ABSTRACT: Congenital heart disease (CHD) is a prevalent birth defect affecting up to 1% of live births. While a genetic etiology is indicated by increased recurrence risk and association with chromosomal abnormalities, the genetics of CHD is poorly understood and likely complex. Here we show hypoplastic left heart syndrome (HLHS), a severe CHD with high morbidity/mortality, has a multigenic etiology. Analysis of 8 HLHS mutant mouse lines recovered from a large-scale mutagenesis screen showed no mutations are shared in common. In the *Ohia* mouse line, mutations in 2 genes, *Pcdha9*/*Sap130*, act synergistically to cause HLHS. Cardiomyocyte proliferation and differentiation defects likely act in concert with valve abnormalities to cause HLHS. Network analysis of mutations recovered in HLHS mice and 68 HLHS patients point to Notch signaling disruption, a pathway previously implicated in HLHS. These findings demonstrate large-scale mutagenesis is an effective systems approach for interrogating the complex genetics of human diseases. SUBMITTER_CITATION: Journal: Nature Genetics Title: The complex genetics of hypoplastic left heart syndrome Author names: Xiaoqin Liu1, Hisato Yagi1, Shazina Saeed1, Abha S. Bais1, George C. Gabriel1, Zhaohan Chen1, Kevin A. Peterson4, You Li1, Molly C. Schwartz1, William T. Reynolds1, Manush Saydmohammed1, Brian Gibbs1, Yijen Wu1, William Devine1, Bishwanath Chatterjee1, Nikolai T. Klena1, Dennis Kostka1, Karen L. de Mesy Bentley5, Madhavi K.Ganapathiraju2, Phillip Dexheimer11, Linda Leatherbury7, Omar Khalifa1, Anchit Bhagat1, Maliha Zahid1, William Pu8, Simon Watkins3, Paul Grossfeld9, Stephen Murray4, George A. Porter, Jr.6, Michael Tsang1, Lisa J.Martin10, D.Woodrow.Benson12, Bruce J. Aronow11, Cecilia W. Lo1
