The CST Complex Mediates End Protection at Double-Strand Breaks and Promotes PARP Inhibitor Sensitivity in BRCA1-Deficient Cells

Selective elimination of BRCA1-deficient cells by inhibitors of poly(ADP-ribose) polymerase (PARP) is a prime example of the concept of synthetic lethality in cancer therapy. This interaction is counteracted by the restoration of BRCA1-independent homologous recombination through loss of factors such as 53BP1, RIF1, and REV7/MAD2L2, which inhibit end resection of DNA double-strand breaks (DSBs). To identify additional factors involved in this process, we performed CRISPR/SpCas9-based loss-of-function screens and selected for factors that confer PARP inhibitor (PARPi) resistance in BRCA1-deficient cells. Loss of members of the CTC1-STN1-TEN1 (CST) complex were found to cause PARPi resistance in BRCA1-deficient cells in vitro and in vivo. We show that CTC1 depletion results in the restoration of end resection and that the CST complex may act downstream of 53BP1/RIF1. These data suggest that, in addition to its role in protecting telomeres, the CST complex also contributes to protecting DSBs from end resection.

Exploiting DNA repair defects for novel cancer therapies

Most human tumors accumulate a multitude of genetic changes due to defects in the DNA damage response. Recently, small-molecule inhibitors have been developed that target cells with specific DNA repair defects, providing hope for precision treatment of such tumors. Here we discuss the rationale behind these therapies and how an important bottleneck—patient selection—can be approached.

Heat-induced BRCA2 degradation in human tumours provides rationale for hyperthermia-PARP-inhibitor combination therapies

PURPOSE

Hyperthermia (40-44 °C) effectively sensitises tumours to radiotherapy by locally altering tumour biology. One of the effects of heat at the cellular level is inhibition of DNA repair by homologous recombination via degradation of the BRCA2-protein. This suggests that hyperthermia can expand the group of patients that benefit from PARP-inhibitors, a drug exploiting homologous recombination deficiency. Here, we explore whether the molecular mechanisms that cause heat-mediated degradation of BRCA2 are conserved in cell lines from various origins and, most importantly, whether, BRCA2 protein levels can be attenuated by heat in freshly biopted human tumours.

EXPERIMENTAL DESIGN

Cells from four established cell lines and from freshly biopsied material of cervical (15), head- and neck (9) or bladder tumours (27) were heated to 42 °C for 60 min ex vivo. In vivo hyperthermia was studied by taking two biopsies of the same breast or cervical tumour: one before and one after treatment. BRCA2 protein levels were measured by immunoblotting.

RESULTS

We found decreased BRCA2-levels after hyperthermia in all established cell lines and in 91% of all tumours treated ex vivo. For tumours treated with hyperthermia in vivo, technical issues and intra-tumour heterogeneity prevented obtaining interpretable results.

CONCLUSIONS

This study demonstrates that heat-mediated degradation of BRCA2 occurs in tumour material directly derived from patients. Although BRCA2-degradation may not be a practical biomarker for heat deposition in situ, it does suggest that application of hyperthermia could be an effective method to expand the patient group that could benefit from PARP-inhibitors.

BRCA1185delAG tumors may acquire therapy resistance through expression of RING-less BRCA1

Heterozygous germline mutations in breast cancer 1 (BRCA1) strongly predispose women to breast cancer. BRCA1 plays an important role in DNA double-strand break (DSB) repair via homologous recombination (HR), which is important for tumor suppression. Although BRCA1-deficient cells are highly sensitive to treatment with DSB-inducing agents through their HR deficiency (HRD), BRCA1-associated tumors display heterogeneous responses to platinum drugs and poly(ADP-ribose) polymerase (PARP) inhibitors in clinical trials. It is unclear whether all pathogenic BRCA1 mutations have similar effects on the response to therapy. Here, we have investigated mammary tumorigenesis and therapy sensitivity in mice carrying the Brca1185stop and Brca15382stop alleles, which respectively mimic the 2 most common BRCA1 founder mutations, BRCA1185delAG and BRCA15382insC. Both the Brca1185stop and Brca15382stop mutations predisposed animals to mammary tumors, but Brca1185stop tumors responded markedly worse to HRD-targeted therapy than did Brca15382stop tumors. Mice expressing Brca1185stop mutations also developed therapy resistance more rapidly than did mice expressing Brca15382stop. We determined that both murine Brca1185stop tumors and human BRCA1185delAG breast cancer cells expressed a really interesting new gene domain-less (RING-less) BRCA1 protein that mediated resistance to HRD-targeted therapies. Together, these results suggest that expression of RING-less BRCA1 may serve as a marker to predict poor response to DSB-inducing therapy in human cancer patients.

Potentiation of Peptide Receptor Radionuclide Therapy by the PARP Inhibitor Olaparib

Metastases expressing tumor-specific receptors can be targeted and treated by binding of radiolabeled peptides (peptide receptor radionuclide therapy or PRRT). For example, patients with metastasized somatostatin receptor-positive neuroendocrine tumors (NETs) can be treated with radiolabeled somatostatin analogues, resulting in strongly increased progression-free survival and quality of life. There is nevertheless still room for improvement, as very few patients can be cured at this stage of disease. We aimed to specifically sensitize replicating tumor cells without further damage to healthy tissues. Thereto we investigated the DNA damaging effects of PRRT with the purpose to enhance these effects through modulation of the DNA damage response. Although PRRT induces DNA double strand breaks (DSBs), a larger fraction of the induced lesions are single strand breaks (expected to be similar to those induced by external beam radiotherapy) that require poly-[ADP-ribose]-polymerase 1 (PARP-1) activity for repair. If these breaks cannot be repaired, they will cause replication fork arrest and DSB formation during replication. Therefore, we used the PARP-1 inhibitor Olaparib to increase the number of cytotoxic DSBs. Here we show that this new combination strategy synergistically sensitized somatostatin receptor expressing cells to PRRT. We observed increased cell death and reduced cellular proliferation compared to the PRRT alone. The enhanced cell death was caused by increased numbers of DSBs that are repaired with remarkably slow kinetics, leading to genome instability. Furthermore, we validated the increased DSB induction after PARP inhibitor addition in the clinically relevant model of living human NET slices. We expect that this combined regimen can thus augment current PRRT outcomes.

Tumor slice culture system to assess drug response of primary breast cancer

BACKGROUND

The high incidence of breast cancer has sparked the development of novel targeted and personalized therapies. Personalization of cancer treatment requires reliable prediction of chemotherapy responses in individual patients. Effective selection can prevent unnecessary treatment that would mainly result in the unwanted side effects of the therapy. This selection can be facilitated by characterization of individual tumors using robust and specific functional assays, which requires development of powerful ex vivo culture systems and procedures to analyze the response to treatment.

METHODS

We optimized culture methods for primary breast tumor samples that allowed propagation of tissue ex vivo. We combined several tissue culture strategies, including defined tissue slicing technology, growth medium optimization and use of a rotating platform to increase nutrient exchange.

RESULTS

We could maintain tissue cultures for at least 7 days without losing tissue morphology, viability or cell proliferation. We also developed methods to determine the cytotoxic response of individual tumors to the chemotherapeutic treatment FAC (5-FU, Adriamycin [Doxorubicin] and Cyclophosphamide). Using this tool we designated tumors as sensitive or resistant and distinguished a clinically proven resistant tumor from other tumors.

CONCLUSION

This method defines conditions that allow ex vivo testing of individual tumor responses to anti-cancer drugs and therefore might improve personalization of breast cancer treatment.

In Vivo Stabilization of a Gastrin-Releasing Peptide Receptor Antagonist Enhances PET Imaging and Radionuclide Therapy of Prostate Cancer in Preclinical Studies

A single tool for early detection, accurate staging, and personalized treatment of prostate cancer (PCa) would be a major breakthrough in the field of PCa. Gastrin-releasing peptide receptor (GRPR) targeting peptides are promising probes for a theranostic approach for PCa overexpressing GRPR. However, the successful application of small peptides in a theranostic approach is often hampered by their fast in vivo degradation by proteolytic enzymes, such as neutral endopeptidase (NEP). Here we show for the first time that co-injection of a NEP inhibitor (phosphoramidon (PA)) can lead to an impressive enhancement of diagnostic sensitivity and therapeutic efficacy of the theranostic (68)Ga-/(177)Lu-JMV4168 GRPR-antagonist. Co-injection of PA (300 µg) led to stabilization of (177)Lu-JMV4168 in murine peripheral blood. In PC-3 tumor-bearing mice, PA co-injection led to a two-fold increase in tumor uptake of (68)Ga-/(177)Lu-JMV4168, 1 h after injection. In positron emission tomography (PET) imaging with (68)Ga-JMV4168, PA co-injection substantially enhanced PC-3 tumor signal intensity. Radionuclide therapy with (177)Lu-JMV4168 resulted in significant regression of PC-3 tumor size. Radionuclide therapy efficacy was confirmed by production of DNA double strand breaks, decreased cell proliferation and increased apoptosis. Increased survival rates were observed in mice treated with (177)Lu-JMV4168 plus PA as compared to those without PA. This data shows that co-injection of the enzyme inhibitor PA greatly enhances the theranostic potential of GRPR-radioantagonists for future application in PCa patients.

REV7 counteracts DNA double-strand break resection and affects PARP inhibition

Error-free repair of DNA double-strand breaks (DSBs) is achieved by homologous recombination (HR), and BRCA1 is an important factor for this repair pathway. In the absence of BRCA1-mediated HR, the administration of PARP inhibitors induces synthetic lethality of tumour cells of patients with breast or ovarian cancers. Despite the benefit of this tailored therapy, drug resistance can occur by HR restoration. Genetic reversion of BRCA1-inactivating mutations can be the underlying mechanism of drug resistance, but this does not explain resistance in all cases. In particular, little is known about BRCA1-independent restoration of HR. Here we show that loss of REV7 (also known as MAD2L2) in mouse and human cell lines re-establishes CTIP-dependent end resection of DSBs in BRCA1-deficient cells, leading to HR restoration and PARP inhibitor resistance, which is reversed by ATM kinase inhibition. REV7 is recruited to DSBs in a manner dependent on the H2AX-MDC1-RNF8-RNF168-53BP1 chromatin pathway, and seems to block HR and promote end joining in addition to its regulatory role in DNA damage tolerance. Finally, we establish that REV7 blocks DSB resection to promote non-homologous end-joining during immunoglobulin class switch recombination. Our results reveal an unexpected crucial function of REV7 downstream of 53BP1 in coordinating pathological DSB repair pathway choices in BRCA1-deficient cells.

An XRCC4 splice mutation associated with severe short stature, gonadal failure, and early-onset metabolic syndrome

CONTEXT

Severe short stature can be caused by defects in numerous biological processes including defects in IGF-1 signaling, centromere function, cell cycle control, and DNA damage repair. Many syndromic causes of short stature are associated with medical comorbidities including hypogonadism and microcephaly.

OBJECTIVE

To identify an underlying genetic etiology in two siblings with severe short stature and gonadal failure.

DESIGN

Clinical phenotyping, genetic analysis, complemented by in vitro functional studies of the candidate gene.

SETTING

An academic pediatric endocrinology clinic.

PATIENTS OR OTHER PARTICIPANTS

Two adult siblings (male patient [P1] and female patient 2 [P2]) presented with a history of severe postnatal growth failure (adult heights: P1, -6.8 SD score; P2, -4 SD score), microcephaly, primary gonadal failure, and early-onset metabolic syndrome in late adolescence. In addition, P2 developed a malignant gastrointestinal stromal tumor at age 28.

INTERVENTION(S)

Single nucleotide polymorphism microarray and exome sequencing.

RESULTS

Combined microarray analysis and whole exome sequencing of the two affected siblings and one unaffected sister identified a homozygous variant in XRCC4 as the probable candidate variant. Sanger sequencing and mRNA studies revealed a splice variant resulting in an in-frame deletion of 23 amino acids. Primary fibroblasts (P1) showed a DNA damage repair defect.

CONCLUSIONS

In this study we have identified a novel pathogenic variant in XRCC4, a gene that plays a critical role in non-homologous end-joining DNA repair. This finding expands the spectrum of DNA damage repair syndromes to include XRCC4 deficiency causing severe postnatal growth failure, microcephaly, gonadal failure, metabolic syndrome, and possibly tumor predisposition.

PARP inhibitors: the journey from research hypothesis to clinical approval

Cancer puts an increasing burden on our healthcare system and is a major cause of death. Therefore, novel approaches are required to improve cancer treatment. Cancer cells have several hallmarks that could be therapeutically targeted. Importantly, every tumor has a different combination of aberrations affecting the different hallmarks. This review focuses on targeting one of these hallmarks, the DNA damage response (DDR). DDR defects can not only cause cancer, but they can also be exploited therapeutically. This plays an important role even in 'classical' (DNA damaging) chemotherapy and radiotherapy, but more precise targeting of specific defects is expected to increase treatment efficacy and decrease normal tissue toxicity. Poly-(ADP-ribose) polymerase (PARP) inhibitors are the first clinical example of such synthetic lethality in tumors having specific DDR defects. They are currently under investigation as DDR-targeting anticancer drugs and they progress quickly in clinical trials.

DNA damage response and anti-apoptotic proteins predict radiosensitization efficacy of HDAC inhibitors SAHA and LBH589 in patient-derived glioblastoma cells

HDAC inhibitors have radiosensitizing effects in established cancer cell lines. This study was conducted to compare the efficacy of SAHA, LBH589, Valproic Acid (VPA), MS275 and Scriptaid in the patient-derived glioblastoma model. In more detail, SAHA and LBH589 were evaluated to determine predictors of response. Acetylated-histone-H3, γH2AX/53BP1, (p)Chek2/ATM, Bcl-2/Bcl-XL, p21(CIP1/WAF1) and caspase-3/7 were studied in relation to response. SAHA sensitized 50% of cultures, LBH589 45%, VPA and Scriptaid 40% and MS275 60%. Differences after treatment with SAHA/RTx or LBH589/RTx in a sensitive and resistant culture were increased acetylated-H3, caspase-3/7 and prolonged DNA damage repair γH2AX/53BP1 foci. pChek2 was found to be associated with both SAHA/RTx and LBH589/RTx response with a positive predictive value (PPV) of 90%. Bcl-XL had a PPV of 100% for LBH589/RTx response. Incubation with HDACi 24 and 48 hours pre-RTx resulted in the best efficacy of combination treatment. In conclusion a subset of patient-derived glioblastoma cultures were sensitive to HDACi/RTx. For SAHA and LBH589 responses were strongly associated with pChek2 and Bcl-XL, which warrant further clinical exploration. Additional information on responsiveness was obtained by DNA damage response markers and apoptosis related proteins.

Abstract 2425: Exploiting DNA repair defects in breast cancer

One of the hallmarks of cancer is genetic instability. Many chemotherapeutic strategies make use of DNA damaging agents, that preferentially target dividing cells. Recently, DNA repair defects have been identified in many different types of tumors, suggesting that the effectiveness of such treatments may be modulated by the precise genetic make-up of the tumor; defects in specific DNA repair pathways can be used as the Achilles' Heel of the tumor. An important new concept is synthetic lethality, the observation that combinations of mutations can be lethal, while both single mutations are viable. In selected tumors, the specific DNA repair defect can be targeted in such a way that the tumor cells can be killed without causing severe side effect on normal tissues. The combination of PARP inhibitors and homologous recombination (HR) deficient hereditary breast cancer is on its way to the clinic and more combinations are currently in the preclinical phase. Selection of breast cancer patients for targeted treatment with PARP inhibitors requires a validated test to identify HR deficient tumors. For this purpose, we have assessed ionizing radiation induced RAD51 foci (IRIF) after ex vivo irradiation of individual breast tumor slices as a marker for HR defects. We validated this approach in known HR- deficient tumors, where we found a severe defect in RAD51 IRIF formation. Subsequently, we used this assay to identify a subgroup of HR deficient primary breast cancers. Out of 42 tumors, we found five HR deficient breast cancers, caused by two distinct modes of BRCA gene inactivation, mutations and promoter hypermethylation. HR deficiency was significantly associated with high grade, triple-negative breast cancer (TNBC). A large fraction of TNBC showed HR defects, suggesting that PARP inhibitors may be particularly promising for this group of tumors that currently have a poor prognosis. We conclude that the RAD51 IRIF assay is a powerful tool to select patients with HR-deficient primary breast cancers eligible for PARP-inhibitor treatment in the clinic. These approaches may change cancer medicine in a fundamental way, from a one-size-fits-all concept to an individualized treatment strategy based on the functional molecular make-up of the tumor for each patient.

Citation Format: Kishan A.T. Naipal, Nicole S. Verkaik, Petra ter Brugge, Najim Ameziane, Carolien H.M. van Deurzen, John W. Martens, Johan P. de Winter, Jos Jonkers, Maaike P. Vreeswijk, Agnes Jager, Jan Hoeijmakers, Roland Kanaar, Dik C. van Gent. Exploiting DNA repair defects in breast cancer. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 2425. doi:10.1158/1538-7445.AM2014-2425

Similar recombination-activating gene (RAG) mutations result in similar immunobiological effects but in different clinical phenotypes

BACKGROUND

V(D)J recombination takes place during lymphocyte development to generate a large repertoire of T- and B-cell receptors. Mutations in recombination-activating gene 1 (RAG1) and RAG2 result in loss or reduction of V(D)J recombination. It is known that different mutations in RAG genes vary in residual recombinase activity and give rise to a broad spectrum of clinical phenotypes.

OBJECTIVE

We sought to study the immunologic mechanisms causing the clinical spectrum of RAG deficiency.

METHODS

We included 22 patients with similar RAG1 mutations (c.519delT or c.368_369delAA) resulting in N-terminal truncated RAG1 protein with residual recombination activity but presenting with different clinical phenotypes. We studied precursor B-cell development, immunoglobulin and T-cell receptor repertoire formation, receptor editing, and B- and T-cell numbers.

RESULTS

Clinically, patients were divided into 3 main categories: T(-)B(-) severe combined immunodeficiency, Omenn syndrome, and combined immunodeficiency. All patients showed a block in the precursor B-cell development, low B- and T-cell numbers, normal immunoglobulin gene use, limited B- and T-cell repertoires, and slightly impaired receptor editing.

CONCLUSION

This study demonstrates that similar RAG mutations can result in similar immunobiological effects but different clinical phenotypes, indicating that the level of residual recombinase activity is not the only determinant for clinical outcome. We postulate a model in which the type and moment of antigenic pressure affect the clinical phenotypes of these patients.

A regulatory role for the cohesin loader NIPBL in nonhomologous end joining during immunoglobulin class switch recombination

DNA double strand breaks (DSBs) are mainly repaired via homologous recombination (HR) or nonhomologous end joining (NHEJ). These breaks pose severe threats to genome integrity but can also be necessary intermediates of normal cellular processes such as immunoglobulin class switch recombination (CSR). During CSR, DSBs are produced in the G1 phase of the cell cycle and are repaired by the classical NHEJ machinery. By studying B lymphocytes derived from patients with Cornelia de Lange Syndrome, we observed a strong correlation between heterozygous loss-of-function mutations in the gene encoding the cohesin loading protein NIPBL and a shift toward the use of an alternative, microhomology-based end joining during CSR. Furthermore, the early recruitment of 53BP1 to DSBs was reduced in the NIPBL-deficient patient cells. Association of NIPBL deficiency and impaired NHEJ was also observed in a plasmid-based end-joining assay and a yeast model system. Our results suggest that NIPBL plays an important and evolutionarily conserved role in NHEJ, in addition to its canonical function in sister chromatid cohesion and its recently suggested function in HR.

Clinical spectrum of LIG4 deficiency is broadened with severe dysmaturity, primordial dwarfism, and neurological abnormalities

DNA double-strand break repair via non-homologous end joining (NHEJ) is involved in recombination of immunoglobulin and T-cell receptor genes. Mutations in NHEJ components result in syndromes that are characterized by microcephaly and immunodeficiency. We present a patient with lymphopenia, extreme radiosensitivity, severe dysmaturity, corpus callosum agenesis, polysyndactily, dysmorphic appearance, and erythema, which are suggestive of a new type of NHEJ deficiency. We identified two heterozygous mutations in LIG4. The p.S205LfsX29 mutation results in lack of the nuclear localization signal and appears to be a null mutation. The second mutation p.K635RfsX10 lacks the C-terminal region responsible for XRCC4 binding and LIG4 stability and activity, and therefore this mutant might be a null mutation as well or have very low residual activity. This is remarkable since Lig4 knockout mice are embryonic lethal and so far in humans no complete LIG4 deficiencies have been described. This case broadens the clinical spectrum of LIG4 deficiencies.

Essential role for DNA-PK-mediated phosphorylation of NR4A nuclear orphan receptors in DNA double-strand break repair

DNA-dependent protein kinase (DNA-PK) is a central regulator of DNA double-strand break (DSB) repair; however, the identity of relevant DNA-PK substrates has remained elusive. NR4A nuclear orphan receptors function as sequence-specific DNA-binding transcription factors that participate in adaptive and stress-related cell responses. We show here that NR4A proteins interact with the DNA-PK catalytic subunit and, upon exposure to DNA damage, translocate to DSB foci by a mechanism requiring the activity of poly(ADP-ribose) polymerase-1 (PARP-1). At DNA repair foci, NR4A is phosphorylated by DNA-PK and promotes DSB repair. Notably, NR4A transcriptional activity is entirely dispensable in this function, and core components of the DNA repair machinery are not transcriptionally regulated by NR4A. Instead, NR4A functions directly at DNA repair sites by a process that requires phosphorylation by DNA-PK. Furthermore, a severe combined immunodeficiency (SCID)-causing mutation in the human gene encoding the DNA-PK catalytic subunit impairs the interaction and phosphorylation of NR4A at DSBs. Thus, NR4As represent an entirely novel component of DNA damage response and are substrates of DNA-PK in the process of DSB repair.

Spatiotemporal aspects of MicroRNA-mediated gene regulation

MicroRNA-mediated modulation of translation has been recently discovered as a new dimension in gene expression regulation. In this chapter we review how this regulation operates in time between the fast protein modification and degradation steps on the one hand and the slow transcriptional reprogramming associated with more stable changes in gene expression patterns on the other hand. We also discuss the additional layer of complexity associated with spatial redistribution of the RNA silencing machinery in subcellular structures. Various stress conditions induce both a transient change in microRNA expression within the first few hours after exposure and a redistribution of the RNA silencing machinery from P-bodies to Stress Granules, which differ in their function and protein content. Insight into the spatiotemporal aspects of the microRNA response will be indispensable for a full understanding of this level of gene regulation.

NBS1 cooperates with homologous recombination to counteract chromosome breakage during replication

Nijmegen breakage syndrome (NBS) is characterized by genome instability and cancer predisposition. NBS patients contain a mutation in the NBS1 gene, which encodes the NBS1 component of the DNA double-strand break (DSB) response complex MRE11/RAD50/NBS1. To investigate the NBS phenotype in more detail, we combined the mouse mimic of the most common patient mutation (Nbs1(Delta B/DeltaB)) with a Rad54 null mutation, which diminishes homologous recombination. Double mutant cells were particularly sensitive to treatments that cause single strand breaks (SSBs), presumably because these SSBs can be converted into detrimental DSBs upon passage of a replication fork. The persistent presence of nuclear RAD51 foci and increased levels of chromatid type breaks in metaphase spreads indicated that replication-associated DSBs are repaired inefficiently in the double mutant cells. We conclude that Nbs1 and Rad54 function cooperatively, but in separate pathways to counteract this type of DNA damage and discuss mechanistic implications of these findings.

MicroRNA-mediated gene silencing modulates the UV-induced DNA-damage response

DNA damage provokes DNA repair, cell-cycle regulation and apoptosis. This DNA-damage response encompasses gene-expression regulation at the transcriptional and post-translational levels. We show that cellular responses to UV-induced DNA damage are also regulated at the post-transcriptional level by microRNAs. Survival and checkpoint response after UV damage was severely reduced on microRNA-mediated gene-silencing inhibition by knocking down essential components of the microRNA-processing pathway (Dicer and Ago2). UV damage triggered a cell-cycle-dependent relocalization of Ago2 into stress granules and various microRNA-expression changes. Ago2 relocalization required CDK activity, but was independent of ATM/ATR checkpoint signalling, whereas UV-responsive microRNA expression was only partially ATM/ATR independent. Both microRNA-expression changes and stress-granule formation were most pronounced within the first hours after genotoxic stress, suggesting that microRNA-mediated gene regulation operates earlier than most transcriptional responses. The functionality of the microRNA response is illustrated by the UV-inducible miR-16 that downregulates checkpoint-gene CDC25a and regulates cell proliferation. We conclude that microRNA-mediated gene regulation adds a new dimension to the DNA-damage response.