The Crk4-Cyc4 complex regulates G2/M transition in Toxoplasma gondii.

A versatile division of apicomplexan parasites and a dearth of conserved regulators have hindered the progress of apicomplexan cell cycle studies. While most apicomplexans divide in a multinuclear fashion, Toxoplasma gondii tachyzoites divide in the traditional binary mode. We previously identified five Toxoplasma CDK-related kinases (Crk). Here, we investigated TgCrk4 and its cyclin partner TgCyc4. We demonstrated that TgCrk4 regulates conventional G2 phase processes, such as repression of chromosome rereplication and centrosome reduplication, and acts upstream of the spindle assembly checkpoint. The spatial TgCyc4 dynamics supported the TgCrk4-TgCyc4 complex role in the coordination of chromosome and centrosome cycles. We also identified a dominant TgCrk4-TgCyc4 complex interactor, TgiRD1 protein, related to DNA replication licensing factor CDT1 but played no role in licensing DNA replication in the G1 phase. Our results showed that TgiRD1 also plays a role in controlling chromosome and centrosome reduplication. Global phosphoproteome analyses identified TgCrk4 substrates, including TgORC4, TgCdc20, TgGCP2, and TgPP2ACA. Importantly, the phylogenetic and structural studies suggest the Crk4-Cyc4 complex is limited to a minor group of the binary dividing apicomplexans.

Habitat type drives the spatial distribution of Australian fish chorus diversitya).

Fish vocalize in association with life functions with many species calling en masse to produce choruses. Monitoring the distribution and behavior of fish choruses provides high-resolution data on fish distribution, habitat use, spawning behavior, and in some circumstances, local abundance. The purpose of this study was to use long-term passive acoustic recordings to obtain a greater understanding of the patterns and drivers of Australian fish chorus diversity at a national scale. This study detected 133 fish choruses from year-long recordings taken at 29 Australian locations with the highest fish chorus diversity identified at a site in the country's northern, tropical waters. A linear model fitted with a generalized least squares regression identified geomorphic feature type, benthic substrate type, and northness (of slope) as explanatory variables of fish chorus diversity. Geomorphic feature type was identified as the significant driver of fish chorus diversity. These results align with broad-scale patterns reported previously in fish biodiversity, fish assemblages, and fish acoustic diversity. This study has highlighted that passive acoustic monitoring of fish chorus diversity has the potential to be used as an indicator of fish biodiversity and to highlight habitats of ecological importance.

A pilot study of the acceptability, feasibility and safety of yoga for chronic pain in sickle cell disease.

OBJECTIVES: To determine the acceptability, feasibility and safety of yoga for chronic pain in sickle cell disease. DESIGN AND SETTING: In Part A of this two-part study, adolescents with SCD and chronic pain (Group 1) and their parent (Group 2) completed a survey designed to capture pain characteristics, attitudes and practices related to yoga, and potential acceptability of a yoga program. In Part B, the study assessed the feasibility and safety of an instructor-led group yoga program. The study was registered on clinicaltrials.gov (NCT03694548). INTERVENTION: Eight instructor-led group yoga sessions. MAIN OUTCOME MEASURES: Feasibility and safety outcomes were chosen a priori, as follows: 1) Proportion of adolescent patients with SCD and chronic pain approached that consent to participate in Part A, 2) Proportion of adolescent participants enrolled in Part A that consent to participate in Part B, 3) Proportion of participants enrolled in Part B that attend at least 6 of 8 yoga sessions, 4) Proportion of participants enrolled in Part B with an ED visit or a hospitalization for pain within 24 h of completion of each yoga session, 5) Proportion of participants in Part B who complete all study assessments before, and at the end of the yoga program, 6) Adherence to submission of pain diary. RESULTS: The median age of 15 patient participants in Part A was 16 (IQR 14-17), and 14 parents was 43.5 (IQR 42-51). Most participants were female. Most participant responses indicated a positive opinion of yoga. Nine adolescents (60 %) from Part A participated in Part B of the study. The median age of 9 participants in Part B was 17 (IQR 15-18), and 5 of the 9 participants were female (53.3 %). Only one participant was able to attend 3 of the 8 yoga sessions offered, and did not experience any ED visits or hospitalizations following the yoga sessions. None of the other feasibility endpoints were met in this study. CONCLUSIONS: Patients with SCD and chronic pain overall have a positive opinion of yoga, but there are challenges with recruitment and retention of participants in a clinical trial of yoga, and barriers to feasibility of an in-person group yoga intervention.

Novel CRK-Cyclin Complex Controls Spindle Assembly Checkpoint in Toxoplasma Endodyogeny.

Opportunistic parasites of the Apicomplexa phylum use a variety of division modes built on two types of cell cycles that incorporate two distinctive mechanisms of mitosis: uncoupled from and coupled to parasite budding. Parasites have evolved novel factors to regulate such unique replication mechanisms that are poorly understood. Here, we have combined genetics, quantitative fluorescence microscopy, and global proteomics approaches to examine endodyogeny in Toxoplasma gondii dividing by mitosis coupled to cytokinesis. In the current study, we focus on the steps controlled by the recently described atypical Cdk-related kinase T. gondii Crk6 (TgCrk6). While inspecting protein complexes, we found that this previously orphaned TgCrk6 kinase interacts with a parasite-specific atypical cyclin, TgCyc1. We built conditional expression models and examined primary cell cycle defects caused by the lack of TgCrk6 or TgCyc1. Quantitative microscopy assays revealed that tachyzoites deficient in either TgCrk6 or the cyclin partner TgCyc1 exhibit identical mitotic defects, suggesting cooperative action of the complex components. Further examination of the mitotic structures indicated that the TgCrk6/TgCyc1 complex regulates metaphase. This novel finding confirms a functional spindle assembly checkpoint (SAC) in T. gondii. Measuring global changes in protein expression and phosphorylation, we found evidence that canonical activities of the Toxoplasma SAC are intertwined with parasite-specific tasks. Analysis of phosphorylation motifs suggests that Toxoplasma metaphase is regulated by CDK, mitogen-activated kinase (MAPK), and Aurora kinases, while the TgCrk6/TgCyc1 complex specifically controls the centromere-associated network. IMPORTANCE The rate of Toxoplasma tachyzoite division directly correlates with the severity of the disease, toxoplasmosis, which affects humans and animals. Thus, a better understanding of the tachyzoite cell cycle would offer much-needed efficient tools to control the acute stage of infection. Although tachyzoites divide by binary division, the cell cycle architecture and regulation differ significantly from the conventional binary fission of their host cells. Unlike the unidirectional conventional cell cycle, the Toxoplasma budding cycle is braided and is regulated by multiple essential Cdk-related kinases (Crks) that emerged in the place of missing conventional cell cycle regulators. How these novel Crks control apicomplexan cell cycles is largely unknown. Here, we have discovered a novel parasite-specific complex, TgCrk6/TgCyc1, that orchestrates a major mitotic event, the spindle assembly checkpoint. We demonstrated that tachyzoites incorporated parasite-specific tasks in the canonical checkpoint functions.

The Silent Epidemic: Causes and Consequences of Medical Learner Burnout.

PURPOSE OF REVIEW: Clinician burnout has significant socioeconomic, health, and quality of life implications. However, there has been little attention directed at medical students and house officers (i.e., medical learners). This review provides pertinent evidence regarding burnout as it relates to medical learners including risk factors and potential interventions. We conclude with recommendations on future research directions and potential approaches to address this epidemic of medical learner burnout. RECENT FINDINGS: Burnout is a significant issue among medical learners that is impacted both by interpersonal and environmental factors. There are points of heightened vulnerability for medical learners throughout their training. However, studies are unable to reach consensus regarding effective interventions to mitigate the impact of burnout. Furthermore, some elements of burnout are not readily reversible even after removing risk factors. Burnout is a significant concern for medical learners with wide-ranging physical, emotional, and psychosocial consequences. However, the current body of literature is sparse and does not provide consistent guidance on how to address burnout in medical learners. It is clear additional attention is needed in understanding burnout among learners and establishing proactive approaches to minimize its negative impact.

Is water-soluble contrast enema examination for integrity of rectal anastomosis necessary prior to ileostomy reversal?

BACKGROUND AND AIM: Routine use of water-soluble contrast enema (WSCE) to assess anastomotic integrity is debated. This study aimed to evaluate the role of WSCE to assess anastomotic integrity following anterior resections (AR) with defunctioning stoma prior to reversal and identify factors to limit its selective use. METHODS: This retrospective study evaluated all WSCE performed over a 7-year period at a high-volume colorectal unit. Risk factors for radiological abnormality/leak, including malignancy, chemoradiotherapy, and immediate postoperative complications, were recorded. A gastrointestinal specialist radiologist and surgeon validated all WSCEs reported as abnormal. RESULTS: Of the 486 WSCE studies identified, 92 were excluded (repeat studies (n = 51), pediatric cases [n = 2], no AR [n = 39]). A total of 394 WSCE studies were evaluated (260 cancer; 134 noncancer patients); 14% (37/260) of cancer patients and 8% (10/134) of noncancer patients had abnormal studies (P = 0.072). Of the 37 abnormal studies in cancer patients, 73% (27/37) radiological leaks were found, and 41% (n = 11/27) of these patients had postoperative complications. Of the 10 abnormal studies in noncancer patients, 20% (2/10) radiological leaks were found, but none of these patients had postoperative complications. Overall leak rates were 7% (29/394), and rates were significantly higher in cancer patients than noncancer patients (10 vs 2%, P = 0.005). CONCLUSION: Routine use of WSCE may not be necessary prior to reversal. WSCE should be selectively used in event of postoperative leak or complications. Noncancer resections are less likely demonstrate a leak.

Patient and family experience with chronic transfusion therapy for sickle cell disease: A qualitative study.

BACKGROUND: There is a limited understanding of the patient and family experience of Chronic Transfusion Therapy (CTT) for prevention of complications of Sickle Cell Disease (SCD). We sought to understand patient and family experience with CTT using qualitative methods. METHODS: Fifteen parents of children < 18 years old and nine children 12-18 years old with SCD who were receiving CTT for > 1 year were interviewed using a semi-structured interview format, and interviews were analyzed using open coding methods. RESULTS: Four themes created a narrative of the patient and family experience of CTT: 1) Burden of CTT, 2) Coping with CTT, 3) Perceived benefits and risks of CTT, and 4) Decision making regarding CTT. Participants reported substantial burden of CTT, including the impact of CTT on daily life and family, distress about venous access, burden of chelation therapy, and anxiety about CTT complications. Participants described how they coped with CTT. Participants reported increased energy, decreased pain, fewer hospitalizations, and stroke prevention with CTT, but also recognized complications of CTT, though awareness was limited in adolescents. Parents described sharing in the informed decision-making process with their healthcare provider about CTT, but adolescent patient participants reported that they were not involved in this process. CONCLUSIONS: CTT is associated with significant patient and family burden. Support from family, healthcare providers and school may help individuals cope with some of this burden. These findings provide the basis for future studies to identify strategies to mitigate the burden of CTT and improve the patient experience with this therapy. Future studies should also systematically assess patient knowledge about the key components of CTT and chelation using quantitative assessments.

Considering Stigma in the Provision of HIV Pre-Exposure Prophylaxis: Reflections from Current Prescribers.

Efforts to identify and address social inequities in HIV pre-exposure prophylaxis (PrEP) access are urgently needed. We investigated early-adopting PrEP prescribers' beliefs about how stigma contributes to PrEP access disparities in health care and explored potential intervention strategies within the context of PrEP service delivery. US-based PrEP prescribers were recruited through professional networks and participant referrals. Qualitative interviews were conducted, transcribed, and thematically analyzed. Participants (n = 18) were primarily male (72%); white (39%) or Asian (33%); and heterosexual (56%). Most practiced in the Northeastern (67%) or Southern (22%) United States; were physicians (94%); and specialized in HIV/infectious disease (89%). Participants described multiple forms of structural and interpersonal stigma impeding PrEP access. The requirement that PrEP be prescribed was a perceived deterrent for populations with medical mistrust and/or low health literacy. Practice norms such as discussing PrEP only in response to patient requests were seen as favoring more privileged groups. When probed about personally held biases, age-related stereotypes were the most readily acknowledged, including assumptions about older adults being sexually inactive and uncomfortable discussing sex. Participants criticized providers who chose not to prescribe PrEP within their clinical practice, particularly those whose decision reflected personal values related to condomless sex or discomfort communicating about sex with their patients. Suggested solutions included standardizing PrEP service delivery across patients and increasing cultural competence training. These early insights from a select sample of early-adopting providers illuminate mechanisms through which stigma could compromise PrEP access for key populations and corresponding points of intervention within the health care system.

Legacy and social media respectively influence risk perceptions and protective behaviors during emerging health threats: A multi-wave analysis of communications on Zika virus cases.

OBJECTIVE: Both legacy media, such as television and newspapers, and online social media are potentially important but incompletely understood sources of information in the face of emerging public health risks. This research aimed to understand media effects on risk perceptions and behaviors concerning the Zika virus in the United States. METHODS: We analyzed a multi-wave nationally representative survey (N = 29,062) and the volume of communications in social and legacy media (i.e., legacy media data from news sources and databases, N = 2,660 and social media data from Twitter, N = 1,605,752) in the United States between April and October 2016, dates coinciding with the early cases of local transmission of Zika in the United States (i.e., 25 weeks). The present study conducted econometric analyses (i.e., Granger causality tests) to assess the associations of legacy and social media coverage with risk perceptions and protective behaviors in the total sample and specific groups separated by pregnancy status/intent, geographic region, income, education level, age, and ethnicity. RESULTS: The results from the overall sample suggested that changes in the volume of information in legacy and social media (i.e., Twitter) were followed by different changes in community risk perceptions and protective behaviors. Specifically, social media coverage correlated with the level of risk perceptions, whereas the legacy media coverage correlated with the level of protective behaviors. Analyses across different subpopulations, including those of different pregnancy status/intent, geographic Zika risk, income, education level, age, and ethnicity, replicated the social media associations with risk perceptions in most cases. However, legacy media and protective behaviors were linked only in some vulnerable subpopulations (e.g., the less-educated populations). CONCLUSION: Understanding how media coverage relates to Zika risk perceptions and protective behaviors will help to facilitate effective risk communications by healthcare professionals and providers, particularly when a health risk emerges.

"Support Your Client at the Space That They're in": HIV Pre-Exposure Prophylaxis (PrEP) Prescribers' Perspectives on PrEP-Related Risk Compensation.

Despite the demonstrated effectiveness of HIV pre-exposure prophylaxis (PrEP) and evidence that most PrEP users do not engage in risk compensation (i.e., increased risk behavior due to a perceived decrease in HIV susceptibility), some healthcare providers report patient risk compensation to be a deterrent to prescribing PrEP. Overcoming this barrier is essential to supporting PrEP access and uptake among people at risk for HIV. To inform such efforts, this qualitative study explored PrEP-related risk compensation attitudes among providers with firsthand experience prescribing PrEP. US-based PrEP providers (n = 18), most of whom were HIV specialists, were recruited through direct outreach and referral from colleagues and other participants. Individual 90-min semistructured interviews were conducted by phone or in person from September 2014 through February 2015, transcribed, and thematically analyzed. Three attitudinal themes emerged: (1) providers' role is to support patients in making informed decisions, (2) risk behavior while taking PrEP does not fully offset PrEP's protective benefit (i.e., PrEP confers net protection, even with added behavioral risk), and (3) PrEP-related risk compensation is unduly stigmatized within and beyond the healthcare community. Participants were critical of other healthcare providers' negative judgment of patients and reluctance to prescribe PrEP due to anticipated risk compensation. Several providers also acknowledged an evolution in their thinking from initial ambivalence toward greater acceptance of PrEP and PrEP-related behavior change. PrEP providers' insights about risk compensation may help to address unsubstantiated concerns about PrEP-related risk compensation and challenge the acceptability of withholding PrEP on these grounds.

Screening entire healthcare system ECG database: Association of deep terminal negativity of P wave in lead V1 and ECG referral with mortality.

BACKGROUND: Each encounter of asymptomatic individuals with the healthcare system presents an opportunity for improvement of cardiovascular disease (CVD) awareness and sudden cardiac death (SCD) risk assessment. ECG sign deep terminal negativity of the P wave in V1 (DTNPV1) was shown to be associated with an increased risk of SCD in the general population. OBJECTIVE: To evaluate association of DTNPV1 with all-cause mortality and newly diagnosed atrial fibrillation (AFib) in the large tertiary healthcare system patient population. METHODS: Retrospective double cohort study compared two levels of exposure (automatically measured amplitude of P-prime (Pp) in V1): DTNPV1 (Pp from -100µV to -200µV) and ZeroPpV1 (Pp=0). An entire healthcare system (2010-2014) ECG database was screened. Medical records of children and patients with previously diagnosed AFib/atrial flutter (AFl), implanted pacemaker or cardioverter-defibrillator were excluded. DTNPV1 (n=3,413) and ZeroPpV1 (n=3,405) cohorts were matched by age and sex. Primary outcome was all-cause mortality. Secondary outcomes were newly diagnosed AFib/AFl. Median follow-up was 2.5 y. RESULTS: DTNPV1 was associated with all-cause mortality (HR 1.95(1.64-2.31); P<0.0001) and newly diagnosed AFib (HR 1.29(1.04-1.59); P=0.021) after adjustment for CVD, comorbidities, other ECG parameters, medications, and index ECG referral. Index ECG referral by a cardiologist was independently associated with 34% relative risk reduction of mortality (HR 0.66(0.52-0.84); P=0.001), as compared to ECG referral by a non-cardiologist. CONCLUSION: DTNPV1 is independently associated with twice higher risk of all-cause death, as compared to patients without P prime in V1. Life-saving effect of the index ECG referral by a cardiologist requires further study.

Putting PrEP into Practice: Lessons Learned from Early-Adopting U.S. Providers' Firsthand Experiences Providing HIV Pre-Exposure Prophylaxis and Associated Care.

Optimizing access to HIV pre-exposure prophylaxis (PrEP), an evidence-based HIV prevention resource, requires expanding healthcare providers' adoption of PrEP into clinical practice. This qualitative study explored PrEP providers' firsthand experiences relative to six commonly-cited barriers to prescription-financial coverage, implementation logistics, eligibility determination, adherence concerns, side effects, and anticipated behavior change (risk compensation)-as well as their recommendations for training PrEP-inexperienced providers. U.S.-based PrEP providers were recruited via direct outreach and referral from colleagues and other participants (2014-2015). One-on-one interviews were conducted in person or by phone, transcribed, and analyzed. The sample (n = 18) primarily practiced in the Northeastern (67%) or Southern (22%) U.S. Nearly all (94%) were medical doctors (MDs), most of whom self-identified as infectious disease specialists. Prior experience prescribing PrEP ranged from 2 to 325 patients. Overall, providers reported favorable experiences with PrEP implementation and indicated that commonly anticipated problems were minimal or manageable. PrEP was covered via insurance or other programs for most patients; however, pre-authorization requirements, laboratory/service provision costs, and high deductibles sometimes presented challenges. Various models of PrEP care and coordination with other providers were utilized, with several providers highlighting the value of clinical staff support. Eligibility was determined through joint decision-making with patients; CDC guidelines were commonly referenced but not considered absolute. Patient adherence was variable, with particularly strong adherence noted among patients who had actively sought PrEP (self-referred). Providers observed minimal adverse effects or increases in risk behavior. However, they identified several barriers with respect to accessing and engaging PrEP candidates. Providers offered a wide range of suggestions regarding content, strategy, and logistics surrounding PrEP training, highlighting sexual history-taking and sexual minority competence as areas to prioritize. These insights from early-adopting PrEP providers may facilitate adoption of PrEP into clinical practice by PrEP-inexperienced providers, thereby improving access for individuals at risk for HIV.

Electrophysiologic Substrate and Risk of Mortality in Incident Hemodialysis.

The single leading cause of mortality on hemodialysis is sudden cardiac death. Whether measures of electrophysiologic substrate independently associate with mortality is unknown. We examined measures of electrophysiologic substrate in a prospective cohort of 571 patients on incident hemodialysis enrolled in the Predictors of Arrhythmic and Cardiovascular Risk in End Stage Renal Disease Study. A total of 358 participants completed both baseline 5-minute and 12-lead electrocardiogram recordings on a nondialysis day. Measures of electrophysiologic substrate included ventricular late potentials by the signal-averaged electrocardiogram and spatial mean QRS-T angle measured on the averaged beat recorded within a median of 106 days (interquartile range, 78-151 days) from dialysis initiation. The cohort was 59% men, and 73% were black, with a mean±SD age of 55±13 years. Transthoracic echocardiography revealed a mean±SD ejection fraction of 65.5%±12.0% and a mean±SD left ventricular mass index of 66.6±22.3 g/m2.7 During 864.6 person-years of follow-up, 77 patients died; 35 died from cardiovascular causes, of which 15 were sudden cardiac deaths. By Cox regression analysis, QRS-T angle ≥75° significantly associated with increased risk of cardiovascular mortality (hazard ratio, 2.99; 95% confidence interval, 1.31 to 6.82) and sudden cardiac death (hazard ratio, 4.52; 95% confidence interval, 1.17 to 17.40) after multivariable adjustment for demographic, cardiovascular, and dialysis factors. Abnormal signal-averaged electrocardiogram measures did not associate with mortality. In conclusion, spatial QRS-T angle but not abnormal signal-averaged electrocardiogram significantly associates with cardiovascular mortality and sudden cardiac death independent of traditional risk factors in patients starting hemodialysis.

R1 changes in the human placenta at 3 T in response to a maternal oxygen challenge protocol.

Oxygen-enhanced MRI non-invasively monitors placental oxygenation in-vivo. This technique has been demonstrated at 1.5 Tesla (T) in healthy pregnancies. The aim of this study was to investigate whether findings are comparable at 3T. Nine pregnant volunteers underwent MRI at 3T. Scans obtained R1 (1/T1) measures from T1 maps under air, followed by a dynamic series breathing 100% oxygen. A statistically significant negative correlation was found between dR1 and gestation (P = 0.0008, r = -0.90, Pearson correlation test). The effect of the field strength was not significant within regression analysis. Placental Oxygen-Enhanced MRI at 3T gives comparable results to those previously obtained at 1.5T.

Wound care basics for the pharmacist.

Elderly skin is prone to wounds as a result of age-related changes and comorbidities such as diabetes and vascular disease. Wound healing is a complex process that can become compromised, leading to the development of chronic, nonhealing wounds in a frail elderly patient. Pharmacists and other practitioners who care for the elderly should be attuned to the wound-healing process and to the types of medications and wound care products available to aid healing.

Detection of lesions during capsule endoscopy: physician performance is disappointing.

OBJECTIVES: Limited data guide capsule endoscopists on how to view the many images collected in each capsule. The objective of this study was to compare the detection rates of clinically significant findings in different capsule endoscopy reading modes and speeds. METHODS: Seventeen capsule endoscopists with experience from 23 to > 1,000 total capsule procedures read 24 clips, 18 of which were abnormal. Clips were read in two different reading modes utilizing two speeds, including SingleView at 15 at frames per second (f.p.s.), SingleView 25 f.p.s., QuadView 20 f.p.s., and QuadView 30 f.p.s. The main outcome measurements were pathology detection rates correlated with reading mode, lesion type, reader experience, and timing order. RESULTS: SingleView 15, QuadView 20, and QuadView 30 had no significant difference in overall detection rate (45, 47, and 43%, respectively). SingleView 25 had a 26% detection rate, which was significantly lower than SingleView 15 (P = 0.04) and QuadView 20 (P = 0.002). The detection rates of angioectasias, ulcers/erosions, masses/polyps, and blood were 69, 38, 46, and 17%, respectively. Reader experience and timing of interpretation did not significantly impact detection rate. LIMITATIONS: Pathology was present on a few frames. Limited modes and speeds were assessed. Lesion types were not confirmed with surgical or deep enteroscopic methods. A relatively small number of readers provided interpretations. CONCLUSIONS: Overall, the detection rates in this study are lower than previously reported and not influenced by increasing experience. Detection rates are significantly higher when reading in SingleView 15 and QuadView 20 compared with reading in SingleView 25. Increasing viewing speed from QuadView 20 to QuadView 30 appears to have no significant effect on detection. Quality control measures to compare and improve lesion detection rates need further study.

Mutation of a single allele of the cancer susceptibility gene BRCA1 leads to genomic instability in human breast epithelial cells.

Biallelic inactivation of cancer susceptibility gene BRCA1 leads to breast and ovarian carcinogenesis. Paradoxically, BRCA1 deficiency in mice results in early embryonic lethality, and similarly, lack of BRCA1 in human cells is thought to result in cellular lethality in view of BRCA1's essential function. To survive homozygous BRCA1 inactivation during tumorigenesis, precancerous cells must accumulate additional genetic alterations, such as p53 mutations, but this requirement for an extra genetic "hit" contradicts the two-hit theory for the accelerated carcinogenesis associated with familial cancer syndromes. Here, we show that heterozygous BRCA1 inactivation results in genomic instability in nontumorigenic human breast epithelial cells in vitro and in vivo. Using somatic cell gene targeting, we demonstrated that a heterozygous BRCA1 185delAG mutation confers impaired homology-mediated DNA repair and hypersensitivity to genotoxic stress. Heterozygous mutant BRCA1 cell clones also showed a higher degree of gene copy number loss and loss of heterozygosity in SNP array analyses. In BRCA1 heterozygous clones and nontumorigenic breast epithelial tissues from BRCA mutation carriers, FISH revealed elevated genomic instability when compared with their respective controls. Thus, BRCA1 haploinsufficiency may accelerate hereditary breast carcinogenesis by facilitating additional genetic alterations.