Atrioventricular septal defect is a complex congenital heart defects (CHD) with a prevalence of approximately 4% of all CHDs. Transitional form of atrio-ventricular septal defect (tAVSD) associates ostium primum atrial septal defect, common atrioventricular annulus with distinct atrioventricular valvar orifices in addition of restrictive ventricular septal defect. We describe in this report clinical and molecular features of a Moroccan boy that carries a novel NK2 homeobox 5 (NKX2-5) germline mutation (Pro141Ala), and exhibits a transitional atrio-ventricular septal defect. This phenotype has never been reported in association with NKX2-5 germline mutations. Pro141Ala is a non-reported pathogenic mutation that alters the nuclear localization signal sequence, leading to disruption of NKX2-5 nuclear translocation mechanism. Such alteration would decrease nuclear transcriptional activity of NKX2-5 and impair cardiogenesis process. The present report comes to widen the phenotypic spectrum of congenital heart disease caused by NKX2-5 germline mutations, and highlights as well the importance of the nuclear localization system in NKX2-5 activity.

Keywords: NKX2-5, atrio-ventricular septal defect (AVSD), nuclear localization site (NLS)