of oral squamous cell carcinoma (OSCC). Of 31 OSCC cell lines studied, more than one-third expressed clonal structural abnormalities involving chromosomes 3, 7, 8, 9, and 11. Eleven OSCC cell lines were evaluated using CGH to identify novel genome-wide gains, losses, or amplifications. By CGH, more than half of the cell lines showed loss of 3p, gain of 3q, 8q, and 20q. Further, molecular cytogenetic analyses by FISH of primary tumors showed that the karyotypes of cell lines derived from those tumors correlated with specific gains and losses in the tumors from which they were derived. The most frequent nonrandom aberration identified by both karyotype and CGH analyses was amplification of chromosomal band 11q13 in the form of a homogeneously staining region. Our data suggest that loss of 9p and 11q13 amplification may be of prognostic benefit in the management of OSCC, which is consistent with the literature. The results of this study validate the relationship between these OSCC cell lines and the tumors from which they were derived. The results also emphasize the usefulness of these cell lines as in vitro experimental models and provide important genetic information on these OSCC cell lines that were recently reported in this journal.

Identification of a tumor suppressive critical region mapping to 3p14.2 in esophageal squamous cell carcinoma and studies of a candidate tumor suppressor gene, ADAMTS9

A gene critical to esophageal cancer has been identified. Functional studies using microcell-mediated chromosome transfer of intact and truncated donor chromosomes 3 into an esophageal cancer cell line and nude mouse tumorigenicity assays were used to identify a 1.61 Mb tumor suppressive critical region (CR) mapping to chromosome 3p14.2. This CR is bounded by D3S1600 and D3S1285 microsatellite markers. One candidate tumor suppressor gene, ADAMTS9, maps to this CR. Further studies showed normal expression levels of this gene in tumor-suppressed microcell hybrids, levels that were much higher than observed in the recipient cells. Complete loss or downregulation of ADAMTS9 gene expression was found in 15 out of 16 esophageal carcinoma cell lines. Promoter hypermethylation was detected in the cell lines that do not express this gene. Re-expression of ADAMTS9 was observed after demethylation drug treatment, confirming that hypermethylation is involved in gene downregulation. Downregulation of ADAMTS9 was also found in 43.5 and 47.6% of primary esophageal tumor tissues from Hong Kong and from the high-risk region of Henan, respectively. Thus, this study identifies and provides functional evidence for a CR associated with tumor suppression on 3p14.2 and provides the first evidence that ADAMTS9, mapping to this region, may contribute to esophageal cancer development.

Methylation of the retinoid response gene TIG1 in prostate cancer correlates with methylation of the retinoic acid receptor beta gene

Methylation of CpG islands and associated gene silencing may lead to malignant progression, but the mechanisms of CpG island methylation in cancer are unknown. The tazarotene-induced gene 1 (TIG1), also known as retinoid acid (RA) receptor-responsive 1 gene was first identified as an RA-responsive gene and was shown to be downregulated in prostate cancer. Here, we show that this downregulation is caused by the methylation of the promoter and CpG island of TIG1. TIG1 was methylated in 26 of 50 (52%) primary prostate cancers, but was not methylated in normal tissues or benign hyperplasias. Three of four tumors that metastasized, five of six that were poorly differentiated and all that were assigned a Gleason score higher than 8 (7/7) were methylated in the promoter of TIG1. The samples with peripheral invasion were more frequently methylated (21/32, 66%) than tissues without peripheral invasion (5/18, 28%). In addition, Gleason 7-10 cancers (21/30, 70%) were significantly more frequently methylated compared with Gleason 4-6 cancers (4/18, 22%) (P<0.01). The retinoic acid receptor beta (RAR-beta) gene was frequently methylated as well (42/50, 84%). When TIG1 showed methylation, RAR-beta was also methylated (25/26 samples). In almost all samples where RAR-beta was not methylated, TIG1 was also in an unmethylated state (14/15 samples). The methylation of TIG1 and RAR-beta was positively correlated (r=0.35; P=0.017). It is possible that the methylation of the retinoid response gene TIG1 occurred in response to the methylation and inactivation of RAR-beta. These observations may contribute to our understanding of mechanistic events leading to CpG island methylation in cancer.

Tumor suppressor genes on chromosome 3p involved in the pathogenesis of lung and other cancers

Loss of heterozygosity (LOH) involving several chromosome 3p regions accompanied by chromosome 3p deletions are detected in almost 100% of small (SCLCs) and more than 90% of non-small (NSCLCs) cell lung cancers. In addition, these changes appear early in the pathogenesis of lung cancer and are found as clonal lesions in the smoking damaged respiratory epithelium including histologically normal epithelium as well as in epithelium showing histologic changes of preneoplasia. These 3p genetic alterations lead to the conclusion that the short arm of human chromosome 3 contains several tumor suppressor gene(s) (TSG(s)). Although the first data suggesting that 3p alterations were involved in lung carcinogenesis were published more than 10 years ago, only recently has significant progress been achieved in identifying the candidate TSGs and beginning to demonstrate their functional role in tumor pathogenesis. Some of the striking results of these findings has been the discovery of multiple 3p TSGs and the importance of tumor acquired promoter DNA methylation as an epigenetic mechanism for inactivating the expression of these genes in lung cancer. This progress, combined with the well known role of smoking as an environmental causative risk factor in lung cancer pathogenesis, is leading to the development of new diagnostic and therapeutic strategies which can be translated into the clinic to combat and prevent the lung cancer epidemic. It is clear now that genetic and epigenetic abnormalities of several genes residing in chromosome region 3p are important for the development of lung cancers but it is still obscure how many of them exist and which of the numerous candidate TSGs are the key players in lung cancer pathogenesis. We review herein our current knowledge and describe the most credible candidate genes.

Frequent 3p allele loss and epigenetic inactivation of the RASSF1A tumour suppressor gene from region 3p21.3 in head and neck squamous cell carcinoma

Studies of allelic imbalance and suppression of tumourigenicity have consistently suggested that the short arm of chromosome three (3p) harbours tumour suppressor genes (TSGs) whose inactivation leads to the development of various types of neoplasia including head and neck squamous cell carcinoma (HNSCC). Previously, we defined a critical minimal region of 120kb at 3p21.3 that contains overlapping homozygous deletions in lung and breast tumour lines and isolated eight genes from the minimal region. Mutation analysis in a large panel of lung and breast cancers revealed only rare mutations, but the majority of lung tumour lines showed loss of expression for one of the eight genes (RASSF1A) due to hypermethylation of a CpG island in the promoter region of RASSF1A. We found RASSF1A to be methylated in the majority of lung tumours, but to a lesser extent in breast and ovarian tumours. In order to define the role of 3p TSGs, in particular RASSF1A in HNSCC, we (a) analysed 43 primary HNSCC for allelic loss in regions proposed to contain 3p TSGs (3p25-26, 3p24, 3p21-22, 3p14 and 3p12), (b) analysed 24 HNSCC for evidence of RASSF1A methylation and (c) undertook mutation analysis of RASSF1A in HNSCC. We found that 81% of HNSCC showed allele loss at one or more 3p markers, 66% demonstrated loss for 3p21.3 markers and 56% showed allelic losses at 3p12 loci. Thus, 3p loss is common in HNSCC and extensive 3p loss occurs even in early stage tumours. RASSF1A promoter region hypermethylation was found in 17% (4/24) of the sporadic HNSCC, but RASSF1A mutations were not identified. Furthermore, we found RASSF1A methylation to be significantly higher in poorly differentiated then in moderate to well differentiated HNSCC (P=0.0048). Three of the four tumours showing RASSF1A methylation also underwent 3p21.3 allelic loss, hence RASSF1A behaves as a classical TSG (two hits, methylation and loss). One tumour with RASSF1A methylation had retention of markers at 3p providing further evidence of specific inactivation of RASSF1A as a critical step in some HNSCC. Although the frequency of 3p21.3 allele loss was substantially higher than that of RASSF1A methylation this does not necessarily suggest that other genes from 3p21.3 are also implicated in HNSCC, as 3p21.3 LOH was invariably found with LOH at other 3p loci. Thus, the presence of 3p21.3 allele loss without RASSF1A methylation might reflect a propensity for 3p21.3 loss to occur as a secondary consequence of large 3p deletions targeted at other 3p TSG regions. Furthermore, in the presence of homozygous inactivation of other 3p TSGs, RASSF1A haploinsufficiency might be sufficient to promote tumourigenesis in many HNSCC.

BPDE-induced lymphocytic 3p21.3 aberrations may predict head and neck carcinoma risk

BACKGROUND

Tobacco exposure is an established risk factor for head and neck squamous cell carcinoma (HNSCC). Benzo[alpha]pyrene diol expoxide (BPDE), a main metabolic product of the tobacco smoke constituent benzo[alpha]pyrene, induces chromosomal aberrations at specific loci. Chromosomal aberrations in peripheral blood lymphocytes (PBLs) induced by BPDE may reflect individuals' genetic susceptibility to tobacco carcinogens.

METHODS

This study was designed to detect BPDE-induced aberrations in PBLs at locus 3p21.3 in cultured lymphocytic cells. Our hypothesis is that the presence of BPDE-induced 3p21.3 aberrations is a biomarker of an individual's genetic susceptibility and that individuals with these aberrations are at an increased risk for HNSCC. PBL cultures from 52 cases and 54 controls were treated with 2 microM BPDE for 24 hours before the 3p21.3 aberrations were assessed by fluorescence in situ hybridization. One thousand lymphocyte interphases were scored for each sample.

RESULTS

We found that BPDE-induced chromosome 3p21.3 aberrations occurred more frequently in cases (mean: 31.4 per 1000 cells) than in controls (mean: 22.1 per 1000 cells; P < 0.001). However, when 6q27 was selected as a control locus, no such difference was observed (P = 0.545). When the 75th percentile value of induced aberrations in the controls was used as a cutoff point to classify 3p21.3 BPDE-induced sensitivity, 30 of the 52 cases (57.69%) and only 14 of the 54 controls (25.93%) were sensitive to BPDE exposure. This approach resulted in an odds ratio of 4.8 (95% confidence interval: 1.87-12.28) for HNSCC risk associated with BPDE-induced 3p21.3 aberrations. There was also a dose-response relationship between the number of BPDE-induced aberrations at 3p21.3 and risk for HNSCC.

CONCLUSIONS

The results from this study demonstrated that 3p21.3 may be a specific molecular target of tobacco carcinogens and that BPDE sensitivity at this locus may reflect an individual's genetic susceptibility to HNSCC.

Homozygous deletions on the short arm of chromosome 3 in human oral squamous cell carcinomas

Recent cytogenetic and allelic deletion analyses have demonstrated that deletions on the short arm of chromosome 3 (3p) are frequently found in various cancers, including oral squamous cell carcinomas (OSCCs). This suggests that one or more tumor suppressor gene(s) for these malignancies might be located on 3p. In the present study, to further define the region(s) on 3p that harbor putative tumor suppressor gene(s) for OSCCs, we have investigated the existence of homozygous deletions (HDs) at 34 loci on 3p, in 14 OSCC cell lines. HDs were detected within the FRA3B region at 3p14.2 in only two cell lines (HSC-4 and TSU). Recently, the human fragile histidine triad (FHIT) gene was isolated from this region, abnormalities of which have been found at high frequencies in several types of human cancers. We also examined the expression of the FHIT gene, using reverse transcription-polymerase chain reaction (RT-PCR) and exon-specific PCR, in the two OSCC cell lines which showed HDs at 3p14.2. There was no detectable expression of exon 5, which was the first protein-coding exon of FHIT gene, in HSC-4 cells, indicating that this region was homozygously deleted in this cell line. On the other hand, HD in the TSU cells did not affect the coding region of the FHIT gene, and the wild-type transcript was detected by RT-PCR. Therefore, several candidate tumor suppressor genes, including the FHIT gene, may reside in these homozygously deleted regions. To our knowledge, this is the first report of HDs on 3p in OSCCs.

Human monochromosome hybrid cell panel characterized by FISH in the JCRB/HSRRB

The human monochromosome hybrid cell panel in the Japanese Collection of Research Bioresources (JCRB) consists of 23 mouse cell clones, each containing a different human chromosome (the Y chromosome is not yet included). The panel is currently distributed by the Human Science Research Resources Bank (HSRRB) in Osaka. In order to determine the state of the human chromosomes and to supply the information to investigators, we characterized the cells by fluorescence in-situ hybridization (FISH) with corresponding human chromosome-specific painting probes, and, in part, by reverse FISH with the hybrid total DNA hybridized onto human metaphase spreads. Here, we report the frequency of intact human chromosomes maintained in each hybrid and the retained subregions of corresponding human chromosomes with relative frequencies estimated by fluorescent intensity. We used specific painted patterns to classify each hybrid into tentative types with their frequencies showing the nature of each hybrid and the state of rearrangements. This characterization will provide valuable information to investigators using the panel.

A review of inherited cancer syndromes and their relevance to oral squamous cell carcinoma

This paper examines the genetic defects associated with inherited cancer syndromes and their relevance to oral cancer. Tumour suppressor genes are now thought of as either gatekeepers or caretakers according to whether they control cell growth directly by inhibiting cell proliferation and/or promoting cell death (gatekeepers) or whether they maintain the integrity of the genome by DNA repair mechanisms (caretakers). In disorders such as xeroderma pigmentosum, ataxia telangiectasia, Bloom syndrome and Fanconi's anaemia, where there are defective caretaker genes, there is an increased incidence of second primary malignancies, including oral cancer. By contrast, with the exception of Li Fraumeni syndrome, abnormalities of gatekeeper genes do not predispose to oral cancer. Not only do Li Fraumeni patients develop second primary malignancies, but defects of the p53 pathway (p53 mutation, MDM2 over-expression, CDKN2A deletion) appear to be a ubiquitous feature of sporadic oral cancer as it occurs in the West. The findings suggest that genetic instability is of fundamental importance in the pathogenesis of oral cancer.

Comparative genomic hybridization analysis detects frequent over-representation of DNA sequences at 3q, 7p, and 8q in head and neck carcinomas

Comparative genomic hybridization (CGH) was used to identify chromosomal imbalances in 19 samples of squamous cell carcinoma of the head and neck (HNSCC). The chromosome arms most often over-represented were 3q (48%), 8q (42%), and 7p (32%); in many cases, these changes were observed at high copy number. Other commonly over-represented sites were 1q, 2q, 6p, 6q, and 18q. The most frequently under-represented segments were 3p and 22q. Loss of heterozygosity of two polymorphic microsatellite loci from chromosome 22 was observed in two tongue tumors, in agreement with the CGH analysis. Gains of 1q and 2q material were detected in patients exhibiting a clinical history of recurrence and/or metastasis followed by terminal disease. This association suggests that gain of 1q and 2q may be a new marker of head and neck tumors with a refractory clinical response.

Losses of heterozygosity in oral and oropharyngeal epithelial carcinomas

We analyzed 25 oral and oropharyngeal epithelial carcinomas for loss of heterozygosity (LOH) and microsatellite instability by using 55 oligonucleotide repeat markers located in 45 chromosomal regions. The aim was to identify which chromosomal regions and tumor-suppressor genes (TSGs) are preferentially lost in these tumors and to relate LOH at specific loci to clinicopathologic data. The analysis was performed on tumor tissue and on a corresponding normal tissue (blood lymphocytes) with the use of the polymerase chain reaction technique followed by microsatellite allele separation with denaturing gel electrophoresis. Thirty-two of 45 chromosomal regions demonstrated a significant (>/=20%) incidence of LOH. An allelic loss of >/=50% was found in 9p21 (77.8%), 8p22-23 (70%), 3p12 (61.5%), 1p36.1 and 12q22 (60%), 3q28 (57.1%), 5q23.3 (54.5%), 3p25-26, 3p24, and 7q35 (50%). We did not find any microsatellite instability. Our results suggest that in addition to a group of TSGs, pleiotropic for several tumor types, other suppressor genes are specifically involved in oral and oropharyngeal carcinogenesis.