Recommendations for Long-Term Follow-up of Adults with Heritable Retinoblastoma.

PURPOSE: To generate recommendations for long-term follow-up of adult survivors of heritable retinoblastoma. DESIGN: We convened a meeting of providers from retinoblastoma centers around the world to review the state of the science and to evaluate the published evidence. PARTICIPANTS: Retinoblastoma is a rare childhood cancer of the retina. Approximately 40% of retinoblastoma cases are heritable, resulting from a germline mutation in RB1. Dramatic improvements in treatment and supportive care have resulted in a growing adult survivor population. However, survivors of heritable retinoblastoma have a significantly increased risk of subsequent malignant neoplasms, particularly bone and soft tissue sarcomas, uterine leiomyosarcoma, melanomas, and radiotherapy-related central nervous system tumors, which are associated with excess morbidity and mortality. Despite these risks, no surveillance recommendations for this population currently are in place, and surveillance practices vary widely by center. METHODS: Following the Institute of Medicine procedure for clinical practice guideline development, a PubMed, EMBASE, and Web of Science search was performed, resulting in 139 articles; after abstract and full-text review, 37 articles underwent detailed data abstraction to quantify risk and evidence regarding surveillance, if available. During an in-person meeting, evidence was presented and discussed, resulting in consensus recommendations. MAIN OUTCOME MEASURES: Diagnosis and mortality from subsequent neoplasm. RESULTS: Although evidence for risk of subsequent neoplasm, especially sarcoma and melanoma, was significant, evidence supporting routine testing of asymptomatic survivors was not identified. Skin examination for melanoma and prompt evaluation of signs and symptoms of head and neck disease were determined to be prudent. CONCLUSIONS: This review of the literature confirmed some of the common second cancers in retinoblastoma survivors but found little evidence for a benefit from currently available surveillance for these malignancies. Future research should incorporate international partners, patients, and family members.

IDH1 mutation is sufficient to establish the glioma hypermethylator phenotype.

Both genome-wide genetic and epigenetic alterations are fundamentally important for the development of cancers, but the interdependence of these aberrations is poorly understood. Glioblastomas and other cancers with the CpG island methylator phenotype (CIMP) constitute a subset of tumours with extensive epigenomic aberrations and a distinct biology. Glioma CIMP (G-CIMP) is a powerful determinant of tumour pathogenicity, but the molecular basis of G-CIMP remains unresolved. Here we show that mutation of a single gene, isocitrate dehydrogenase 1 (IDH1), establishes G-CIMP by remodelling the methylome. This remodelling results in reorganization of the methylome and transcriptome. Examination of the epigenome of a large set of intermediate-grade gliomas demonstrates a distinct G-CIMP phenotype that is highly dependent on the presence of IDH mutation. Introduction of mutant IDH1 into primary human astrocytes alters specific histone marks, induces extensive DNA hypermethylation, and reshapes the methylome in a fashion that mirrors the changes observed in G-CIMP-positive lower-grade gliomas. Furthermore, the epigenomic alterations resulting from mutant IDH1 activate key gene expression programs, characterize G-CIMP-positive proneural glioblastomas but not other glioblastomas, and are predictive of improved survival. Our findings demonstrate that IDH mutation is the molecular basis of CIMP in gliomas, provide a framework for understanding oncogenesis in these gliomas, and highlight the interplay between genomic and epigenomic changes in human cancers.

Loss of the tyrosine phosphatase PTPRD leads to aberrant STAT3 activation and promotes gliomagenesis.

PTPRD, which encodes the protein tyrosine phosphatase receptor-δ, is one of the most frequently inactivated genes across human cancers, including glioblastoma multiforme (GBM). PTPRD undergoes both deletion and mutation in cancers, with copy number loss comprising the primary mode of inactivation in GBM. However, it is unknown whether loss of PTPRD promotes tumorigenesis in vivo, and the mechanistic basis of PTPRD function in tumors is unclear. Here, using genomic analysis and a glioma mouse model, we demonstrate that loss of Ptprd accelerates tumor formation and define the oncogenic context in which Ptprd loss acts. Specifically, we show that in human GBMs, heterozygous loss of PTPRD is the predominant type of lesion and that loss of PTPRD and the CDKN2A/p16(INK4A) tumor suppressor frequently co-occur. Accordingly, heterozygous loss of Ptprd cooperates with p16 deletion to drive gliomagenesis in mice. Moreover, loss of the Ptprd phosphatase resulted in phospho-Stat3 accumulation and constitutive activation of Stat3-driven genetic programs. Surprisingly, the consequences of Ptprd loss are maximal in the heterozygous state, demonstrating a tight dependence on gene dosage. Ptprd loss did not increase cell proliferation but rather altered pathways governing the macrophage response. In total, we reveal that PTPRD is a bona fide tumor suppressor, pinpoint PTPRD loss as a cause of aberrant STAT3 activation in gliomas, and establish PTPRD loss, in the setting of CDKN2A/p16(INK4A) deletion, as a driver of glioma progression.

Protein tyrosine phosphatase receptor delta acts as a neuroblastoma tumor suppressor by destabilizing the aurora kinase A oncogene.

BACKGROUND: Protein tyrosine phosphatase receptor delta (PTPRD) is a member of a large family of protein tyrosine phosphatases which negatively regulate tyrosine phosphorylation. Neuroblastoma is a major childhood cancer arising from precursor cells of the sympathetic nervous system which is known to acquire deletions and alterations in the expression patterns of PTPRD, indicating a potential tumor suppressor function for this gene. The molecular mechanism, however, by which PTPRD renders a tumor suppressor effect in neuroblastoma is unknown. RESULTS: As a molecular mechanism, we demonstrate that PTPRD interacts with aurora kinase A (AURKA), an oncogenic protein that is over-expressed in multiple forms of cancer, including neuroblastoma. Ectopic up-regulation of PTPRD in neuroblastoma dephosphorylates tyrosine residues in AURKA resulting in a destabilization of this protein culminating in interfering with one of AURKA's primary functions in neuroblastoma, the stabilization of MYCN protein, the gene of which is amplified in approximately 25 to 30% of high risk neuroblastoma. CONCLUSIONS: PTPRD has a tumor suppressor function in neuroblastoma through AURKA dephosphorylation and destabilization and a downstream destabilization of MYCN protein, representing a novel mechanism for the function of PTPRD in neuroblastoma.

High-Level MYCN-Amplified RB1-Proficient Retinoblastoma Tumors Retain Distinct Molecular Signatures.

Purpose: Retinoblastomas are malignant eye tumors diagnosed in young children. Most retinoblastomas are genetically characterized by biallelic inactivation of the RB1 gene. However, 1.5% of tumors demonstrate high-level amplification of the proto-oncogene MYCN. Patients with MYCN-amplified RB1-proficient retinoblastoma receive a diagnosis at an earlier age and show a clinically and histologically more malignant phenotype. This study aimed to identify genome-wide molecular features that distinguish this subtype from other retinoblastomas. Design: Cohort study. Participants: Forty-seven retinoblastoma tumors, comprising 36 RB1 -/-, 4 RB1 +/-, and 7 RB1 +/+ tumors. In total, 5 retinoblastomas displayed high-level MYCN amplification, with 3 being RB1 +/+, 1 being RB1 +/-, and 1 being RB1 -/- . Methods: Integrated analysis, based on gene expression, methylation, and methylation-expression correlations, was performed to identify distinct molecular components of MYCN-amplified RB1-proficient retinoblastomas compared with other retinoblastoma subtypes. The methylation and methylation-expression correlation analysis was initially conducted within a subset of samples (n = 15) for which methylation profiles were available. The significant findings were cross-validated in the entire cohort (n = 47) and in publicly available data. Main Outcome Measures: Differentially expressed genes/pathways, differentially methylated genes, and methylation-driven differential gene expression. Results: A large number of genes (n = 3155) were identified with distinct expression patterns in MYCN-amplified RB1-proficient retinoblastomas. The upregulated and downregulated genes were associated with translation and cell-cycle processes, respectively. Methylation analysis revealed distinct methylated patterns in MYCN-amplified RB1-proficient tumors, many of which showing significant impact on gene expression. Data integration identified a 40-gene expression signature with hypermethylated state resulting in a significant downregulation in MYCN-amplified RB1-proficient retinoblastomas. Cross-validation using the entire cohort and the public domain expression data verified the overall lower expression of these genes not only in retinoblastomas with a MYCN-amplified RB1-proficient background, but also in MYCN-amplified neuroblastomas. These include the metabolism-associated TSTD1 gene and the cyclin-dependent kinase inhibitor gene CDKN2C. Conclusions: MYCN-amplified RB1-proficient retinoblastomas display significantly distinct molecular features compared with other retinoblastomas, including a set of 40 hypermethylation-driven downregulated genes. This gene set can give insight into the biology of MYCN-amplified retinoblastomas and may help us to understand the more aggressive clinical behavior.

Subsequent Malignant Neoplasms in Retinoblastoma Survivors.

Retinoblastoma (Rb) is a pediatric malignant eye tumor. Subsequent malignant neoplasms (SMNs) and trilateral Rb (TRb) are the leading cause of death in heritable Rb patients in developed countries. The high rate of SMNs in heritable Rb patients is attributed to the presence of a mutation in the RB1 tumor suppressor gene. In addition, Rb therapy choices also influence SMN incidence in this patient group. The incidence rates and age of occurrence for the most frequent SMNs and TRb will be discussed. In addition, the impact of genetic predisposition and Rb treatments on the development of SMNs will be evaluated. Furthermore, screening and other prevention methods will be reviewed.

Optical coherence tomography (OCT) to image active and inactive retinoblastomas as well as retinomas.

PURPOSE: To illustrate Optical Coherence Tomography (OCT) images of active and inactive retinoblastoma (Rb) tumours. METHODS: Current observational study included patients diagnosed with retinoblastoma and retinoma who were presented at Amsterdam UMC and Jules-Gonin Eye Hospital, between November 2010 and October 2017. Patients aged between 0 and 4 years were imaged under general anaesthesia with handheld OCT in supine position. Patients older than 4 years were imaged with the conventional OCT (Heidelberg Engineering, Heidelberg Spectralis, Germany). All patients included were divided into two groups: active and inactive tumours (retinoma and regression patterns). Patients' medical records and OCT images were analysed during meetings via discussions by ophthalmologists and physicists. RESULTS: Twelve Dutch and 8 Swiss patients were divided into two groups: 2 patients with active tumour versus 18 patients with inactive tumour. Subsequently, inactive group could be divided in two groups, which consisted of 10 patients with retinoma and 8 patients with different regression pattern types. Of all included patients, 15 were male (75%). Median age at diagnosis was 18.0 months (range 0.19-715.2 months). A total of 12 retinoblastoma (active and inactive) and 8 retinoma foci were investigated by OCT. No distinction could be made between active and inactive tumours using only OCT. CONCLUSION: Optical coherence tomography alone cannot distinguish between active and inactive Rbs. However, handheld OCT adds useful information to the established imaging techniques in the monitoring and follow-up of retinoblastoma patients. With this study, we provide an overview of OCT images of active and inactive Rbs.

Mutant-IDH1-dependent chromatin state reprogramming, reversibility, and persistence.

Mutations in IDH1 and IDH2 (encoding isocitrate dehydrogenase 1 and 2) drive the development of gliomas and other human malignancies. Mutant IDH1 induces epigenetic changes that promote tumorigenesis, but the scale and reversibility of these changes are unknown. Here, using human astrocyte and glioma tumorsphere systems, we generate a large-scale atlas of mutant-IDH1-induced epigenomic reprogramming. We characterize the reversibility of the alterations in DNA methylation, the histone landscape, and transcriptional reprogramming that occur following IDH1 mutation. We discover genome-wide coordinate changes in the localization and intensity of multiple histone marks and chromatin states. Mutant IDH1 establishes a CD24+ population with a proliferative advantage and stem-like transcriptional features. Strikingly, prolonged exposure to mutant IDH1 results in irreversible genomic and epigenetic alterations. Together, these observations provide unprecedented high-resolution molecular portraits of mutant-IDH1-dependent epigenomic reprogramming. These findings have substantial implications for understanding of mutant IDH function and for optimizing therapeutic approaches to targeting IDH-mutant tumors.

Feasibility of RetinoQuest: e-health application to facilitate and improve additional care for retinoblastoma survivors.

PURPOSE: The current study aimed to evaluate the feasibility of RetinoQuest in clinical practice, from survivors and healthcare professionals' (HCPs) point of view. METHODS: RetinoQuest is a touch screen computer program to monitor health-related quality of life (HRQoL) of retinoblastoma survivors via patient-reported outcome measures (PROMs) targeting children (4-10 years) as evaluated by their parents (proxy measures), adolescents (11-18 years), and adults. Feasibility was evaluated by the actual time taken to complete the PROMs, acceptability of the time as perceived by the users, the content of PROMs in RetinoQuest, and overall satisfaction with RetinoQuest. RESULTS: Ninety-six survivors participated: 41 parents of children, 38 adolescents, and 17 adults. Mean time to complete the evaluation form was 7.8 min (median 6.7, range 2.4-24.5), and 90% of the users stated that the time needed to complete PROMs in RetinoQuest was acceptable. The majority of users reported that it was important to answer the questions (88% of the parents, 66% of the adolescents, and 76% of the adult survivors) and that all important issues were covered, e.g., no missing questions (78, 84, and 76%, respectively). Satisfaction rate was high, 7.8 according to parents, 8.1 according to adolescents, and 7.7 for adults. CONCLUSIONS: RetinoQuest is a feasible e-health application to monitor HRQoL in retinoblastoma survivors in clinical practice. IMPLICATIONS FOR CANCER SURVIVORS: This tool allows for open and structured communication which can lead to early detection of psychosocial impacts on quality of life and referral of the retinoblastoma survivors.

Ophthalmic artery chemosurgery for eyes with advanced retinoblastoma.

BACKGROUND: Surgical removal of one or both eyes has been the most common way to treat children with retinoblastoma worldwide for more than 100 years. Ophthalmic artery chemosurgery (OAC) was introduced 10 years ago and it has been used as an alternative to enucleation for eyes with advanced retinoblastoma. The purpose of this report is to analyze our 9-year experience treating advanced retinoblastoma eyes with OAC. MATERIALS AND METHODS: Single-arm retrospective study from a single center of 226 eyes with eyes of retinoblastoma patients with advanced intraocular disease defined as both Reese-Ellsworth (RE) "Va" or "Vb" and International Classification Retinoblastoma (ICRb) group "D" or "E" (COG Classification). Ocular survival, patient survival, second cancers, and electroretinography (ERG) were assessed. RESULTS: Ocular survival at five years for these advanced eyes was 70.2% (95% confidence interval, 57.3%-79.8%). When eyes were divided into groups either by RE classification or ICRb, no significant differences in ocular survival were seen. Ocular survival was significantly better in naïve compared to non-naïve eyes (80.2% vs 58.4%, p = 0.041). The ERG distribution was very similar before and after OAC treatment for the patient population that did not receive intravitreal chemotherapy. Three patients (1.5%) have developed metastatic retinoblastoma (previously reported) and were successfully treated (no deaths). CONCLUSION: Using OAC for advanced eyes (the majority of such eyes have been enucleated in the past) enables 70% 5-year ocular survival. Treated eyes have a similar ERG distribution before and after treatment. No patient has died of metastatic retinoblastoma.

Simultaneous Bilateral Ophthalmic Artery Chemosurgery for Bilateral Retinoblastoma (Tandem Therapy).

OBJECTIVE: Report on the 7-year experience with bilateral ophthalmic artery chemosurgery (OAC-Tandem therapy) for bilateral retinoblastoma. DESIGN: Retrospective, single institution study. SUBJECTS: 120 eyes of 60 children with bilateral retinoblastoma treated since March 2008. METHODS: Retrospective review of all children treated at Memorial Sloan Kettering with bilateral ophthalmic artery chemosurgery (Melphalan, Carboplatin, Topotecan, Methotrexate) delivered in the same initial session to both naïve and previously treated eyes. MAIN OUTCOME MEASURES: Ocular survival, metastatic disease, patient survival from metastases, second cancers, systemic adverse effects, need for transfusion of blood products, electroretinogram before and after treatment. RESULTS: 116 eyes were salvaged (4 eyes were enucleated: 3 because of progressive disease, 1 family choice). Kaplan Meier ocular survival was 99.2% at one year, 96.9% at 2 and 3 years and 94.9% for years 4 through 7. There were no cases of metastatic disease or metastatic deaths with a mean follow-up of 3.01 years. Two children developed second cancers (both pineoblastoma) and one of them died. Transfusion of blood products was required in 3 cases (4 transfusions), 1.9%. Two children developed fever/neutropenia requiring hospitalization (0.95%). ERGs were improved in 21.6% and unchanged after treatment in 52.5% of cases (increase or decrease of less than 25µV). CONCLUSIONS: Bilateral ophthalmic artery chemosurgery is a safe and effective technique for managing bilateral retinoblastoma-even when eyes are advanced bilaterally, and if both eyes have progressed after systemic chemotherapy. Ocular survival was excellent (94.9% at 8 years), there were no cases of of metastatic disease and no deaths from metastatic disease, but children remain at risk for second cancers. In 21.6% of cases ERG function improved. Despite using chemotherapy in both eyes in the same session, systemic toxicity was low.

Advanced Unilateral Retinoblastoma: The Impact of Ophthalmic Artery Chemosurgery on Enucleation Rate and Patient Survival at MSKCC.

PURPOSE: To report on the influence of ophthalmic artery chemosurgery (OAC) on enucleation rates, ocular and patient survival from metastasis and impact on practice patterns at Memorial Sloan Kettering for children with advanced intraocular unilateral retinoblastoma. PATIENTS AND METHODS: Single-center retrospective review of all unilateral retinoblastoma patients with advanced intraocular retinoblastoma treated at MSKCC between our introduction of OAC (May 2006) and December 2014. End points were ocular survival, patient survival from metastases and enucleation rates. RESULTS: 156 eyes of 156 retinoblastoma patients were included. Primary enucleation rates have progressively decreased from a rate of >95% before OAC to 66.7% in the first year of OAC use to the present rate of 7.4%. The percent of patients receiving OAC has progressively increased from 33.3% in 2006 to 92.6% in 2014. Overall, ocular survival was significantly better in eyes treated with OAC in the years 2010-2014 compared to 2006-2009 (p = 0.023, 92.7% vs 68.0% ocular survival at 48 months). There have been no metastatic deaths in the OAC group but two patients treated with primary enucleation have died of metastatic disease. CONCLUSION: OAC was introduced in 2006 and its impact on patient management is profound. Enucleation rates have decreased from over 95% to less than 10%. Our ocular survival rate has also significantly and progressively improved since May 2006. Despite treating more advanced eyes rather then enucleating them patient survival has not been compromised (there have been no metastatic deaths in the OAC group). In our institution, enucleation is no longer the most common treatment for advanced unilateral retinoblastoma.

Efficient induction of differentiation and growth inhibition in IDH1 mutant glioma cells by the DNMT Inhibitor Decitabine.

Mutation in the IDH1 or IDH2 genes occurs frequently in gliomas and other human malignancies. In intermediate grade gliomas, IDH1 mutation is found in over 70% of tumors. These mutations impart the mutant IDH enzyme with a neomorphic activity - the ability to synthesize 2-hydroxyglutarate (2-HG). This ability leads to a reprogramming of chromatin state, a block in differentiation, and the establishment of the glioma hypermethylator phenotype (G-CIMP). It has been hypothesized but not proven that the extensive DNA methylation that occurs in G-CIMP tumors helps maintain and "lock in" glioma cancer cells in a dedifferentiated state. Here, we tested this hypothesis by treating patient derived IDH1 mutant glioma initiating cells (GIC) with non-cytotoxic, epigenetically targeted doses of the DNMT inhibitor decitabine. Global methylome analysis of treated IDH1 mutant GICs showed that DAC treatment resulted in reversal of DNA methylation marks induced by IDH and the re-expression of genes associated with differentiation. Accordingly, treatment of IDH1 mutant glioma cells resulted in a dramatic loss of stem-like properties and efficient adoption of markers of differentiation, effects not seen in decitabine treated IDH wild-type GICs. Induction of differentiation was much more efficient than that seen following treatment with a specific inhibitor of mutant IDH enzyme (Agios). Decitabine also decreased replicative potential and tumor growth in vivo. Reexpression of polycomb regulated genes accompanied these DAC-induced phenotypes. In total, our data indicates that targeting the pathologic DNA methylation in IDH mutant cells can reverse mutant IDH induced hypermethylation and block in differentiation and promote tumor control. These findings have substantial impact for exploring new treatment strategies for patients with IDH mutant gliomas.

5-azacytidine reduces methylation, promotes differentiation and induces tumor regression in a patient-derived IDH1 mutant glioma xenograft.

Somatic mutations in Isocitrate Dehydrogenase 1 (IDH1) are frequent in low grade and progressive gliomas and are characterized by the production of 2-hydroxyglutarate (2-HG) from α-ketoglutarate by the mutant enzyme. 2-HG is an "oncometabolite" that competitively inhibits α-KG dependent dioxygenases resulting in various widespread cellular changes including abnormal hypermethylation of genomic DNA and suppression of cellular differentiation. Despite the growing understanding of IDH mutant gliomas, the development of effective therapies has proved challenging in part due to the scarcity of endogenous mutant in vivo models. Here we report the generation of an endogenous IDH1 anaplastic astrocytoma model which rapidly grows in vivo, produces 2-HG and exhibits DNA hypermethylation. Using this model, we have demonstrated the preclinical efficacy and mechanism of action of the FDA approved demethylating drug 5-azacytidine in vivo. Long term administration of 5-azacytidine resulted in reduction of DNA methylation of promoter loci, induction of glial differentiation, reduction of cell proliferation and a significant reduction in tumor growth. Tumor regression was observed at 14 weeks and subsequently showed no signs of re-growth at 7 weeks despite discontinuation of therapy. These results have implications for clinical trials of demethylating agents for patients with IDH mutated gliomas.

Interleukin-1R-associated kinase 2 is a novel modulator of the transforming growth factor beta signaling cascade.

The transforming growth factor beta (TGFbeta) pathway orchestrates an extensive transcriptional program that is important for many processes in the cell. For example, TGFbeta regulates cell cycle, migration, and epithelial-to-mesenchymal transition. The TGFbeta pathway has a dual role in cancer: it is involved in early-stage tumor suppression but also contributes to tumor progression by promoting invasion. To identify the novel genes involved in TGFbeta pathway signaling, we have performed a functional genetic loss-of-function screen. We screened a small interfering RNA library targeting 700 kinases and kinase-related genes in a TGFbeta-responsive reporter assay. Several genes were identified that upon knockdown could repress the reporter signal; among these are the two cellular receptors for TGFbeta. In addition to these two known components of the TGFbeta pathway, several genes were identified that were previously not linked to the TGFbeta signaling. Knockdown of one of these genes, the IRAK2 kinase, resulted not only in an impaired TGFbeta target gene response but also in a reduction of the nuclear accumulation and phosphorylation of SMAD2. In addition, suppression of interleukin-1R-associated kinase 2 expression led to a partial override of a TGFbeta-induced cell cycle arrest. Our data show that interleukin-1R-associated kinase 2 is a novel and critical component of TGFbeta signaling.

A large scale shRNA barcode screen identifies the circadian clock component ARNTL as putative regulator of the p53 tumor suppressor pathway.

BACKGROUND: The p53 tumor suppressor gene is mutated in about half of human cancers, but the p53 pathway is thought to be functionally inactivated in the vast majority of cancer. Understanding how tumor cells can become insensitive to p53 activation is therefore of major importance. Using an RNAi-based genetic screen, we have identified three novel genes that regulate p53 function. RESULTS: We have screened the NKI shRNA library targeting 8,000 human genes to identify modulators of p53 function. Using the shRNA barcode technique we were able to quickly identify active shRNA vectors from a complex mixture. Validation of the screening results indicates that the shRNA barcode technique can reliable identify active shRNA vectors from a complex pool. Using this approach we have identified three genes, ARNTL, RBCK1 and TNIP1, previously unknown to regulate p53 function. Importantly, ARNTL (BMAL1) is an established component of the circadian regulatory network. The latter finding adds to recent observations that link circadian rhythm to the cell cycle and cancer. We show that cells having suppressed ARNTL are unable to arrest upon p53 activation associated with an inability to activate the p53 target gene p21(CIP1). CONCLUSIONS: We identified three new regulators of the p53 pathway through a functional genetic screen. The identification of the circadian core component ARNTL strengthens the link between circadian rhythm and cancer.

An shRNA barcode screen provides insight into cancer cell vulnerability to MDM2 inhibitors.

The identification of the cellular targets of small molecules with anticancer activity is crucial to their further development as drug candidates. Here, we present the application of a large-scale RNA interference-based short hairpin RNA (shRNA) barcode screen to gain insight in the mechanism of action of nutlin-3 (1). Nutlin-3 is a small-molecule inhibitor of MDM2, which can activate the p53 pathway. Nutlin-3 shows strong antitumor effects in mice, with surprisingly few side effects on normal tissues. Aside from p53, we here identify 53BP1 as a critical mediator of nutlin-3-induced cytotoxicity. 53BP1 is part of a signaling network induced by DNA damage that is frequently activated in cancer but not in healthy tissues. Our results suggest that nutlin-3's tumor specificity may result from its ability to turn a cancer cell-specific property (activated DNA damage signaling) into a weakness that can be exploited therapeutically.