TP53 allele loss, mutations and expression in malignant melanoma

The Genetics of Early-Stage Melanoma in a Veteran Population

To improve understanding of the genetic signature of early-stage melanomas in Veterans, hotspot mutation profiling using next-generation sequencing (NGS) was performed on melanoma tissue samples from patients at the Iowa City Veterans Affairs Medical Center (VAMC). Genetic analysis identified BRAF (36.3%), TP53 (25.9%), NRAS (19.3%), CDKN2A (11.1%), KIT (8.1%), and BAP1 (7.4%) mutations with the highest prevalence. Although common variants in BRAF were detected at lower rates than what is reported for the general population, 55.6% of cases showed activating mutations in the RAS/RAF pathways. Variants in TP53 and KIT were detected at higher rates than in the general population. Veterans with prior history of melanoma were at significantly higher odds of having TP53 mutation (OR = 2.67, p = 0.04). This suggests that TP53 may be a marker for recurrent melanoma and possibly alternative exposures in the military population. This study provides new information regarding the genetics of melanoma in a Veteran population and early-stage melanomas, highlighting risk factors unique to this population and contributing to the conversation about preventing melanoma deaths in US Military personnel.

Immunohistochemistry as an accurate tool for the assessment of BRAF V600E and TP53 mutations in primary and metastatic melanoma

Metastatic melanoma is a fatal disease with poor prognosis. Ever since targeted therapy against oncogenic BRAF was approved, molecular profiling has become an integral part of the management of such patients. While molecular testing is not available in all pathology laboratories, immunohistochemistry (IHC) is a reliable screening option. The major objective of the present study was to evaluate whether IHC detection of BRAF and the tumor (suppressor) protein 53 gene (TP53) are reliable surrogates for mutation detection. Formalin-fixed paraffin-embedded samples of melanomas for which molecular data were previously obtained by targeted next-generation sequencing (NGS) between January 2014 and February 2019 were immunostained with BRAF V600E and p53 antibodies. A blinded evaluation of the IHC slides was performed by two pathologists in order to evaluate inter-observer concordance (discordant cases were reviewed by a third observer). The associations between the results of IHC and molecular profiling were evaluated. The study included a series of 37 cases of which 15 harbored a BRAF mutation and five a TP53 mutation. IHC had an overall diagnostic accuracy of 93.9% for BRAF V600E and 68.8% for TP53 compared to NGS. A statistically significant association between the two diagnostic methods was obtained for BRAF V600E (P=0.0004) but not for p53 (P=0.3098) IHC. The κ coefficient for IHC assessment of p53 was 0.55 and that for BRAF V600E was 0.72. In conclusion, the present results evidenced that IHC staining is a reliable surrogate for NGS in identifying the BRAF V600E mutation, which may become an efficient screening tool. Aberrant expression of p53 on IHC is at times associated with TP53 mutations but it was not possible to establish a direct link.

G-quadruplexes: a promising target for cancer therapy

DNA and RNA can fold into a variety of alternative conformations. In recent years, a particular nucleic acid structure was discussed to play a role in malignant transformation and cancer development. This structure is called a G-quadruplex (G4). G4 structure formation can drive genome instability by creating mutations, deletions and stimulating recombination events. The importance of G4 structures in the characterization of malignant cells was currently demonstrated in breast cancer samples. In this analysis a correlation between G4 structure formation and an increased intratumor heterogeneity was identified. This suggests that G4 structures might allow breast cancer stratification and supports the identification of new personalized treatment options. Because of the stability of G4 structures and their presence within most human oncogenic promoters and at telomeres, G4 structures are currently tested as a therapeutic target to downregulate transcription or to block telomere elongation in cancer cells. To date, different chemical molecules (G4 ligands) have been developed that aim to target G4 structures. In this review we discuss and compare G4 function and relevance for therapeutic approaches and their impact on cancer development for three cancer entities, which differ significantly in their amount and type of mutations: pancreatic cancer, leukemia and malignant melanoma. G4 structures might present a promising new strategy to individually target tumor cells and could support personalized treatment approaches in the future.

Emerging roles of the MAGE protein family in stress response pathways

The melanoma antigen (MAGE) proteins all contain a MAGE homology domain. MAGE genes are conserved in all eukaryotes and have expanded from a single gene in lower eukaryotes to ∼40 genes in humans and mice. Whereas some MAGEs are ubiquitously expressed in tissues, others are expressed in only germ cells with aberrant reactivation in multiple cancers. Much of the initial research on MAGEs focused on exploiting their antigenicity and restricted expression pattern to target them with cancer immunotherapy. Beyond their potential clinical application and role in tumorigenesis, recent studies have shown that MAGE proteins regulate diverse cellular and developmental pathways, implicating them in many diseases besides cancer, including lung, renal, and neurodevelopmental disorders. At the molecular level, many MAGEs bind to E3 RING ubiquitin ligases and, thus, regulate their substrate specificity, ligase activity, and subcellular localization. On a broader scale, the MAGE genes likely expanded in eutherian mammals to protect the germline from environmental stress and aid in stress adaptation, and this stress tolerance may explain why many cancers aberrantly express MAGEs Here, we present an updated, comprehensive review on the MAGE family that highlights general characteristics, emphasizes recent comparative studies in mice, and describes the diverse functions exerted by individual MAGEs.

Pathway analysis of genomic pathology tests for prognostic cancer subtyping

Genomic test results collected during the provision of medical care and stored in Electronic Health Record (EHR) systems represent an opportunity for clinical research into disease heterogeneity and clinical outcomes. In this paper, we evaluate the use of genomic test reports ordered for cancer patients in order to derive cancer subtypes and to identify biological pathways predictive of poor survival outcomes. A novel method is proposed to calculate patient similarity based on affected biological pathways rather than gene mutations. We demonstrate that this approach identifies subtypes of prognostic value and biological pathways linked to survival, with implications for precision treatment selection and a better understanding of the underlying disease. We also share lessons learned regarding the opportunities and challenges of secondary use of observational genomic data to conduct such research.

Redox effects and cytotoxic profiles of MJ25 and auranofin towards malignant melanoma cells

Malignant melanoma is the most dangerous type of skin cancer. Although recent progress in treatment has been achieved, lack of response, drug resistance and relapse remain major problems. The tumor suppressor p53 is rarely mutated in melanoma, yet it is inactive in the majority of cases due to dysregulation of upstream pathways. Thus, we screened for compounds that can activate p53 in melanoma cells. Here we describe effects of the small molecule MJ25 (2-{[2-(1,3-benzothiazol-2-ylsulfonyl)ethyl]thio}-1,3-benzoxazole), which increased the level of p53-dependent transactivation both as a single agent and in combination with nutlin-3. Furthermore, MJ25 showed potent cytotoxicity towards melanoma cell lines, whilst having weaker effects against human normal cells. MJ25 was also identified in an independent screen as an inhibitor of thioredoxin reductase 1 (TrxR1), an important selenoenzyme in the control of oxidative stress and redox regulation. The well-characterized TrxR inhibitor auranofin, which is FDA-approved and currently in clinical trials against leukemia and a number of solid cancers, displayed effects comparable with MJ25 on cells and led to eradication of cultured melanoma cells at low micromolar concentrations. In conclusion, auranofin, MJ25 or other inhibitors of TrxR1 should be evaluated as candidate compounds or leads for targeted therapy of malignant melanoma.

Whole genome and exome sequencing of melanoma: a step toward personalized targeted therapy

Melanoma has historically been refractive to traditional therapeutic approaches. As such, the development of novel drug strategies has been needed to improve rates of overall survival in patients with melanoma, particularly those with late stage or disseminated disease. Recent success with molecularly based targeted drugs, such as Vemurafenib in BRAF-mutant melanomas, has now made "personalized medicine" a reality within some oncology clinics. In this sense, tailored drugs can be administered to patients according to their tumor "mutation profiles." The success of these drug strategies, in part, can be attributed to the identification of the genetic mechanisms responsible for the development and progression of metastatic melanoma. Recently, the advances in sequencing technology have allowed for comprehensive mutation analysis of tumors and have led to the identification of a number of genes involved in the etiology of metastatic melanoma. As the methodology and costs associated with next-generation sequencing continue to improve, this technology will be rapidly adopted into routine clinical oncology practices and will significantly impact on personalized therapy. This review summarizes current and emerging molecular targets in metastatic melanoma, discusses the potential application of next-generation sequencing within the paradigm of personalized medicine, and describes the current limitations for the adoption of this technology within the clinic.

MAGE-RING protein complexes comprise a family of E3 ubiquitin ligases

The melanoma antigen (MAGE) family consists of more than 60 genes, many of which are cancer-testis antigens that are highly expressed in cancer and play a critical role in tumorigenesis. However, the biochemical and cellular functions of this enigmatic family of proteins have remained elusive. Here, we identify really interesting new gene (RING) domain proteins as binding partners for MAGE family proteins. Multiple MAGE family proteins bind E3 RING ubiquitin ligases with specificity. The crystal structure of one of these MAGE-RING complexes, MAGE-G1-NSE1, reveals structural insights into MAGE family proteins and their interaction with E3 RING ubiquitin ligases. Biochemical and cellular assays demonstrate that MAGE proteins enhance the ubiquitin ligase activity of RING domain proteins. For example, MAGE-C2-TRIM28 is shown to target p53 for degradation in a proteasome-dependent manner, consistent with its tumorigenic functions. These findings define a biochemical and cellular function for the MAGE protein family.

Similar nucleotide excision repair capacity in melanocytes and melanoma cells

Sunlight UV exposure produces DNA photoproducts in skin that are repaired solely by nucleotide excision repair in humans. A significant fraction of melanomas are thought to result from UV-induced DNA damage that escapes repair; however, little evidence is available about the functional capacity of normal human melanocytes, malignant melanoma cells, and metastatic melanoma cells to repair UV-induced photoproducts in DNA. In this study, we measured nucleotide excision repair in both normal melanocytes and a panel of melanoma cell lines. Our results show that in 11 of 12 melanoma cell lines tested, UV photoproduct repair occurred as efficiently as in primary melanocytes. Importantly, repair capacity was not affected by mutation in the N-RAS or B-RAF oncogenes, nor was a difference observed between a highly metastatic melanoma cell line (A375SM) or its parental line (A375P). Lastly, we found that although p53 status contributed to photoproduct removal efficiency, its role did not seem to be mediated by enhanced expression or activity of DNA binding protein DDB2. We concluded that melanoma cells retain capacity for nucleotide excision repair, the loss of which probably does not commonly contribute to melanoma progression.

The genome and epigenome of malignant melanoma

Malignant melanoma originates in melanocytes, the pigment-producing cells of the skin and eye, and is one of the most deadly human cancers with no effective cure for metastatic disease. Like many other cancers, melanoma has both environmental and genetic components. For more than 20 years, the melanoma genome has been subject to extensive scrutiny, which has led to the identification of several genes that contribute to melanoma genesis and progression. Three molecular pathways have been found to be nearly invariably dysregulated in melanocytic tumors, including the RAS-RAF-MEK-ERK pathway (through mutation of BRAF, NRAS or KIT), the p16 INK4A-CDK4-RB pathway (through mutation of INK4A or CDK4) and the ARF-p53 pathway (through mutation of ARF or TP53). Less frequently targeted pathways include the PI3K-AKT pathway (through mutation of NRAS, PTEN or PIK3CA) and the canonical Wnt signaling pathway (through mutation of CTNNB1 or APC). Beyond the specific and well-characterized genetic events leading to activation of proto-oncogenes or inactivation of tumor suppressor genes in these pathways, systematic high-resolution genomic analysis of melanoma specimens has revealed recurrent DNA copy number aberrations as well as perturbations of DNA methylation patterns. Melanoma provides one of the best examples of how genomic analysis can lead to a better understanding of tumor biology. We review current knowledge of the genes involved in the development of melanoma and the molecular pathways in which these genes operate.

The role of p53 in pigmentation, tanning and melanoma

p53 has a central role in skin pigmentation and may impact on melanoma at all stages, however, as it's mutation frequency in melanoma is low, it's role has been somewhat under-appreciated. During normal skin function, p53 in the keratinocyte is a transducer of the skin tanning signal and an essential component of what is effectively a keratinocyte-melanocyte signaling cycle that regulates skin pigmentation. It is clear that this cycle functions optimally in skin of dark pigmentation. When melanin biosynthesis is genetically disrupted in skin of white complexion, we propose that this cycle operates as a promoter of melanocyte proliferation. The cell autonomous function of p53 in melanocytes is not well described, however, the balance of the evidence suggests that p53 is an effective tumor suppressor and the myriad of mechanisms by which the p53 pathway may be dysregulated in tumors attests to it importance as a tumor suppressor. In this review, we outline the known mechanisms that impair p53 itself and its immediate regulators or target genes during melanomagenesis. Due to the importance of this pathway, it is clear that p53 disruptions may relate directly to a patient's prognosis. This pathway will continue to be a focus of investigation, particularly with respect to targeted experimental chemotherapeutics.

p53 plays a central role in UVA and UVB induced cell damage and apoptosis in melanoma cells

We investigated whether p53 plays a role in UV induced apoptosis in melanoma. UVA and UVB induced apoptosis in dose dependent and wild type p53 melanoma cells were much more vulnerable than the mutant cells, indicating that p53 played a role in UV-induced apoptosis in melanoma. No difference in p53 expression pattern between the primary and matched metastatic melanomas was noticed in tumour tissue or cell lines from the same patients. Our findings indicate that expression of p53 plays a role in UV-induced apoptosis in melanoma cells, but not important in melanoma progression from primary to metastasis.

Comprehensive analysis of the p53 status in mucosal and cutaneous melanomas

The abrogation of the function of the "gatekeeper of the genome", p53, is the most prevalent molecular alteration in solid human tumors. Regarding melanomas the involvement of p53 alterations is discussed controversially to date. In order to evaluate the status of p53 in detail, primary tumors and metastases of 63 sporadic cutaneous (CM) and mucosal (MuM) melanomas were examined by immunohistochemistry and sequence analysis of the entire coding region of the p53 transcript, i.e., exons 2 to 11. In addition, loss of heterozygosity (LOH) and loss of allele-specific transcription (LOT) were determined. Accumulation of the p53 protein occurred in most of the CM and MuM specimens (71% and 58%, respectively). In contrast, protein stabilizing p53 mutations were observed in 14% of the CM and no mutation was found in MuM specimens. Two of the aberrations located outside the core domain. LOH was detected in 22% CM and 58% MuM, and LOT in 25% of the CM specimens. The genotype distribution at the polymorphic p53 codon 72 in melanoma patients differed significantly from control subjects. The calculation of odds ratios (OR) and 95% confidence intervals (CI) indicated an increased risk for developing cutaneous melanomas in individuals carrying the Pro-coding allele. Altogether, aberrant p53 expression appears to be a common event in both CM and MuM.

Comparative Analysis of Loss of Heterozygosity and Microsatellite Instability in Adult and Pediatric Melanoma

Although 0.3% of melanomas occur in children, the incidence has risen in past decades. In adult melanoma, some chromosomal regions in 1p, 6q, 9p, 10q, and 11q are frequently deleted. Microsatellite instability (MSI), which reflects impaired DNA repair, has been found at low levels in adult melanoma and melanocytic nevi. To investigate the molecular changes in pediatric melanoma, a screening for loss of heterozygosity and microsatellite instability was performed and compared with changes found in adult melanoma. Formalin-fixed, paraffin-embedded tissues from 10 adult melanomas, 9 melanocytic nevi, and 8 pediatric melanomas were microdissected and the DNA was extracted. Loss of heterozygosity and microsatellite instability were evaluated using 13 microsatellite repeat polymorphisms located in 1p36, 1q32, 2p12, 2p22-25, 2q33-37, 9p21, 10q23.3, 11q23, 13q14, 17p13, and 17q21. The overall frequency of loss of heterozygosity was 0.09 for nevi, 0.30 for adult melanoma, and 0.43 for pediatric melanoma (nevi vs. adult melanoma, P = 0.0082; nevi vs. pediatric melanoma, P = 0.0092). Pediatric melanoma has more loss of heterozygosity (44%) in 11q23 than adult melanoma (7%, P = 0.046). The microsatellite instability overall frequency was greater in pediatric melanoma (0.24) than nevi (0.05, P = 0.0031) and adult melanoma (0.09, P = 0.0195). Our findings suggest that pediatric melanoma has a different abnormal pattern than adult melanoma. Pediatric melanoma has more microsatellite instability than adult melanoma. 11q23 could contain genes related to the early age onset of melanoma. The high frequency of microsatellite instability is coincidental with the finding of higher levels of microsatellite instability in pediatric brain tumors and could play a role in the pathogenesis of pediatric melanoma.

p53 protein expression and TP53 mutations in malignant melanomas of sun-sheltered mucosal membranes versus chronically sun-exposed skin

In this paper, we compare the expression of the TP53 gene product, p53 protein (p53p), in primary malignant melanomas from sun-shielded mucous membranes and from chronically sun-exposed skin. Archival tissues from 29 patients with mucosal melanomas and from 27 with cutaneous melanomas in facial skin were subjected to immunohistochemical procedures using the monoclonal antibody DO-1. p53p expression did not differ significantly between the two groups of melanomas. A comparison with previously obtained data on TP53 mutations from the same tumours showed closer concordance amongst mucosal than amongst skin tumours. Primary mucosal melanomas and their satellites showed identical patterns, focal or diffuse, of p53p expression. Thus, expression of altered p53p could well participate in the clonal expansion of these mucosal melanomas and in tumour progression. The p53p characteristics recognized in our investigations are amongst the first hallmarks in the emerging molecular pathological profiling of mucosal melanomas, and may therefore be useful in exploring the aetiology of UV-independent melanomas.

Molecular markers in malignant cutaneous melanoma: Gift horse or one-trick pony?

The management of malignant cutaneous melanoma is problematic. Current clinical prognostic factors do not adequately predict disease recurrence and overall survival in a significant subset of patients. Adjuvant therapies for melanoma are notoriously toxic and associated with significant morbidity. Furthermore, it has been difficult to predict which patients will respond best to these treatments, if at all. DNA and RNA biomarkers have been developed to help overcome these problems. Biomarkers have been shown to upstage patients with melanoma, but are the assays sensitive and specific enough for clinical use as predictors of disease outcome or treatment response? We review our experience with DNA and RNA biomarkers in terms of their prognostic and predictive capabilities in malignant melanoma and outline their likely role in the future of melanoma staging, surveillance, and treatment.

Telomere erosion triggers growth arrest but not cell death in human cancer cells retaining wild-type p53: implications for antitelomerase therapy

Telomerase activity in tumours is often associated with p53 mutation. Many antitelomerase therapies take advantage of the inability of cells expressing mutant p53 to undergo replicative senescence, since this allows telomere erosion to continue until 'crisis', hence providing the desired cytotoxic effect. However, some tumour types, including breast, melanomas and thyroid, retain wild-type p53 function and the effectiveness of antitelomerase therapies in such tumour cells have not been adequately addressed. To explore this, we made use of two thyroid cancer cell lines K1 and K2, which retain wt p53. Telomere erosion induced by the expression of a dominant-negative (DN) hTERT resulted in delayed onset of growth arrest in K1 and K2 cells, reminiscent of replicative senescence, with low levels of BrdU labelling and apoptosis, associated with high p21(WAF1) and senescence-associated beta galactosidase expression. In contrast, abrogation of p53 function by the expression of HPV16 E6 in K1 and K2 cells either at the same time as DNhTERT or just prior to the onset of senescence allowed cells to continue growing until 'crisis'. Likewise, microinjection of a p53 neutralizing antibody into 'senescent' K1 DNhTERT cells permitted re-entry into the cell cycle. We conclude that thyroid tumour cells with wild-type p53 retain an intact p53-mediated growth arrest response to telomere erosion. This raises the intriguing question of why, therefore, p53 mutation is not selected for in such cancers, and also calls into question the therapeutic value of telomerase inhibitors in such cases.

The TP53 Tumor Suppressor Gene and Melanoma Tumorigenesis: Is There a Relationship?

Mutations in the TP53 gene are found in about 11% of melanomas. Although nearly 600 papers have been published with varying degrees of consensus, there does not appear to be any comparable analysis that facilitates more than a glimpse into the role of p53 in melanomagenesis. This article reviews p53 alterations (at the gene and protein levels) in melanocytic skin lesions and discusses the following points: (i) p53 alterations commence as early as at the stage of benign and dysplastic nevi; (ii) these alterations are frequent in melanomas, and gradually increase with their progression; (iii) there is no concordance between the frequent p53 protein expression and the rarity of both TP53 gene mutations in melanomas, and (iv) the entire p53 pathway is a more critical determinant of the fate of the melanocytic skin lesions than the status of the p53 protein or the gene itself.

p53-related pathways and the molecular pathogenesis of melanoma

Cutaneous malignant melanoma (CMM) is a life-threatening cancer that can have a poor prognosis with high metastatic potential. Its incidence is rapidly increasing worldwide. Its molecular alterations involve multiple pathways, including those related to p53. Since 1981, more than 380 papers containing the terms 'p53 and melanoma' as key words in the Abstract have been published in the literature. However, in spite of these extensive investigations, a review of p53 and associated genes in CMM is still lacking. To remedy this issue, this review seeks to provide a brief overview of p53 and discuss the genes targeted along its related pathways.

Expression and significance of p53, rb, p21/waf-1, p16/ink-4a, and PTEN tumor suppressors in canine melanoma

The role of tumor suppressor genes in the pathogenesis of canine melanoma is incompletely understood. The genes encoding the tumor suppressors p53, Rb, p21 (waf-1), p16 (ink-4a), and PTEN have been postulated to contribute to the pathogenesis of melanoma in humans and experimental animal models. To assess whether inactivation of these genes similarly contributes to the origin and progression of canine melanoma, we examined their expression in seven distinct canine melanoma cell lines and in 31 retrospective samples (representing 29 dogs) of spontaneous canine melanoma. Various patterns suggestive of loss of tumor suppressor function emerged in these cell lines. The most frequently observed abnormality was loss or significant reduction of p16 expression in six of seven cell lines and in 21 of 26 tumor samples. Loss or significant reduction of PTEN expression was seen in four of seven cell lines and in 13 of 27 tumor samples. Although p53 was detectable in all the cell lines and in 24 of 30 tumors, exclusion of p53 from the nuclear compartment was observed in each of the cell lines and in 18 of 25 tumor samples. These results indicate that loss of function of these tumor suppressor proteins is a common occurrence that may contribute to the origin of canine melanoma. In our sample population, abnormalities in the expression or localization of one or more tumor suppressor proteins occurred with similar frequency in malignant and benign tumors; thus, additional work is necessary to determine how these proteins may impact disease progression and response to therapy.

Deregulation of the Rb and p53 pathways in uveal melanoma

Uveal melanoma is the most common primary eye cancer, yet its molecular pathogenesis is poorly understood. In this study, we investigated the immunohistochemical expression of proteins in the Rb and p53 tumor suppressor pathways in 33 uveal melanomas from enucleated eyes. Strong nuclear staining for Rb was present in most tumors. However, a few cases displayed weak nuclear staining and strong cytoplasmic staining (possibly indicating Rb mutation), and this aberrant staining correlated strongly with failed radiotherapy or thermotherapy before enucleation. Staining for cyclin D1 was positive in most tumors and was associated with advanced age and larger tumor size, which are both poor prognostic factors. Generally, immunostaining for p53 was weak (suggesting a lack of p53 mutations), although p53 positivity correlated strongly with staining for phosphorylated Rb, supporting the notion that inappropriate phosphorylation of Rb can induce p53. Strong immunostaining for MDM2, which can functionally block p53 activity, was observed in most tumors and correlated significantly with female sex. Strong cytoplasmic staining was observed for Bcl2, which can inhibit both p53-dependent and -independent apoptosis. We conclude that Rb and p53 are mutated infrequently in uveal melanoma, but their respective pathways may be functionally inactivated.

Cell cycle and melanoma--two different tumours from the same cell type

Melanoma of the uvea of the eye and melanoma of the skin share a common cell of origin, but differ substantially in their behaviour and response to chemotherapy. There is increasing evidence that this is related to differences in their molecular phenotype, particularly in relation to the expression of cell cycle-associated proteins. Since many cytotoxic agents act by damaging DNA, resistance is often associated with intact mechanisms which allow the neoplastic cells to arrest their growth while DNA is repaired, or to resist apoptosis in response to detection of DNA damage. p53 is important to these processes, but mutation appears to be a less common event in uveal melanoma than in skin melanoma, probably due to the lack of UV exposure in the uvea. There are also differences in proliferation-associated proteins such as c-myc and cyclin D1. Overexpression of the former molecule is associated with a poor prognosis in skin melanoma, but is associated with a good prognosis in uveal melanoma, although there is considerable genetic heterogeneity within each type. While prognostic studies may therefore be of little relevance to individual patients, they continue to inform our understanding of tumour biology.

Lack of p73 mutations and late occurrence of p73 allelic deletions in melanoma tissues and cell lines

A novel gene, termed p73 with significant homology to p53, has been identified at 1p36, a chromosomal region which is frequently deleted in malignant melanoma. To determine whether p73 is involved in melanoma development we analyzed 8 benign melanocytic nevi, 17 primary melanomas, 34 melanoma metastases and 9 melanoma cell lines for p73 alterations. Allelic loss at the p73 locus was observed in 2 of 10 cases (20%) and occurred only in metastatic tumors. Mutation analysis of the DNA-binding domain of p73 revealed no somatic mutations in the tumor specimens and melanoma cell lines analyzed, whereas the p53 gene was mutated in 5 of 9 melanoma cell lines. Expression analysis of p73 using semiquantitative RT-PCR demonstrated that p73 is not expressed or at exceedingly low levels in benign melanocytic nevi, primary melanomas and lymph node metastases, but at various levels in melanoma cell lines. Our data indicate that p73 does not play a role as a tumor suppressor in melanoma development.

In squamous cell carcinoma of the vulva, overexpression of p53 is a late event and neither p53 nor mdm2 expression is a useful marker to predict lymph node metastases

To offer more tailored treatment to individual patients with squamous cell carcinoma of the vulva, more accurate prediction of lymph node metastases is required. As p53 and mdm2 are genes known to be involved in the development of other tumours, we studied expression of p53 and mdm2 in carcinogenesis of squamous cell carcinoma of the vulva and their clinical relevance. Archival material of 141 T1 and T2 vulvar tumours were used. Of the 141 primary tumours, the corresponding 39 lymph node metastases (LNM) were studied, and in 90 cases the pre-existent epithelia adjacent to the tumour (EAT) and in 14 cases vulvar intraepithelial neoplasia adjacent to the tumour (VIN) was also investigated. Detection of p53 and mdm2 protein was immunohistochemically performed. Scoring categories were: negative (1); weakly positive (2); moderately to markedly positive (3); and markedly positive (4). Overexpression of p53 was seen in 56% of the LNM, 39% of the primary tumours, 21% of the VIN lesions and 0% in the group of EAT. No relation was found between overexpression of p53 in the primary tumour and LNM. Expression of mdm2 was seen in 14% of the primary tumours, of which four cases were marked positive. In the group of LNM no mdm2-positive staining was observed. In the group of EAT, 25% was mdm2-positive, of which six cases were marked positive. In the group of VIN, 36% showed moderate (score 3) mdm2 expression. No relation was found between expression of mdm2 and LNM. In squamous cell carcinoma, overexpression of p53 is a late event in carcinogenesis. Marked expression of mdm2 is rarely seen in vulvar carcinomas, indicating that aberrant p53 cannot induce mdm2 expression. LNM cannot be predicted by detection of these proteins.

p53 mutations in human cutaneous melanoma correlate with sun exposure but are not always involved in melanomagenesis

In melanoma, the relationship between sun exposure and the origin of mutations in either the N-ras oncogene or the p53 tumour-suppressor gene is not as clear as in other types of skin cancer. We have previously shown that mutations in the N-ras gene occur more frequently in melanomas originating from sun-exposed body sites, indicating that these mutations are UV induced. To investigate whether sun exposure also affects p53 in melanoma, we analysed 81 melanoma specimens for mutations in the p53 gene. The mutation frequency is higher than thus far reported: 17 specimens (21%) harbour one or more p53 mutations. Strikingly, 17 out of 22 mutations in p53 are of the C:G to TA or CC:GG to TT:AA transitional type, strongly suggesting an aetiology involving UV exposure. Interestingly, the p53 mutation frequency in metastases was much lower than in primary tumours. In the case of metastases, a role for sun exposure was indicated by the finding that the mutations are present exclusively in skin metastases and not in internal metastases. Together with a relatively frequent occurrence of silent third-base pair mutations in primary melanomas, this indicates that the p53 mutations, at least in these tumours, have not contributed to melanomagenesis and may have originated after establishment of the primary tumour.

p53 expression and risk factors for cutaneous melanoma: a case-control study

Epidemiological data strongly implicate sunlight as the principal environmental cause of melanoma; however, critical molecular targets for ultraviolet (UV)-induced melanoma remain to be identified. The p53 tumor suppressor gene is one possible target, being abnormally expressed in 20-40% of primary melanomas. We undertook a population-based molecular epidemiological study with the aim of determining the environmental and phenotypic factors associated with p53-positive and p53-negative melanomas. One hundred fifty cases of melanoma were randomly ascertained from the Queensland Cancer Registry and matched to 150 electoral roll controls. Data on environmental and phenotypic exposures were collected through interviews and physical examination of all participants. Sections of tumor tissue were obtained from 134 (89%) cases and stained with the anti-p53 DO-7 monoclonal antibody (MAb) following microwave antigen retrieval. Of 121 useable sections, 22 tumors (18%) had more than 1% cells with positive staining consistent with abnormalities in p53 expression. Strongest predictors of p53-positive melanoma were inability to tan [odds ratio (OR) 6.8], history of non-melanoma skin cancer (OR 3.2) and site of melanoma: head/neck (OR 2.2) and lower limbs (OR 2.3). In contrast, factors such as nevus density and freckling propensity were strongly associated only with p53-immunonegative melanoma (OR 8.6 for >25 moles; OR 3.0 for heavy facial freckling). Overall, the determinants of p53-positive and p53-negative melanomas were independent and complementary, the former being associated with features of sun-sensitivity and chronic sun exposure, the latter with phenotypic markers of melanocytic proliferation. Our findings are consistent with at least 2 independent pathways in the pathogenesis of melanoma, characterized by environmental induction and p53 overexpression on the one hand and pigment cell instability on the other.

Different patterns of cell proliferation and death and oncogene expression in cutaneous malignant melanoma

Ninety-six cutaneous melanomas (CMs) were investigated aiming at finding differences, if any, among the main four clinicopathological types, for Bcl-2, c-myc and p53 protein expression, and for tumor cell proliferation and death indices. Proliferation was assessed by calculating the mitotic index (MI, number of mitoses) and the MIB1 labelling index (M-LI, number of MIB1+ nuclei), and tumor cell death by calculating the apoptotic index (AI, number of apoptoses) among 1000 tumor cells. CMs were subdivided into thin (<1 mm) and intermediate thickness (1-4 mm) tumors. Bcl-2 expression did not significantly change among different types. c-myc Expression decreased especially in thicker superficial spreading (SSM) and lentigo maligna melanoma (LMM) types. p53 Expression was higher in nodular melanoma (NM) and in acral lentiginous melanoma(ALM), which also showed the highest degrees of proliferation. AI was significantly higher in thin rather than in intermediate thickness SSMs, LMMs and ALMs (8.4 vs. 2; 6.1 vs. 2.3, and 5.8 vs. 3.6, respectively). AI was low in thin (1.7) and intermediate thickness (1.9) NMs, which also showed high MI (3.9 and 4.5, respectively), and M-LI (16.7 and 2.9, respectively). Thin and intermediate thickness ALMs also showed high MI and M-LI (4.1 vs. 5.2 and 11.3 vs. 14.6, respectively). Bcl-2 is among genes which inhibit apoptotic death, whereas c-myc and p-53 genes promote this process. In CMs, no relation was found between Bcl-2 expression, MI, PI, and AI. All SSMs, LMMs, and ALMs with a high AI showed a high c-myc expression and were negative for p53. c-myc, Although highly expressed, did not promote a significant apoptotic death in NM type. Bc12, c-myc, and p53 were not equally expressed nor equally related to tumor cell turnover in all CMs, suggesting their different influence on the various types and stages, and the role of other factors in CM growth control.

Frequent mutations of the p53 gene in cutaneous melanoma of the nodular type

The frequency and significance of p53 alterations in cutaneous melanoma have not been completely clarified. In the present study, 31 primary melanomas of the nodular type and 15 metastases occurring between 1981 and 1983 were studied with respect to mutations in exons 7 and 8, as well as to p53 protein immunostaining using different antibodies. Altogether 13% of the primary tumors showed strong p53 staining using the DO-7 antibody. Different results were obtained with other antibodies. Seven mutations were found in primary and metastatic tumors; all of these were single base changes, most of which occurred in the core domain of the p53 protein responsible for sequence-specific DNA binding (residues 102-293). The mutations were not significantly associated with p53 staining results, and p53 alterations (mutations or marked immunopositivity) had no prognostic value. Our results indicate that point mutations in exons 7 and 8 are more frequent than previously reported in primary melanomas, and such changes may be important for the development of certain melanoma subgroups.

Alterations and prognostic significance of p16 and p53 protein expression in subgroups of cutaneous melanoma

The role of p16 and p53 alterations in cutaneous melanoma has been recently discussed, but it remains to be clarified. In the present immunohistochemical study, the expression of p16 and p53 proteins and their possible prognostic relevance have been examined in 102 melanomas of the aggressive nodular type. Twelve percent showed a strong expression of p53 protein, and these cases were significantly more frequent in the head/neck area compared with other sites (32% vs. 6%). Expression of p16 protein was negative or weak in 9% of the cases, and this tended to be less frequent in head/neck tumors compared with the others (0% vs. 12%). Whereas p53 staining was not prognostically important, loss of p16 staining was significantly associated with markedly reduced recurrence free and patient survival in univariate analysis (product-limit method). In multivariate analysis, lack of p16 staining was significantly associated with recurrent disease (p = 0.013). Our findings indicate an important role of altered p16 protein expression in a subgroup of melanoma patients.

Expression patterns of the p53 tumor suppressor gene and the mdm2 proto-oncogene in human meningiomas

Meningiomas represent a common class of tumors of the central nervous system. However, the molecular events underlying their formation are poorly understood. Because altered expression of the p53 tumor suppressor gene and the mdm2 proto-oncogene have been demonstrated in a wide variety of tumors, we carried out studies to assess the possible involvement of these two genes in meningioma tumorigenesis. We used Western blot analysis to examine the level of expression of the mdm2 and p53 proteins in a series of sixteen primary meningiomas and four meningioma cell lines. The data obtained from these studies suggest that elevated expression of the p53 or mdm2 protein products does not represent a common event in the development of human meningiomas.

Cyclin kinase inhibitor p21WAF1/CIP1 in malignant melanoma: reduced expression in metastatic lesions

Immunohistochemical analysis of the expression of the cyclin kinase inhibitor p21WAF1/CIP1 in a panel of primary and metastatic human melanocytic tumors was performed. It was found that, independent of the p53 status, approximately 30% of the primary melanomas and 40% of the metastases completely lacked expression of this cell cycle inhibitor. Some tumors were also analyzed by Northern blotting, and in most of the cases a consistant correlation between mRNA and protein expression was observed. In four benign nevi studied, WAF1/CIP1 mRNA was expressed whereas the protein was not detected, suggesting a post-transcriptional regulation of the inhibitor in these cases. In superficial spreading melanomas, a significant correlation between protein expression and tumor thickness was found, with thin lesions showing low protein levels. Interestingly, by comparing primary and metastatic specimens obtained from the same patient, a reduction in p21WAF1/CIP1 antibody staining was observed in the latter, probably reflecting a more aggressive phenotype of the metastases. In conclusion, our results demonstrate the complexity in the relationship between p21WAF1/CIP1 expression and tumor phenotype and furthermore suggest that aberrant expression of the cyclin-dependent kinase inhibitor may be of importance in the development and progression of sporadic malignant melanoma.

Concordance between p53 protein overexpression and gene mutation in a large series of common human carcinomas

Immunohistochemical (IHC) detection of p53 protein was compared with the presence of p53 gene mutation in many colorectal (n = 100), breast (n = 92), endometrial (n = 122), and gastric (n = 116) carcinomas. Two commercially available antibodies, DO7 and CM1, were used for IHC analysis of paraffin-embedded tissue sections. Screening for gene mutations in frozen and paraffin-embedded tumor samples was carried out using polymerase chain reaction-single-strand conformation polymorphism (PCR-SSCP). The frequency of nuclear staining with DO7 or CM1 for each tumor type, respectively, was colorectal (36%, 23%); breast (15%, 19%); endometrial (21%, 33%); and gastric (23%,-). Overall correlation between the two antibodies for nuclear staining was 90% for the 314 tumors analyzed. Cytoplasmic staining was observed with DO7 in 7% of breast and 5% of gastric carcinomas and with CM1 in 17% of breast and 54% of endometrial carcinomas. p53 gene mutation was found in 39% of colorectal, 28% of breast, 13% of endometrial, and 25% of gastric cancers. The concordance between p53 nuclear overexpression and gene mutation (both positive or both negative) was 68% for colorectal, 79% for breast, 76% for endometrial, and 73% for gastric carcinomas. This study provides further evidence that IHC detection of p53 protein accumulation does not always indicate the presence of a gene mutation and vice versa. Discordant results were observed in approximately 20% to 30% of the tumors studied, highlighting the need for careful characterization of both p53 gene and protein alterations when assessing the relationship between p53 status and tumor behavior.

Lack of p53 mutations and loss of heterozygosity in non-cultured human melanocytic lesions

In this study we analysed snap-frozen surgical resections of 16 superficial spreading melanomas, 13 nodular malignant melanomas, 2 lentigo maligna melanomas, 1 dysplastic nevus, 1 congenital nevus and 5 normal nevi from 38 patients for point mutations in the human p53 gene at exons 5-8 by polymerase chain reaction/single-strand conformation polymorphism as well as for loss of heterozygosity of p53 by restriction-fragment-length polymorphism/polymerase chain reaction in order to determine whether p53 aberrations are associated with melanoma subtypes. In addition, we analysed six melanoma cell lines for point mutations in p53. Our results revealed the absence of point mutations and loss of heterozygosity in all fresh resected lesions. However, a TAC (Tyr) to TGC (Cys) transition at codon 163 in exon 5 was found in one cell line.

Genes involved in cell cycle G1 checkpoint control are frequently mutated in human melanoma metastases

A common characteristic of cancer cells is unrestrained cell division. This may be caused by mutational changes in genes coding for components of cell cycle-controlling networks. Alterations in genes involved in G1 checkpoint control have been registered in many human tumours, and investigations from several laboratories show that such alterations, taken together, are the most frequent changes detected in cancer cells. The present paper describes mutational analysis by polymerase chain reaction-single-strand conformation polymorphism (PCR/SSCP) and nucleotide sequence analysis of the genes coding for the p15, p53 and N-ras proteins in 26 metastases from 25 melanoma patients. The registered mutation frequencies add together with previously registered mutations in p16 in the same patient samples to a substantial total frequency of 44% of patients with mutation in at least one of the investigated genes. These results show the occurrence of heterogeneous defects among components of the cell cycle controlling machinery in a human melanoma tumour sample collection and demonstrate that the total frequency of detected alterations increases with the number of cell cycle controlling genes included in the screening panel.

Immunohistochemical expression of p53 protein, mitotic index and nuclear morphometry in primary malignant melanoma of the skin

The immunohistochemical expression of the p53 tumor suppressor protein and the nuclear morphometric parameters were studied in 80 primary skin melanomas. The mitotic index was counted in 64 tumors. In 95% of the tumors p53 positive nuclei were detected, but in only 31% of the cases was the proportion of positive nuclei 3% or higher. The mitotic indices and the mean nuclear areas were not significantly different in melanomas with different p53 expression levels. The p53 positive nuclei had a larger mean nuclear area than the p53 negative nuclei. p53 expression did not increase with melanoma thickness. In Cox's stepwise proportional hazards model, the ratio of mean nuclear long and short axis and melanoma thickness had independent prognostic value, while the level of p53 expression, the mitotic index, and the mean nuclear area were not significantly associated with survival. We conclude that the nuclei expressing p53 protein are larger than p53 negative nuclei. The level of immunohistochemical expression of p53 is low in primary skin melanoma, and it is not valuable as a general prognostic marker for this tumor. p53 expression is not associated with melanoma thickness, indicating that high p53 expression is not a late phenomenon in the progression of this tumor.

Nuclear morphometry, immunohistochemical staining with Ki-67 antibody and mitotic index in the assessment of proliferative activity and prognosis of primary malignant melanomas of the skin

Nuclear morphometry, immunohistochemical staining with Ki-67 antibody and mitotic index were studied in primary cutaneous malignant melanomas. The number of Ki-67 positive cells/ 200 tumor cells did not correlate with any nuclear morphometrical parameters, and it only approached but did not reach significant correlation with melanoma thickness according to Breslow. The nuclear area, short axis and long axis correlated with melanoma thickness, but the nuclear axis ratio (which reflects the sphericity of nuclei) and melanoma thickness did not show significant correlation. Mitotic index was higher in thick melanomas and in melanomas with high Ki-67 positivity, large nuclear area, long nuclear short axis, and small nuclear axis ratio. In Cox's stepwise proportional hazard model, melanoma thickness and the nuclear axis ratio were significant independent prognostic factors for patient survival, while the nuclear area, short axis and long axis, gender, age, Clark level, mitotic index and Ki-67 positivity lacked significant independent prognostic value. The results suggest that the proliferative activity of tumor cells does not alone explain the great importance of tumor thickness as prognosticator in melanoma. The thickness of melanoma measured according to Breslow and the nuclear axis ratio are more efficient prognosticators in melanoma than parameters associated with proliferation.

Overexpression and mutations of p53 in metastatic malignant melanomas

Alterations of the p53 tumor suppressor gene are the most frequent genetic abnormalities in human malignancies, but the role of p53 in the etiology of malignant melanomas is unclear. Fifty unselected malignant melanomas were analyzed for p53 overexpression by immunohistochemistry using 3 monoclonal antibodies (MAbs). Fifteen tumors (29.4%) showed positive staining with at least 2 different antibodies. In the first 20 consecutive tumors exons 5-9 and adjacent splice sites of the p53 gene were analyzed by genomic sequencing. There were 4 mutations in 20 metastatic melanomas. Three of 4 mutations were C:G-->T:A transitions. A search of our database of p53 mutations revealed that out of 8 p53 mutations reported by others, 4 are C:G-->T:A transitions at dipyrimidine sites, and one is a tandem CC-->TT mutation. This mutational pattern is comparable with the pattern of p53 mutations in squamous cell and basal cell carcinomas of the skin and is related to exposure to ultraviolet B (UV-B) wavelength radiation. Taken together with a predominance of UV-induced mutations in the CDKN2/ p16 gene demonstrated in melanoma cell lines, our data support a role of sunlight exposure in the etiology of malignant melanoma. The low frequency of p53 mutants in melanomas compared with other types of skin cancers suggests that although mutations in this gene are likely to be involved in the development of some malignant melanomas, they do not play as large a role as in squamous and basal cell carcinomas of the skin.

Expression of p53 protein in melanoma progression

p53, A tumor suppressor gene, has been documented as the most frequently mutated gene in human cancers including non-melanoma skin tumors. It has been controversial whether the p53 gene mutation plays a major role for melanoma genesis. To examine the role of p53 in human malignant melanoma carcinogenesis, we performed immunohistochemical analysis using anti-p53 antibodies (CM-1 and DO-7) in microwaved paraffin sections. When cases having more than 1% reactive cells were regarded as positive, immunohistochemical analysis revealed that in primary melanomas 14 of 51 (27%) were positive with CM-1 or 15 of 51 (29%) were positive with DO-7. Tumor thickness of primary melanomas in p53 positive cases was significantly thicker than that in p53 negative cases. In metastatic melanomas, 35 of 41 (85%) lymph node metastases were positive with either antibody and in skin metastases 16 of 28 (57%) lesions with CM-1 or 18 of 28 (64%) lesions with DO-7 were positive. The mean percentages of reactive cells were 2.3% in primary lesions and 4.9% in metastases. The incidence of positivity was significantly higher in metastases than primary lesions. In 10 cases examined, with both primary and metastatic melanoma, 3 cases were negative in both lesions and 1 case was positive in both lesions, while 6 cases were negative in the primary lesions and positive only in metastatic lesions. Four Spitz nevi, 6 dysplastic nevi and 11 common nevi were all negative. These data suggest that the expression of p53 protein may be a late event in melanoma progression.

p53 gene and cell cycling in uveal melanoma

PURPOSE

To determine whether alterations of p53, a tumor suppressor gene, were present in uveal melanoma, and to characterize further the nature of those changes.

METHODS

Immunohistochemical analysis with a monoclonal antibody was used to determine whether alterations of p53 were present in 35 enucleated archival uveal melanomas. Further characterization was done by comparing the p53 gene and cell cycling status by using bromodeoxyuridine staining. The alterations in p53 were characterized using polymerase chain reaction single-strand conformational polymorphism analysis and sequencing.

RESULTS

Four of 35 uveal melanomas showed low levels (0.5% to 5.0%) of positive immunostaining for altered p53 in tumor cell nuclei using monoclonal antibody DO-7. These four tumors had the three highest and the 14th highest bromodeoxyuridine labeling indices, ranging from 1.3% to 7.0%. Polymerase chain reaction single-strand conformational polymorphism analysis of p53 exons 5 to 8 was performed on three p53-positive and six p53-negative tumors, and no altered motility shifts were detected. Sequencing of one of the positive staining specimens confirmed no mutations in exons 5 through 8 in the p53 gene. Double immunohistochemical labeling for both bromodeoxyuridine and p53 in one tumor showed that most of p53-positive cells were in S phase.

CONCLUSIONS

Mutation of p53 is an uncommon event in uveal melanomas. Nuclear accumulation of p53 protein was found in three of the four tumors with the highest levels of cell cycling.

Involvement of the pRb/p16/cdk4/cyclin D1 pathway in the tumorigenesis of sporadic malignant melanomas

Biopsies from 61 sporadic metastatic malignant melanomas and five melanoma cell lines were examined for homozygous deletions and mutations in the CDKN2 gene (p16). As the p16 protein is involved in a cell cycle regulatory pathway consisting of at least pRb, cdk4 and cyclin D1, the tumours were also screened for amplifications of the last two genes. Moreover, the transcript levels of the genes were determined and the results compared with the immunohistochemically assessed expression of pRb. Altogether, homozygous deletions of CDKN2 were found in seven tumours (11%) and two of five cell lines, whereas a mutation was detected in only one biopsy, indicating that in sporadic melanomas the former mechanism is predominant for inactivating this gene. Notably, in total 59% of the metastatic lesions lacked detectable expression of p16 mRNA, whereas all the biopsies were found to express pRb. In accordance with the postulated negative feedback loop between p16 and pRb, one melanoma cell line showed overexpression of CDKN2 mRNA together with very low levels of the Rb protein. Amplification of the other two genes may not be important in the tumorigenesis of melanomas, as only one CDK4 and no CCND1 amplification was observed. However, highly elevated CDK4 mRNA levels, compared with that seen in a panel of normal tissues, were observed in 76% of the tumours, accompanied in 71% of the cases by high expression of the CCND1 cyclin activator. Although a low frequency of CDKN2 DNA aberrations was observed, the high number of tumours that lacked CDKN2 expression but showed overexpression of CDK4 and/or CCND1, suggest that functional inactivation of pRb through this pathway may be involved in the development or progression of sporadic human melanomas.

P53-mediated radioresistance does not correlate with metastatic potential in tumorigenic rat embryo cell lines following oncogene transfection

PURPOSE

Changes in wild-type p53 protein function occur in the majority of human tumors, and may alter genomic stability and the cellular response to ionizing radiation. Whether oncoproteins can render tumor cells both radioresistant and metastatic, may have implications for clinical strategies designed to improve local tumor control. In the studies reported here, we tested the hypothesis that acquired radioresistance correlates with metastatic potential within a large panel of transformed rat embryo cell (REF) lines following transfection with activated H-ras, mutant p53, and HPV16-E7 alleles.

METHODS AND MATERIALS

Rat embryo cells (REF cells) were transfected using the calcium-phosphate technique with an activated H-ras gene alone, or in combination with human papillomavirus HPV16-E7 and/or human or murine mutant p53 sequences. Other rat embryo cell clones expressing transfected HPV-E7 and activated ras sequences subsequently acquired endogenous p53 gene mutations during culture in vitro. The relative expression of p21ras and p53 protein for each REF transformant was determined by Western blot analysis following transfection. REF clones were phenotypically characterized at early passage (i.e., passages 5-7) and late passage (i.e., passages 10-20) for their: (a) relative tumor growth rate, and (b) their ability to undergo spontaneous metastasis following intramuscular injection into the hind legs of SCID mice. In vivo phenotypic end points were then compared to previously measured parameters of in vitro radiosensitivity for each cell line. Additionally, the expression of the cellular protease, plasminogen activator, was determined for a number of metastatic and nonmetastatic cell lines.

RESULTS

We found no evidence that selected oncogene-transfected REF transformants that were radioresistant in culture had a greater spontaneous metastatic potential than nonradioresistant REF transformants. Neither the level of expression of the p21ras protein nor that of the p53 protein was correlated with the spontaneous metastatic phenotype when tested at early passage. The metastatic phenotype appeared to be independent of p53 genotype. The majority of metastatic REF clones tested (7 out of 9 clones) expressed plasminogen activator following oncogene transfection, in contrast to nonmetastatic REF transformed cell lines.

CONCLUSIONS

Our results suggest that (a) intrinsic radioresistance does not correlate with spontaneous metastatic potential in oncogene-expressing REF transformant cell lines, and (b), novel clinical strategies designed to overcome oncogene-mediated radioresistance could potentially impact on overall survival, as gains in local tumor control may not be offset by a greater risk of distant metastasis.

MDM2 overexpression is rare in ovarian carcinoma irrespective of TP53 mutation status

Somatic mutations in TP53 are seen in many human cancers. In addition, the protein product of the wild-type TP53 can be sequestered by the protein MDM2 (murine double minute 2). This protein is commonly overexpressed in human sarcomas and gliomas, usually as a result of gene amplification. In this study, 43 ovarian carcinomas (OCs) were analysed for aberrations in the TP53 gene by immunohistochemistry (IHC), loss of heterozygosity (LOH) or mutation analysis. The MDM2 gene and its product was studied by Southern blotting and IHC. Over 50% of the OCs studied showed mutations in TP53 by either direct sequencing (19/36, 53%), positive IHC (23,43, 53%) or both, whereas 0/32 had amplification of MDM2 and only 1/37 tumours had positive IHC using the anti-MDM2 antibody IF-2. The solitary example of positive IHC in this series was seen in a mixed müllerian tumour with sarcomatous differentiation and was not accompanied by MDM2 DNA amplification. These results support previous data showing that around 50% of OCs have mutations in TP53 and in addition, suggest that MDM2 is not amplified in OC, but the presence of sarcomatous features in mixed müllerian tumours may result in positive immunohistochemistry with IF-2.

The expression of the MDM2 gene, a p53 binding protein, in thyroid carcinogenesis

BACKGROUND

The authors previously found p53 mutations in 24% of malignant thyroid tumors, representing a wide stating spectrum. Overexpression of MDM2, most often due to gene amplification, has been suggested to be an additional mechanism for abrogation of the p53 function. In the current study, MDM2 gene expression and amplification were examined in a randomly selected subset of these tumors to explore the possibility that wild-type p53 may be inactivated by complexing with MDM2 in specimens without p53 mutations.

METHODS

MDM2 gene expression and amplification were studied by Northern and Southern blot analysis, respectively. Twenty-two thyroid tumors were included: 16 papillary carcinomas, 1 follicular carcinoma, 3 anaplastic carcinomas, and 2 multinodular goiters (adenomatous goiters).

RESULTS

A two- to threefold increase in MDM2 expression in 4 of 20 thyroid carcinomas was found. It was noteworthy that all of these four samples harbored p53 mutations. The association between increased MDM2 expression and p53 mutation was statistically significant (P < 0.005). No evidence of MDM2 gene amplification or rearrangement accounting for such an increase in MDM2 expression was found.

CONCLUSIONS

Genetic and/or environmental factors contributing to random p53 mutations also may cause increased MDM2 expression. Given the moderate increase in MDM2 expression without associated genetic alterations such as gene amplification and rearrangement, MDM2 may not play any significant role in the development and progression of thyroid carcinoma.