UV-induced DNA damage and mutations in Hupki (human p53 knock-in) mice recapitulate p53 hotspot alterations in sun-exposed human skin.

The major etiological agent contributing to human nonmelanoma skin cancer is sunlight. The p53 tumor suppressor gene is usually mutated in these tumors, and the mutations are "UV signature" single or tandem transitions at dipyrimidine sequences in the DNA-binding domain (DBD). Cells that harbor these characteristic mutations are already present in sun-exposed skin areas of healthy individuals, and small epidermal patches that are immunoreactive to anti-p53 antibody accrue as exposure increases. To explore carcinogen-specific human p53 mutation patterns experimentally, we generated a knock-in (Hupki) mouse in which the murine DBD of the p53 gene has been replaced by the homologous human p53 DBD segment; thus, the precise base sequence context frequently targeted by mutagens or endogenous mutagenic processes in human carcinogenesis is present in this strain (J. L. Luo et al., Oncogene, 20: 320-328, 2001). Here we show that when epidermal cells of Hupki mice (p53(ki/ki)) are irradiated in vivo with a single acute dose of UVB light, they accumulate UV photoproducts at the same locations of the p53 gene as human cells. Chronic exposure of Hupki mice (4.5 kJ/m(2) 5x/week for 4 weeks) results in the appearance of cell patches that stain intensely with the anti-p53 antiserum CM1. DNA preparations from 2 cm(2) sections of chronically irradiated Hupki epidermis harbor C to T and CC to TT mutations at two mutation hotspots identified in human skin cancer, one at codons 278-279, and one at codons 247-248; the latter is the most frequent UVB-associated mutation site in humans but not in p53 wild-type mice. Thus, Hupki keratinocytes with these p53 mutations encode an aberrant DBD identical in amino acid sequence to the mutant p53 molecules in human UV-induced tumors. The Hupki mouse model offers a new experimental tool in molecular epidemiology and biomedical research.     

Aflatoxin-B exposure does not lead to p53 mutations but results in enhanced liver cancer of Hupki (human p53 knock-in) mice

Hepatocellular carcinoma (HCC) is a common human malignancy that is often associated with risk factors such as aflatoxin-B1 (AFB1) exposure and Hepatitis-B virus infection in developing countries. There is a strong correlation between these risk factors and mutation of the tumor-suppressor gene p53 at codon 249. In vitro experiments have also shown that treatment of human liver cells with AFB1 results in p53 mutations. A tumor-promoting role for mutant p53 was demonstrated using transgenic mice models, in which HCC development was accelerated upon AFB1-exposure. However, wild-type mice in which AFB1 alone was used to induce liver cancers have failed to recapitulate p53 mutations, raising the possibility that mouse DNA context may not be appropriate for the generation of AFB1-induced p53 mutations. We have now tested this hypothesis using the Hupki mice (human p53 knock-in) in which the mouse DNA-binding domain has been replaced by the homologous human p53 segment. Mice were followed for 80 weeks after AFB1 injection for survival and HCC formation. Hupki mice were found to be more susceptible to AFB1 than wild-type mice. Moreover, only 19% of wild-type mice developed HCCs compared to 44% in Hupki mice. However, none of the liver tumors and normal tissues from Hupki mice contained any mutations in the DNA-binding domain of p53. These findings suggest that the human DNA context of the p53 gene alone may not be the sole determinant of AFB1-induced mutagenesis. Furthermore, humanized p53 appears not to be as effective as murine p53 in the mouse cellular environment in preventing malignant transformation.     

Common tumour p53 mutations in immortalized cells from Hupki mice heterozygous at codon 72

Codon 72 of human p53 gene is polymorphic, encoding arginine or proline. Here we report construction of a human p53 knock-in (Hupki) mouse encoding the codon 72(pro) variant. The new strain was crossed with the original Hupki mice (codon 72(arg/arg)) to obtain primary embryonic fibroblasts polymorphic at codon 72 or homozygous for codon 72(pro). The fibroblasts, cultured under standard conditions, immortalized within 12 weeks and acquired p53 mutations similarly to Hupki codon 72(arg/arg) cells investigated previously. Sequencing of human p53 exons 4-9 in immortalized cultures revealed missense mutations found repeatedly in human tumours. In cell lines ensuing from benzo(a)pyrene-treated cultures the combined p53 mutation pattern from experiments with the 3 codon 72 genotypes showed a predominance of strand-biased G to T transversions (18 of 36 mutations), and mutations recurring at smokers' lung tumour hotspot codons 157 and 273, supporting involvement of tobacco carcinogens in shaping the mutation signature in lung cancers of smokers. Mutations in cell lines from unexposed cultures did not cluster at these codons and G to T transversions were uncommon (2 of 52 mutations) (Fisher's exact test P<0.0001). Most mutations (13/16) in cell lines derived from cells polymorphic at codon 72 were found on the proline allele, with loss of the arginine allele.     

Quantitative detection of p53 mutations in plasma DNA from tobacco smokers

In lung tumors, the p53 tumor suppressor gene is commonly mutated with a characteristic mutation spectrum. The amount of and alterations in plasma DNA, such as mutations in p53, were associated with several cancers. Few studies used quantitative methods of high sensitivity. Previously, we observed p53 mutations in the noncancerous tissue that differed from those in lung tumors using the highly sensitive p53 mutation load assay. Based on our observation of an increased p53 mutation load in nontumorous lung tissue in smokers, we hypothesized that plasma DNA may contain mutant p53 indicative of tobacco smoke exposure and will be an effective biomarker of lung cancer or smoking exposure. We modified the p53 mutation load assay to detect mutations at p53 codons 248 and 249, common mutations in lung cancer, in plasma DNA samples with a sensitivity of 1:5,000. The assay was applied to a set of lung cancer cases (n = 39), hospital controls (n = 21), and population controls (n = 20) from a larger study. Controls were selected to consist of equal numbers of both ever and never smokers. The p53 mutation load (mutated p53 copies per total number of p53 copies) was associated with smoking (P = 0.06), but not with lung cancer (P = 0.59). Most of the individuals with p53 mutations observed in plasma DNA were ever smokers and the p53 mutation load was higher in those who smoked for longer durations (P = 0.04). In summary, we were able to detect p53 mutations in plasma DNA from healthy individuals and our data suggest that p53 mutations in plasma DNA may be a marker of carcinogen exposure from tobacco smoke.     

Ultraviolet-light induced p53 mutational spectrum in yeast is indistinguishable from p53 mutations in human skin cancer

Ultraviolet (UV) light has been associated with the development of human non-melanoma skin cancers (NMSC). Such cancers often exhibit mutations in the p53 tumour suppressor gene. In order to determine the UV-induced p53 mutation spectrum, a yeast expression vector that harbours a human wild-type p53 cDNA was UV-irradiated in vitro and transfected into a yeast strain that contained the ADE2 gene regulated by a p53-responsive promoter. Forty-five mutant clones contained 51 mutations. Seven mutations were tandem base pair substitutions, four of which being CC-->TT, hallmark mutations of UV mutagenesis. Eighty percent (41/51) of the mutations were single or non-tandem base pair substitutions, the majority of which (27/41) were C-->T transitions. Ninety-five percent of such mutations occurred at dipyrimidine sites. Through a rigorous statistical test, the UV-induced p53 mutation spectrum appears to differ significantly (P < 0.008) from the one induced by the antineoplastic drug chloroethyl-cyclohexyl-nitrosourea, and to be indistinguishable from the one observed in NMSC (P = 0.4). These results demonstrate that the assay allows the determination of carcinogen-specific p53 mutation fingerprints and represents a new tool for molecular epidemiology.      

p53 mutations in human lung tumors.

Mutation of one p53 allele and loss of the normal p53 allele [loss of heterozygosity (LOH)] occur in many tumors including lung cancers. These alterations apparently contribute to development of cancer by interfering with the tumor suppressor activity of p53. We directly sequenced amplified DNA in the mutational hot spots (exons 4-8) of p53 in DNA samples from 40 lung cancers. Most (31 of 40) samples were preselected for LOH in the region of p53. We detected 23 p53 mutations within these exons in 22 lung cancers; no p53 mutations were found in normal tissue of the patients. One-half of the mutations were G to T transversions on the nontranscribed strand, consistent with mutagenesis by tobacco smoke. Mutations of C to A on the nontranscribed strand, which would result from G to T mutations on the transcribed strand, were detected only in one sample. Three of 23 mutations were nonsense mutations; to date, nonsense mutations of p53 have not been reported in lung cancer. Mutation of this p53-coding region was detected in 20 of 27 small cell lung cancer samples, representing a 70% occurrence. Mutation of the p53 gene is apparently very frequent in small cell lung cancers. When LOH in the p53 region could be determined, complete concordance occurred between a sample having both a p53 mutation and LOH in the region of p53 (18 of 18 samples). Twelve samples of lung cancer had LOH in the region of p53, but the samples had no detectable p53 mutations, suggesting either alterations outside the known mutational hot spots of p53 or alterations of another unidentified tumor suppressor gene in the region of p53.     

p53 mutations in primary human lung tumors and their metastases

In a total of 26 primary human lung tumors and 60 metastases derived from them, exons 5–8 of the p53 tumor suppressor gene were analyzed by single-strand conformation polymorphism and subsequent direct DNA sequencing of amplified DNA. Mutational inactivation of the p53 gene was identified in four of five squamous cell carcinomas, three of nine adenocarcinomas, and two of nine small-cell carcinomas, the overall incidence being 35%. Point mutations occurred at a similar incidence in exons 5–8, with a preference for G←T transversions. In seven of nine cases (78%), mutations were identical in the primary tumor and all of its metastases, indicating that in lung tumors, p53 mutations usually precede metastasis and that hematogenic and lymphogenic dissemination of tumor cells to other tissues is not associated with a selection against p53 inactivation. In one case, a kidney metastasis had the same mutation as the primary squamous cell carcinoma, whereas a liver metastasis had no mutation, indicating heterogeneity of the primary lung neoplasm and selective metastasis of mutated and nonmutated tumor cells to kidney and liver, respectively. Only in one liver metastasis was a mutation identified that was neither present in the primary lung tumor nor in a kidney metastasis, suggesting that occasionally p53 mutations occur after metastatic spread. © 1994 Wiley-Liss, Inc.     

p53 gene mutations in human endometrial carcinoma.

Although carcinoma of the uterine endometrium is the most frequently diagnosed malignancy of the female reproductive tract, the molecular genetic features of this tumor have yet to be described in significant detail. Since mutations of the p53 tumor suppressor gene are the single most common genetic alteration found in human malignancies, we examined the hypothesis that p53 mutations occur in human endometrial carcinoma. Sequencing analysis of exons 5-8 revealed point mutations in 3 of 21 (14%) tumors; one mutation was an unusual single-base insertion at codons 176-177, resulting in a premature stop codon, whereas the other two were CGG----TGG transitions at codon 248. Two of these tumors showed reduction to homozygosity at the p53 allele, but one tumor apparently retained heterozygosity. These data indicate that p53 mutations occur in human endometrial carcinoma, although relatively infrequently, and that loss of the normal p53 allele does not necessarily occur with point mutation of the p53 gene in this tumor type.     

Unveiling the methylation status of CpG dinucleotides in the substituted segment of the human p53 knock-in (Hupki) mouse genome

Methylated cytosines within CpG dinucleotides (mCpGs) along the DNA-binding domain of the TP53 tumor suppressor gene (exons ~5-8) are the single most significant mutational target in human cancers. The human p53 knock-in (Hupki) mouse model was constructed using gene-targeting technology to create a mouse strain that harbors human wild-type TP53 DNA sequences spanning exons 4-9 in both copies of the mouse p53 gene. To date, however, the methylation status of cytosines within CpGs in the substituted segment of the Hupki mouse genome has not been determined. This lack of information deserves special attention because DNA methylation in mammals, which occurs almost exclusively within CpG dinucleotides, is a dynamic process throughout developmental stages and may vary among different species. Here, we have investigated the status of CpG methylation in the substituted segment of the Hupki mouse genome, and compared it to the methylation profile of the corresponding segment in the human genome using the combined bisulfite-restriction analysis and sodium bisulfite genomic sequencing. We found that all cytosines within CpGs of the TP53 DNA-binding domain, on both the coding and noncoding strands, were heavily methylated in Hupki fibroblasts, as they were in human fibroblasts. This is in keeping with the fully methylated status of TP53 CpGs that is known to prevail in adult human tissues. The remarkably similar patterns of cytosine methylation within CpG dinucleotides in Hupki cells and human cells further validates the suitability of mutagenesis assays in Hupki cells for experimental induction of TP53 mutations that have been observed in human tumors.     

Nutlin‐3a selects for cells harbouring TP53 mutations

TP53 mutations occur in half of all human tumours. Mutagen‐induced or spontaneous TP53 mutagenesis can be studied in vitro using the human TP53 knock‐in (Hupki) mouse embryo fibroblast (HUF) immortalisation assay (HIMA). TP53 mutations arise in up to 30% of mutagen‐treated, immortalised HUFs; however, mutants are not identified until TP53 sequence analysis following immortalisation (2–5 months) and much effort is expended maintaining TP53‐WT cultures. In order to improve the selectivity of the HIMA for HUFs harbouring TP53 mutations, we explored the use of Nutlin‐3a, an MDM2 inhibitor that leads to stabilisation and activation of wild‐type (WT) p53. First, we treated previously established immortal HUF lines carrying WT or mutated TP53 with Nutlin‐3a to examine the effect on cell growth and p53 activation. Nutlin‐3a induced the p53 pathway in TP53‐WT HUFs and inhibited cell growth, whereas most TP53‐mutated HUFs were resistant to Nutlin‐3a. We then assessed whether Nutlin‐3a treatment could discriminate between TP53‐WT and TP53‐mutated cells during the HIMA (n = 72 cultures). As immortal clones emerged from senescent cultures, each was treated with 10 µM Nutlin‐3a for 5 days and observed for sensitivity or resistance. TP53 was subsequently sequenced from all immortalised clones. We found that all Nutlin‐3a‐resistant clones harboured TP53 mutations, which were diverse in position and functional impact, while all but one of the Nutlin‐3a‐sensitive clones were TP53‐WT. These data suggest that including a Nutlin‐3a counter‐screen significantly improves the specificity and efficiency of the HIMA, whereby TP53‐mutated clones are selected prior to sequencing and TP53‐WT clones can be discarded.     

p53 status in multiple human urothelial cancers: assessment for clonality by the yeast p53 functional assay in combination with p53 immunohistochemistry.

Multifocal synchronous or metachronous tumor development is a common observation in human urothelial cancer cases. However, the underlying mechanism has remained obscure. We have employed a new tool to investigate the p53 gene status, the yeast p53 functional assay, in combination with immunohistochemistry in a total of 50 tumor samples from 32 cases with urothelial cancers, including 8 with multiple synchronous tumor development and 2 demonstrating metachronous tumors. p53 mutations were found in 13 cases (9 with missense mutations, 3 with deletion, 1 with splicing mutation) by the yeast p53 functional assay. p53 protein overexpression was seen in all 9 cases with missense mutations, but in only one of the 4 cases with nonsense mutations. Two tumors without p53 mutation also showed positive p53 immunoreactivity. Overall, p53 abnormalities including mutations and/or protein overexpression were found in 15 (47%) cases. p53 abnormalities were significantly more frequent in non-papillary and in high grade tumors. Loss of the wild type allele in addition to a p53 mutation was suggested in 8 of the 15 (53%) cases. All 4 cases with mutations in multiple synchronous tumors had identical p53 mutations in the separate urothelial cancers, strongly suggestive of monoclonality. The one case with multiple metachronous tumors, in contrast, was characterized by variation in the p53 status, indicative of different clonal origins. In conclusion, combined assessment for p53 status as used here (yeast p53 functional assay plus immunohistochemistry) may provide insights into the molecular mechanisms of urothelial carcinogenesis.     

p53 mutations, ras mutations, and p53-heat shock 70 protein complexes in human lung carcinoma cell lines.

The p53 tumor suppressor gene is frequently mutated and the K-ras oncogene is occasionally mutated in primary specimens of human lung carcinomas. These mutated genes also cooperate in the immortalization and neoplastic transformation of rodent cells. To determine whether these mutations are necessary for maintenance of the immortalized and/or neoplastically transformed states of human bronchial epithelial cells, the p53 gene and regions of the ras (K-, H-, and N-) genes were sequenced in nine human lung carcinoma cell lines. Detection of p53 mutations by polymerase chain amplification and direct DNA sequencing was corroborated by p53 immunocytochemistry and coimmunoprecipitation of p53 with heat shock protein 70. p53 and ras genes were frequently, but not always, mutated in the carcinoma cell lines. These data are consistent with the hypothesis that multiple genetic changes involving both protooncogenes and tumor suppressor genes occur during lung carcinogenesis.     

Significant Correlation of Nitric Oxide Synthase Activity and p53 Gene Mutation in Stage I Lung Adenocarcinoma

Nitric oxide (NO) and its derivatives can directly cause DNA damage and mutation in vitro and may play a role in the multistage carcinogenic process. It has been reported that NO induces mutation in the p53 tumor suppressor gene; we therefore analyzed the relationship between NO synthase 9 activity and p53 gene status in early-stage lung adenocarcinoma. Surgical samples were classified into two categories: 14 lung adenocarcinomas with high NOS activity (>25 pmol/min/g tissue, category A), and 16 with low NOS activity (<25 pmol/min/g tissue, category B). A yeast functional assay for p53 mutations disclosed a red colony that corresponded to a mutation in the p53 gene in 8 cases (57.1%) in category A and 3 cases (18.8%) in category B, the frequency being significantly higher in the former ( P <0.05). A p53 DNA sequence analysis revealed that 5 of the 8 p53 mutation-positive samples in category A had a G:C-to-T:A transversion, which is reported to be a major target of NO. The mechanism of carcinogenesis of adenocarcinoma is not fully understood, but these results suggest that an excess of endogenously formed NO may induce a p53 gene mutation containing mainly G:C-to-T:A transversion in the early stage of lung adenocarcinoma. Our results suggest that NO has potential mutagenic and carcinogenic activity, and may play important roles in human lung adenocarcinoma.     

Distribution of p53 and K-ras mutations in human lung cancer tissues

Studies were performed to examine the mutational pattern of K-ras exons 1 and 2 and p53 exons 5-8 in lung cancer tissues from 27 Chinese patients (10 smokers, 17 non-smokers) using single-stranded conformational polymorphism and DNA sequencing. K-ras mutations were found in 13/27 tumors (48%); all mutations were clustered in exon 1 and distributed between codons 9 and 32. The frequency and number of patients with K-ras mutations between smokers and non-smokers were not different, except that a high frequency of G --> A transitions (11/11) was found in non-smokers. Among cell types, K-ras mutations were found in 7/13 (54%) squamous cell carcinoma (SC) and 5/12 (42%) adenocarcinoma (AC) patients. A --> T transversions (all six transversions) were present only in SC. In p53, 18/27 (67%) tumors contained mutations in exons 7 and 8, frequently at codons 226, 270, 275 and 281. The number of tumors with p53 mutations in smokers (70%) and in non-smokers (65%) was similar, and the mutation frequency did not differ except for a higher number of G --> A (6/7) and T --> C (5/6) transitions in non-smokers. Among cell types, the number of tumors with p53 mutations was 9/13 (69%) in SC and 8/12 (67%) in AC. The A --> G (11/16) transitions and A --> C (4/4) transversions in p53 were more frequent in SC than in AC (P < 0.04 for A --> G; P < 0.02 for A --> C). The varying mutation patterns in both the K-ras and p53 genes between smokers and non-smokers and among cell types suggest that other than cigarette smoke, environmental and dietary factors may also be involved in the genesis of lung cancer among these patients.      

p53 mutations in human aggressive and nonaggressive basal and squamous cell carcinomas.

PURPOSE: The purpose is to investigate whether aggressive basal cell carcinoma (BCC) and squamous cell carcinoma (SCC) differ from nonaggressive BCC and SCC with respect to the p53 mutation spectrum and whether specific mutations can serve as prognostic indicators of tumor aggressiveness. EXPERIMENTAL DESIGN: We analyzed 342 tissues from patients with aggressive and nonaggressive BCCs and SCCs for p53 mutations by single-strand conformation polymorphism and nucleotide sequencing. RESULTS: p53 mutations were detected in 33 of 50 aggressive BCCs (66%), 37 of 98 nonaggressive BCCs (38%), 28 of 80 aggressive SCCs (35%), 28 of 56 nonaggressive SCCs (50%), and 3 of 29 samples of sun-exposed skin (10%). About 71% of the p53 mutations detected in aggressive and nonaggressive BCCs and SCCs were UV signature mutations. The frequency of CC to TT mutations in aggressive (36%) and nonaggressive SCCs (39%) was 2-fold higher than in aggressive (18%) and nonaggressive (14%) BCCs. In contrast, aggressive BCCs had a higher frequency (24%) of transversions than nonaggressive BCCs (8%) and aggressive (14%) and nonaggressive (11%) SCCs did. CONCLUSIONS: Our results indicate that UV radiation is responsible for the induction of p53 mutations and perhaps for the initiation of both aggressive and nonaggressive BCCs and SCCs. Although some differences in p53 mutation frequency, types of mutation, and hot spots were seen between aggressive and nonaggressive BCCs and SCCs, these factors do not constitute as clear-cut diagnostic or prognostic indicators of tumor aggressiveness. Tumor aggressiveness may be attributable to other genetic changes or events that occur during tumor progression.     

Mutability of p53 hotspot codons to benzo(a)pyrene diol epoxide (BPDE) and the frequency of p53 mutations in nontumorous human lung

p53 mutations are common in lung cancer. In smoking-associated lung cancer,the occurrence of G:C to T:A transversions at hotspot codons, e.g., 157, 248, 249,and 273, has been linked to the presence of carcinogenic chemicalsin tobacco smoke including polycyclic aromatic hydrocarbons suchas benzo(a)pyrene (BP). In the present study, we have used a highly sensitive mutation assay to determine the p53 mutation load in nontumorous human lung and to study the mutability of p53 codons 157, 248, 249, and 250 to benzo(a)pyrene-diol-epoxide (BPDE), an active metabolite of BP in human bronchial epithelial BEAS-2B cells. We determined the p53 mutational load at codons 157, 248, 249, and 250 in nontumorous peripheral lung tissue either from lung cancer cases among smokers or noncancer controls among smokers and nonsmokers. A 5-25-fold higher frequency of GTC(val) to TTC(phe) transversions at codon 157 was found in nontumorous samples (57%) from cancer cases (n = 14) when compared with noncancer controls (n = 8; P < 0.01). Fifty percent (7/14) of the nontumorous samples from lung cancer cases showed a high frequency of codon 249 AGG(arg) to AGT(ser) mutations (P < 0.02). Four of these seven samples with AGT(ser) mutations also showed a high frequency of codon 249 AGG(arg) to ATG(met) mutations, whereas only one sample showed a codon 250 CCC to ACC transversion. Tumor tissue from these lung cancer cases (38%) contained p53 mutations but were different from the above mutations found in the nontumorous pair. Noncancer control samples from smokers or nonsmokers did not contain any detectable mutations at codons 248, 249, or 250. BEAS-2B bronchial epithelial cells exposed to doses of 0.125, 0.5, and 1.0 microM BPDE, showed G:C to T:A transversions at codon 157 at a frequency of 3.5 x 10(-7), 4.4 x 10(-7), and 8.9 x 10(-7), respectively. No mutations at codon 157 were found in the DMSO-treated controls. These doses of BPDE induced higher frequencies, ranging from 4-12-fold, of G:C to T:A transversions at codon 248, G:C to T:A transversions and G:C to A:T transitions at codon 249, and C:G to T:A transitions at codon 250 when compared with the DMSO-treated controls. These data are consistent with the hypothesis that chemical carcinogens such as BP in cigarette smoke cause G:C to T:A transversions at p53 codons 157, 248, and 249 and that nontumorous lung tissues from smokers with lung cancer carry a high p53 mutational load at these codons.     

Functional evaluation of p53 and PTEN gene mutations in gliomas.

We screened mutations of two major tumor suppressor genes, p53 and PTEN, in 66 human brain tumors using a yeast-based functional assay and cDNA-based direct sequencing, respectively. The frequency of p53 mutations was 28.8% (19 of 66) and was higher in anaplastic astrocytoma (9 of 14, 64.3%,) than in glioblastoma multiforme (GBM; 7 of 27, 25.9%,), supporting previous speculation that there are at least two genetic pathways leading to GBM, a de novo pathway without p53 mutation and a "progressive" pathway with p53 mutation. PTEN mutation was observed in 8 of 64 tumors (12.5%), mainly GBMs (7 of 26, 26.9%), both with and without p53 mutation. These results suggest that mutation of the PTEN gene is a later event than that of the p53 gene in glioma progression and is associated with both the genetic pathways. All of the detected PTEN missense mutations and an in-frame small deletion inactivated PTEN phosphoinositide phosphatase activity in vitro. Because the tumors containing PTEN mutations also showed loss of heterozygosity in the chromosome 10q23 region flanking the PTEN gene, our data clearly indicate that inactivation of both PTEN alleles occurs in a subset of high-grade gliomas, therefore confirming the previous idea that PTEN acts as a tumor suppressor gene.     

High frequency and heterogeneous distribution of p53 mutations in aflatoxin B1-induced mouse lung tumors.

Inactivation of the p53 tumor suppressor gene is one of the most frequent genetic alterations observed in human lung cancers. However, p53 mutations are more rarely detected in chemically induced mouse lung tumors. In this study, 62 female AC3F1 (A/J x C3H/HeJ) mice were treated with aflatoxin B1 (AFB1; 150 mg/kg i.p. divided into 24 doses over 8 weeks). At 6-14 months after dosing, mice were killed, and tumors were collected. A total of 71 AFB1-induced lung tumors were examined for overexpression of p53 protein by immunohistochemical staining. Positive nuclear p53 staining was observed in 79% of the AFB1-induced tumors, but the pattern was highly heterogeneous. In approximately 73% of the positively stained tumors, fewer than 5% of cells demonstrated positive staining; in the other 27%, between 10% and 60% of the cells stained positively, with staining localized to the periphery of the tumors in many cases. Single-strand conformational polymorphism analysis of the evolutionarily conserved regions of the p53 gene (exons 5-8) from AFB1-induced whole lung tumor DNA revealed banding patterns consistent with point mutations in 20 of 76 (26%) tumors, with 85% of the mutations in exon 7 and 15% of the mutations in exon 6. Identification of point mutations could not be confirmed by direct sequence analysis because bands representing putative mutations appeared only weakly on autoradiograms. This was presumably due to the heterogeneous nature of the DNA analyzed. Single-strand conformational polymorphism analysis of DNA from laser capture microdissected cells of paraffin-embedded AFB1-induced tumor tissue sections stained for p53 produced banding patterns consistent with point mutations in 18 of 30 (60%) DNA samples. Direct sequencing of the microdissected samples revealed mutations at numerous different codons in exons 5, 6, and 7. Of 26 mutations found in microdissected regions from adenomas and carcinomas, 9 were G:C-->A:T transitions, 11 were A:T-->G:C transitions, and 5 were transversions (2 G:C-->T:A, 2 T:A-->A:T, and 1 A:T-->C:G), whereas 1 deletion mutation was identified. The concordance between immunostaining and molecular detection of p53 alterations was 72% when laser capture microdissection was used versus 17% based on whole tumor analysis. The high mutation frequency and heterogeneous staining pattern suggest that p53 mutations occur relatively late in AFB1-induced mouse lung tumorigenesis and emphasize the value of analyzing different staining regions from paraffin-embedded mouse lung tumors.     

p53 Status in Multiple Human Urothelial Cancers: Assessment for Clonality by the Yeast p53 Functional Assay in Combination with p53 Immunohistochemistry

Multifocal synchronous or metachronous tumor development is a common observation in human urothelial cancer cases. However, the underlying mechanism has remained obscure. We have employed a new tool to investigate the p53 gene status, the yeast p53 functional assay, in combination with immunohistochemistry in a total of 50 tumor samples from 32 cases with urothelial cancers, including 8 with multiple synchronous tumor development and 2 demonstrating metachronous tumors. p53 mutations were found in 13 cases (9 with missense mutations, 3 with deletion, 1 with splicing mutation) by the yeast p53 functional assay. p53 protein overexpression was seen in all 9 cases with missense mutations, but in only one of the 4 cases with nonsense mutations. Two tumors without p53 mutation also showed positive p53 immunoreactivity. Overall, p53 abnormalities including mutations and/or protein overexpression were found in 15 (47%) cases. p53 abnormalities were significantly more frequent in non-papillary and in high grade tumors. Loss of the wild type allele in addition to a p53 mutation was suggested in 8 of the 15 (53%) cases. All 4 cases with mutations in multiple synchronous tumors had identical p53 mutations in the separate urothelial cancers, strongly suggestive of monoclonality. The one case with multiple metachronous tumors, in contrast, was characterized by variation in the p53 status, indicative of different clonal origins. In conclusion, combined assessment for p53 status as used here (yeast p53 functional assay plus immunohistochemistry) may provide insights into the molecular mechanisms of urothelial carcinogenesis.     

Analysis of K-ras p53 and c-raf-1 mutations in beryllium-induced rat lung tumors

Beryllium (Be) metal and several of its analogues have beenshown to be carcinogenic in rats. In addition, workers employedat Be processing plants have been shown to have a slight excessof lung cancer. In this study, a single inhalation exposureto Be metal produced a 64% incidence of lung tumors in the F344/Nrat. The most frequent tumor type observed was adenocarcinoma.These Be metal-induced lung carcinomas were examined for geneticalterations in the K-ras, p53, and c-raf-1 genes. DNA isolatedfrom lung neoplasms was analyzed by PCR amplification and directDNA sequence analysis, immunohistochemical analysis and Southernblot analysis. No K-ras codon 12,13 or 61 mutations were detectedin 24 lung tumors by direct sequencing. Using a more sensitiveK-ras codon 12 mutation selection assay, K-ras codon 12 GGT-GTTtransversions were detected in two of 12 adeno-carcinomas. Theseresults suggest that activation of the K-ras protooncogene isboth a rare and late event, possibly stemming from genomic instabilityduring the progression of some Be-induced rat adenocarcinomasof the lung. No mutant p53 nuclear immunoreactivity was observedin any Be-induced tumor. Because immunohistochemical analysisof the p53 protein only detects missense mutations, exons 5–8of this gene were also analyzed by direct DNA sequencing. Inorder to perform the p53 sequence analysis, it was necessaryto first characterize and sequence the/p53 intron sequencesflanking exons 5–8 and their splice sites. Details ofthis expanded intron DNA sequence information are given here.No mutations were detected within exons 5–8 of the p53gene. No rearrangement of the c-raf-1 protooncogene was detectedby Southern blot analysis. These results indicate that the mechanismsunderlying the development of Be-induced lung cancer in ratsdo not involve gene dysfunctions commonly associated with humannon-small-cell lung cancer.