Fungal Depletion Bolsters Anti-Tumor Immune Response Elicited by Anti-PD1 Alone and in Combination with Radiation Therapy

PURPOSE/OBJECTIVE(S)

Pembrolizumab in combination with chemotherapy has become the standard of care treatment for both metastatic and early-stage triple negative breast cancer (TNBC). Clinical trials are currently underway investigating the use of pembrolizumab with radiation in the neoadjuvant setting in early TNBC. Several groups have described a link between the microbiome and the efficacy of chemotherapy and anti-PD1 immune checkpoint inhibitors (ICIs) in preclinical models. Recent work from our lab has shown that targeting commensal fungi in the microbiome enhances the radiation induced antitumor immune response. Therefore, we hypothesized that fungal depletion might positively impact anti-PD1 therapy and combination treatment with anti-PD1 and radiation therapy (RT).

MATERIALS/METHODS

This study utilized an orthotopic syngeneic breast tumor model in which the syngeneic cell line E0771 was injected into the mammary fat pad of female C57BL/6 mice. Tumor-bearing mice were then treated with and without the antifungal fluconazole, anti-PD1, and radiation (16 Gy single fraction) using the X-RAD SmART platform with CT guidance. Tumor volumes were compared using 2-way ANOVA and survival curves analyzed using log rank. In a separate set of experiments, tumor-infiltrating immune cells were isolated and analyzed by high-dimensional multiplex flow cytometry.

RESULTS

We found that fungal depletion with fluconazole prior to treatment with anti-PD1 reduced the tumor volume and significantly improved survival in comparison to those treated with anti-PD1 alone (P = 0.0016). To identify what changes in the tumor immune microenvironment is driving this increased anti-tumor response, we performed flow cytometry on immune cells isolated from the tumors. We found that the use of fluconazole prior to anti-PD1 treatment reduced the proportion of CD11b+F4/80+ tumor-associated macrophages (TAMs) (P = 0.01) and increased tumor infiltrating cytotoxic T cell population (P = 0.04) when compared with the use of anti-PD1 alone. We also evaluated the effect of fungal depletion on combination therapy with RT and anti-PD1. Strikingly, we found that mice depleted of fungi with fluconazole prior to radiation and anti-PD1 therapy, have decreased tumor burden and significantly increased survival when compared to their fungally-intact counterparts (P = 0.043).

CONCLUSION

Our data indicates that the depletion of the gut fungal populations induces an increased antitumor response following anti-PD1 alone and in combination with radiation. This increased antitumor immune response is associated with an increase in the cytotoxic CD8+ T cell compartment with concomitant decrease in immunosuppressive tumor associated macrophages.

Variation in Left Anterior Descending Coronary Artery Accumulated Dose Estimates during Breast Cancer Radiotherapy

PURPOSE/OBJECTIVE(S)

Left anterior descending (LAD) coronary artery RT dose has been associated with the risk of coronary ischemic events in patients with breast cancer treated with radiotherapy (RT). However, consensus dose constraints commonly utilize mean heart dose, which has been shown to be an inadequate surrogate for LAD dose. Given the LAD is adjacent to the steep dose gradients of the left breast/chest wall, we hypothesize that variations in patient positioning or depth of breath hold may contribute to significant deviations in daily LAD dose exposure compared to predicted. Our objective was to investigate variations in accumulated LAD dose in patients with left-sided breast cancer treated with RT.

MATERIALS/METHODS

Retrospective analysis of 10 consecutive patients with left-sided breast cancer treated between 2019-2022 with volumetric modulated arc therapy (VMAT) in the supine position. RT was delivered using daily cone beam computed tomography (CBCT) image guidance with deep-inspiratory breath hold technique and an optical surface monitoring system. Daily CBCT scans were individually registered to each planning CT based on daily positional changes. The LAD was manually segmented and transformed to each CBCT. Daily fractional dose was calculated (mean, volume receiving 15 Gy [V15 Gy], V30 Gy, and max) and summed to produce an accumulated dose which was compared to the predicted dose. Significant deviations in accumulated dose were defined as at least ±15% from predicted.

RESULTS

The RT targets included breast/chest wall only (n = 1), supraclavicular nodes (n = 8), and/or internal mammary chain (n = 5). All plans were prescribed to 50 Gy in 25 fractions. The median predicted mean heart dose was 5.1 Gy. Overall, there were no significant differences between the median predicted vs. accumulated LAD doses: mean 10.4 vs. 10.2 Gy, V15 Gy 21% vs. 25%, V30 Gy 0% vs. 1%, max 24.5 vs 25.7 Gy (all p>.05). However, there was a subset of patients (n = 5, 50%) with significant deviations in accumulated vs. predicted dose (at least ±15%). For LAD mean, n = 2 had higher accumulated vs. predicted doses (16.6 vs. 11.6 Gy; 14.6 vs. 12.8 Gy), while n = 3 had lower (21.8 vs. 26.5 Gy; 2.7 vs. 4.2 Gy; 13.9 vs. 16.5 Gy). For LAD V15 Gy, n = 3 had higher accumulated vs. predicted doses (43 vs. 35%; 51% vs. 25%; 14% vs. 9%), while n = 2 had lower (49% vs. 61%; 37% vs. 56%). For LAD V30 Gy, n = 4 had higher accumulated vs. predicted doses (12% vs. 5%; 5% vs. 0%; 4% vs. 0%; 4% vs. 2%), while n = 1 had lower (32% vs. 44%).

CONCLUSION

Daily setup differences, including the extent of inspiratory breath hold, may contribute to deviations in accumulated LAD dose (more than 15% of predicted) in approximately half of patients and were more pronounced in V15 and V30 Gy metrics. The potential for significant LAD dose uncertainty in a clinically meaningful subset of patients should be recognized and warrants further analysis in an expanded cohort.

Non-Redundant Mechanisms of Immune Resistance to Radiotherapy Converge on Innate Immunity

PURPOSE/OBJECTIVE(S)

Despite evidence of preclinical synergy between radiotherapy (RT) and immune checkpoint blockade (ICB), randomized trials of RT/ICB have demonstrated limited benefit in solid tumors. We performed single-cell RNA sequencing (scRNA-seq) and CITE-seq (cellular indexing of transcriptomes and epitopes) to address the discordance between preclinical and clinical data. We hypothesized that multiple orthogonal inhibitory immune pathways restrain the local and systemic efficacy of RT beyond T-cell oriented immune checkpoints.

MATERIALS/METHODS

We used the EO771 syngeneic murine model of breast cancer to characterize the immune tumor microenvironment following RT with or without ICB. RT (16 Gy x 1) was delivered using the X-RAD SmART platform with CT image guidance. Neutralizing antibodies (anti-PD-1/Ly6G/Gr-1/CD47) were delivered by intraperitoneal injections. scRNA-seq analysis were performed by Seurat and BBrowser (BioTuring).

RESULTS

We found that adaptive ICB (anti-PD-1) reprogrammed the immune response to RT by promoting an M1-like interferon-primed state (ISG15, CXCL10) in tumor associated macrophages (TAMs) and by increasing the late recruitment of intratumoral neutrophils. Given that neutrophils may drive resistance to RT in other models, we evaluated the effect of intratumoral neutrophil depletion using anti-Ly6G or anti-Gr-1 on the antitumor efficacy of RT/ICB. Both neutrophil depletion strategies led to enhanced tumor control and improved survival in advanced EO771 tumors compared to RT/ICB alone (P<0.001). In parallel to this approach, we found that TAMs upregulated several innate immune checkpoints including SIRPα in response to RT. Disruption of the SIRPα-CD47 interaction by anti-CD47 antibodies similarly enhanced the antitumor efficacy of RT/ICB by improving tumor control and survival (P<0.001). Using scRNA-seq and unbiased clustering, we found that anti-CD47 eliminated an entire cluster of chronically inflamed TAMs, characterized by pro-inflammatory markers (IL1A, NOS2) and chemokines (CCL3, CXCL1/2/3). Anti-CD47 also reduced intratumoral neutrophils by eliminating a cluster of pathologically activated neutrophils, termed myeloid-derived suppressor cells (PMN-MDSCs) that expressed several markers of ferroptosis (TFRC, PTGS2, SLC3A2). Consistent with the potent immunosuppressive capacity of PMN-MDSCs, we found that anti-CD47 increased tumor-infiltrating lymphocytes including central memory TCF7+ T cells and CD19+ B cells. Lastly, by inference and analysis of cell-cell communication (CellChat), we found that anti-CD47 strengthened the interactions between TAMs and CD8+ T cells compared to RT/ICB alone.

CONCLUSION

Our data collectively indicate that resistance to RT/ICB in the EO771 model Is driven by innate immune cells including neutrophils and chronically inflamed TAMs. Targeted disruption of the CD47-SIRPα axis is a promising approach to overcoming immune resistance by reprogramming TAMs and eliminating PMN-MDSCs.

Association of Left Anterior Descending Coronary Artery Calcium Progression and Radiation Dose with Major Adverse Cardiac Events in Breast Cancer

PURPOSE/OBJECTIVE(S)

Coronary artery calcium (CAC) is associated with increased risk of major adverse cardiac events (MACE). Accelerated CAC progression has been observed in patients with breast cancer after radiotherapy (RT) and there is a relationship between left anterior descending (LAD) coronary artery RT dose and the risk of coronary events. However, there is lack of consensus on LAD dose constraints for breast RT and limited data on the extent and impact of CAC progression. Our objective was to evaluate the association of LAD dose exposure and CAC progression with the risk of MACE in patients with breast cancer following RT.

MATERIALS/METHODS

Retrospective analysis of 181 patients with breast cancer treated with RT between 2008 and 2019. CAC was manually measured on RT planning and follow-up CTs (with at least one-year interval) using the Agatston method. Coronary arteries were segmented using a deep learning-based automated algorithm and dosimetric parameters collected. MACE cumulative incidence was estimated, and Fine and Gray regressions performed, accounting for non-cardiac death as a competing risk.

RESULTS

The median follow-up following RT was 70 months (interquartile range [IQR], 53-86). The median age was 63 years (IQR, 53-72), 43% had hypertension, 40% hyperlipidemia, 8% coronary heart disease (CHD). Most had pathologic stage I-II disease (76%). RT was targeted to breast/chest wall only in 60% and included regional nodes in 40% (internal mammary chain in 4%). The most common dose/fractionation was 48-50.4 Gy/25-28 fractions (67%) and 42.6-42.7 Gy/16 fractions (30%). At the time of RT, 68 (38%) had at least moderate CAC burden (CAC >100; statin-therapy indicated), but only 29 (43%) were on statin therapy. At a median interval of 44 months (IQR, 26-63), 55% (n = 84) had CAC progression, with a median increase of 52%/year (IQR, 18-193). The median time to MACE was 68 months (IQR, 53-85), with a 5-year cumulative incidence of 7.3% (15 MACE overall). Accounting for age and CHD, there was an increased risk of MACE with LAD CAC progression (subdistribution hazard ratio [SHR] 1.02/10 CAC points; 95% confidence interval [CI] 1.01 = 1.03; p = .007) and the volume of LAD receiving 15 Gy (LAD V15 Gy; SHR 1.03/%; 95% CI, 1.01-1.06; p = .004). There was no association between mean heart dose, chemotherapy, or Her2 therapy exposure and MACE (p>.05).

CONCLUSION

LAD CAC progression and LAD V15 Gy dose exposure were associated with an increased risk of MACE following RT. Accelerated CAC progression was commonly observed, however most patients were under-optimized for cardiovascular (CV) risk, with less than half of statin-eligible patients with at least moderate CAC burden on statin therapy. Together, these data support more aggressive cardiac risk mitigation approaches, including guidelines-based CV risk factor modification and optimized sparing of LAD radiation dose.

Reshaping Macrophage Polarization Potential Enhances Antitumor Immune Response to Radiation Therapy

PURPOSE/OBJECTIVE(S)

Although radiation therapy (RT) remains a cornerstone in the treatment of breast cancer, many trials combining RT with immune checkpoint blockade (ICB) have failed to demonstrate benefit in solid tumors including breast cancer. Maximal efficacy of RT relies on the generation of antitumor immunity following treatment which largely consists of cytotoxic T cells and macrophages. Broad depletion of macrophages modestly enhances tumor responses to RT suggesting that they can shape RT-induced antitumor immunity. Although IL4 signaling through GATA-3 is known to polarize T cells into the protumor Th2 phenotype, such central drivers of macrophage polarization are not well established. Given that macrophages abundantly express IL4 receptor, we hypothesized that GATA-3 may direct the transition of macrophages to M2/alternative phase and that genetic ablation of GATA-3 in macrophages can enhance antitumor immunity by arresting macrophage transition to an M2-like pro-tumor state.

MATERIALS/METHODS

We generated a macrophage specific GATA-3 KO mouse model (mG3KO) driven by the LysM-Cre promoter. Using a syngeneic orthotopic murine model of breast cancer (EO771), we evaluated the differential effect of RT (16Gy x 1) in WT and mG3KO mice. Multiparametric flow cytometry was performed to investigate the immune changes within the tumor microenvironment on day 3, day 5 and day 10 after RT. T cell depletion was performed using antibodies to CD4 and CD8 by intraperitoneal injections to understand the role of adaptive immunity in the response to RT in WT and mG3KO mice.

RESULTS

We found that mG3KO mice bearing advanced EO771 tumors demonstrated significantly improved tumor regression compared to WT mice (p<0.001), which translated to increased overall survival. In vitro characterization of bone-marrow derived macrophages from mG3KO and WT mice suggest that macrophages with ablated GATA-3 expressed increased levels of iNOS and decreased levels of Arginase (Arg-1), consistent with an M1-like phenotype. Immune profiling of the tumors also revealed that mGATA-3 KO animals have significant enrichment of CD8+ T cells in the tumor milieu post RT and these CD8+ T cells express higher amounts of interferon gamma (p<0.001) and Granzyme B (p<0.0015) than their WT counterparts. Using neutralizing antibodies to deplete CD8+ T cells, we show that anti-tumor effects in the mG3KO mice were abolished, suggesting that mG3KO macrophages impact survival, at least, in part by enhancing cytotoxic CD8+T cells. Studies are currently ongoing to reveal the detailed mechanism of GATA-3 ablation in improving the efficacy of RT.

CONCLUSION

Our data indicates that GATA-3 is a central regulator of macrophage polarization in response to RT. Further, directed ablation of GATA-3 appears to drive macrophages towards an M1-like phenotype, which enhances T cell recruitment to irradiated tumors. These data suggest that the antitumor efficacy of RT can be prolonged by targeting GATA-3-dependent signaling within myeloid cells.

Magnetic resonance tumour regression grade and pathological correlates in patients with rectal cancer

BACKGROUND

Evidence to support the specific use of magnetic resonance tumour regression grade (mrTRG) is inadequate. The aim of this study was to investigate the pathological characteristics of mrTRG after chemoradiotherapy (CRT) for rectal cancer and the implications for surgery.

METHODS

Patients undergoing long-course CRT (45-50 Gy plus a booster dose of 4-6 Gy) for mid or low rectal cancer (cT3-4 or cN+ without metastasis) between 2011 and 2015 who had post-CRT rectal MRI before surgery were included retrospectively. Three board-certified experienced radiologists assessed mrTRG. mrTRG was correlated with pathological tumour regression grade (pTRG), ypT and ypN. In a subgroup of patients with mrTRG1-2 and no tumour spread (such as nodal metastasis) on MRI, the projected rate of completion total mesorectal excision (TME) if they underwent transanal excision (TAE) and had a ypT status of ypT2 or higher was estimated, and recurrence-free survival was calculated according to the operation (TME or TAE) that patients had actually received.

RESULTS

Some 439 patients (290 men and 149 women of mean(s.d.) age 62·2(11·4) years) were analysed. The accuracy of mrTRG1 for predicting pTRG1 was 61 per cent (40 of 66), and that for ypT1 or less was 74 per cent (49 of 66). For mrTRG2, these values were 22·3 per cent (25 of 112) and 36·6 per cent (41 of 112) respectively. Patients with mrTRG1 and mrTRG2 without tumour spread were ypN+ in 3 per cent (1 of 29) and 16 per cent (8 of 50) respectively. Assuming mrTRG1 or mrTRG1-2 with no tumour spread on post-CRT MRI as the criteria for TAE, the projected completion TME rate was 26 per cent (11 of 43) and 41·0 per cent (41 of 100) respectively. For the 100 patients with mrTRG1-2 and no tumour spread, recurrence-free survival did not differ significantly between TME (79 patients) and TAE (21) (adjusted hazard ratio 1·86, 95 per cent c.i. 0·42 to 8·18).

CONCLUSION

Patients with mrTRG1 without tumour spread may be suitable for TAE.

Anatomical relationship of maxillary posterior teeth with the sinus floor and buccal cortex

This study aimed to evaluate the possibility of root fenestration or oroantral communication by evaluating the distance from root apex to the sinus floor and buccal cortex in maxillary posterior teeth using cone-beam computed tomography (CBCT) images. The study included 2182 roots of the maxillary posterior teeth from 219 patients after reviewing CBCT images of 462 patients according to the location of roots by two endodontists. The distances from each root apex to the maxillary sinus floor and buccal and palatal cortices were evaluated according to sex and age, and the mean values were compared by one-way analysis of variance and Mann-Whitney U-test. The distance between root apex and maxillary sinus floor was the greatest in maxillary first premolars and shortest in the mesio-buccal roots of maxillary second molars. The distances from root apex to the buccal and palatal cortical bones were significantly greater in male patients than those in female patients (P < 0·05). The palatal roots of maxillary first molars exhibited the highest incidence as well as the greatest mean length (1·96 mm) of protrusion into the maxillary sinus. The distance from root apex to the sinus floor was found to increase with age, except in case of maxillary second premolars. Understanding the relationship of maxillary posterior teeth with the sinus floor and buccal cortex could provide clinicians valuable information to help reduce iatrogenic damage.

Measurement of microresonator frequency comb coherence by spectral interferometry

We experimentally investigate the spectral coherence of microresonator optical frequency combs. Specifically, we use a spectral interference method, typically used in the context of supercontinuum generation, to explore the variation of the magnitude of the complex degree of first-order coherence across the full comb bandwidth. We measure the coherence of two different frequency combs and observe wholly different coherence characteristics. In particular, we find that the observed dynamical regimes are similar to the stable and unstable modulation instability regimes reported in previous theoretical studies. Results from numerical simulations are found to be in good agreement with experimental observations. In addition to demonstrating a new technique to assess comb stability, our results provide strong experimental support for previous theoretical analyses.

Protective Informal Social Control of Child Maltreatment and Child Abuse Injury in Seoul

Previous findings on the relationship between neighborhood informal social control and child abuse have been mixed. We implemented a scale created by Emery, Trung, and Wu to study protective informal social control of child maltreatment (ISC_CM) by neighbors in a three-stage random cluster sample of 541 families in Seoul, South Korea. Random-effects regression models found that protective ISC_CM significantly moderated the relationship between very severe abuse and child injuries. Very severe abuse was associated with fewer injuries when levels of protective ISC_CM were higher. Implications are discussed.

Strong Brillouin suppression in a passive fiber ring resonator

We demonstrate the passive suppression of stimulated Brillouin scattering in meter-length fiber-ring cavities through careful control of the fiber length. Experimentally we are able to demonstrate an over sixty-times increase in the Brillouin threshold of a 0.4 m fiber ring. This very simple suppression technique greatly simplifies the design of optical parametric devices based on fiber-ring cavities.

The kinesinlike protein Subito contributes to central spindle assembly and organization of the meiotic spindle in Drosophila oocytes

In the oocytes of many species, bipolar spindles form in the absence of centrosomes. Drosophila melanogaster oocyte chromosomes have a major role in nucleating microtubules, which precedes the bundling and assembly of these microtubules into a bipolar spindle. Here we present evidence that a region similar to the anaphase central spindle functions to organize acentrosomal spindles. Subito mutants are characterized by the formation of tripolar or monopolar spindles and nondisjunction of homologous chromosomes at meiosis I. Subito encodes a kinesinlike protein and associates with the meiotic central spindle, consistent with its classification in the Kinesin 6/MKLP1 family. This class of proteins is known to be required for cytokinesis, but our results suggest a new function in spindle formation. The meiotic central spindle appears during prometaphase and includes passenger complex proteins such as AurB and Incenp. Unlike mitotic cells, the passenger proteins do not associate with centromeres before anaphase. In the absence of Subito, central spindle formation is defective and AurB and Incenp fail to properly localize. We propose that Subito is required for establishing and/or maintaining the central spindle in Drosophila oocytes, and this substitutes for the role of centrosomes in organizing the bipolar spindle.

Increased risk of peristomal wound infection after percutaneous endoscopic gastrostomy in patients with diabetes mellitus

BACKGROUND

Results of prospective studies on the effect of prophylactic antibiotics before percutaneous endoscopic gastrostomy are conflicting. Factors for increased risk of peristomal wound infection have not been clearly identified.

AIM

To evaluate the incidence of complications of percutaneous endoscopic gastrostomy and to determine the predictors of wound infection.

PATIENTS AND METHODS

Percutaneous endoscopic gastrostomy was performed on 134 patients in different disease groups between January 1996 and June 2000. Medical records were carefully reviewed for demographic data, indications for percutaneous endoscopic gastrostomy, use of prophylactic antibiotics, complications and comorbid conditions predisposing to wound infection.

RESULTS

Of 134 patients, 22 (16.4%) developed complications after percutaneous endoscopic gastrostomy Wound infection, the most common complication, occurred in 19 patients (14.2%) and Pseudomonas aeruginosa was the most frequently isolated microorganism. In univariate analysis, non-malignant disease and diabetes mellitus were significantly associated with peristomal wound infection after percutaneous endoscopic gastrostomy. In multivariate analysis, only diabetes mellitus was an independent risk factor for the development of peristomal wound infection after percutaneous endoscopic gastrostomy (p = 0.035)

CONCLUSIONS

Patients with diabetes mellitus have a higher risk of peristomal wound infection after percutaneous endoscopic gastrostomy.

Separation of PCBs and PAHs in sediment samples using silica gel fractionation chromatography

The aim of this research is to develop a silica gel fractionation procedure for sediment sample extracts, which separates polycyclic aromatic hydrocarbons (PAHs) and polychlorinated biphenyls (PCBs) into two groups, while simultaneously eliminating most interfering substances for subsequent instrumental analysis. This is achieved by optimizing the fraction cut-off volume of eluting solvent and the deactivation level of the silica gel. Using fully activated silica gel and cutting off PCB collection after passing 60-65 ml eluting solvent (pentane or hexane) through the column resulted in satisfactory separation of PCBs and PAHs. This procedure tends to have a higher reliability for PCBs and PAHs with higher molecular mass. This approach deviates only slightly from the standard methods of the USEPA, and it is less expensive due to reduced sample pre-treatment time and solvent consumption.

mei-41 is required for precocious anaphase in Drosophila females

This paper reports on a new role for mei-41 in cell cycle control during meiosis. This function is revealed by the requirement of mei-41 for the precocious anaphase observed in crossover-defective mutants. Normally in Drosophila oocytes, tension on the meiotic spindle causes a metaphase I arrest. This tension results because crossovers, and the resulting chiasmata, hold homologs together that are being pulled by kinetochore microtobules toward opposite spindle poles. In the absence of tension, such as in a recombination-defective mutant, metaphase arrest is not observed and meiosis proceeds through the two divisions. Here we show that in some recombination-defective mutants, the precocious anaphase requires the mei-41 gene product. For example, metaphase arrest is not observed in mei-218 mutants because of the severe reduction in crossing over. In mei-41 mei-218 double mutants, however, metaphase arrest was restored. The effect of mei-41 is dependent on double-strand break formation. Thus, in mutants that fail to initiate meiotic recombination the absence of mei-41 has no effect.

Two genes required for meiotic recombination in Drosophila are expressed from a dicistronic message

We have isolated two alleles of a previously unidentified meiotic recombination gene, mei-217. Genetic analysis of these mutants shows that mei-217 is a typical "precondition" gene. The phenotypes of the mutants are meiosis specific. The strongest allele has <10% of the normal level of crossing over, and the residual events are distributed abnormally. We have used double mutant analysis to position mei-217 in the meiotic recombination pathway. In general, mutations causing defects in the initiation of meiotic recombination are epistatic to mutations in mei-41 and spnB. These two mutations, however, are epistatic to mei-217, suggesting that recombination is initiated normally in mei-217 mutants. It is likely that mei-217 mutants are able to make Holliday junction intermediates but are defective in the production of crossovers. These phenotypes are most similar to mutants of the mei-218 gene. This is striking because mei-217 and mei-218 are part of the same transcription unit and are most likely produced from a dicistronic message.

Identification of novel Drosophila meiotic genes recovered in a P-element screen

The segregation of homologous chromosomes from one another is the essence of meiosis. In many organisms, accurate segregation is ensured by the formation of chiasmata resulting from crossing over. Drosophila melanogaster females use this type of recombination-based system, but they also have mechanisms for segregating achiasmate chromosomes with high fidelity. We describe a P-element mutagenesis and screen in a sensitized genetic background to detect mutations that impair meiotic chromosome pairing, recombination, or segregation. Our screen identified two new recombination-deficient mutations: mei-P22, which fully eliminates meiotic recombination, and mei-P26, which decreases meiotic exchange by 70% in a polar fashion. We also recovered an unusual allele of the ncd gene, whose wild-type product is required for proper structure and function of the meiotic spindle. However, the screen yielded primarily mutants specifically defective in the segregation of achiasmate chromosomes. Although most of these are alleles of previously undescribed genes, five were in the known genes alphaTubulin67C, CycE, push, and Trl. The five mutations in known genes produce novel phenotypes for those genes.

Induction of metaphase arrest in Drosophila oocytes by chiasma-based kinetochore tension

In normal Drosophila melanogaster oocytes, meiosis arrests at metaphase I and resumes after oocyte passage through the oviduct. Thus, metaphase arrest defines a control point in the meiotic cell cycle. Metaphase arrest only occurs in oocytes that have undergone at least one meiotic exchange. Here it is shown that crossovers between homologs attached to the same centromere do not induce metaphase arrest. Hence, exchanges induce metaphase arrest only when they physically conjoin two separate kinetochores. Thus, the signal that mediates metaphase arrest is not the exchange event per se but the resulting tension on homologous kinetochores.

Mechanical basis of meiotic metaphase arrest

Control of the metaphase to anaphase transition is a central component of cell-cycle regulation. Arrest at either metaphase I or II before fertilization is a common component of oogenesis in many organisms. In Drosophila melanogaster females, this arrest occurs at meiosis I with the chiasmate bivalents tightly massed at the metaphase plate and the nonexchange chromosomes positioned between the plate and the poles on long tapered spindles. Meiosis resumes only after passage through the oviduct. Thus, metaphase arrest defines an important checkpoint in the meiotic cell cycle. We report here that this arrest results from the balancing of chiasmate bivalents at the metaphase plate. Two meiotic mutations, mei-9b and mei-218a4, both of which greatly reduce the frequency of chiasma formation, bypass the metaphase block and allow stage 14 oocytes to finish both meiotic divisions without arrest. We conclude that metaphase arrest results from the balancing of kinetochore forces due to chiasmata.