Ribonucleotide Incorporation by Eukaryotic B-Family Replicases and Its Implications for Genome Stability.

Our current view of how DNA-based genomes are efficiently and accurately replicated continues to evolve as new details emerge on the presence of ribonucleotides in DNA. Ribonucleotides are incorporated during eukaryotic DNA replication at rates that make them the most common noncanonical nucleotide placed into the nuclear genome, they are efficiently repaired, and their removal impacts genome integrity. This review focuses on three aspects of this subject: the incorporation of ribonucleotides into the eukaryotic nuclear genome during replication by B-family DNA replicases, how these ribonucleotides are removed, and the consequences of their presence or removal for genome stability and disease.

Processing ribonucleotides incorporated during eukaryotic DNA replication.

The information encoded in DNA is influenced by the presence of non-canonical nucleotides, the most frequent of which are ribonucleotides. In this Review, we discuss recent discoveries about ribonucleotide incorporation into DNA during replication by the three major eukaryotic replicases, DNA polymerases α, δ and ε. The presence of ribonucleotides in DNA causes short deletion mutations and may result in the generation of single- and double-strand DNA breaks, leading to genome instability. We describe how these ribonucleotides are removed from DNA through ribonucleotide excision repair and by topoisomerase I. We discuss the biological consequences and the physiological roles of ribonucleotides in DNA, and consider how deficiencies in their removal from DNA may be important in the aetiology of disease.

Targetability of the neurovascular unit in inflammatory diseases of the central nervous system.

The blood-brain barrier (BBB) is a selectively permeable barrier separating the periphery from the central nervous system (CNS). The BBB restricts the flow of most material into and out of the CNS, including many drugs that could be used as potent therapies. BBB permeability is modulated by several cells that are collectively called the neurovascular unit (NVU). The NVU consists of specialized CNS endothelial cells (ECs), pericytes, astrocytes, microglia, and neurons. CNS ECs maintain a complex "seal" via tight junctions, forming the BBB; breakdown of these tight junctions leads to BBB disruption. Pericytes control the vascular flow within capillaries and help maintain the basal lamina. Astrocytes control much of the flow of material that has moved beyond the CNS EC layer and can form a secondary barrier under inflammatory conditions. Microglia survey the border of the NVU for noxious material. Neuronal activity also plays a role in the maintenance of the BBB. Since astrocytes, pericytes, microglia, and neurons are all able to modulate the permeability of the BBB, understating the complex contributions of each member of the NVU will potentially uncover novel and effective methods for delivery of neurotherapies to the CNS.

Targeting IFN-λ Signaling Promotes Recovery from Central Nervous System Autoimmunity.

Type III IFNs (IFNLs) are newly discovered cytokines, acting at epithelial and other barriers, that exert immunomodulatory functions in addition to their primary roles in antiviral defense. In this study, we define a role for IFNLs in maintaining autoreactive T cell effector function and limiting recovery in a murine model of multiple sclerosis (MS), experimental autoimmune encephalomyelitis. Genetic or Ab-based neutralization of the IFNL receptor (IFNLR) resulted in lack of disease maintenance during experimental autoimmune encephalomyelitis, with loss of CNS Th1 effector responses and limited axonal injury. Phenotypic effects of IFNLR signaling were traced to increased APC function, with associated increase in T cell production of IFN-γ and GM-CSF. Consistent with this, IFNL levels within lesions of CNS tissues derived from patients with MS were elevated compared with MS normal-appearing white matter. Furthermore, expression of IFNLR was selectively elevated in MS active lesions compared with inactive lesions or normal-appearing white matter. These findings suggest IFNL signaling as a potential therapeutic target to prevent chronic autoimmune neuroinflammation.

Nbs1 flexibly tethers Ctp1 and Mre11-Rad50 to coordinate DNA double-strand break processing and repair.

The Nijmegen breakage syndrome 1 (Nbs1) subunit of the Mre11-Rad50-Nbs1 (MRN) complex protects genome integrity by coordinating double-strand break (DSB) repair and checkpoint signaling through undefined interactions with ATM, MDC1, and Sae2/Ctp1/CtIP. Here, fission yeast and human Nbs1 structures defined by X-ray crystallography and small angle X-ray scattering (SAXS) reveal Nbs1 cardinal features: fused, extended, FHA-BRCT(1)-BRCT(2) domains flexibly linked to C-terminal Mre11- and ATM-binding motifs. Genetic, biochemical, and structural analyses of an Nbs1-Ctp1 complex show Nbs1 recruits phosphorylated Ctp1 to DSBs via binding of the Nbs1 FHA domain to a Ctp1 pThr-Asp motif. Nbs1 structures further identify an extensive FHA-BRCT interface, a bipartite MDC1-binding scaffold, an extended conformational switch, and the molecular consequences associated with cancer predisposing Nijmegen breakage syndrome mutations. Tethering of Ctp1 to a flexible Nbs1 arm suggests a mechanism for restricting DNA end processing and homologous recombination activities of Sae2/Ctp1/CtIP to the immediate vicinity of DSBs.

Primary cutaneous EBV+ extranodal NK/T-cell lymphoma of gamma/delta T-cell lineage in the posttransplantation setting.

Posttransplantation primary cutaneous T-cell lymphomas (PT-CTCL) are a rare complication of sustained immunosuppression in the posttransplant setting. When present, PT-CTCLs are typically EBV- and exhibit features of mycosis fungoides/Sézary syndrome or CD30+ lymphoproliferative disorders. We present a case of a 75-year-old individual who developed skin lesions 30 years after liver transplantation. Pathologic evaluation of the skin biopsy revealed involvement by a clonal, EBV+ T-cell population of gamma/delta lineage with no evidence of systemic disease. Comprehensive genomic profiling was performed, confirming focal one-copy loss of 6q23.3, altogether consistent with the extremely rare and unusual diagnosis of primary cutaneous EBV+ extranodal NK/T-cell lymphoma of gamma/delta T-cell lineage in the posttransplantation setting.

mTORC1 Coordinates Protein Synthesis and Immunoproteasome Formation via PRAS40 to Prevent Accumulation of Protein Stress.

Reduction of translational fidelity often occurs in cells with high rates of protein synthesis, generating defective ribosomal products. If not removed, such aberrant proteins can be a major source of cellular stress causing human diseases. Here, we demonstrate that mTORC1 promotes the formation of immunoproteasomes for efficient turnover of defective proteins and cell survival. mTORC1 sequesters precursors of immunoproteasome ß subunits via PRAS40. When activated, mTORC1 phosphorylates PRAS40 to enhance protein synthesis and simultaneously to facilitate the assembly of the ß subunits for forming immunoproteasomes. Consequently, the PRAS40 phosphorylations play crucial roles in clearing aberrant proteins that accumulate due to mTORC1 activation. Mutations of RAS, PTEN, and TSC1, which cause mTORC1 hyperactivation, enhance immunoproteasome formation in cells and tissues. Those mutations increase cellular dependence on immunoproteasomes for stress response and survival. These results define a mechanism by which mTORC1 couples elevated protein synthesis with immunoproteasome biogenesis to protect cells against protein stress.

Injury-related emergency department use among people with intellectual and developmental disabilities insured by Medicaid from 2010 to 2016.

OBJECTIVES: Data on non-fatal injuries and visits to the emergency department (ED) for injuries are not readily available. The objective of this paper is to describe injury-related ED visits for people with intellectual and developmental disabilities who are covered by the Medicaid insurance programme. METHODS: We aggregated 2010-2016 Medicaid claims data from eight states. Using these data, we identified individuals with intellectual and developmental disabilities and then determined an all-cause ED visit rate, ED visit due to injury rate and admission from ED due to injury rate. Data were stratified by sex and age group. Results were compared with national rates. RESULTS: Medicaid members with intellectual and developmental disabilities visited EDs at approximately 1.8 times the rate of the general population. The ED visit rate due to injury was approximately 1.5 times that observed in the population overall. When ED visits due to injury data were stratified by age and sex, the largest discrepancy was observed in women ages 45-64, who visited EDs due to injury at a rate 2.1 times that of women of the same age in the general population. The admission rate from ED due to injury increased over the study period most notably in the older age groups. CONCLUSIONS: While rates and patterns of ED utilisation among Medicaid members with intellectual and developmental disabilities vary by age and gender, our findings suggest this group visits the ED due to injury at rates well above the general population.

Orchestrating Stress Responses in Multiple Sclerosis: A Role for Astrocytic IFNγ Signaling.

Multiple sclerosis (MS) is an inflammatory and neurodegenerative disease that is characterized by the infiltration of peripheral immune cells into the central nervous system (CNS), secretion of inflammatory factors, demyelination, and axonal degeneration. Inflammatory mediators such as cytokines alter cellular function and activate resident CNS cells, including astrocytes. Notably, interferon (IFN)γ is a prominent pleiotropic cytokine involved in MS that contributes to disease pathogenesis. Astrocytes are dynamic cells that respond to changes in the cellular microenvironment and are highly responsive to many cytokines, including IFNγ. Throughout the course of MS, intrinsic cell stress is initiated in response to inflammation, which can impact the pathology. It is known that cell stress is pronounced during MS; however, the specific mechanisms relating IFNγ signaling to cell stress responses in astrocytes are still under investigation. This review will highlight the current literature regarding the impact of IFNγ signaling alone and in combination with other immune mediators on astrocyte synthesis of free oxygen radicals and cell death, and cover what is understood regarding astrocytic mitochondrial dysfunction and endoplasmic reticulum stress.

Do Palatoplasty Procedures Resolve Hypernasality as Effectively as Pharyngoplasty Procedures in Patients with 22q11.2 Deletion Syndrome?

OBJECTIVE: Compare the effectiveness of palatoplasty and pharyngoplasty procedures at resolving hypernasality in patients with 22q11.2 deletion syndrome (22q). DESIGN: Retrospective cohort study. SETTING: Metropolitan children's hospital. PATIENTS: Fourteen patients with 22q presenting for management of velopharyngeal insufficiency. INTERVENTIONS: Palatoplasty or pharyngoplasty procedure. MAIN OUTCOME MEASURE: Resolution of hypernasality 12 months postoperatively. RESULTS: Both procedure groups had a mean preoperative velopharyngeal gap of 6.2 mm during phonation. No patient who underwent palatoplasty achieved resolution of hypernasality; 1/7 patients had worse hypernasality, 4/7 had no change, and 2/7 had improved hypernasality. In contrast, hypernasality was resolved in 6/7 patients in the pharyngoplasty group, which was significantly (P = .03) higher than the palatoplasty group. CONCLUSIONS: In patients with 22q, palatoplasty procedures may be less effective than pharyngoplasty procedures at resolving hypernasality. This may be due to underlying anatomic or physiologic differences, such as increased pharyngeal depth and hypodynamic muscles.

Feeding Management and Palate Repair Timing in Infants with Cleft Palate with and without Pierre Robin Sequence: A Multisite Study.

OBJECTIVES: Compare the feeding management practices in infants with cleft palate with and without Pierre Robin sequence (PRS) and determine if specific feeding difficulties or interventions predict delayed palate repair. DESIGN: Retrospective cross-sectional study. SETTING: Seventeen cleft palate teams contributed data. PATIENTS: 414 infants were included in this study: 268 infants with cleft palate only and 146 infants with cleft palate and PRS. PROCEDURES: Data were collected via parent interview and electronic health records. MAIN OUTCOME MEASURES: Outcomes for the primary objective included categorical data for: history of poor growth, feeding therapy, milk fortification, use of enteral feeding, and feeding difficulties. The outcome for the secondary objective was age in months at primary palate repair. RESULTS: Infants with PRS had a significantly higher prevalence of feeding difficulties (81% versus 61%) and poor growth (29% versus 15%) compared to infants with cleft palate only. Infants with PRS received all feeding interventions-including feeding therapy, milk fortification, and enteral feeding-at a significantly higher frequency. Infants with PRS underwent primary palate repair at a mean age of 13.55 months (SD = 3.29) which was significantly (P < .00001) later than infants with cleft palate only who underwent palate repair at a mean age of 12.05 months (SD = 2.36). Predictors of delayed palate repair included diagnosis of PRS as well as Hispanic ethnicity and a history of poor growth. CONCLUSIONS: These findings can be used to establish clinical directives focused on providing early, multimodal feeding interventions to promote optimal growth and timely palate repair for infants with PRS.

The Levator Veli Palatini: Are all Segments Created Equal?

INTRODUCTION: The levator veli palatini (LVP) muscle has two segments with distinct roles in velopharyngeal function. Previous research suggests longer extravelar segments with shorter intravelar segments may lead to a more advantageous mechanism for velopharyngeal closure. The purpose of this study was to examine whether the distribution of the LVP intravelar and extravelar segments differs between children with cleft palate with and without VPI and controls. METHODS: The study included 97 children: 37 with cleft palate +/- lip with VPI, 37 controls, and 19 with cleft palate with normal resonance. Measures included mean LVP length, mean extravelar LVP length, and intravelar LVP length. RESULTS: Overall mean LVP length was similar (P = .267) between controls and children with cleft palate (with and without VPI). However, there was a significant difference (P < .001) between group for both intravelar and extravelar LVP lengths: the intravelar segment was significantly longer in those with VPI compared to controls and children with cleft palate and normal resonance; and the extravelar segment was significantly shorter in those with VPI compared to controls and children with cleft palate and normal resonance. CONCLUSIONS: Results from this study demonstrate a significant difference between the distribution of the functional segments of the LVP among children with VPI, with a more disadvantageous distribution of the muscle segments among those with VPI.

Can MRI Replace Nasopharyngoscopy in the Evaluation of Velopharyngeal Insufficiency?

OBJECTIVE: To investigate whether flexible nasopharyngoscopy, when performed in addition to magnetic resonance imaging (MRI), influences the type of surgery selected or success of surgery in patients with velopharyngeal insufficiency (VPI). DESIGN: Cohort study. SETTING: A metropolitan children's hospital. PATIENTS: Patients with non-syndromic, repaired cleft palate presenting for management of VPI. INTERVENTIONS: MRI and nasopharyngoscopy or MRI alone for preoperative imaging of the velopharyngeal mechanism. MAIN OUTCOME MEASURES: (1) Surgical selection and (2) resolution of hypernasality. All speech, MRI, and nasopharyngoscopy measurements were performed by raters blinded to patients' medical and surgical history. RESULTS: Of the 25 patients referred for nasopharyngoscopy, 76% completed the exam. Of the 41 patients referred for MRI, the scan was successfully completed by 98% of patients. Completion of nasopharyngoscopy was significantly (p=0.01) lower than MRI. Surgical selection did not significantly differ (p=0.73) between the group receiving MRI and nasopharyngoscopy and the group receiving MRI alone, nor was there a significant difference between these groups in the proportion of patients achieving resolution of hypernasality postoperatively (p=0.63). Percent total velopharyngeal closure assessments on nasopharyngoscopy and MRI were strongly correlated (r=0.73). CONCLUSIONS: In patients receiving MRI as part of their preoperative VPI evaluation, the addition of nasopharyngoscopy did not result in a difference in surgical selection or resolution of hypernasality. Routine inclusion of nasopharyngoscopy may not be necessary for the evaluation of velopharyngeal anatomy when MRI is available.

A simple, fast, and cost-effective smartphone-based digital imaging method for quantification of lidocaine hydrochloride in pharmaceutical formulations.

OBJECTIVE: A simple, highly specific, accurate and fast method by smartphone-based digital imaging was developed for estimating lidocaine hydrochloride in pharmaceutical formulations. MATERIAL AND METHODS: To obtain the images, a Galaxy A03 Core smartphone and an image acquisition device developed in the laboratory were used to control the incident factors in reproducibility of the measurements. The processing of the images was carried out with the Color Grab application. Finally, the absorbance values were calculated using the RGB intensity values of blank, standard, and sample solutions. The proposed method was compared with spectroscopic and chromatographic methods. RESULTS: The reaction between copper and lidocaine hydrochloride was characterized, showing better results in an equimolar ratio and maintaining the pH of the solution above 11.5. The use of the device for the capture of digital images allowed to control those sensitive parameters for reproducibility so that the analytical measurements showed adequate precision and accuracy. Validation of the main parameters of the method showed compliance with acceptance criteria. The application of the method for the analysis of injectable samples achieved reliable results, which were statistically similar to other reference instrumental methods. CONCLUSION: The proposed method presented figures of merit in relation to linearity, precision, selectivity, accuracy, and robustness; it was carried out by designing and manufacturing a device for capturing digital images on a smartphone, which were analyzed to obtain RGB intensity values. These data are finally used to calculate absorbance values of solutions. All these elements provide this work with innovative characteristics in the field of analysis for control of pharmaceutical formulations.

Ribonucleotide incorporation into DNA during DNA replication and its consequences.

Ribonucleotides are the most abundant non-canonical nucleotides in the genome. Their vast presence and influence over genome biology is becoming increasingly appreciated. Here we review the recent progress made in understanding their genomic presence, incorporation characteristics and usefulness as biomarkers for polymerase enzymology. We also discuss ribonucleotide processing, the genetic consequences of unrepaired ribonucleotides in DNA and evidence supporting the significance of their transient presence in the nuclear genome.

Immunity impacts cognitive deficits across neurological disorders.

Cognitive deficits are a common comorbidity with neurological disorders and normal aging. Inflammation is associated with multiple diseases including classical neurodegenerative dementias such as Alzheimer's disease (AD) and autoimmune disorders such as multiple sclerosis (MS), in which over half of all patients experience some form of cognitive deficits. Other degenerative diseases of the central nervous system (CNS) including frontotemporal lobe dementia (FTLD), and Parkinson's disease (PD) as well as traumatic brain injury (TBI) and psychological disorders like major depressive disorder (MDD), and even normal aging all have cytokine-associated reductions in cognitive function. Thus, there is likely commonality between these secondary cognitive deficits and inflammation. Neurological disorders are increasingly associated with substantial neuroinflammation, in which CNS-resident cells secrete cytokines and chemokines such as tumor necrosis factor (TNF)α and interleukins (ILs) including IL-1ß and IL-6. CNS-resident cells also respond to a wide variety of cytokines and chemokines, which can have both direct effects on neurons by changing the expression of ion channels and perturbing electrical properties, as well as indirect effects through glia-glia and immune-glia cross-talk. There is significant overlap in these cytokine and chemokine expression profiles across diseases, with TNFα and IL-6 strongly associated with cognitive deficits in multiple disorders. Here, we review the involvement of various cytokines and chemokines in AD, MS, FTLD, PD, TBI, MDD, and normal aging in the absence of dementia. We propose that the neuropsychiatric phenotypes observed in these disorders may be at least partially attributable to a dysregulation of immunity resulting in pathological cytokine and chemokine expression from both CNS-resident and non-resident cells.

Defining the serum proteomic signature of hepatic steatosis, inflammation, ballooning and fibrosis in non-alcoholic fatty liver disease.

BACKGROUND & AIMS: Despite recent progress, non-invasive tests for the diagnostic assessment and monitoring of non-alcoholic fatty liver disease (NAFLD) remain an unmet need. Herein, we aimed to identify diagnostic signatures of the key histological features of NAFLD. METHODS: Using modified-aptamer proteomics, we assayed 5,220 proteins in each of 2,852 single serum samples from 636 individuals with histologically confirmed NAFLD. We developed and validated dichotomized protein-phenotype models to identify clinically relevant severities of steatosis (grade 0 vs. 1-3), hepatocellular ballooning (0 vs. 1 or 2), lobular inflammation (0-1 vs. 2-3) and fibrosis (stages 0-1 vs. 2-4). RESULTS: The AUCs of the four protein models, based on 37 analytes (18 not previously linked to NAFLD), for the diagnosis of their respective components (at a clinically relevant severity) in training/paired validation sets were: fibrosis (AUC 0.92/0.85); steatosis (AUC 0.95/0.79), inflammation (AUC 0.83/0.72), and ballooning (AUC 0.87/0.83). An additional outcome, at-risk NASH, defined as steatohepatitis with NAFLD activity score ≥4 (with a score of at least 1 for each of its components) and fibrosis stage ≥2, was predicted by multiplying the outputs of each individual component model (AUC 0.93/0.85). We further evaluated their ability to detect change in histology following treatment with placebo, pioglitazone, vitamin E or obeticholic acid. Component model scores significantly improved in the active therapies vs. placebo, and differential effects of vitamin E, pioglitazone, and obeticholic acid were identified. CONCLUSIONS: Serum protein scanning identified signatures corresponding to the key components of liver biopsy in NAFLD. The models developed were sufficiently sensitive to characterize the longitudinal change for three different drug interventions. These data support continued validation of these proteomic models to enable a "liquid biopsy"-based assessment of NAFLD. CLINICAL TRIAL NUMBER: Not applicable. IMPACT AND IMPLICATIONS: An aptamer-based protein scan of serum proteins was performed to identify diagnostic signatures of the key histological features of non-alcoholic fatty liver disease (NAFLD), for which no approved non-invasive diagnostic tools are currently available. We also identified specific protein signatures related to the presence and severity of NAFLD and its histological components that were also sensitive to change over time. These are fundamental initial steps in establishing a serum proteome-based diagnostic signature of NASH and provide the rationale for using these signatures to test treatment response and to identify several novel targets for evaluation in the pathogenesis of NAFLD.

Astrocyte interferon-gamma signaling dampens inflammation during chronic central nervous system autoimmunity via PD-L1.

Multiple sclerosis (MS) is an inflammatory and neurodegenerative disease of the central nervous system (CNS). Infiltrating inflammatory immune cells perpetuate demyelination and axonal damage in the CNS and significantly contribute to pathology and clinical deficits. While the cytokine interferon (IFN)γ is classically described as deleterious in acute CNS autoimmunity, we and others have shown astrocytic IFNγ signaling also has a neuroprotective role. Here, we performed RNA sequencing and ingenuity pathway analysis on IFNγ-treated astrocytes and found that PD-L1 was prominently expressed. Interestingly, PD-1/PD-L1 antagonism reduced apoptosis in leukocytes exposed to IFNγ-treated astrocytes in vitro. To further elucidate the role of astrocytic IFNγ signaling on the PD-1/PD-L1 axis in vivo, we induced the experimental autoimmune encephalomyelitis (EAE) model of MS in Aldh1l1-CreERT2, Ifngr1fl/fl mice. Mice with conditional astrocytic deletion of IFNγ receptor exhibited a reduction in PD-L1 expression which corresponded to increased infiltrating leukocytes, particularly from the myeloid lineage, and exacerbated clinical disease. PD-1 agonism reduced EAE severity and CNS-infiltrating leukocytes. Importantly, PD-1 is expressed by myeloid cells surrounding MS lesions. These data support that IFNγ signaling in astrocytes diminishes inflammation during chronic autoimmunity via upregulation of PD-L1, suggesting potential therapeutic benefit for MS patients.

The impact of social determinants of health on the presentation, management and outcomes of systemic lupus erythematosus.

Disparities in SLE rates and outcomes have been attributed to genetic and hormonal factors, cigarette smoking and environmental pollutants. However, a growing body of research indicates that social determinants of health (SDH) also have substantial impact on the disparities that characterize SLE. According to the World Health Organization, SDH are defined as 'the conditions in which people are born, grow, work, live, and age', account for 30-55% of health outcomes, and adversely impact health outcomes among those of low socioeconomic status and stigmatized racial/ethnic groups. We reviewed the impact of key SDH on SLE presentation, management and outcomes, including income, education, neighbourhood factors, healthcare access, discrimination and social support. We found that adverse SDH conditions may lead to more severe SLE with increased morbidity and mortality, and that SDH affect SLE management by dictating the most feasible monitoring and treatment plan for each individual patient based on his or her specific life circumstances (for example, based on health insurance status, distance to nearest provider and/or drug affordability). SDH also have a significant impact on SLE outcomes, with worse disease and psychosocial outcomes associated with lower income level, lower educational attainment, disadvantaged neighbourhoods, lack of health insurance or public health insurance in the USA, travel burden to nearest provider, anti-Black racism and lower social support. Future efforts to improve the management and outcomes of patients with SLE must combat the societal, economic and political forces that perpetuate these inequities.

Mre11 dimers coordinate DNA end bridging and nuclease processing in double-strand-break repair.

Mre11 forms the core of the multifunctional Mre11-Rad50-Nbs1 (MRN) complex that detects DNA double-strand breaks (DSBs), activates the ATM checkpoint kinase, and initiates homologous recombination (HR) repair of DSBs. To define the roles of Mre11 in both DNA bridging and nucleolytic processing during initiation of DSB repair, we combined small-angle X-ray scattering (SAXS) and crystal structures of Pyrococcus furiosus Mre11 dimers bound to DNA with mutational analyses of fission yeast Mre11. The Mre11 dimer adopts a four-lobed U-shaped structure that is critical for proper MRN complex assembly and for binding and aligning DNA ends. Further, mutations blocking Mre11 endonuclease activity impair cell survival after DSB induction without compromising MRN complex assembly or Mre11-dependant recruitment of Ctp1, an HR factor, to DSBs. These results show how Mre11 dimerization and nuclease activities initiate repair of DSBs and collapsed replication forks, as well as provide a molecular foundation for understanding cancer-causing Mre11 mutations in ataxia telangiectasia-like disorder (ATLD).