Atrial-specific reduction of Kir2.1 channel pore diameter in addition to loss of inward-going rectification underlies inducible atrial fibrillation in a mouse model of short QT syndrome type 3

Introduction

Short QT Syndrome Type 3 (SQTS3) is an extremely rare arrhythmogenic disease caused by gain-of-function mutations in the KCNJ2 gene coding the inward rectifier potassium channel Kir2.1. We investigated arrhythmogenic mechanisms associated with a de-novo mutation (E299V) in Kir2.1 in an 11-year-old boy presenting an extremely abbreviated QT interval, paroxysmal atrial fibrillation, and mild left ventricular dysfunction. Amino acid E299 in the Kir2.1 sequence is necessary for polyamine binding induced inward rectification.

Purpose

Test the hypothesis that Kir2.1E299V induces reduced conductance and lack of rectification that causes electrical defects in atrial cardiomyocytes, predisposing patients to atrial arrhythmias.

Methods

We used intravenous adeno-associated virus-mediated gene transfer to generate mice expressing wild-type (WT) and the E299V mutant protein. We used ECG, intracardiac stimulation, patch-clamp, molecular biology and computational modelling to characterize the models and study arrhythmia mechanisms in the atria and ventricles.

Results

We confirmed WT or mutant Kir2.1 gene expression specifically in the mouse heart. On ECG, the corrected QT (QTc) interval of Kir2.1E299V mice was significantly shorter than Kir2.1WT mice (p<0.0001). The PR interval in Kir2.1E299V was also significantly shorter than WT mice (p<0.0001). On intracardiac stimulation, the largest proportion of arrhythmic events occurred in the atria, as 7 out of 9 Kir2.1E299V mice presented >1 second atrial flutter/fibrillation, while only 2 out of 10 Kir2.1WT mice showed this type of arrhythmia (p=0.023). On patch clamping, both atrial and ventricular cardiomyocytes expressing Kir2.1E299V had extremely abbreviated action potential durations (APD90) at all frequencies studied (p<0.0001). The current/voltage relation of ventricular Kir2.1E299V cardiomyocytes revealed an absence of inward-going rectification and increased IK1 at voltages positive to −80 mV compared to Kir2.1WT cardiomyocytes (p<0.0001). In contrast, while in the atrial Kir2.1E299V cardiomyocytes the outward IK1 was increased at voltages positive to −80 mV with loss of rectification, IK1 was significantly reduced at voltages negative to −80 mV (p<0.0001), suggesting a loss of function leading to atrial arrhythmia inducibility. A higher proportion of Kir2.2 at atrial level and atomic in-silico 3D simulations suggested that the mutation impaired polyamine block of the Kir2.1E299V-Kir2.2 channel while reducing the pore diameter.

Conclusions

This first in-vivo mouse model of cardiac-specific SQTS3 recapitulates the electrophysiological phenotype of a patient with the Kir2.1E299V mutation. The mutation results in a Kir2.1 gain-of-function mediated by and absence of rectification. The predominant arrhythmias induced in these SQTS3 mice were supraventricular likely due to the combined lack of inward rectification and atrial-specific reduced pore diameter of the Kir2.1E299V-Kir2.2 channel.

Funding Acknowledgement

Type of funding sources: Private company. Main funding source(s): La Caixa FoundationLa Maratό TV3 Foundation

Three dimensional modelling of mutant Kir2.1 channel PIP2 interactions help stratify arrhythmia severity in Andersen Tawil syndrome type 1

Background

Andersen-Tawil type 1 (ATS1) is associated with loss-of-function mutations in the inward rectifier potassium channel Kir2.1, which controls cardiac excitability and impulse conduction. Phosphatidylinositol-4,5-bisphosphate (PIP2) acts as an essential cofactor regulating the opening of Kir2.1 channels. Fifty percent of reported ATS1 mutations affect Kir2.1-PIP2 interactions, leading to ECG defects, ventricular arrhythmias and sudden cardiac death (SCD) by mechanisms that are poorly understood.

Purpose

To test the hypothesis that the degree of arrhythmogenic severity of ATS1 mutations disrupting PIP2-Kir2.1 binding may be predicted by the level of polarization of the mutant Kir2.1 channel pore.

Methods

We first used a statistical mean value approach to classify the 40 known arrhythmogenic ATS1 mutations impacting Kir2.1-PIP2 interaction (N=260 individuals) according to arrhythmogenic severity, ranging from SCD through ventricular bigeminy and QT prolongation. We then generated 3D in-silico atomic models of the wildtype channel and the 10 mutant channels with the most severe arrhythmic phenotype to assess the mechanism of the structural defects associated with Kir2.1-PIP2 disruption.

Results

Our cardiac lethality scoring stratifying Kir2.1 mutations according to arrhythmogenic severity was validated by three additional biostatical quantitative measures. On in-silico modelling, wildtype Kir2.1 channels without PIP2 binding had transmembrane and cytoplasmic pore radius of 1.5 and 3 Å, respectively. Kir2.1-PIP2 interactions increased transmembrane and cytoplasmic pore radius to 3 and 6 Å, respectively. All 10 Kir2.1 mutations had similar transmembrane and cytoplasmic pore radius of ∼1.0 and ∼3.0 Å, respectively. The most severe mutations yielded pore channels with highly polarized electrostatic forces. Remarkably, simulations showed a descending electrostatic pattern at the transmembrane region of PIP2 binding, where the more severe the mutation, the more positive that region was. Structural changes produced by mutations correlated with cardiac severity (R2=0.51; p<0.005) in that the most drastically altered protein structure correlated with the most severe arrhythmic phenotype.

Conclusions

Computer simulations of mutant Kir2.1 channel structure from the most arrhythmogenic to the least arrhythmogenic predict a gradual decrease in polarization of electrostatic forces along the Kir2.1 channel pore. The results reveal a novel mechanistic stratification of arrhythmogenic severity of ATS1 mutant Kir2.1 channel-PIP2 interactions and open new pathways for developing more personalized ATS1 patient therapies.

Funding Acknowledgement

Type of funding sources: Private grant(s) and/or Sponsorship. Main funding source(s): La Caixa Banking Foundation under the project code HR18-00304Fundaciόn La Marato TV3: Ayudas a la investigaciόn en enfermedades raras 2020

Arrhythmogenic and antiarrhythmic actions of late sustained sodium current in the adult human heart

Late sodium current (late INa) inhibition has been proposed to suppress the incidence of arrhythmias generated by pathological states or induced by drugs. However, the role of late INa in the human heart is still poorly understood. We therefore investigated the role of this conductance in arrhythmias using adult primary cardiomyocytes and tissues from donor hearts. Potentiation of late INa with ATX-II (anemonia sulcata toxin II) and E-4031 (selective blocker of the hERG channel) slowed the kinetics of action potential repolarization, impaired Ca2+ homeostasis, increased contractility, and increased the manifestation of arrhythmia markers. These effects could be reversed by late INa inhibitors, ranolazine and GS-967. We also report that atrial tissues from donor hearts affected by atrial fibrillation exhibit arrhythmia markers in the absence of drug treatment and inhibition of late INa with GS-967 leads to a significant reduction in arrhythmic behaviour. These findings reveal a critical role for the late INa in cardiac arrhythmias and suggest that inhibition of this conductance could provide an effective therapeutic strategy. Finally, this study highlights the utility of human ex-vivo heart models for advancing cardiac translational sciences.

The multikinase inhibitor RXDX-105 is effective against neuroblastoma in vitro and in vivo

Neuroblastoma is the most common extracranial solid tumor of childhood and accounts for 15% of all pediatric cancer-related deaths. New therapies are needed to improve outcomes for children with high-risk and relapsed tumors. Inhibitors of the RET kinase and the RAS-MAPK pathway have previously been shown to be effective against neuroblastoma, suggesting that combined inhibition may have increased efficacy. RXDX-105 is a small molecule inhibitor of multiple kinases, including the RET and BRAF kinases. We found that treatment of neuroblastoma cells with RXDX-105 resulted in a significant decrease in cell viability and proliferation in vitro and in tumor growth and tumor vascularity in vivo. Treatment with RXDX-105 inhibited RET phosphorylation and phosphorylation of the MEK and ERK kinases in neuroblastoma cells and xenograft tumors, and RXDX-105 treatment induced both apoptosis and cell cycle arrest. RXDX-105 also showed enhanced efficacy in combination with 13-cis-retinoic acid, which is currently a component of maintenance therapy for children with high-risk neuroblastoma. Our results demonstrate that RXDX-105 shows promise as a novel therapeutic agent for children with high-risk and relapsed neuroblastoma.

Cognitive declines following bilateral subthalamic nucleus deep brain stimulation for the treatment of Parkinson's disease

BACKGROUND

We investigated the cognitive and psychiatric outcome 6 months after bilateral subthalamic nucleus deep brain stimulation (DBS) for the treatment of Parkinson's disease (PD) using a disease control group.

METHODS

23 patients who underwent DBS were compared with 28 medically treated patients with PD at baseline and at 6 months for neuropsychological measures. In addition to the group outcomes, we report reliable change indices (RCI) and a dementia caseness analysis.

RESULTS

Patients who underwent DBS demonstrated a significant decline in verbal memory compared with the control group (p<0.003), and trends for decline on oral information processing, including verbal fluency, timed transcription and word naming. Patients who underwent DBS demonstrated declines in attention, set shifting and semantic fluency but these changes were similar to the rate of decline in the PD group. RCI indicated that patients who underwent DBS demonstrated clinically significant declines in verbal fluency (p<0.01) and inhibition of a dominant response (p<0.003), with trends for declines in set shifting (p<0.02) and verbal long term recall (p<0.08), indicative of frontostriatal dysfunction. Patients who underwent DBS did not demonstrate significant changes in depression, anxiety or psychological distress scores. The caseness analysis revealed that one of the patients who underwent DBS (4%) converted to dementia over 6 months compared with none of the PD controls.

CONCLUSIONS

Our findings demonstrated that patients who underwent DBS experienced declines in verbal recall and trends for declines in oral information processing 6 months following surgery, even when good motor outcome was achieved. Potential candidates should be counselled about the risk of mild frontostriatal cognitive declines following DBS to weigh the risks and benefits of surgery.

Short and long-term motor and cognitive outcome of staged bilateral pallidotomy: a retrospective analysis

BACKGROUND

We investigated retrospectively the short and long-term motor and cognitive functioning of staged bilateral pallidotomy using motor testing and a comprehensive neuropsychological battery before and after each procedure.

METHODS

Fifteen patients with idiopathic Parkinson's disease were assessed at baseline and at least 3 months after each of their two staged surgeries. Motor and neuropsychological results were compared to 15 non-surgical Parkinson's disease patients matched for disease stage and mental status. In addition, nine bilateral pallidotomy patients were evaluated for long-term cognitive changes (>2 years).

FINDINGS

Bilateral pallidotomy patients demonstrated significant improvements in motor functioning in the "on" and "off" states and with dyskinesias after the first surgery, with an additional improvement reported for dyskinesias after the second procedure. On long-term follow-up, dyskinesia improvements were maintained. Bilateral pallidotomy patients did not show significant cognitive declines following both procedures on the short-term follow-up and when compared to the Parkinson's disease group. However, significant cognitive declines were found on the long-term follow-up evaluation.

CONCLUSIONS

Parkinson's disease patients received significant short- and long-term motor benefits, particularly reduced dyskinesias, following staged bilateral pallidotomy without significant short-term cognitive consequences. Two years following the second procedure, bilateral pallidotomy patients tended to show an increase in both motor and non-motor symptoms of Parkinson's disease, particularly cognitive decline.

AINTEGUMENTA promotes petal identity and acts as a negative regulator of AGAMOUS

The Arabidopsis AINTEGUMENTA (ANT) gene has been shown previously to be involved in ovule development and in the initiation and growth of floral organs. Here, we show that ANT acts in additional processes during flower development, including repression of AGAMOUS (AG) in second whorl cells, promotion of petal epidermal cell identity, and gynoecium development. Analyses of ap2-1 ant-6 double mutants reveal that ANT acts redundantly with AP2 to repress AG in second whorl cells. The abaxial surface of ant petals contains features such as stomata and elongated, interdigitated cells that are not present on wild-type petals. The loss of petal identity in these second whorl cells does not result from ectopic AG expression, suggesting that ANT acts in a pathway promoting petal cell identity that is independent of its role in repression of AG. These data suggest that ANT may function as a class A gene.

Novel cross-reactive anti-idiotype antibodies with properties close to the human intravenous immunoglobulin (IVIg)

The most important link between the immune network theory and clinically useful therapies so far is the use of human intravenous immunoglobulin (IVIg) in the treatment of autoimmune diseases. Although still controversial, one of the main mechanisms that has been postulated for the in vivo effects of IVIg, is the selection of immune repertoires through idiotypic interactions. We describe here anti-idiotype IgG monoclonal antibodies (MAbs), which were obtained by immunization of syngeneic mice (Balb/c) with an anti-ganglioside antibody. These anti-idiotype MAbs show multiple idiotypic connections and share some of the properties of the IVIg pool. The antiidiotype (Ab2) MAbs B7 and 34B7 showed heterogeneous binding with the idiotypes of several anti-ganglioside antibodies, MAbs obtained from splenocytes of nonimmunized newborn mice, F(ab')2 fragments of IgG human myeloma proteins, and nonimmunoglobulin antigens. The recognition pattern of the B7 MAb to the idiotypes of human immunoglobulins was also studied using a phage display library obtained from the variable region genes of an asymptomatic AIDS patient and also F(ab')2 fragments obtained from an IVIg pool of healthy human donors. We also demonstrated that these MAbs produced some of the in vitro effects reported for the human IVIg pool, such as the inhibition of cell proliferation of human B and T cell lines and of normal human lymphocytes activated with different mitogens. Another striking property of the MAb B7 was its ability to induce a dose-dependent specific antibody T-cell response in vivo in syngeneic mice. Both anti-idiotype MAbs showed anti-metastatic effect in vivo when injected intravenously to mice inoculated with MB16-F10 melanoma cells. The antimetastatic effect of the antiidiotype MAbs was not observed in athymic mice inoculated with the same tumor. This kind of antibody can become an interesting tool for further exploration of the role of idiotypic network connections in the regulation of the immune system and to study the effects of interventions on network connectivity in experimental autoimmune disease, using a reagent better chemically defined than the IVIg pool.

Expression and regulation of the abd-A gene of Drosophila

We have developed a specific polyclonal antibody that recognizes the protein products of the abdominal-A (abd-A) gene, a member of the bithorax complex of Drosophila. The normal expression domain extends from parasegments 7 to 13, in good correspondence with previous genetic and molecular results. However, while the anterior border of expression is precisely demarcated by a parasegmental boundary, the posterior border does not coincide with a lineage boundary. Within the normal domain, the expression of abd-A shows intrametameric modulation; the amount of product is higher in posterior compartments and in the most anterior cells of the anterior compartments and then gradually decreases. We have examined the effect on abd-A expression of a number of mutations, some mapping within and others outside the abd-A transcription unit. Those mapping to the transcription unit eliminate or severely reduce the amount of abd-A antigen, while those mapping outside produce an abnormal distribution of abd-A protein. Finally, we show that the abd-A gene is down-regulated in part of the Abdominal-B (Abd-B) domain, precisely in those regions where the Abd-B gene is expressed at high levels.

Different forms of Ultrabithorax proteins generated by alternative splicing are functionally equivalent

The Ubx gene of Drosophila normally produces several forms of Ubx proteins through alternative splicing of two microexons. We describe here two new viable Ubx mutations that show similar and almost wild-type adult phenotypes. Molecular characterization has shown that one of them, UbxMX17, is an inversion within the Ubx transcription unit including one of the microexons involved in alternative splicing. This results in mutant flies possessing a very abnormal array of Ubx proteins, probably including spliced forms not present in wild-type flies. Yet these protein products successfully substitute for the normal ones and allow virtually normal Ubx function. We argue that the different Ubx proteins are developmentally equivalent and that the slight mutant phenotype observed in UbxMX17 flies is not due to the abnormal set of Ubx proteins but to a breakpoint in a cis-regulatory region.

Receptors for epidermal growth factor and sex steroid hormones in human colorectal carcinomas

Iodinated epidermal growth factor was used to identify its specific receptor in 32 human colorectal carcinomas. The receptor was found in 25% of the tumors with values between 6.9 - 104.0 fmoles/mg DNA. Receptors for estrogen and progesterone were detected in 45 and 25% of the tumors respectively. No correlation was observed between short term survival and content of receptors for sex steroid hormones. Four of the 8 patients with detectable levels of epidermal growth factor receptor had experienced relapse.

Transforming growth factor alpha in human mammary carcinomas and their metastases

A binding competition assay was used to measure the content of transforming growth factor alpha in human mammary carcinomas and their metastases. Thirty one percent of the primary and 50 percent of the metastases expressed the transforming growth factor alpha with values ranging between 1.6 to 278 EGF equivalent ng/mg tumour extract protein. There was no correlation between the transforming growth factor alpha content and the estrogen receptor value.

Identification of transforming growth factor alpha in human primary breast carcinomas

Primary human mammary carcinomas were found to contain an acid stable polypeptide which bound to the receptor for epidermal growth factor (EGF). This polypeptide which was antigenetically different from EGF and thus identified as Tumor Growth Factor (TGF) alpha, could be identified in 25% of the tumors. Among these tumors 40% contained measurable levels of the EGF receptor. Thus these tumors exhibit the molecular prerequisite for autocrine growth stimulation.

Prognostic significance of the receptor for epidermal growth factor in human mammary carcinomas

125I-EGF binding technique was used to demonstrate high affinity receptor binding for epidermal growth factor (EGF-R) in 72 human mammary carcinomas. 27% of the tumors were EGF-R positive and the presence of this receptor was correlated with receptor levels for estradiol, DNA-pattern, proliferative index, thymidine kinase, tumor size, number of lymph node metastases and 6-year relapse probability. Our results confirm previous reports of an inverse relation between the cellular content of EGF-R and ER. In addition, we could associate EGF-R positivity with an aneuploid DNA-pattern and an increased growth rate, as measured by proliferative index. No correlation was found between EGF-R positivity in the primary tumor and presence of axillary lymph node metastasis at the time of operation. The 6-year relapse rate was somewhat higher for patients whose primary tumors were EGF-R positive. Moreover patients who had lymph node metastases at the time of operation and EGF-R positive tumors experienced a significantly lower 6-year disease free survival rate as compared to those who were node negative and had receptor negative tumors. It remains to be shown whether EGF-R alone can be used as an independent prognostic factor.