Left ventricular non-compaction cardiomyopathy (LVNC) is a congenital cardiomyopathy,
characterized by excessive ventricular trabeculation, with a ‘spongy’ appearance of
the myocardium, deep intertrabecular recesses, and a thin compacted myocardial layer.
LVNC has recently gained more attention because it has been increasingly recognized
as a cause of heart failure, thromboembolism, or ventricular arrhythmia, which can
all lead to sudden cardiac death. However, due to its heterogeneous anatomical and
clinical features, LVNC is still a poorly understood condition. In addition, LVNC
is suggested to be due to a premature arrest in the myocardial compaction process,
but its genetic basis and pathologic mechanism remain largely unknown.
METHODS AND RESULTS
Here we introduce novel genetic factors implicated in ventricular compaction. Specifically,
combinatorial loss of two Iroquois homeobox (Irx) transcription factors, Irx3, critical
for development and function of the ventricular conduction system (e.g., His-bundle,
bundle branches and Purkinje fibers) and Irx4, essential for establishing ventricular
identity, recapitulates the clinical hallmarks of LVNC in mice. While mice lacking
either Irx3 or Irx4 do not exhibit gross congenital cardiac malformations, mice lacking
both Irx3 and Irx4 (Irx3;Irx4DKO) show postnatal lethality with abnormal muscular
growths that resemble the polypoid pattern of LVNC, characterized by multiple muscular
nodes in the left ventricle. Optical projection tomography in postnatal day 14 (P14)
mutant hearts unveiled increased trabeculation, abnormal bridging below the papillary
muscles, and disorganized muscle structures (Fig. 1). During embryogenesis, Irx3 and
Irx4 exhibit overlapping expression in the developing ventricles. Consistent with
the hypothesis that LVNC can be caused by a defect in embryonic heart growth, Irx3;Irx4DKO
embryonic hearts showed abnormal trabeculation with thinner ventricular walls as early
as E14.5. RNA sequencing on E14.5 Irx3;Irx4DKO ventricles revealed altered ventricular
identity with atrial gene activation and enriched Bmp2/Tbx2 pathway that is critical
for endocardial cushion development by suppressing cardiomyocyte differentiation.
Moreover, co-immunoprecipitation discovered that Irx4 physically interacts with Irx3
and Gata4, and promoter assay further showed that Irx3, Irx4, and Gata4 synergistically
suppress Tbx2 promoter activity, suggesting the repressive role of this transcriptional
regulatory complex in the developing myocardium.
Our study demonstrates a novel mechanistic interaction of Irx3 and Irx4, in cooperation
with Gata4, during the ventricular compaction process, thereby ensuring proper ventricular
development. This finding can promote improvement of genetic testing for LVNC and
prediction of individual outcomes with LVNC.