Re-evaluation of hypoplastic left heart syndrome from a developmental and morphological perspective

Area of Science:

• Developmental Biology
• Congenital Heart Disease
• Genetics

Background:

• Hypoplastic left heart syndrome (HLHS) encompasses rare congenital anomalies affecting left ventricular development and outflow tract valves.
• The precise etiology of HLHS remains unclear, with debated roles for flow, valvar, or myocardial factors.
• Existing classifications do not fully align with cardiac progenitor populations.

Purpose of the Study:

• To re-evaluate HLHS malformation patterns in relation to cardiac progenitor populations.
• To establish aetiologically relevant sub-groupings for genomic studies of HLHS.
• To investigate the developmental origins of distinct HLHS phenotypes.

Main Methods:

• Retrospective examination and re-categorization of 78 HLHS hearts based on ventricular phenotype.
• Analysis of valve patency and myocardial characteristics.
• Lineage tracing studies in mice using Cre-driver lines (Nkx2-5-Cre, Mef2c-AHF-Cre, Wnt1-Cre) to track cardiac progenitor contributions.

Main Results:

• Three distinct HLHS subgroups identified: slit-like (24%), miniaturised (6%), and thickened left ventricle with endocardial fibroelastosis (70%).
• Slit-like ventricles correlated with mitral and aortic atresia; miniaturised ventricles had smaller but normally formed valves.
• Lineage tracing indicated distinct cardiac progenitor contributions (second heart field, Wnt1-derived) to specific HLHS phenotypes and valve formation.

Conclusions:

• Current HLHS nomenclature aligns only with the slit-like ventricle subgroup (mitral and aortic atresia).
• Slit-like and miniaturised ventricles represent discrete subgroups based on phenotype.
• Reclassification of HLHS by ventricular phenotype may facilitate genetic and developmental investigations into its etiology.