Abstract 2681: Mutations in Smooth Muscle-Specific Contractile Proteins Cause Familial Thoracic Aortic Aneurysms and Dissections and Lead to Increased IGF-1 Expression
Aortic aneurysms and dissections are the most common disorders affecting the aorta, and are a major cause of morbidity and mortality in the United States. Familial thoracic aortic aneurysms and dissections (FTAAD) are inherited in an autosomal dominant manner with variable expression and decreased penetrance. The disorder is genetically heterogeneous with four loci and three genes identified. Mutations in either TGFBR2, encoding the transforming growth factor β (TGF-β) type II receptor, or MYH11, encoding the smooth muscle cell (SMC)-specific β-myosin heavy chain, were previously found to cause FTAAD. Recently, positional cloning identified smooth muscle α-actin (ACTA2) mutations as a novel cause in 10% of FTAAD. Mutations in ACTC and MYH7 cause hypertrophic cardiomyopathy (HCM), characterized by myocyte disarray and upregulation of mitotic and trophic factors. Histologic examination of aortic tissue from patients with ACTA2 (n = 6) and MYH11 (n = 2) mutations revealed SMC disarray in the aortic media similar to that seen in HCM. Furthermore, we hypothesized that mutations in ACTA2 and MYH11 cause a similar increase of mitotic and trophic factors in SMCs. The expression of two factors known to be increased in HCM, TGF-β1 and insulin-like growth factor 1 (IGF-1), were analyzed in patients’ aortic SMCs and dermal myofibroblasts. No changes in TGF-β1 were found; however, both mRNA, as measured by Q-PCR (p<0.05), and protein levels, as assessed by immunostaining, of IGF-1 were markedly increased in MYH11 and two ACTA2 mutant SMCs and aortic tissue compared with control SMCs and tissue. Differentiation of dermal fibroblasts into myofibroblasts was accomplished using TGF-β1 treatment; myofibroblast differentiation was confirmed by assessing α-actin mRNA and protein levels in untreated vs. TGF-β1-treated fibroblasts. Upon differentiation, patients’ myofibroblasts (n = 3) demonstrated increased IGF-1 expression compared with controls (p<0.05), similar to the increased IGF-1 expression by SMCs. In conclusion, IGF-1 secretion is increased in response to defects in SMC contractile proteins in SMCs and myofibroblasts. Future studies will clarify the role of IGF-1 in FTAAD and identify the pathways leading to increased IGF-1 expression.