Influence of Incomplete Death Information on Cumulative Risk Estimates in United States Claims Data.

Administrative claims databases often do not capture date or fact of death, so studies using these data may inappropriately treat death as a censoring event-equivalent to other withdrawal reasons-rather than a competing event. We examined 1-, 3-, and 5-year inverse-probability-of-treatment-weighted cumulative risks of a composite cardiovascular outcome among 34,527 initiators of telmisartan (exposure) and ramipril (referent) ages ≥55 in Optum claims from 2003 to 2020. Differences in cumulative risks of the cardiovascular endpoint due to censoring of death (cause-specific), as compared to treating death as a competing event (sub-distribution), increased with greater follow-up time and older age, where event and mortality risks were higher. Among ramipril users (selected results), 5-year cause-specific and sub-distribution cumulative risk estimates per 100, respectively, were 16.4 (95% CI 15.3, 17.5) and 16.2 (95% CI 15.1, 17.3) among ages 55-64 (difference=0.2) and were 43.2 (95% CI 41.3, 45.2) and 39.7 (95% CI 37.9, 41.4) among ages ≥75 (difference=3.6). Plasmode simulation results demonstrated the differences in cause-specific versus sub-distribution cumulative risks to increase with increasing mortality rate. We suggest researchers consider the cohort's baseline mortality risk when deciding whether real-world data with incomplete death information can be used without concern.

MSUT2 regulates tau spreading via adenosinergic signaling mediated ASAP1 pathway in neurons.

Inclusions comprised of microtubule-associated protein tau (tau) are implicated in a group of neurodegenerative diseases, collectively known as tauopathies, that include Alzheimer's disease (AD). The spreading of misfolded tau "seeds" along neuronal networks is thought to play a crucial role in the progression of tau pathology. Consequently, restricting the release or uptake of tau seeds may inhibit the spread of tau pathology and potentially halt the advancement of the disease. Previous studies have demonstrated that the Mammalian Suppressor of Tauopathy 2 (MSUT2), an RNA binding protein, modulates tau pathogenesis in a transgenic mouse model. In this study, we investigated the impact of MSUT2 on tau pathogenesis using tau seeding models. Our findings indicate that the loss of MSUT2 mitigates human tau seed-induced pathology in neuron cultures and mouse models. In addition, MSUT2 regulates many gene transcripts, including the Adenosine Receptor 1 (A1AR), and we show that down regulation or inhibition of A1AR modulates the activity of the "ArfGAP with SH3 Domain, Ankyrin Repeat, and PH Domain 1 protein" (ASAP1), thereby influencing the internalization of pathogenic tau seeds into neurons resulting in reduction of tau pathology.

Real-World Risk of Severe Cytopenias in Multiple Myeloma Patients Sequentially Treated with Immunomodulatory Drugs.

INTRODUCTION: Most multiple myeloma (MM) patients experience cytopenias, likely driven by both disease and treatment-related factors. Immunomodulatory agents (IMiDs), which form the backbone of most anti-myeloma regimens, are known to cause higher grade cytopenias. In this context, the impact of sequential IMiD treatments on cytopenia risk is unknown. METHODS: We evaluated the cumulative risks of severe cytopenias following second line of therapy (LOT) initiation in 5573 MM patients in the Flatiron Health database. Patients for whom both LOTs 1 and 2 contained IMiDs were considered "sequentially exposed"; those for whom neither contained IMiDs were "never exposed." RESULTS: For the neutropenia outcome, compared to the never exposed, the sequentially exposed had the highest 1-year risk (risk difference [RD] 12%), followed by those only recently exposed during LOT 2 (RD 8%), then by those with only past exposure during LOT 1 (RD 5%). A similar pattern was observed for leukopenia, but no meaningful differences were observed for anemia or thrombocytopenia. The associations between sequential exposure, versus never, with neutropenia and leukopenia were even stronger among those with a recent cytopenia history. CONCLUSION: Results suggest that sequential exposure to IMiDs is a risk factor for higher grade cytopenias. These findings have profound clinical implications in choosing newer LOTs with potential risks of cytopenia.

Linking a cell-division gene and a suicide gene to define and improve cell therapy safety.

Human pluripotent cell lines hold enormous promise for the development of cell-based therapies. Safety, however, is a crucial prerequisite condition for clinical applications. Numerous groups have attempted to eliminate potentially harmful cells through the use of suicide genes1, but none has quantitatively defined the safety level of transplant therapies. Here, using genome-engineering strategies, we demonstrate the protection of a suicide system from inactivation in dividing cells. We created a transcriptional link between the suicide gene herpes simplex virus thymidine kinase (HSV-TK) and a cell-division gene (CDK1); this combination is designated the safe-cell system. Furthermore, we used a mathematical model to quantify the safety level of the cell therapy as a function of the number of cells that is needed for the therapy and the type of genome editing that is performed. Even with the highly conservative estimates described here, we anticipate that our solution will rapidly accelerate the entry of cell-based medicine into the clinic.

The MOTION study: Two-year results of a real-world randomized controlled trial of the mild® procedure for treatment of lumbar spinal stenosis.

OBJECTIVE: The MOTION study is designed to measure the impact of percutaneous image-guided lumbar decompression as a first-line therapy on patients otherwise receiving real-world conventional medical management for lumbar spinal stenosis with neurogenic claudication secondary to hypertrophic ligamentum flavum. This prospective, multicenter randomized controlled trial uses objective and patient-reported outcome measures to compare the combination of the mild® percutaneous treatment and nonsurgical conventional medical management (CMM) to CMM-Alone. METHODS: Test group patients received the mild procedure after study enrollment. Test and control groups were allowed conventional conservative therapies and low-risk interventional therapies as recommended by their physicians. Subjective outcomes included the Oswestry Disability Index, Numeric Pain Rating Scale, and Zurich Claudication Questionnaire. Objective outcomes included a validated Walking Tolerance Test, the rate of subsequent lumbar spine interventions, and safety data. RESULTS: Two-year follow-up included 64 mild + CMM and 67 CMM-Alone patients. All outcome measures showed significant improvement from baseline for mild + CMM, whereas the majority of CMM-Alone patients had elected to receive mild treatment or other lumbar spine interventions by 2 years, precluding valid 2-year between-group comparisons. Neither group reported any device- or procedure-related adverse events. CONCLUSIONS: The durability of mild + CMM for this patient population was demonstrated for all efficacy outcomes through 2 years. Improvements in walking time from baseline to 2 years for patients treated with mild + CMM were significant and substantial. The lack of reported device or procedure-related adverse events reinforces the strong safety profile of the mild procedure. These results provide support for early interventional treatment of symptomatic LSS with the mild procedure.

Reticulocytes in donor blood units enhance red blood cell alloimmunization.

Although red blood cell (RBC) transfusions save lives, some patients develop clinically-significant alloantibodies against donor blood group antigens, which then have adverse effects in multiple clinical settings. Few effective measures exist to prevent RBC alloimmunization and/or eliminate alloantibodies in sensitized patients. Donor-related factors may influence alloimmunization; thus, there is an unmet clinical need to identify which RBC units are immunogenic. Repeat volunteer blood donors and donors on iron supplements have elevated reticulocyte counts compared to healthy non-donors. Early reticulocytes retain mitochondria and other components, which may act as danger signals in immune responses. Herein, we tested whether reticulocytes in donor RBC units could enhance RBC alloimmunization. Using a murine model, we demonstrate that transfusing donor RBC units with increased reticulocyte frequencies dose-dependently increased RBC alloimmunization rates and alloantibody levels. Transfusing reticulocyte-rich RBC units was associated with increased RBC clearance from the circulation and a robust proinflammatory cytokine response. As compared to previously reported post-transfusion RBC consumption patterns, erythrophagocytosis from reticulocyte-rich units was increasingly performed by splenic B cells. These data suggest that reticulocytes in a donated RBC unit impact the quality of blood transfused, are targeted to a distinct compartment, and may be an underappreciated risk factor for RBC alloimmunization.

Telemedicine use for pediatric asthma care: a mixed methods study.

OBJECTIVES: To identify factors associated with telemedicine use for asthma care among children and young adults, and to describe the parent and patient experience of asthma care over telemedicine. METHODS: Our mixed methods study consisted of an electronic health record analysis and a qualitative focus group analysis. We analyzed records for all patients aged 2-24 seen at UC Davis Health between March 19, 2020 and September 30, 2020 for a primary diagnosis of asthma. We performed multivariable logistic regression to quantify the relationships between patient characteristics and telemedicine use. We also conducted focus groups with parents and patients who received asthma care during the study period and used qualitative content analysis to identify themes from the transcripts. RESULTS: 502 patients met the inclusion criteria. Telemedicine use was significantly lower among patients with a primary language other than English (OR = 0.12, 95% CI: 0.025-0.54, p = 0.006), school-aged children (OR = 0.43, 95% CI: 0.24-0.77, p = 0.005), and patients who received asthma care from a primary care provider instead of a specialist (OR = 0.55, 95% CI: 0.34-0.91, p = 0.020). Six thematic categories emerged from focus groups: engaging with the patient, improving access to care, experience of visit, measurements, scheduling, and the future of telemedicine in asthma care. CONCLUSIONS: Alternating telemedicine with in-person visits for asthma care may result in improved access to care and reduced burdens on patients and families. Providers and researchers should work to understand the specific reasons for low telemedicine use among non-English speaking patients so that these patients receive equitable access to care.

Training Spiking Neural Networks in the Strong Coupling Regime.

Recurrent neural networks trained to perform complex tasks can provide insight into the dynamic mechanism that underlies computations performed by cortical circuits. However, due to a large number of unconstrained synaptic connections, the recurrent connectivity that emerges from network training may not be biologically plausible. Therefore, it remains unknown if and how biological neural circuits implement dynamic mechanisms proposed by the models. To narrow this gap, we developed a training scheme that, in addition to achieving learning goals, respects the structural and dynamic properties of a standard cortical circuit model: strongly coupled excitatory-inhibitory spiking neural networks. By preserving the strong mean excitatory and inhibitory coupling of initial networks, we found that most of trained synapses obeyed Dale's law without additional constraints, exhibited large trial-to-trial spiking variability, and operated in inhibition-stabilized regime. We derived analytical estimates on how training and network parameters constrained the changes in mean synaptic strength during training. Our results demonstrate that training recurrent neural networks subject to strong coupling constraints can result in connectivity structure and dynamic regime relevant to cortical circuits.

Rotator cuff repair in HIV-positive patients ages 65 and older: only slight increase in risk of general postoperative surgical complications.

PURPOSE: To examine postoperative complications associated with rotator cuff repair (RCR) in HIV-positive patients ages 65 and older. METHODS: Data were collected from the Medicare Standardized Analytic Files between 2005 and 2015 using the PearlDiver Patient Records Database. Subjects were selected using Current Procedural Terminology (CPT) and International Classification of Diseases (ICD) codes. Demographics including age, sex, medical comorbidities, and smoking status were collected. Complications were examined at 7-day, 30-day, and 90-day postoperative time points. Data were examined with univariate and multivariate analyses. RESULTS: The study included 152,114 patients who underwent RCR, with 24,486 (16.1%) patients who were HIV-positive. Following univariate analysis, patients with HIV were observed to be more likely to develop 7-day, 30-day, and 90-day postoperative complications. However, the absolute risk of each complication was quite low for HIV-positive patients. Univariate and multivariate analysis showed that within 7 days following surgery, patients with HIV were more likely to develop myocardial infarction (OR 2.5, AR 0.1%) and sepsis (OR 2.5, AR 0.04%). Within 30 days, HIV-positive patients were at increased risk for postoperative anemia (OR 2.8, AR 0.1%), blood transfusion (OR 3.3, AR 0.1%), heart failure (OR 2.3, AR 0.8%), and sepsis (OR 2.7, AR 0.1%). Within 90 days, mechanical complications (OR 2.1, AR 0.1%) were increased in the HIV-positive group. CONCLUSION: Postoperative complications of RCR occurred at increased rates in the HIV-positive group compared to the HIV-negative group in patients ages 65 and older. In particular, increased risk for myocardial infarction, sepsis, heart failure, anemia, and mechanical complications was noted in HIV-positive patients. However, the actual percentage of patients who experienced each complication was low, indicating RCR is likely safe to perform even in older HIV-positive patients. As more older adults living with HIV present for elective orthopedic procedures, the results of the present study may reassure physicians who are considering RCR as an option for patients in this particular population, while also informing providers about potential complications. LEVEL OF EVIDENCE: III.

Roles of D1-Glu189 and D1-Glu329 in O2 Formation by the Water-Splitting Mn4Ca Cluster in Photosystem II.

During the catalytic step that precedes O-O bond formation in Photosystem II, a water molecule deprotonates and moves next to the water-splitting Mn4Ca cluster's O5 oxo bridge. The relocated oxygen, known as O6 or Ox, may serve as a substrate, combining with O5 to form O2 during the final step in the catalytic cycle, or may be positioned to become a substrate during the next catalytic cycle. Recent serial femtosecond X-ray crystallographic studies show that the flexibility of D1-E189 plays a critical role in facilitating the relocation of O6/Ox. In this study, the D1-E189G and D1-E189S mutations were characterized with FTIR difference spectroscopy. The data show that both mutations support Mn4Ca cluster assembly, substantially inhibit advancement beyond the S2 state, and alter the network of H bonds that surrounds the Mn4Ca cluster. Previously, the D1-E189Q, D1-E189K, and D1-E189R mutations were shown to have little impact on the activity, electron transfer rates, or spectral properties of Photosystem II. A rationale for this behavior is presented. The residue D1-E329 interacts with water molecules in the O1 water network that has been suggested recently to supply substrate during the catalytic cycle. Characterization of the D1-E329A mutant with FTIR difference spectroscopy shows that this mutation does not substantially perturb the structure of PSII or the water molecules whose O-H stretching modes change during the catalytic cycle. This result provides additional evidence that the water molecules whose vibrational properties change during the S1 to S2 transition are confined approximately to the region bounded by D1-N87, D1-N298, and D2-K317.

Blinatumomab compared with standard of care for the treatment of adult patients with relapsed/refractory Philadelphia chromosome-positive B-precursor acute lymphoblastic leukemia.

BACKGROUND: A single-arm, phase 2 trial demonstrated the efficacy and safety of blinatumomab, a bispecific T-cell-engaging antibody construct, in patients with relapsed/refractory (r/r) Philadelphia chromosome-positive (Ph+) acute lymphoblastic leukemia (ALL), a rare hematologic malignancy with limited treatment options. This study compared outcomes with blinatumomab with those of a historical control treated with the standard of care (SOC). METHODS: The blinatumomab trial enrolled adult patients with Ph+ ALL who were r/r to at least 1 second-generation tyrosine kinase inhibitor (n = 45). Propensity score analysis (PSA) was used to compare outcomes with blinatumomab with those of an external cohort of similar patients receiving SOC chemotherapy (n = 55). The PSA mitigated confounding variables between studies by adjusting for imbalances in the age at diagnosis and start of treatment, sex, duration from diagnosis to most recent treatment, prior allogeneic hematopoietic stem cell transplantation, prior salvage therapy, and number of salvage therapies. Bayesian data augmentation was applied to improve power to 80% with data from a phase 3 blinatumomab study in r/r Philadelphia chromosome-negative ALL. RESULTS: In the PSA, the rate of complete remission or complete remission with partial hematologic recovery was 36% for blinatumomab and 25% for SOC, and this resulted in an odds ratio of 1.54 (95% confidence interval [CI], 0.61-3.89) or 1.70 (95% credible interval [CrI], 0.94-2.94) with Bayesian data augmentation. Overall survival favored blinatumomab over SOC, with a hazard ratio of 0.81 (95% CI, 0.57-1.14) or 0.77 (95% CrI, 0.61-0.96) with Bayesian data augmentation. CONCLUSIONS: These results further support blinatumomab as a treatment option for patients with r/r Ph+ ALL.

Identification of patients and plaques vulnerable to future coronary events with near-infrared spectroscopy intravascular ultrasound imaging: a prospective, cohort study.

BACKGROUND: Near-infrared spectroscopy (NIRS) intravascular ultrasound imaging can detect lipid-rich plaques (LRPs). LRPs are associated with acute coronary syndromes or myocardial infarction, which can result in revascularisation or cardiac death. In this study, we aimed to establish the relationship between LRPs detected by NIRS-intravascular ultrasound imaging at unstented sites and subsequent coronary events from new culprit lesions. METHODS: In this prospective, cohort study (LRP), patients from 44 medical centres were enrolled in Italy, Latvia, Netherlands, Slovakia, UK, and the USA. Patients with suspected coronary artery disease who underwent cardiac catheterisation with possible ad hoc percutaneous coronary intervention were eligible to be enrolled. Enrolled patients underwent scanning of non-culprit segments using NIRS-intravascular ultrasound imaging. The study had two hierarchal primary hypotheses, patient and plaque, each testing the association between maximum 4 mm Lipid Core Burden Index (maxLCBI4mm) and non-culprit major adverse cardiovascular events (NC-MACE). Enrolled patients with large LRPs (≥250 maxLCBI4mm) and a randomly selected half of patients with small LRPs (<250 maxLCBI4mm) were followed up for 24 months. This study is registered with ClinicalTrials.gov, NCT02033694. FINDINGS: Between Feb 21, 2014, and March 30, 2016, 1563 patients were enrolled. NIRS-intravascular ultrasound device-related events were seen in six (0·4%) patients. 1271 patients (mean age 64 years, SD 10, 883 [69%] men, 388 [31%]women) with analysable maxLCBI4mm were allocated to follow-up. The 2-year cumulative incidence of NC-MACE was 9% (n=103). Both hierarchical primary hypotheses were met. On a patient level, the unadjusted hazard ratio (HR) for NC-MACE was 1·21 (95% CI 1·09-1·35; p=0·0004) for each 100-unit increase maxLCBI4mm) and adjusted HR 1·18 (1·05-1·32; p=0·0043). In patients with a maxLCBI4mm more than 400, the unadjusted HR for NC-MACE was 2·18 (1·48-3·22; p<0·0001) and adjusted HR was 1·89 (1·26-2·83; p=0·0021). At the plaque level, the unadjusted HR was 1·45 (1·30-1·60; p<0·0001) for each 100-unit increase in maxLCBI4mm. For segments with a maxLCBI4mm more than 400, the unadjusted HR for NC-MACE was 4·22 (2·39-7·45; p<0·0001) and adjusted HR was 3·39 (1·85-6·20; p<0·0001). INTERPRETATION: NIRS imaging of non-obstructive territories in patients undergoing cardiac catheterisation and possible percutaneous coronary intervention was safe and can aid in identifying patients and segments at higher risk for subsequent NC-MACE. NIRS-intravascular ultrasound imaging adds to the armamentarium as the first diagnostic tool able to detect vulnerable patients and plaques in clinical practice. FUNDING: Infraredx.

Blinatumomab vs historic standard-of-care treatment for minimal residual disease in adults with B-cell precursor acute lymphoblastic leukaemia.

OBJECTIVES: Survival outcomes from a single-arm phase 2 blinatumomab study in patients with minimal residual disease (MRD)-positive B-cell precursor (BCP)-acute lymphoblastic leukaemia (ALL) were compared with those receiving standard of care (SOC) in a historic data set. METHODS: The primary analysis comprised adult Philadelphia chromosome (Ph)-negative patients in first complete haematologic remission (MRD ≥ 10-3 ). Relapse-free survival (RFS) and overall survival (OS) were compared between blinatumomab- and SOC-treatment groups. Baseline differences between groups were adjusted by propensity scores. RESULTS: The primary analysis included 73 and 182 patients from the blinatumomab and historic data sets, respectively. When weighted by age to the blinatumomab-treatment group, median RFS was 7.8 months and median OS was 25.9 months in the SOC-treated group. In the blinatumomab study, median RFS was 35.2 months; median OS was not evaluable. Propensity score weighting achieved balance with seven baseline prognostic factors. With adjustment for haematopoietic stem cell transplantation (HSCT) status, a 50% reduction in risk of relapse or death was observed with blinatumomab vs SOC. Median RFS, unadjusted for HSCT status, was 35.2 months with blinatumomab and 8.3 months with SOC. CONCLUSIONS: These analyses suggest that blinatumomab improves RFS, and possibly OS, in adults with MRD-positive Ph-negative BCP-ALL vs SOC.

Trends in the use of primary prophylactic colony-stimulating factors and neutropenia-related hospitalization in elderly cancer patients receiving myelosuppressive chemotherapy in the USA: 1995-2015.

PURPOSE: To assess changes in neutropenia-related hospitalization, myelosuppressive chemotherapy, and primary prophylactic colony-stimulating factor (PP-CSF) use in elderly cancer patients receiving myelosuppressive chemotherapy. METHODS: We identified annual cohorts of patients aged ≥ 66 years with breast cancer, lung cancer, or non-Hodgkin lymphoma (NHL) initiating myelosuppressive chemotherapy during 1995-2015 using Medicare 5% (1994-2008) and 20% (2007-2015) data. We described myelosuppressive chemotherapy changes by febrile neutropenia (FN) risk category (high, intermediate, unclassified), PP-CSF use, and, in the first cycle of myelosuppressive chemotherapy, neutropenia-related hospitalization (ICD-9-CM: 288.0X, first 5 positions). We evaluated hospitalization trends using a logistic regression model with spline curve of calendar year adjusting for baseline characteristics. RESULTS: Annual cohorts included 1451-2114 eligible patients for 1995-2007 and 5272-7603 for 2008-2015. Myelosuppressive chemotherapy use with high/intermediate FN risk increased from 31% in 1995 to 56% in 1999, stabilized through 2008 (range 56-61%), then decreased to 52% in 2015. PP-CSF use increased from 5.5% in 1995 to 52.7% in 2015, mainly due to pegfilgrastim introduction in 2002. Crude neutropenia-related hospitalization incidence decreased from 5.2% in 1995 to 2.7% in 2015; adjusted incidence decreased, on average, by 4.7% yearly before 2010 (p < 0.0001) and was flat from 2010 onward (p = 0.53). CONCLUSIONS: Among elderly patients with breast cancer, lung cancer, or NHL receiving myelosuppressive chemotherapy, PP-CSF use increased substantially after 2002. Neutropenia-related hospitalization incidence in the first cycle decreased yearly before 2010 and was flat afterward. Further studies are needed to understand overall decreasing neutropenia-related hospitalization trends and effects of changes in myelosuppressive chemotherapy and FN management.

Febrile neutropenia-related care and associated costs in elderly patients with breast cancer, lung cancer, or non-Hodgkin lymphoma.

PURPOSE: Limited information is available regarding elderly patients experiencing febrile neutropenia (FN). This study evaluated FN-related care among elderly cancer patients who received high/intermediate FN-risk chemotherapy and experienced ≥ 1 FN episodes. METHODS: We used Medicare data to identify patients aged ≥ 66 years who initiated high/intermediate FN-risk chemotherapy between 1 January 2008 and 31 August 2015 to treat breast cancer (BC), lung cancer (LC), or non-Hodgkin lymphoma (NHL) and had ≥ 1 FN episodes. We identified within-cycle FN episodes for each chemotherapy cycle on Part A inpatient claims or outpatient or Part B claims. We described the FN-related care setting (inpatient hospital, outpatient emergency department [ED], or outpatient non-ED) and reported mean total cost of FN-related care per episode overall and by care setting (adjusted to 2015 US$). RESULTS: We identified 2138, 3521, and 2862 patients with BC, LC, and NHL, respectively, with ≥ 1 FN episodes (total episodes: 2407, 3840, 3587, respectively). Most FN episodes required inpatient care (BC, 88.1%; LC, 93.0%; NHL, 93.2%) with mean hospital length of stay (LOS) 6.2, 6.5, and 6.8 days, respectively. Intensive care unit admission was required for 20.4% of BC, 29.0% of LC, and 25.7% of NHL hospitalizations (mean LOS: 4.7, 4.7, 5.5 days, respectively). The mean total cost of FN care per episode was $11,959 BC, $14,388 LC, and $15,006 NHL, with inpatient admission the costliest care component ($11,826; $14,294; and $14,873; respectively). CONCLUSIONS: Among elderly patients with BC, LC, or NHL who experienced FN, most FN episodes required costly hospital care, highlighting the FN burden on healthcare systems.

Coordination of Rad1-Rad10 interactions with Msh2-Msh3, Saw1 and RPA is essential for functional 3' non-homologous tail removal.

Double strand DNA break repair (DSBR) comprises multiple pathways. A subset of DSBR pathways, including single strand annealing, involve intermediates with 3' non-homologous tails that must be removed to complete repair. In Saccharomyces cerevisiae, Rad1-Rad10 is the structure-specific endonuclease that cleaves the tails in 3' non-homologous tail removal (3' NHTR). Rad1-Rad10 is also an essential component of the nucleotide excision repair (NER) pathway. In both cases, Rad1-Rad10 requires protein partners for recruitment to the relevant DNA intermediate. Msh2-Msh3 and Saw1 recruit Rad1-Rad10 in 3' NHTR; Rad14 recruits Rad1-Rad10 in NER. We created two rad1 separation-of-function alleles, rad1R203A,K205A and rad1R218A; both are defective in 3' NHTR but functional in NER. In vitro, rad1R203A,K205A was impaired at multiple steps in 3' NHTR. The rad1R218A in vivo phenotype resembles that of msh2- or msh3-deleted cells; recruitment of rad1R218A-Rad10 to recombination intermediates is defective. Interactions among rad1R218A-Rad10 and Msh2-Msh3 and Saw1 are altered and rad1R218A-Rad10 interactions with RPA are compromised. We propose a model in which Rad1-Rad10 is recruited and positioned at the recombination intermediate through interactions, between Saw1 and DNA, Rad1-Rad10 and Msh2-Msh3, Saw1 and Msh2-Msh3 and Rad1-Rad10 and RPA. When any of these interactions is altered, 3' NHTR is impaired.

One of the Substrate Waters for O2 Formation in Photosystem II Is Provided by the Water-Splitting Mn4CaO5 Cluster's Ca2+ Ion.

During the catalytic step immediately prior to O-O bond formation in Photosystem II, a water molecule deprotonates and moves next to the water-splitting Mn4CaO5 cluster's O5 oxo bridge. Considerable evidence identifies O5 as one of the two substrate waters that ultimately form O2. The relocated oxygen, known as O6 or Ox, may be the second. It is currently debated whether O6 or Ox originates as the Mn-bound water denoted W2 or as the Ca2+-bound water denoted W3. To distinguish between these two possibilities, we analyzed the D-O-D bending mode of the water molecule that deprotonates/relocates to become O6/Ox. We show that this D-O-D bending mode is not altered by the D1-S169A mutation. Previously, we showed that this D-O-D bending mode is altered substantially when Sr2+ is substituted for Ca2+. Because Sr2+/Ca2+ substitution alters this D-O-D bending mode but the D1-S169A mutation does not, we conclude that the water-derived oxygen that relocates and becomes O6/Ox derives from the Ca2+-bound W3. This conclusion provides an important constraint for proposed mechanisms of O-O bond formation in Photosystem II.

D1-S169A Substitution of Photosystem II Perturbs Water Oxidation.

In photosystem II (PSII), photosynthetic water oxidation occurs at the tetramanganese-calcium cluster that cycles through light-induced intermediates (S0-S4) to produce oxygen from two substrate waters. The surrounding hydrogen-bonded amino acid residues and waters form channels that facilitate proton transfer and substrate water delivery, thereby ensuring efficient water oxidation. The residue D1-S169 lies in the "narrow" channel and forms hydrogen bonds with the Mn4CaO5 cluster via waters W1 and Wx. To probe the role of the narrow channel in substrate-water binding, we studied the D1-S169A mutation. PSII core complexes isolated from mutant cells exhibit inefficient S-state cycling and delayed oxygen evolution. The S2-state multiline EPR spectrum of D1-S169A PSII core complexes differed significantly from that of wild-type, and FTIR difference spectra showed that the mutation strongly perturbs the extensive network of hydrogen bonds that extends at least from D1-Y161 (YZ) to D1-D61. These results imply a possible role of D1-S169 in proton egress or substrate water delivery.

Substitution of the D1-Asn87 site in photosystem II of cyanobacteria mimics the chloride-binding characteristics of spinach photosystem II.

Photoinduced water oxidation at the O2-evolving complex (OEC) of photosystem II (PSII) is a complex process involving a tetramanganese-calcium cluster that is surrounded by a hydrogen-bonded network of water molecules, chloride ions, and amino acid residues. Although the structure of the OEC has remained conserved over eons of evolution, significant differences in the chloride-binding characteristics exist between cyanobacteria and higher plants. An analysis of amino acid residues in and around the OEC has identified residue 87 in the D1 subunit as the only significant difference between PSII in cyanobacteria and higher plants. We substituted the D1-Asn87 residue in the cyanobacterium Synechocystis sp. PCC 6803 (wildtype) with alanine, present in higher plants, or with aspartic acid. We studied PSII core complexes purified from D1-N87A and D1-N87D variant strains to probe the function of the D1-Asn87 residue in the water-oxidation mechanism. EPR spectra of the S2 state and flash-induced FTIR spectra of both D1-N87A and D1-N87D PSII core complexes exhibited characteristics similar to those of wildtype Synechocystis PSII core complexes. However, flash-induced O2-evolution studies revealed a decreased cycling efficiency of the D1-N87D variant, whereas the cycling efficiency of the D1-N87A PSII variant was similar to that of wildtype PSII. Steady-state O2-evolution activity assays revealed that substitution of the D1 residue at position 87 with alanine perturbs the chloride-binding site in the proton-exit channel. These findings provide new insight into the role of the D1-Asn87 site in the water-oxidation mechanism and explain the difference in the chloride-binding properties of cyanobacterial and higher-plant PSII.