Mitochondrial MICOS complex genes, implicated in hypoplastic left heart syndrome, maintain cardiac contractility and actomyosin integrity

Hypoplastic left heart syndrome (HLHS) is a severe congenital heart
disease (CHD) with a likely oligogenic etiology, but our understanding of
the genetic complexities and pathogenic mechanisms leading to HLHS is
limited. We therefore performed whole genome sequencing (WGS) on a large
cohort of HLHS patients and their families to identify candidate genes
that were then tested in Drosophila heart model for functional and
structural requirements. Bioinformatic analysis of WGS data from an index
family comprised of a HLHS proband born to consanguineous parents and
postulated to have a homozygous recessive disease etiology, prioritized 9
candidate genes with rare, predicted damaging homozygous variants. Of the
candidate HLHS gene homologs tested, cardiac-specific knockdown (KD) of
mitochondrial MICOS complex subunit dCHCHD3/6 resulted in drastically
compromised heart contractility, diminished levels of sarcomeric actin and
myosin, reduced cardiac ATP levels, and mitochondrial fission-fusion
defects. Interestingly, these heart defects were similar to those
inflicted by cardiac KD of ATP synthase subunits of the electron transport
chain (ETC), consistent with the MICOS complex’s role in maintaining
cristae morphology and ETC complex assembly. Analysis of 183 genomes of
HLHS patient-parent trios revealed five additional HLHS probands with
rare, predicted damaging variants in CHCHD3 or CHCHD6. Hypothesizing an
oligogenic basis for HLHS, we tested 60 additional prioritized candidate
genes in these cases for genetic interactions with CHCHD3/6 in sensitized
fly hearts. Moderate KD of CHCHD3/6 in combination with Cdk12 (activator
of RNA polymerase II), RNF149 (E3 ubiquitin ligase), or SPTBN1
(scaffolding protein) caused synergistic heart defects, suggesting the
potential involvement of a diverse set of pathways in HLHS. Further
elucidation of novel candidate genes and genetic interactions of
potentially-disease-contributing pathways is expected to lead to a better
understanding of HLHS and other CHDs.