Background/aims Alveolar capillary dysplasia (ACD) is characterised by pulmonary veins misalignment, capillary paucity and alveolar misdevelopment, and caused by FOXF1 mutations only in 40% of cases. Objectives were 1. to identify known and new gene defects and 2. to correlate them with molecular/cellular mechanisms.

Methods We recruited a cohort of 23 pathology-confirmed cases. When DNA was available, genome-wide copy number variation was analysed through Array Comparative Genomic Hybridization (aCGH). Mutations were tested by direct sequencing of FOXF1 and candidate genes identified by aCGH; Molecular pathways were analysed by multi-channel immunofluorescence microscopy of ACD cases compared to human fetal/neonatal lung tissue at various development stages.

Results

1. Genomic deletions or mutations were identified in 57% of tested cases. Besides FOXF1, two of the genes involved stand out as potential candidates: MEOX2 and TBX4.

2. ACD cases showed a markedly decreased expression of c-kit, a marker expressed in pulmonary small arteries and capillaries in fetal lung controls. In normal fetal lungs FOXF1 and TBX4 were prevalently expressed at the mesenchymal-epithelial border, and MEOX2 in pulmonary vascular smooth muscle cells (PVSMC). Their expression pattern and intensity were altered in all ACD cases, indicating that decreased FOXF-1 and/or its downstream transcription factor TBX4 disrupt lung micro vessel formation and homing to alveolar epithelium, and that a similar phenotype may derive from dysregulated PVSMC proliferation and angiogenesis related to MEOX2 insufficiency.

Conclusion Genetic defects affecting the FOXF1 pathway affect the mesenchymal, endothelial and epithelial cross-talk leading to lung developmental disruption, pulmonary hypertension and hypoxic respiratory failure.