Glypican-6 deficiency causes dose-dependent conotruncal congenital heart malformations through abnormal remodelling of the endocardial cushions

Tetralogy of Fallot (TOF) is considered to be the commonest type of cyanotic congenital heart disease (CHD). A previous GWAS showed significant association between TOF and single nucleotide polymorphisms in chromosome 13q31. Here through integration of population genomic and chromosomal interaction data we identify the heparan sulfate proteoglycan glypican-6 (GPC6) as the potentially responsible gene at the associated locus. We showed that GPC6 is expressed in the endocardial cushions at the appropriate time in development to contribute to TOF risk. We generated mice homozygous for a Gpc6 KO allele, which exhibit 100% neonatal lethality with severe cardiac malformations, namely TOF-type double outlet right ventricle (DORV) with rightward mal-positioned aorta and perimembranous ventricular septal defect (VSD), together with right ventricular (RV) hypertrophy and narrowing of the pulmonary artery. We established a dose-response relationship between Gpc6 expression and the anatomical severity of cardiac malformations. We showed the mouse knockout phenotype arises from abnormal morphology of the endocardial cushions, and tissue-specific knockout of Gpc6 in endothelial and neural crest cell lineages produces a phenotype featuring VSD and aortic malposition analogous to human TOF. This successful identification of a CHD gene from GWAS data suggests that larger GWA studies may find additional causative genes.