Molecular cloning of the akt oncogene and its human homologues AKT1 and AKT2: amplification of AKT1 in a primary human gastric adenocarcinoma

A previous report described the isolation of a directly transforming retrovirus, AKT8, from a spontaneous thymoma of an AKR mouse. The AKT8 provirus has now been molecularly cloned from a transformed, nonproducer cell line. The virus genome contains both viral and nonviral, cell-related sequences; the nonviral sequence has been designated v-akt, the presumed viral oncogene of the AKT8 virus. This gene lacks homology to the 16 other oncogenes tested. The cloned provirus has undergone a partial deletion, during cell passage in vitro, that prevents direct demonstration of the transforming ability of this molecular clone. Two human homologues of the v-akt oncogene, AKT1 and AKT2, were cloned. A survey of 225 human tumors for changes involving AKT1 led to the discovery of a 20-fold amplification of this gene in one of the five gastric adenocarcinomas tested. The results demonstrate that AKT8 has the characteristic structure of a directly transforming retrovirus and that it contains a gene derived from highly conserved cellular sequences that may be involved in the pathogenesis of some human malignancies.

E2A deficiency leads to abnormalities in alphabeta T-cell development and to rapid development of T-cell lymphomas

The E2A gene products, E12 and E47, are critical for proper early B-cell development and commitment to the B-cell lineage. Here we reveal a new role for E2A in T-lymphocyte development. Loss of E2A activity results in a partial block at the earliest stage of T-lineage development. This early T-cell phenotype precedes the development of a T-cell lymphoma which occurs between 3 and 9 months of age. The thymomas are monoclonal and highly malignant and display a cell surface phenotype similar to that of immature thymocytes. In addition, the thymomas generally express high levels of c-myc. As assayed by comparative genomic hybridization, each of the tumor populations analyzed showed a nonrandom gain of chromosome 15, which contains the c-myc gene. Taken together, the data suggest that the E2A gene products play a role early in thymocyte development that is similar to their function in B-lineage determination. Furthermore, the lack of E2A results in development of T-cell malignancies, and we propose that E2A inactivation is a common feature of a wide variety of human T-cell proliferative disorders, including those involving the E2A heterodimeric partners tal-1 and lyl-1.

The Integrated Genomic Landscape of Thymic Epithelial Tumors

Thymic epithelial tumors (TETs) are one of the rarest adult malignancies. Among TETs, thymoma is the most predominant, characterized by a unique association with autoimmune diseases, followed by thymic carcinoma, which is less common but more clinically aggressive. Using multi-platform omics analyses on 117 TETs, we define four subtypes of these tumors defined by genomic hallmarks and an association with survival and World Health Organization histological subtype. We further demonstrate a marked prevalence of a thymoma-specific mutated oncogene, GTF2I, and explore its biological effects on multi-platform analysis. We further observe enrichment of mutations in HRAS, NRAS, and TP53. Last, we identify a molecular link between thymoma and the autoimmune disease myasthenia gravis, characterized by tumoral overexpression of muscle autoantigens, and increased aneuploidy.

Frequent provirus insertional mutagenesis of Notch1 in thymomas of MMTVD/myc transgenic mice suggests a collaboration of c-myc and Notch1 for oncogenesis

The MMTVD/myc transgenic mice spontaneously develop oligoclonal CD4+CD8+ T-cell tumors. We used provirus insertional mutagenesis in these mice to identify putative collaborators of c-myc. We found that Notch1 was mutated in a high proportion (52%) of these tumors. Proviruses were inserted upstream of the exon coding for the transmembrane domain and in both transcriptional orientations. These mutations led to high expression of truncated Notch1 RNAs and proteins (86-110 kD). In addition, many Notch1-rearranged tumors showed elevated levels of full-length Notch1 transcripts, whereas nearly all showed increased levels of full-length (330-kD) or close to full-length (280-kD) Notch1 proteins. The 5' end of the truncated RNAs were determined for some tumors by use of RT-PCR and 5' RACE techniques. Depending on the orientation of the proviruses, viral LTR or cryptic promoters appeared to be utilized, and coding potential began in most cases in the transmembrane domain. Pulse-chase experiments revealed that the 330-kD Notch1 proteins were processed into 110- and 280-kD cleavage products. These results suggest that Notch1 can be a frequent collaborator of c-myc for oncogenesis. Furthermore, our data indicate that Notch1 alleles mutated by provirus insertion can lead to increased expression of truncated and full-length (330/280-kD) Notch1 proteins, both being produced in a cleaved and uncleaved form.

A common region for proviral DNA integration in MoMuLV-induced rat thymic lymphomas

Similarly to other mammalian and avian retroviruses that lack a transforming gene, moloney murine leukaemia virus (MoMuLV) causes no morphological transformation in infected tissue culture cells. However, following injection in an appropriate animal host, MoMuLV induces mainly thymic lymphomas after a long latency period. A common characteristic of neoplasms induced by retroviruses lacking transforming genes is their clonal origin. Here we have generated MoMuLV-induced rat thymic lymphomas and confirmed their clonal nature. Furthermore, we took advantage of the clonality of these tumours to investigate the specificity of provirus integration in the tumour DNA. We reasoned that if several independently derived thymic lymphomas would contain the provirus integrated in the same region of cellular DNA, this would be a strong indication that this integration event is a contributing factor in oncogenesis. The results indicate that there is indeed a cellular DNA region (termed the MLVI-1 locus) that serves as the substrate for proviral DNA integration in 5 out of 16 tumours we examined.

Tumor immunotherapy targeting fibroblast activation protein, a product expressed in tumor-associated fibroblasts

Murine studies have shown that immunologic targeting of the tumor vasculature, a key element of the tumor stroma, can lead to protective immunity in the absence of significant pathology. In the current study, we expand the scope of stroma-targeted immunotherapy to antigens expressed in tumor-associated fibroblasts, the predominant component of the stroma in most types of cancer. Mice were immunized against fibroblast activation protein (FAP), a product up-regulated in tumor-associated fibroblasts, using dendritic cells transfected with FAP mRNA. Using melanoma, carcinoma, and lymphoma models, we show that tumor growth was inhibited in tumor-bearing mice vaccinated against FAP and that the magnitude of the antitumor response was comparable to that of vaccination against tumor cell-expressed antigens. Both s.c. implanted tumors and lung metastases were susceptible to anti-FAP immunotherapy. The antitumor response could be further enhanced by augmenting the CD4+ T-cell arm of the anti-FAP immune response, achieved by using a lysosomal targeting sequence to redirect the translated FAP product into the class II presentation pathway, or by covaccination against FAP and a tumor cell-expressed antigen, tyrosinase-related protein 2. No morbidity or mortality was associated with anti-FAP vaccination except for a small delay in wound healing. The study suggests that FAP, a product which is preferentially expressed in tumor-associated fibroblasts, could function as a tumor rejection antigen in a broad range of cancers.

A molecular approach to leukemogenesis: mouse lymphomas contain an activated c-ras oncogene

By inducing mouse thymomas with carcinogens and gamma-radiation, we have studied the potential of tumor DNA to induce foci in rodent fibroblasts. A high percentage of the tumors used transformed the cultured cells, and the oncogenic phenotype segregated with extra copies of the c-ras gene family. There appears to be selectivity in the activated gene because so far all analyzed tumors induced by carcinogen have activated the N-ras gene, and those induced by radiation have activated the K-ras gene. The K-ras gene is the cellular counterpart of the viral ras oncogene in Kirsten murine sarcoma virus, but the N-ras has not yet been found in a retrovirus. The transformed cells have a marked increase in expression of the oncogene at the RNA and protein level. This model system might be a powerful tool in the study of leukemogenesis.

Thymoma and paraneoplastic myasthenia gravis

Paraneoplastic autoimmune diseases associate occasionally with small cell lung cancers and gynecologic tumors. However, myasthenia gravis (MG) occurs in at least 30% of all patients with thymomas (usually present at MG diagnosis). These epithelial neoplasms almost always have numerous admixed maturing polyclonal T cells (thymocytes). This thymopoiesis-and export of mature CD4(+)T cells-particularly associates with MG, though there are rare/puzzling exceptions in apparently pure epithelial WHO type A thymomas. Other features potentially leading to inefficient self-tolerance induction include defective epithelial expression of the autoimmune regulator (AIRE) gene and/or of major histocompatibility complex class II molecules in thymomas, absence of myoid cells, failure to generate FOXP3(+) regulatory T cells, and genetic polymorphisms affecting T-cell signaling. However, the strong focus on MG/neuromuscular targets remains unexplained and suggests some biased autoantigen expression, T-cell selection, or autoimmunization within thymomas. There must be further clues in the intriguing serological and cellular parallels in some patients with late-onset MG but without thymomas-and in others with AIRE mutations-and in the contrasts with early-onset MG, as discussed here.

Thymic tumorigenesis induced by overexpression of p56lck

The lck gene encodes a membrane-associated protein tyrosine kinase (p56lck) that is believed to participate in lymphocyte-specific signal transduction pathways. To investigate the function of this molecule, transgenic mice were generated carrying the wild-type lck gene or a mutated lck gene encoding a constitutively activated form of p56lck (p56lckF505). Transgene expression in thymocytes was achieved in each case using the lck proximal promoter element. Mice expressing high levels of either p56lckF505 or p56lckY505 reproducibly developed thymic tumors. The sensitivity of thymocytes to p56lck-induced transformation suggests that disturbances in lck expression may contribute to the pathogenesis of some human neoplastic diseases.

Recurrent genetic aberrations in thymoma and thymic carcinoma

Apart from single reported aberrant karyotypes, genetic alterations in thymic epithelial neoplasms have not been investigated so far. In this study, 12 World Health Organization classification type A thymomas (medullary thymomas), 16 type B3 thymomas (well-differentiated thymic carcinomas), and nine type C thymomas, all of them primary thymic squamous cell carcinomas, were analyzed by comparative genomic hybridization and fluorescence in situ hybridization. With the exception of one single case, type A thymomas did not reveal chromosomal gains or losses in comparative genomic hybridization. In contrast, all type B3 thymomas showed chromosomal imbalances, with gain of 1q, loss of chromosome 6, and loss of 13q occurring in 11 (69%), six (38%), and five (31%) of 16 cases, respectively. In primary thymic squamous cell carcinoma, the most frequent chromosomal losses were observed for 16q (six of nine cases, 67%), 6 (4 of 9, 44%), and 3p and 17p (three of nine each, 33%), whereas recurrent gains of chromosomal material were gains of 1q (5 of 9, 56%), 17q, and 18 (three of nine each, 33%). This study shows that the distinct histological thymoma types A and B3 exhibit distinct genetic phenotypes, whereas type B3 thymoma and primary thymic squamous cell carcinoma partially share genetic aberrations. In addition to the possible tumorigenic role, the deletion in type B3 thymoma of chromosome 6, harboring the HLA locus, might play a role in the pathogenesis of paraneoplastic autoimmunity characteristic of thymoma.

Trisomy of chromosome 15 in spontaneous leukemia of AKR mice

Karyotypes of spontaneous thymomas of AKR mice were determined by trypsin-Giemsa banding methods. Trisomy of chromosome 15 occurred in 10 of 11 leukemic mice. Seven of the thymomas were predominantly trisomic for chromosome 15, one was trisomic for chromosome 12, and one exhibited multiple trisomies of chromosomes 3, 12, 15, and 17. Trisomy was not found in the norm-l AKR tissues examined.

Runx3 regulates integrin alpha E/CD103 and CD4 expression during development of CD4-/CD8+ T cells

During thymic T cell development, immature CD4+CD8+ double-positive (DP) thymocytes develop either into CD4+CD8- Th cells or CD4-CD8+ CTLs. Differentially expressed primary factors inducing the fate of these cell types are still poorly described. The transcription factor Runx3/AML-2 Runx, runt [corrected] dominant factor; AML, acute myeloid leukemia is expressed specifically during the development of CD8 single-positive (SP) thymocytes, where it silences CD4 expression. Deletion of murine Runx3 results in a reduction of CD8 SP T cells and concomitant accumulation of CD4+CD8+ T cells, which cannot down-regulate CD4 expression in the thymus and periphery. In this study we have investigated the role of Runx3 during thymocyte development and CD4 silencing and have identified integrin alpha(E)/CD103 on CD8 SP T cells as a new potential target gene of Runx3. We demonstrate that Runx3 is necessary not only to repress CD4, but also to induce CD103 expression during development of CD8 SP T cells. In addition, transgenic overexpression of Runx3 reduced CD4 expression during development of DP thymocytes, leading to a reduced number of CD4 SP thymocytes and an increased number of CD8 SP thymocytes. This reversal is not caused by redirection of specific MHC class II-restricted cells to the CD8 lineage. Overexpression of Runx3 also up-regulated CD103 expression on a subpopulation of CD4 SP T cells with characteristics of regulatory T cells. Thus, Runx3 is a main regulator of CD4 silencing and CD103 induction and thus contributes to the phenotype of CD8 SP T cells during thymocyte development.

Ectopic expression of E47 or E12 promotes the death of E2A-deficient lymphomas

Mice with null mutations in the E2A gene are highly susceptible to the spontaneous development of thymic lymphomas. To understand better how E2A deficiency may contribute to lymphomagenesis, we have observed the consequences of enforced expression of the E2A gene products E12 and E47 in cell lines derived from lymphomas that arose spontaneously in E2A-deficient mice. E2A-expressing cells are steadily eliminated from lymphoma cultures into which E47 or E12 was introduced. The mechanism underlying the loss of E2A-expressing cells does not involve an arrest in cell-cycle progression. Rather, the E2A proteins activate a programmed cell death pathway in these lymphomas. This E2A-mediated cell death appears to be preceded by a loss of mitochondrial transmembrane potential. These data provide direct evidence that E2A gene products can act as tumor suppressors.

Critical role for Atm in suppressing V(D)J recombination-driven thymic lymphoma

Chromosome translocations involving T cell receptor (TCR) loci have been found in tumors from Ataxia telangiectasia (AT) patients and in mouse Atm-/- thymoma, suggesting the involvement of V(D)J recombination in these malignancies. By introducing a RAG-1 deficiency into Atm-/- mice in the presence of a TCR transgene, we show that V(D)J recombination is critical for thymoma development in these mice. Therefore, aberrant V(D)J recombination, normally suppressed by Atm, facilitates tumorigenic events leading to cancer. Because V(D)J recombination is dispensable for lymphomagenesis upon p53 deficiency, this study also indicates that Atm and p53 function by distinct mechanisms in suppressing thymoma.

Pathogenesis of myasthenia gravis

Various studies over the last 25 years in Man and animal models have revealed many steps in the pathogenesis of myasthenia gravis (MG) which is now considered the classical organ specific, autoantibody mediated and T cell dependent human autoimmune disease. Though not a disease entity, MG is associated with pathological alterations of the thymus in about 80% of cases. These are described here with reference to distinct models of autoimmunization against the acetylcholine receptor (AChR). In MG with thymitis, intrathymic production of AChR-specific autoantibodies is the result of a classical antigen-driven immune reaction that occurs completely inside the thymus and probably involves AChR on myoid cells as the triggering (myasthenogenic) antigen. Genetic factors contribute essentially to the pathogenesis of this form of MG. In thymoma-associated MG genetic factors are probably of marginal significance. Neither intratumour autoantibody production nor T cell activation seem to occur and the AChR is not the myasthenogenic antigen. Instead, abnormal neurofilaments that share epitopes with the AChR and other auto-antigen targets in paraneoplastic MG are expressed in thymomas and may trigger autoantigen-specific, non-tolerogenic T cell selection by molecular mimicry. These data support the hypothesis that initial steps in the pathogenesis of most MG cases take place within abnormal thymic microenvironments, be they inflammatory or neoplastic. Where these initial steps occur in MG cases without thymic pathology is not known. Likewise, the factors involved in the initial triggering of MG remain enigmatic in all MG subtypes.

Targeting apoptotic tumor cells to Fc gamma R provides efficient and versatile vaccination against tumors by dendritic cells

Dendritic cells (DCs) loaded with tumor-associated Ags (TAAs) act as potent adjuvant that initiates antitumor immune responses in vivo. However, TAA-based DC vaccination requires prior identification of TAAs. Apoptotic tumor cells (ATCs) can be an excellent source for DC loading because their potential uncharacterized Ags would be efficiently presented to T cells without any prior characterization and isolation of these Ags. However, ATCs alone are considered to be inefficient for activating antitumor immunity, possibly because of their inability to induce DC maturation. In this study, the aim was to enhance antitumor immune response by taking advantage of ATCs that have been opsonized with IgG (ATC-immune complexes, ATC-ICs) so as to target them to FcR for IgG (FcgammaRs) on DCs. It was found that when compared with ATCs, ATC-ICs were efficiently internalized by DCs via FcgammaRs, and this process induced maturation of DCs, which was more efficient than that of ATCs. Importantly, ATC-IC loading was shown to be more efficient than ATCs alone in its capacity for inducing antitumor immunity in vivo, in terms of cytotoxic T cell induction and tumor rejection. These results show that using ATC-ICs may overcome the limitations and may enhance the immune response of current ATC-based DC vaccination therapy.

Imatinib mesylate in patients with WHO B3 thymomas and thymic carcinomas

Thymic malignancies are rare tumors of the mediastinum. c-KIT is highly expressed in thymic carcinomas (TC) but infrequently in thymomas. Anecdotal experience suggests activity of imatinib mesylate in TC. Patients with unresectable World Health Organization B3 thymomas or TC, performance status 0 to 2, good organ function, and measurable disease were enrolled in this study. Imatinib was administered at 600 mg PO daily. Seven patients were recruited at one institution: two World Health Organization B3 thymomas and five TC. Imatinib treatment was generally well tolerated. Two patients had stable disease and five progressed. Median survival was 4 months, and median time to progression was 2 months. c-KIT expression was found in one of four samples by immunohistochemistry. No mutations were detected in the c-KIT or PDGFRA genes in three samples analyzed. Imatinib has no major activity in this rare tumor. Given the small number of patients treated in this study, selection based on presence of c-KIT mutations might be warranted.

Mucocutaneous candidiasis and autoimmunity against cytokines in APECED and thymoma patients: clinical and pathogenetic implications

Much has been learnt about the mechanisms of thymic self-tolerance induction from work on both the rare autosomal recessive disease autoimmune polyendocrinopathy candidiasis ectodermal dystrophy (APECED) and the autoimmune regulator (AIRE) protein mutated in this disease. Normally, AIRE drives low-level expression of huge numbers of peripheral tissue-specific antigens (TSAgs) in medullary thymic epithelial cells (mTECs), leading to the deletion of TSAg-reactive thymocytes maturing nearby. The very recently discovered neutralizing autoantibodies (autoAbs) against Th17-related cells and cytokines in two autoimmunity-related syndromes associated with AIRE-mutant thymi or AIRE-deficient thymomas help to explain the chronic mucocutaneous candidiasis (CMC) seen in both syndromes. The surprising parallels between these syndromes also demand new hypotheses and research into the consequences of AIRE deficiency and the ensuing autoimmunizing pathways, and suggest more appropriate treatment regimens as discussed in this review.

Disruption of thymocyte development and lymphomagenesis induced by SV40 T-antigen

The lck gene encodes a membrane-associated protein tyrosine kinase that is expressed specifically in lymphoid cells, especially thymocytes. Structural analysis of the murine and human lck genes previously identified conserved 5' flanking sequences that were proposed to represent transcriptional regulatory elements. Here we demonstrate that a murine lck promoter construct containing these sequences directs the expression of the SV40 T-antigen gene in lymphoid cells. Remarkably, expression of SV40 T-antigen in transgenic animals dramatically disturbs thymic development, resulting in preferential loss of CD4+CD8+ thymocytes. In contrast, immature cells lacking both CD4 and CD8 markers are present in near-normal numbers. Thus SV40 T-antigen expression appears partially to arrest thymopoiesis. Mice bearing the lck-SV40 transgene develop readily explantable thymic tumors at 12-18 weeks of age. Fluorocytometric analyses of lck-SV40 tumor cells reveal that immature thymocytes are frequently immortalized. The lck-SV40 mouse may therefore provide materials for the in vitro investigation of thymocyte differentiation.

No requirement for V(D)J recombination in p53-deficient thymic lymphoma

The p53 tumor suppressor is activated in response to a variety of cellular stress signals, although specific in vivo signals that trigger tumor suppression are unknown. In mouse thymocytes, where p53 inactivation leads to tumorigenesis, several observations suggest that V(D)J recombination of T-cell receptor (TCR) loci could provide a DNA damage signal triggering p53-dependent apoptosis and tumor suppression. Inactivation of p53 would allow V(D)J driven mutation of additional cancer genes, facilitating tumorigenesis. Here, we show that mice with a p53 deficiency in thymocytes and unable to carry out V(D)J recombination are not impaired in the development of thymoma. Recombination-activating gene (RAG) deficiencies were introduced into both p53-/- mice and TgTDeltaN transgenic mice, a strain in which 100% of the mice develop thymoma due to thymocyte-specific inactivation of p53 by a simian virus 40 T-antigen variant. V(D)J recombination was dispensable for tumorigenesis since thymomas developed with or without the RAG-1 or RAG-2 gene, although some delay was observed. When V(D)J recombination was suppressed by expression of rearranged TCR transgenes, 100% of the TgTDeltaN mice developed thymoma, surprisingly with reduced latency. Further introduction of a RAG deficiency into these mice had no impact on the timing or frequency of tumorigenesis. Finally, karyotype and chromosome painting analyses showed no evidence for TCR gene translocations in p53-deficient thymomas, although abundant aneuploidy involving frequent duplication of certain chromosomes was present. Thus, contrary to the current hypothesis, these studies indicate that signals other than V(D)J recombination promote p53 tumor suppression in thymocytes and that the mechanism of tumorigenesis is distinct from TCR translocation oncogene activation.

Thymoma and Thymic Carcinoma: An Update of the WHO Classification 2004

When dealing with tumors of the thymus and mediastinum, both the surgeon and the pathologist are confronted with a vast variety of different pathohistologic entities, among which epithelial tumors, namely thymomas and thymic carcinomas, are the most frequent. Intimate knowledge of the different entities and their anticipated clinical outcome is required to meet diagnostic and therapeutic challenges. We herein briefly update the reader with the most important new changes in the 2004 WHO classification of tumors of the thymus and to important new trends in the treatment of some of these tumors.

Genetic association of Ctla-4 to myasthenia gravis with thymoma

Cytotoxic T lymphocyte associated antigen-4 (CTLA-4) plays a pivotal role in downregulating both the cellular and the humoral response by suppressing ongoing responses of activated T cells. Our earlier study showed that genetic variations in interleukin-1 genes confer susceptibility to myasthenia gravis, especially in patients having the lowest risk from major histocompatibility complex genes. Here we describe an association of Ctla-4 gene to the disease with thymoma and a higher prevalence of CTLA-4 gene polymorphism allele 104 in patients positive for IL-1beta TaqI allele 2, an IL-1beta 'high secretor' phenotype. There was no association in patients with hyperplasia and normal thymic histology. These results further advocate that MG is a polygenetic disease and suggest that co-stimulators such as CTLA-4 and CD28 might have an important role in the pathogenesis of the disease.

A role for Brca1 in chromosome end maintenance

The role of BRCA1 in breast and ovarian tumor suppression has been primarily ascribed to the maintenance of genome integrity. BRCA1 interacts with components of the non-homologous end-joining pathway previously shown to play a role in telomere maintenance in yeast. Here, we provide evidence that links Brca1 with telomere integrity. Brca1(-/-) T-cells display telomere dysfunction in both loss of telomere repeats as well as defective telomere capping. Loss of Brca1 synergizes with p53 deficiency in the onset and frequency of tumorigenesis. Karyotyping of tBrca1(-/-)p53(-/-) thymic lymphomas revealed the presence of telomere dysfunction accompanied by clonal chromosomal translocations. The telomere dysfunction phenotype in Brca1-deficient cells suggests that loss of telomere integrity might contribute to chromosome end dysfunction and permit the formation of potentially oncogenic translocations.

p16INK4, pRB, p53 and cyclin D1 expression and hypermethylation of CDKN2 gene in thymoma and thymic carcinoma

There have been few reports on genetic alterations in thymomas. To investigate the expression of p16INK4A, RB, p53 and cyclin D1 in thymomas, we first examined 36 thymomas (non-invasive type, 16 cases; invasive type, 20 cases) and 3 thymic carcinomas, using immunohistochemistry. Abnormal expression of p16INK4A, RB, p53 and cyclin D1 was observed in 18, 8, 10 and 7 cases, respectively. Only a subgroup of invasive thymomas and thymic carcinomas showed an inverse correlation between p16INK4A and RB expression. Subsequently, we examined the 36 thymomas and 4 thymic carcinomas for mutations in p53 and CDKN2 genes, using PCR-SSCP and direct-sequencing analyses. No mutation of these genes was detected in the thymomas and thymic carcinomas examined. A polymorphism in the 3' untranslated region of exon 3 of CDKN2 was detected in 5 cases of thymoma. We searched for hypermethylation in the promoter region of CDKN2, observing it in 4 thymomas and 1 thymic carcinoma. Our data suggest that, unlike other more common cancers, alteration of the p53 gene may not play a significant role in the tumorigenesis of thymoma. However, inactivation of p16INK4A and RB may play a role in the progression of thymoma and thymic carcinoma.