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Contains a more detailed method section as well as all additional supplementary figures S1 to S8

Tumour growth upon ATRi and Gemcitabine Treatment in Allografts model

Tumour growth upon Olaparib and Gemcitabine Treatment in Allografts model

Pancreatic ductal adenocarcinoma (PDAC) still presents with a dismal prognosis despite intense research. Better understanding of cellular homeostasis could identify druggable targets to improve therapy. Here we propose RAD50-interacting protein 1 (RINT1) as an essential mediator of cellular homeostasis in PDAC. In a cohort of resected PDAC, low RINT1 protein expression correlated significantly with better survival. Accordingly, RINT1 depletion caused severe growth defects in vitro associated with accumulation of DNA double-strand breaks (DSB), G2 cell cycle arrest, disruption of Golgi–endoplasmic reticulum homeostasis, and cell death. Time-resolved transcriptomics corroborated by quantitative proteome and interactome analyses pointed toward defective SUMOylation after RINT1 loss, impairing nucleocytoplasmic transport and DSB response. Subcutaneous xenografts confirmed tumor response by RINT1 depletion, also resulting in a survival benefit when transferred to an orthotopic model. Primary human PDAC organoids licensed RINT1 relevance for cell viability. Taken together, our data indicate that RINT1 loss affects PDAC cell fate by disturbing SUMOylation pathways. Therefore, a RINT1 interference strategy may represent a new putative therapeutic approach.Significance:These findings provide new insights into the aggressive behavior of PDAC, showing that RINT1 directly correlates with survival in patients with PDAC by disturbing the SUMOylation process, a crucial modification in carcinogenesis.

Pancreatic ductal adenocarcinomas (PDAC) harbor recurrent functional mutations of the master DNA damage response kinase ATM, which has been shown to accelerate tumorigenesis and epithelial–mesenchymal transition. To study how ATM deficiency affects genome integrity in this setting, we evaluated the molecular and functional effects of conditional Atm deletion in a mouse model of PDAC. ATM deficiency was associated with increased mitotic defects, recurrent genomic rearrangements, and deregulated DNA integrity checkpoints, reminiscent of human PDAC. We hypothesized that altered genome integrity might allow synthetic lethality-based options for targeted therapeutic intervention. Supporting this possibility, we found that the PARP inhibitor olaparib or ATR inhibitors reduced the viability of PDAC cells in vitro and in vivo associated with a genotype-selective increase in apoptosis. Overall, our results offered a preclinical mechanistic rationale for the use of PARP and ATR inhibitors to improve treatment of ATM-mutant PDAC. Cancer Res; 77(20); 5576–90. ©2017 AACR.

Description of the genomic alterations in KC and AKC PDAC cell lines

GSEA Analysis

Tumour growth upon Olaparib and Gemcitabine Treatment in Allografts model

© Author(s) (or their employer(s)) 2020., [Objective]: ATM serine/threonine kinase (ATM) is the most frequently mutated DNA damage response gene, involved in homologous recombination (HR), in pancreatic ductal adenocarcinoma (PDAC)., [Design]: Combinational synergy screening was performed to endeavour a genotype-tailored targeted therapy., [Results]: Synergy was found on inhibition of PARP, ATR and DNA-PKcs (PAD) leading to synthetic lethality in ATM-deficient murine and human PDAC. Mechanistically, PAD-induced PARP trapping, replication fork stalling and mitosis defects leading to P53-mediated apoptosis. Most importantly, chemical inhibition of ATM sensitises human PDAC cells toward PAD with long-term tumour control in vivo. Finally, we anticipated and elucidated PARP inhibitor resistance within the ATM-null background via whole exome sequencing. Arising cells were aneuploid, underwent epithelial-mesenchymal-transition and acquired multidrug resistance (MDR) due to upregulation of drug transporters and a bypass within the DNA repair machinery. These functional observations were mirrored in copy number variations affecting a region on chromosome 5 comprising several of the upregulated MDR genes. Using these findings, we ultimately propose alternative strategies to overcome the resistance., [Conclusion]: Analysis of the molecular susceptibilities triggered by ATM deficiency in PDAC allow elaboration of an efficient mutation-specific combinational therapeutic approach that can be also implemented in a genotype-independent manner by ATM inhibition., Main funding is provided by the German Cancer Aid grant to AK (111879). Additional funding came from the Deutsche Forschungsgemeinschaft (DFG) ’Sachbeihilfe’ (KL 2544/1–1, 1–2, 5–1, 7-1) and ’Heisenberg-Programm’ (KL 2544/6–1), the Baden-Württemberg-Foundation ExPoChip and the INDIMEDVerbund PancChip. AK, FA, MI, SB, LW and TS are either Principal Investigators or students of HEIST RTG funded by the DFG GRK 2254/1. AK is an Else-KrönerFresenius Excellence fellow. LP received funds by the Bausteinprogramm of Ulm University. PCH is supported by a Max Eder Fellowship of the German Cancer Aid (111746), a German Cancer Aid Priority Program ’Translational Oncology’ 70112505 and by a Collaborative Research Centre grant (316249678 – SFB 1279) of the German Research Foundation. EH received funding from the German Cancer Aid (PiPAC, 70112505) and the Volkswagenstiftung/Ministry for Science and Culture in Lower Saxony (ZN3222). This work was also supported by the Deutsche Forschungsgemeinschaft (AZ.96/1–3) to NA, by the Deutsche Krebshilfe (111264) to AL and by the German Cancer Aid Priority Program Translational Oncology (70112504) to LW.

The maintenance of genomic stability during the cell cycle of progenitor cells is essential for the faithful transmission of genetic information. Mutations in genes that ensure genome stability lead to human developmental syndromes. Mutations in Ataxia Telangiectasia and Rad3-related (ATR) or in ATR-interacting protein (ATRIP) lead to Seckel syndrome, which is characterized by developmental malformations and short life expectancy. While the roles of ATR in replicative stress response and chromosomal segregation are well established, it is unknown how ATRIP contributes to maintaining genomic stability in progenitor cells in vivo. Here, we generated the first mouse model to investigate ATRIP function. Conditional inactivation of Atrip in progenitor cells of the CNS and eye led to microcephaly, microphthalmia and postnatal lethality. To understand the mechanisms underlying these malformations, we used lens progenitor cells as a model and found that ATRIP loss promotes replicative stress and TP53-dependent cell death. Trp53 inactivation in Atrip-deficient progenitor cells rescued apoptosis, but increased mitotic DNA damage and mitotic defects. Our findings demonstrate an essential role of ATRIP in preventing DNA damage accumulation during unchallenged replication.

To date, it is still unclear how cancer stem cells (CSCs) regulate their stemness properties, and to what extent they share common features with normal stem cells. Telomerase regulation is a key factor in stem cell maintenance. In this study, we investigate how telomerase regulation affects cancer stem cell biology in pancreatic ductal adenocarcinoma (PDAC), and delineate the mechanisms by which telomerase activity and CSC properties are linked. Using primary patient-derived pancreatic cancer cells, we show that CSCs have higher telomerase activity and longer telomeres than bulk tumor cells. Inhibition of telomerase activity, using genetic TERT-knockdown or pharmacological inhibitor (BIBR1532) resulted in CSC marker depletion in vitro, and reduced tumorigenicity in vivo. Furthermore, we identify a positive feedback loop between stemness factors (KLF4, SOX2, OCT3/4, NANOG) and telomerase, which is essential for the self-renewal of pancreatic CSCs. Disruption the balance between telomerase activity and stemness factors, eliminates CSCs via induction of DNA damage and apoptosis, opening future perspectives to avoid CSC driven therapy resistance and tumor relapse in PDAC patients.

Simple Summary Pancreatic ductal adenocarcinoma (PDAC) causes massive medical problems because of late diagnosis and limited responsiveness to standard chemotherapeutic treatments. This makes PDAC one of the major causes of death by cancer. To address this problem, novel tools for early diagnosis and therapy are needed. The recent development of PDAC organoids, which represent micro-scale mini-tumors, offers promising new options for personalized drug-testing based on primary PDAC patient material. This overview article summarizes and discusses the current state-of-the-art in exploiting the organoid technology to improve clinical management of PDAC. Abstract Pancreatic ductal adenocarcinoma (PDAC) represents 90% of pancreatic malignancies. In contrast to many other tumor entities, the prognosis of PDAC has not significantly improved during the past thirty years. Patients are often diagnosed too late, leading to an overall five-year survival rate below 10%. More dramatically, PDAC cases are on the rise and it is expected to become the second leading cause of death by cancer in western countries by 2030. Currently, the use of gemcitabine/nab-paclitaxel or FOLFIRINOX remains the standard chemotherapy treatment but still with limited efficiency. There is an urgent need for the development of early diagnostic and therapeutic tools. To this point, in the past 5 years, organoid technology has emerged as a revolution in the field of PDAC personalized medicine. Here, we are reviewing and discussing the current technical and scientific knowledge on PDAC organoids, their future perspectives, and how they can represent a game change in the fight against PDAC by improving both diagnosis and treatment options.

Pancreatic ductal adenocarcinoma (PDAC) still presents with a dismal prognosis despite intense research. Better understanding of cellular homeostasis could identify druggable targets to improve therapy. Here we propose RAD50-interacting protein 1 (RINT1) as an essential mediator of cellular homeostasis in PDAC. In a cohort of resected PDAC, low RINT1 protein expression correlated significantly with better survival. Accordingly, RINT1 depletion caused severe growth defects in vitro associated with accumulation of DNA double-strand breaks (DSB), G2 cell cycle arrest, disruption of Golgi–endoplasmic reticulum homeostasis, and cell death. Time-resolved transcriptomics corroborated by quantitative proteome and interactome analyses pointed toward defective SUMOylation after RINT1 loss, impairing nucleocytoplasmic transport and DSB response. Subcutaneous xenografts confirmed tumor response by RINT1 depletion, also resulting in a survival benefit when transferred to an orthotopic model. Primary human PDAC organoids licensed RINT1 relevance for cell viability. Taken together, our data indicate that RINT1 loss affects PDAC cell fate by disturbing SUMOylation pathways. Therefore, a RINT1 interference strategy may represent a new putative therapeutic approach. Significance: These findings provide new insights into the aggressive behavior of PDAC, showing that RINT1 directly correlates with survival in patients with PDAC by disturbing the SUMOylation process, a crucial modification in carcinogenesis.

Supplemental material, UEG905183 Supplemental material for Pancreatic cancer-derived organoids – a disease modeling tool to predict drug response by Pierre-Olivier Frappart, Karolin Walter, Johann Gout, Alica K Beutel, Mareen Morawe, Frank Arnold, Markus Breunig, Thomas FE Barth, Ralf Marienfeld, Lucas Schulte, Thomas Ettrich, Thilo Hackert, Michael Svinarenko, Reinhild Rösler, Sebastian Wiese, Heike Wiese, Lukas Perkhofer, Martin Müller, André Lechel, Bruno Sainz Jr Patrick C Hermann, Thomas Seufferlein and Alexander Kleger in United European Gastroenterology Journal

[Background]: Organotypic cultures derived from pancreatic ductal adenocarcinoma (PDAC) termed pancreatic ductal cancer organoids (PDOs) recapitulate the primary cancer and can be derived from primary or metastatic biopsies. Although isolation and culture of patient-derived pancreatic organoids were established several years ago, pros and cons for individualized medicine have not been comprehensively investigated to date., [Methods]: We conducted a feasibility study, systematically comparing head-to-head patient-derived xenograft tumor (PDX) and PDX-derived organoids by rigorous immunohistochemical and molecular characterization. Subsequently, a drug testing platform was set up and validated in vivo. Patient-derived organoids were investigated as well., [Results]: First, PDOs faithfully recapitulated the morphology and marker protein expression patterns of the PDXs. Second, quantitative proteomes from the PDX as well as from corresponding organoid cultures showed high concordance. Third, genomic alterations, as assessed by array-based comparative genomic hybridization, revealed similar results in both groups. Fourth, we established a small-scale pharmacotyping platform adjusted to operate in parallel considering potential obstacles such as culture conditions, timing, drug dosing, and interpretation of the results. In vitro predictions were successfully validated in an in vivo xenograft trial. Translational proof-of-concept is exemplified in a patient with PDAC receiving palliative chemotherapy., [Conclusion]: Small-scale drug screening in organoids appears to be a feasible, robust and easy-to-handle disease modeling method to allow response predictions in parallel to daily clinical routine. Therefore, our fast and cost-efficient assay is a reasonable approach in a predictive clinical setting., The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: Main funding is provided by the German Cancer Aid grant to A. Kleger (111879). Additional funding came from the Deutsche Forschungsgemeinschaft (DFG, K.L. 2544/1-1, and 1-2; GRK 2254/1 to T. Seufferlein), the BIU fund (Böhringer Ingelheim), the NDIMED-Verbund PancChip, and the Else-Kröner-Fresenius Memorial funding to A. Kleger. AK receives also funding from the DFG within the Heisenberg program and from the Baden-Württemberg Foundation via ExPo Chip. This project was also funded by ANR-DFG collaborative research project (ANR-18-CE92-0031, DFG KL 2544/5-1) to CJ and AK and via additional DFG funding KL 2544/6-1, KL 2544/7-1, KL 2544/1-1, and KL 2544/1-2 to AK. L. Perkhofer is funded by Bausteinprogramm of the Ulm University hospital. This work was supported by a project grant for André Lechel (Deutsche Krebshilfe/111264), for Patrick C. Hermann (Max Eder Fellowship 111746, Projektnummer 316249678 – SFB 1279, and Hector Foundation Cancer Research grant M65.1). Reinhild Rösler and parts of proteomics method development were funded by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) – SFB 1074. Bruno Sainz Jr was funded by a Ramón y Cajal Merit Award from the Ministerio de Economía y Competitividad, Spain and a coordinated grant from the Fundación Asociación Española Contra el Cáncer (AECC).

Seckel syndrome is a type of microcephalic primordial dwarfism (MPD) that is characterized by growth retardation and neurodevelopmental defects, including reports of retinopathy. Mutations in key mediators of the replication stress response, the mutually dependent partners ATR and ATRIP, are among the known causes of Seckel syndrome. However, it remains unclear how their deficiency disrupts the development and function of the central nervous system (CNS). Here, we investigated the cellular and molecular consequences of ATRIP deficiency in different cell populations of the developing murine neural retina. We discovered that conditional inactivation of Atrip in photoreceptor neurons did not affect their survival or function. In contrast, Atrip deficiency in retinal progenitor cells (RPCs) led to severe lamination defects followed by secondary photoreceptor degeneration and loss of vision. Furthermore, we showed that RPCs lacking functional ATRIP exhibited higher levels of replicative stress and accumulated endogenous DNA damage that was accompanied by stabilization of TRP53. Notably, inactivation of Trp53 prevented apoptosis of Atrip-deficient progenitor cells and was sufficient to rescue retinal dysplasia, neurodegeneration and loss of vision. Together, these results reveal an essential role of ATRIP-mediated replication stress response in CNS development and suggest that the TRP53-mediated apoptosis of progenitor cells might contribute to retinal malformations in Seckel syndrome and other MPD disorders. This article has an associated First Person interview with the first author of the paper., Summary: Retinopathies have been reported in primordial dwarfism syndromes. We show that the loss of Atrip, a gene mutated in Seckel syndrome, causes photoreceptor degeneration owing to p53-dependent apoptosis of retinal progenitors during development.

The establishment of organoid culture systems represents a milestone on the route toward successful personalized medicine. This mini review provides an update on the current status of organoid technology and summarizes their applications in personalized medicine. Organoids can be defined as 3D structures derived either from pluripotent or organ restricted stem cells harboring the ability to mimic in vivo architecture and multi lineage differentiation of terminally differentiated tissues. Due to their unique ability of virtually unlimited self-renewal, organoid cultures should be distinguished from previous ‘sphere’-culture assays, for example, ‘tumor spheres’ that have already been described and applied over the last decades.

Pancreatic ductal adenocarcinomas (PDAC) harbor recurrent functional mutations of the master DNA damage response kinase ATM, which has been shown to accelerate tumorigenesis and epithelial–mesenchymal transition. To study how ATM deficiency affects genome integrity in this setting, we evaluated the molecular and functional effects of conditional Atm deletion in a mouse model of PDAC. ATM deficiency was associated with increased mitotic defects, recurrent genomic rearrangements, and deregulated DNA integrity checkpoints, reminiscent of human PDAC. We hypothesized that altered genome integrity might allow synthetic lethality-based options for targeted therapeutic intervention. Supporting this possibility, we found that the PARP inhibitor olaparib or ATR inhibitors reduced the viability of PDAC cells in vitro and in vivo associated with a genotype-selective increase in apoptosis. Overall, our results offered a preclinical mechanistic rationale for the use of PARP and ATR inhibitors to improve treatment of ATM-mutant PDAC. Cancer Res; 77(20); 5576–90. ©2017 AACR.

Pancreatic ductal adenocarcinoma (PDAC) is still the Achilles heel in modern oncology, with an increasing incidence accompanied by a persisting high mortality. The developmental process of PDAC is thought to be stepwise via precursor lesions and sequential accumulation of mutations. Thereby, current sequencing studies recapitulate this genetic heterogeneity in PDAC and show besides a handful of driver mutations (KRAS, TP53) a plethora of passenger mutations that allow to define subtypes. However, modeling the mutations of interest and their effects is still challenging. Interestingly, organoids have the potential to recapitulate in vitro, the in vivo characteristics of the tissue they originate from. Here, we could establish and develop tools allowing us to isolate, culture, and genetically modify ductal mouse organoids. Transferred to known effectors in the IPMN-PDAC sequence, we could reveal significantly increased proliferative and self-renewal capacities for PTEN and RNF43 deficiency in the context of oncogenic KRASG12D in mouse pancreatic organoids. Overall, we were able to obtain promising data centering ductal organoids in the focus of future PDAC research.

// Philipp Seidel 1,2 , Martina Remus 3 , Michael Delacher 4 , Paulius Grigaravicius 3 , David E. Reuss 5 , Lucien Frappart 6 , Andreas von Deimling 3,5 , Markus Feuerer 4 , Amir Abdollahi 1,2 and Pierre-Olivier Frappart 3 1 Molecular and Translational Radiation Oncology, National Center for Tumor Diseases (NCT), Heidelberg University Medical School (HUMS), Heidelberg, Germany 2 German Cancer Consortium (DKTK) and Heidelberg Institute of Radiation Oncology (HIRO), German Cancer Research Center (DKFZ), Heidelberg, Germany 3 Clinical Cooperation Unit Neuropathology, German Cancer Research Center (DKFZ), Heidelberg, Germany 4 Helmholtz Young Investigator Group Immune Tolerance, Tumor Immunology Program, German Cancer Research Center, Heidelberg, Germany 5 Department of Neuropathology, Institute of Pathology, Ruprecht-Karls-Universitat Heidelberg, Heidelberg, Germany 6 Leibniz Institute for Age Research - Fritz Lipmann Institute (FLI), Jena, Germany Correspondence to: Pierre-Olivier Frappart, email: // Keywords : inflammation, Nbn, psoriasiform dermatitis, skin, Gerotarget Received : February 17, 2016 Accepted : March 22, 2016 Published : March 30, 2016 Abstract Nijmegen Breakage Syndrome is a disease caused by NBN mutations. Here, we report a novel function of Nbn in skin homeostasis. We found that Nbn deficiency in hair follicle (HF) progenitors promoted increased DNA damage signaling, stimulating p16 Ink4a up-regulation, Trp53 stabilization and cytokines secretion leading to HF-growth arrest and hair loss. At later stages, the basal keratinocytes layer exhibited also enhanced DNA damage response but in contrast to the one in HF progenitor was not associated with pro-inflammatory cytokines expression, but rather increased proliferation, lack of differentiation and immune response resembling psoriasiform dermatitis. Simultaneous Nbn and Trp53 inactivation significantly exacerbated this phenotype, due to the lack of inhibition of pro-inflammatory cytokines secretion by Trp53. Altogether, we demonstrated novel functions of Nbn in HF maintenance and prevention of skin inflammation and we provide a mechanistic explanation that links cell intrinsic DNA maintenance with large scale morphological tissue alterations.

Embryonic stem cells (ESCs) and induced pluripotent stem cells (iPSCs) are characterized by their unique capacity to stepwise differentiate towards any particular cell type in an adult organism. Pluripotent stem cells provide a beneficial platform to model hereditary diseases and even cancer development. While the incidence of pancreatic diseases such as diabetes and pancreatitis is increasing, the understanding of the underlying pathogenesis of particular diseases remains limited. Only a few recent publications have contributed to the characterization of human pancreatic development in the fetal stage. Hence, most knowledge of pancreatic specification is based on murine embryology. Optimizing and understanding current in vitro protocols for pancreatic differentiation of ESCs and iPSCs constitutes a prerequisite to generate functional pancreatic cells for better disease modeling and drug discovery. Moreover, human pancreatic organoids derived from pluripotent stem cells, organ-restricted stem cells, and tumor samples provide a powerful technology to model carcinogenesis and hereditary diseases independent of genetically engineered mouse models. Herein, we summarize recent advances in directed differentiation of pancreatic organoids comprising endocrine cell types. Beyond that, we illustrate up-and-coming applications for organoid-based platforms.

RINT1 was first identified as an RAD50-interacting protein and its function was therefore linked to the maintenance of genomic stability. It was also shown that RINT1 was a key player in ER-Golgi trafficking as a member of an ER tethering complex interacting with STX18. However, due to early embryonic lethality of rint1-null mice, the in vivo functions of RINT1 remained for the most part elusive. We recently described the consequences of Rint1 inactivation in various neuronal cells of the central nervous system. We observed that lack of RINT1 in vivo triggers genomic instability and ER stress leading to depletion of the neural progenitor pool and neurodegeneration. Surprisingly, we also observed inhibition of autophagy in RINT1-deficient neurons, indicating an involvement of RINT1 in the regulation of neuronal autophagy. Here, we summarize our main RINT1 findings and discuss its putative roles in autophagy.

Malignant peripheral nerve sheath tumors (MPNST) derive from the Schwann cell or perineurial cell lineage and occur either sporadically or in association with the tumor syndrome neurofibromatosis type 1 (NF1). MPNST often pose a diagnostic challenge due to their frequent lack of pathognomonic morphological or immunohistochemical features. Mutations in the NF1 tumor suppressor gene are found in all NF1-associated and many sporadic MPNST. The presence of NF1 mutation may have the potential to differentiate MPNST from several morphologically similar neoplasms; however, mutation detection is hampered by the size of the gene and the lack of mutational hot spots. Here we describe a newly developed monoclonal antibody binding to the C-terminus of neurofibromin (clone NFC) which was selected for optimal performance in routinely processed formalin-fixed and paraffin-embedded tissue. NFC immunohistochemistry revealed loss of neurofibromin in 22/25 (88 %) of NF1-associated and 26/61 (43 %) of sporadic MPNST. There was a strong association of neurofibromin loss with deletions affecting the NF1 gene (P < 0.01). In a series of 256 soft tissue tumors of different histotypes NFC staining showed loss of neurofibromin in 2/8 myxofibrosarcomas, 2/12 (16 %) pleomorphic liposarcomas, 1/16 (6 %) leiomyosarcomas, and 4/28 (14 %) unclassified undifferentiated pleomorphic sarcomas. However, loss of neurofibromin was not observed in 22 synovial sarcomas, 27 schwannomas, 23 solitary fibrous tumors, 14 low-grade fibromyxoid sarcomas, 50 dedifferentiated liposarcomas, 27 myxoid liposarcomas, 13 angiosarcomas, 9 extraskeletal myxoid chondrosarcomas, and 7 epitheloid sarcomas. Immunohistochemistry using antibody NFC may substantially facilitate sarcoma research and diagnostics.

The ATR (ATM (ataxia telangiectasia mutated) and rad3related) checkpoint kinase is considered critical for signalling DNA replication stress and its dysfunction can lead to the neurodevelopmental disorder, ATR-Seckel syndrome. To understand how ATR functions during neurogenesis, we conditionally deleted Atr broadly throughout the murine nervous system, or in a restricted manner in the dorsal telencephalon. Unexpectedly, in both scenarios, Atr loss impacted neurogenesis relatively late during neural development involving only certain progenitor populations. Whereas the Atr-deficient embryonic cerebellar external germinal layer underwent p53- (and p16 Ink4a/Arf )-independent proliferation arrest, other brain regions suffered apoptosis that was partially p53 dependent. In contrast to other organs, in the nervous system, p53 loss did not worsen the outcome of Atr inactivation. Coincident inactivation of Atm also did not affect the phenotype after Atr deletion, supporting non-overlapping physiological roles for these related DNA damage-response kinases in the brain. Rather than an essential general role in preventing replication stress, our data indicate that ATR functions to monitor genomic integrity in a selective spatiotemporal manner during neurogenesis.

Inactivation of homologous recombination (HR) or nonhomologous end-joining (NHEJ) predisposes to a spectrum of tumor types. Here, we inactivated DNA double-strand break repair (DSBR) proteins, DNA Ligase IV (Lig4), Xrcc2, and Brca2, or combined Lig4/Xrcc2 during neural development using Nestin-cre. In all cases, inactivation of these repair factors, together with p53 loss, led to rapid medulloblastoma formation. Genomic analysis of these tumors showed recurring chromosome 13 alterations via chromosomal loss or translocations involving regions containing Ptch1 . Sequence analysis of the remaining Ptch1 allele showed a variety of inactivating mutations in all tumors analyzed, highlighting the critical tumor suppressor function of this hedgehog-signaling regulator. We also observed genomic amplification or up-regulation of either N-Myc or cyclin D2 in all medulloblastomas. Additionally, chromosome 19, which contains Pten, was also selectively deleted in medulloblastoma arising after disruption of HR. Thus, our data highlight the preeminence of Ptch1 as a tumor suppressor in cerebellar granule cells and reveal other genomic events central to the genesis of medulloblastoma.

The repair of DNA damage is essential for the prevention of disease. The DNA double-strand break (DSB) is a particularly hazardous lesion. DNA DSBs activate a coordinated cellular response involving cell cycle checkpoint activation and repair of the DNA break, or alternatively apoptosis. In the nervous system the inability to respond to DNA DSBs may lead to neurodegenerative disease or brain tumors. Therefore, understanding the DNA DSB response mechanism in the nervous system is of high importance for developing new treatments for neurodegeneration and cancer. In this regard, the use of mouse models represents an important approach for advancing our understanding of the biology of the DNA damage response in the nervous system.

Defective responses to DNA double strand breaks (DSBs) in the nervous system can lead to neurodegeneration or tumorigenesis. A key player in the repair of DNA DSBs is the tumor suppressor BRCA2, an essential component of the homologous recombination repair pathway and the Fanconi Anemia complex. We recently demonstrated that BRCA2 was required for normal neurogenesis and prevention of medulloblastoma brain tumors. Here, we discuss how this study contributes both to our understanding of BRCA2 functions in vivo, and the tissue-specific requirements for DNA repair and damage-signaling pathways.

Defective DNA damage responses in the nervous system can result in neurodegeneration or tumorigenesis. Despite the importance of DNA damage signalling, the neural function of many critical DNA repair factors is unclear. BRCA2 is necessary for homologous recombination repair of DNA and the prevention of diseases including Fanconi Anemia and cancer. We determined the role of BRCA2 during brain development by inactivating murine Brca2 throughout neural tissues. In striking contrast to early embryonic lethality after germ-line inactivation, Brca2(LoxP/LoxP);Nestin-cre mice were viable. However, Brca2 loss profoundly affected neurogenesis, particularly during embryonic and postnatal neural development. These neurological defects arose from DNA damage as Brca2(LoxP/LoxP);Nestin-cre mice showed extensive gammaH2AX in neural tissue and p53 deficiency restored brain histology but lead to rapid formation of medulloblastoma brain tumors. In contrast, loss of the Atm kinase did not markedly attenuate apoptosis after Brca2 loss, but did partially restore cerebellar morphology, supporting a genomic surveillance function for ATM during neurogenesis. These data illustrate the importance of Brca2 during nervous system development and underscore the tissue-specific requirements for DNA repair factors.

The Nbs1 protein, hypomorphic mutant in Nijmegen breakage syndrome (NBS), is a component of the Mre11/Rad50/Nbs1 (M/R/N) complex that acts as a DNA double-strand break sensor and functions in cell cycle checkpoint in response to DNA damage and DNA repair. Here we report that targeted disruption of murine NBS1 gene (Nbn) in the lens alters the M/R/N complex nuclear localization and results in microphthalmia in mice due to reduced proliferation of the lens epithelial cells. Unexpectedly, all Nbn-deficient lenses develop cataracts at an early age due to altered lens fibre cell differentiation, including disruption of normal lens epithelial and fibre cell architecture and incomplete denucleation of fibre cells, and these changes are independent of the p53 pathway. In addition, Nbn-deficient lenses show dysregulated transcription of various crystallins. Thus, this study implicates a novel function of Nbs1 in terminal differentiation of the lens fibre cells and in cataractogenesis.

Appropriate cellular signaling responses to DNA damage and the ability to repair DNA are fundamental processes that are required for organismal survival. Ataxia-telangiectasia (A-T) is a rare neurodegenerative disease that results from defective DNA damage signaling. Understanding the molecular basis of A-T has provided many critical insights into the cellular response to DNA double-strand breaks (DSBs). A-T is a syndrome that shows pronounced neurodegeneration of the nervous system coincident with immune deficiency, radiosensitivity, and cancer proneness. A-T results from inactivation of the A-T mutated (ATM) kinase, a critical protein kinase that regulates the response to DNA-DSBs by selective phosphorylation of a variety of substrates. Therefore, understanding the ATM signaling program has important biological ramifications for nervous system homeostasis. Underscoring the importance of the DNA-DSBs response in the nervous system are other diseases related to A-T that also result from defects in this signaling pathway. In particular, defects in the DNA damage sensor, the Mre11-RAD50-NBS1 complex, also lead to syndromes with neurological deficits and overlapping phenotypes to A-T. Collectively, these diseases highlight the critical importance of appropriate responses to DNA-DSBs to maintain homeostasis in the nervous system.

Nijmegen breakage syndrome (NBS) is a rare autosomal recessive disorder characterized by predisposition to hematopoietic malignancy, cell-cycle checkpoint defects, and ionizing radiation sensitivity. NBS is caused by a hypomorphic mutation of the NBS1 gene, encoding nibrin, which forms a protein complex with Mre11 and Rad50, both involved in DNA repair. Nibrin localizes to chromosomal sites of class switching, and B cells from NBS patients show an enhanced presence of microhomologies at the sites of switch recombination. Because nibrin is crucial for embryonic survival, direct demonstration by targeted deletion that nibrin functions in class switch recombination has been lacking. Here, we show by cell-type-specific conditional inactivation of Nbn , the murine homologue of NBS1 , that nibrin plays a role in the repair of γ-irradiation damage, maintenance of chromosomal stability, and the recombination of Ig constant region genes in B lymphocytes.

The human genetic disorder, Nijmegen breakage syndrome, is characterized by radiosensitivity, immunodeficiency, chromosomal instability and an increased risk for cancer of the lymphatic system. The NBS1 gene codes for a protein, nibrin, involved in the processing/repair of DNA double strand breaks and in cell cycle checkpoints. Most patients are homozygous for a founder mutation, a 5 bp deletion, which might not be a null mutation, as functionally relevant truncated nibrin proteins are observed, at least in vitro. In agreement with this hypothesis, null mutation of the homologous gene, Nbn, is lethal in mice. Here, we have used Cre recombinase/loxP technology to generate an inducible Nbn null mutation allowing the examination of DNA-repair and cell cycle-checkpoints in the complete absence of nibrin. Induction of Nbn null mutation leads to the loss of the G2/M checkpoint, increased chromosome damage, radiomimetic-sensitivity and cell death. In vivo, this particularly affects the lymphatic tissues, bone marrow, thymus and spleen, whereas liver, kidney and muscle are hardly affected. In vitro, null mutant murine fibroblasts can be rescued from cell death by transfer of human nibrin cDNA and, more significantly, by a cDNA carrying the 5 bp deletion. This demonstrates, for the first time, that the common human mutation is hypomorphic and that the expression of a truncated protein is sufficient to restore nibrin's vital cellular functions.