p53 mutations increase resistance to ionizing radiation

A mutant p53 transgene accelerates tumour development in heterozygous but not nullizygous p53-deficient mice

To test the behaviour of a mutant form of p53 in the presence and absence of wild-type p53 in vivo, we mated p53-deficient mice containing a p53 null allele to transgenic mice containing multiple copies of a mutant p53 gene (Val 135). Animals hemizygous for the endogenous wild-type p53 gene with the mutant transgene exhibited accelerated tumour development and an altered tumour spectrum compared to their non-transgenic counterparts. In contrast, transgenic and non-transgenic animals nullizygous for endogenous p53 developed tumours at the same rate. Thus, the mutant Val-135 p53 allele may act in vivo in a dominant negative manner in the presence of wild-type p53 but does not display gain of function activity in the absence of wild-type p53.

Use of semiquantitative reverse transcription-polymerase chain reaction to study gene expression in normal human skin fibroblasts following low dose-rate irradiation

One way to study the effect of radiation on gene expression is to monitor changes in the levels of specific messenger RNAs. We describe the use of reverse transcription-polymerase chain reaction (RT-PCR) analysis, a faster and more sensitive procedure than the traditional techniques to monitor RNA levels. Using RT-PCR, we confirmed previous results showing increased levels of GADD45 transcripts after high dose-rate X-irradiation in normal human fibroblasts. No differences were observed in the transcript levels of beta-ACTIN, beta-MICROGLOBULIN, Cu-Zn SUPEROXIDE DISMUTASE (SOD-1) and CATALASE. In cells exposed to 3-6 Gy low dose-rate gamma-irradiation we observed increased levels of the GADD45 transcript and lower transcript levels of the genes TOPOISOMERASE II alpha, FACC, CYCLIN A and CYCLIN B. No differences were detected in the transcript levels of beta-ACTIN, beta-MICROGLOBULIN, SOD-1, URACYL-DNA GLYCOSYLASE, CYCLIN C, CYCLIN E, CYCLIN D1, CYCLIN D2, CYCLIN D3, TOPOISOMERASE I and TOPOISOMERASE II beta.

Molecular characterization of the in vivo alkylating agent resistant murine EMT-6 mammary carcinoma tumors

The expression of several early-response genes and genes associated with malignant disease was assessed in the EMT-6/parent tumor and the EMT-6/CTX and EMT-6/CDDP in vivo resistant tumor lines growing as tumors or as monolayers in culture. In the absence of treatment the levels of mRNA for the genes c-jun, c-fos, c-myc, Ha-ras and p53 were increased in the EMT-6/CTX and EMT-6/CDDP as compared with the EMT-6/parent tumor, whereas the expression of erb-2 was similar in all three tumors. Although the cells from each of the three tumors show increased expression of early response genes after exposure to cisplatin (CDDP; 100 microM, 2 h) or 4-Hydroxyperoxycyclophosphamide (4-HC; 100 microM, 2 h) in culture, in mRNA extracted from tumor tissue these changes are absent or very small. Both C-jun and erb-2 were detectable in liver. There was increased expression of both of these genes in the livers of tumor-bearing animals as compared with non-tumor-bearing animals. The highest expression of both c-jun and erb-2 occurred in the livers of animals bearing the EMT-6/CDDP tumor. Treatment of the animals with CDDP or cyclophosphamide, in general, resulted in increased expression of both genes at 6 h post treatment. The increased expression of these genes may impart metabolic changes in the tumors and/or hosts that contribute to the resistance of these tumors to specific antitumor alkylating agents.

Tumor suppressor p53 mutations and breast cancer: a critical analysis

Alterations in the tumor suppressor gene p53 are the most commonly identified changes in cancer, including neoplasia of the breast. The activity of p53 is regulated post-translationally. Phosphorylation state, subcellular localization, and interaction with any of a number of cellular proteins are likely to influence the function of p53. The exact effect of p53-mediated growth suppression seems to be cell-type specific but appears to be directly related to the ability of p53 to act as a specific transcriptional activator. The role that transcriptional repression plays in the function of WT p53 is less clear. It is also possible that p53 has a more direct activity in DNA replication and repair. Most documented p53 mutations result in single amino acid substitutions which may confer one or more of a spectrum of transforming abilities on the protein. Mutation may lead to nuclear accumulation of p53 protein; however, inactivation of p53 by nuclear exclusion and interaction with the mdm2 protein also appear to be important in tumorigenesis. Used in conjunction with other established factors, accumulation of cellular p53 may be a useful prognostic indicator in breast cancer. A syngeneic mouse model system yielded evidence that p53 mutations are important in the early, preneoplastic stages of mammary tumorigenesis. This murine system may provide the ability to investigate the functions of p53 in the early stages of breast cancer which are technically difficult to examine in the human system.

Frequent p53 mutation in mouse tumors induced by repeated beta-irradiation

On examination of cDNA of the p53 gene in 65 murine tumors of the skin and bone produced by repeated exposure to a suprathreshold dose of 1-8 Gy of 90Sr-90Y beta-radiation per exposure, we found 20 cases of mutation: 11 minute deletions (loss of 1-24 bp), including two cases possibly due to aberrant splicing; three insertions of 4-8 bp; and six base-pair substitutions, including four at CpG sites, three being identical changes at codon 122 (a p53 hot spot). All but one of these mutations were confined to the central region of the p53 gene. From frameshifts created by deletions and insertions, the minimum size of the mutant p53 proteins found in these tumors was estimated to be 55% of the intact size. Cells of 17 of 19 tested tumors with p53 mutation were positive for immunostaining of p53 proteins accumulated in the nuclei and so were clearly different from nonneoplastic cells. Ha-ras mutation was absent in these tumors, indicating that repeated beta-irradiation created a cell-growth stimulating effect similar to that of ras mutation.

Radiation-induced genomic instability

Quantitative assessment of the heritable somatic effects of ionizing radiation exposures has relied upon the assumption that radiation-induced lesions were 'fixed' in the DNA prior to the first postirradiation mitosis. Lesion conversion was thought to occur during the initial round of DNA replication or as a consequence of error-prone enzymatic processing of lesions. The standard experimental protocols for the assessment of a variety of radiation-induced endpoints (cell death, specific locus mutations, neoplastic transformation and chromosome aberrations) evaluate these various endpoints at a single snapshot in time. In contrast with the aforementioned approaches, some studies have specifically assessed radiation effects as a function of time following exposure. Evidence has accumulated in support of the hypothesis that radiation exposure induces a persistent destabilization of the genome. This instability has been observed as a delayed expression of lethal mutations, as an enhanced rate of accumulation of non-lethal heritable alterations, and as a progressive intraclonal chromosomal heterogeneity. The genetic controls and biochemical mechanisms underlying radiation-induced genomic instability have not yet been delineated. The aim is to integrate the accumulated evidence that suggests that radiation exposure has a persistent effect on the stability of the mammalian genome.

p53 status, DNA double-strand break repair proficiency, and radiation response of mouse lymphoid and myeloid cell lines

The p53 status of a panel of 10 mouse lymphoid or myeloid cell lines was determined by immunoprecipitation with mutant- and wild-type-specific antibodies and was compared with the radiation response of the lines. The more rapidly dying cell lines all contained p53 displaying the wild-type epitope. By contrast, four of six more slowly dying cell lines contained either no or mutant p53 protein. It was of interest that radiation-induced apoptosis occurred, albeit at a considerable time after irradiation, in cells ostensibly lacking p53 protein. DNA double-strand break (dsb) repair was examined in both a rapidly and more slowly dying cell line. The rapidly dying cell line was capable of DNA dsb rejoining, however this repair was interrupted by postirradiation DNA degradation.

DNA damages processed by base excision repair: biological consequences

Base damages, sugar damages, and single-strand breaks produced by free radicals are the preponderant lesions produced in DNA by ionizing radiation. These lesions have been individually introduced into substrate, template, and biologically active DNA molecules and enzymatic processing and biological consequences determined. Free radical-induced DNA lesions are processed by base excision repair and many are potentially lethal in simple viral systems. Furthermore, a number of free radical modifications of purine and pyrimidine bases are premutagenic lesions. The results of the enzymatic and biological processing of a number of the more well-studied and stable lesions are summarized.

Characterization of G1 checkpoint control in the yeast Saccharomyces cerevisiae following exposure to DNA-damaging agents

The delay of S-phase following treatment of yeast cells with DNA-damaging agents is an actively regulated response that requires functional RAD9 and RAD24 genes. An analysis of cell cycle arrest indicates the existence of (at least) two checkpoints for damaged DNA prior to S-phase; one at START (a G1 checkpoint characterized by pheromone sensitivity of arrested cells) and one between the CDC4- and CDC7-mediated steps (termed the G1/S checkpoint). When a dna1-1 mutant (that affects early events of replicon initiation) also carries a rad9 deletion mutation, it manifests a failure to arrest in G1/S following incubation at the restrictive temperature. This failure to execute regulated G1/S arrest is correlated with enhanced thermosensitivity of colony-forming ability. In an attempt to characterize the signal for RAD9 gene-dependent G1 and G1/S cell cycle arrest, we examined the influence of the continued presence of unexcised photoproducts. In mutants defective in nucleotide excision repair, cessation of S-phase was observed at much lower doses of UV radiation compared to excision-proficient cells. However, this response was not RAD9-dependent. We suggest that an intermediate of nucleotide excision repair, such as DNA strand breaks or single-stranded DNA tracts, is required to activate RAD9-dependent G1 and G1/S checkpoint controls.

Autoimmune disease. A problem of defective apoptosis

Human autoimmune diseases share the common feature of an imbalance between the production and destruction of various cell types including lymphocytes (SLE), synovial cells (RA), and fibroblasts (scleroderma). Patients with SLE have increased levels of soluble Fas that inhibit proper apoptosis of lymphocytes. In animal models of autoimmune diseases, mutations of genes involved in apoptosis including Fas, Fas ligand, and the hematopoietic cell phosphatase gene have been identified. Oncogenes, including bcl-2, p53, and myc, that regulate apoptosis are also expressed abnormally. Potent inducers of apoptosis including steroids, azathioprine, cyclophosphamide, and methotrexate are the most efficacious therapies for autoimmune disease currently known. Specific therapies that induce apoptosis without incurring side effects should improve treatment of autoimmune disease.

p53-deficient mice are extremely susceptible to radiation-induced tumorigenesis

Mice constitutively lacking alleles of the p53 tumour suppressor gene spontaneously develop lymphomas and sarcomas. We report here that a single dose of 4 Gy radiation dramatically decreases the latency for tumour development in p53 heterozygous mice. The pattern of genetic alterations at the remaining wild type allele in these tumours differs substantially from spontaneous tumours from similar mice indicating that p53 itself may have been a target for radiation-induced alterations. Lower dose irradiation (1 Gy) of preweanling p53 null mice also significantly decreases tumour latency, suggesting that there are additional genetic targets involved in radiation-induced malignancy. Thus p53-deficient mice provide a sensitive model system for studies of the consequences of radiation exposure.

Cell cycle control and cancer chemotherapy

As detailed information accumulates about how cell cycle events are regulated, we can expect new opportunities for application to cancer therapy. The altered expression of oncogenes and tumor suppressor genes that commonly occurs in human cancers may impair the ability of the cells to respond to metabolic perturbations of stress. Impaired cell cycle regulation would make cells vulnerable to pharmacologic intervention by drug regimens tailored to the defects existing in particular tumors. Recent findings that may become applicable to therapy are reviewed, and the possible form of new therapeutic stratagems is considered.

Mitotic checkpoint genes in budding yeast and the dependence of mitosis on DNA replication and repair

In eukaryotes a cell-cycle control termed a checkpoint causes arrest in the S or G2 phases when chromosomes are incompletely replicated or damaged. Previously, we showed in budding yeast that RAD9 and RAD17 are checkpoint genes required for arrest in the G2 phase after DNA damage. Here, we describe a genetic strategy that identified four additional checkpoint genes that act in two pathways. Both classes of genes are required for arrest in the G2 phase after DNA damage, and one class of genes is also required for arrest in S phase when DNA replication is incomplete. The G2-specific genes include MEC3 (for mitosis entry checkpoint), RAD9, RAD17, and RAD24. The genes common to both S phase and G2 phase pathways are MEC1 and MEC2. The MEC2 gene proves to be identical to the RAD53 gene. Checkpoint mutants were identified by their interactions with a temperature-sensitive allele of the cell division cycle gene CDC13; cdc13 mutants arrested in G2 and survived at the restrictive temperature, whereas all cdc13 checkpoint double mutants failed to arrest in G2 and died rapidly at the restrictive temperature. The cell-cycle roles of the RAD and MEC genes were examined by combination of rad and mec mutant alleles with 10 cdc mutant alleles that arrest in different stages of the cell cycle at the restrictive temperature and by the response of rad and mec mutant alleles to DNA damaging agents and to hydroxyurea, a drug that inhibits DNA replication. We conclude that the checkpoint in budding yeast consists of overlapping S-phase and G2-phase pathways that respond to incomplete DNA replication and/or DNA damage and cause arret of cells before mitosis.

Increased accumulation of p53 protein in cisplatin-resistant ovarian cell lines

We have examined p53 protein levels in cell lines selected for resistance to the chemotherapeutic drug cis-diamminedichloroplatinum (II), cisplatin. The majority of the independent cisplatin-resistant clones isolated by a single selection with cisplatin from the ovarian tumour cell line A2780 showed increased levels of p53 protein compared to the parental cell line. Elevated p53 protein levels were also observed in cisplatin-resistant ovarian human tumour lines isolated after multiple exposures to cisplatin (A2780/cp70 and OVIP/DDP). Direct PCR sequencing of p53 cDNAs showed that both the A2780/cp70 and the parental A2780 cell lines had a wild-type p53 gene sequence. The OVIP and OVIP/DDP lines both had a heterozygous mutation at codon 126. Cell-cycle analysis after gamma-irradiation or cisplatin treatment showed evidence of a G1/S and G2/M cell-cycle checkpoint in both A2780/cp70 and the sensitive parental cell lines. However, the resistant cell line A2780/cp70 showed less inhibition of DNA synthesis after gamma-irradiation than the sensitive cell line. Transfection of a mutant p53 gene construct (containing a mutation at codon 143, val to ala) into the A2780/cp70 resistant cells conferred a significantly increased sensitivity to cisplatin, suggesting that p53 is a direct determinant of cisplatin resistance in these cells. However, expression of this mutant p53 in the A2780 cells did not affect sensitivity.