Cancer-associated mutations at the INK4a locus cancel cell cycle arrest by p16INK4a but not by the alternative reading frame protein p19ARF

Methylation status of p14ARF, p15INK4b, and p16INK4a genes in human hepatocellular carcinoma

BACKGROUND

The INK4 locus consisting of three genes involved in the regulation of cell cycle, p16INK4a, p15INK4b, and p14ARF is often disrupted in human neoplasms.

METHODS

We analyzed the promoter methylation of each gene by methylation-specific PCR in hepatocellular carcinoma (HCC).

RESULTS

The methylation of p16INK4a, p15INK4b, and p14ARF was found to occur in 27 (69.2%), seven (17.9%), and none out of 39 HCC tumors, respectively. Regarding corresponding nontumorous liver tissues, the promoter regions of p16INK4a, p15INK4b, and p14ARF were methylated in three (17.6%), three (17.6%), and none out of 17 samples, respectively. Analysis of mRNA expression revealed that loss of p16INK4a expression was frequently observed in HCC. In contrast, transcripts of p14ARF and p15INK4b were detected in 16 (88.9%) and 16 (88.9%) of 18 tumors, respectively.

CONCLUSIONS

The frequent loss of transcription of p16INK4a with promoter methylation not only in the advanced but also in the early stages of HCC suggests that the epigenetic alteration of p16INK4a promoter is likely to be involved in hepatocarcinogenesis. Together with the result of RT-PCR analysis, the role of aberrant methylation of p14ARF or p15INK4a promoter in hepatocarcinogenesis is thought to be limited.

Comprehensive analysis of CDKN2A (p16INK4A/p14ARF) and CDKN2B genes in 53 melanoma index cases considered to be at heightened risk of melanoma

OBJECTIVE

Comprehensive analysis of the 9p21 locus including the CDKN2A, ARF, and CDKN2B genes in 53 individuals from melanoma index cases considered to be at heightened risk of melanoma.

METHODS AND RESULTS

Using a combination of DNA sequencing, gene copy number by real time quantitative PCR, linkage analysis, and transcript analysis in haploid somatic cell hybrids, we found no evidence for germline alteration in either coding or non-coding domains of CDKN2A and CDKN2B. However, we identified a p14ARF exon 1beta missense germline mutation (G16D) in a melanoma-neural system tumour syndrome (CMM+NST) family and a 8474 bp germline deletion from 196 bp upstream of p14ARF exon 1beta initiation codon to 11233 bp upstream of exon 1alpha of p16(INK4A) in a family with five melanoma cases. For three out of 10 families with at least three melanoma cases, the disease gene was unlinked to the 9p21 region, while linkage analysis was not fully conclusive for seven families.

CONCLUSIONS

These data reinforce the hypothesis that ARF is a melanoma susceptibility gene and suggest that germline deletions specifically affecting p14ARF may not be solely responsible for NST susceptibility. Predisposition to CMM+NST could either be due to complete disruption of the CDKN2A locus or be the result of more complex genetic inheritance. In addition, the absence of any genetic alteration in 50 melanoma prone families or patients suggests the presence of additional tumour suppressor genes possibly in the 9p21 region, and on other chromosomes.

Monoclonal antibodies to the mouse p19(Arf) tumor suppressor protein

The Arf tumor suppressor protein inhibits cell proliferation through both p53-dependent and -independent mechanisms. Two rat monoclonal antibodies raised against a peptide corresponding to amino acids 54-75 of the mouse p19(Arf) protein reacted with the native protein expressed in mammalian cells. Neither antibody detected human or golden hamster Arf proteins. The two antibodies, both of IgG2b isotype, are directed to adjacent epitopes contained within residues 54-62 and 62-75, respectively, of the mouse p19(Arf) protein, and both were highly efficient in detecting p19(Arf) by immunoprecipitation, immunoblotting, and immunofluorescence. One antibody proved useful for immunohistochemical staining of p19(Arf) in fixed sections of mouse testes, revealing low levels of protein expression within the nucleoli of spermatogonia. This indicates that these antibodies should be useful in detecting the endogenous p(19Arf) protein at specific stages of mouse development and during early stages of tumorigenesis.

N-terminal polyubiquitination and degradation of the Arf tumor suppressor

Unknown mechanisms govern degradation of the p19Arf tumor suppressor, an activator of p53 and inhibitor of ribosomal RNA processing. Kinetic metabolic labeling of cells with [3H]-leucine indicated that p19Arf is a relatively stable protein (half-life approximately 6 h) whose degradation depends upon the ubiquitin-proteasome pathway. Although p19Arf binds to the Mdm2 E3 ubiquitin protein ligase to activate p53, neither of these molecules regulates p19Arf turnover. In contrast, the nucleolar protein nucleophosmin/B23, which binds to p19Arf with high stoichiometry, retards its turnover, and Arf mutants that do not efficiently associate with nucleophosmin/B23 are unstable and functionally impaired. Mouse p19Arf, although highly basic (22% arginine content), contains only a single lysine residue absent from human p14ARF, and substitution of arginine for lysine in mouse p19Arf had no effect on its rate of degradation. Mouse p19Arf (either wild-type or lacking lysine) and human p14ARF undergo N-terminal polyubiquitination, a process that has not as yet been documented in naturally occurring lysine-less proteins. Re-engineering of the p19Arf N terminus to provide consensus sequences for N-acetylation limited Arf ubiquitination and decelerated its turnover.

Cyclin-dependent kinase inhibition by the KLF6 tumor suppressor protein through interaction with cyclin D1

Kruppel-like factor 6 (KLF6) is a tumor suppressor gene inactivated in prostate and colon cancers, as well as in astrocytic gliomas. Here, we establish that KLF6 mediates growth inhibition through an interaction with cyclin D1, leading to reduced phosphorylation of the retinoblastoma protein (Rb) at Ser(795). Furthermore, introduction of KLF6 disrupts cyclin D1-cyclin-dependent kinase (cdk) 4 complexes and forces the redistribution of p21(Cip/Kip) onto cdk2, which promotes G(1) cell cycle arrest. Our data suggest that KLF6 converges with the Rb pathway to inhibit cyclin D1/cdk4 activity, resulting in growth suppression.

Different requirements for the cytostatic and apoptotic effects of type I interferons. Induction of apoptosis requires ARF but not p53 in osteosarcoma cell lines

The regulation of cell growth is one of the most important effects of type I interferons (IFNs). This response may involve a cytostatic effect or the induction of apoptosis depending on the cell context. Often the growth-inhibitory response of type I IFNs is studied in tumor cell lines carrying mutations of tumor suppressor genes, and therefore, the growth-inhibitory effect can be influenced by inactivation of these important regulators of cell proliferation. In this report, we explored the role of the ARF-p53 pathway in the growth-inhibitory effect of type I IFNs. We found that p53 is only induced in cells that express p14(ARF) (p19(ARF) in mouse cells). Surprisingly, mouse embryonal fibroblasts that are null for p19(ARF) or P53, even after transformation with oncogenic RAS, respond as well as wild type to the growth-inhibitory effect of type I IFNs. Similarly, human ARF(-/-) U2OS and P53(-/-) SAOS-2 cells show a significant decrease in cell proliferation. However, only SAOS-2 or U2OS reconstituted with inducible p14(ARF) undergo apoptosis in response to IFN beta treatment, and this effect was not inhibited by expression of dominant negative p53. These data suggest that (i) at least in specific cell types, the induction of apoptosis by type I IFNs requires an ARF pathway that is p53-independent and (ii) the cytostatic and pro-apoptotic effects of type I IFNs employ different pathways.

p16(Ink4a) interferes with Abelson virus transformation by enhancing apoptosis

Pre-B-cell transformation by Abelson virus (Ab-MLV) is a multistep process in which primary transformants are stimulated to proliferate but subsequently undergo crisis, a period of erratic growth marked by high levels of apoptosis. Inactivation of the p53 tumor suppressor pathway is an important step in this process and can be accomplished by mutation of p53 or down-modulation of p19(Arf), a p53 regulatory protein. Consistent with these data, pre-B cells from either p53 or Ink4a/Arf null mice bypass crisis. However, the Ink4a/Arf locus encodes both p19(Arf) and a second tumor suppressor, p16(Ink4a), that blocks cell cycle progression by inhibiting Cdk4/6. To determine if p16(Ink4a) plays a role in Ab-MLV transformation, primary transformants derived from Arf(-/-) and p16(Ink4a(-/-)) mice were compared. A fraction of those derived from Arf(-/-) animals underwent crisis, and even though all p16(Ink4a(-/-)) primary transformants experienced crisis, these cells became established more readily than cells derived from +/+ mice. Analyses of Ink4a/Arf(-/-) cells infected with a virus that expresses both v-Abl and p16(Ink4a) revealed that p16(Ink4a) expression does not alter cell cycle profiles but does increase the level of apoptosis in primary transformants. These results indicate that both products of the Ink4a/Arf locus influence Ab-MLV transformation and reveal that in addition to its well-recognized effects on the cell cycle, p16(Ink4a) can suppress transformation by inducing apoptosis.

Physical and functional interactions of the Arf tumor suppressor protein with nucleophosmin/B23

The Arf tumor suppressor inhibits cell cycle progression through both p53-dependent and p53-independent mechanisms, including interference with rRNA processing. Using tandem-affinity-tagged p19(Arf), we purified Arf-associated proteins from mouse NIH 3T3 fibroblasts undergoing cell cycle arrest. Tagged p19(Arf) associated with nucleolar and ribosomal proteins, including nucleophosmin/B23 (NPM), a protein thought to foster the maturation of preribosomal particles. NPM is an abundant protein, only a minor fraction of which binds to p19(Arf); however, a significant proportion of p19(Arf) associates with NPM. The interaction between p19(Arf) and NPM requires amino acid sequences at the Arf amino terminus, which are also required for Mdm2 binding, as well as the central acidic domain of NPM and an adjacent segment that regulates NPM oligomerization. The interaction between p19(Arf) and NPM occurs in primary mouse embryonic fibroblasts, including those lacking both Mdm2 and p53. In an NIH 3T3 derivative cell line (MT-Arf) engineered to conditionally express an Arf transgene, induced p19(Arf) associates with NPM and colocalizes with it in high-molecular-weight complexes (2 to 5 MDa). An NPM mutant lacking its carboxyl-terminal nucleic acid-binding domain oligomerizes with endogenous NPM, inhibits p19(Arf) from entering into 2- to 5-MDa particles, and overrides the ability of p19(Arf) to retard rRNA processing.

Targeted therapies in myeloid leukemia

Targeted therapies for hematological malignancies have come of age since the advent of all trans retinoic acid (ATRA) for treating APL and STI571/Imatinib Mesylate/Gleevec for CML. There are good molecular targets for other malignancies and several new drugs are in clinical trials. In this review, we will concentrate on individual abnormalities that exist in the myelodysplastic syndromes (MDS) and myeloid leukemias that are targets for small molecule therapies (summarised in Fig. 1). We will cover fusion proteins that are produced as a result of translocations, including BCR-ABL, the FLT3 tyrosine kinase receptor and RAS. Progression of diseases such as MDS to secondary AML occur as a result of changes in the balance between cell proliferation and apoptosis and we will review targets in both these areas, including reversal of epigenetic silencing of genes such as p15(INK4B).

Germline splicing mutations of CDKN2A predispose to melanoma

Coding mutations of the CDKN2A gene on chromosome 9p21 cosegregate with 25-60% of familial melanoma cases, but there remains a number of 9p21-linked kindreds that lack germline coding mutations of CDKN2A. We sequenced CDKN2A exons 1alpha, 2, 3, and the adjacent intronic regions in 167 melanoma-prone families (at least two affected first-degree relatives), and detected four splice site variations, three of which cosegregate with the disease. RT-PCR experiments verified that these three variants, including an AGgt to ATgt mutation that demonstrates a founder effect, do affect splicing. While an exon 1alpha splice donor site mutation incompletely abolishes splicing, the correctly spliced mRNA yields a protein (Q50P) that cannot effectively interact with CDK4. We also performed RT-PCR on mRNA from 16 melanoma-prone kindreds to search for cryptic splice sites deep within introns, but identified no splice variants. Meanwhile, we screened 139 affected families using allele-specific PCR for the recently discovered IVS2-105A>G mutation, but found only one family that possesses this alteration. We conclude that splice site mutations do predispose to disease in a subset of melanoma-prone kindreds. Characterization of additional splice site variants and other noncoding alterations of CDKN2A should allow us to detect a wider range of mutations in at-risk patients.

The genetics of malignant melanoma: lessons from mouse and man

Therapeutic resistance and proclivity for metastasis are hallmarks of malignant melanoma. Genetic, epidemiological and genomic investigations are uncovering the spectrum of stereotypical mutations that are associated with melanoma and how these mutations relate to risk factors such as ultraviolet exposure. The ability to validate the pathogenetic relevance of these mutations in the mouse, coupled with advances in rational drug design, has generated optimism for the development of effective prevention programmes, diagnostic measures and targeted therapeutics in the near future.

CDKN2A point mutations D153spl(c.457G>T) and IVS2+1G>T result in aberrant splice products affecting both p16INK4a and p14ARF

The CDKN2A gene, which encodes the proteins p16(INK4a) and p14(ARF), is located on chromosome 9p21. Germline mutations at this locus increase susceptibility to cutaneous malignant melanoma (CMM). In general, missense and nonsense mutations are primarily responsible for defective p16(INK4a) and possibly p14(ARF) protein function and account for approximately 20% of inherited CMM cases. We report a G>T transversion mutation in the last nucleotide of exon 2, affecting the aspartic acid residue at position 153 of CDKN2A-p16(INK4a) in a proband with melanoma. If splicing were unaffected, this mutation would change Asp to Tyr. RT-PCR analysis, however, revealed that this mutation, which we have termed D153spl(c.457G>T), and a previously described mutation at the next nucleotide, IVS2+1G>T, result in identical aberrant splicing affecting both p16(INK4a) and p14(ARF). The two main alternate splice products for each of the two normal transcripts includes a 74 bp deletion in exon 2, revealing a cryptic splice site, and the complete skipping of exon 2. The dual inactivation of p16(INK4a) and p14(ARF) may contribute to the CMM in these families.

Involvement of the INK4a/Arf gene locus in senescence

The INK4a/ARF locus encodes two proteins whose expression limits cellular proliferation. Whilst the biochemical activities of the two proteins appear very different, they both converge on regulating the retinoblastoma and p53 tumour suppressor pathways. Neither protein is required for normal development, but lack of either predisposes to the development of malignancy. Both proteins have also been implicated in the establishment of senescence states in response to a variety of stresses, signalling imbalances and telomere shortening. The INK4a/Arf regulatory circuits appear to be partially redundant and show evidence of rapid evolution. Especially intriguing are the large number of biological differences documented between mice and man. We review here the brief history of INK4a/Arf and explore possible links with organismal aging and the evolution of longevity.

Reduced hepatic tumor incidence in cyclin G1-deficient mice

Cyclin G1 is a transcriptional target of the tumor suppressor p53, and its expression is increased after DNA damage. Recent data show that cyclin G1 can regulate the levels of p53 by a mechanism that involves dephosphorylation of Mdm2 by protein phosphatase 2A. To understand the biologic role of cyclin G1, we have generated cyclin G1-deficient mice. In agreement with previous results, we showed that these mice develop normally, and that proliferation and induction of cellular senescence in cyclin G1-deficient mouse embryo fibroblasts are indistinguishable from wild-type fibroblasts. However, we found that the p53 levels in the cyclin G1-deficient mice are 2-fold higher that in wild-type mice. Moreover, we showed that treatment of mice with the alkylating agent 1,4-bis[N,N'-di(ethylene)-phosphamide]piperazine (Dipin), followed by partial hepatectomy, decreased G1-S transition in cyclin G1-null hepatocytes as compared with wild type. Finally, we found a significant decrease in tumor incidence, mass, and malignancy in both male and female cyclin G1-null mice after treatment with the potent hepatocarcinogen N-diethylnitrosamine. Taken with recent published data, our results suggest that cyclin G1, together with Mdm2, constitute a part of a negative feedback system that attenuates the activity of p53. In conclusion, our data suggest that the decreased tumor susceptibility after loss of cyclin G1 function is caused by the increased tumor suppressor action of p53.

Ewing's sarcoma: diagnostic, prognostic, and therapeutic implications of molecular abnormalities

The identification of the non-random chromosome rearrangements between the EWS gene on chromosome 22q12 and members of the ETS gene family in Ewing's sarcoma, peripheral primitive neuroectodermal tumour, Askin tumour, and neuroepithelioma has been a key advance in understanding their common histogenesis and defining the Ewing's sarcoma family of tumours (ESFT). In addition to improvements in diagnosis and potentially the stratification of patients for risk, biological investigations of these gene fusions may define targets for much needed therapeutic strategies to eliminate minimal residual disease or metastatic disease. Insight into their relation with other oncogenic events in ESFT will advance risk group analysis and ultimately may improve clinical management and survival for patients with this disease.

Absence of p16INK4a and truncation of ARF tumor suppressors in chickens

The INK4b-ARF-INK4a locus on human chromosome 9p21 (Human Genome Organization designation CDKN2B-CDKN2A), and the corresponding locus on mouse chromosome 4, encodes three distinct products: two members of the INK4 cyclin-dependent kinase inhibitor family and a completely unrelated protein, ARF, whose carboxyl-terminal half is specified by the second exon of INK4a but in an alternative reading frame. As INK4 proteins block the phosphorylation of the retinoblastoma gene product and ARF protects p53 from degradation, the locus plays a key role in tumor suppression and the control of cell proliferation. To gain further insights into the relative importance of INK4a and ARF in different settings, we have isolated and characterized the equivalent locus in chickens. Surprisingly, although we identified orthologues of INK4b and ARF, chickens do not encode an equivalent of INK4a. Moreover, the reading frame for chicken ARF does not extend into exon 2, because splicing occurs in a different register to that used in mammals. The resultant 60-aa product nevertheless shares functional attributes with its mammalian counterparts. As well as indicating that the locus has been subject to dynamic evolutionary pressures, these unexpected findings suggest that in chickens, the tumor-suppressor functions of INK4a have been compensated for by other genes.

A major functional difference between the mouse and human ARF tumor suppressor proteins

Suppression of tumorigenesis is considerably more stringent in the human than in the much shorter lived mouse species, and the reasons for this difference are poorly understood. We investigated functional differences in the control of the ARF (alternative reading frame) protein that acts upstream of p53 and is encoded along with p16(INK4a) at a major tumor suppressor locus in both the human and mouse genomes. The mouse and human ARF proteins are substantially divergent at their carboxyl termini. We have shown that the mouse ARF protein (p19ARF) interacts with Pex19p in the cell cytoplasm leading to its nuclear exclusion and repression of its p53 activation function. The human ARF protein (p14ARF) is substantially smaller than its mouse counterpart and is not subject to this functional inactivation by Pex19p. In an identical cellular background, ribozymes directed against Pex19p enhanced p19ARF- but not p14ARF-activated p53 function. This is the first demonstration of a functional difference between the mouse and human ARF proteins. In view of the major role of ARF in tumor suppression, this distinction may contribute to the different levels of tumor proneness of these species.

Escape of p53 protein from E6-mediated degradation in HeLa cells after cisplatin therapy

We previously reported that therapy of human cervical carcinoma HeLa cells with CP induced segregation of nucleoli and changes of nuclei characteristic of apoptosis. We raised the question of whether p53 can be reactivated by chemotherapy in HeLa cells despite the presence of HPV-encoded E6 activity. Cellular levels of p53 protein increased after CP treatment, reaching a maximum after 6 hr. p53 protein accumulated preferentially in the nucleoli, with a peak after 15 hr. CP-induced nucleolar targeting of p53 appears to be selective because p73, another member of the p53 gene family, accumulated primarily in nuclei in response to CP. Monitoring of the intranuclear distribution of Hdm-2, a negative regulator of p53, revealed this protein in the nucleoli of untreated controls translocated into chromatin during CP therapy. Interestingly, p14(ARF) showed an inverse intranuclear redistribution. Proteasome inhibitors were not able to mimic the effect of CP on p53 levels. Since the reduced stability of wild-type p53 protein in HeLa cells is a consequence of its enhanced ubiquitination by virally encoded E6 protein, resulting in its accelerated degradation, we checked the cellular level of E6 during CP therapy. Six hours after application of CP, E6 protein expression was markedly reduced. This coincided with the increase of cellular p53 and preceded the nucleolar accumulation of p53 protein, indicating that repression of virally coded E6 protein by CP contributes to the restoration of p53 expression.

Multiple interacting domains contribute to p14ARF mediated inhibition of MDM2

The small basic protein p14ARF, encoded by one of the alternative transcripts from the human INK4A/ARF locus, interferes with MDM2-mediated ubiquitination of the p53 tumour suppressor protein. The resultant stabilization of p53 leads to increased expression of p53-regulated genes, such as MDM2 itself and the cyclin-dependent kinase inhibitor p21(CIP1). Here we relate physical interactions between p14ARF and MDM2, as determined using synthetic peptides and systematic deletions of p14ARF, with consequential effects on p53 stabilization and transcriptional activity. The data imply that the amino terminal half of p14ARF, encoded by the alternative first exon (exon 1beta) contacts MDM2 through multiple domains that can independently impede MDM2-mediated degradation of p53, provided that they are localized in the cell nucleus. As well as identifying previously unrecognized functional domains, our findings offer an explanation for the relative paucity of missense mutations in exon 1beta in human tumours.

Human p14(ARF)-mediated cell cycle arrest strictly depends on intact p53 signaling pathways

The tumor suppressor ARF is transcribed from the INK4a/ARF locus in partly overlapping reading frames with the CDK inhibitor p16(Ink4a). ARF is able to antagonize the MDM2-mediated ubiquitination and degradation of p53, leading to either cell cycle arrest or apoptosis, depending on the cellular context. However, recent data point to additional p53-independent functions of mouse p19(ARF). Little is known about the dependency of human p14(ARF) function on p53 and its downstream genes. Therefore, we analysed the mechanism of p14(ARF)-induced cell cycle arrest in several human cell types. Wild-type HCT116 colon carcinoma cells (p53(+/+)p21(CIP1+/+) 14-3-3sigma(+/+)), but not p53(-/-) counterparts, underwent G(1) and G(2) cell cycle arrest following infection with a p14(ARF)-adenovirus. In p21(CIP1-/-) cells, p14(ARF) did not induce G(1) or G(2) arrest, while 14-3-3sigma(-/-) counterparts were mainly arrested in G(1), pointing to essential roles of p21(CIP1) in G(1) and G(2) arrest and cooperative roles of p21 and 14-3-3sigma in ARF-mediated G(2) arrest. Our data demonstrate a strict p53 and p21(CIP1) dependency of p14(ARF)-induced cell cycle arrest in human cells.

Adenovirus-mediated overexpression of p14(ARF) induces p53 and Bax-independent apoptosis

The human INK4a gene locus encodes two structurally unrelated tumor suppressor proteins, p16(INK4a) and p14(ARF), which are frequently inactivated in human cancer. Whereas p16(INK4a) acts through engagement of the Rb-cdk4/6-cyclin D pathway, both the pro-apoptotic and cell cycle-regulatory functions of p14(ARF) were shown to be primarily dependent on the presence of functional p53. Recent reports have also implicated p14(ARF) in p53-independent mechanisms of cell cycle regulation and apoptosis induction, respectively. To further explore the pro-apoptotic function of p14(ARF) in relation to functional cellular p53, we constructed a replication-deficient adenoviral vector for overexpression of p14(ARF) (Ad-p14(ARF)). As expected, Ad-p14(ARF) efficiently induced apoptosis in p53/Rb wild-type U-2OS osteosarcoma cells at low multiplicities of infection. Interestingly, Ad-p14(ARF) also induced apoptosis in both p53-deleted SAOS-2 osteosarcoma cells and HCT116 colon cancer cells with a bi-allelic knock-out of p53 (HCT116-p53(-/-)). Similarly, adenovirus-mediated overexpression of p14(ARF) induced apoptosis in p53/Bax-mutated DU145 prostate cancer cells as well as in HCT116 cells devoid of functional Bax (HCT116-Bax(-/-)). Restoration of Bax expression by retroviral gene transfer in DU145 cells did not further enhance p14(ARF)-triggered cell death. Infection with Ad-p14(ARF) induced activation of mitochondrial permeability shift transition, caspase activation and apoptotic DNA fragmentation irrespective of the presence or absence of either Bax or functional cellular p53. Nevertheless, overexpression of the anti-apoptotic Bcl-2 homolog Bcl-x(L) markedly inhibited p14(ARF)-induced apoptosis. This may indicate that p14(ARF) triggers a so far unknown activator of mitochondrial apoptosis which can be inhibited by Bcl-2 but which acts either independently or downstream of Bax. Taken together, this report demonstrates the participation of signaling pathways apart from the p53/Mdm-2 rheostat and Bax in p14(ARF)-mediated apoptosis.

Alterations in the INK4a/ARF locus and their effects on the growth of human osteosarcoma cell lines

Two different proteins, p16(INK4a) and p14(ARF), encoded by the INK4a/ARF locus play important roles in the RB and p53 pathways, respectively. This study was performed to determine genetic and epigenetic alterations in the INK4a/ARF locus and their effects on the growth of osteosarcoma. Among six cell lines examined, both p16(INK4a) and p14(ARF) exons were homozygously deleted in two cell lines, MG63 and HOS, and both p16(INK4a) and p14(ARF) promoters were methylated in one cell line, U2OS. Wild-type mRNA and proteins for p16(INK4a) and p14(ARF) were expressed in three other cell lines, SaOS2, HuO9, and G292. Transfection studies were performed using two cell lines, U2OS and MG63. Both the RB and p53 genes were wild types in U2OS, whereas p53 but not RB was mutated in MG63. Both p16(INK4a) and p14(ARF) suppressed the growth of U2OS, whereas p16(INK4a) but not p14(ARF) suppressed the growth of MG63. p53 only did not suppress the growth of MG63 either; however, coexpression of p14(ARF) with p53 increased the fraction of the G0/G1 phase in MG63 cells. The data presented here demonstrate the importance of genetic and epigenetic alterations in the INK4a/ARF locus for the growth of osteosarcoma and thus will be useful to further understand the biologic behavior of osteosarcoma in association with the defects in the p53 and RB pathways.

p16(INK4a) is selectively silenced in the tumoral progression of mycosis fungoides

Knowledge about the molecular mechanisms involved in the pathogenesis of tumoral progression in mycosis fungoides (MF) is still scarce. Because the 9p21 locus seems to be a good target for a detailed study in MF, this prompted us to compare the mechanisms of inactivation of the p16(INK4a), p15(INK4b), and p14(ARF) genes in aggressive and stable forms of MF, performing microsatellite analysis, methylation-specific polymerase chain reaction, direct sequencing, and p16(INK4a) protein expression by immunohistochemistry. Additionally, the p53 gene was also sequenced in tumoral lesions. Thirty-nine patients with stable MF were studied. Alterations in p16(INK4a) and p15(INK4b) genes were detected in 18% and 5% of the cases, respectively. None of the cases analyzed showed alterations of the p14(ARF) gene. In contrast with these findings, in the 11 patients with aggressive MF, alterations of the p16(INK4a), p15(INK4b), or p14(ARF) genes were found in 8 (73%), 3 (27%), and 2 (18%) cases, respectively. A significant proportion (4/11) of these alterations were already present in the p16(INK4a) gene in the initial plaque lesions in these aggressive forms of MF. Alterations in the p16(INK4a) gene, either methylation or loss of heterozygosity, were clearly more frequent than those in the p15(INK4b) and p14(ARF) genes. These p16(INK4A) alterations were confirmed using immunohistochemistry. None of the nine tumoral lesions analyzed showed mutations in exons 1-2 of the p16(INK4a) gene or in exons 5-8 of the p53 gene. These results seem to suggest that 9p21 alterations, and selectively p16(INK4a) silencing, could be a characteristic phenomenon in MF progression.

Preferential loss of maternal 9p alleles in childhood acute lymphoblastic leukemia

Germ-line events, such as paternal mutation or genomic imprinting, contribute to the early onset of childhood cancers such as retinoblastoma, Wilms tumors, and neuroblastoma. Given the high frequency of deletion involving chromosome 9p in childhood acute lymphoblastic leukemia (ALL), this study investigated whether 9p deletion might reflect preexisting germ-line gene inactivation. To do this the parental origin of deletion was determined in 10 cases of ALL with 9p21 loss of heterozygosity. Of these cases, 9 showed loss of the maternally derived allele, suggesting that a germ-line event involving a 9p gene may play a role in the onset of childhood ALL.

ARF function does not require p53 stabilization or Mdm2 relocalization

It is generally accepted that the ARF tumor suppressor induces p53-dependent growth arrest by sequestering the p53 antagonist Mdm2 in the nucleolus. Previous mutagenic studies of murine ARF suggested that residues 1 through 14 and 26 through 37 were critical for Mdm2 binding, while the latter domain also governed ARF nucleolar localization. We show that mouse ARF residues 6 to 10 and 21 to 25 are required for ARF-induced growth arrest whereas residues 1 to 5 and 29 to 34 are dispensable. Deletion of the putative nucleolar localization signal (31)RRPR(34) did not prevent nucleolar localization. Surprisingly, unlike wild-type ARF, growth-inhibitory mutants D1-5 and D29-34 failed to stabilize p53 yet induced its transcriptional activation in reporter assays. This suggests that p53 stabilization is not essential for ARF-mediated activation of p53. Like wild-type ARF, both mutants also exhibited p53-independent function since they were able to arrest p53/Mdm2-null cells. Notably, other mutants lacking conserved residues 6 to 10 or 21 to 25 were unable to suppress growth in p53-positive cells despite nucleolar localization and the ability to import Mdm2. Those observations stood in apparent contrast to the ability of wild-type ARF to block growth in some cells without relocalizing endogenous Mdm2 to nucleoli. Together, these data show a lack of correlation between ARF activity and Mdm2 relocalization, suggesting that additional events other than Mdm2 import are required for ARF function.

[Cyclin dependent kinase inhibitors and replicative senescence]

Replicative senescence is defined for human diploid fibroblasts in culture as a cell growth arrest appearing beyond 50 +/- 10 population doublings and associated with telomeres' shortening. This phenomenon shows an increased expression of growth cell inhibitors: p21Waf1 described as an universal CDK inhibitor and p16INK4a as a specific inhibitor for both G1 phase kinases CDK4 and CDK6. The cell proliferation inhibitor p14ARF, product of INK4a/ARF locus is involved in replicative senescence too. Overexpression or homozygotic deletion of these inhibitors demonstrated their role in senescence induction. These proteins are involved in two different metabolic pathways, the first including p53, represented by E2F, ARF, MDM2, p53, p21Waf1, and the second concerning pRb and p16INK4a. These two pathways present numerous interactions and the polymerase (PARP) in relation with p53 and activated by telomere shortening might represent via p21Waf1 a link between this shortening and cell cycle control. An another metabolic pathway involving PTEN and p27KIP1 is discussed in senescent-like phenotype induction, but its activity in replicative senescent is uncertain.

Mutations in the INK4a/ARF melanoma susceptibility locus functionally impair p14ARF

The INK4a/ARF locus encodes two cell cycle regulatory proteins, the cyclin-dependent kinase inhibitor, p16(INK4a), and the p53 activator, p14(ARF). Germline mutations in this locus are associated with melanoma susceptibility in 20-40% of multiple case melanoma families. Many of these mutations specifically impair p16(INK4a), whereas mutations uniquely targeting p14(ARF) are rare. Nevertheless, the importance of p14(ARF) has not been excluded because more than 40% of INK4a/ARF alterations affect p16(INK4a) and p14(ARF). We now report that p14(ARF) is functionally impaired in melanoma kindreds carrying INK4a/ARF mutations. Of the seven INK4a/ARF mutations tested, three altered the subcellular distribution of p14(ARF) and diminished the ability of p14(ARF) to activate the p53 pathway. This work establishes the importance of p14(ARF) in melanoma predisposition.

Molecular analysis of the INK4A and INK4B gene loci in human breast cancer cell lines and primary carcinomas

The INK4A and INK4B loci are located at 9p21 and have been implicated in the tumorigenesis of various human malignancies. The INK4A gene encodes two cell cycle regulators, p16(INK4A) and ARF, while INK4B encodes p15(INK4B). Previously, we have shown that the p16(INK4) tumor suppressor was not mutated or deleted in primary breast carcinomas. However, primary and metastatic breast carcinomas exhibited a relative hypomethylation of p16(INK4A), which is associated with expression, compared to normal breast tissue. The present study was conducted to determine if inactivation of p15(INK4B) and INK4A exon 1beta (ARF) are common events in breast carcinoma. Mutational analysis was performed by PCR-SSCP, and mRNA expression was evaluated by RT-PCR. Methylation-specific PCR was used to determine the methylation status of the p15(INK4B) promoter. Our results demonstrate that the p15(INK4B) gene was altered in 3 (21%) of the 14 breast cell lines; one had a silent mutation and two had homozygous deletion of the gene. None of the cell lines showed methylation of p15(INK4B). Two (14%) cell lines had homozygous deletion of INK4A exon 1beta. All normal and malignant breast tissue samples were wild-type and non-methylated for p15(INK4B) and wild-type for exon 1beta. Our results show that these structurally and functionally related genes are not invariably affected together, and the most frequently observed alteration at the INK4A and INK4B loci in breast carcinoma appears to be p16(INK4A) hypomethylation.

Identification of a 1.2 Kb cDNA fragment from a region on 9p21 commonly deleted in multiple tumor types

Chromosome band 9p21 is a frequent target of homozygous deletion in many tumor types. Putative tumor suppressor genes, CDKN2A (p16), p14(ARF) and CDKN2B (p15), were localized to 9p21. However, there have been reports that suggest that there may be other genes targeted for inactivation in the region. We have developed a method to search for transcribed sequences within large genomic regions. We tested our approach in a 100-kilobase region on 9p21, which is 40 kilobases telomeric to CDKN2A. The method, termed expressed sequence selection (ESS), resulted in the isolation of genomic fragments known to be from 9q21 that are homologous to transcribed sequences. One fragment was used to obtain a 1.2 kilobase cDNA. The sequence of the 5' half of the cDNA was almost identical to exons 3-5 of the MTAP gene, which maps to chromosome band 9p21. The 3' portion of the cDNA had sequence homology to the ALA gene, which maps to chromosome arm 9q. Using Northern blot analysis, the 1.2 Kb cDNA identified several widely expressed transcripts ranging from 1 Kb to 8.5 Kb and displayed a complex pattern of alternative splicing in which certain exons of the 1.2 Kb cDNA are excluded from some of the splice products. Using cancer tissue Northern blots, we could show that all of the transcripts are absent from a leukemia cell line and a lung cancer cell line (K562, A549) with homozygous, genomic deletions within chromosome band 9p21. In addition, the 7 Kb transcript is also absent from two additional tumor cell lines (Molt4, a leukemia derived cell line, and in G361, a melanoma derived cell line) with homozygous deletions. Further investigation will determine whether the difference in the expression pattern between the 7 Kb transcript compared with the other sized transcripts could be due to specific targeting for alteration in certain tumor types.

Aberrant expression of tumor suppressor proteins in the Ewing family of tumors

BACKGROUND

Deregulation of tumor suppressor gene function and abrogation of cell cycle control are common features of malignant neoplasms, but corresponding data on Ewing sarcomas and primitive neuroectodermal tumors are relatively scarce. We studied the expression of 4 tumor suppressor proteins in the Ewing family of tumors (EFTs).

DESIGN

We examined a series of 20 pediatric EFTs for abnormal expression of p16(INK4a), p14(ARF), p21(WAF1), and pRB by immunohistochemical analysis of pretreatment, nondecalcified archival specimens. Clinical follow up was available in all cases (median, 21 months; range, 5-103 months). Five patients presented with metastatic disease, 8 had no evidence of disease at last follow up, and 12 had an adverse outcome (death or progressive tumor posttherapy).

RESULTS

Twelve cases (60%) demonstrated abnormal expression of at least one tumor suppressor protein. There were 11 cases (55%) with loss of p21(WAF1) expression, 4 (20%) with down-regulation of p16(INK4a), 2 (10%) with absence of pRB, and one case (5%) with loss of p14(ARF) expression. Loss of p16(INK4a) expression correlated with metastatic disease at presentation (P =.026), and showed a trend toward shortened survival (P =.20). The p21(WAF1), p14(ARF), and pRB status was not significantly correlated with either metastatic disease at presentation or outcome.

CONCLUSION

Abrogation of the G1 checkpoint was common in this series of EFTs, and down-regulation of p21(WAF1) and p16(INK4a) were the most frequent findings. Loss of p16(INK4a) expression may identify a subset of cases with a more aggressive phenotype.

Resistance of primary cultured mouse hepatic tumor cells to cellular senescence despite expression of p16(Ink4a), p19(Arf), p53, and p21(Waf1/Cip1)

Primary cultured mouse hepatic cells become senescent within a short period, although rare cells form colonies from which continuously proliferating cell lines can be established. In contrast, hepatic tumor (HT) cells show little senescence and higher colony-forming capacity. To assess this difference, we investigated p16(Ink4a)/p19(Arf)/p53/p21(Waf1/Cip1) expression in primary normal and HT cells, together with cell lines established from both. In primary normal cells, p16(Ink4a)/p19(Arf) were expressed only in association with senescence and disappeared at later stages of colony formation. In contrast, primary HT cells showed sustained p16(Ink4a)/p19(Arf) expression from the beginning. No p16(Ink4a)/p19(Arf) alterations, such as deletion, mutations, or hypermethylation, were detected in the primary HT cells, although most cell lines derived from either normal or HT cell colonies lost p16(Ink4a) or p19(Arf) expression owing to hypermethylation or homozygous deletion of p16(Ink4a)/p19(Arf). On the other hand, primary normal and HT cells and most cell lines showed constitutively elevated expression of p53/p21(Waf1/Cip1), with a further increment after ultraviolet ir-radiation, indicating a functionally normal p53 pathway. These results indicate that primary HT cells are resistant to senescence despite retaining p16(Ink4a)/p19(Arf)/p53/p21(Waf1/Cip1) expression and that loss of p16(Ink4a)/p19(Arf) function is associated only with establishment of the cell lines.

Identification of the gene immediately downstream of the murine INK4a/ARF locus

The tumor suppressor gene ARF is formed by three exons, namely exons 1 beta, 2 and 3. Here, we show that embryo fibroblasts from mice genetically deficient in exons 2 and 3 (Delta 2,3) express a transcript formed by exon 1 beta followed by the 3'-terminal exon of the gene immediately downstream of the INK4a/ARF locus, which we have called NTp16 (Next-To-p16). The chimeric ARF-NTp16 transcript is not detectable in wild-type fibroblasts but its expression level in Delta 2,3 fibroblasts is 30% compared to the level of the normal ARF transcript in wild-type cells. Expression of the ARF-NTp16 transcript in Delta 2,3 cells is subject to normal regulatory features, such as upregulation by the accumulation of cell doublings, and by the presence of oncogenic Ras or E1a. The chimeric ARF-NTp16 transcript has the potential to encode a 17kDa peptide; however, this peptide is not accumulated in cells at detectable levels, probably reflecting poor codon usage or protein instability. We conclude that Delta 2,3 cells do not retain ARF functionality, at least to a significant extent. Interestingly, the expression pattern of the full-length NTp16 gene is altered in several tissues by the presence of the Delta 2,3 mutation. Finally, these data identify the gene immediately downstream of the INK4a/ARF locus, a region that has been previously proposed to contain another tumor suppressor different from the INK4a/ARF genes.

CDKN2A germline splicing mutation affecting both p16(ink4) and p14(arf) RNA processing in a melanoma/neurofibroma kindred

The CDKN2A locus encodes two tumor suppressor proteins, p16(ink4) and p14(arf), through use of alternative first exons. CDKN2A mutations detected in melanoma families are usually missense or nonsense changes which mainly impair p16(ink4) function. Large genomic deletions spanning the entire locus have been observed in two pedigrees with melanomas and nervous tumors. We have detected a novel splice site mutation in a family with melanomas, neurofibromas, and multiple dysplastic nevi. Both alternative mRNAs produced by the mutant allele lacked shared sequences from exon 2, which encodes a substantial portion (>50%) of both p16(ink4) and p14(arf) proteins. The development of neurofibromas can be explained by cooperative effects of combined inactivation of p16(ink4) and p14(arf) or, alternatively, of p14(arf) alone.

Analysis of genetic and epigenetic processes that influence p14ARF expression in breast cancer

The INK4a/ARF locus encodes two unrelated cell cycle-regulatory proteins that both function in tumor suppression, p16INK4a and p14ARF. In human tumors including breast cancer, alterations affecting selectively p14ARF have been poorly analysed. We have performed a comprehensive analysis of the inactivation mechanisms (mutation, homozygous and hemizygous deletion, and promoter hypermethylation) in a large series of 100 primary breast carcinomas. RT-PCR showed expression variable of the p14ARF transcript, with 17% demonstrating overexpression and 26% demonstrating decreased expression. No detectable alterations were observed in the majority of cases with overexpressed p14ARF mRNA, but 77% of tumors with decreased expression presented at least one of these genetic/epigenetic alterations. Nevertheless, a statistically significant correlation was observed between decreased p14ARF expression and several poor prognostic parameters.

Selective deletion of p14(ARF) exon 1beta of the INK4a locus in oral squamous cell carcinomas of Indians

The tumor suppressor gene - p16 INK4/CDKN2/MTS1 and its alternate splice product p14 (ARF), constitute the INK4a locus. We have examined the integrity of exon 1beta of p14(ARF) gene of oral squamous cell carcinomas (n=58) in untreated Indian patients. No mutations were detected in this region by PCR-SSCP analysis of the tumor DNA's. Further, PCR-based analysis revealed homozygous deletions of exon 1beta in 14 of the 58 tumors; these results were confirmed by hybridization of tumor DNAs with exon 1beta specific probe. The deletions were limited to the exon 1beta while the exons coding p16/INK4 were not affected. Except in two cases these deletions were mutually exclusive to the p53 inactivating mutations. These observations suggest an alternate mechanism of loss of p14(ARF) in the genesis of oral squamous cell carcinomas.

Characterization of the CDKN2A and ARF genes in UV-induced melanocytic hyperplasias and melanomas of an opossum (Monodelphis domestica)

We examined the involvement of the cyclin-dependent kinase inhibitor 2A (CDKN2A) locus in the pathogenesis of ultraviolet (UV) radiation-induced melanomas in an opossum (Monodelphis domestica) melanoma model in which suckling young were exposed to UVB to produce melanocytic lesions. Monodelphis CDKN2A and alternated reading frame (ARF) cDNAs were cloned and sequenced, and the expression patterns of these genes were determined by reverse transcription-polymerase chain reaction in normal tissues, 39 primary melanocytic skin lesions, and two tumor-derived cell lines, one nonmetastatic and one metastatic. Primary melanocytic lesions, including hyperplasias, benign melanomas, melanomas metastatic to lymph nodes, and melanomas metastatic to nodes and additional visceral organs, were categorized accordingly as types I-IV. Levels of CDKN2A transcripts were most abundant in type III tumor samples and the metastatic cell line but absent in the nonmetastatic cell line. ARF transcripts were expressed in all tumors and cell lines. A UV-signature mutation was detected with the wild-type allele at the CDKN2A locus in type II and III primary tumor samples and in the nonmetastatic cell line. Interestingly, in the metastatic cell line, only the mutant allele was present and expressed. These data suggest dynamic changes in the expression and/or structure of the CDKN2A and ARF genes represent one molecular defect associated with the etiology of melanoma formation and progression in the Monodelphis model system.

Stabilization of p53 by p14ARF without relocation of MDM2 to the nucleolus

The alternative product of the human INK4a/ARF locus, p14ARF, has the potential to act as a tumour suppressor by binding to and inhibiting the p53 antagonist MDM2. Current models propose that ARF function depends on its ability to sequester MDM2 in the nucleolus. Here we describe situations in which stabilization of MDM2 and p53 occur without relocalization of endogenous MDM2 from the nucleoplasm. Conversely, forms of ARF that do not accumulate in the nucleolus retain the capacity to stabilize MDM2 and p53. We therefore propose that nucleolar localization is not essential for ARF function but may enhance the availability of ARF to inhibit MDM2.

p16(MTS-1/CDKN2/INK4a) in cancer progression

Since its discovery as an inhibitor of cyclin-dependent kinases 4 and 6, the tumor suppressor p16 has continued to gain widespread importance in cancer. The high frequency of deletions of p16 in tumor cell lines first suggested an important role for p16 in carcinogenesis. This initial genetic evidence was subsequently strengthened by numerous studies documenting p16 inactivation in kindreds with familial melanoma. Moreover, a high frequency of p16 gene alterations was found in primary tumors, while recent studies have identified p16 promoter methylation as a major mechanism of tumor-suppressor-gene silencing. Additional insight into p16's role in cancer has come from the genetic analysis of precancerous lesions and various tissue culture models. It is now believed that loss of p16 is an early and often critical event in tumor progression. Consequently, p16 is a major tumor-suppressor gene whose frequent loss occurs early in many human cancers.

NF-kappaB and cell-cycle regulation: the cyclin connection

The cyclins are a family of proteins that are centrally involved in cell cycle regulation and which are structurally identified by conserved "cyclin box" regions. They are regulatory subunits of holoenzyme cyclin-dependent kinase (CDK) complexes controlling progression through cell cycle checkpoints by phosphorylating and inactivating target substrates. CDK activity is controlled by cyclin abundance and subcellular location and by the activity of two families of inhibitors, the cyclin-dependent kinase inhibitors (CKI). Many hormones and growth factors influence cell growth through signal transduction pathways that modify the activity of the cyclins. Dysregulated cyclin activity in transformed cells contributes to accelerated cell cycle progression and may arise because of dysregulated activity in pathways that control the abundance of a cyclin or because of loss-of-function mutations in inhibitory proteins.Analysis of transformed cells and cells undergoing mitogen-stimulated growth implicate proteins of the NF-kappaB family in cell cycle regulation, through actions on the CDK/CKI system. The mammalian members of this family are Rel-A (p65), NF-kappaB(1) (p50; p105), NF-kappaB(2) (p52; p100), c-Rel and Rel-B. These proteins are structurally identified by an amino-terminal region of about 300 amino acids, known as the Rel-homology domain. They exist in cytoplasmic complexes with inhibitory proteins of the IkappaB family, and translocate to the nucleus to act as transcription factors when activated. NF-kappaB pathway activation occurs during transformation induced by a number of classical oncogenes, including Bcr/Abl, Ras and Rac, and is necessary for full transforming potential. The avian viral oncogene, v-Rel is an NF-kappaB protein. The best explored link between NF-kappaB activation and cell cycle progression involves cyclin D(1), a cyclin which is expressed relatively early in the cell cycle and which is crucial to commitment to DNA synthesis. This review examines the interactions between NF-kappaB signaling and the CDK/CKI system in cell cycle progression in normal and transformed cells. The growth-promoting actions of NF-kappaB factors are accompanied, in some instances, by inhibition of cellular differentiation and by inhibition of programmed cell death, which involve related response pathways and which contribute to the overall increase in mass of undifferentiated tissue.

Quantitative real-time PCR does not show selective targeting of p14(ARF) but concomitant inactivation of both p16(INK4A) and p14(ARF) in 105 human primary gliomas

In many human cancers, the INK4A locus is frequently mutated by homozygous deletions. By alternative splicing this locus encodes two non-related tumor suppressor genes, p16(INK4A) and p14(ARF) (p19(ARF) in mice), which regulate cell cycle and cell survival in the retinoblastoma protein (pRb) and p53 pathways, respectively. In mice, the role of p16(INK4A) as the critical tumor suppressor gene at the INK4A locus was challenged when it was found that p19(ARF) only knock-out mice developed tumors, including gliomas. We have analysed the genetic status of the INK4A locus in 105 primary gliomas using both microsatellite mapping (MSM) and quantitative real-time PCR (QRT-PCR). Comparison of the results of the two methods revealed agreement in 67% of the tumors examined. In discordant cases, fluorescence in situ hybridization (FISH) analysis was always found to support QRT-PCR classification. Direct assessment of p14(ARF) exon 1beta, p16(INK4A) exon 1alpha and exon 2 by QRT-PCR revealed 43 (41%) homozygous and eight (7%) hemizygous deletions at the INK4A locus. In 49 (47%) gliomas, both alleles were retained. In addition, QRT-PCR, but not MSM, detected hyperploidy in five (5%) tumors. Deletion of p14(ARF) was always associated with co-deletion of p16(INK4A) and increased in frequency upon progression from low to high grade gliomas. Shorter survival was associated with homozygous deletions of INK4A in the subgroup of glioblastoma patients older than 50 years of age (P=0.025, Anova test single factor, alpha=0.05).

The INK4A/ARF locus: role in cell cycle control and apoptosis and implications for glioma growth

The unique INK4A/ARF locus at chromosome 9p21 encodes two distinct proteins that intimately link the pRB and p53 tumour suppressor pathways. p16INK4A has been identified as an inhibitor of the cell cycle, capable of inducing arrest in G1 phase. p14/p19ARF on the other hand can induce both G1 and G2 arrest due to its stabilizing effects on the p53 transcription factor. In addition to their roles in growth arrest, both proteins are involved in cellular senescence and apoptosis. The frequent mutation or deletion of INK4A/ARF in human tumours as well as the occurence of tumours in the murine knockout models have identified both p16 and ARF as bona fide tumour suppressors.

Methylation silencing and mutations of the p14ARF and p16INK4a genes in colon cancer

The INK4a-ARF locus encodes two tumor suppressor proteins involved in cell-cycle regulation, p16INK4a and p14ARF, whose functions are inactivated in many human cancers. The aim of this study was to evaluate p14ARF and p16INK4a gene inactivation and its association with some clinocopathological parameters in colon cancer. The mutational and methylation status of the p14ARF and p16INK4a genes was analyzed in 60 primary colon carcinomas and 8 colon cancer cell lines. We have identified the first two reported mutations affecting exon 1beta of p14ARF in the HCT116 cell line and in one of the primary colon carcinomas. Both mutations occur within the N-terminal region of p14ARF, documented as important for nucleolar localization and interaction with Mdm2. Tumor-specific methylation of the p14ARF and p16INK4a genes was found in 33% and 32% of primary colon carcinomas, respectively. Methylation of the p14ARF was inversely correlated with p53 overexpression (p = 0.02). p14ARF and p16INK4a gene methylation was significantly more frequent in right-sided than in left-sided tumors (p = 0.02). Methylation of the p14ARF gene occurred more frequently in well-differentiated adenocarcinomas (p = 0.005), whereas the p16INK4a gene was more often methylated in poorly differentiated adenocarcinomas (p = 0.002). The present results underline the role of p14ARF and p16INK4a gene inactivation in the development of colon carcinoma. They suggest that the methylation profile of specific genes, in particular p14ARF and p16INK4a, might be related to biologically distinct subsets of colon carcinomas and possibly to different tumorigenic pathways.

Genetic alterations of the p14ARF -hdm2-p53 regulatory pathway in breast carcinoma

TP53 is the most commonly mutated tumor suppressor gene in human cancers. The amplification and overexpression of HDM2 plays a role in tumorigenesis via inactivation of p53-dependent cell cycle arrest. p14ARF, an alternate transcript of the INK4A tumor suppressor locus, prevents hdm2-induced transcriptional silencing of p53 by binding hdm2. The role of this p14ARF-hdm2-p53 regulatory pathway in breast carcinoma is unknown. We hypothesized that p14ARF mutations and HDM2 gene amplification may be alternative mechanisms of p53 inactivation in breast cancer. Mutational analysis of TP53 (exons 5-9) and exon 1beta of pl4ARF was performed by PCR-SSCP and putative mutations were confirmed by sequencing. p14ARF mRNA expression was evaluated by RT-PCR and the presence of HDM2 gene amplification by differential PCR. Among the cell lines, 7/14 (50%) harbored TP53 mutations and 2/14 (14%) had a deletion ofp14ARF exon 1beta with no detectable p14ARF mRNA. None demonstrated HDM2 gene amplification. TP53 mutations were identified in 7/36 (19%) breast tumors and HDM2 amplification in 2/30 (7%) tumors. All the tumors contained an intact p14ARF exon 1beta with corresponding expression of the mRNA. Alterations in the various components of this regulatory pathway were identified in nine (64%) cell lines and 25% of the 36 breast cancers with TP53 mutation being the predominant aberration. Although p14ARF mutations and HDM2 gene amplification appear to be uncommon events in breast carcinoma, deregulation of this pathway may occur via alternative mechanisms in breast carcinogenesis.

HTLV-I Tax related dysfunction of cell cycle regulators and oncogenesis of adult T cell leukemia

HTLV-I is causually related to the oncogenesis of adult T cell leukemia (ATL). However, the precise mechanism of HTLV-I oncogenesis is unclear. HTLV-I Tax protein functions as an activator of various cellular genes, including IL-2, IL-2 receptor-alpha, and c-fos through the activation of nuclear transfer factors such as NF-kappaB and SRF, and also potently activates trascription of viral genes through CREB/ATF sites in the viral LTR. However, Tax activation of HTLV-I infected T cells through the above pathways induces polyclonal proliferation of the cells in vitro; Tax however may function only transiently in the immediate post-infection period following infection in vivo. The long latent period of 60 years from infection to onset of disease suggests other mechanisms for ATL oncogenesis. Recent studies suggest that the malignant transformation of ATL is a multi-hit phenomena, suggesting that discrete genetic events are responsible for ATL oncogenesis. These genetic events could be responsible for the different stages of ATL: smoldering, chronic, lymphoma, and acute type, p16 and p53 genes are important negative regulators of the cell cycle and are often found to be mutated in neoplasms. Recent studies including ours demonstrated a high frequency of alteration of these two genes in primary ATL cells. Furthermore, alteration of the two genes is associated with acute but not chronic type ATL. In addition, p16 gene alteration is linked to the growth rate of ATL cells, suggesting that the alteration of these cell cycle regulatory genes may be related to progression from smoldering or chronic to acute or lymphoma type ATL. Tax may be involved in mutagenesis of these genes through suppression of DNA-beta polymerase gene expression during the process from latent period to acute/lymphoma type. Once transformation occurs, activation of the pathway between Tax and the three nuclear transfer factors, NF-kappaB, SRF, and CREB/ATF, contributes to establish the aggressive manifestations of acute/lymphoma type ATL cells.

Efficiency alleles of the Pctr1 modifier locus for plasmacytoma susceptibility

The susceptibility of BALB/c mice to pristane-induced plasmacytomas is a complex genetic trait involving multiple loci, while DBA/2 and C57BL/6 strains are genetically resistant to the plasmacytomagenic effects of pristane. In this model system for human B-cell neoplasia, one of the BALB/c susceptibility and modifier loci, Pctr1, was mapped to a 5.7-centimorgan (cM) chromosomal region that included Cdkn2a, which encodes p16(INK4a) and p19(ARF), and the coding sequences for the BALB/c p16(INK4a) and p19(ARF) alleles were found to be polymorphic with respect to their resistant Pctr1 counterparts in DBA/2 and C57BL/6 mice (45). In the present study, alleles of Pctr1, Cdkn2a, and D4Mit15 from a resistant strain (BALB/cDAG) carrying DBA/2 chromatin were introgressively backcrossed to the susceptible BALB/c strain. The resultant C.DAG-Pctr1 Cdkn2a D4Mit15 congenic was more resistant to plasmacytomagenesis than BALB/c, thus narrowing Pctr1 to a 1.5-cM interval. Concomitantly, resistant C57BL/6 mice, from which both gene products of the Cdkn2a gene have been eliminated, developed pristane-induced plasma cell tumors over a shorter latency period than the traditionally susceptible BALB/cAn strain. Biological assays of the p16(INK4a) and p19(ARF) alleles from BALB/c and DBA/2 indicated that the BALB/c p16(INK4a) allele was less active than its DBA/2 counterpart in inducing growth arrest of mouse plasmacytoma cell lines and preventing ras-induced transformation of NIH 3T3 cells, while the two p19(ARF) alleles displayed similar potencies in both assays. We propose that the BALB/c susceptibility/modifier locus, Pctr1, is an "efficiency" allele of the p16(INK4a) gene.