Abstract 67: A Rare Variant in Myh11, R247C, Affects Smooth Muscle Cell Phenotype and Acts as a Modifier for Aortic Disease

Abstract

Thoracic aortic aneurysms and dissections (TAAD) affect more than 30,000 Americans annually. One-fifth of TAAD patients have a first-degree relative with the disease, suggesting a genetic component to its pathogenesis. Missense mutations in a number of genes lead to TAAD, including MYH11 ,which encodes the smooth muscle isoform of myosin heavy chain. However, potential genetic contributors in the four-fifths of patients without affected relatives have not been explored. Variants of unknown significance in MYH11 are prevalent in the population, and are enriched in patients with TAAD and other vascular diseases. The goal of this study was to evaluate one of these variants as a potential contributor to TAAD. A knock-in mouse model of the Myh11 R247C rare variant was generated, and these mice survive and reproduce normally. They have no structural abnormalities of the aorta or signs of aortic disease, but do have decreased aortic contractility. Myh11 R247C/R247C mice also have increased neointimal proliferation following carotid ligation and increased proliferation of vascular smooth muscle cells (SMCs) in vitro. Myh11 R247C/R247C SMCs have decreased contractile gene and protein expression and are dedifferentiated. In fibroblasts, myosin force generation is required for maturation of focal adhesions, and enhancers of RhoA activity replace enhancers of Rac1 activity as maturation occurs. Consistent with these previous findings, adhesions are smaller in Myh11 R247C/R247C SMCs, and there is increased Rac1 and focal adhesion kinase (FAK) activation with decreased RhoA activation. A RhoA activator (CN03) rescues the dedifferentiated phenotype of Myh11 R247C/R247C SMCs, and FAK inhibition reduces proliferation. Since this rare variant occurs in the general population, we sought to determine if it acts as a modifier gene for aortic disease. The Myh11 R247C allele was bred into Acta2 -/- mice, which develop aortic aneurysms, and into Acta2 +/- mice, which do not. In both cases, the presence of the R247C allele increases aortic diameter. These preliminary data suggest that the MYH11 R247C rare variant may be a genetic contributor to TAAD in patients without an inherited mutation, or may affect the clinical presentation of TAAD in patients with Mendelian disease.