Prostaglandin E2 activates melanin-concentrating hormone neurons to drive diet-induced obesity.

Hypothalamic inflammation reduces appetite and body weight during inflammatory diseases, while promoting weight gain when induced by high-fat diet (HFD). How hypothalamic inflammation can induce opposite energy balance outcomes remains unclear. We found that prostaglandin E2 (PGE2), a key hypothalamic inflammatory mediator of sickness, also mediates diet-induced obesity (DIO) by activating appetite-promoting melanin-concentrating hormone (MCH) neurons in the hypothalamus in rats and mice. The effect of PGE2 on MCH neurons is excitatory at low concentrations while inhibitory at high concentrations, indicating that these neurons can bidirectionally respond to varying levels of inflammation. During prolonged HFD, endogenous PGE2 depolarizes MCH neurons through an EP2 receptor-mediated inhibition of the electrogenic Na+/K+-ATPase. Disrupting this mechanism by genetic deletion of EP2 receptors on MCH neurons is protective against DIO and liver steatosis in male and female mice. Thus, an inflammatory mediator can directly stimulate appetite-promoting neurons to exacerbate DIO and fatty liver.

The perinatal outcomes of women treated for Asherman syndrome: a propensity score-matched cohort study.

STUDY QUESTION: Do the perinatal outcomes of patients following hysteroscopic treatment for Asherman syndrome (AS) differ from that of a control population? SUMMARY ANSWER: Perinatal complications including placental issues, high blood loss, and prematurity in women after treatment for AS should be considered as moderate to high risk, especially in patients who have undergone more than one hysteroscopy (HS) or repeated postpartum instrumental revisions of the uterine cavity (Dilation and Curettage; D&C). WHAT IS KNOWN ALREADY: The detrimental impact of AS on obstetrics outcomes is commonly recognized. However, prospective studies evaluating perinatal/neonatal outcomes in women with AS history are sparse, and the characteristics accounting for the respective morbidity of AS patients remain to be elucidated. STUDY DESIGN, SIZE, DURATION: We conducted a prospective cohort study utilizing data from patients who underwent HS treatment for moderate to severe AS in a single tertiary University-affiliated hospital (enrolled between 01 January 2009 and March 2021), and who consequently conceived and progressed to at least 22nd gestational week of pregnancy. Perinatal outcomes were compared to a control population without an AS history, retrospectively enrolled concomitantly at the time of delivery for each patient with AS. Maternal and neonatal morbidity was assessed as well as the characteristics-related risk factors of AS patients. PARTICIPANTS/MATERIALS, SETTING, METHODS: Our analytic cohort included a total of 198 patients, 66 prospectively enrolled patients with moderate to severe AS and 132 controls. We used multivariable logistic regression to calculate a propensity score to match 1-1 women with and without AS history based on demographic and clinical factors. After matching, 60 pairs of patients were analysed. Chi-square test was used to compare perinatal outcomes between the pairs. Spearman's correlation analysis was utilized to investigate the correlation between perinatal/neonatal morbidity and the characteristics-related factors of AS patients. The odds ratio (OR) for the associations was calculated by logistic regression. MAIN RESULTS AND THE ROLE OF CHANCE: Among the 60 propensity matched pairs, the AS group more frequently experienced overall perinatal morbidity, including abnormally invasive placenta (41.7% vs 0%; P < 0.001), retained placenta requiring manual or surgical removal (46.7% vs 6.7%; P < 0.001), and peripartum haemorrhage occurrence (31.7% vs 3.3%; P < 0.001). Premature delivery (<37 gestational weeks) was reported more frequently also for patients with AS (28.3% vs 5.0%; P < 0.001). However, no increased frequency of intra-uterine growth restriction or worsened neonatal outcomes were observed in AS group. Univariable analysis of risk factors for AS group morbidity outcomes revealed that the main factor related to abnormally invasive placenta was two or more HS procedures (OR 11.0; 95% CI: 1.33-91.23), followed by two or more D&Cs preceding AS treatment (OR 5.11; 95% CI: 1.69-15.45), and D&C performed postpartum as compared to post abortion (OR 3.0; 95% CI: 1.03-8.71). Similarly, two or more HS procedures were observed as the most important factor for retained placenta (OR 13.75; 95% CI: 1.66-114.14), followed by two or more preceding D&Cs (OR 5.16; 95% CI: 1.67-15.9). Premature birth was significantly associated with the number of preceding D&Cs (OR for two or more, 4.29; 95% CI: 1.12-14.91). LIMITATIONS, REASONS FOR CAUTION: Although the cohort of patients with AS was enrolled prospectively, a baseline imbalance was intrinsically involved in the retrospective enrolment of the control group. However, to reduce the risk of bias, confounding factors were adjusted for using propensity score matching. The limitation to the generalization of our reported results is the single institution design in which all patients were treated for AS in one tertiary medical centre. WIDER IMPLICATIONS OF THE FINDINGS: Within our search scope, our study represents one of the first and largest prospective studies of perinatal and neonatal outcomes in moderate to severe AS patients with a prospectively analysis of the risks factors of characteristics significantly influencing reported morbidities among patients with AS. STUDY FUNDING/COMPETING INTEREST(S): The study was supported by the Charles University in Prague [UNCE 204065] and by the institutional grant of The General Faculty Hospital in Prague [00064165]. No competing interests were declared. TRIAL REGISTRATION NUMBER: N/A.

Patients with telomere biology disorders show context specific somatic mosaic states with high frequency of U2AF1 variants.

Somatic mosaic states in telomere biology disorders are characterized by somatic variants in the spliceosome and DNA damage response and repair pathways. A likely maladaptive response to short telomeres that may lead to increased hematological cancer.

Comparative study of therapy-related and de novo adult b-cell acute lymphoblastic leukaemia.

We report a comparative analysis of patients with therapy-related acute lymphoblastic leukaemia (tr-ALL) vs de novo ALL. We identified 331 patients with B-ALL; 69 (21%) were classified as tr-ALL. The most common prior malignancies were breast (23·2%) and plasma cell disorders (20·3%). Patients with tr-ALL were older (median 63·2 vs. 46·2 years, P < 0.001), more often female (66·7% vs. 43·5%, P < 0·001), and more likely to have hypodiploid cytogenetics (18·8% vs. 5·0%, P < 0·001). In multivariable analysis, patients with tr-ALL were less likely to achieve complete remission [odds ratio (OR) = 0·16, P < 0·001] and more likely to be minimal residual disease-positive (OR = 4·86, P = 0·01) but had similar OS after diagnosis and allo-haematopoietic cell transplantation.

Gilteritinib clinical activity in relapsed/refractory FLT3 mutated acute myeloid leukemia previously treated with FLT3 inhibitors.

Gilteritinib is approved for the treatment of relapsed/refractory (R/R) acute myeloid leukemia (AML) with an FLT3-mutation (FLT3mut+ ). However, the gilteritinib phase 3 ADMIRAL study (Perl et al NEJM 2019) was conducted prior to widespread adoption of either midostaurin as a component of standard intensive induction and consolidation or posttransplant FLT3 inhibitor maintenance. We performed a retrospective analysis using data from 11 US centers and where we identified 113 patients who received gilteritinib alone or as combination therapy for the treatment of R/R FLT3mut+ AML. The composite complete remission (CR) rate (CRc, defined as CR + CRi + CR with incomplete platelet recovery [CRp]) was 48.7% (n = 55). The CRc rate after treatment with gilteritinib in patients who were treated with only prior 7+3 and midostaurin with or without consolidation was 58% with a median survival of 7.8 months. Survival was longest in patients who obtained a CR, particularly a cMRD (clinical minimal or measurable residual disease) negative response; this remained significant after censoring at the time of stem cell transplant. The mitogen-activated protein kinase pathway activating mutations that are known for gilteritinib resistance (NRAS, KRAS, and PTPN11) had lower CRc (35% vs. 60.5%) and lower median overall survival than patients' whose leukemia did not express these mutations (4.9 months vs. 7.8 months) (HR 2.4; 95% CI 1. 5.4) p value <.01.

Interprofessional Attitudes and Interdisciplinary Practices for Older Adults With Back Pain Among Doctors of Chiropractic: A Descriptive Survey.

OBJECTIVE: The purpose of this study is to report on attitudes of doctors of chiropractic (DCs) toward integrative medicine and their self-reported interdisciplinary practices for older adults with back pain. METHODS: This descriptive survey was conducted with licensed DCs in a Midwestern community in the United States. Respondents completed a 53-item postal survey of demographics, practice characteristics, referral and co-management patterns, attitudes toward interdisciplinary practice, and the Integrative Medicine-30 Questionnaire (IM-30). Descriptive statistical analysis was performed. RESULTS: Fifty-seven DCs completed the survey (29% response). Geriatric-focused chiropractic practices were uncommon (<15%), although 56% reported that 25% to 49% of the patients treated each week were older adults. Respondents had a moderate orientation toward collaboration with other health care providers (IM-30 mean [standard deviation] 61.3 [11.5]). The IM-30 subscales placed DCs high on measures of integrative medicine safety; moderate on patient-centeredness, openness to working with other providers, and referral readiness; and low on learning from alternative paradigms. Doctors of chiropractic most referred older patients to neurologists, family physicians, massage therapists, orthopedists, and other chiropractors. Doctors of chiropractic reported the highest levels of co-management with family physicians, physical therapists, and massage therapists. Most DCs (92%) were confident in their own ability to manage back pain in older adults, with modest confidence expressed for treatments from professionals using manual therapies. Most (77%) responded that older patients would experience the most improvement if DCs collaborated with another chiropractor, rather than with medical professionals. CONCLUSION: Doctors of chiropractic in one geographic community are moderately oriented toward interprofessional practice with other health care providers for older adults with back pain. Follow-up studies in representative national and international samples are recommended.

Uterine leiomyomas with bizarre nuclei: analysis of 37 cases after laparoscopic or open myomectomy.

OBJECTIVE: The aim of this study was to analyse the clinical outcome of patients with diagnosis of leiomyoma with bizarre nuclei (LBN) undergoing uterus saving surgery due to fertility preservation. DESIGN: Retrospective clinical study. SETTING: Department of Obstetrics and Gynaecology, First Faculty of Medicine, Charles University, General University Hospital, Prague. METHODS: This was a retrospective clinical study of patients with LBN diagnosis after myomectomy between January 2002 and June 2017 which were searched in our database. The data were obtained from medical documentation and from correspondence with patients. RESULTS: We identified 37 patients meeting the criteria in our database. The median age of the patients was 34.0 years. 30 patients (81.1%) underwent laparoscopic procedure, 7 (18.9.%) had open myomectomy. The perioperative appearance of fibroid was found normal in 27 cases (73.0%), in the rest the appearance was described somehow abnormal. The follow-up data were obtained from 35 women; the median follow-up time was 48 months. 9 patients (25.7%) needed re-intervention for fibroids with 2 specimens (22.2%) classified as LBN again. The overall pregnancy rate was 63.6% and life birth rate was 33.3%. We did not observe any distant recurrence of the disease or malignant recurrence or death related to the diagnosis. CONCLUSION: Uterus sparing surgery for treatment of LBN seems to be safe and reasonable therapy for women wishing to preserve fertility.

Short-term high-fat diet primes excitatory synapses for long-term depression in orexin neurons.

KEY POINTS: High-fat diet consumption is a major cause of obesity. Orexin neurons are known to be activated by a high-fat diet and in turn promote further consumption of a high-fat diet. Our study shows that excitatory synapses to orexin neurons become amenable to long-term depression (LTD) after 1 week of high-fat diet feeding. However, this effect reverses after 4 weeks of a high-fat diet. This LTD may be a homeostatic response to a high-fat diet to curb the activity of orexin neurons and hence caloric consumption. Adaptation seen after prolonged high-fat diet intake may contribute to the development of obesity. ABSTRACT: Overconsumption of high-fat diets is one of the strongest contributing factors to the rise of obesity rates. Orexin neurons are known to be activated by a palatable high-fat diet and mediate the activation of the mesolimbic reward pathway, resulting in further food intake. While short-term exposure to a high-fat diet is known to induce synaptic plasticity within the mesolimbic pathway, it is unknown if such changes occur in orexin neurons. To investigate this, 3-week-old male rats were fed a palatable high-fat western diet (WD) or control chow for 1 week and then in vitro patch clamp recording was performed. In the WD condition, an activity-dependent long-term depression (LTD) of excitatory synapses was observed in orexin neurons, but not in chow controls. This LTD was presynaptic and depended on postsynaptic metabotropic glutamate receptor 5 (mGluR5) and retrograde endocannabinoid signalling. WD also increased extracellular glutamate levels, suggesting that glutamate spillover and subsequent activation of perisynaptic mGluR5 may occur more readily in the WD condition. In support of this, pharmacological inhibition of glutamate uptake was sufficient to prime chow control synapses to undergo a presynaptic LTD. Interestingly, these WD effects are transient, as extracellular glutamate levels were similar to controls and LTD was no longer observed in orexin neurons after 4 weeks of WD. In summary, excitatory synapses to orexin neurons become amenable to LTD under a palatable high-fat diet, which may represent a homeostatic mechanism to prevent overactivation of these neurons and to curtail high-fat diet consumption.

Patient-centered professional practice models for managing low back pain in older adults: a pilot randomized controlled trial.

BACKGROUND: Low back pain is a debilitating condition for older adults, who may seek healthcare from multiple providers. Few studies have evaluated impacts of different healthcare delivery models on back pain outcomes in this population. The purpose of this study was to compare clinical outcomes of older adults receiving back pain treatment under 3 professional practice models that included primary medical care with or without chiropractic care. METHODS: We conducted a pilot randomized controlled trial with 131 community-dwelling, ambulatory older adults with subacute or chronic low back pain. Participants were randomly allocated to 12 weeks of individualized primary medical care (Medical Care), concurrent medical and chiropractic care (Dual Care), or medical and chiropractic care with enhanced interprofessional collaboration (Shared Care). Primary outcomes were low back pain intensity rated on the numerical rating scale and back-related disability measured with the Roland-Morris Disability Questionnaire. Secondary outcomes included clinical measures, adverse events, and patient satisfaction. Statistical analyses included mixed-effects regression models and general estimating equations. RESULTS: At 12 weeks, participants in all three treatment groups reported improvements in mean average low back pain intensity [Shared Care: 1.8; 95% confidence interval (CI) 1.0 to 2.6; Dual Care: 3.0; 95% CI 2.3 to 3.8; Medical Care: 2.3; 95% CI 1.5 to 3.2)] and back-related disability (Shared Care: 2.8; 95% CI 1.6 to 4.0; Dual Care: 2.5; 95% CI 1.3 to 3.7; Medical Care: 1.5; 95% CI 0.2 to 2.8). No statistically significant differences were noted between the three groups on the primary measures. Participants in both models that included chiropractic reported significantly better perceived low back pain improvement, overall health and quality of life, and greater satisfaction with healthcare services than patients who received medical care alone. CONCLUSIONS: Professional practice models that included primary care and chiropractic care led to modest improvements in low back pain intensity and disability for older adults, with chiropractic-inclusive models resulting in better perceived improvement and patient satisfaction over the primary care model alone. TRIAL REGISTRATION: Clinicaltrials.gov, NCT01312233 , 4 March 2011.

Best Practices for Chiropractic Care for Older Adults: A Systematic Review and Consensus Update.

OBJECTIVE: The purpose of this study was to update evidence-based recommendations on the best practices for chiropractic care of older adults. METHODS: The project consisted of a systematic literature review and a consensus process. The following were searched from October 2009 through January 2016: MEDLINE, Index to Chiropractic Literature, CINAHL (Cumulative Index to Nursing and Allied Health Literature), AMED (Allied and Complementary Medicine Database), Alt HealthWatch, Cochrane Database of Systematic Reviews, and Cochrane Registry of Controlled Trials. Search terms were: (manipulation, spinal OR manipulation, chiropractic OR chiropract*) AND (geriatric OR "older adult*"). Two reviewers independently screened articles and abstracts using inclusion and exclusion criteria. The systematic review informed the project steering committee, which revised the previous recommendations. A multidisciplinary panel of experts representing expertise in practice, research, and teaching in a variety of health professions serving older adults rated the revised recommendations. The RAND Corporation/University of California, Los Angeles methodology for a modified Delphi consensus process was used. RESULTS: A total of 199 articles were found; after exclusion criteria were applied, 6 articles about effectiveness or efficacy and 6 on safety were added. The Delphi process was conducted from April to June 2016. Of the 37 Delphi panelists, 31 were DCs and 6 were other health care professionals. Three Delphi rounds were conducted to reach consensus on all 45 statements. As a result, statements regarding the safety of manipulation were strengthened and additional statements were added recommending that DCs advise patients on exercise and that manipulation and mobilization contribute to general positive outcomes beyond pain reduction only. CONCLUSIONS: This document provides a summary of evidence-informed best practices for doctors of chiropractic for the evaluation, management, and manual treatment of older adult patients.

Genome of the pathogen Porphyromonas gingivalis recovered from a biofilm in a hospital sink using a high-throughput single-cell genomics platform.

Although biofilms have been shown to be reservoirs of pathogens, our knowledge of the microbial diversity in biofilms within critical areas, such as health care facilities, is limited. Available methods for pathogen identification and strain typing have some inherent restrictions. In particular, culturing will yield only a fraction of the species present, PCR of virulence or marker genes is mainly focused on a handful of known species, and shotgun metagenomics is limited in the ability to detect strain variations. In this study, we present a single-cell genome sequencing approach to address these limitations and demonstrate it by specifically targeting bacterial cells within a complex biofilm from a hospital bathroom sink drain. A newly developed, automated platform was used to generate genomic DNA by the multiple displacement amplification (MDA) technique from hundreds of single cells in parallel. MDA reactions were screened and classified by 16S rRNA gene PCR sequence, which revealed a broad range of bacteria covering 25 different genera representing environmental species, human commensals, and opportunistic human pathogens. Here we focus on the recovery of a nearly complete genome representing a novel strain of the periodontal pathogen Porphyromonas gingivalis (P. gingivalis JCVI SC001) using the single-cell assembly tool SPAdes. Single-cell genomics is becoming an accepted method to capture novel genomes, primarily in the marine and soil environments. Here we show for the first time that it also enables comparative genomic analysis of strain variation in a pathogen captured from complex biofilm samples in a healthcare facility.

When communication fails: resolving end-of-life impasses.

Various protocols and advance directives can prepare physician assistants for difficult conversations with patients about end-of-life care. This case scenario explores the roles and influences that complicated one patient's care and how the situation was resolved.

Updating the striatal-pallidal wiring diagram.

The striatal and pallidal complexes are basal ganglia structures that orchestrate learning and execution of flexible behavior. Models of how the basal ganglia subserve these functions have evolved considerably, and the advent of optogenetic and molecular tools has shed light on the heterogeneity of subcircuits within these pathways. However, a synthesis of how molecularly diverse neurons integrate into existing models of basal ganglia function is lacking. Here, we provide an overview of the neurochemical and molecular diversity of striatal and pallidal neurons and synthesize recent circuit connectivity studies in rodents that takes this diversity into account. We also highlight anatomical organizational principles that distinguish the dorsal and ventral basal ganglia pathways in rodents. Future work integrating the molecular and anatomical properties of striatal and pallidal subpopulations may resolve controversies regarding basal ganglia network function.

Evaluation of Novel Targets, Including CC-Chemokine Receptor 4, in Adult T-Cell Acute Lymphoblastic Leukemia/Lymphoma: A Mayo Clinic Clinical and Pathologic Study.

CONTEXT.­: Unlike B-cell acute lymphoblastic leukemia/lymphoma (ALL/LBL), there have been few therapeutic advances in T-cell ALL (T-ALL)/LBL, an aggressive ALL/LBL subtype. OBJECTIVE.­: To perform a focused tissue array study to elucidate tumor markers of therapeutic potential in T-ALL/LBL. DESIGN.­: Using immunohistochemistry, we evaluated expression of leukemic antigens of interest, specifically CC-chemokine receptor 4 (CCR4), among others, on available remnant diagnostic material, including tumor tissue slides obtained from formalin-fixed, paraffin-embedded preserved tissues. RESULTS.­: Our analysis identified, for the first time, expression of CCR4 in T-ALL/LBL in 11 of 27 cases (40.7%) and confirmed common expression of BCL2, CD38, and CD47, as reported previously. We also identified the expression of CD123 in 4 of 26 cases (15.4%), whereas BCL6 and PDL1 were expressed in a small number of T-ALL/LBL cases. The potential novel target CCR4 was significantly more common in the Pre/Pro-T immunophenotypic subtype, 6 of 9 (66.7%, P = .01). No additional differences in clinical and epidemiologic variables were noted among positive or negative CCR4 cases. CONCLUSIONS.­: These findings support preclinical and clinical testing of therapies targeting CCR4, CD47, BCL2, CD38, and CD123 in T-ALL/LBL, and may help guide the development of targeted clinical trials in T-ALL/LBL, a rare disease in urgent need of novel therapies.

Transcriptomic landscape of mammalian ventral pallidum at single-cell resolution.

The ventral pallidum (VP) is critical for motivated behaviors. While contemporary work has begun to elucidate the functional diversity of VP neurons, the molecular heterogeneity underlying this functional diversity remains incompletely understood. We used snRNA-seq and in situ hybridization to define the transcriptional taxonomy of VP cell types in mice, macaques, and baboons. We found transcriptional conservation between all three species, within the broader neurochemical cell types. Unique dopaminoceptive and cholinergic subclusters were identified and conserved across both primate species but had no homolog in mice. This harmonized consensus VP cellular atlas will pave the way for understanding the structure and function of the VP and identified key neuropeptides, neurotransmitters, and neuro receptors that could be targeted within specific VP cell types for functional investigations.

High-fat diet-induced elevation of body weight set point in male mice.

OBJECTIVE: High-fat diets (HFD) are thought to disrupt energy homeostasis to drive overeating and obesity. However, weight loss resistance in individuals with obesity suggests that homeostasis is intact. This study aimed to reconcile this difference by systematically assessing body weight (BW) regulation under HFD. METHODS: Male C57BL/6 N mice were fed diets with varying fat and sugar in different durations and patterns. BW and food intake were monitored. RESULTS: BW gain was transiently accelerated by HFD (≥40%) prior to plateauing. The plateau was consistent regardless of starting age, HFD duration, or fat/sugar content. Reverting to a low-fat diet (LFD) caused transiently accelerated weight loss, which correlated with how heavy mice were before the diet relative to LFD-only controls. Chronic HFD attenuated the efficacy of single or repetitive dieting, revealing a defended BW higher than that of LFD-only controls. CONCLUSIONS: This study suggests that dietary fat modulates the BW set point immediately upon switching from LFD to HFD. Mice defend a new elevated set point by increasing caloric intake and efficiency. This response is consistent and controlled, suggesting that hedonic mechanisms contribute to rather than disrupt energy homeostasis. An elevated floor of the BW set point after chronic HFD could explain weight loss resistance in individuals with obesity.

Community laboratories in the United States: BioMakerspaces for life science learning.

Informal learning environments play a critical role in science, technology, engineering, and mathematics learning across the lifespan and are consequential in informing public understanding and engagement. This can be difficult to accomplish in life science where expertise thresholds and logistics involved with handling biological materials can restrict access. Community laboratories are informal learning environments that provide access to the resources necessary to carry out pursuits using enabling biotechnologies. We investigate a group of these spaces in order to ascertain how this occurs-with specific attention to how material and intellectual resources are structured and shape learning. Using surveys and focus group interviews, we explore a group of these spaces located in the United States. We found that the spaces examined offer learning activities that are sufficiently scaffolded and flexible as to promote personalized and community-driven practice. We discuss these findings in relation to informal learning environment design and learning.

CLONEQC: lightweight sequence verification for synthetic biology.

Synthetic biology projects aim to produce physical DNA that matches a designed target sequence. Chemically synthesized oligomers are generally used as the starting point for building larger and larger sequences. Due to the error rate of chemical synthesis, these oligomers can have many differences from the target sequence. As oligomers are joined together to make larger and larger synthetic intermediates, it becomes essential to perform quality control to eliminate intermediates with errors and retain only those DNA molecules that are error free with respect to the target. This step is often performed by transforming bacteria with synthetic DNA and sequencing colonies until a clone with a perfect sequence is identified. Here we present CloneQC, a lightweight software pipeline available as a free web server and as source code that performs quality control on sequenced clones. Input to the server is a list of desired sequences and forward and reverse reads for each clone. The server generates summary statistics (error rates and success rates target-by-target) and a detailed report of perfect clones. This software will be useful to laboratories conducting in-house DNA synthesis and is available at http://cloneqc.thruhere.net/ and as Berkeley Software Distribution (BSD) licensed source.

Retrotransposon overdose and genome integrity.

Yeast and mammalian genomes are replete with nearly identical copies of long dispersed repeats in the form of retrotransposons. Mechanisms clearly exist to maintain genome structure in the face of potential rearrangement between the dispersed repeats, but the nature of this machinery is poorly understood. Here we describe a series of distinct "retrotransposon overdose" (RO) lineages in which the number of Ty1 elements in the Saccharomyces cerevisiae genome has been increased by as much as 10 fold. Although these RO strains are remarkably normal in growth rate, they demonstrate an intrinsic supersensitivity to DNA-damaging agents. We describe the identification of mutants in the DNA replication pathway that enhance this RO-specific DNA damage supersensitivity by promoting ectopic recombination between Ty1 elements. Abrogation of normal DNA replication leads to rampant genome instability primarily in the form of chromosomal aberrations and confirms the central role of DNA replication accuracy in the stabilization of repetitive DNA.