Retuning the Core of the Atrial Symphony

Abstract

Atrial fibrillation (AF) is one of the most common age-acquired cardiac arrhythmias, with around 15% of patients having a genetic predisposition. Genetic variants associated with AF predominantly affect ion channels or structural proteins that shape and support cardiomyocytes. Two examples are LMNA and DES, encoding the intermediate filament proteins lamin A/C and desmin, respectively. Lamin A/C forms part of the nuclear lamina, supporting nuclear structure and protecting DNA, whereas desmin forms cytoskeletal filaments that connect the contractile apparatus to the nucleus, plasma membrane, and other organelles. However, how variants in these genes contribute to AF remains poorly understood. Cardiomyocytes are highly specialized cells exposed to continuous mechanical stress. Their long-term function therefore depends on maintaining protein homeostasis (proteostasis) and structural integrity. Heat shock proteins (HSPs) are molecular chaperones that support proteostasis by refolding or facilitating degradation of damaged proteins. Chapter 2 explores their potential role in AF and shows that boosting HSPB1 function with geranylgeranylacetone (GGA) protects the microtubule network in experimental AF models. Chapter 3 further reviews the role of cytoskeletal and cytoskeletal-associated proteins in AF, highlighting parallels between cytoskeletal disruption and features of AF, including electrophysiological alterations, mitochondrial dysfunction, DNA damage, and metabolic stress. Lamin A/C is among the structural proteins most strongly associated with genetic AF. In Chapter 4, Drosophila models expressing four AF-associated LamC variants were electrically tachypaced. Two variants increased vulnerability to arrhythmogenesis, while taxol had opposite effects depending on the variant. These findings demonstrate that different LMNA variants can produce distinct, and sometimes opposing, effects on arrhythmia susceptibility. This variant-specificity was further investigated in Chapter 5 using HL-1 cardiomyocytes expressing human lamin A variants R331Q or Q493X. Although neither variant significantly altered nuclear shape, they produced opposing changes in cellular viscoelasticity. Both variants nevertheless increased arrhythmicity of calcium transients, suggesting that different structural consequences can converge on a common arrhythmic phenotype. Together, the Drosophila and cellular models indicate that therapeutic strategies for LMNA-associated arrhythmia may need to be tailored to individual variants. In Chapter 6, the heterozygous Lmna knockout mouse model was investigated using in vivo and ex vivo electrical stimulation. In contrast to the Drosophila model, no significant differences in atrial arrhythmogenesis or sinus node responses were observed compared with wild-type littermates. These findings suggest that this mouse model may have limited value for studying the effects of individual LMNA variants on AF. Chapter 7 examined the relationship between lamin A/C and another intermediate filament protein, desmin. AF-associated DES variants p.N342D and p.S13F caused severe filament assembly defects, perinuclear aggregation, disruption of nuclear-associated proteins, and increased nuclear envelope rupture and DNA leakage. They also impaired action potential duration and calcium handling. Treatment with GGA partially improved these phenotypes, particularly in p.N342D-expressing cells, supporting HSP induction as a potential therapeutic strategy. Finally, Chapter 8 investigated DNA damage in patient-derived atrial tissue and serum using LORD-Q PCR. DNA lesions were increased in paroxysmal and persistent AF, particularly during the first three years of disease. Nuclear DNA damage correlated with fractionated potentials, while mitochondrial DNA lesions correlated with slower conduction velocity and lower potential amplitudes. These findings suggest that DNA damage may not merely accompany atrial remodeling but may actively contribute to the electrophysiological alterations underlying AF. Collectively, this thesis demonstrates that disruption of intermediate filament proteins, proteostasis, cytoskeletal organization, and genome stability are interconnected features of AF. Importantly, the effects of genetic variants are highly context- and variant-dependent, highlighting the need for a more individualized understanding of the molecular mechanisms underlying AF.