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The mechanisms by which cells accurately distinguish between DNA double-strand break (DSB) ends and telomeric DNA ends remain poorly defined. Recent investigations have revealed intriguing interactions between DNA repair and telomeres. We were the first to report a requirement for the nonhomologous end-joining (NHEJ) protein DNA-dependent protein kinase (DNA-PK) in the effective end-capping of mammalian telomeres. Here, we report our continued characterization of uncapped (as opposed to shortened) dysfunctional telomeres in cells deficient for the catalytic subunit of DNA-PK (DNA-PKcs) and shed light on their consequence. We present evidence in support of our model that uncapped telomeres in this repair-deficient background are inappropriately detected and processed as DSBs and thus participate not only in spontaneous telomere-telomere fusion but, importantly, also in ionizing radiation-induced telomere-DSB fusion events. We show that phosphorylation of DNA-PKcs itself (Thr-2609 cluster) is a critical event for proper telomere end-processing and that ligase IV (NHEJ) is required for uncapped telomere fusion. We also find uncapped telomeres in cells from the BALB/c mouse, which harbors two single-nucleotide polymorphisms that result in reduced DNA-PKcs abundance and activity, most markedly in mammary tissue, and are both radiosensitive and susceptible to radiogenic mammary cancer. Our results suggest mechanistic links between uncapped/dysfunctional telomeres in DNA-PKcs-deficient backgrounds, radiation-induced instability, and breast cancer. These studies provide the first direct evidence of genetic susceptibility and environmental insult interactions leading to a unique and ongoing form of genomic instability capable of driving carcinogenesis.

The absence of p53 function increases risk for spontaneous tumorigenesis in the mammary gland. Hormonal stimulation enhances tumor risk in p53-null mammary epithelial cells as well as the incidence of aneuploidy. Aneuploidy appears in normal p53-null mammary epithelial cells within 5 weeks of hormone stimulation. Experiments reported herein assessed a possible mechanism of hormone-induced aneuploidy. Hormones increased DNA synthesis equally between wild-type (WT) and p53-null mammary epithelial cells. There were two distinct responses in p53-null cells to hormone exposure. First, Western blot analysis demonstrated that the levels of two proteins involved in regulating sister chromatid separation and the spindle checkpoint, Mad2 and separase (ESPL1) were increased in null compared with WT cells. In contrast, the levels of securin and Rad21 proteins were not increased in hormone-stimulated p53-null compared with WT cells. ESPL1 RNA was also increased in p53-null mouse mammary cells in vivo by 18 h of hormone stimulation and in human breast MCF7 cells in monolayer culture by 8 h of hormone stimulation. Furthermore, both promoters contained p53 and steroid hormone response elements. Mad2 protein was increased as a consequence of the absence of p53 function. The increase in Mad2 protein was observed also at the cellular level by immunohistochemistry. Second, hormones increased gene amplication in the distal arm of chromosome 2, as shown by comparative genomic hybridization. These results support the hypothesis that hormone stimulation acts to increase aneuploidy by several mechanisms. First, by increasing mitogenesis in the absence of the p53 checkpoint in G2, hormones allow the accumulation of cells that have experienced chromosome missegregation. Second, the absolute rate of chromosome missegregation may be increased by alterations in the levels of two proteins, separase and Mad2, which are important for maintaining chromosomal segregation and the normal spindle checkpoint during mitosis.

Female BALB/c mice are unusually radiosensitive and more susceptible than C57BL/6 and other tested inbred mice to ionizing radiation (IR)-induced mammary tumors. This breast cancer susceptibility is correlated with elevated susceptibility for mammary cell transformation and genomic instability following irradiation. In this study, we report the identification of two BALB/c strain-specific polymorphisms in the coding region of Prkdc, the gene encoding the DNA-dependent protein kinase catalytic subunit, which is known to be involved in DNA double-stranded break repair and post-IR signal transduction. First, we identified an A --> G transition at base 11530 resulting in a Met --> Val conversion at codon 3844 (M3844V) in the phosphatidylinositol 3-kinase domain upstream of the scid mutation (Y4046X). Second, we identified a C --> T transition at base 6418 resulting in an Arg --> Cys conversion at codon 2140 (R2140C) downstream of the putative leucine zipper domain. This unique PrkdcBALB variant gene is shown to be associated with decreased DNA-dependent protein kinase catalytic subunit activity and with increased susceptibility to IR-induced genomic instability in primary mammary epithelial cells. The data provide the first evidence that naturally arising allelic variation in a mouse DNA damage response gene may associate with IR response and breast cancer risk.

We have studied the efficiency of DNA double strand break (DSB) rejoining in primary cells from mouse strains that show large differences in in vivo radiosensitivity and tumor susceptibility. Cells from radiosensitive, cancer-prone BALB/c mice showed inefficient end joining of gamma ray-induced DSBs as compared with cells from all of the other commonly used strains and F1 hybrids of C57BL/6 and BALB/c mice. The BALB/c repair phenotype was accompanied by a significantly reduced expression level of DNA-PKcs protein as well as a lowered DNA-PK activity level as compared with the other strains. In conjunction with published reports, these data suggest that natural genetic variation in nonhomologous end joining processes may have a significant impact on the in vivo radiation response of mice.

A radiosensitive DNA repair-deficient xrs-5 cell line was used to study asbestos cytotoxicity and DNA double strand breaks (DSBs). Although xrs-5 cells did not show any increase in sensitivity to chrysotile fibers in short-term (4-h) treatment when compared with wild-type CHO cells, longer-term exposure (24 h) gave significantly lower cell survival accompanied by a cell growth delay as well as a higher DNA DSB induction in this mutant cell line. These results suggest an important role played by DNA DSBs at the initial stage of asbestos injury.

EF42 is a clonally derived preneoplastic cell lineage from irradiated mouse mammary tissue, which becomes neoplastic with time in vitro or in vivo. We now report that multiple mutations in p53 occur before the acquisition of the neoplastic phenotype. The selective expansion of mutant cells is accompanied by loss of heterozygosity at the p53 locus and c-myc amplification. Although p53 mutations represent critical early events, our data argue these mutations were not directly induced by radiation but arose in the progeny of irradiated cells several cell generations later. The data are consistent with a multistep model of carcinogenesis that identifies genomic instability as the earliest step.

Virgin female BALB/c mice were exposed in vivo to whole body gamma-radiation and/or to 7,12-dimethylbenz[a]anthracene (DMBA) p.o. Mammary epithelial cells were isolated and assayed for carcinogen altered cell populations both in vitro by an epithelial focus assay and in vivo by injection into cleared fat pads of syngeneic host mice. Five groups of mice were exposed as follows: (a) sham controls; (b) 50-rad gamma-radiation; (c) 100-rad gamma-radiation; (d) 75 micrograms DMBA; or (e) 50-rad gamma-radiation followed in 1 week by 75 micrograms DMBA. Mammary epithelial cells were isolated and assayed at 24 h and at 1, 4, 16, and 52 weeks after in vivo exposure. Four to 12 mice per treatment per time point were individually assayed. Altered in vitro growth potential was characterized by the proliferation of carcinogen exposed (but not control) cells as epithelial foci which persisted at least 12 weeks in primary culture. Epithelial foci which could then be subcultured at least four times were termed subculturable epithelial foci. Altered in vivo morphogenic potential was characterized by dysplastic or neoplastic growth in host fat pads. With increased time in situ between exposure and assay, cell populations emerged which exhibited both increased in vitro subculturability and enhanced tumorigenic potential including a host response upon injection in vivo. Further, combined radiation and DMBA resulted in higher frequencies of subculturable epithelial foci than either treatment alone. The relevance of these progressive cellular changes to the process of mammary tumor development is discussed.

These studies were undertaken to determine if altered growth potential of mammary epithelial cells could be detected in outgrowths derived from monodispersed mammary cells of virgin female BALB/c mice previously exposed to ionizing radiation or 7, 12-dimethylbenz(a)anthracene (DMBA). Monodispersed mammary epithelial cells were obtained by enzymatic dissociation of mammary tissues of 12-week-old virgin female BALB/c mice. Twenty-four hr prior to cell dissociation, donor animals were exposed to either 100 rads of gamma-ray irradiation, 0.25 mg of DMBA, or 0.075 mg of DMBA. Control donors were untreated. Mammary outgrowths were then derived from these donor cells by injecting either 10(5) or 10(4) cells into the gland-free mammary fat pads of three-week-old virgin female BALB/c mice. Ten weeks after the injection of cells, the outgrowths were examined and classified. Mammary outgrowths were classified either as having a normal ductal architecture or as having ductal dysplasia. Ductal dysplasias were further classified on the basis of an index of severity, which was an arbitrary index based on the number of abnormal ductal structures within each lesion. The data indicated that treatment of donor animals with either gamma-radiation or DMBA increased the frequency of ductal lesions over control levels; however, both the frequency and severity of the lesions depended on the number of cells which were injected into the fat pad. When outgrowths were derived by injection of 10(5) cells into the gland-free fat pads, lesion frequencies in outgrowths from control and treated cells were: 3.3%, control; 15.7%, gamma-rays; 5.3%, 0.25 mg DMBA; in these groups only a few severe lesions were detected. In outgrowths derived from 10(4) cells, less severe lesions (Class I lesions) were common in all groups and occurred in approximately 10 to 15% of the outgrowths. The frequency of severe (Class II and III) ductal dysplasia, however, was increased by treatment in these groups, occurring in 4.5% of control outgrowths in 15.6, 14.9, an 14.3% of the outgrowths derived from donor cells treated with 100 rads gamma-rays, 0.075 mg DMBA, nd 0.25 mg DMBA, respectively. Thus, these data indicated that ductal dysplasias were more common and more severe in outgrowths derived from 10(4) rather than 10(5) cells. The ductal lesions observed in this study resembled both morphologically and histologically ductal abnormalities which have been associated with the pathogenesis of mammary carcinoma in both rats and mice.

The antitumor properties of Corynebacterium parvum have been studied in young (3- to 8-month-old) and aged 18 or more months old) BALB/c mice given s.c., i.m., i.p., or i.v. transplants of the highly malignant, weakly immunogenic line 1 lung carcinoma, and in aged (25- to 33-month-old) BALB/c mice bearing primary mammary tumors. These aged BALB/c mice were shown to be less immunoresponsive than their younger counterparts, and this, in combination with nonimmunological factors, made them more sensitive to the lethal effects of the line 1 carcinoma. Correspondingly, C. parvum proved to have less antitumor activity in aged mice than it did in young mice. In spite of this relatively weaker antitumor activity for C. parvum in aged mice, repeated injections of this agent were able to induce temporary regressions of the primary mammary tumors studied and therby prolong survival time.

The expression of mammary ductal dysplasia has been shown to be enhanced in mammary outgrowths derived from enzymatically dissociated mammary cells and influenced by the number of cells used to derive the outgrowths. The present study was designed to examine this cell dose effect further and to determine if the developmental state of the outgrowths or the time between carcinogen administration and cell dissociation affects the expression and persistence of the ductal dysplasias. Mammary outgrowths were derived by injecting 10(4) or 10(5) enzymatically dissociated mammary cells into gland-free fat pads of 3-week-old female BALB/c mice. Donor animals were untreated or were exposed to either 7,12-dimethylbenz(a)anthracene or gamma-ray irradiation. The outgrowths were examined at 4.5, 8, 10, or 16 weeks after transplantation, depending on the experiment, and classified as having normal or dysplastic growth. The data indicated that expression of ductal dysplasia was greater in outgrowths derived from 10(4) than from 10(5) cells regardless of the developmental state of the outgrowths. When 24 hr elapsed between carcinogen exposure and cell dissociation, expression of lesions in outgrowths derived from 10(4) cells required active ductal growth, in that lesions that were present in developing outgrowths remodeled and were no longer detectable when the outgrowths completely filled the fat pad. When second-transplant-generation outgrowths were derived from cells of full (non-growing) first-generation outgrowths, ductal dysplasias were reexpressed but, once again, only within developing outgrowths. Increasing the time between carcinogen treatment and cell dissociation, i.e., 6 days or longer, resulted in both increased frequencies of ductal dysplasias and substantial numbers of lesions which persisted within full outgrowths. These results suggested that the acquisition of the altered growth potential which resulted in ductal dysplasias and the ability of these lesions to gain some autonomy from growth-regulatory mechanisms were separate events that occurred at different times after carcinogen treatment.

The morphological and histological characteristics of ductal dysplasias that were observed in mammary outgrowths derived from monodispersed mammary cells of carcinogen-treated mice are described. Mammary outgrowths were derived by injecting either 10(4) or 10(5) enzymatically dissociated mammary cells, obtained from control or carcinogen-treated BALB/c mice, into gland-free mammary fat pads of syngeneic hosts. The mammary dysplasias observed varied considerably in morphological and histological characteristics. The majority of the lesions were ductal in origin and were associated with epithelial hyperplasia which ranged from mild hyperplasia, in which only a few extra layers of epithelium were present, to severe hyperplasia, in which the ducts and end buds were occluded and distended with epithelial cells. In addition, papillary and lobular lesions were observed which were also associated with varying degrees of hyperplasia. The range of mammary dysplasias observed in these outgrowths closely resembles that of lesions associated with the pathogenesis of mammary carcinoma in mice, rats, and humans.

Two rat monoclonal antibodies, 34A and 201B, have been isolated and shown to bind preferentially to capillary endothelial cells in the lung. Administration of these antibodies to mice increases the number of lung colonies derived from i.v. injection of tumor cells. The antibodies increase lung colonization in C57BL/6 mice following i.v. injection of B16-F10 melanoma cells and in BALB/c mice following injection of line 1 lung carcinoma cells. Neither 34A nor 201B monoclonal antibody binds to B16 melanoma or line 1 carcinoma and so must exert its effect by interaction with endothelial cells. Antibodies injected i.v., s.c., or i.p. are active from 1 h to 1 wk if injected before cell injection. The effect is optimal when 0.1 ml of ascites fluid containing 120 micrograms of antigen binding capacity of both MoAbs 34A and 201B is injected. Significant damage to endothelial cells could not be documented by histopathological examination at the light microscope level or by protein leakage into the air space as measured by lung lavage. However, electron micrographs taken 3 h after monoclonal antibody injection show minor damage to endothelial cell membranes throughout the lung with some areas of mild edema. The increased colonization may be mediated by this subtle damage to endothelial cells, or antibody interactions with endothelial cells may trigger secondary reactions such as altered expression of growth factors.