Ataxia-telangiectasia mutated is not required for p53 induction and apoptosis in irradiated epithelial tissues

The HSP90/R2TP assembly chaperone promotes cell proliferation in the intestinal epithelium

The R2TP chaperone cooperates with HSP90 to integrate newly synthesized proteins into multi-subunit complexes, yet its role in tissue homeostasis is unknown. Here, we generated conditional, inducible knock-out mice for Rpap3 to inactivate this core component of R2TP in the intestinal epithelium. In adult mice, Rpap3 invalidation caused destruction of the small intestinal epithelium and death within 10 days. Levels of R2TP substrates decreased, with strong effects on mTOR, ATM and ATR. Proliferative stem cells and progenitors deficient for Rpap3 failed to import RNA polymerase II into the nucleus and they induced p53, cell cycle arrest and apoptosis. Post-mitotic, differentiated cells did not display these alterations, suggesting that R2TP clients are preferentially built in actively proliferating cells. In addition, high RPAP3 levels in colorectal tumors from patients correlate with bad prognosis. Here, we show that, in the intestine, the R2TP chaperone plays essential roles in normal and tumoral proliferation.

The Role of Caspase-2 in Regulating Cell Fate

Caspase-2 is the most evolutionarily conserved member of the mammalian caspase family and has been implicated in both apoptotic and non-apoptotic signaling pathways, including tumor suppression, cell cycle regulation, and DNA repair. A myriad of signaling molecules is associated with the tight regulation of caspase-2 to mediate multiple cellular processes far beyond apoptotic cell death. This review provides a comprehensive overview of the literature pertaining to possible sophisticated molecular mechanisms underlying the multifaceted process of caspase-2 activation and to highlight its interplay between factors that promote or suppress apoptosis in a complicated regulatory network that determines the fate of a cell from its birth and throughout its life.

Adverse outcome pathways for ionizing radiation and breast cancer involve direct and indirect DNA damage, oxidative stress, inflammation, genomic instability, and interaction with hormonal regulation of the breast

Knowledge about established breast carcinogens can support improved and modernized toxicological testing methods by identifying key mechanistic events. Ionizing radiation (IR) increases the risk of breast cancer, especially for women and for exposure at younger ages, and evidence overall supports a linear dose-response relationship. We used the Adverse Outcome Pathway (AOP) framework to outline and evaluate the evidence linking ionizing radiation with breast cancer from molecular initiating events to the adverse outcome through intermediate key events, creating a qualitative AOP. We identified key events based on review articles, searched PubMed for recent literature on key events and IR, and identified additional papers using references. We manually curated publications and evaluated data quality. Ionizing radiation directly and indirectly causes DNA damage and increases production of reactive oxygen and nitrogen species (RONS). RONS lead to DNA damage and epigenetic changes leading to mutations and genomic instability (GI). Proliferation amplifies the effects of DNA damage and mutations leading to the AO of breast cancer. Separately, RONS and DNA damage also increase inflammation. Inflammation contributes to direct and indirect effects (effects in cells not directly reached by IR) via positive feedback to RONS and DNA damage, and separately increases proliferation and breast cancer through pro-carcinogenic effects on cells and tissue. For example, gene expression changes alter inflammatory mediators, resulting in improved survival and growth of cancer cells and a more hospitable tissue environment. All of these events overlap at multiple points with events characteristic of "background" induction of breast carcinogenesis, including hormone-responsive proliferation, oxidative activity, and DNA damage. These overlaps make the breast particularly susceptible to ionizing radiation and reinforce that these biological activities are important characteristics of carcinogens. Agents that increase these biological processes should be considered potential breast carcinogens, and predictive methods are needed to identify chemicals that increase these processes. Techniques are available to measure RONS, DNA damage and mutation, cell proliferation, and some inflammatory proteins or processes. Improved assays are needed to measure GI and chronic inflammation, as well as the interaction with hormonally driven development and proliferation. Several methods measure diverse epigenetic changes, but it is not clear which changes are relevant to breast cancer. In addition, most toxicological assays are not conducted in mammary tissue, and so it is a priority to evaluate if results from other tissues are generalizable to breast, or to conduct assays in breast tissue. Developing and applying these assays to identify exposures of concern will facilitate efforts to reduce subsequent breast cancer risk.

Adverse outcome pathways for ionizing radiation and breast cancer involve direct and indirect DNA damage, oxidative stress, inflammation, genomic instability, and interaction with hormonal regulation of the breast

Knowledge about established breast carcinogens can support improved and modernized toxicological testing methods by identifying key mechanistic events. Ionizing radiation (IR) increases the risk of breast cancer, especially for women and for exposure at younger ages, and evidence overall supports a linear dose-response relationship. We used the Adverse Outcome Pathway (AOP) framework to outline and evaluate the evidence linking ionizing radiation with breast cancer from molecular initiating events to the adverse outcome through intermediate key events, creating a qualitative AOP. We identified key events based on review articles, searched PubMed for recent literature on key events and IR, and identified additional papers using references. We manually curated publications and evaluated data quality. Ionizing radiation directly and indirectly causes DNA damage and increases production of reactive oxygen and nitrogen species (RONS). RONS lead to DNA damage and epigenetic changes leading to mutations and genomic instability (GI). Proliferation amplifies the effects of DNA damage and mutations leading to the AO of breast cancer. Separately, RONS and DNA damage also increase inflammation. Inflammation contributes to direct and indirect effects (effects in cells not directly reached by IR) via positive feedback to RONS and DNA damage, and separately increases proliferation and breast cancer through pro-carcinogenic effects on cells and tissue. For example, gene expression changes alter inflammatory mediators, resulting in improved survival and growth of cancer cells and a more hospitable tissue environment. All of these events overlap at multiple points with events characteristic of "background" induction of breast carcinogenesis, including hormone-responsive proliferation, oxidative activity, and DNA damage. These overlaps make the breast particularly susceptible to ionizing radiation and reinforce that these biological activities are important characteristics of carcinogens. Agents that increase these biological processes should be considered potential breast carcinogens, and predictive methods are needed to identify chemicals that increase these processes. Techniques are available to measure RONS, DNA damage and mutation, cell proliferation, and some inflammatory proteins or processes. Improved assays are needed to measure GI and chronic inflammation, as well as the interaction with hormonally driven development and proliferation. Several methods measure diverse epigenetic changes, but it is not clear which changes are relevant to breast cancer. In addition, most toxicological assays are not conducted in mammary tissue, and so it is a priority to evaluate if results from other tissues are generalizable to breast, or to conduct assays in breast tissue. Developing and applying these assays to identify exposures of concern will facilitate efforts to reduce subsequent breast cancer risk.

Synergy between Prkdc and Trp53 regulates stem cell proliferation and GI-ARS after irradiation

Ionizing radiation (IR) is one of the most widely used treatments for cancer. However, acute damage to the gastrointestinal tract or gastrointestinal acute radiation syndrome (GI-ARS) is a major dose-limiting side effect, and the mechanisms that underlie this remain unclear. Here we use mouse models to explore the relative roles of DNA repair, apoptosis, and cell cycle arrest in radiation response. IR induces DNA double strand breaks and DNA-PK mutant Prkdc^scid/scid mice are sensitive to GI-ARS due to an inability to repair these breaks. IR also activates the tumor suppressor p53 to trigger apoptotic cell death within intestinal crypt cells and p53 deficient mice are resistant to apoptosis. To determine if DNA-PK and p53 interact to govern radiosensitivity, we compared the response of single and compound mutant mice to 8 Gy IR. Compound mutant Prkdc^scid/scid/Trp53^-/-mice died earliest due to severe GI-ARS. While both Prkdc^scid/scid and Prkdc^scid/scid/Trp53^-/-mutant mice had higher levels of IR-induced DNA damage, particularly within the stem cell compartment of the intestinal crypt, in Prkdc^scid/scid/Trp53^-/-mice these damaged cells abnormally progressed through the cell cycle resulting in mitotic cell death. This led to a loss of Paneth cells and a failure to regenerate the differentiated epithelial cells required for intestinal function. IR-induced apoptosis did not correlate with radiosensitivity. Overall, these data reveal that DNA repair, mediated by DNA-PK, and cell cycle arrest, mediated by p53, cooperate to protect the stem cell niche after DNA damage, suggesting combination approaches to modulate both pathways may be beneficial to reduce GI-ARS. As many cancers harbor p53 mutations, this also suggests targeting DNA-PK may be effective to enhance sensitivity of p53 mutant tumors to radiation.

Regulation of the Mdm2-p53 signaling axis in the DNA damage response and tumorigenesis

The p53 tumor suppressor acts as a guardian of the genome in mammalian cells undergoing DNA double strand breaks induced by a various forms of cell stress, including inappropriate growth signals or ionizing radiation. Following damage, p53 protein levels become greatly elevated in cells and p53 functions primarily as a transcription factor to regulate the expression a wide variety of genes that coordinate this DNA damage response. In cells undergoing high amounts of DNA damage, p53 can promote apoptosis, whereas in cells undergoing less damage, p53 promotes senescence or transient cell growth arrest and the expression of genes involved in DNA repair, depending upon the cell type and level of damage. Failure of the damaged cell to undergo growth arrest or apoptosis, or to respond to the DNA damage by other p53-coordinated mechanisms, can lead to inappropriate cell growth and tumorigenesis. In cells that have successfully responded to genetic damage, the amount of p53 present in the cell must return to basal levels in order for the cell to resume normal growth and function. Although regulation of p53 levels and function is coordinated by many proteins, it is now widely accepted that the master regulator of p53 is Mdm2. In this review, we discuss the role(s) of p53 in the DNA damage response and in tumor suppression, and how post-translational modification of Mdm2 regulates the Mdm2-p53 signaling axis to govern p53 activities in the cell.

A novel mouse model for ataxia-telangiectasia with a N-terminal mutation displays a behavioral defect and a low incidence of lymphoma but no increased oxidative burden

Ataxia-telangiectasia (A-T) is a rare multi-system disorder caused by mutations in the ATM gene. Significant heterogeneity exists in the underlying genetic mutations and clinical phenotypes. A number of mouse models have been generated that harbor mutations in the distal region of the gene, and a recent study suggests the presence of residual ATM protein in the brain of one such model. These mice recapitulate many of the characteristics of A-T seen in humans, with the notable exception of neurodegeneration. In order to study how an N-terminal mutation affects the disease phenotype, we generated an inducible Atm mutant mouse model (Atm(tm1Mmpl/tm1Mmpl), referred to as A-T [M]) predicted to express only the first 62 amino acids of Atm. Cells derived from A-T [M] mutant mice exhibited reduced cellular proliferation and an altered DNA damage response, but surprisingly, showed no evidence of an oxidative imbalance. Examination of the A-T [M] animals revealed an altered immunophenotype consistent with A-T. In contrast to mice harboring C-terminal Atm mutations that disproportionately develop thymic lymphomas, A-T [M] mice developed lymphoma at a similar rate as human A-T patients. Morphological analyses of A-T [M] cerebella revealed no substantial cellular defects, similar to other models of A-T, although mice display behavioral defects consistent with cerebellar dysfunction. Overall, these results suggest that loss of Atm is not necessarily associated with an oxidized phenotype as has been previously proposed and that loss of ATM protein is not sufficient to induce cerebellar degeneration in mice.

Mapping genetic alterations causing chemoresistance in cancer: identifying the roads by tracking the drivers

Although new agents are implemented to cancer therapy, we lack fundamental understandings of the mechanisms of chemoresistance, the main obstacle to cure in cancer. Here we review clinical evidence linking molecular defects to drug resistance across different tumour forms and discuss contemporary experimental evidence exploring these mechanisms. Although evidence, in general, is sparse and fragmentary, merging knowledge links drug resistance, and also sensitivity, to defects in functional pathways having a key role in cell growth arrest or death and DNA repair. As these pathways may act in concert, there is a need to explore multiple mechanisms in parallel. Taking advantage of massive parallel sequencing and other novel high-throughput technologies and base research on biological hypotheses, we now have the possibility to characterize functional defects related to these key pathways and to design a new generation of studies identifying the mechanisms controlling resistance to different treatment regimens in different tumour forms.

Maintenance of genomic integrity after DNA double strand breaks in the human prostate and seminal vesicle epithelium: the best and the worst

Prostate cancer is one of the most frequent cancer types in men, and its incidence is steadily increasing. On the other hand, primary seminal vesicle carcinomas are extremely rare with less than 60 cases reported worldwide. Therefore the difference in cancer incidence has been estimated to be more than a 100,000-fold. This is astonishing, as both tissues share similar epithelial structure and hormonal cues. Clearly, the two epithelia differ substantially in the maintenance of genomic integrity, possibly due to inherent differences in their DNA damage burden and DNA damage signaling. The DNA damage response evoked by DNA double strand breaks may be relevant, as their faulty repair has been implicated in the formation of common genomic rearrangements such as TMPRSS2-ERG fusions during prostate carcinogenesis. Here, we review DNA damaging processes of both tissues with an emphasis on inflammation and androgen signaling. We discuss how benign prostate and seminal vesicle epithelia respond to acute DNA damage, focusing on the canonical DNA double strand break-induced ATM-pathway, p53 and DNA damage induced checkpoints. We propose that the prostate might be more prone to the accumulation of genetic aberrations during epithelial regeneration than seminal vesicles due to a weaker ability to enforce DNA damage checkpoints.

ATM phosphorylation of Mdm2 Ser394 regulates the amplitude and duration of the DNA damage response in mice

DNA damage induced by ionizing radiation activates the ATM kinase, which subsequently stabilizes and activates the p53 tumor suppressor protein. Although phosphorylation of p53 by ATM was found previously to modulate p53 levels and transcriptional activities in vivo, it does not appear to be a major regulator of p53 stability. We have utilized mice bearing altered Mdm2 alleles to demonstrate that ATM phosphorylation of Mdm2 serine 394 is required for robust p53 stabilization and activation after DNA damage. In addition, we demonstrate that dephosphorylation of Mdm2 Ser394 regulates attenuation of the p53-mediated response to DNA damage. Therefore, the phosphorylation status of Mdm2 Ser394 governs p53 protein levels and functions in cells undergoing DNA damage.

Differential contextual responses of normal human breast epithelium to ionizing radiation in a mouse xenograft model

Radiotherapy is a key treatment option for breast cancer, yet the molecular responses of normal human breast epithelial cells to ionizing radiation are unclear. A murine subcutaneous xenograft model was developed in which nonneoplastic human breast tissue was maintained with the preservation of normal tissue architecture, allowing us to study for the first time the radiation response of normal human breast tissue in situ. Ionizing radiation induced dose-dependent p53 stabilization and p53 phosphorylation, together with the induction of p21(CDKN1A) and apoptosis of normal breast epithelium. Although p53 was stabilized in both luminal and basal cells, induction of Ser392-phosphorylated p53 and p21 was higher in basal cells and varied along the length of the ductal system. Basal breast epithelial cells expressed ΔNp63, which was unchanged on irradiation. Although stromal responses themselves were minimal, the response of normal breast epithelium to ionizing radiation differed according to the stromal setting. We also demonstrated a dose-dependent induction of γ-H2AX foci in epithelial cells that was similarly dependent on the stromal environment and differed between basal and luminal epithelial cells. The intrinsic differences between human mammary cell types in response to in vivo irradiation are consistent with clinical observation that therapeutic ionizing radiation is associated with the development of basal-type breast carcinomas. Furthermore, there may be clinically important stromal-epithelial interactions that influence DNA damage responses in the normal breast. These findings demonstrate highly complex responses of normal human breast epithelium following ionizing radiation exposure and emphasize the importance of studying whole-tissue effects rather than single-cell systems.

DNA damage recognition via activated ATM and p53 pathway in nonproliferating human prostate tissue

DNA damage response (DDR) pathways have been extensively studied in cancer cell lines and mouse models, but little is known about how DNA damage is recognized by different cell types in nonmalignant, slowly replicating human tissues. Here, we assess, using ex vivo cultures of human prostate tissue, DDR caused by cytotoxic drugs (camptothecin, doxorubicin, etoposide, and cisplatin) and ionizing radiation (IR) in the context of normal tissue architecture. Using specific markers for basal and luminal epithelial cells, we determine and quantify cell compartment-specific damage recognition. IR, doxorubicin, and etoposide induced the phosphorylation of H2A.X on Ser(139) (γH2AX) and DNA damage foci formation. Surprisingly, luminal epithelial cells lack the prominent γH2AX response after IR when compared with basal cells, although ATM phosphorylation on Ser(1981) and 53BP1 foci were clearly detectable in both cell types. The attenuated γH2AX response seems to result from low levels of total H2A.X in the luminal cells. Marked increase in p53, a downstream target of the activated ATM pathway, was detected only in response to camptothecin and doxorubicin. These findings emphasize the diversity of pathways activated by DNA damage in slowly replicating tissues and reveal an unexpected deviation in the prostate luminal compartment that may be relevant in prostate tumorigenesis. Detailed mapping of tissue and cell type differences in DDR will provide an outlook of relevant responses to therapeutic strategies.

p53 and gamma radiation in the normal breast

PURPOSE

With the increasing use of radiation as adjuvant therapy in breast cancer, the effects of gamma radiation on the remaining normal breast are of increasing importance. The complexities of multiple cellular types within breast tissues and the role of the pleiotropic Tumour Protein 53 (TP53, p53) protein with its downstream transcriptional targets and cellular processes may be central to the effects on residual normal breast tissues.

CONCLUSION

While a detailed understanding of p53 protein-mediated responses in normal breast tissues remains elusive, p53 appears to have a pivotal role in the effects of gamma radiation on normal breast epithelium, but not stromal cells, which may account for the differing clinical effects of gamma radiation in women treated for breast cancer.

DNA-PK suppresses a p53-independent apoptotic response to DNA damage

p53 is required for DNA damage-induced apoptosis, which is central to its function as a tumour suppressor. Here, we show that the apoptotic defect of p53-deficient cells is nearly completely rescued by inactivation of any of the three subunits of the DNA repair holoenzyme DNA-dependent protein kinase (DNA-PK). Intestinal crypt cells from p53 nullizygous mice were resistant to radiation-induced apoptosis, whereas apoptosis in DNA-PK(cs)/p53, Ku80/p53 and Ku70/p53 double-null mice was quantitatively equivalent to that seen in wild-type mice. This p53-independent apoptotic response was specific to the loss of DNA-PK, as it was not seen in ligase IV (Lig4)/p53 or ataxia telangiectasia mutated (Atm)/p53 double-null mice. Furthermore, it was associated with an increase in phospho-checkpoint kinase 2 (CHK2), and cleaved caspases 3 and 9, the latter indicating engagement of the intrinsic apoptotic pathway. This shows that there are two separate, but equally effective, apoptotic responses to DNA damage: one is p53 dependent and the other, engaged in the absence of DNA-PK, does not require p53.

CHEK2 mutations affecting kinase activity together with mutations in TP53 indicate a functional pathway associated with resistance to epirubicin in primary breast cancer

Background

Chemoresistance is the main obstacle to cure in most malignant diseases. Anthracyclines are among the main drugs used for breast cancer therapy and in many other malignant conditions. Single parameter analysis or global gene expression profiles have failed to identify mechanisms causing in vivo resistance to anthracyclines. While we previously found TP53 mutations in the L2/L3 domains to be associated with drug resistance, some tumors harboring wild-type TP53 were also therapy resistant. The aim of this study was; 1) To explore alterations in the TP53 gene with respect to resistance to a regular dose epirubicin regimen (90 mg/m² every 3 week) in patients with primary, locally advanced breast cancer; 2) Identify critical mechanisms activating p53 in response to DNA damage in breast cancer; 3) Evaluate in vitro function of Chk2 and p14 proteins corresponding to identified mutations in the CHEK2 and p14^(ARF) genes; and 4) Explore potential CHEK2 or p14^(ARF) germline mutations with respect to family cancer incidence.

Methods and Findings

Snap-frozen biopsies from 109 patients collected prior to epirubicin (as preoperative therapy were investigated for TP53, CHEK2 and p14^(ARF) mutations by sequencing the coding region and p14^(ARF) promoter methylations. TP53 mutastions were associated with chemoresistance, defined as progressive disease on therapy (p = 0.0358; p = 0.0136 for mutations affecting p53 loop domains L2/L3). Germline CHEK2 mutations (n = 3) were associated with therapy resistance (p = 0.0226). Combined, mutations affecting either CHEK2 or TP53 strongly predicted therapy resistance (p = 0.0101; TP53 mutations restricted to the L2/L3 domains: p = 0.0032). Two patients progressing on therapy harbored the CHEK2 mutation, Arg95Ter, completely abrogating Chk2 protein dimerization and kinase activity. One patient (Epi132) revealed family cancer occurrence resembling families harboring CHEK2 mutations in general, the other patient (epi203) was non-conclusive. No mutation or promoter hypermethylation in p14^(ARF) were detected.

Conclusion

This study is the first reporting an association between CHEK2 mutations and therapy resistance in human cancers and to document mutations in two genes acting direct up/down-stream to each other to cause therapy failure, emphasizing the need to investigate functional cascades in future studies.

Tumor suppression by p53 in the absence of Atm

Oncogenes can induce p53 through a signaling pathway involving p19/Arf. It was recently proposed that oncogenes can also induce DNA damage, and this can induce p53 through the Atm DNA damage pathway. To assess the relative roles of Atm, Arf, and p53 in the suppression of Ras-driven tumors, we examined susceptibility to skin carcinogenesis in 7,12-dimethylbenz(a)anthracene/12-O-tetradecanoylphorbol-13-acetate (TPA)-treated Atm- and p53-deficient mice and compared these results to previous studies on Arf-deficient mice. Mice with epidermal-specific deletion of p53 showed increased papilloma number and progression to malignant invasive carcinomas compared with wild-type littermates. In contrast, Atm-deficient mice showed no increase in papilloma number, growth, or malignant progression. gamma-H2AX and p53 levels were increased in both Atm(+/+) and Atm(-/-) papillomas, whereas Arf(-/-) papillomas showed much lower p53 expression. Thus, although there is evidence of DNA damage, signaling through Arf seems to regulate p53 in these Ras-driven tumors. In spontaneous and radiation-induced lymphoma models, tumor latency was accelerated in Atm(-/-)p53(-/-) compound mutant mice compared with the single mutant Atm(-/-) or p53(-/-) mice, indicating cooperation between loss of Atm and loss of p53. Although p53-mediated apoptosis was impaired in irradiated Atm(-/-) lymphocytes, p53 loss was still selected for during lymphomagenesis in Atm(-/-) mice. In conclusion, in these models of oncogene- or DNA damage-induced tumors, p53 retains tumor suppressor activity in the absence of Atm.