Correlation of Transient Elastography with Liver Iron Concentration and Serum Ferritin Levels in Patients with Transfusion-Dependent Thalassemia Major from Oman

Aims

In a longitudinal study, we aimed to assess the correlation between ultrasound transient elastography (TE), serum ferritin (SF), liver iron content (LIC) by magnetic resonance imaging (MRI) T2* along with the fibrosis-4 (FIB-4) score as a screening tool to detect significant liver fibrosis among chronically transfusion-dependent beta-thalassemia (TDT) patients.

Methods

The study was conducted at a tertiary health center treating TDT patients. Transient elastography was performed within 3 months of Liver MRI T2* examinations at the radiology department over a median of one-year duration. T-test for independent data or Mann-Whitney U test was used to analyze group differences. Spearman correlation with linear regression analysis was used to evaluate the correlation between TE liver stiffness measurements, Liver MRI T2* values, and SF levels.

Results

In this study on 91 patients, the median age (IQR) of the subjects was 33 (9) years, and the median (IQR) body mass index was 23.8 (6.1) kg/m2. Median (IQR) TE by fibroscan, MRI T2*(3T), Liver iron concentration (LIC) by MRI Liver T2*, and SF levels were 6.38 (2.6) kPa, 32.4 (18) milliseconds, 7(9) g/dry wt., and 1881 (2969) ng/mL, respectively. TE measurements correlated with LIC g/dry wt. (rS =0.39, p=0.0001) and with SF level (rS =0.43, P=0.001) but not with MRI T2* values (rS =-0.24; P=0.98).

Conclusion

In TDT patients, liver stiffness measured as TE decreased significantly with improved iron overload measured as LIC by MRI and SF levels. However, there was no correlation of TE with the fibrosis-4 (FIB-4) score.

Defining the rate of nutritional and metabolic derangements after pancreatic resection

BACKGROUND AND AIMS

Pancreatic resection is associated with pancreatic exocrine insufficiency (PEI) leading to nutritional consequences. The Pancreatic Nutrition Clinic was established to diagnose and manage PEI through standardised nutritional assessment. In this prospective observational study, we aimed to define the rate of PEI, diabetes mellitus and nutritional abnormalities in patients who underwent pancreatic resection.

METHODS

All Pancreatic Nutrition Clinic patients were included for analysis. Clinical data were prospectively obtained at initial assessment. Biochemical data included micronutrient levels, faecal elastase-1 and haemoglobin A1c. Bone mineral density and nutritional assessment were undertaken.

RESULTS

Ninety-eight patients were included. Fifty-nine per cent (58/98) had undergone a pancreatoduodenectomy. Ninety-three patients had a faecal elastase-1 result, 65% (60/93) of which had a faecal elastase-1 less than 200 μg/g of faeces. Seventy-five patients (76%) of the total population required PERT, and thirty-nine (40%) were classified as malnourished using the patient-generated subjective global assessment tool. Seventy-two per cent (70/97) had a biochemical deficiency of one or more micronutrients. Thirty-eight people (39%) had diabetes mellitus. Of the seventy-eight patients with a bone mineral density scan available for analysis, 29% (23/78) had osteoporosis and 49% (38/78) osteopenia.

CONCLUSIONS

Pancreatic exocrine insufficiency, micronutrient deficiency, bone disease, diabetes mellitus and malnutrition are highly prevalent in patients who have undergone pancreatic resection.

Evaluation of prognostic risk models for postoperative pulmonary complications in adult patients undergoing major abdominal surgery: a systematic review and international external validation cohort study

BACKGROUND

Stratifying risk of postoperative pulmonary complications after major abdominal surgery allows clinicians to modify risk through targeted interventions and enhanced monitoring. In this study, we aimed to identify and validate prognostic models against a new consensus definition of postoperative pulmonary complications.

METHODS

We did a systematic review and international external validation cohort study. The systematic review was done in accordance with the Preferred Reporting Items for Systematic Reviews and Meta-Analyses guidelines. We searched MEDLINE and Embase on March 1, 2020, for articles published in English that reported on risk prediction models for postoperative pulmonary complications following abdominal surgery. External validation of existing models was done within a prospective international cohort study of adult patients (≥18 years) undergoing major abdominal surgery. Data were collected between Jan 1, 2019, and April 30, 2019, in the UK, Ireland, and Australia. Discriminative ability and prognostic accuracy summary statistics were compared between models for the 30-day postoperative pulmonary complication rate as defined by the Standardised Endpoints in Perioperative Medicine Core Outcome Measures in Perioperative and Anaesthetic Care (StEP-COMPAC). Model performance was compared using the area under the receiver operating characteristic curve (AUROCC).

FINDINGS

In total, we identified 2903 records from our literature search; of which, 2514 (86·6%) unique records were screened, 121 (4·8%) of 2514 full texts were assessed for eligibility, and 29 unique prognostic models were identified. Nine (31·0%) of 29 models had score development reported only, 19 (65·5%) had undergone internal validation, and only four (13·8%) had been externally validated. Data to validate six eligible models were collected in the international external validation cohort study. Data from 11 591 patients were available, with an overall postoperative pulmonary complication rate of 7·8% (n=903). None of the six models showed good discrimination (defined as AUROCC ≥0·70) for identifying postoperative pulmonary complications, with the Assess Respiratory Risk in Surgical Patients in Catalonia score showing the best discrimination (AUROCC 0·700 [95% CI 0·683-0·717]).

INTERPRETATION

In the pre-COVID-19 pandemic data, variability in the risk of pulmonary complications (StEP-COMPAC definition) following major abdominal surgery was poorly described by existing prognostication tools. To improve surgical safety during the COVID-19 pandemic recovery and beyond, novel risk stratification tools are required.

FUNDING

British Journal of Surgery Society.

Atrial fibrillation in unexplained syncope: observations from the Reveal LINQ registry

Background

Implantable loop recorders (ILRs) have come to play an important role in the workup of patients with recurrent syncope of uncertain origin. In addition to detecting bradyarrhythmias related to syncope, which is the main diagnostic focus in these patients, ILRs are also capable of uncovering subclinical atrial fibrillation (AF).

Purpose

We sought to determine the percentage of patients monitored with an ILR for unexplained syncope who have AF detected and to describe clinical actions taken in these patients.

Methods

Patients enrolled in the Reveal LINQ Registry who received an ILR for unexplained syncope and had at least one follow-up form were included. The device automatically detects AF episodes lasting ≥2 minutes. Patients were considered to have AF based on an AF diagnosis made by the treating physician during follow-up or if device-detected AF was adjudicated as true AF by an external reviewer. AF detection rates were calculated using Kaplan-Meier methods.

Results

In total, 498 patients (aged 61.8±20.0 years, 49.6% female, CHA2DS2VASc score 2.2±1.7) were included and followed for 22±12 months. A history of AF was present in 97 (20%) patients, while 401 patients had no history. By 18 months, the incidence of AF was 70.9% (95% CI, 60.8%, 80.3%) in patients with a history of AF and 21.4% (95% CI, 17.4%, 26.1%) in patients without (Figure). AF detection in those with (30.4%) and without (30.1%) syncope during follow-up was similar. By the end of follow-up, and among patients with newly detected AF, 29/86 (33.7%) were on oral anticoagulation, 7 (8.1%) underwent AF ablation, 6 (7.0%) underwent other type of ablation, and 2 (2.3%) received cardioversion. Other actions among the whole cohort included implant of an IPG, ICD, or CRT in 98/498 (19.7%).

Conclusion

Among patients monitored with ILRs to determine the cause of recurrent syncope episodes, approximately 1 in 5 patients had new AF detected. In addition to improving the management of patients with syncope, ILR data served to support AF-related clinical decisions.

Funding Acknowledgement

Type of funding sources: Private company. Main funding source(s): Medtronic Inc Incidence of AF according to baseline AF

Dose-dense Paclitaxel and Carboplatin as Neoadjuvant Chemotherapy for Stage IIB/IIIA Non-small Cell Lung Cancer - A Phase II trial

AIMS

The approach to potentially resectable non-small cell lung cancer (NSCLC) remains controversial. There is a benefit of neoadjuvant chemotherapy (NACT), but the ideal regimen is unknown. We evaluated the efficacy and safety of dose-dense NACT in potentially resectable NSCLC in this phase II trial.

MATERIALS AND METHODS

Paclitaxel at 80 mg/m² on days 1, 8 and 15 with AUC-6 carboplatin on day 1, 3 weekly for four cycles was evaluated as NACT. Patients with Eastern Cooperative Oncology Group performance status 0-2, stage IIB and IIIA (with only non-bulky N2 nodes) were included. The primary end point was the objective response rate. Secondary end points included toxicity, progression-free survival, recurrence-free survival, complete resection rate and overall survival. The relative dose intensity (RDI) was calculated to define tolerability (CTRI/2016/05/006916).

RESULTS

In total, 37 patients were enrolled (median age 55 years). Most (78.8%) were smokers. Most patients had adenocarcinoma (57.6%) and stage IIIA disease (81.0%) according to the seventh American Joint Committee on Cancer staging system. Seventy-eight per cent of patients completed four cycles. The objective response rate was 75.6% with a complete response in 10.8%. The mean RDI of paclitaxel was 88.61%, with 68.0% of patients able to maintain an RDI ≥85.0%. In total, 187 toxicity events were recorded (120 grade 1, 64 grade 2 and three grade 3 events). Common toxicities were peripheral neuropathy (20.3%), myalgia (19.8%), nausea (15.7%) and neutropenia (10.2%). There were no treatment-related deaths. Seventeen patients underwent surgery (lobectomy 82.4%). After a median follow-up of 47 months (95% confidence interval 27-50.7 months), the median progression-free survival was 9.6 months (7.4-17.4) and overall survival was 29.2 months (16.0-37.2).

CONCLUSION

Dose-dense paclitaxel-carboplatin is feasible, safe and efficacious and should be evaluated further in potentially resectable NSCLC.

Management of peri-prosthetic joint infection and severe bone loss after total hip arthroplasty using a long-stemmed cemented custom-made articulating spacer (CUMARS)

Background

There is little evidence on techniques for management of peri-prosthetic infection (PJI) in the context of severe proximal femoral bone loss. Custom-made articulating spacers (CUMARS) utilising cemented femoral stems as spacers was described providing better bone support and longer survival compared to conventional articulating spacers. We retrospectively report our experience managing PJI by adaptation of this technique using long cemented femoral stems where bone loss precludes use of standard stems.

Methods

Patients undergoing 1st stage revision for infected primary and revision THA using a cemented long stem (> 205 mm) and standard all-polyethylene acetabulum between 2011 and 2018 were identified. After excluding other causes of revision (fractures or aseptic loosening), Twenty-one patients remained out of total 721 revisions. Medical records were assessed for demographics, initial microbiological and operative treatment, complications, eradication of infection and subsequent operations. 2nd stage revision was undertaken in the presence of pain or subsidence.

Results

Twenty-one patients underwent 1st stage revision with a cemented long femoral stem. Mean follow up was 3.9 years (range 1.7–7.2). Infection was eradicated in 15 (71.4%) patients. Two patients (9.5%) required repeat 1st stage and subsequently cleared their infection. Three patients (14.3%) had chronic infection and are on long term suppressive antibiotics. One patient (4.8%) was lost to follow up before 2 years. Complications occurred in seven patients (33%) during or after 1st stage revision. Where infection was cleared, 2nd stage revision was undertaken in 12 patients (76.5%) at average of 9 months post 1st stage. Five (23.8%) CUMARS constructs remained in-situ at an average of 3.8 years post-op (range 2.6–5.1).

Conclusions

Our technique can be used in the most taxing of reconstructive scenarios allowing mobility, local antibiotic delivery, maintenance of leg length and preserves bone and soft tissue, factors not afforded by alternative spacer options.

Extending Horizon of Robotic Surgery to Bladder-Preserving Approach for Vesical Paraganglioma: Rare Case with Unusual Presentation

Background: Vesical paraganglioma is rare and accounts for <0.1% of all urinary bladder tumors. They are mostly functional because of secretion of catecholamines and clinical presentation may mimic like a hyperfunctioning adrenal pheochromocytoma. They are easily misdiagnosed as urothelial malignancy and adequate perioperative attention is not provided. Case presentation: We hereby report a case of 55-year-old Indian lady with silent vesical paraganglioma at anatomically difficult location of bladder neck managed with robot-assisted excision of mass and bladder preservation. Conclusion: Surgery is the mainstay of the treatment that requires total excision of mass. However, minimally invasive bladder-preserving approach should be always kept as an option, if feasible. Robot assistance can help in bladder preservation even in difficult anatomic locations.

0535 Evaluation of Insomnia Symptoms in a Double-Blind, Randomized, Placebo-Controlled Phase 3 Trial of Sage-217 in Postpartum Depression

Introduction

Postpartum depression (PPD) is a specifier of major depressive disorder (MDD) with peripartum onset. SAGE-217, an investigational, oral neuroactive steroid GABAA receptor positive allosteric modulator, demonstrated improvements in depressive and anxiety symptoms versus placebo in a Phase 3 trial in PPD (NCT02978326; ROBIN) and a pivotal trial in MDD (NCT03000530). In PPD and MDD, insomnia symptoms are key diagnostic features, comorbid sleep disorders are frequent, and insomnia is a common residual symptom. Here we conducted post-hoc analyses to assess insomnia symptoms in the ROBIN trial.

Methods

Women (n=151) ages 18-45, ≤6 months postpartum, with PPD (major depressive episode beginning in 3rd trimester or ≤4 weeks postpartum) and a Hamilton Rating Scale for Depression (HAM-D) total score ≥26, were randomized 1:1 to receive outpatient SAGE-217 30mg or placebo for two weeks, with 4 weeks follow-up. Change from baseline (CFB) in HAM-D score at Day 15 was the primary endpoint. Secondary endpoints included CFB in HAM-D at other time points and the Montgomery-Åsberg Depression Rating Scale (MADRS). Post-hoc analyses assessed HAM-D insomnia subscale (HAM-D-Ins) and MADRS individual insomnia item (MADRS-Ins) scores. HAM-D and MADRS measures were evaluated using a mixed-effects model for repeated measures. Safety and tolerability were assessed by adverse event (AE) reporting.

Results

SAGE-217 demonstrated statistically significant Day 15 CFB versus placebo in HAM-D (primary endpoint: -17.8 vs. -13.6, p=0.0028), MADRS (-22.1 vs. -17.6, p=0.0180), and associated insomnia sub-scales/items (difference SAGE-217 vs. placebo; HAM-D-Ins: -1.003, p=0.0038; MADRS-Ins: -0.773, p=0.0116). Significant CFB in insomnia sub-scales/items favoring SAGE-217 were observed by Day 3 (HAM-D-Ins: -0.841, p=0.0142; MADRS-Ins: -0.710, p=0.017) and at Day 45 (HAM-D-Ins: -0.730, p=0.0207; MADRS-Ins: -0.636, p=0.0221). Most common (≥5%) AEs were somnolence, headache, dizziness, upper respiratory tract infection, diarrhea, and sedation.

Conclusion

SAGE-217 demonstrated improvements in depression symptoms, including insomnia symptoms, and was generally well tolerated.

Support

This study was sponsored by Sage Therapeutics, Inc.

Do pediatricians routinely perform genitourinary examinations during well-child visits? A review from a large tertiary pediatric hospital

BACKGROUND

The male genital examination is a common source of discomfort for the patient and medical provider. Performance of male genital examination is imperative; however, as many treatable diagnoses can be made. Undescended testicles (UDTs), hernias, testicular tumors, and urethral abnormalities are all potentially concerning findings which can be discovered on routine examination.

OBJECTIVE

The objectives of this study are to determine the rate at which general pediatricians perform routine genitourinary (GU) examinations in the pediatric population and to determine the rate at which UDT are diagnosed or documented in the patient's history. The authors hypothesize the rate of pediatric GU examination during routine well-child visits to be in line with the previously reported rates in the adult literature.

STUDY DESIGN

Nine hundred ninety-six consecutive male well-child visits conducted by general pediatricians at the study institution were reviewed. These visits were evaluated for documentation of a detailed GU examination as well as the presence of UDT from these examinations. In addition, past medical and surgical histories were reviewed to determine if a diagnosis of UDT was noted.

RESULTS

Pediatricians at the study institution documented GU examinations 99.1% of the time during male well-child visits. Only 1.1% of the cohort had a documentation of UDT at any time point. Of the 11 patients with UDT, 6 boys (54.5%) had spontaneous descent with no referral to urology, whereas 5 (45.5%) required orchidopexy.

DISCUSSION

Prior reports suggest 70-75% of routine office visits include a genital examination. None of these reports reviewed the pediatric population, thus making this review novel in this respect. In addition, the results are vastly different from these prior studies as the authors demonstrated over 99% of male well-child examinations included documentation of a thorough genital examination. A limitation of the study is its retrospective nature, which creates a lack of standardization across the data set. In addition, without being physically present in the examination room, one cannot discern whether an examination is simply being documented without actual performance because of the template format of the electronic medical record (EMR). Furthermore, the study was not designed to best evaluate the true rate of UDTs; therefore, the reported rate of 1.1% cannot be accurately associated with a particular age at diagnosis.

CONCLUSIONS

Pediatricians do, in fact, document GU examinations on a routine basis. This finding cannot be taken with complete certainty as verification of actual examination performance is impractical. While the data demonstrated a lower than expected rate of UDT, depending upon age at diagnosis, this could indicate that although examinations are being documented, their accuracy may be diminished because of various factors at play in the healthcare system as a whole, including improper exam performance and EMR templates. Follow-up studies are required to verify these potentially changing rates of UDT and to determine if there is discordance between documentation and performance of GU examinations.

The structure of the RbBP5 β-propeller domain reveals a surface with potential nucleic acid binding sites

The multi-protein complex WRAD, formed by WDR5, RbBP5, Ash2L and Dpy30, binds to the MLL SET domain to stabilize the catalytically active conformation required for histone H3K4 methylation. In addition, the WRAD complex contributes to the targeting of the activated complex to specific sites on chromatin. RbBP5 is central to MLL catalytic activation, by making critical contacts with the other members of the complex. Interestingly its only major structural domain, a canonical WD40 repeat β-propeller, is not implicated in this function. Here, we present the structure of the RbBP5 β-propeller domain revealing a distinct, feature rich surface, dominated by clusters of Arginine residues. Our nuclear magnetic resonance binding data supports the hypothesis that in addition to the role of RbBP5 in catalytic activation, its β-propeller domain is a platform for the recruitment of the MLL complexes to chromatin targets through its direct interaction with nucleic acids.

Dietary Intervention to Control Vitamin A Deficiency in Seven- to Twelve-Year-Old Children

The efficacy of a dietary intervention programme to control vitamin A deficiency through inexpensive, locally available sources of β-carotene was evaluated in 121 children 7–12 years old. The subjects were randomly divided into experimental and control groups. A three-day food intake was first recorded for each subject using a 24-hour recall method and repeated at the end of the study on a randomly selected subsample. The intervention period lasted one month, during which carrots, papayas, coriander, and mint were offered daily as sources of β-carotene. There was no significant difference in the dietary intakes of the groups before the study. After the intervention period, the serum vitamin A values of the experimental subjects were significantly higher than those of the controls. These results indicate that consumption of small amounts of inexpensive, readily available vegetable sources of β-carotene could help prevent and control vitamin A deficiency. Nutrition education programmes are needed to encourage the use of these foods for home consumption as well as in feeding programmes for schoolchildren.

An antenatal diagnosis: Congenital high airway obstruction

Congenital high airway obstruction (CHAOS) is a rare lethal fetal malformation characterised by obstruction to the fetal upper airway, which can be partial or complete. Antenatal diagnosis of CHAOS is important due to recent management options. Diagnosis is made with secondary changes such as hyperechoic enlarged lungs resulting in mediastinal compression, ascites, hydrops, flattened or everted diaphragms and dilated distal airways. We reported a case of CHAOS, antenatally on ultrasonography (USG) at 20 weeks of gestation.

Skin Cancers in Organ Transplant Recipients

Long-term utilization of immunosuppression in organ transplant recipients (OTRs) leads to decreased immune-mediated tumor surveillance and development of malignant tumors. A delicate balance needs to be maintained in the intensity of immunosuppression to keep the risk of malignancy low without jeopardizing life-saving graft function. OTRs are prone to developing skin cancers that exhibit unique epidemiologic, pathophysiologic, and prognostic characteristics. In this review, we discuss the most commonly reported skin cancers in OTRs: squamous cell carcinoma (SCC), basal cell carcinoma (BCC), Kaposi sarcoma, Merkel cell carcinoma, and malignant melanoma (MM). Tumors in this high-risk population are aggressive and may respond poorly to standard therapies; however, new targeted therapies are promising. Checkpoint inhibitor antibodies have been used for treatment of cutaneous SCC, Merkel cell carcinoma, and MM; epidermal growth factor receptor inhibitors have been used for cutaneous SCC; hedgehog pathway inhibitors have been used for BCC; and BRAF and MEK inhibitors are being used increasingly in the management of MM. Guidelines for dermatologic screening are variable and primarily based on expert opinion. Prospective evidence-based trials by multidisciplinary groups are needed to better define surveillance schedules for pre- and posttransplant cutaneous malignancies.

The validity and reproducibility of cross table radiographs compared with CT scans for the measurement of anteversion of the acetabular component after total hip arthroplasty

Aims

The aim of this study was to assess the reproducibility and validity of cross table radiographs for measuring the anteversion of the acetabular component after total hip arthroplasty (THA) and to compare it with measurements using CT scans.

Patients and Methods

A total of 29 patients who underwent THA between June 2010 and January 2016 were included. There were 17 men and 12 women. Their mean age was 43 years (26 to 65). Seven patients underwent a bilateral procedure. Thus, 36 THAs were included in the study. Lateral radiographs and CT scans were obtained post-operatively and radiographs repeated three weeks later. The anteversion of the acetabular component was measured using the method described by Woo and Morrey and the ischiolateral method described by Pulos et al and these were compared with the results obtained from CT scans.

Results

The mean anteversion was 18.35° (3° to 38°) using Woo and Morrey’s method, 51.45° (30° to 85°) using the ischiolateral method and 21.22° (2° to 48°) using CT scans. The Pearson correlation coefficient was 0.754 for Woo and Morrey’s method and 0.925 for the ischiolateral method. There was a linear correlation between the measurements using the ischiolateral method and those using CT scans. We derived a simple linear equation between the value of the CT scan and that of ischiolateral method to deduce the CT scan value from that of ischiolateral method and vice versa.

Conclusion

The anteversion of the acetabular component measured using both plain radiographic methods was consistently valid with good interobserver reproducibility, but the ischiolateral method which is independent of pelvic tilt was more accurate. As CT is costly, associated with a high dose of radiation and not readily available, the ischiolateral method can be used for assessing the anteversion of the acetabular component. Cite this article: Bone Joint J 2017;99-B:1006–11.

Retrospective cohort analysis of neoadjuvant treatment and survival in resectable and borderline resectable pancreatic ductal adenocarcinoma in a high volume referral centre

BACKGROUND

Pancreatic ductal adenocarcinoma (PDAC) is a deadly disease. Neoadjuvant therapy (NA) with chemotherapy (NAC) and radiotherapy (RT) prior to surgery provides promise. In the absence of prospective data, well annotated clinical data from high-volume units may provide pilot data for randomised trials.

METHODS

Medical records from a tertiary hospital in Sydney, Australia, were analysed to identify all patients with resectable or borderline resectable PDAC. Data regarding treatment, toxicity and survival were collected.

RESULTS

Between January 1 2010 and April 1 2016, 220 sequential patients were treated: 87 with NA and 133 with upfront operation (UO). Forty-three NA patients (52%) and 5 UO patients (4%) were borderline resectable at diagnosis. Twenty-four borderline patients received NA RT, 22 sequential to NAC. The median overall survival (OS) in the NA group was 25.9 months (mo); 95% CI (21.1-43.0 mo) compared to 26.9 mo (19.7, 32.7) in the UO; HR 0.89; log-ranked p-value = 0.58. Sixty-nine NA patients (79%) were resected, mOS was 29.2 mo (22.27, not reached (NR)). Twenty-two NA (31%) versus 22 UO (17%) were node negative at operation (N0). In those managed with NAC/RT the mOS was 29.0 mo (17.3, NR). There were no post-operative deaths with NA within 90-days and three in the UO arm.

DISCUSSION

This is a hypothesis generating retrospective review of a selected real-world population in a high-throughput unit. Treatment with NA was well tolerated. The long observed survival in this group may be explained by lymph node sterilisation by NA, and the achievement of R0 resection in a greater proportion of patients.

Exploring conformational equilibria of a heterodimeric ABC transporter

ABC exporters pump substrates across the membrane by coupling ATP-driven movements of nucleotide binding domains (NBDs) to the transmembrane domains (TMDs), which switch between inward- and outward-facing (IF, OF) orientations. DEER measurements on the heterodimeric ABC exporter TM287/288 from Thermotoga maritima, which contains a non-canonical ATP binding site, revealed that in the presence of nucleotides the transporter exists in an IF/OF equilibrium. While ATP binding was sufficient to partially populate the OF state, nucleotide trapping in the pre- or post-hydrolytic state was required for a pronounced conformational shift. At physiologically high temperatures and in the absence of nucleotides, the NBDs disengage asymmetrically while the conformation of the TMDs remains unchanged. Nucleotide binding at the degenerate ATP site prevents complete NBD separation, a molecular feature differentiating heterodimeric from homodimeric ABC exporters. Our data suggest hydrolysis-independent closure of the NBD dimer, which is further stabilized as the consensus site nucleotide is committed to hydrolysis.

DOI:http://dx.doi.org/10.7554/eLife.20236.001