Stability of p53 tumor suppressor gene mutations during the process of metastasis and during chemotherapy

Genetic alterations in cancer knowledge system: analysis of gene mutations in mouse and human liver and lung tumors

Mutational incidence and spectra for genes examined in both human and mouse lung and liver tumors were analyzed using the National Institute of Environmental Health Sciences (NIEHS) Genetic Alterations in Cancer (GAC) knowledge system. GAC is a publicly available, web-based system for evaluating data obtained from peer-reviewed studies of genetic changes in tumors associated with exposure to chemical, physical, or biological agents, as well as spontaneous tumors. In mice, mutations in Kras2 and Hras-1 were the most common events reported for lung and liver tumors, respectively, whether chemically induced or spontaneous. There was a significant difference in Kras2 mutation incidence for spontaneous versus induced mouse lung tumors and in Hras-1 mutation incidence and spectrum for spontaneous versus induced mouse liver tumors. The major gene changes reported for human lung and liver tumors were in KRAS2 (lung only) and TP53. The KRAS2 mutation incidence was similar for spontaneous and asbestos-induced human lung tumors, while the TP53 mutation incidence differed significantly. Aflatoxin B1, hepatitis B virus, hepatitis C virus, and vinyl chloride all caused TP53 mutations in human liver tumors, but the mutation spectrum for each agent differed. The incidence of KRAS2 mutations in human compared to mouse lung tumors differed significantly, as did the incidence of Hras and p53 gene mutations in human compared to mouse liver tumors. Differences observed in the mutation spectra for agent-induced compared to spontaneous tumors and similarities in spectra for structurally similar agents support the concept that mutation spectra can serve as a "fingerprint" of exposure based on chemical structure.

Mutational Analysis of the Macrophage Scavenger Receptor 1 (MSR1) Gene in Primary Lung Cancer

Allelic deletion at chromosome 8p21-25 is an early and frequent event in the carcinogenesis and development of various cancers. To facilitate investigation of alterations of the macrophage scavenger receptor 1 (MSR1), which is located on 8p22, and to determine the role of this gene in human carcinogenesis and tumor progression, we determined intronic primers designed to amplify the coding region. Since frequent deletion of 8p21-23 has been previously reported in lung cancer, we searched for mutations throughout the coding sequence of the MSR1 gene within a panel of genomic DNA samples obtained from 30 primary lung cancers. Our approach, which involved polymerase chain reaction-single strand conformation polymorphism (PCR-SSCP) analysis and direct DNA sequencing, revealed nucleotide variants of the MSR1 gene in only one of the 30 cases examined, with this sample displaying both a 6 bp deletion and a thymine-to-cytosine substitution, the latter occurring within intron 7. The 6 bp deletion was located at a DNA microsatellite region and the thymine-to-cytosine substitution seemed to be a polymorphism. These results suggest that the MSR1 gene is not commonly mutated in lung cancer and not important in susceptibility to lung cancer. Further studies may focus on alternative mechanisms through which the MSR1 gene might be inactivated, such as aberrant DNA methylation, and/or pursue analyses of other genes on 8p21-23 for mutational events. Nevertheless, the panel of intronic PCR primer pair sequences presented here will facilitate future studies to determine the full spectrum and frequency of genetic events that may affect expression/activity of the MSR1 gene in human tumors.

Genomic structure of the human MAD2 gene and mutation analysis in human lung and breast cancers

Some of the many human cancers that exhibit chromosomal instability also carry mutations in mitotic checkpoint genes and/or reveal reduced expression of some of those genes, such as hMAD2. To facilitate investigation of alterations of hMAD2, we determined its genomic structure and intronic primers designed to amplify the entire coding region. Since general impairment of the mitotic checkpoint is frequently reported in lung cancers, and reduced expression of hMAD2 has been reported in breast cancers as well, we searched for mutations throughout the coding sequence of this gene in the genomic DNA of 30 primary lung tumors, 30 lung-cancer cell lines and 48 primary breast cancers. Our approach, which involved polymerase chain reaction-single strand conformation polymorphism (PCR-SSCP) analysis and direct sequencing, revealed nucleotide variants in only two of the 108 specimens. One was a cytosine-to-adenine substitution 3 bp upstream of exon 4 that occurred in one lung cancer cell line and one primary breast tumor, a change that did not alter transcriptional sequence. The other was an adenine-to-guanine substitution within exon 4, of the same lung cell line; this change already had been reported as a polymorphism. The results suggested that the hMAD2 gene is not commonly mutated in either lung nor breast cancers. Further studies should focus on other mechanisms that might account for reduced expression of the hMAD2 gene, and/or pursue analyses of other mitotic checkpoint genes for mutations in human cancer. Nevertheless, the genomic structure, the intronic primer sequences, and polymorphisms of the hMAD2 gene presented here will facilitate future studies to determine the full spectrum and frequency of the genetic events that can affect expression of the hMAD2 gene in human tumors.

Somatic mutation of the hBUB1 mitotic checkpoint gene in primary lung cancer

Mutations in mitotic checkpoint genes have been detected in several human cancers, and these cancers exhibit chromosomal instability. Aneuploid stem cells seem to result from chromosomal instability and have been reported in many lung cancers. To determine whether alteration of mitotic checkpoint regulators is involved in carcinogenesis and tumor progression in primary lung cancer, we screened the genomic DNA sequence of 30 human lung cancer cell lines and 30 primary lung cancer tumors for a mutation in the hBUB1 mitotic checkpoint gene. First, we designed 26 sets of intron-based primers to amplify each of the 25 exons of the hBUB1 gene to examine the entire coding region of the hBUB1 gene. Using these primers, we performed polymerase chain reaction-single strand conformation polymorphism (PCR-SSCP) analysis as well as direct sequencing in the mutation analysis of the hBUB1 gene. Three different nucleotide substitutions were detected in the coding region of the hBUB1 gene in some of the cancer cell lines and primary tumors as follows. The hBUB1 gene of one adenocarcinoma tumor contained a somatic missense mutation, a cytosine-to-guanine substitution in codon 51 of exon 5 that resulted in a histidine-to-aspartic acid amino acid substitution. The hBUB1 gene of three lung cancer cell lines contained a thymine-to-cytosine substitution in codon 430 of exon 12, which did not result in an amino-acid substitution. We were unable to determine whether the nucleotide substitution in exon 12 was a polymorphism or a silent mutation because matched normal tissue was not available. A polymorphism in codon 93 of exon 4, a guanine-to-thymine substitution, in hBUB1 was found in one lung cancer cell line and one primary lung tumor. This is the first report of a somatic missense mutation of a gene involved in a mitotic checkpoint in primary lung cancer. The presence of a point mutation in the hBUB1 gene is consistent with the hypothesis that alteration of mitotic checkpoint genes is involved in the development of primary lung cancers. Because the frequency of hBUB1 gene mutations was low, future studies should focus on other mechanisms of inactivation of the hBUB1 gene as well as mutation analysis of other mitotic checkpoint genes in lung cancers.

Alteration of the PTEN/MMAC1 gene locus in primary lung cancer with distant metastasis

The PTEN/MMAC1 gene located at 10q23, has been proposed to be a tumor suppressor gene. To determine the involvement of alteration of the PTEN/MMAC1 gene in carcinogenesis and the progression of primary lung cancers, we analyzed tumor samples of primary and distant metastatic sites and normal lung tissue samples of 30 patients with advanced lung cancer with distant metastasis. The tissues were analyzed for allelic deletion and mutational inactivation of PTEN/MMAC1 by loss of heterozygosity (LOH) analysis, polymerase chain reaction-single strand conformation polymorphism (PCR-SSCP), and direct sequence analysis. LOH of the PTEN/MMAC1 locus was common in each histologic type of primary lung cancer. In this study, the overall allelic deletion rate was 33.3% (7/21). Allelic loss at the primary site and that at the metastatic site of each patient, were identical; in most cases, it seemed that the allelic loss had occurred before metastasis. Sequence analysis of the PTEN/MMAC1 gene revealed a G to C substitution located 8 bp upstream of the coding region of exon 1 and which seems to be a polymorphism, in 4 of the 30 cases. Somatic mutations of the PTEN/MMAC1 gene were not identified in any of the tumors at the primary and metastatic sites. These data indicate that point mutations in the PTEN/MMAC1 gene are probably not an important factor in tumorigenesis and the progression of a major subset of lung cancers. Due to frequent allelic loss at the PTEN/MMAC1 locus occurring at a stage earlier than the metastatic process, alternative mechanisms in which the remaining allele is inactivated such as methylation or homozygous deletion of a small region of the gene that can not be detected by the usual analysis, or alteration of other important tumor suppressor genes lying close to the PTEN/MMAC1 gene on 10q23, may be involved in the tumorigenesis of lung cancers of all histologic subtypes.

p53 tumor suppressor gene as a clonal marker in head and neck squamous cell carcinoma: p53 mutations in primary tumor and matched lymph node metastases

In order to define the diagnostic value of p53 tumor suppressor gene as a clonal marker in head and neck squamous cell carcinoma (HNSCC), we investigated p53 mutations in primary tumors (PT) and matched lymph node metastases (LNM); the underlying question being whether differentiation between metastatic disease of a known PT or (a metastasis of) a synchronous or metachronous second tumor is possible by means of p53 sequencing-based mutation analysis. In 15 PT, the p53 status was analyzed, following RNA isolation, cDNA synthesis and polymerase chain reaction amplification, by direct sequencing full-length mRNA. Mutations thus found were confirmed by DNA sequencing analysis of the corresponding exon in the PT. When RNA isolation was defective, DNA sequencing analysis of exons 1 through 11 was performed. In the matched LNM, DNA analysis of the corresponding exon was performed to prove the presence of the same p53 mutation. In the event of small clones not detectable by direct sequencing, an oligo ligation assay was developed to detect a specific mutation. The presence of germline mutations was excluded by DNA sequencing analysis of the corresponding exon of peripheral blood leucocytes. In 14 PT (94%), a mutation was identified. In one PT, no p53 mutation could be identified either after full-length mRNA sequencing or after sequencing exons 1 through 11. In all cases of PT and matched LNM, the mutations proved to be identical. We conclude that p53 mutations develop in carcinogenesis before metastases occur and are maintained during metastasis. Consequently, p53 may serve as a clonal marker not susceptible to change during tumor metastasis. This merits further exploration of the application of p53 mutation analysis in differentiating between metastatic disease from a known PT versus a metastasis of another second PT.