Slow progressive conduction and contraction defects in loss of Nkx2–5 mice after cardiomyocyte terminal differentiation

Mutations in homeoprotein NKX2–5 are linked to human congenital heart disease resulting in various cardiac anomalies, as well as postnatal progressive conduction defects and occasional left ventricular dysfunction, yet the function of Nkx2–5 in the postnatal period is largely unexplored. In the heart, the majority of cardiomyocytes are believed to complete cell-cycle withdrawal shortly after birth, which is generally accompanied by re-organization of chromatin structure demonstrated in other tissues. We reasoned that effects of loss of Nkx2–5 in mice may be different after cell-cycle withdrawal compared to perinatal loss of Nkx2–5, which results in rapid conduction and contraction defects within 4 days after deletion of Nkx2–5 alleles (Circ Res. 2008;103:580). In this study, floxed-Nkx2–5 alleles were deleted using tamoxifen-inducible Cre transgene (Cre-ER(™)) beginning at 2 weeks of age. Loss of Nkx2–5 beginning at 2 weeks of age resulted in conduction and contraction defects similar to perinatal loss of Nkx2–5, however with substantially slower disease progression demonstrated by 1° atrioventricular block at 6 weeks of age (4 weeks after tamoxifen injections), and heart enlargement after 12 weeks of age (10 weeks after tamoxifen injections). The phenotypes were accompanied by slower and smaller degree of reduction of several critical Nkx2–5 downstream targets that were observed in mice with perinatal loss of Nkx2–5. These results suggest that Nkx2–5 is necessary for proper conduction and contraction after 2 weeks of age, but with substantially distinct level of necessity at 2 weeks of age compared to the perinatal period.