Outcomes of Radiofrequency Ablation Compared to Liquid Nitrogen Spray Cryotherapy for the Eradication of Dysplasia in Barrett's Esophagus

INTRODUCTION

Current guidelines recommend endoscopic eradication therapy (EET) for Barrett's esophagus (BE) with dysplasia and intramucosal adenocarcinoma using either radiofrequency ablation (RFA) or liquid nitrogen spray cryotherapy (LNSC). The aims of this multicenter study are to compare the rate and number of treatment sessions of RFA vs. LNSC to achieve CE-D and CE-IM and assess outcomes for those who switched therapy.

METHODS

This is a retrospective cohort study of patients with BE undergoing EET. Demographics, baseline variables, endoscopy details, and histology information were abstracted.

RESULTS

One hundred and sixty-two patients were included in this study with 100 patients in the RFA group and 62 patients in the LNSC group. The rate of CE-D and CE-IM did not differ between the RFA group and LNSC group (81% vs. 71.0%, p = 0.14) and (64% vs. 66%, p = 0.78), respectively. The number of sessions to achieve CE-D and CE-IM was higher with LNSC compared to RFA (4.2 vs. 3.2, p = 0.05) and (4.8 vs. 3.5, p = 0.04), respectively. The likelihood of developing recurrent dysplasia was higher among patients who did not achieve CE-IM (12%) compared to those who did achieve CE-IM (4%), p = 0.04. Similar findings were found in those who switched treatment modalities.

DISCUSSION

EET is highly effective in eradication of Barrett's associated dysplasia and neoplasia. Both RFA and LNSC achieved similar rates of CE-D and CE-IM although LNSC required more sessions. Also, achievement of CE-IM was associated with less recurrence rates of dysplasia.

Post-transplant biliary complications: advances in pathophysiology, diagnosis, and treatment

Liver transplantation (LT) is the only curative therapy in patients with end-stage liver disease. Long-term survival is excellent, yet LT recipients are at risk of significant complications. Biliary complications are an important source of morbidity after LT, with an estimated incidence of 5%-32%. Post-LT biliary complications include strictures (anastomotic and non-anastomotic), bile leaks, stones, and sphincter of Oddi dysfunction. Prompt recognition and management is critical as these complications are associated with mortality rates up to 20% and retransplantation rates up to 13%. This review aims to summarise our current understanding of risk factors, natural history, diagnostic testing, and treatment options for post-transplant biliary complications.

Post-Transplant Biliary Strictures: An Updated Review

Liver transplantation (LT) is the only curative therapy in patients with end-stage liver disease with excellent long-term survival; however, LT recipients are at risk of significant complications. Among these complications are biliary complications with an incidence ranging from 5 to 32% and associated with significant post-LT morbidity and mortality. Prompt recognition and management are critical as these complications have been associated with mortality rates up to 19% and retransplantation rates up to 13%. An important limitation of published studies is that a large proportion does not discriminate between anastomotic strictures and nonanastomotic strictures. This review aims to summarize our current understanding of risk factors and natural history, diagnostic testing, and treatment options for post-LT biliary strictures.

Sarcopenia in Chronic Pancreatitis - Prevalence, Diagnosis, Mechanisms and Potential Therapies

PURPOSE OF REVIEW

To investigate the prevalence, pathogenesis, diagnosis, clinical sequelae, and management of sarcopenia to improve mortality and quality of life in those with Chronic Pancreatitis.

RECENT FINDINGS

Sarcopenia is prevalent in chronic pancreatitis and can significantly affect clinical outcomes. Sarcopenia is prevalent in chronic pancreatitis. While effects on some clinical outcomes is has been shown, there is a significant gap in knowledge regarding effects on outcomes, pathophysiology, and options for management.

Early laboratory values after liver transplantation are associated with anastomotic biliary strictures

Backgrounds/Aims

The aim of this study was to evaluate longitudinal changes of post-liver transplantation (LT) biliary anatomy and to assess the association of increased laboratory values after LT with the development of post-LT anastomotic biliary stricture (ABS).

Methods

Adult deceased donor LT recipients from 2008 and 2019 were evaluated. ABS was defined after blinded review of endoscopic cholangiograms. Controls were patients who underwent LT for hepatocellular carcinoma who did not have any clinical or biochemical concerns for ABS.

Results

Of 534 patients who underwent LT, 57 patients had ABS and 57 patients served as controls. On MRI, ABS patients had a narrower anastomosis (2.47 ± 1.32 mm vs. 3.38 ± 1.05 mm; p < 0.01) and wider bile duct at 1-cm proximal to the anastomosis (6.73 ± 2.45 mm vs. 5.66 ± 1.95 mm; p = 0.01) than controls. Association between labs at day 7 and ABS formation was as follows: aspartate aminotransferase hazard ratio (HR): 1.014; 95% confidence interval (CI): 1.008–1.020, p = 0.001; total bilirubin HR: 1.292, 95% CI: 1.100–1.517, p = 0.002; and conjugated bilirubin HR: 1.467, 95% CI: 1.216–1.768, p = 0.001. Corresponding analysis results for day 28 were alanine aminotransferase HR: 1.004, 95% CI: 1.002–1.006, p = 0.001; alkaline phosphatase HR: 1.005, 95% CI: 1.003–1.007, p = 0.001; total bilirubin HR: 1.233, 95% CI: 1.110–1.369, p = 0.001; and conjugated bilirubin HR: 1.272, 95% CI: 1.126–1.437, p = 0.001.

Conclusions

Elevation of laboratory values early after LT is associated with ABS formation.

Co-existing inflammatory bowel disease and Barrett's esophagus is associated with esophageal dysplasia: a propensity score-matched cohort

Background and study aims  Barrett's esophagus (BE) and inflammatory bowel disease (IBD) predispose to the development of dysplasia and cancer. It is unclear if the inflammatory cascade seen in IBD affects disease progression in BE. We aimed to determine if patients with BE who have co-existing IBD had a higher risk of dysplasia, nodular disease, or longer segments than BE patients without IBD. Patients and methods  This was a multicenter, retrospective propensity score-matched cohort study. We compared rates of dysplasia, nodular disease, and segment length in patients with BE and IBD (cases) to patients with BE who did not have IBD (controls). Controls were 1:1 propensity score matched with controls for age, sex, body mass index (BMI), smoking, and hiatal hernia. Results  A total of 132 patients were included in the IBD + BE group and 132 patients in the BE group. Patients with IBD + BE had higher rates of esophageal dysplasia compared to controls (15.9 % vs. 6.1 % [adjusted odds ratio [OR]: 2.9, 95 % CI: 1.2-6.9]) and more nodules (9.8 % vs. 3.0 % [adjusted OR: 3.5, 95 % CI: 1.1-11.0]). IBD + BE group was also associated with longer BE segments (43.9 % vs. 12.1 % [OR: 5.7, 95 % CI: 3.0-10.6]). Conclusions  Co-existing IBD may increase the risk of dysplasia and esophageal nodules in patients with BE. Our findings may have implications for BE surveillance intervals in IBD patients. Prospective studies are needed to confirm our findings.

Deep learning systems detect dysplasia with human-like accuracy using histopathology and probe-based confocal laser endomicroscopy

Probe-based confocal laser endomicroscopy (pCLE) allows for real-time diagnosis of dysplasia and cancer in Barrett's esophagus (BE) but is limited by low sensitivity. Even the gold standard of histopathology is hindered by poor agreement between pathologists. We deployed deep-learning-based image and video analysis in order to improve diagnostic accuracy of pCLE videos and biopsy images. Blinded experts categorized biopsies and pCLE videos as squamous, non-dysplastic BE, or dysplasia/cancer, and deep learning models were trained to classify the data into these three categories. Biopsy classification was conducted using two distinct approaches-a patch-level model and a whole-slide-image-level model. Gradient-weighted class activation maps (Grad-CAMs) were extracted from pCLE and biopsy models in order to determine tissue structures deemed relevant by the models. 1970 pCLE videos, 897,931 biopsy patches, and 387 whole-slide images were used to train, test, and validate the models. In pCLE analysis, models achieved a high sensitivity for dysplasia (71%) and an overall accuracy of 90% for all classes. For biopsies at the patch level, the model achieved a sensitivity of 72% for dysplasia and an overall accuracy of 90%. The whole-slide-image-level model achieved a sensitivity of 90% for dysplasia and 94% overall accuracy. Grad-CAMs for all models showed activation in medically relevant tissue regions. Our deep learning models achieved high diagnostic accuracy for both pCLE-based and histopathologic diagnosis of esophageal dysplasia and its precursors, similar to human accuracy in prior studies. These machine learning approaches may improve accuracy and efficiency of current screening protocols.

Deep learning systems detect dysplasia with human-like accuracy using histopathology and probe-based confocal laser endomicroscopy

Probe-based confocal laser endomicroscopy (pCLE) allows for real-time diagnosis of dysplasia and cancer in Barrett's esophagus (BE) but is limited by low sensitivity. Even the gold standard of histopathology is hindered by poor agreement between pathologists. We deployed deep-learning-based image and video analysis in order to improve diagnostic accuracy of pCLE videos and biopsy images. Blinded experts categorized biopsies and pCLE videos as squamous, non-dysplastic BE, or dysplasia/cancer, and deep learning models were trained to classify the data into these three categories. Biopsy classification was conducted using two distinct approaches-a patch-level model and a whole-slide-image-level model. Gradient-weighted class activation maps (Grad-CAMs) were extracted from pCLE and biopsy models in order to determine tissue structures deemed relevant by the models. 1970 pCLE videos, 897,931 biopsy patches, and 387 whole-slide images were used to train, test, and validate the models. In pCLE analysis, models achieved a high sensitivity for dysplasia (71%) and an overall accuracy of 90% for all classes. For biopsies at the patch level, the model achieved a sensitivity of 72% for dysplasia and an overall accuracy of 90%. The whole-slide-image-level model achieved a sensitivity of 90% for dysplasia and 94% overall accuracy. Grad-CAMs for all models showed activation in medically relevant tissue regions. Our deep learning models achieved high diagnostic accuracy for both pCLE-based and histopathologic diagnosis of esophageal dysplasia and its precursors, similar to human accuracy in prior studies. These machine learning approaches may improve accuracy and efficiency of current screening protocols.

Semi-Supervised Classification of Noisy, Gigapixel Histology Images

One of the greatest obstacles in the adoption of deep neural networks for new medical applications is that training these models typically require a large amount of manually labeled training samples. In this body of work, we investigate the semi-supervised scenario where one has access to large amounts of unlabeled data and only a few labeled samples. We study the performance of MixMatch and FixMatch-two popular semi-supervised learning methods-on a histology dataset. More specifically, we study these models' impact under a highly noisy and imbalanced setting. The findings here motivate the development of semi-supervised methods to ameliorate problems commonly encountered in medical data applications.

Proton pump inhibitors increase the severity of hepatic encephalopathy in cirrhotic patients

BACKGROUND

Liver cirrhosis is the late stage of hepatic fibrosis and is characterized by portal hypertension that can clinically lead to decompensation in the form of ascites, esophageal/gastric varices or encephalopathy. The most common sequelae associated with liver cirrhosis are neurologic and neuropsychiatric impairments labeled as hepatic encephalopathy (HE). Well established triggers for HE include infection, gastrointestinal bleeding, constipation, and medications. Alterations to the gut microbiome is one of the leading ammonia producers in the body, and therefore may make patients more susceptible to HE.

AIM

To investigate the relationship between the use of proton pump inhibitors (PPIs) and HE in patients with cirrhosis.

METHODS

This is a single center, retrospective analysis. Patients were included in the study with an admitting diagnosis of HE. The degree of HE was determined from subjective and objective portions of hospital admission notes using the West Haven Criteria. The primary outcome of the study was to evaluate the grade of HE in PPI users versus non-users at admission to the hospital and throughout their hospital course. Secondary outcomes included rate of infection, gastrointestinal bleeding within the last 12 mo, mean ammonia level, and model for end-stage liver disease scores at admission.

RESULTS

The HE grade at admission using the West Haven Criteria was 2.3 in the PPI group compared to 1.7 in the PPI nonuser group (P = 0.001). The average length of hospital stay in PPI group was 8.3 d compared to 6.5 d in PPI nonusers (P = 0.046). Twenty-seven (31.8%) patients in the PPI user group required an Intensive Care Unit admission during their hospital course compared to 6 in the PPI nonuser group (16.7%) (P = 0.138). Finally, 10 (11.8%) patients in the PPI group expired during their hospital stay compared to 1 in the PPI nonuser group (2.8%) (P = 0.220).

CONCLUSION

Chronic PPI use in cirrhotic patients is associated with significantly higher average West Haven Criteria for HE compared to patients that do not use PPIs.

Comparing efficacy of lumen-apposing stents to plastic stents in the endoscopic management of mature peripancreatic fluid collections: a single-center experience

Introduction

Mature peripancreatic fluid collection (MPFC) is a known and often challenging consequence of acute pancreatitis and often requires intervention. The most common method accepted is the “step-up approach,” which consists of percutaneous drainage followed, if necessary, by minimally invasive retroperitoneal necrosectomy. Our paper aims to distinguish between plastic stents and lumen-apposing stents in the endoscopic management of MPFC in terms of morbidity, mortality, and haste of fluid collection resolution.

Methods

A retrospective analysis was performed at UMass Memorial Medical Center in patients with a diagnosis of MPFC. Utilizing medical records, clinical data, radiology, as well as endoscopic evidence, patients were differentiated by stent type used (plastic versus lumen-apposing) for the management of the MPFC. The primary outcome of the study was to assess the time to MPFC resolution following the placement of either plastic or lumen-apposing stents (on endoscopic ultrasound or computerized tomography scan) using a multivariate analysis with a logistic regression model.

Results

A total of 54 patients were included in this study from UMass Memorial Medical Center between 2012 and 2015. Twelve (22%) of these patients received lumen-apposing stents and 42 (78%) of these patients received plastic pigtail stents. For the lumen-apposing stent group, the mean interval between stent placement and resolution of MPFC was 57 days as compared to 102 days for plastic pigtail stents (p=0.02). The mean interval for placement/removal of lumen-apposing stents was 48 days as compared to 81 days for plastic pigtail stents (p=0.01). Stent migration was seen in 5 patients (11%) who received a plastic pigtail stent compared to 0 (0%) patients who received a lumen-apposing stent.

Discussion

Our study demonstrates that lumen-apposing stents result in a significant reduction in the interval between stent placement and MPFC resolution as well as the time from stent placement to removal, when compared to plastic pigtail stents, the prior standard-of-care. Our study reached similar conclusions regarding the number of stents placed. However, we did not find a significant difference between the complication rates, specifically peri- and postprocedural bleeding or perforation, between the 2 study groups, as demonstrated in prior papers.

The Morbidity and Mortality of Laparoscopic Appendectomy in Patients with Cirrhosis

INTRODUCTION

The perioperative mortality is significantly higher in patients with cirrhosis undergoing certain surgical procedures. In this study, we examined the inpatient perioperative mortality and morbidities in cirrhotic people who underwent laparoscopic appendectomy.

METHODS

We performed a retrospective analysis using the National Inpatient Sample database for 2010. Inclusion criteria were all race and sex who are 18 years or older. Those who have laparoscopic appendectomy and have a history of liver cirrhosis were assigned to case group. An equal random number of appendectomy-related admissions and those who have no history of liver cirrhosis were selected and placed in the control group. A binary logistic regression statistical test was used to examine the odds ratio for the mortality difference and postoperative complication including pneumonia, urinary tract infection (UTI), surgical site infection, postoperative bleeding. IBM SPSS statistics was used to execute the analysis. A confidence interval of 95% and P value less than .05 were determined to define the statistical significance.

RESULT

A total of 754 appendectomy-related admissions were identified-376 appendectomy-related admissions and history of cirrhosis and 378 admissions with appendectomy and no history of cirrhosis. Control group was not found to be statistically different from the case group when it comes to age, race, and sex. Of 754, 520 were white (73.5%), 334 (44.3%) were men. The mean age was 43.75 years for the case group and 46.68 years for the control group. Comparing cirrhotic with noncirrhotic group, the mean length of stay was 1.1 vs 1.52 days, inpatient mortality was 2 (0.5%) vs 1 (0.3%) (P = .56), pneumonia 8 (2.1%) vs 3 (0.8%) (P = .142), surgical site infection 3 (0.8%) vs 2 (0.5%) (P = .652), UTI 18 (4.8%) vs 12 (3.2%) (P = .26), and postoperative bleeding 3 (0.8%) vs 2 (0.5%) (P = .65).

CONCLUSIONS

Appendectomy-related morbidity and mortality in cirrhotic patients are not different from noncirrhotic patients.

Risks of PEG tube placement in patients with cirrhosis-associated ascites

This study examined the safety of placing percutaneous endoscopic gastrostomy (PEG) tube in people with liver cirrhosis. The target population was further subdivided into people with ascites (case group) and people without ascites (control). We compare the morbidity and the mortality difference of PEG placement in cirrhotic patients with ascites vs cirrhotic patients without ascites. We then examined multiple factors including sex, race, chronic illness including hypertension, congestive heart failure, and others and their influence on the inpatient mortality of all cirrhotic patients who had PEG placement. A total of 38,175 inpatient PEG tube placements were identified. Only 583 patients out of 38,175 had a history of cirrhosis. One hundred seven had ascites and the rest did not. Mean age of the patients was 61.14 years. Patient demography included (65.2%) male and the rest were female, 359 were white (64.4%), 90 black (14.8%), 84 Hispanic (13.7%), 23 Asians (3.3%), 7 Native Americans (0.4%), and 20 others (3.5%). Complications from PEG procedure in cirrhosis with ascites vs non-ascites included bleeding of 4 (0.8%) vs 2 (1.9%) (P=0.35), surgical site infection 2 (0.4%) vs 1 (0.9%) (P=0.51), and urinary tract infection 105 (22.1%) vs 34 (23.8%) (P=0.34), respectively. There was no colonic injury in either group. The total inpatient mortality was 75 out of the 583. Fifty-six (11.8%) were in the ascites group and 19 (17.8%) in the non-ascites group (P=0.097). Factors including ascites, postsurgical bleeding, and surgical site infection did not have influence on the inpatient mortality and there were no statistical differences between the two groups.

Saccharomyces cerevisiae rad51 mutants are defective in DNA damage-associated sister chromatid exchanges but exhibit increased rates of homology-directed translocations

Saccharomyces cerevisiae Rad51 is structurally similar to Escherichia coli RecA. We investigated the role of S. cerevisiae RAD51 in DNA damage-associated unequal sister chromatid exchanges (SCEs), translocations, and inversions. The frequency of these rearrangements was measured by monitoring mitotic recombination between two his3 fragments, his3-Delta5' and his3-Delta3'::HOcs, when positioned on different chromosomes or in tandem and oriented in direct or inverted orientation. Recombination was measured after cells were exposed to chemical agents and radiation and after HO endonuclease digestion at his3-Delta3'::HOcs. Wild-type and rad51 mutant strains showed no difference in the rate of spontaneous SCEs; however, the rate of spontaneous inversions was decreased threefold in the rad51 mutant. The rad51 null mutant was defective in DNA damage-associated SCE when cells were exposed to either radiation or chemical DNA-damaging agents or when HO endonuclease-induced double-strand breaks (DSBs) were directly targeted at his3-Delta3'::HOcs. The defect in DNA damage-associated SCEs in rad51 mutants correlated with an eightfold higher spontaneous level of directed translocations in diploid strains and with a higher level of radiation-associated translocations. We suggest that S. cerevisiae RAD51 facilitates genomic stability by reducing nonreciprocal translocations generated by RAD51-independent break-induced replication (BIR) mechanisms.

Radiosensitive and mitotic recombination phenotypes of the Saccharomyces cerevisiae dun1 mutant defective in DNA damage-inducible gene expression

The biological significance of DNA damage-induced gene expression in conferring resistance to DNA-damaging agents is unclear. We investigated the role of DUN1-mediated, DNA damage-inducible gene expression in conferring radiation resistance in Saccharomyces cerevisiae. The DUN1 gene was assigned to the RAD3 epistasis group by quantitating the radiation sensitivities of dun1, rad52, rad1, rad9, rad18 single and double mutants, and of the dun1 rad9 rad52 triple mutant. The dun1 and rad52 single mutants were similar in terms of UV sensitivities; however, the dun1 rad52 double mutant exhibited a synergistic decrease in UV resistance. Both spontaneous intrachromosomal and heteroallelic gene conversion events between two ade2 alleles were enhanced in dun1 mutants, compared to DUN1 strains, and elevated recombination was dependent on RAD52 but not RAD1 gene function. Spontaneous sister chromatid exchange (SCE), as monitored between truncated his3 fragments, was not enhanced in dun1 mutants, but UV-induced SCE and heteroallelic recombination were enhanced. Ionizing radiation and methyl methanesulfonate (MMS)-induced DNA damage did not exhibit greater recombinogenicity in the dun1 mutant compared to the DUN1 strain. We suggest that one function of DUN1-mediated DNA damage-induced gene expression is to channel the repair of UV damage into a nonrecombinogenic repair pathway.

Expression of Saccharomyces cerevisiae MATa and MAT alpha enhances the HO endonuclease-stimulation of chromosomal rearrangements directed by his3 recombinational substrates

Radiation resistance in Saccharomyces cerevisiae is greater in a/alpha diploids than in aa or alpha alpha diploids, and higher levels of radiation resistance correlates with more mitotic recombination. Specifically, we investigated whether the stimulation of directed translocations, inversions, and unequal sister chromatid exchanges (SCEs) by HO endonuclease-induced double-strand breaks (DSBs) is enhanced in a/alpha cells. These rearrangements result from mitotic recombination between two truncated his3 genes, his3-delta 5' and his3-delta 3'::HOcs, positioned on non-homologous chromosomes or positioned in juxtaposition on the same chromosome in inverted or direct orientation. Mitotic recombination was initiated by HO endonuclease-induced DSBs at the HO cut site (HOcs) located at his3-delta 3'::HOcs, and His+ recombinants were selected. In MATa-inc haploid strains, which do not switch mating-type, the DSB reduced viability, relative to undamaged cells, and increases the frequency of His+ recombinants containing translocations to 2.4 x 10(-4) (seven-fold), SCEs to 5.4 x 10(-4) (five-fold), and inversions to 1.8 x 10(-3) (six-fold). Compared to a haploids, DSB-stimulated frequencies in a/alpha haploids were three-fold higher for translocations, two-fold higher for SCEs, and ten-fold higher for inversions; however DSB-induced lethality was greater in a/alpha haploids. Compared to aa diploids, DSB-stimulated frequencies of translocations and viability after chromosome cleavage were greater in a/alpha diploids. We suggest that heterozygosity at MAT may elevate the frequency of DSB-initiated reciprocal exchange events in both haploid and diploid cells, but may only increase viability after chromosome cleavage in diploid cells.

A role for RAD51 in the generation of immunoglobulin gene diversity in rabbits

Ig VDJ genes in rabbit somatically diversify by both hyperpointmutation and gene conversion. To elucidate the mechanism of gene conversion of IgH genes, we cloned a rabbit homologue of RAD51, a gene involved in gene conversion in Saccharomyces cerevisiae (yeast), and tested whether it could complement a yeast rad51 mutant deficient in recombination repair. We found that rabbit RAD51 partially complemented the defect in switching mating types by gene conversion as well as in DNA double-strand break repair after gamma-irradiation. Further, by Western blot analysis, we found that levels of Rad51 were higher in appendix-derived B lymphocytes of 6-wk-old rabbits, a time at which IgH genes diversify by somatic gene conversion. We suggest that Rad51 is involved in somatic gene conversion of rabbit Ig genes.

The Saccharomyces cerevisiae RAD9 checkpoint reduces the DNA damage-associated stimulation of directed translocations

Genetic instability in the Saccharomyces cerevisiae rad9 mutant correlates with failure to arrest the cell cycle in response to DNA damage. We quantitated the DNA damage-associated stimulation of directed translocations in RAD9+ and rad9 mutants. Directed translocations were generated by selecting for His+ prototrophs that result from homologous, mitotic recombination between two truncated his3 genes, GAL1::his3-delta5' and trp1::his3-delta3'::HOcs. Compared to RAD9+ strains, the rad9 mutant exhibits a 5-fold higher rate of spontaneous, mitotic recombination and a greater than 10-fold increase in the number of UV- and X-ray-stimulated His+ recombinants that contain translocations. The higher level of recombination in rad9 mutants correlated with the appearance of nonreciprocal translocations and additional karyotypic changes, indicating that genomic instability also occurred among non-his3 sequences. Both enhanced spontaneous recombination and DNA damage-associated recombination are dependent on RAD1, a gene involved in DNA excision repair. The hyperrecombinational phenotype of the rad9 mutant was correlated with a deficiency in cell cycle arrest at the G2-M checkpoint by demonstrating that if rad9 mutants were arrested in G2 before irradiation, the numbers both of UV- and gamma-ray-stimulated recombinants were reduced. The importance of G2 arrest in DNA damage-induced sister chromatid exchange (SCE) was evident by a 10-fold reduction in HO endonuclease-induced SCE and no detectable X-ray stimulation of SCE in a rad9 mutant. We suggest that one mechanism by which the RAD9-mediated G2-M checkpoint may reduce the frequency of DNA damage-induced translocations is by channeling the repair of double-strand breaks into SCE.

Genotoxicity of aflatoxin B1: evidence for a recombination-mediated mechanism in Saccharomyces cerevisiae

The potent liver carcinogen aflatoxin B1 (AFB1) is metabolized by cytochrome P450 to the mutagenic epoxide. We have observed that activated AFB1 also strongly induced mitotic recombination in the yeast Saccharomyces cerevisiae. To compare the recombinogenicity of AFB1 to its mutagenicity, three metabolically competent S. cerevisiae strains have been constructed. The frequencies of induced recombinants resulting from gene conversion or chromosomal translocations were determined by different prototrophic selections using two strains, whereas the inducibility of forward mutations was determined by the frequency of drug resistance in the third strain. Human cytochrome P4501A1- (CYP1A) and NADPH-cytochrome P450-oxidoreductase cDNAs were expressed in the strains to ensure intracellular metabolism to the epoxide. Exposure of the strains to AFB1 resulted in a 139- and 24-fold increase in the translocation and gene conversion frequencies, respectively, whereas the mutation frequency was increased only 3-fold. In contrast, benzo[a]pyrene-7,8-dihydrodiol and ethyl methanesulfonate induced mutation and mitotic recombination to similar degrees. We conclude that AFB1 exerted a strong recombinogenic, but only a weak mutagenic, effect. The recombinogenicity of AFB1 in yeast may indicate a mechanism for the high proportion of loss of heterozygosity that has been detected in AFB1-related human liver cancers.

Mating type regulates the radiation-associated stimulation of reciprocal translocation events in Saccharomyces cerevisiae

Both ultraviolet (UV) and ionizing radiation were observed to stimulate mitotic, ectopic recombination between his3 recombinational substrates, generating reciprocal translocations in Saccharomyces cerevisiae (yeast). The stimulation was greatest in diploid strains competent for sporulation and depends upon both the ploidy of the strain and heterozygosity at the MATlocus. The difference in levels of stimulation between MATa/MAT alpha diploid and MAT alpha haploid strains increases when cells are exposed to higher levels of UV radiation (sevenfold at 150 J/m2), whereas when cells are exposed to higher levels of ionizing radiation (23.4 krad), only a twofold difference is observed. When the MAT alpha gene was introduced by DNA transformation into a MATa/mat alpha::LEU2+ diploid, the levels of radiation-induced ectopic recombination approach those obtained in a strain that is heterozygous at MAT. Conversely, when the MATa gene was introduced by DNA transformation into a MAT alpha haploid, no enhanced stimulation of ectopic recombination was observed when cells were irradiated with ionizing radiation but a threefold enhancement was observed when cells were irradiated with UV. The increase in radiation-stimulated ectopic recombination resulting from heterozygosity at MAT correlated with greater spontaneous ectopic recombination and higher levels of viability after irradiation. We suggest that MAT functions that have been previously shown to control the level of mitotic, allelic recombination (homolog recombination) also control the level of mitotic, radiation-stimulated ectopic recombination between short dispersed repetitive sequences on non-homologous chromosomes.

DNA-damaging agents stimulate the formation of directed reciprocal translocations in Saccharomyces cerevisiae

DNA-damaging agents can stimulate the formation of directed reciprocal translocations in strains of Saccharomyces cerevisiae containing his3 recombinational substrates to generate chromosomal rearrangements. Such agents were compared with those that can stimulate sister-chromatid recombination. We show that chemicals and environmental agents that produce a variety of DNA lesions, including bulky adduct, thymidine dimers, interstrand cross-links, double-strand breaks alkylated bases, can stimulate recombination to yield reciprocal translocations. Of the agents tested, only the alkylating agents methyl methanesulfonate (MMS) and N-methyl-N'-nitro-N-nitrosoguanidine (MNNG), and a bifunctional agent that causes bulky DNA adducts, 4-nitroquinoline-N-oxide (4-NQO), significantly stimulate sister-chromatid recombination in our assay. Factors that contribute to the stimulation of interchromosomal recombination include strain genetic background and ploidy.