Mitral valve prolapse (MVP) is one of the most common forms of cardiac valve disease and affects ~2-3% of the human population. There are no effective nonsurgical treatments for MVP and therapeutic efforts have been hindered by an incomplete understanding of its fundamental causes. However, we now have compelling genetic and functional evidence that significantly advances our understanding of MVP pathogenesis. Our group was the first to identify a genetic cause for MVP through identification of mutations in the atypical cadherin gene, DCHS1, in multiple families with non-syndromic MVP and have traced the origin of disease back to defects in fetal valve morphogenesis. The distinct functional and molecular consequences of DCHS1 deficiency are not currently known, but recent two-hybrid studies have revealed a novel protein complex between DCHS1, Lix-1 like (LIX1L), and Septin-9 (SEPT9) (DLS). Preliminary evidence supports a mechanism in which this complex links DCHS1- based cell adhesions to the actin cytoskeleton through its interactions with cytoplasmic LIX1L and SEPT9. Thus, we hypothesize that valve remodeling occurs through a DCHS1-LIX1L-SEPT9-actin mechanism, which may provide a molecular and cellular origin for MVP. This hypothesis will be tested by defining mechanisms by which the DLS complex regulates actin organization (Aim 1), directs proper valve remodeling ex vivo (Aim 2) and genetically interacts within the same pathway to regulate proper valve geometry and ECM organization (Aim 3).
Aim 1 of this proposal involves an in vitro approach to define the effect of DLS on actin filament organization by quantifying septin-actin network formation and the resulting intracellular tension in genetically modified mouse cardiac fibroblasts. The functional consequences of DLS interactions with the actin cytoskeleton and its role in valve remodeling will be measured through application of a novel ex vivo approach in Aim 2. Here, valve interstitial cells (VICs) will be isolated from control and global Dchs1 and/or LIX1L heterozygote mouse hearts and seeded into a 3D bioengineered valve construct that recapitulates the native valve environment. Readouts including cell alignment, nuclear shape, actin organization, ECM production and formation, and force generation will be measured and allow for quantification of the remodeling processes that are crucial for valve morphogenesis. In vivo epistasis experiments performed in Aim 3 will add credence to each approach and will define the genetic interaction between Dchs1 and Lix1L and their role in our proposed pathway. These studies are significant since they are based on mutations identified in MVP patients and will define the molecular and cellular origins of one of the most common cardiovascular diseases in the world.
Mitral valve prolapse is a common cardiac valve disease that necessitates thousands of open-heart surgeries each year. The proposed studies aim to establish an origin for valve disease by defining the functional role of Dchs1 in the developing valve and by uncovering how valves remodel into functionally competent leaflets. Completion of this proposal will provide new mechanistic insights into the molecular and developmental origins of valve disease, which may inform the advancement of new therapeutics to treat mitral valve prolapse.