The autonomic nervous system (ANS) plays an important role in creating a vulnerability to atrial fibrillation in otherwise healthy and young individuals. This is commonly referred to as vagal atrial fibrillation. The ANS regulates the parasympathetic (vagal) tone of the heart via fast synaptic transmission of neural inputs into the cardiac neural ganglia, which surround the heart. The molecular mediators of fast synaptic transmission of neurons comprising these ganglia are nictotinic acetylcholine receptors (nAChRs), especially α3β4 and α3α5β4 subtypes, which bind acetylcholine and lead to rapid transmission of cholinergic signalling onto atrial myocardial tissue. This vagal signalling leads to altered electrical properties of the atria, specifically slowing of heart and shortening of tissue refractoriness, which in turn set up a vulnerable substrate for initiation of AF. In this research, we hypothesized that rare genetic variants in the molecular mediators of parasympathetic transmission to the heart may be present in some patients with AF, and imbue a hypersensitivity of vagal tone on the heart contributing to an increased vulnerability to arrhythmia. In a large cohort of other healthy patients with lone AF, we identified and functionally characterized rare genetic variants of both the neuronal nicotinic receptor B4 subunit gene (CHRNB4) and the neuronal nicotinic receptor chaperone gene, Ric-3.
Methods and Results
We comprehensively sequenced the CHRNB4 and Ric-3 genes in 315 patients with lone AF and identified 4 rare mutations in each gene, absent from 600 elderly (age > 70 years) controls with no reported history of arrhythmia. Mutant or wild-type CHRNB4 were co-expressed in heterologous HEK tsa-201 cells with the alpha-3 nicotinic receptor subunit (CHRNA3) cell and the functional response to acetylcholine and nicotine agonists was measured. For Ric-3, mutant or wild-type Ric-3 were co-expressed in HEK293 cells stably expressing α3α5β4 nAChRs. Mutations in CHRNB4 showed significant differences in biophysical function, including alterations in receptor current density and sensitivity in response to neuroligand. A unifying observation among 4 mutant receptors was a significant increase in macroscopic current as compared to wild-type channels following repetitive application of agonist, indicating loss of receptor desensitization. All 4 rare mutations in the Ric-3 gene led to significant enhancement of both nAChR current and surface expression.
These observations are consistent with a gain-of-function in nAChR behavior secondary to the identified rare mutations in the CHRNB4 and Ric-3 genes, which suggest that genetic influences on autonomic tone may contribute to AF vulnerability.|
|dc.publisher||Université d'Ottawa / University of Ottawa|
|dc.title||Novel Molecular Mechanisms in the Pathophysiology of Human Atrial Fibrillation|
|thesis.degree.discipline||Médecine / Medicine|
|uottawa.department||Médecine cellulaire et moléculaire / Cellular and Molecular Medicine|
|Collection||Thèses - Embargo // Theses - Embargo|