Mechanisms Underlying Sinus Node Dysfunction in a Rat Model of Genetic Atrial Cardiomyopathy

Sinoatrial node (SAN) dysfunction is commonly associated with atrial dysrhythmia (tachy-brady syndrome) and is a particularly important feature of inherited atrial cardiomyopathies leading to artificial pacemaker implantation. Essential MYL4 (myosin light chain-4) is an atrial-selective protein that associates with the myosin light chain and participates importantly in cardiacmuscle contraction. MYL4 gene variants encoding dysfunctional versions of MYL4 cause familial atrial cardiomyopathy with a high incidence of early SAN dysfunction (SND) and pacemaker requirement. In this study, we used a rat line, genetically modified to express an E11K gene mutation responsible for familial atrial cardiomyopathy, to address the mechanisms underlying SND.

Cardiac structure and function were assessed by echocardiography and in vivo telemetry recording. SAN function was studied in vivo with intracardiac electrophysiology and ex vivo with optical mapping. Mechanisms underlying SND were interrogated in vitro with the use of voltage and current clamp with tight-seal patch-clamp and Ca²⁺ imaging of isolated SAN cardiomyocytes. Gene expression was assessed by quantitative polymerase chain reaction, and fibrosis was determined with Masson's trichrome stain.

Mutant Myl4-p.E11K^+/+ rats exhibited worse SAN function compared with wild-type controls. In vivo, SND was demonstrated by ≈63% increase in sinus node recovery time compared with wild type. In vitro, SAN conduction velocity was reduced by ≈ 50% for Myl4-p.E11K^+/+ compared with wild type. Isolated SAN cells showed ≈50% reduction in funny current and L-type Ca²⁺-current densities. Dysregulation of Ca²⁺ homeostasis was observed in Myl4-p.E11K^+/+, with ≈30% slower time to peak and Ca²⁺ decay. Masson's trichrome staining showed ≈45% increase in SAN region collagen deposition in Myl4-p.E11K^+/+.

Myl4-p.E11K^+/+ mutation causes progressive SND with aging, as a result of extensive abnormalities in the underlying determinants of SAN function, including ion-channel properties, Ca²⁺-homeostasis, and SAN structure. These observations provide new insights into the mechanisms of SAN abnormality in atrial cardiomyopathy.