Alternative reading frames of the INK4a tumor suppressor gene encode two unrelated proteins capable of inducing cell cycle arrest

The leptin receptor promoter controls expression of a second distinct protein

The leptin receptor (OB-R) is a single membrane- spanning protein that mediates the weight-regulatory effects of leptin (OB protein). Several mRNA splice variants have been described which either encode OB-R proteins with cytoplasmic domains of different length or the OB-R and B219/OBR variants, which have different 5'-untranslated regions. Here we report evidence for the synthesis of a human mRNA splice variant of the OB-R gene that potentially encodes a novel protein, leptin receptor gene-related protein (OB-RGRP), which displays no sequence similarity to the leptin receptor itself. This OB-RGRP transcript contains the first two OB-R gene 5'-untranslated exons, but then is alternatively spliced to two novel exons which were mapped to a yeast artificial chromosome containing the leptin receptor gene. First identified by analysis of a large human expressed sequence tag database, the OB-RGRP transcript has now also been found in human and mouse tissues by the use of PCR. Preliminary experiments suggest that OB-RGRP and the OB-R variants share similar patterns of expression that are distinct from that of the B219/OBR variant. OB-RGRP is highly homologous to putative open reading frames in both yeast and Caenorhabditis elegans , suggesting a phylogenetically conserved role for this novel protein.

Distinct roles for E2F proteins in cell growth control and apoptosis

E2F transcription activity is composed of a family of heterodimers encoded by distinct genes. Through the overproduction of each of the five known E2F proteins in mammalian cells, we demonstrate that a large number of genes encoding proteins important for cell cycle regulation and DNA replication can be activated by the E2F proteins and that there are distinct specificities in the activation of these genes by individual E2F family members. Coexpression of each E2F protein with the DP1 heterodimeric partner does not significantly alter this specificity. We also find that only E2F1 overexpression induces cells to undergo apoptosis, despite the fact that at least two other E2F family members, E2F2 and E2F3, are equally capable of inducing S phase. The ability of E2F1 to induce apoptosis appears to result from the specific induction of an apoptosis-promoting activity rather than the lack of induction of a survival activity, because co-expression of E2F2 and E2F3 does not rescue cells from E2F1-mediated apoptosis. We conclude that E2F family members play distinct roles in cell cycle control and that E2F1 may function as a specific signal for the initiation of an apoptosis pathway that must normally be blocked for a productive proliferation event.

Screening of germline mutations in the CDKN2A and CDKN2B genes in Swedish families with hereditary cutaneous melanoma

BACKGROUND

Approximately 10% of human cutaneous melanomas occur in families in which several members are affected. The familial predisposition to this disease is often associated with dysplastic nevus syndrome, a condition in which afflicted family members have multiple dysplastic nevi (atypical moles). The chromosome region 9p21 and markers on chromosomes 1p and 6p have been linked to melanoma susceptibility. The tumor suppressor genes CDKN2A and CDKN2B have been mapped to the 9p21 region, and genetic analyses have revealed the presence of germline CDKN2A alterations in melanoma families. The reported frequencies of such alterations, however, vary among these families.

PURPOSE

The present investigation was carried out to determine the frequencies of CDKN2A and CDKN2B germline gene mutations among members in a population-based cohort of Swedish melanoma families (i.e., melanoma kindreds).

METHODS

DNA was prepared from blood samples obtained from 181 individuals belonging to 100 melanoma kindreds. The polymerase chain reaction (PCR) technique, followed by single-strand conformation polymorphism (SSCP) and nucleotide sequence analyses, were used to identify the types and frequencies of mutations in exons 1, 1beta, 2, and 3 of the CDKN2A gene and in exons 1 and 2 of the CDKN2B gene.

RESULTS

CDKN2A gene aberrations were independently identified by both SSCP and nucleotide-sequence analyses. Nucleotide-sequence analysis identified a single point mutation leading to a substitution of leucine for proline in codon 48 of exon 1 in a family with a history of melanoma and several other cancers. A second abnormality, leading to an insertion of an extra arginine residue at codon number 113 of exon 2, was seen in four separate families. The CDKN2A exon-3 coding region had the wild-type sequence in all samples. No germline mutations were found in the alternative exon 1beta of the CDKN2A gene or in exons 1 and 2 of the CDKN2B gene.

CONCLUSIONS

The present investigation demonstrates that CDKN2A germline gene mutations were observed in 7.8% of the 64 Swedish melanoma kindreds that each included at least two first-degree relatives with melanoma and dysplastic nevus syndrome. No CDKN2A exon 1beta or CDKN2B mutations were identified. The critical genes responsible for the inheritance of a susceptibility to develop melanoma among family members in this population have yet to be identified.

A molecular genetic model of astrocytoma histopathology

As the molecular events responsible for astrocytoma formation and progression are being clarified, it is becoming possible to correlate these alterations with the specific histopathological and biological features of astrocytoma, anaplastic astrocytoma and glioblastoma multiforme. In WHO grade II astrocytomas, autocrine stimulation by the plateletderived growth factor system coupled with inactivation of the p53 gene may lead to a growth stimulus in the face of decreased cell death with slow net growth ensuing. Such cells would also have defective responses to DNA damage and impaired DNA repair, setting the stage for future malignant change. Such biological scenarios recapitulate many of the clinicopathological features of WHO grade II astrocytomas. Anaplastic astrocytomas further display release of a critical cell cycle brake that involves the CDKN2/p16, RB and CDK4 genes. This results in mitoses seen histologically; clinically, there is more conspicuous, rapid growth. Finally, glioblastomas may emerge from the microenvironmental outgrowth of more malignant clones in a complex vicious cycle that involves necrosis, hypoxia, growth factor release, angiogenesis and clonal selection; growth signals mediated by activation of epidermal growth factor receptors may precipitate glioblastomas. It is clear as well that glioblastoma multiforme can arise via a number of independent genetic pathways, although the clinical significance of these distinctions remains unclear.

Regulation of CDK/cyclin complexes during the cell cycle

The eukaryotic cell cycle is regulated by the temporal activation of different cyclin-dependent kinase (CDK)/cyclin complexes. Whilst the level of the catalytic subunit of the complex, the CDK, remains relatively constant through the cycle, the level of the cyclin subunit generally oscillates. Cyclins are synthesized, bind and activate the CDK and are then destroyed. In this review, we summarize the current knowledge of the regulation of the cell cycle by CDK/cyclin complexes with special emphasis on new developments in cyclin biosynthesis and destruction, the structural analysis of the CDK/cyclin complexes and the role of a set of inhibitors of CDK/cyclin complexes that are important for the coordination of the different stages of the cell cycle.

Mutations of the INK4a locus in squamous cell carcinomas of human skin

The INK4a locus encodes two different, cell cycle-regulating proteins, p16INK4a and p19ARF. In this study, we screened mutations in all coding regions of the INK4a locus (exons 1 beta, 1 alpha, 2, and 3) in 21 squamous cell carcinomas (SCCs) of human skin by polymerase chain reaction-single strand conformation polymorphism analysis. Mutations were detected in 3 SCCs in exon 2, which is common to both p16INK4a and p19ARF. These included an in-frame deletion of 21 base pairs from codon 84 to 90, a frameshift mutation of CCC-->TC at codon 75, and a nonsense mutation of CGA-->TGA at codon 80 of the p16INK4a gene. These results suggest that inactivation of the INK4a locus has some relevance to the carcinogenesis in at least some of SCCs of human skin. This is the first demonstration of aberrations in the INK4a locus in SCCs of human skin.

Frequent aberrant methylation of p16INK4a in primary rat lung tumors

The p16INK4a (p16) tumor suppressor gene is frequently inactivated by homozygous deletion or methylation of the 5' CpG island in cell lines derived from human non-small-cell lung cancers. However, the frequency of dysfunction in primary tumors appears to be significantly lower than that in cell lines. This discordance could result from the occurrence or selection of p16 dysfunction during cell culture. Alternatively, techniques commonly used to examine tumors for genetic and epigenetic alterations may not be sensitive enough to detect all dysfunctions within the heterogeneous cell population present in primary tumors. If p16 inactivation plays a central role in development of non-small-cell lung cancer, then the frequency of gene inactivation in primary tumors should parallel that observed in cell lines. The present investigation addressed this issue in primary rat lung tumors and corresponding derived cell lines. A further goal was to determine whether the aberrant p16 gene methylation seen in human tumors is a conserved event in this animal model. The rat p16 gene was cloned and sequenced, and the predicted amino acid sequence of its product found to be 62% homologous to the amino acid sequence of the human analog. Homozygous deletion accounted for loss of p16 expression in 8 of 20 cell lines, while methylation of the CpG island extending throughout exon 1 was observed in 9 of 20 cell lines. 2-Deoxy-5-azacytidine treatment of cell lines with aberrant methylation restored gene expression. The methylated phenotype seen in cell lines showed an absolute correlation with detection of methylation in primary tumors. Aberrant methylation was also detected in four of eight primary tumors in which the derived cell line contained a deletion in p16. These results substantiate the primary tumor as the origin for dysfunction of the p16 gene and implicate CpG island methylation as the major mechanism for inactivating this gene in the rat lung tumors examined. Furthermore, rat lung cancer appears to be an excellent model in which to investigate the mechanisms of de novo gene methylation and the role of p16 dysfunction in the progression of neoplasia.

Alterations of the p16INK4A gene in human ovarian cancers

The p16INK4A gene, which encodes the cell-cycle regulatory protein cyclin-dependent kinase 4 inhibitor, is a putative tumor-suppressor gene. We examined p16 gene alterations in 30 primary ovarian cancers and 11 ovarian cancer cell lines. Five of the primary cancers (16.7%) had lost both p16INK4A genes. In addition, four cancers (13.3%) contained five kinds of missense mutations and a one-base deletion. Three cell lines had homozygous deletions of p16 genes, and one cell line had multiple intragenic mutations. There was also suppressed transcription of the p16 gene in one cell line. Some point mutations occurred in the conserved ankylin consensus region. These observations suggest that p16 is a functional target for ovarian carcinogenesis and that p16 alterations occurred in the primary cancers.

Multiple types of aberrations in the p16 (INK4a) and the p15(INK4b) genes in 30 esophageal squamous-cell-carcinoma cell lines

To determine the role and mode of inactivation of the p16 and p15 genes in human esophageal tumors, we examined alterations and expression of the alpha and beta forms of the p16 gene, 5' CpG island methylation of p16 exon 1 alpha, and alterations of the p15 gene in 30 esophageal squamous-cell-carcinoma cell lines. Of 30 such cell lines examined, 28 (93%) showed aberrations of the alpha form of the p16 gene: 18 homozygous deletions, 6 point mutations and 4 hypermethylation. Methylation was exclusively observed in cell lines with the wild-type alpha form. Of the 6 point mutations, one was observed in exon 1 alpha, one in the splice acceptor site of intron 1 and the remaining 4 were in exon 2. In the beta form, 18 homozygous deletions and 3 point mutations in exon 2 were detected, but no point mutation was found in exon 1 beta. All mutations in exon 2 gave rise to premature termination codons in the reading frame of the alpha transcript, while no non-sense mutations were observed in the reading frame of the beta transcript. Among 12 cell lines without homozygous deletions of the alpha and beta forms of the p16 gene, the expected wild-type beta transcript was observed in 8 cell lines, whereas only one cell line expressed the expected wild-type alpha transcript. Homozygous deletions of the p15 gene were observed in 16 cell lines (53%), and no point mutations were detected. Twelve cell lines had alterations only in the alpha form of the p16 gene, while none showed aberrations exclusively in the p15 gene. Taken together, these results indicate that inactivation of the beta form of the p16 gene and the p15 gene are not so frequent as that of the alpha form of the p16 gene in ESC cell lines, suggesting that aberration of the alpha form of p16 gene is the primary target of 9p loss in ESC.

CDKN2 (MTS1/p16INK4A) gene alterations in hematological malignancies

Cyclin dependent kinases (CDKs) make complexes with cyclins, and regulate cell cycle progression by their serine/threonine kinase activities. CDK inhibitors (CDKIs) arrest the inappropriate progression of the cell cycle by combining with CDKs. Because the functional loss of CDKIs may permit unlimited cell growth, their disruptions are thought to be associated with tumorigenesis. Recently, one CDKI, p16, was found, and its gene, CDKN2 (MTS1/p16INK4A), was identified on chromosome 9p21. Intensive investigations of the CDKN2 gene in various tumors have shown that alterations frequently occur in this gene, thus suggesting that the CDKN2 gene is a tumor suppressor gene. In hematological malignancies, CDKN2 gene alterations may be limited to lymphoid malignancies, especially T-cell type acute lymphocytic leukemias, in which frequent chromosomal abnormalities in the 9p21 region have been reported. The CDKN2 gene is also inactivated in some patients with non-Hodgkin's lymphomas, adult T-cell leukemias, and lymphoid blastic crisis of chronic myelogenous leukemias. The main mechanism of CDKN2 gene inactivation is thought to be homozygous deletion, but point mutations may also inactivate it in some cases. The CDKN2 gene appears to be the major tumor suppressor gene on chromosome 9p21, and it is thought to be involved in the tumorigenesis of various lymphoid malignancies.

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

The INK4a gene, one of the most frequently disrupted tumor suppressor loci in human cancer, encodes two unrelated proteins, p16INK4a and p19ARF, each of which is capable of inducing cell cycle arrest. Splicing of alternative first exons (1 alpha vs. 1 beta) to a common second exon within INK4a generates mRNAs in which exon 2 sequences are translated in two different reading frames. One of the products, the cyclin D-dependent kinase inhibitor p16INK4a, is functionally inactivated by mutations or deletions in a wide variety of cancers. However, because many such mutations reside in exon 2, they also affect the alternative reading frame (ARF) protein. To determine whether such mutations disrupt p19ARF function, we introduced naturally occurring missense mutations into mouse INK4a exon 2 sequences and tested mutant p16INK4a and p19ARF proteins for their ability to inhibit cell cycle progression. Six p19ARF point mutants remained fully active in mediating cell cycle arrest in NIH 3T3 fibroblasts, whereas two of the corresponding mutations within p16INK4a resulted in complete loss of activity. Analysis of p19ARF deletion mutants indicated that the unique aminoterminal domain encoded by exon 1 beta was both necessary and sufficient for inducing G1 arrest. Therefore, cancer-associated mutations within exon 2 of the INK4a gene specifically target p16INK4a, and not p19ARF, for inactivation.

Infrequent methylation of CDKN2A(MTS1/p16) and rare mutation of both CDKN2A and CDKN2B(MTS2/p15) in primary astrocytic tumours

In a series of 46 glioblastomas, 16 anaplastic astrocytomas and eight astrocytomas, all tumours retaining one or both alleles of CDKN2A (48 tumours) and CDKN2B (49 tumours) were subjected to sequence analysis (entire coding region and splice acceptor and donor sites). One glioblastoma with hemizygous deletion of CDKN2A showed a missense mutation in exon 2 (codon 83) that would result in the substitution of tyrosine for histidine in the protein. None of the tumours retaining alleles of CDKN2B showed mutations of this gene. Glioblastomas with retention of both alleles of CDKN2A (14 tumours) and CDKN2B (16 tumours) expressed transcripts for these genes. In contrast, 7/13 glioblastomas with hemizygous deletions of CDKN2A and 8/11 glioblastomas with hemizygous deletions of CDKN2B showed no or weak expression. Anaplastic astrocytomas and astrocytomas showed a considerable variation in the expression of both genes, regardless of whether they retained one or two copies of the genes. The methylation status of the 5' CpG island of the CDKN2A gene was studied in all 15 tumours retaining only one allele of CDKN2A as well as in the six tumours showing no significant expression of transcript despite their retaining both CDKN2A alleles. Three tumours (one of each malignancy grade studied) were found to have partially methylated the 5' CpG island of CDKN2A. It appears that in human astrocytic gliomas point mutations of the CDKN2A and CDKN2B genes are uncommon and hypermethylation of the 5' CpG region of CDKN2A does not appear to be a major mechanism for inhibiting transcription of this gene.

Isolation of DNA markers for the rat Sai 1 gene for suppression of anchorage independence by using representational difference analysis

We have applied the representational difference analysis (RDA) to isolate genetic markers for a deletion on the rat chromosome RNO5q22-33. This deletion occurred in anchorage independent sublines of a normal rat fibroblast x mouse hepatoma cell hybrid (BS181) (Islam 1989). Normal rat tissue DNA provided the "tester" and the BS181 hybrid DNA the "driver" in the RDA hybridization/selection reactions. Out of twelve RDA derived DNA sequences that were analyzed in detail using a rat X mouse cell hybrid panel for chromosome mapping, nine (75%) were found to represent RNO5 deletions, whereas the other three were new RFLPs mapping to other chromosomes. In two cases, the RDA sequences were also analyzed by fluorescence in situ hybridization (FISH) and found to give distinct signals in the RNOq22-33 region. This result emphasizes teh significance of the previous cytogenetic analysis of this hybrid, which indicated the presence of a gene for the suppression of anchorage independence, Sai 1, in this deletion region. The RDA derived sequences isolated by this work will provide a valuable source of new genetic markers for the further detailed analysis of the Sai 1 deletion region.

Tumor suppressor gene mutations in mice

Over the past several years, a number of human tumor suppressor genes have been cloned and characterized. Germline mutations in tumor suppressor genes strongly predispose to cancer, and they are also mutated somatically in sporadic forms of the disease. In order to create animal models for the familial cancer syndromes caused by inherited mutations in these genes as well as to determine their role in embryogenesis, the homologues of several members of this class have been mutated in the mouse. The initial characterization of the heterozygous and homozygous phenotypes caused by these mutations has led to important insights into the mechanisms by which tumor suppressor genes participate in normal development and how their loss contributes to tumorigenesis.

Involvement of the cyclin-dependent kinase inhibitor p16 (INK4a) in replicative senescence of normal human fibroblasts

Human diploid fibroblasts (HDFs) can be grown in culture for a finite number of population doublings before they cease proliferation and enter a growth-arrest state termed replicative senescence. The retinoblastoma gene product, Rb, expressed in these cells is hypophosphorylated. To determine a possible mechanism by which senescent human fibroblasts maintain a hypophosphorylated Rb, we examined the expression levels and interaction of the Rb kinases, CDK4 and CDK6, and the cyclin-dependent kinase inhibitors p21 and p16 in senescent HDFs. Cellular p21 protein expression increased dramatically during the final two to three passages when the majority of cells lost their growth potential and neared senescence but p21 levels declined in senescent HDFs. During this period, p16 mRNA and cellular protein levels gradually rose with the protein levels in senescent HDFs reaching nearly 40-fold higher than early passage cells. In senescent HDFs, p16 was shown to be complexed to both CDK4 and CDK6. Immunodepletion analysis of p21 and p16 from the senescent cell extracts revealed that p16 is the major CDK inhibitor for both CDK4 and CDK6 kinases. Immunoprecipitation of CDK4 and CDK6 and their associated proteins from radiolabeled extracts from senescent HDFs showed no other CDK inhibitors. Based upon these results, we propose that senescence is a multistep process requiring the expression of both p21 and p16. p16 up-regulation is a key event in the terminal stages of growth arrest in senescence, which may explain why p16 but not p21 is commonly mutated in immortal cells and human tumors.

Novel form of p21(WAF1/CIP1/SDI1) protein in phorbol ester-induced G2/M arrest

Cell cycle progression requires activation of different cyclin-dependent kinases (CDKs) which are positively regulated by cyclins and negatively regulated by CDK inhibitors. Growth inhibition of the Calu-1 lung carcinoma cells induced with the phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA), a potent activator of protein kinase C, is associated with G2/M arrest and induction of expression of a novel, faster-migrating form of p21(WAF1/CIP1/SDI1) (p21) protein, an inhibitor of cyclin-dependent kinases. This faster-migrating p21 protein was also expressed in TPA-treated A549 lung carcinoma cells which also exhibited G2/M arrest but not in TPA-treated U937 leukemia cells, which only expressed a slower-migrating form of p21 protein. However, reverse transcriptase-polymerase chain reaction and Southern analysis demonstrated no evidence of novel splice in TPA-treated Calu-1 cells. On the other hand, immunoblotting analysis demonstrated that the faster-migrating p21 protein could be detected only by peptide antibody directed against the N terminus but not the C terminus, suggestive of truncation of the latter or protein modification that results in the loss of the C-terminal epitope. Correlation of G2/M arrest with expression of the faster-migrating p21 protein suggests that this novel form of p21 protein may be a mediator of G2/M arrest and growth inhibition.

The 5'-flanking region of the E1 alpha form of the murine p16INK4a (MTS1) gene

We have PCR-amplified and sequenced the immediate (841 bp) 5'-flanking region of murine p16INK4a (MTS1, CDKN2) tumor suppressor gene. Comparing to recently published 5'-flanking region of the human alpha form of p16INK4a, homologies were found in several regions of murine p16INK4a-alpha putative promoter sequence.

Inactivation of the p15INK4B and p16INK4 genes in hematologic malignancies

The recently discovered p15INK4B and p16INK4 genes encoding cell cycle regulating proteins, map to a region on chromosome 9p21 that is commonly deleted in a variety of malignant diseases. The p16INK4 gene has now been shown to be a tumor suppressor gene. It is frequently inactivated in cancer and is possibly the second most often mutated gene in human malignant disease after p53. The role of the p15INK4B and p16INK4 genes in hematologic malignancies has been the subject of intense investigation since their discovery. In this review we address the function and possible role in tumorigenesis of the p15INK4B and p16INK4 genes and discuss their significance as prognostic markers in hematologic malignancies.

Prevalence of germ-line mutations in p16, p19ARF, and CDK4 in familial melanoma: analysis of a clinic-based population

Five to ten percent of individuals with melanoma have another affected family member, suggesting familial predisposition. Germ-line mutations in the cyclin-dependent kinase (CDK) inhibitor p16 have been reported in a subset of melanoma pedigrees, but their prevalence is unknown in more common cases of familial melanoma that do not involve large families with multiple affected members. We screened for germ-line mutations in p16 and in two other candidate melanoma genes, p19ARF and CDK4, in 33 consecutive patients treated for melanoma; these patients had at least one affected first or second degree relative (28 independent families). Five independent, definitive p16 mutations were detected (18%, 95% confidence interval: 6%, 37%), including one nonsense, one disease-associated missense, and three small deletions. No mutations were detected in CDK4. Disease-associated mutations in p19ARF, whose transcript is derived in part from an alternative codon reading frame of p16, were only detected in patients who also had mutations inactivating p16. We conclude that germ-line p16 mutations are present in a significant fraction of individuals who have melanoma and a positive family history.

Temperature-sensitive mutants of p16CDKN2 associated with familial melanoma

Altered expression or function of the p16CDKN2 tumor suppressor gene on chromosome 9p21 occurs in a wide range of human tumors, and mutations in the gene have been shown to segregate with familial predisposition to malignant melanoma. We have used a variety of assays to examine the functional properties of tumor-associated alleles, including eight premature termination mutants, eight missense mutants, and three isoforms of p16 initiated at different amino-terminal methionine codons. The amino- and carboxy-terminal domains of the protein, outside the ankyrin-like repeats, appeared to be dispensable, but the majority of the premature termination mutations led to loss of function. Of the missense mutations tested, four displayed clear loss of function whereas two behaved like the wild type under all conditions tested. The remaining two mutations, a G-to-W mutation at position 101 (Gl01W) and V126D, both of which are associated with familial melanoma, were found to be temperature sensitive for binding to Cdk4 and Cdk6 in vitro, for inhibiting cyclin D1-Cdk4 in a reconstituted pRb-kinase assay, and for increasing the proportion of G1-phase cells following transfection. These findings clarify previous disparities and argue strongly that p16CDKN2 is a bona fide tumor suppressor associated with familial melanoma.

Cyclin-dependent kinases are inactivated by a combination of p21 and Thr-14/Tyr-15 phosphorylation after UV-induced DNA damage

The cyclin-dependent kinase (CDK) inhibitor p21 is induced by the tumor suppressor gene product p53 and is thought to be important for the arrest of the cell cycle following DNA damage. Here we have investigated the contribution of p21 in inhibiting different cyclin-CDK complexes that drive different cell cycle transitions following UV irradiation-induced DNA damage in normal human fibroblasts and immortalized rodent fibroblasts. When cells were exposed to a low dose of UV irradiation, both p53 and p21 were induced; the protein kinase activities associated with Cdc2, Cdk2, and Cdk4 were inhibited; and there was a good correlation between their inhibition and binding to p21. p21 alone is likely to be sufficient for the inhibition of Cdk2 because all the cyclin-complexed forms of Cdk2 were associated with p21 after irradiation. In contrast, only a small proportion of Cdk4 and Cdc2 was complexed with p21, although the level of Cdk4 associated with either p21 or p27 was increased after irradiation. Furthermore, recombinant p21 added to an unirradiated cell lysate at the same level as that induced by irradiation damage inhibited only the kinase activity associated with Cdk2. Cdc2 is likely to be inhibited by Thr-14/Tyr-15 phosphorylation after irradiation because Cdc2 was tyrosine-phosphorylated, and recombinant Cdc25 was able to increase its kinase activity significantly. Taken together, these results suggest that different CDKs are inhibited by different mechanisms following UV-induced DNA damage: Cdk2 is inhibited by the elevated level of p21; Cdk4 is inhibited by cooperation of p21 with other CDK inhibitors, like p27, and possibly by phosphorylation; and Cdc2 is inhibited by Thr-14/Tyr-15 phosphorylation. It is likely that these underlying mechanisms that inactivate CDKs are similar for other kinds of DNA damage.

Two tracks but one race? Cancer genetics

The tumor suppressor gene encoding the cyclin-dependent kinase inhibitor p16 has, remarkably, been found to encode a second protein, p19, with a distinct sequence translated from an alternative reading frame; like p16, p19 can block the cell cycle in G1 phase.

Role of the INK4a locus in tumor suppression and cell mortality

The cell cycle inhibitor p16INK4a is inactivated in many human tumors and in families with hereditary melanoma and pancreatic cancer. Tumor-associated alterations in the INK4a locus may also affect the overlapping gene encoding p19ARF and the adjacent gene encoding p15I1NK4b, both negative regulators of cell proliferation. We report the phenotype of mice carrying a targeted deletion of the INK4a locus that eliminates both p16INK4a and p19ARF. The mice are viable but develop spontaneous tumors at an early age and are highly sensitive to carcinogenic treatments. INK4a-deficient primary fibroblasts proliferate rapidly and have a high colony-formation efficiency. In contrast with normal cells, the introduction of activated Ha-ras into INK4a-deficient fibroblasts can result in neoplastic transformation. These findings directly demonstrate that the INK4a locus functions to suppress neoplastic growth.

Utilization of an alternative open reading frame of a normal gene in generating a novel human cancer antigen

Tumor infiltrating lymphocytes (TILs) derived from tumor-bearing patients recognize tumor-associated antigens presented by major histocompatibility complex (MHC) class I molecules. The infusion of TIL586 along with interleukin (IL) 2 into an autologous patient with metastatic melanoma resulted in the objective regression of tumor. A gene encoding a tumor antigen recognized by TIL586 was recently isolated and shown to encode gp75. Here we report that an antigenic peptide, MSLQRQFLR, recognized by TIL586 was not derived from the normal gp75 protein. Instead, this nonamer peptide resulted from translation of an alternative open reading frame of the same gene. Thus, the gp75 gene encodes two completely different polypeptides, gp75 as an antigen recognized by immunoglobulin G antibodies in sera from a patient with cancer, and a 24-amino acid product as a tumor rejection antigen recognized by T cells. This represents the first demonstration that a human tumor rejection antigen can be generated from a normal cellular gene using an open reading frame other than that used to encode the normal protein. These findings revealed a novel mechanism for generating tumor antigens, which may be useful as vaccines to induce tumor-specific cell-mediated immunity against cancer.