BACKGROUND:
Mitral valve (MV) disease including myxomatous degeneration is the most common form of valvular heart disease with an age-dependent frequency. Genetic evidence indicates that mutations of the human transcription factor
FOXC1
are associated with MV defects, including MV regurgitation. In this study, we sought to determine whether murine
Foxc1
and its closely related factor,
Foxc2
, are required in valvular endothelial cells (VECs) for the maintenance of MV leaflets, including VEC junctions and the stratified trilaminar ECM (extracellular matrix).
METHODS:
Adult mice carrying tamoxifen-inducible, vascular endothelial cell (EC), and lymphatic EC–specific, compound
Foxc1;Foxc2
mutations (ie, EC-
Foxc
-DKO and lymphatic EC-
Foxc
-DKO mice, respectively) were used to study the function of
Foxc1
and
Foxc2
in the maintenance of MVs. The EC and lymphatic EC mutations of
Foxc1/c2
were induced at 7 to 8 weeks of age by tamoxifen treatment, and abnormalities in the MVs of these mutant mice were assessed via whole-mount immunostaining, immunohistochemistry/RNAscope, Movat pentachrome/Masson Trichrome staining, and Evans blue injection.
RESULTS:
EC deletions of
Foxc1
and
Foxc2
in mice resulted in abnormally extended and thicker MVs by causing defects in the regulation of ECM organization with increased proteoglycan and decreased collagen. Notably, reticular adherens junctions were found in VECs of control MV leaflets, and these reticular structures were severely disrupted in EC-
Foxc
-DKO mice. PROX1 (prospero homeobox protein 1), a key regulator in a subset of VECs on the fibrosa side of MVs, was downregulated in EC-
Foxc1/c2
mutant VECs. Furthermore, we determined the precise location of lymphatic vessels in murine MVs, and these lymphatic vessels were aberrantly expanded and dysfunctional in EC-
Foxc1/c2
mutant MVs. Lymphatic EC deletion of
Foxc1/c2
also resulted in similar structural/ECM abnormalities as seen in EC-
Foxc1/c2
mutant MVs.
CONCLUSIONS:
Our results indicate that
Foxc1
and
Foxc2
are required for maintaining the integrity of the MV, including VEC junctions, ECM organization, and lymphatic vessel formation/function to prevent myxomatous MV degeneration.