Project Summary
Hypoplastic left heart syndrome (HLHS) is a fatal form of congenital heart disease characterized by the
underdevelopment of left-sided structures, including the left ventricle (LV), mitral and aortic valves, and
ascending aorta. These three structural deficiencies result in diminished circulation that invariably leads to
morbidity without surgical intervention. It is currently thought that HLHS is an oligogenic disease of co-
occurring structural defects caused by different genetic lesions. In some cases, however, one structural defect
might be primary and the others secondary. As HLHS is clinically heterogeneous, there are likely multiple paths
to disease pathogenesis caused by diverse, and often unknown, genetic etiologies. To begin studying the
affected genes in the context of heart development, I mined published databases for de novo and rare
inherited variants in HLHS probands for follow-up in zebrafish. I initially selected RBFOX2, a highly conserved
RNA binding protein (RBP), based on its strong statistical link to HLHS and lack of study in cardiac
development. I published that rbfox2 mutant zebrafish display all three HLHS-like structural heart defects that
arise secondary to compromised pump function (Huang et al. Nat Commun 2022). However, unlike patients
that are heterozygous for RBFOX2, heterozygous zebrafish and mice are healthy. Therefore, no appropriate
model exists to investigate how halving the dose of RBFOX2 affects CM biology. Moreover, many variants
statistically linked to HLHS remain unvalidated or unstudied in whole animal models. To address these
knowledge gaps, I engineered and characterized RBFOX2 het and null human (h) iPSC-CMs. Both genetic
cohorts show dose-dependent reductions in cell size, cell adhesion, calcium handling, and oxygen
consumption rates. Myofibril alignment is also perturbed. Based on this phenotypic characterization and
integration of several sequencing datasets, I propose to test the hypothesis that RBFOX2 directly regulates CM
cell adhesion and growth by controlling transcript stability and splicing of an ECM-cytoskeletal-myofibril gene
network in Aim 1. To identify additional HLHS-linked variants that cause cardiac phenotypes in zebrafish, I
conducted a pilot CRISPR screen and recovered an ortholog of FOXC2, which encodes a transcription factor
expressed in the endocardium and myocardium. Preliminary phenotyping revealed HLHS-like cardiac
phenotypes in foxc2 global knock-outs that also fail to activate a Notch reporter in the endocardium. In Aim 2, I
propose to test the hypothesis that endocardial-specific FOXC2-deficiency leads to primary valve and
secondary myocardial defects caused by defective Notch signaling. Overall, I anticipate learning that both
myocardial- and endocardial-intrinsic defects recapitulate specific aspects of HLHS. More broadly, implicating
human variants as causal for HLHS pathogenesis and deciphering their mechanism of action in whole animal
and human iPSC models will ultimately lead to better predictors of heart failure following surgical intervention
and new therapeutic inroads for patient-specific clinical management of the disease.