Rebecca C. Ahrens-Nicklas

Re-evaluating the efficacy of β-adrenergic agonists and antagonists in long QT-3 syndrome through computational modelling

AIMS Long QT syndrome (LQTS) is a heterogeneous collection of inherited cardiac ion channelopathies characterized by a prolonged electrocardiogram QT interval and increased risk of sudden cardiac death. Beta-adrenergic blockers are the mainstay of treatment for LQTS. While their efficacy has been demonstrated in LQTS patients harbouring potassium channel mutations, studies of beta-blockers in subtype 3 (LQT3), which is caused by sodium channel mutations, have produced ambiguous results. In this modelling study, we explore the effects of beta-adrenergic drugs on the LQT3 phenotype. METHODS AND RESULTS In order to investigate the effects of beta-adrenergic activity and to identify sources of ambiguity in earlier studies, we developed a computational model incorporating the effects of beta-agonists and beta-blockers into an LQT3 mutant guinea pig ventricular myocyte model. Beta-activation suppressed two arrhythmogenic phenomena, transmural dispersion of repolarization and early after depolarizations, in a dose-dependent manner. However, the ability of beta-activation to prevent cardiac conduction block was pacing-rate-dependent. Low-dose beta-blockade by propranolol reversed the beneficial effects of beta-activation, while high dose (which has off-target sodium channel effects) decreased arrhythmia susceptibility. CONCLUSION These results demonstrate that beta-activation may be protective in LQT3 and help to reconcile seemingly conflicting results from different experimental models. They also highlight the need for well-controlled clinical investigations re-evaluating the use of beta-blockers in LQT3 patients.

Molecular annotation of integrative feeding neural circuits.

The identity of higher-order neurons and circuits playing an associative role to control feeding is unknown. We injected pseudorabies virus, a retrograde tracer, into masseter muscle, salivary gland, and tongue of BAC-transgenic mice expressing GFP in specific neural populations and identified several CNS regions that project multisynaptically to the periphery. MCH and orexin neurons were identified in the lateral hypothalamus, and Nurr1 and Cnr1 in the amygdala and insular/rhinal cortices. Cholera toxin β tracing showed that insular Nurr1(+) and Cnr1(+) neurons project to the amygdala or lateral hypothalamus, respectively. Finally, we show that cortical Cnr1(+) neurons show increased Cnr1 mRNA and c-Fos expression after fasting, consistent with a possible role for Cnr1(+) neurons in feeding. Overall, these studies define a general approach for identifying specific molecular markers for neurons in complex neural circuits. These markers now provide a means for functional studies of specific neuronal populations in feeding or other complex behaviors.

Anthropomorphizing the Mouse Cardiac Action Potential via a Novel Dynamic Clamp Method

Interspecies differences can limit the translational value of excitable cells isolated from model organisms. It can be difficult to extrapolate from a drug- or mutation-induced phenotype in mice to human pathophysiology because mouse and human cardiac electrodynamics differ greatly. We present a hybrid computational-experimental technique, the cell-type transforming clamp, which is designed to overcome such differences by using a calculated compensatory current to convert the macroscopic electrical behavior of an isolated cell into that of a different cell type. We demonstrate the techniques utility by evaluating drug arrhythmogenicity in murine cardiomyocytes that are transformed to behave like human myocytes. Whereas we use the cell-type transforming clamp in this work to convert between mouse and human electrodynamics, the technique could be adapted to convert between the action potential morphologies of any two cell types of interest.

Pathologic Variants of the Mitochondrial Phosphate Carrier SLC25A3: Two New Patients and Expansion of the Cardiomyopathy/Skeletal Myopathy Phenotype With and Without Lactic Acidosis

Variants in the SLC25A3 gene, which codes for the mitochondrial phosphate transporter (PiC), lead to a failure of inorganic phosphate (Pi) transport across the mitochondrial membrane, which is required in the final step of oxidative phosphorylation. The literature described two affected sibships with variants in SLC25A3; all cases had skeletal myopathy and cardiomyopathy (OMIM 610773). We report here two new patients who had neonatal cardiomyopathy; one of whom did not have skeletal myopathy nor elevated lactate. Patient 1 had a homozygous splice site variant, c.158-9A>G, which has been previously reported in a Turkish family. Patient 2 was found to be a compound heterozygote for two novel variants, c.599T>G (p.Leu200Trp) and c. 886_898delGGTAGCAGTGCTTinsCAGATAC (p.Gly296_Ser300delinsGlnIlePro). Protein structure analysis indicated that both variants are likely to be pathogenic. Sequencing of SLC25A3 should be considered in patients with isolated cardiomyopathy, even those without generalized skeletal myopathy or lactic acidosis.

Precision therapy for a new disorder of AMPA receptor recycling due to mutations inATAD1

Objective: ATAD1 encodes Thorase, a mediator of α-amino-3-hydroxy-5-methylisoxazole-4-proprionate (AMPA) receptor recycling; in this work, we characterized the phenotype resulting from ATAD1 mutations and developed a targeted therapy in both mice and humans. Methods: Using exome sequencing, we identified a novel ATAD1 mutation (p.E276X) as the etiology of a devastating neurologic disorder characterized by hypertonia, seizures, and death in a consanguineous family. We postulated that pathogenesis was a result of excessive AMPA receptor activity and designed a targeted therapeutic approach using perampanel, an AMPA-receptor antagonist. Results: Perampanel therapy in ATAD1 knockout mice reversed behavioral defects, normalized brain MRI abnormalities, prevented seizures, and prolonged survival. The ATAD1 patients treated with perampanel showed improvement in hypertonicity and resolution of seizures. Conclusions: This work demonstrates that identification of novel monogenic neurologic disorders and observation of response to targeted therapeutics can provide important insights into human nervous system functioning.

Efficacy of early treatment in patients with cobalamin C disease identified by newborn screening: a 16-year experience

Purpose:Despite implementation of newborn screening (NBS), outcomes in cobalamin C disease (cblC) remain poor. Therapy with hydroxycobalamin and betaine is widely used, but dietary recommendations vary among metabolic centers. We present a longitudinal analysis of the relationship between metabolic control, diet, and outcomes in a cohort of cblC patients.Methods:We completed a retrospective analysis of 12 patients with cblC referred for abnormal NBS results and followed in our center between 1999 and 2015.Results:Of the patients, 87.5% had intellectual disability and 75% had retinopathy; 16.7% had one episode of mild acidosis. However, no patients manifested major metabolic decompensation. Developmental outcomes correlated more closely with initial metabolic abnormalities than with long-term metabolic control. Increased intake of medical foods resulted in better control but also perturbations in the ratios of essential amino acids and lower z-scores for head circumference. We found no relationship between diet and cognitive outcomes.Conclusions:Although dietary therapy for cblC patients improves metabolic control, few patients experience metabolic decompensation regardless of diet. Increased incomplete protein intake is not correlated with improvements in outcomes. Overall, outcomes are poor despite early initiation of therapy and regardless of the dietary strategy used.Genet Med advance online publication 02 February 2017

Utility of genetic evaluation in infants with congenital heart defects admitted to the cardiac intensive care unit.

Congenital heart defects (CHDs) are heterogeneous and present with a spectrum of severity, with roughly 25% of patients requiring intervention before age 1. The etiology of disease is unknown in many individuals; however, there is a rapidly expanding understanding of genetic risk factors that may contribute to pathogenesis. Through this work, we sought to evaluate the diagnostic yield of a clinical genetics evaluation and associated genetic testing among infants with critical CHDs. Furthermore, we aimed to both determine the utility of microarray and establish a strong baseline that can be used in future studies of the impact of exome sequencing in this population. We completed a retrospective chart review of 364 infants with CHDs admitted to the Cardiac Intensive Care Unit who underwent a clinical genetics evaluation. A genetic diagnosis was established in 25% of patients: 9% of infants were diagnosed prenatally, while 16% were diagnosed postnatally. Cardiac lesion subtype greatly influenced the diagnostic yield. On physical exam, the presence of dysmorphic features, as assessed by a clinical geneticist, was associated with a sevenfold increased likelihood of reaching a diagnosis. Directed by clinical acumen, diagnostic rates varied by testing modality with rates of 23% for karyotype, 12% for fluorescent in situ hybridization or multiplex‐dependent ligation probe analysis, 9% for genome wide microarray, and 17% for targeted gene sequencing. Careful consideration of lesion subtype and physical exam findings clarify populations of infants with CHD that benefit from a genetics evaluation and inform an efficient testing paradigm.

Adolescent Presentations of Inborn Errors of Metabolism

Several studies have shown that a large percentage of inborn errors of metabolism is present in adolescent patients. Individually, each diagnosis in this category of diseases is rare; therefore, there is often a significant delay in determining the etiology of a patients complaints. These disorders can have a wide variety of multisystemic presentations, several of which overlap with more common disorders of adolescence. This review highlights the red-flag findings on history and physical examination indicating a possible inborn error of metabolism. In addition, a systematic approach for evaluating and categorizing these disorders is introduced and demonstrated through case examples. Primary care physicians play a crucial role in the early detection and prompt treatment of patients with late-onset inborn errors of metabolism.

Cobalamin C Disease Missed by Newborn Screening in a Patient with Low Carnitine Level.

Cobalamin C (CblC) disease is the most common inherited disorder of intracellular cobalamin metabolism. It is a multisystemic disorder mainly affecting the eye and brain and characterized biochemically by methylmalonic aciduria, low methionine level, and homocystinuria. We report a patient found to have CblC disease who initially presented with low carnitine and normal propionylcarnitine (C3) levels on newborn screen. Newborn screening likely failed to detect CblC in this patient because of both his low carnitine level and the presence of a mild phenotype.

Stimulation of entorhinal cortex–dentate gyrus circuitry is antidepressive

Major depressive disorder (MDD) is considered a ‘circuitopathy’, and brain stimulation therapies hold promise for ameliorating MDD symptoms, including hippocampal dysfunction. It is unknown whether stimulation of upstream hippocampal circuitry, such as the entorhinal cortex (Ent), is antidepressive, although Ent stimulation improves learning and memory in mice and humans. Here we show that molecular targeting (Ent-specific knockdown of a psychosocial stress-induced protein) and chemogenetic stimulation of Ent neurons induce antidepressive-like effects in mice. Mechanistically, we show that Ent-stimulation-induced antidepressive-like behavior relies on the generation of new hippocampal neurons. Thus, controlled stimulation of Ent hippocampal afferents is antidepressive via increased hippocampal neurogenesis. These findings emphasize the power and potential of Ent glutamatergic afferent stimulation—previously well-known for its ability to influence learning and memory—for MDD treatment.In mouse models of stress-induced depression, molecular and chemogenetic stimulation of the entorhinal cortex induces the production of adult-born hippocampal neurons and generates antidepressive-like effects.