Abstract 4275: TGFBR2 Mutations Reduce Expression of Contractile Proteins in Aortic Smooth Muscle Cells and Predispose Individuals to Thoracic Aortic Aneurysms and Dissections

Mutations in the TGF-β receptor type II gene ( TGFBR2 ) cause thoracic aortic aneurysms and dissections (TAAD). Studies have suggested a gain of function effect for these mutations, leading to increased TGF-β signaling in the aortic media and resulting in vascular disease. We sought to characterize the phenotype of smooth muscle cells (SMCs) harboring heterozygous missense TGFBR2 mutations and our data suggest that instead of a gain of function, TGFBR2 mutations cause TAAD as a result of a loss of function resulting in defective SMC differentiation. Using primary aortic SMCs from patients harboring TGFBR2 mutations (n=4), we show a global decrease in expression of SMC contractile proteins ( ACTA2 , MYH11 , CNN1 , SMTN , TPM1 , TPM2 , p <0.001) by quantitative PCR analysis when these cells are compared with age and gender matched control SMCs (n=4), along with no change in the expression of cytoskeletal proteins. Consistent with the decreased expression of contractile proteins in the mutant cells, there was increased expression of S100A4, a marker of de-differentiated SMCs (p<0.001). Analysis of fixed and frozen aortas from patients with TGFBR2 mutations (n=3) confirmed decreased in vivo expression of SMC contractile proteins when compared to control aortas (n=3). In control SMCs, addition of TGF- β significantly increased the expression of the SMC contractile proteins but the TGFBR2 SMCs showed no significant increase in expression of these proteins with TGF-β stimulation. We found that fibroblasts explanted from patients with TGFBR2 mutations (n=8) consistently fail to transform into myofibroblasts as assessed by expression of SMC contractile proteins after TGF-β stimulation, when compared with age and gender matched control fibroblasts (n=8). Finally, introduction of TGFBR2 missense mutations into a mouse mesenchymal embryonic cell line that is used as a model of SMC differentiation (10T1/2 cells) disrupts the expression of contractile proteins in these cells when assessed post-differentiation. These data suggest that TGFBR2 mutations disrupt differentiation of SMCs and myofibroblasts. This is the first genetic defect identified to lead to defective SMC differentiation.