HELB Is a Feedback Inhibitor of DNA End Resection.

DNA double-strand break repair by homologous recombination is initiated by the formation of 3' single-stranded DNA (ssDNA) overhangs by a process termed end resection. Although much focus has been given to the decision to initiate resection, little is known of the mechanisms that regulate the ongoing formation of ssDNA tails. Here we report that DNA helicase B (HELB) underpins a feedback inhibition mechanism that curtails resection. HELB is recruited to ssDNA by interacting with RPA and uses its 5'-3' ssDNA translocase activity to inhibit EXO1 and BLM-DNA2, the nucleases catalyzing resection. HELB acts independently of 53BP1 and is exported from the nucleus as cells approach S phase, concomitant with the upregulation of resection. Consistent with its role as a resection antagonist, loss of HELB results in PARP inhibitor resistance in BRCA1-deficient tumor cells. We conclude that mammalian DNA end resection triggers its own inhibition via the recruitment of HELB.

Molecular mechanisms of assembly and TRIP13-mediated remodeling of the human Shieldin complex.

The Shieldin complex, composed of REV7, SHLD1, SHLD2, and SHLD3, protects DNA double-strand breaks (DSBs) to promote nonhomologous end joining. The AAA+ ATPase TRIP13 remodels Shieldin to regulate DNA repair pathway choice. Here we report crystal structures of human SHLD3-REV7 binary and fused SHLD2-SHLD3-REV7 ternary complexes, revealing that assembly of Shieldin requires fused SHLD2-SHLD3 induced conformational heterodimerization of open (O-REV7) and closed (C-REV7) forms of REV7. We also report the cryogenic electron microscopy (cryo-EM) structures of the ATPγS-bound fused SHLD2-SHLD3-REV7-TRIP13 complexes, uncovering the principles underlying the TRIP13-mediated disassembly mechanism of the Shieldin complex. We demonstrate that the N terminus of REV7 inserts into the central channel of TRIP13, setting the stage for pulling the unfolded N-terminal peptide of C-REV7 through the central TRIP13 hexameric channel. The primary interface involves contacts between the safety-belt segment of C-REV7 and a conserved and negatively charged loop of TRIP13. This process is mediated by ATP hydrolysis-triggered rotatory motions of the TRIP13 ATPase, thereby resulting in the disassembly of the Shieldin complex.

REV7 counteracts DNA double-strand break resection and affects PARP inhibition.

Error-free repair of DNA double-strand breaks (DSBs) is achieved by homologous recombination (HR), and BRCA1 is an important factor for this repair pathway. In the absence of BRCA1-mediated HR, the administration of PARP inhibitors induces synthetic lethality of tumour cells of patients with breast or ovarian cancers. Despite the benefit of this tailored therapy, drug resistance can occur by HR restoration. Genetic reversion of BRCA1-inactivating mutations can be the underlying mechanism of drug resistance, but this does not explain resistance in all cases. In particular, little is known about BRCA1-independent restoration of HR. Here we show that loss of REV7 (also known as MAD2L2) in mouse and human cell lines re-establishes CTIP-dependent end resection of DSBs in BRCA1-deficient cells, leading to HR restoration and PARP inhibitor resistance, which is reversed by ATM kinase inhibition. REV7 is recruited to DSBs in a manner dependent on the H2AX-MDC1-RNF8-RNF168-53BP1 chromatin pathway, and seems to block HR and promote end joining in addition to its regulatory role in DNA damage tolerance. Finally, we establish that REV7 blocks DSB resection to promote non-homologous end-joining during immunoglobulin class switch recombination. Our results reveal an unexpected crucial function of REV7 downstream of 53BP1 in coordinating pathological DSB repair pathway choices in BRCA1-deficient cells.

Subunit composition of VRAC channels determines substrate specificity and cellular resistance to Pt-based anti-cancer drugs.

Although platinum-based drugs are widely used chemotherapeutics for cancer treatment, the determinants of tumor cell responsiveness remain poorly understood. We show that the loss of subunits LRRC8A and LRRC8D of the heteromeric LRRC8 volume-regulated anion channels (VRACs) increased resistance to clinically relevant cisplatin/carboplatin concentrations. Under isotonic conditions, about 50% of cisplatin uptake depended on LRRC8A and LRRC8D, but neither on LRRC8C nor on LRRC8E. Cell swelling strongly enhanced LRRC8-dependent cisplatin uptake, bolstering the notion that cisplatin enters cells through VRAC. LRRC8A disruption also suppressed drug-induced apoptosis independently from drug uptake, possibly by impairing VRAC-dependent apoptotic cell volume decrease. Hence, by mediating cisplatin uptake and facilitating apoptosis, VRAC plays a dual role in the cellular drug response. Incorporation of the LRRC8D subunit into VRAC substantially increased its permeability for cisplatin and the cellular osmolyte taurine, indicating that LRRC8 proteins form the channel pore. Our work suggests that LRRC8D-containing VRACs are crucial for cell volume regulation by an important organic osmolyte and may influence cisplatin/carboplatin responsiveness of tumors.

Proteomics of genetically engineered mouse mammary tumors identifies fatty acid metabolism members as potential predictive markers for cisplatin resistance.

In contrast to various signatures that predict the prognosis of breast cancer patients, markers that predict chemotherapy response are still elusive. To detect such predictive biomarkers, we investigated early changes in protein expression using two mouse models for distinct breast cancer subtypes who have a differential knock-out status for the breast cancer 1, early onset (Brca1) gene. The proteome of cisplatin-sensitive BRCA1-deficient mammary tumors was compared with that of cisplatin-resistant mammary tumors resembling pleomorphic invasive lobular carcinoma. The analyses were performed 24 h after administration of the maximum tolerable dose of cisplatin. At this time point, drug-sensitive BRCA1-deficient tumors showed DNA damage, but cells were largely viable. By applying paired statistics and quantitative filtering, we identified highly discriminatory markers for the sensitive and resistant model. Proteins up-regulated in the sensitive model are involved in centrosome organization, chromosome condensation, homology-directed DNA repair, and nucleotide metabolism. Major discriminatory markers that were up-regulated in the resistant model were predominantly involved in fatty acid metabolism, such as fatty-acid synthase. Specific inhibition of fatty-acid synthase sensitized resistant cells to cisplatin. Our data suggest that exploring the functional link between the DNA damage response and cancer metabolism shortly after the initial treatment may be a useful strategy to predict the efficacy of cisplatin.

Enhancing CAR T-cell therapies against solid tumors: Mechanisms and reversion of resistance.

Chimeric antigen receptor (CAR) T-cell therapy, belonging to adoptive immune cells therapy, utilizes engineered immunoreceptors to enhance tumor-specific killing. By now new generations of CAR T-cell therapies dramatically promote the effectiveness and robustness in leukemia cases. However, only a few CAR T-cell therapies gain FDA approval till now, which are applied to hematologic cancers. Targeting solid tumors through CAR T-cell therapies still faces many problems, such as tumor heterogeneity, antigen loss, infiltration inability and immunosuppressive micro-environment. Recent advances provide new insights about the mechanisms of CAR T-cell therapy resistance and give rise to potential reversal therapies. In this review, we mainly introduce existing barriers when treating solid tumors with CAR T-cells and discuss the methods to overcome these challenges.

[Application of pueumatic tourniquet in the operation of lower tibiofibular fracture].

OBJECTIVE: To investigate the complications of tourniquet in the clinical application of lower tibiofibular fracture. METHODS: From June 2018 to September 2019, 33 cases of closed lower tibiofibular fractures (AO type 43A) were treated with plates and screws and were divided into two groups according to whether pueumatic tourniquet was used:16 cases in the observation group, 13 males and 3 females, aged 18 to 69 (38.8±17.0) years, the operation time after injury was (6.9±1.7) days, and tourniquet was not used during operation. There were 17 cases in the control group, 13 males and 4 females, aged from 21 to 71 (43.8±12.4) years, the operation time after injury was (6.5±1.0) days, automatic pneumatic tourniquetwas routinely used in the operation. The operation time, blood loss, postoperative swelling, pain and other complications were compared between two groups. RESULTS: Total of 33 patients were followed up for an average of 15 months. There was no significant difference in operation time and blood loss between two groups (P>0.05). The VAS scores of limb pain in the observation group were 5.13±1.70 and 2.25±1.60 respectively 1 and 3 days after operation, which were significantly lower than those in the control group 7.35±1.30 and 4.18±1.60;the swelling was (3.67±0.70) cm and (2.02±0.90) cm respectively, which was significantly lower than(4.54±0.40) cm and(3.54±0.40) cm in the control group (P<0.05);there were 1 case of tourniquet pain, 1 case of numbness, 1 case of blister and 1 case of poor wound healing in the control group, there were no such complications in the observation group (P<0.05). CONCLUSION: The fracture of lower tibiofibular segment is superficial and easy to be exposed and fixed during operation. In order to avoid tourniquet complications, it is not recommended to use air bag tourniquet routinely or minimize the application time of tourniquet.

Cell Line-Specific Network Models of ER+ Breast Cancer Identify Potential PI3Kα Inhibitor Resistance Mechanisms and Drug Combinations.

Durable control of invasive solid tumors necessitates identifying therapeutic resistance mechanisms and effective drug combinations. In this work, we used a network-based mathematical model to identify sensitivity regulators and drug combinations for the PI3Kα inhibitor alpelisib in estrogen receptor positive (ER+) PIK3CA-mutant breast cancer. The model-predicted efficacious combination of alpelisib and BH3 mimetics, for example, MCL1 inhibitors, was experimentally validated in ER+ breast cancer cell lines. Consistent with the model, FOXO3 downregulation reduced sensitivity to alpelisib, revealing a novel potential resistance mechanism. Cell line-specific sensitivity to combinations of alpelisib and BH3 mimetics depended on which BCL2 family members were highly expressed. On the basis of these results, newly developed cell line-specific network models were able to recapitulate the observed differential response to alpelisib and BH3 mimetics. This approach illustrates how network-based mathematical models can contribute to overcoming the challenge of cancer drug resistance. SIGNIFICANCE: Network-based mathematical models of oncogenic signaling and experimental validation of its predictions can identify resistance mechanisms for targeted therapies, as this study demonstrates for PI3Kα-specific inhibitors in breast cancer.

Defining the therapeutic selective dependencies for distinct subtypes of PI3K pathway-altered prostate cancers.

Previous studies have suggested that PTEN loss is associated with p110ß signaling dependency, leading to the clinical development of p110ß-selective inhibitors. Here we use a panel pre-clinical models to reveal that PI3K isoform dependency is not governed by loss of PTEN and is impacted by feedback inhibition and concurrent PIK3CA/PIK3CB alterations. Furthermore, while pan-PI3K inhibition in PTEN-deficient tumors is efficacious, upregulation of Insulin Like Growth Factor 1 Receptor (IGF1R) promotes resistance. Importantly, we show that this resistance can be overcome through targeting AKT and we find that AKT inhibitors are superior to pan-PI3K inhibition in the context of PTEN loss. However, in the presence of wild-type PTEN and PIK3CA-activating mutations, p110α-dependent signaling is dominant and selectively inhibiting p110α is therapeutically superior to AKT inhibition. These discoveries reveal a more nuanced understanding of PI3K isoform dependency and unveil novel strategies to selectively target PI3K signaling nodes in a context-specific manner.

Genomic Alterations in PIK3CA-Mutated Breast Cancer Result in mTORC1 Activation and Limit the Sensitivity to PI3Kα Inhibitors.

PI3Kα inhibitors have shown clinical activity in PIK3CA-mutated estrogen receptor-positive (ER+) patients with breast cancer. Using whole genome CRISPR/Cas9 sgRNA knockout screens, we identified and validated several negative regulators of mTORC1 whose loss confers resistance to PI3Kα inhibition. Among the top candidates were TSC1, TSC2, TBC1D7, AKT1S1, STK11, MARK2, PDE7A, DEPDC5, NPRL2, NPRL3, C12orf66, SZT2, and ITFG2. Loss of these genes invariably results in sustained mTOR signaling under pharmacologic inhibition of the PI3K-AKT pathway. Moreover, resistance could be prevented or overcome by mTOR inhibition, confirming the causative role of sustained mTOR activity in limiting the sensitivity to PI3Kα inhibition. Cumulatively, genomic alterations affecting these genes are identified in about 15% of PIK3CA-mutated breast tumors and appear to be mutually exclusive. This study improves our understanding of the role of mTOR signaling restoration in leading to resistance to PI3Kα inhibition and proposes therapeutic strategies to prevent or revert this resistance. SIGNIFICANCE: These findings show that genetic lesions of multiple negative regulators of mTORC1 could limit the efficacy of PI3Kα inhibitors in breast cancer, which may guide patient selection strategies for future clinical trials.

ARID1A determines luminal identity and therapeutic response in estrogen-receptor-positive breast cancer.

Mutations in ARID1A, a subunit of the SWI/SNF chromatin remodeling complex, are the most common alterations of the SWI/SNF complex in estrogen-receptor-positive (ER+) breast cancer. We identify that ARID1A inactivating mutations are present at a high frequency in advanced endocrine-resistant ER+ breast cancer. An epigenome CRISPR-CAS9 knockout (KO) screen identifies ARID1A as the top candidate whose loss determines resistance to the ER degrader fulvestrant. ARID1A inactivation in cells and in patients leads to resistance to ER degraders by facilitating a switch from ER-dependent luminal cells to ER-independent basal-like cells. Cellular plasticity is mediated by loss of ARID1A-dependent SWI/SNF complex targeting to genomic sites of the luminal lineage-determining transcription factors including ER, forkhead box protein A1 (FOXA1) and GATA-binding factor 3 (GATA3). ARID1A also regulates genome-wide ER-FOXA1 chromatin interactions and ER-dependent transcription. Altogether, we uncover a critical role for ARID1A in maintaining luminal cell identity and endocrine therapeutic response in ER+ breast cancer.

PI3K Inhibition Activates SGK1 via a Feedback Loop to Promote Chromatin-Based Regulation of ER-Dependent Gene Expression.

The PI3K pathway integrates extracellular stimuli to phosphorylate effectors such as AKT and serum-and-glucocorticoid-regulated kinase (SGK1). We have previously reported that the PI3K pathway regulates estrogen receptor (ER)-dependent transcription in breast cancer through the phosphorylation of the lysine methyltransferase KMT2D by AKT. Here, we show that PI3Kα inhibition, via a negative-feedback loop, activates SGK1 to promote chromatin-based regulation of ER-dependent transcription. PI3K/AKT inhibitors activate ER, which promotes SGK1 transcription through direct binding to its promoter. Elevated SGK1, in turn, phosphorylates KMT2D, suppressing its function, leading to a loss of methylation of lysine 4 on histone H3 (H3K4) and a repressive chromatin state at ER loci to attenuate ER activity. Thus, SGK1 regulates the chromatin landscape and ER-dependent transcription via the direct phosphorylation of KMT2D. These findings reveal an ER-SGK1-KMT2D signaling circuit aimed to attenuate ER response through a role for SGK1 to program chromatin and ER transcriptional output.

Double PIK3CA mutations in cis increase oncogenicity and sensitivity to PI3Kα inhibitors.

Activating mutations in PIK3CA are frequent in human breast cancer, and phosphoinositide 3-kinase alpha (PI3Kα) inhibitors have been approved for therapy. To characterize determinants of sensitivity to these agents, we analyzed PIK3CA-mutant cancer genomes and observed the presence of multiple PIK3CA mutations in 12 to 15% of breast cancers and other tumor types, most of which (95%) are double mutations. Double PIK3CA mutations are in cis on the same allele and result in increased PI3K activity, enhanced downstream signaling, increased cell proliferation, and tumor growth. The biochemical mechanisms of dual mutations include increased disruption of p110α binding to the inhibitory subunit p85α, which relieves its catalytic inhibition, and increased p110α membrane lipid binding. Double PIK3CA mutations predict increased sensitivity to PI3Kα inhibitors compared with single-hotspot mutations.

The Genomic Landscape of Endocrine-Resistant Advanced Breast Cancers.

We integrated the genomic sequencing of 1,918 breast cancers, including 1,501 hormone receptor-positive tumors, with detailed clinical information and treatment outcomes. In 692 tumors previously exposed to hormonal therapy, we identified an increased number of alterations in genes involved in the mitogen-activated protein kinase (MAPK) pathway and in the estrogen receptor transcriptional machinery. Activating ERBB2 mutations and NF1 loss-of-function mutations were more than twice as common in endocrine resistant tumors. Alterations in other MAPK pathway genes (EGFR, KRAS, among others) and estrogen receptor transcriptional regulators (MYC, CTCF, FOXA1, and TBX3) were also enriched. Altogether, these alterations were present in 22% of tumors, mutually exclusive with ESR1 mutations, and associated with a shorter duration of response to subsequent hormonal therapies.

[PRELIMINARY APPLICATION OF VIRTUAL PREOPERATIVE RECONSTRUCTION PLANNING IN Pilon FRACTURES].

OBJECTIVE: To investigate the application value of three-dimensional reconstruction and virtual preoperative planning for Pilon fractures. METHODS: Between July 2010 and June 2014, 16 patients with closed Pilon fracture were treated, including 12 males and 4 females with an average age of 36.5 years (range, 22-53 years) and a mean disease duration of 10.2 days (range, 6-14 days). According to AO/Orthopaedic Trauma Association (AO/OTA) typing, 2 cases were rated as 43.B2 type, 3 cases as 43.B3 type, 3 cases as 43.C1 type, 2 cases as 43.C2 type, and 6 cases as 43.C3 type. The preoperative CT data from 16 patients were imported into Mimics10.01 software to establish the detailed fracture three-dimensional digital models. Virtual operation of fracture reduction and implanting internal fixation was performed on the models, and the optional surgical planning was made. Based on the virtual preoperative planning, operations were performed. RESULTS: Established detailed three-dimensional Pilon fracture digital models could perfectly reflect the fracture characteristics, could be observed at any direction, and aided for fracture classification accurately. Virtual fracture operations of reduction, internal fixation and other could be performed to simulate the clinical operation, which could assist the surgeon better preoperative planning in achieving visual presentation and improving the communication. The operation time was 70-130 minutes (mean, 87.8 minutes); intraoperative blood loss volume was 30-150 mL (mean, 71.9 mL). The wounds healed by first intension in all patients. The mean follow-up time was 11.6 months (range, 8-18 months). Postoperative radiological results at 3 groups showed good anatomic reduction according to the Burwell-Charnley criteria, and the fracture healing time was 3-6 months (mean, 3.7 months). There was no complication of internal fixation loosening or breakage during follow-up. The American Orthopedic Foot and Ankle Society (AOFAS) score was 71-100 (mean, 92.3); the results were excellent in 10 cases, good in 5 cases, and fair in 1 case, with an excellent and good rate of 93.8% at last follow-up. No loss of fracture reduction was observed on the X-ray film. CONCLUSION: The clinical feasibility of virtual reconstruction preoperative planning is good in the treatment of Pilon fractures, which helps surgeons better understanding Pilon fracure and making the surgical planning.

[Application of a self-made steel wire guide in the treatment of avulsion fractures of tibial posterior cruciate ligament].

OBJECTIVE: To explore the effect of a self-made guiding needle of steel wire in guiding the wire through the tibial tunnel for the treatment of avulsion fractures of tibial posterior cruciate ligament with open reduction and wire fixation. METHODS: From February 2011 to June 2014, a total of 22 patients with avulsion fractures of tibial posterior cruciate ligament underwent surgical treatments were analyzed, including 14 males and 8 females with an average age of 35.6 years old (ranged, 17 to 63 years old). According to Meyers classification, 9 patients were classified as type II, 13 patients were classified as type III. All the patients underwent open reduction and wire fixation with medial knee "L" shape approach. A wire guiding needle was used to guide the wire through the tibial tunnel during operation. RESULTS: With the assistance of wire guidance needles, wires passed through the tibial tunnel rapidly during the operation in all the 22 patients. All the patients were followed up, X-ray imagings 6 months after operation showed the fractures healed well. The average follow-up time in all patients was 6 months (ranged, 6 to 12 months). The averaged Lysholm knee score in 22 knee was 92.7 +/- 3.4. All patients' posterior drawer test were negative. CONCLUSION: Self-made wire guiding needle can simplify the operation procedures in which the wires pass through the tibial tunnel, shorten the operation time, reduce the surgical trauma and complications, and be worthy of clinical application.

BRCA2-deficient sarcomatoid mammary tumors exhibit multidrug resistance.

Pan- or multidrug resistance is a central problem in clinical oncology. Here, we use a genetically engineered mouse model of BRCA2-associated hereditary breast cancer to study drug resistance to several types of chemotherapy and PARP inhibition. We found that multidrug resistance was strongly associated with an EMT-like sarcomatoid phenotype and high expression of the Abcb1b gene, which encodes the drug efflux transporter P-glycoprotein. Inhibition of P-glycoprotein could partly resensitize sarcomatoid tumors to the PARP inhibitor olaparib, docetaxel, and doxorubicin. We propose that multidrug resistance is a multifactorial process and that mouse models are useful to unravel this.

CopywriteR: DNA copy number detection from off-target sequence data.

Current methods for detection of copy number variants (CNV) and aberrations (CNA) from targeted sequencing data are based on the depth of coverage of captured exons. Accurate CNA determination is complicated by uneven genomic distribution and non-uniform capture efficiency of targeted exons. Here we present CopywriteR, which eludes these problems by exploiting 'off-target' sequence reads. CopywriteR allows for extracting uniformly distributed copy number information, can be used without reference, and can be applied to sequencing data obtained from various techniques including chromatin immunoprecipitation and target enrichment on small gene panels. CopywriteR outperforms existing methods and constitutes a widely applicable alternative to available tools.

Dominant negative STAT3 suppresses the growth and invasion capability of human lung cancer cells.

Signal transducer and activator of transcription 3 (STAT3) is constitutively activated in diverse human cancers, including human lung cancer. In this study, STAT3ß, a dominant negative (DN) form of STAT3, was used to block activated STAT3 in human lung cancer cells and to confirm the effects of DN STAT3 on lung cancer cell proliferation and invasion in?vitro. The results showed that the pIRES-STAT3ß plasmid is efficiently transfected into and overexpressed in human lung cancer cells. Overexpressed STAT3ß specifically blocked STAT3 transcriptional activation, inhibiting the proliferation and augmenting the apoptosis of human lung cancer cells. This was associated with the down-regulation of the anti-apoptotic gene bcl-xl and the cell cycle gene cyclin D1. Additionally, the invasive activity of A549 and PG cells was significantly inhibited by overexpressed STAT3ß, and was accompanied by a decrease in matrix metalloproteinase-2 activation. These findings suggest that interfering with the DN STAT3, STAT3ß, may induce potent antitumor activity. STAT3? is therefore a potential candidate for the treatment of human lung cancers with high metastasic ability.