1. ¹Department of Physiology and Functional Genomics, University of Florida College of Medicine, Gainesville, FL, USA
2. ²Department of Pediatrics, Tokyo Medical and Dental School, Tokyo, Japan
3. ³Cardiology Division, UCSF, San Francisco, CA, USA
4. ⁴Cardiovascular Research, Boston University Medical Center, Boston, MA, USA
5. ⁵Department of Biochemistry and Molecular Biology, University of Florida College of Medicine, Gainesville, FL, USA
6. ⁶Cardiovascular Research Center, Massachusetts General Hospital, Boston, MA, USA

Correspondence: Dr H Kasahara, MD, PhD, Department of Physiology and Functional Genomics, University of Florida College of Medicine, PO Box 100274, 1600 SW Archer Rd. M540, Gainesville, FL 32610-0274, USA. E-mail: hkasahar@phys.med.ufl.edu

⁷These authors contributed equally to this work.

Received 17 February 2009; Revised 29 April 2009; Accepted 21 May 2009; Published online 22 June 2009.

Abstract

Mutations in homeoprotein NKX2-5 are linked to human congenital heart disease, resulting in various cardiac anomalies, as well as in 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 a re-organization of chromatin structure shown in other tissues. We reasoned that the effects of the loss of Nkx2-5 in mice may be different after cell-cycle withdrawal compared with those of the perinatal loss of Nkx2-5, which results in rapid conduction and contraction defects within 4 days after the 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. The loss of Nkx2-5 beginning at 2 weeks of age resulted in conduction and contraction defects similar to the perinatal loss of Nkx2-5, however, with a substantially slower disease progression shown 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 a slower and smaller degree of reduction of several critical Nkx2-5 downstream targets that were observed in mice with a 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 a substantially distinct level of necessity at 2 weeks of age compared with that in the perinatal period.