Chromosome 7 suppresses indefinite division of nontumorigenic immortalized human fibroblast cell lines KMST-6 and SUSM-1

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.

GNG11 (G-protein subunit γ 11) suppresses cell growth with induction of reactive oxygen species and abnormal nuclear morphology in human SUSM-1 cells

Enforced expression of GNG11, G-protein subunit γ 11, induces cellular senescence in normal human diploid fibroblasts. We here examined the effect of the expression of GNG11 on the growth of immortalized human cell lines, and found that it suppressed the growth of SUSM-1 cells, but not of HeLa cells. We then compared these two cell lines to understand the molecular basis for the action of GNG11. We found that expression of GNG11 induced the generation of reactive oxygen species (ROS) and abnormal nuclear morphology in SUSM-1 cells but not in HeLa cells. Increased ROS generation by GNG11 would likely be caused by the down-regulation of the antioxidant enzymes in SUSM-1 cells. We also found that SUSM-1 cells, even under normal culture conditions, showed higher levels of ROS and higher incidence of abnormal nuclear morphology than HeLa cells, and that abnormal nuclear morphology was relevant to the increased ROS generation in SUSM-1 cells. Thus, SUSM-1 and HeLa cells showed differences in the regulation of ROS and nuclear morphology, which might account for their different responses to the expression of GNG11. Thus, SUSM-1 cells may provide a unique system to study the regulatory relationship between ROS generation, nuclear morphology, and G-protein signaling.

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.

Telomerase promotes efficient cell cycle kinetics and confers growth advantage to telomerase-negative transformed human cells

Constitutive telomerase activity maintains telomere length and confers immortal phenotypes to human cancers. The prevalence of telomerase, rather than a homologous recombination-based mechanism, in telomere length maintenance suggests that telomerase also has auxiliary roles in tumorigenesis. Here, we investigate growth advantages provided by the telomerase enzyme in oncogene-transformed human cells that do not require telomerase activity for telomere length control. Our data suggest that in oncogene-transformed cells, telomerase activity accelerates cell growth kinetics in a cell cycle phase-specific manner and promotes anchorage-independent growth. Coculture experiments demonstrated that this growth advantage conferred by telomerase activity is not due to increased cellular cross-talk. Growth advantages provided by telomerase required all functional aspects of the enzyme. Dissociation-of-activity-in-telomerase mutants and other functionally defective versions of telomerase were unable to promote oncogene-transformed cell growth, suggesting that canonical telomerase activities may be involved. We conclude that telomerase provides advantages to oncogene-transformed human cells, thereby supporting the development of telomerase-based anticancer chemotherapies targeting these growth-promoting effects.

Localization and characterization of ST7 in cancer

PURPOSE

ST7 has been proposed to be a tumor suppressor gene in the chromosome region 7q31.1-q31.2. In order to gain some insight into its role in cancer, the localization and verification of the ST7 expression levels were determined.

METHODS

Various types of ST7 expression vectors tagged with the sequences of GFP, YFP or V5 were created using a gateway cloning system and full-length ST7 cDNA isolated from a human adult brain cDNA library. Cell cycle synchronization was also performed to analyze the expression of endogenous ST7 and its potentially related genes at each stage of the cell cycle.

RESULTS

Cytosolic ST7 expression in HCT-116, MCF-7 and PC-3 cancer cell lines was detected via the fluorescence signal of the fusion proteins. ST7 translocation from the cytoplasm to the nucleus has not been observed in any of the conditions assayed. A cell cycle synchronization study demonstrated that both ST7 and SERPINE1 were overexpressed when cells were arrested. Expression of these genes was found to be diminished when the cells re-entered cell division status. In addition, we also found that Survivin, MMP-13 and Cyclin D1 were differentially expressed during the cell cycle.

CONCLUSION

Our findings suggest that ST7 mediates tumor suppression through the regulation of the genes involved in maintaining the cellular structure of the cell and involved in oncogenic pathways.

The role of the MORF/MRG family of genes in cell growth, differentiation, DNA repair, and thereby aging

The discovery that replicative cellular senescence is a dominant phenotype over immortality led to the discovery that there are at least four unique genetic subgroups of immortal cell lines that use distinct mechanistic pathways to evade cell cycle exit. Study of one of these genetic complementation groups demonstrated that one gene, MORF4, possessed the ability to induce senescence in group B cell lines. The MRG family of genes, of which MORF4 is a member, has since proven important for cellular aging, proliferation, positive and negative transcriptional regulation, and DNA damage repair. MRG15, the evolutionary ancestor of the family, is highly conserved in yeast, C. elegans, drosophila, plants, and mammals and has been implicated in chromatin remodeling in these species. Our proteomics studies have found that MRG15 is unique among mammalian genes in that it associates with both histone deacetylases and histone acetyl transferase complexes, and thus potentially plays a role in both transcriptional silencing and activation. Its knockout in mice is embryonic lethal, resulting in improper organogenesis, as well as cell proliferation and DNA damage repair defects. Future study of these genes will help clarify the role of chromatin remodeling in aging, cellular proliferation, and DNA damage repair.

Stat1 expression is not sufficient to regulate the interferon signaling pathway in cellular immortalization

DNA hypermethylation in gene promoters is an epigenetic mechanism regulating gene expression in cellular immortalization, an important step in carcinogenesis. Previously, we studied the genes dysregulated during immortalization using spontaneously immortalized fibroblasts from patients with Li-Fraumeni syndrome (LFS), who carry a germline mutation in the tumor suppressor gene p53. We found that multiple interferon (IFN) signaling pathway genes were regulated by epigenetic silencing. In this study we focused on a key regulator of that pathway, the signal transducer and transcription activator 1 (Stat1) gene. Although Stat1 is downregulated after cellular immortalization and upregulated in immortal MDAH041 cells after 5-aza-2'-deoxycytidine (5-aza-dC) treatment, we detected no methylation of the Stat1 promoter region in these cells before or after immortalization. To analyze the function of Stat1 in immortalization, we expressed Stat1 in immortal MDAH041 cells by stable infection, expecting to induce IFN-regulated genes or cellular senescence or both. However, the overexpression of Stat1 alone was not sufficient to repress the proliferation rate of immortal MDAH041 cells or induce senescence in immortal MDAH041 cells. We concluded that factor(s) additional to Stat1 (whether IFN dependent or not) are required for the immortalization of LFS fibroblasts.

ST7-mediated suppression of tumorigenicity of prostate cancer cells is characterized by remodeling of the extracellular matrix

Multiple lines of evidence have provided compelling evidence for the existence of a tumor suppressor gene (TSG) on chromosome 7q31.1. ST7 may be the target of this genetic instability but its designation as a TSG is controversial. In this study, we show that, functionally, ST7 behaves as a tumor suppressor in human cancer. ST7 suppressed growth of PC-3 prostate cancer cells inoculated subcutaneously into severe combined immunodeficient mice, and increased the latency of tumor detection from 13 days in control tumors to 23 days. Re-expression of ST7 was also associated with suppression of colony formation under anchorage-independent conditions in MDA-MB-231 breast cancer cells and ST7 mRNA expression was downregulated in 44% of primary breast cancers. Expression profiling of PC-3 cells revealed that ST7 predominantly induces changes in genes involved in re-modeling the extracellular matrix such as SPARC, IGFBP5 and several matrix metalloproteinases. These data indicate that ST7 may mediate tumor suppression through modification of the tumor microenvironment.

Expression and functional characterization ofLRRC4, a novel brain-specific member of the LRR superfamily

LRRC4, a novel member of LRR superfamily thought to be involved in development and tumorigenesis of the nervous tissue, has the potential to suppress tumorigenesis and cell proliferation of U251MG cells. This study aimed at revealing the correlation between expression of LRRC4 and the maintenance of normal function and tumorigenesis suppression within the central nervous system. We systematically analyzed the expression and tissue distributions of the gene in tissues. Results showed that LRRC4 expression was limited to normal adult brain, both in human and in mouse, and exhibited a development-regulated pattern, but was down-regulated in brain tumor tissues and U251MG cell line. Furthermore, dynamic alterations in gene expression associated with cell cycle progression were investigated by using Tet-on system. Results showed that LRRC4 induced a cell cycle delay at the late G1 phase, probably through the alteration of the expression of different cell cycle regulating proteins responsible for mediating G1-S progression, such as p21(Waf1/Cip1) and p27(Kip1), Cdk2 and PCNA, p-ERK1/2. These findings suggest that LRRC4 may play an important role in maintaining normal function and suppressing tumorigenesis in the central nervous system.

Early BrdU-responsive genes constitute a novel class of senescence-associated genes in human cells

We identified genes that immediately respond to 5-bromodeoxyuridine (BrdU) in SUSM-1, an immortal fibroblastic line, with DNA microarray and Northern blot analysis. At least 29 genes were found to alter gene expression greater than twice more or less than controls within 36 h after addition of BrdU. They took several different expression patterns upon addition of BrdU, and the majority showed a significant alteration within 12 h. When compared among SUSM-1, HeLa, and TIG-7 normal human fibroblasts, 19 genes behaved similarly upon addition of BrdU. In addition, 14 genes, 9 of which are novel as regards senescence, behaved similarly in senescent TIG-7 cells. The genes do not seem to have a role in proliferation or cell cycle progression. These results suggest that the early BrdU-responsive genes represent early signs of cellular senescence and can be its new biomarkers.

Human chromosome 5 carries a putative telomerase repressor gene

Telomerase, the ribonucleoprotein enzyme that maintains the telomere, is active in human germ and stem cells and in a majority of tumor tissues and immortalized cell lines. In contrast, telomerase activity is not detected in most somatic cells, suggesting that normal human cells contain a regulatory factor(s) to repress this activity. To identify which human chromosomes carry a gene or genes that function as telomerase repressors, we investigated telomerase activity in hybrids of the B16-F10 cell line, which contain individual human chromosomes transferred previously by microcell fusion and therefore represent a hybrid panel for the entire genome except for the Y chromosome. Microcell hybrids with an introduced normal human chromosome 5 showed inhibition of telomerase activity, but clones at a late passage exhibited reactivation of telomerase activity. Reactivation of telomerase activity was accompanied by deletion and/or rearrangement of the transferred human chromosome 5. The introduction of other human chromosomes did not significantly affect the telomerase activity of B16-F10 cells. The effect of suppression of telomerase activity in microcell hybrids containing chromosome 5 was accompanied by a reduction in the level of mTERT mRNA, which encodes a component of the telomerase complex. The putative telomerase repressor gene was mapped to human chromosome bands 5p11-p13 by a combination of functional analysis using transfer of subchromosomal transferable fragments of chromosome 5 into B16-F10 cells and deletion mapping of revertant clones with reactivated telomerase activity. Thus, these results suggest that loss of a gene(s) on this chromosome was responsible for telomerase reactivation, indicating that human chromosome 5 contains a gene or genes that can regulate the expression of mTERT in B16-F10 cells.

A mortality gene(s) for the human adenocarcinoma line HeLa maps to a 130-kb region of human chromosome 4q22-q23

Human chromosome 4 was previously shown to elicit features of senescence when introduced into cell lines that map to complementation group B for senescence, including HeLa cells. Subsequently, a DNA segment encoding the pseudogene Mortality Factor 4 (MORF4) was shown to reproduce some of the effects of the intact chromosome 4 and was suggested to be a candidate mortality gene. We have identified multiple MORF4 alleles in several cell lines and tissues by sequencing and have failed to detect any cancer-specific mutations in three of the complementation group B lines (HeLa, T98G, and J82). Furthermore, MORF4 was heterozygous in these lines. These results question whether MORF4 is the chromosome 4 mortality gene. To map other candidate mortality gene(s) on this chromosome, we employed microcell-mediated monochromosome transfer to introduce either a complete copy, or defined fragments of the chromosome into HeLa cells. The introduced chromosome 4 fragments mapped the mortality gene to a region between the centromere and the marker D4S2975 (4q27), thus excluding MORF4, which maps to 4q33-q34.1. Analysis of microsatellite markers on the introduced chromosome in 59 immortal segregants identified a frequently deleted region, spanning the markers BIR0110 and D4S1557. This defines a new candidate interval of 130 kb at 4q22-q23.

Chromosome studies of in vitro senescent lymphocytes: nonrandom trisomy 2

Chromosome studies were carried out in long-term (142 and 184 d) human lymphocyte in vitro cultures in order to investigate the cytogenetic status of aging lymphocytes. The female donors were subdivided into three subgroups according to their age: 20-40 year-old (three individuals), 70-90 year-old (five persons), and centenarians (three persons). Besides some aneuploidy and structural abnormalities, telomere fusions were detected in all donor cells, and associations of acrocentric chromosomes were found in six persons in the three age-groups. Clonal trisomy 2 was present in three individuals (two from the 70-90 year-group and one centenarian with a clone +2, +8). While telomeric fusions and acrocentric associations seem to be more related to in vitro aging, trisomy 2 also appears dependent on the age of the cell donors.

Functional evidence for a squamous cell carcinoma mortality gene(s) on human chromosome 4

Squamous cell carcinoma (SCC) immortality is associated with p53 and INK4A dysfunction, high levels of telomerase and loss of heterozygosity (LOH) of other chromosomes, including chromosome 4. To test for a functional cancer mortality gene on human chromosome 4 we introduced a complete or fragmented copy of the chromosome into SCC lines by microcell-mediated chromosome transfer (MMCT). Human chromosome 4 caused a delayed crisis, specifically in SCC lines with LOH on chromosome 4, but chromosomes 3, 6, 11 and 15 were without effect. The introduction of the telomerase reverse transcriptase into the target lines extended the average telomere terminal fragment length but did not affect the frequency of mortal hybrids following MMCT of chromosome 4. Furthermore, telomerase activity was still present in hybrids displaying the mortal phenotype. The MMCT of chromosomal fragments into BICR6 mapped the mortality gene to between the centromere and 4q23. Deletion analysis of the introduced chromosome in immortal segregants narrowed the candidate interval to 2.7 Mb spanning D4S423 and D4S1557. The results suggest the existence of a gene on human chromosome 4 whose dysfunction contributes to the continuous proliferation of SCC and that this gene operates independently from telomeres, p53 and INK4A.

Lack of mutations within ST7 gene in tumour-derived cell lines and primary epithelial tumours

ST7 is a candidate tumour suppressor gene at human chromosome locus 7q31.1. We have performed mutational analysis of ST7 in a wide-range of cell lines and primary epithelial cancers and detected only one missense change in a breast cancer cell line. Other mutations previously found in cell lines and primary tumours were not evident in our analysis. These results imply that another tumour suppressor gene at this locus may be more important than ST7 in carcinogenesis.

Allelic loss and reduced expression of the ING3, a candidate tumor suppressor gene at 7q31, in human head and neck cancers

Loss of heterozygosity (LOH) has been frequently detected at chromosome 7q31 region in human head and neck squamous cell carcinomas (HNSCC) and many other cancers, suggesting the existence of tumor suppressor genes (TSG). We analysed LOH at 7q31 region in 49 HNSCC by using six polymorphic microsatellite markers and found allelic deletion in 48% (22/46) of the informative cases. We detected two preferentially deleted regions, one is around D7S643 and the other around D7S486. When we redefined the map of 7q31 region according to the contiguous sequences, a recently identified gene, ING3, was found in the proximity of D7S643. ING3 protein harbors the PHD domain highly homologous among ING family proteins, in which we previously found mutations in a related gene, ING1. As only one missense mutation of the ING3 gene was found in HNSCC, we examined the expression level. Reverse-transcription-PCR analysis demonstrated decreased or no expression of ING3 mRNA in 50% of primary tumors as compared with that of matched normal samples. Especially, about 63% of tongue and larynx tumors showed the decrease and a tendency of higher mortality was observed in cases with decreased ING3 expression. All these findings suggest a possibility that the ING3 gene functions as a TSG in a subset of HNSCC.

Genetics of cellular senescence

Cellular senescence or replicative senescence is a state of irreversible growth arrest that somatic cells enter as a result of replicative exhaustion. This can be mimicked by culture manipulations such as Ras oncogene overexpression or treatment with various agents such as sodium butyrate and 5-azacytidine. It is believed that cellular senescence is one of the protective mechanisms against tumor formation. Genetic analyses of cellular senescence have revealed that it is dominant over immortality because whole cell fusion of normal with immortal cells yields hybrids with limited division potential. Only four complementation groups for indefinite division have been identified from extensive studies fusing different immortal human cell lines with each other. The senescence-related genes for three of the complementation groups B-D have been identified on human chromosomes 4, 1, and 7, respectively, by microcell-mediated chromosome transfer, though the existence of senescence-related genes on other chromosomes has been suggested. MORF4 was cloned as the senescence-related gene on human chromosome 4 and is a member of a new gene family, which has multiple transcription factor-like motifs. This gene family may affect cell division by modulating gene expression. Study of this novel gene family should lead to new insights regarding the mechanisms and function of cellular senescence in aging and immortalization.

Alternative lengthening of telomeres in mammalian cells

Some immortalized mammalian cell lines and tumors maintain or increase the overall length of their telomeres in the absence of telomerase activity by one or more mechanisms referred to as alternative lengthening of telomeres (ALT). Characteristics of human ALT cells include great heterogeneity of telomere size (ranging from undetectable to abnormally long) within individual cells, and ALT-associated PML bodies (APBs) that contain extrachromosomal telomeric DNA, telomere-specific binding proteins, and proteins involved in DNA recombination and replication. Activation of ALT during immortalization involves recessive mutations in genes that are as yet unidentified. Repressors of ALT activity are present in normal cells and some telomerase-positive cells. Telomere length dynamics in ALT cells suggest a recombinational mechanism. Inter-telomeric copying occurs, consistent with a mechanism in which single-stranded DNA at one telomere terminus invades another telomere and uses it as a copy template resulting in net increase in telomeric sequence. It is possible that t-loops, linear and/or circular extrachromosomal telomeric DNA, and the proteins found in APBs, may be involved in the mechanism. ALT and telomerase activity can co-exist within cultured cells, and within tumors. The existence of ALT adds some complexity to proposed uses of telomere-related parameters in cancer diagnosis and prognosis, and poses challenges for the design of anticancer therapeutics designed to inhibit telomere maintenance.

Absence of ST7 mutations in tumor-derived cell lines and tumors

The gene ST7 has been proposed as the multi-tissue tumor-suppressor gene (TSG) at chromosome 7q31.1. However, we have sought and failed to detect the truncating mutations reported to exist in this gene.

Mutation of the ST7 tumor suppressor gene on 7q31.1 is rare in breast, ovarian and colorectal cancers

The gene ST7 has recently been implicated as the broad-range tumor suppressor on human chromosome 7q31.1. We did not detect somatic mutations in ST7 in any of 149 primary ovarian, breast or colon carcinomas. These data suggest that epigenetic downregulation or haploinsufficiency, rather than somatic genetic alterations, may be the primary mechanism of abrogation of ST7 function in these tumor types.

Telomeres and telomerase: basic science implications for aging

Life expectancy in the United States and other developed nations has increased remarkably over the past century, and continues to increase. However, lifespan has remained relatively unchanged over this period. As life expectancy approaches maximum human lifespan, further increase in life expectancy would only be possible if lifespan could also be increased. Although little is known about the aging process, increasing lifespan and delaying aging are the research challenges of the new century, and have caused intense debate and research activities among biogerontologists. Many theories have been proposed to explain the aging process. However, damage to deoxyribonucleic acid (DNA) is the centerpiece of most of these. Recently telomere shortening has been described to be associated with DNA damage. Located at the ends of eukaryotic chromosomes and synthesized by telomerase, telomeres maintain the length of chromosomes. The loss of telomeres can lead to DNA damage. The association between cellular senescence and telomere shortening in vitro is well established. In the laboratory, telomerase-negative differentiated somatic cells maintain a youthful state, instead of aging, when transfected with vectors encoding telomerase. Many human cancer cells demonstrate high telomerase activity. Evidence is also accumulating that telomere shortening is associated with cellular senescence in vivo. What causes changes in expression of telomerase in different cell types and premature aging syndromes? Does the key to "youthfulness" lie in our ability to control the expression of telomerase? We have reviewed the contemporary literature to find answers to these questions and explore the association between aging, telomeres, and telomerase.

Clonal chromosomal aberrations in simian virus 40-transfected human thyroid cells and in derived tumors developed after in vitro irradiation

In vitro model cell systems are important tools for studying mechanisms of radiation-induced neoplastic transformation of human epithelial cells. In our study, the human thyroid epithelial cell line HTori-3 was analyzed cytogenetically following exposure to different doses of alpha- and gamma-irradiation and subsequent tumor formation in athymic nude mice. Combining results from G-banding, comparative genomic hybridization, and spectral karyotyping, chromosome abnormalities could be depicted in the parental line HTori-3 and in nine different HTori lines established from the developed tumors. A number of chromosomal aberrations were found to be characteristic for simian virus 40 immortalization and/or radiation-induced transformation of human thyroid epithelial cells. Common chromosomal changes in cell lines originating from different irradiation experiments were loss of 8q23 and 13cen-q21 as well as gain of 1q32-qter and 2q11.2-q14.1. By comparison of chromosomal aberrations in cell lines exhibiting a different tumorigenic behavior, cytogenetic markers important for the tumorigenic process were studied. It appeared that deletions on chromosomes 9q32-q34 and 7q21-q31 as well as an increased copy number of chromosome 20 were important for the tumorigenic phenotype. A comparative breakpoint analysis of the marker chromosomes found and those observed in radiation-induced childhood thyroid tumors from Belarus revealed a coincidence for a number of chromosome bands. Thus, the data support the usefulness of the established cell system as an in vitro model to study important steps during radiation-induced malignant transformation in human thyroid cells.

Microcell-mediated transfer of chromosome 4 into HeLa cells suppresses telomerase activity

Telomerase activity can be detected in most human cancers and immortal cell lines. In contrast, the lack of telomerase activity in normal diploid fibroblasts has been correlated with progressive reduction of telomere lengths to critically short sizes followed by the cessation of cell division and the onset of senescence. Several investigators have provided evidence for the localization of a telomerase suppressor gene on chromosome 3. The aim of our study was to determine whether other chromosomes are involved in telomerase repression. Beside human chromosome 3 (serving as positive control), chromosomes 4, 6, and 11 were introduced into HeLa cells via microcell-mediated chromosome transfer. Telomerase activity from different hybrid cell lysates was determined at an early time point after fusion using a Telomerase ELISA kit. Strong repression of telomerase activity was only found in a subset of HeLa hybrids in which chromosome 3 or chromosome 4 had been introduced. Telomerase suppression induced by chromosome 3 or 4 transfer was paralleled by a high frequency (30% or 43%, respectively) of a senescent-like phenotype. Chromosomes 6 and 11, the functional loss of which is also implicated in cervical cancer, had no effect. These results indicate that normal human chromosomes 3 and 4 carry a gene or genes that suppress telomerase activity and induce cellular senescence in HeLa cells.

The TES gene at 7q31.1 is methylated in tumours and encodes a novel growth-suppressing LIM domain protein

Many studies suggest that a multi-tissue tumour suppressor gene is located at human chromosome 7q31.1. We have cloned and characterized a novel gene at this locus. The TES gene lies within the minimal region of overlap of several LOH studies and appears to possess the properties of a tumour suppressor. TES is widely expressed and is predicted to encode a protein of 421 amino acids, with three C-terminal LIM domains. Mutation analysis of the coding TES exons in 21 human tumour-derived cell lines revealed the presence of a frameshift mutation in one allele in the breast cancer cell line ZR-75. Methylation of the CpG island at the 5' end of TES appears to be a remarkably frequent finding, occurring in seven out of 10 ovarian carcinomas and in each of the 30 tumour-derived cell lines tested. Moreover, forced expression of TES in HeLa or OVCAR5 cells, resulted in a profound reduction in growth potential, as determined by the colony formation assay. We believe that TES is a tumour suppressor gene that is inactivated primarily by transcriptional silencing resulting from CpG island methylation.

Mutational and functional analyses reveal that ST7 is a highly conserved tumor-suppressor gene on human chromosome 7q31

Loss of heterozygosity (LOH) of markers on human chromosome 7q31 is frequently encountered in a variety of human neoplasias, indicating the presence of a tumor-suppressor gene (TSG). By a combination of microcell-fusion and deletion-mapping studies, we previously established that this TSG resides within a critical region flanked by the genetic markers D7S522 and D7S677. Using a positional cloning strategy and aided by the availability of near-complete sequence of this genomic interval, we have identified a TSG within 7q31, named ST7 (for suppression of tumorigenicity 7; this same gene was recently reported in another context and called RAY1). ST7 is ubiquitously expressed in human tissues. Analysis of a series of cell lines derived from breast tumors and primary colon carcinomas revealed the presence of mutations in ST7. Introduction of the ST7 cDNA into the prostate-cancer-derived cell line PC3 had no effect on the in vitro proliferation of the cells, but abrogated their in vivo tumorigenicity. Our data indicate that ST7 is a TSG within chromosome 7q31 and may have an important role in the development of some types of human cancer.

Lack of Correlation Between Survival and Allele Loss on Chromosome 7q31-32 in Primary Breast Cancer

High incidence of loss of heterozygosity (LOH), affecting the 7q31-32 chromosome region in sporadic primary human breast carcinomas suggests the presence of a tumor suppressor gene in this region which seems relevant to the development of breast cancer. To further determine the possible role of this region in the pathogenesis of human primary breast cancer and association with survival, LOH analysis was performed on 52 primary breast cancer patients using a set of highly polymorphic microsatellite markers. Our panel contained twenty biopsy cases of unknown survival, nineteen cases with more than five years survival and fourteen cases with less than two years survival. Corresponding normal and tumor DNAs were analyzed by polymerase chain reaction (PCR). The data presented here demonstrate that all patients were informative at least at one locus and 20 (38%) out of 53 cases showed LOH at one or more loci on chromosome 7q31-32. Relatively high incidence of LOH (34%) was detected at the D7S522 microsatellite marker located near to the cMet proto-oncogene while lower frequencies were observed at D7S523 (19%) and D7S495 (17%) loci, supporting the existence of a putative tumor suppressor gene at the chromosome 7q31.1 region. Our results suggest that allelic imbalance on 7q may occur at an early stage of breast carcinogenesis, as no correlation was observed between allelic loss and clinico-pathological data.

Loss of nuclear localization of the S100C protein in immortalized human fibroblasts

It is well known that cancer develops through a multistep process. In vitro transformation studies of normal human cells have shown that the immortalization step is critical for neoplastic transformation of cells. Furthermore, studies of cell fusion between normal and immortalized cells have indicated that the normal phenotype is dominant and the immortal phenotype is recessive. Thus we looked for cellular proteins that were down-regulated in immortalized human cells by two-dimensional gel electrophoresis to elucidate the mechanisms of immortalization of human cells. We found that the S100C protein was down-regulated in immortalized cells. This protein was localized in the cytoplasm of cells at the semiconfluent stage, while at the confluent stage it moved into the nuclei of normal cells but not into those of immortalized cells. Microinjection of an S100C antibody into normal confluent cells diminished the level of nuclear S100C protein, resulting in DNA synthesis. Taken together, loss of nuclear localization of the S100C protein, which may be related to DNA synthesis, is thought to be one of the mechanisms of immortalization.

Molecular characterization of pancreatic serous microcystic adenomas: evidence for a tumor suppressor gene on chromosome 10q

Pancreatic serous microcystic adenomas (SCAs) are rare, benign tumors with a striking female preference. Virtually no information is available about chromosomal or genetic anomalies in this disease. We performed extensive molecular characterization of 21 cases of formalin-fixed, paraffin-embedded sporadic SCAs consisting in genome-wide allelic loss analysis with 79 microsatellite markers covering all 22 autosomes, assessment of microsatellite instability, and mutational analysis of the VHL, K-ras, and p53 genes in nine cases for which frozen tissue was available. Although no case showed microsatellite instability of the type seen in mismatch repair-deficient tumors, a relatively low fractional allelic loss of 0.08 was found. Losses on chromosome 10q were the most frequent event in SCAs (50% of cases), followed by allelic losses on chromosome 3p (40% of cases). Moderately frequent losses (>25% of cases) were found on chromosomes 1q, 2q, and 7q. The VHL gene, located on chromosome 3p, had somatic inactivating mutations in two of nine cases (22%), whereas no mutations were found in either K-ras or p53, in agreement with the finding that all 21 cases stained negative for p53 by immunohistochemistry. Our study indicates that the involvement of chromosomal arms 10q and 3p is characteristic of SCAs and that the VHL gene is involved in a subset of sporadic cases.

Definitive functional evidence for a tumor suppressor gene on human chromosome 7q31.1 neighboring the Fra7G site

We have previously shown that loss of heterozygosity (LOH) on human chromosome (hchr) 7 at q31.1 is common in a variety of tumors of epithelial origin. Frequent LOH of a specific chromosomal marker is indicative of a closely linked tumor suppressor gene (TSG). However, recent reports have also indicated that such a high frequency of LOH could be due to the presence in this region of the second most common aphidicolin-inducible fragile site in the human genome (Fra7G). To address this controversy, we introduced single copies of hchr7 or hchr12 into a highly aggressive human prostate carcinoma cell line (PC3) by microcell-mediated transfer. The tumorigenicity of six clones of PC3/hchr7 hybrids and three clones of PCRhchr12 hybrids, obtained in four separate fusion experiments, were studied in BALB/c nude mice. All but one of the PC3/hchr7 hybrids increased tumor latency by at least twofold, whereas none of the PC3/hchr12 hybrids delayed tumor onset. No differences in the in vitro growth rate were observed among any of the cell lines assayed (parental and hybrids) suggesting that the observed tumor suppression was due to factors other than cell cycle regulation. Deletion mapping of the PC3/hchr7 tumors obtained after reversion to the malignant phenotype revealed a common region of loss centred around 7q31.1, supporting the TSG hypothesis. The smallest commonly deleted region was approximately 1.5 Mb in size and flanked by the markers D7S486 and D7S655.

A REIC gene shows down-regulation in human immortalized cells and human tumor-derived cell lines

Normal human cells stop proliferation after a certain number of cell divisions. This phenomenon is called cellular aging. The fact that the senescence phenotype is dominant and the immortal one is recessive indicates that immortalization of human cells may be caused by loss of functions of certain genes in normal cells. Based on this evidence, several cDNA clones whose expression was down-regulated during the immortalization process of human cells were isolated by the representative difference analysis (RDA) system in our laboratory. One of them, which was named REIC, was expressed to a lower degree in three human immortalized cell lines as compared with their parental normal counterparts. In addition, the expression of REIC was markedly lower in eight human tumor-derived cell lines (Hep3B and HuH-7 hepatocellular carcinomas, HuH-6 Clone 5 hepatoblastoma, HuCCT-1 cholangiocarcinoma, A549 lung cancer, HaCaT immortalized keratinocyte, HeLa cervical carcinoma, and Saos-2 osteosarcoma). In contrast, among the human tissues examined, the heart and brain, which contain a large number of post-mitotic cells, showed the highest expression of REIC. The full-length REIC cDNA revealed that the predicted protein is 350 amino acids in length and possesses coiled-coil tertiary structures in each of the amino- and carboxyl-termini. Furthermore, a search of the protein database revealed a match of this gene product with Dkk-3, which is a novel inhibitor of Wnt oncogene. These results indicate that the REIC cloned by us may function as a tumor suppressor.

Cellular senescence of a human bladder carcinoma cell line (JTC-32) induced by a normal chromosome 11

Human chromosome 11 is expected to carry tumor suppressor genes for a variety of human cancers, including bladder carcinoma. To examine the functional role of a putative tumor suppressor gene(s) on this chromosome in the development of bladder carcinoma, we performed microcell-mediated transfer of chromosome 11 into the bladder carcinoma cell line, JTC-32. Fifteen of 20 colonies formed by the transfer experiment showed a remarkable change in cell morphology. They flattened and ceased growing, or senesced, prior to 10 population doublings. The presence of transferred chromosome 11-derived fragments in the growth-arrested cells was confirmed by PCR-based polymorphism analyses. The remaining 5 microcell hybrid clones exhibited a parental cell-like morphology, and presumably escaped from senescence, which was accompanied by deletions and/or rearrangements of the transferred chromosome 11. On the other hand, a transferred normal chromosome 7 neither changed the cell morphology nor arrested the cell growth. These results support the hypothesis that chromosome 11 contains a gene or genes which restore the senescence program lost during the immortalization process of JTC-32 cells.

Identification of a gene at 16q24.3 that restores cellular senescence in immortal mammary tumor cells

We have mapped a cellular senescence gene, SEN16, within a genetic distance of 3 - 7 cM, at 16q24.3. Microcell mediated transfer of a normal human chromosome 16, 16q22-qter or 16q23-qter restored cellular senescence in four immortal cell lines, derived from human and rat mammary tumors. The resumption of indefinite cell proliferation, concordant with the segregation of the donor chromosome, confirmed the presence of a senescence gene at 16q23-qter. While microcell hybrids were maintained in selection medium to retain the donor chromosome, sporadic immortal revertant clones arose among senescent cells. Reversion to immortal growth could occur due to inactivation of the senescence gene either by a mutation or a deletion. The analysis for chromosome 16 specific DNA markers, in revertant clones of senescent microcell hybrids, revealed a consensus deletion, spanning a genetic interval of approximately 3 - 7 cM at 16q24.3.

Repression of an alternative mechanism for lengthening of telomeres in somatic cell hybrids

Some immortalized cell lines maintain their telomeres in the absence of detectable telomerase activity by an alternative (ALT) mechanism. To study how telomere maintenance is controlled in ALT cells, we have fused an ALT cell line GM847 (SV40 immortalized human skin fibroblasts) with normal fibroblasts or with telomerase positive immortal human cell lines and have examined their proliferative potential and telomere dynamics. The telomeres in ALT cells are characteristically very heterogeneous in length, ranging from very short to very long. The ALT x normal hybrids underwent a rapid reduction in telomeric DNA and entered a senescence-like state. Immortal segregants rapidly reverted to the ALT telomere phenotype. Fusion of ALT cells to telomerase-positive immortal cells in the same immortalization complementation group resulted in hybrids that appeared immortal and also exhibited repression of the ALT telomere phenotype. In these hybrids, which were all telomerase-positive, we observed an initial rapid loss of most long telomeres, followed either by gradual loss of the remaining long telomeres at a rate similar to the rate of telomere shortening in normal telomerase-negative cells, or by maintenance of shortened telomeres. These data indicate the existence of a mechanism of rapid telomere deletion in human cells. They also demonstrate that normal cells and at least some telomerase-positive immortal cells contain repressors of the ALT telomere phenotype.

Construction of a high-resolution physical map of the approximate 1-Mb region of human chromosome 7q31.1-q31.2 harboring a putative tumor suppressor gene

Reports of frequent loss of heterozygosity (LOH) of markers on human chromosome 7q in malignant myeloid disorders as well as breast, prostate, ovarian, colon, head and neck, gastric, pancreatic, and renal cell carcinomas suggest the presence of a tumor suppressor gene (TSG). Functional assays have demonstrated that the introduction of an intact copy of human chromosome 7 (hchr7) can restore senescence to immortalized human fibroblast cell lines having LOH of markers within 7q31-q32 and can inhibit the tumorigenic phenotype of a murine squamous cell carcinoma cell line. To facilitate the cloning of the putative TSG, we have constructed a high-resolution physical map of this region of hchr7, specifically that encompassing the markers D7S522 and D7S677 within 7q31.1-q31.2. By using a lower resolution yeast artificial chromosome-based map as a starting framework, we established complete clone coverage of the implicated critical region in bacterial-artificial chromosomes (BACs) and P1-derived artificial chromosomes (PACs). The resulting BAC/PAC-based contig map has provided suitable clones for the systematic sequencing of the entire interval. In addition, we have already identified 29 clusters of overlapping expressed-sequence tags (ESTs) and 4 known genes contained within these clones. Together, the physical map reported here coupled with the evolving sequence and gene maps should hasten the identification of the putative TSG residing within this region of hchr7.

Analysis of the CAVEOLIN-1 gene at human chromosome 7q31.1 in primary tumours and tumour-derived cell lines

We identified CAVEOLIN-1 as a candidate for a tumour suppressor gene mapping to human chromosome 7q31.1. A number of studies suggest that caveolin could function as a tumour suppressor. Expression of caveolin, and in turn the number of caveolae within a cell, are inversely correlated with the transforming ability of numerous oncoproteins, including H-ras, v-abl, and bcr-abl, and caveolin is a major transformation-dependent substrate of v-src. Heterologous expression of caveolin has been shown to abrogate anchorage-independent growth and induce apoptosis in transformed fibroblasts and also to suppress anchorage-independent growth in human mammary carcinoma cells. We have analysed the status and expression of the human CAVEOLIN-1 gene in primary tumours and tumour-derived cell lines. We found no evidence for mutation of CAVEOLIN-1 in human cancers. Additionally, we found that while the first two exons of CAVEOLIN-1 are associated with a CpG island, this is not methylated in either primary tumours or in tumour-derived cell lines in which Caveolin-1 expression is low or undetectable. The level of expression of Caveolin-1 does not correlate with loss of heterozygosity at the CAVEOLIN-1 locus in these same cell lines. Contrary to other published studies, we have shown that CAVEOLIN-1 is not expressed in normal breast ductal epithelial cells in vivo. CAVEOLIN-1 is however highly expressed in breast myoepithelial cells and its expression is retained in tumours derived from breast myoepithelium. Together our data refute a role for CAVEOLIN-1 as a breast tumour suppressor gene in vivo.

DNA topoisomerase inhibitors induce reversible senescence in normal human fibroblasts

Inhibitors of DNA topoisomerases I and II induced arrest in cell division in normal human fibroblasts depending on cell divisions. Arrested cells showed morphology similar to those of normally senesced cells and strongly induced senescence-associated beta-galactosidase. In these cells, p16ink4a was upregulated, whereas p21waf1 or p53 was not altered. Upon removal of the inhibitors, the cells resumed growth but their cumulative population doublings were reduced dose dependently. Accelerated telomere shortening was not observed in the arrested cells. These results suggest that DNA topoisomerase inhibitors are efficient and reversible inducers of premature senescence in normal human cells.

A reassessment of the telomere hypothesis of senescence

According to the telomere hypothesis of senescence, the telomeric shortening that accompanies the replication of normal somatic cells acts as the mitotic clock that eventually results in their permanent exit from the cell cycle. Although evidence consistent with the telomere hypothesis continues to accumulate, on the basis of recent findings it is suggested that instead of a single clock mechanism there are multiple inducers of senescence.

Genes involved in the control of cellular proliferative potential

Evidence that control of cellular proliferative potential may be linked to telomere length, along with data indicating that other factors may also be involved, will be reviewed. According to the telomere hypothesis of senescence, the sequential loss of telomeric repeat DNA that occurs during the replication of normal somatic cells eventually dictates the onset of the permanently nonreplicative state known as senescence. Many immortalized cells express telomerase, a ribonucleoprotein enzyme that replaces the telomeric DNA that would otherwise be lost due to replication. However, some immortalized human cells may avoid telomeric shortening without using telomerase. The mechanism involved is currently unknown, but other eukaryotes are able to replace telomeric DNA through (1) recombination and copy switching or (2) retrotransposition. Human fibroblasts that lose p53 function proliferate a limited number of times beyond the population-doubling level at which their normal counterparts become senescent. Lack of functional retinoblastoma (Rb) protein (or equivalent events, such as loss of p16INK4 function, resulting in abrogation of Rb regulatory activity) also permits a temporary extension of proliferative potential. The p53 and pRb effects are additive, indicating that they exert their control on proliferative potential separately. The temporary life span extension associated with loss of p53 and/or Rb pathway function is accompanied by continued telomere shortening. The proliferation arrest that eventually ensues in p53-minus cells or in p53-minus/Rb-minus cells may be regarded as terminal proliferation arrest states serving as a backup to senescence. p53-minus/Rb-minus cells cannot proliferate further unless they acquire the ability to prevent telomeric shortening. Somatic cell hybridization and microcell-mediated chromosome transfer experiments indicate that immortalization involves the loss of function of other, as yet unidentified, genes; some of these may normally repress telomerase expression in somatic cells.

Evidence for a putative telomerase repressor gene in the 3p14.2-p21.1 region

Telomeres, which are the repeated sequences located on both ends of chromosomes in eukaryotes, are known to shorten with each cell division, and their eventual loss is thought to result in cellular senescence. Unlike normal somatic cells, most tumor cells show activation of telomerase, a ribonucleoprotein enzyme that stably maintains telomere length by addition of the sequences of TTAGGG repeats to telomeres. The KC12 cell line derived from a renal cell carcinoma in a patient with von Hippel-Lindau disease showed telomerase activity and loss of heterozygosity on the short arm of chromosome 3. Introduction of a normal human chromosome 3 into KC12 cells by microcell fusion induced cellular senescence, accompanied by suppression of telomerase activity and shortening of telomere length. Microcell hybrids that escaped from cellular senescence maintained telomere length and telomerase activity similar to those of the parental KC12 cells. We previously showed a similar suppression of telomerase activity by introduction of chromosome 3 into another renal cell carcinoma cell line, RCC23. The putative telomerase repressor gene was mapped to chromosome region 3p14.2-p21.1 by deletion mapping of KC12 + chromosome 3 revertants that escaped from cellular senescence and by transfer of subchromosomal fragments of chromosome 3 into RCC23 cells.

Mapping a novel cellular-senescence gene to human chromosome 2q37 by irradiation microcell-mediated chromosome transfer

To identify the subchromosomal region that carries the cellular-senescence-restoring program of the human cervical carcinoma cell line SiHa, we constructed by irradiation microcell-mediated chromosome transfer a library of mouse A9 cells containing various fragments of human chromosome 2 tagged with pSV2neo in 2p11-p12. Eighty-seven clones were isolated and screened for the presence of human sequences by inter-Alu and inter-L1 polymerase chain reaction (PCR), and six clones exhibiting PCR-laddering patterns that differed from those of the A9 cells containing an intact chromosome 2 were examined further. Chromosome analysis and fluorescence in situ hybridization (FISH) using human-specific repetitive sequences revealed that four of these clones contained single subchromosomal transferable fragments (STFs). Southern blot hybridization of 14 cosmid markers revealed that the STFs in A9 cells were derived from human chromosome 2. These STFs were transferred into SiHa cells by microcell fusion, and one of the STFs restored the cellular-senescence program. The concordance of the cellular-senescence-restoring program with the presence or absence of specific DNA fragments of chromosome 2 indicated that the putative cellular-senescence gene was located in 2q32-qter. For more detailed mapping, we constructed mouse A9 cells containing STFs derived from human chromosome 2 tagged with pSTneo at different regions in 2q31-qter. PCR-laddering and FISH analyses were used to identify six clones that contained different STFs. These STFs were transferred into SiHa cells, and one of the three clones that restored cellular senescence contained a small fragment of human chromosome 2. This STF was shown by PCR analysis using 14 human chromosome 2-specific primer pairs to be smaller than 12.2 cM and was mapped to the 2q37 region by FISH analysis with inter-Alu PCR. Beta-galactosidase activity, which is a biomarker of senescent cells, and telomerase activity similar to that found in parental SiHa cells were detected in SiHa microcell hybrids, suggesting that the putative cellular-senescence gene was not involved in a telomerase pathway but rather in an alternate pathway of cellular senescence.

Frequent allelic loss at 7p14-15 associated with aggressive histologic types of breast cancer

We examined 142 primary human breast cancers to determine their patterns of loss of heterozygosity (LOH) at 19 microsatellite markers over the entire length of chromosome 7. Allelic loss at one or more loci on the short arm of chromosome 7 was observed in 37 of the tumors (26%). We found a new target region of allelic loss on 7p between D7S1802 and D7S817 at 7p14-15. LOH on 7p was found more frequently in tumors of the invasive solid tubular and scirrhous type (31 of 87); 36%) than in other less aggressive types (2 of 27; 7%) (P = 0.0047). The results suggest that inactivation of putative tumor suppressor gene(s) located at 7p14-15 may play a role in the development and/or progression of primary breast cancers, particularly those of the invasive solid tubular and scirrhous type. Allelic loss was also found in 56 of 142 tumors on the long arm, and a commonly deleted region was defined between D7S522 and D7S1801 at 7q31.

A repressor function for telomerase activity in telomerase-negative immortal cells

Human telomerase, a ribonucleoprotein that adds TTAGGG repeats onto telomeres and compensates for their shortening, is repressed in most normal human somatic cells. Human somatic cells are considered to have a limited proliferation capacity because of the telomere shortening. Although immortalization of somatic cells is often associated with telomerase reactivation, there are some immortal cells in which telomerase activity is undetectable. In these cells, telomeres may be maintained by an unknown mechanism other than telomerase reactivation. To examine the genetic regulation of telomerase activity, we constructed hybrids between immortal cells with (HepG2) and without (KMST6) telomerase activity. These two cell lines had relatively short and long telomeres, respectively. The hybrid cells continued to proliferate without detectable telomerase activity even after 100 population doublings. Telomerase-positive subpopulations occasionally appeared after serial passages. Southern blot analysis revealed that the hybrids had long terminal restriction fragments similar to that of KMST6, regardless of telomerase activity, and fluorescence in situ hybridization with a telomeric probe showed high-intensity hybridization signals on telomeres, indicating relatively long telomeric repeats. These results suggest that the telomerase-negative immortal cells contain a gene or genes functioning as a telomerase repressor and maintain telomere length by a dominant mechanism other than telomerase reactivation.

Reassessment of immortalization complementation group D

Previous somatic cell hybridization studies have assigned many human cell lines to one of four complementation groups (A-D) for immortalization. We report here that the A1698DM cell line, which contains selectable markers and has previously been defined as the immortalization group D representative, was derived from T24 cells rather than A1698. A1698DM did not undergo senescence when fused with cell lines assigned to groups A, B, or C. This raises the possibility that this cell line has undergone further evolution and lost multiple putative senescence genes so that it is now unable to complement any, or most, other cell lines for senescence. Cell lines previously assigned to group D may, therefore, be heterogeneous with respect to the genetic changes that resulted in their immortalization. This has important implications for strategies to clone senescence genes based on complementation groups.

Differential gene expression in SV40-mediated immortalization of human fibroblasts

Normal human diploid fibroblasts (HF) have a limited life span, undergo senescence, and rarely, if ever, spontaneously immortalize in culture. Introduction of the gene for T antigen encoded by the DNA virus SV40 extends the life span of HF and increases the frequency of immortalization; however, immortalization requires both T-dependent and T-independent functions. We previously generated independent SV40-transformed non-immortal (pre-immortal) HF cell lines from which we then obtained immortal sublines as part of a multifaceted approach to identify functions responsible for immortalization. In this study we undertook a search for cellular mRNAs which are differentially expressed upon immortalization. A lambda cDNA library was prepared from a pre-immortal SV40-transformed HF (HF-C). We screened the library with a subtracted probe enriched for sequences present in HF-C and reduced in immortal AR5 cells. A more limited screen was also employed for sequences overexpressed in AR5 using a different strategy. Alterations in the level of mRNAs in AR5 encoding functions relevant to signal transduction pathways were identified; however, most cDNAs encoded novel sequences. In an effort to clarify which of the altered mRNAs are most relevant to immortalization, we performed Northern analysis with RNA prepared from three paired sets of independent pre-immortal and immortal (4 cell lines) SV40-transformants using eight cloned cDNAs which show reduced expression in AR5. Three of these were reduced in additional immortal cell lines as well; one, J4-4 (unknown function) is reduced in all the immortal cell lines tested; a second, J4-3 (possible PP2C type phosphatase) is reduced in 2 of the 3 matched sets; and a third, J2-2 (unknown function) is reduced in 2 unrelated immortal cell lines. Although the roles of these genes are as yet unclear, their further analysis should extend our understanding of the molecular bases for immortalization. In particular, the patterns of expression of J4-4 and J4-3 strongly suggest that they are involved in the process of immortalization and/or can serve as target genes for assessing regulators of gene expression in this process.