Genetic modifiers predisposing to congenital heart disease in the sensitized Down syndrome population.

Background —About half of people with Down syndrome (DS) exhibit some form of congenital heart disease (CHD). However, trisomy for human chromosome 21 (Hsa21) alone is insufficient to cause CHD as half of all people with DS have a normal heart, suggesting that genetic modifiers interact with dosage sensitive gene(s) on Hsa21 to result in CHD. We hypothesize that a threshold exists in both Down syndrome and euploid populations for the number of genetic perturbations that can be tolerated before CHD results. Methods and Results —We ascertained a group of individuals with DS and complete atrioventricular septal defect (AVSD) and sequenced two candidate genes for CHD, CRELD1 , which is associated with AVSD in people with or without DS, and HEY2 , whose mouse ortholog produces septal defects when mutated. Several deleterious variants were identified but the frequency of these potential modifiers was low. We crossed mice with mutant forms of these potential modifiers to the Ts65Dn mouse model of Down syndrome. Crossing loss-of-function alleles of either Creld1 or Hey2 onto the trisomic background caused a significant increase in the frequency of CHD, demonstrating an interaction between the modifiers and trisomic genes. We showed further that although either of these mutant modifiers is benign by itself, they interact to affect heart development when inherited together. Conclusions —Using mouse models of Down syndrome and of genes associated with congenital heart disease we demonstrate a biological basis for an interaction that supports a threshold hypothesis for additive effects of genetic modifiers in the sensitized trisomic population.