Postnatal Progressive Atrioventricular Block By Heterozygous Nkx2-5 Homeodomain Missense Mutation With Atrioventricular Nodal Atrophy

Introduction: Heterozygous human NKX2-5 homeodomain (DNA binding domain) missense mutations are highly penetrant for varied cardiac anomalies and progressive atrioventricular block (AVB). We replicated many elements of the cardiac anomalies in a murine heterozygous knock-in model (Nkx2-5+/R52G, hereafter Nkx2-5-KI) in a 129/Sv background. Hypothesis: We hypothesized that Nkx2-5-KI mice demonstrate progressive AVB during postnatal life similar to human patients. Methods: Nkx2-5-KI mouse model and control Nkx2-5+/+ mice were analyzed by histopathology, surface and telemetry ECG, and in vivo electrophysiology studies (EPS), and compared at postnatal day 1 (P1) to 17 months. We re-evaluated a pedigree of a human family having Arg52(189)Gly mutation in 5 generations. Results: Nkx2-5-KI mice demonstrated PR-prolongation (1st degree AVB) at 4 weeks, 7 months, and 17 months of age but not at P1 (control n=35, mutant n=38). Advanced AVB was occasionally observed. EPS showed that AV node function, and right ventricular effective refractory period were impaired in Nkx2-5-KI mice, while sinus node function was normal. AV node size was significantly smaller in Nkx2-5-KI mice relative to control mice (maximum area size 2.4 ± 0.2 vs. 1.6 ± 0.1 x 10-2 mm2, 36% reduction; calculated volume 1.50 ± 0.1 vs. 1.11 ± 0.03 x 10-4 mm3, 24% reduction, n=6 each) at 4 weeks of age but not P1, similar to age-dependence of PR-prolongation. AV node in mutant mice was composed of smaller cardiomyocytes without fibrosis, suggesting in part, that conduction abnormalities are the result of an AV node that becomes morphologically atrophic during postnatal life. All 6 genotype-positive humans demonstrated AVB (1st, 2nd, and 3rd degree). QRS prolongation was uniquely demonstrated in Nkx2-5-KI mice from the neonatal stage onward but not in human patients, suggesting substantial discordance in ventricular conduction system between mouse and human. Conclusion: Highly penetrant and progressive AVB similar to that seen in humans with heterozygous NKX2-5 homeodomain missense mutations was replicated in mice by knocking-in a comparable missense mutation. The progressive AVB phenotype is likely due to an AV node composed of smaller cardiomyocytes that becomes atrophic during postnatal life.