The most common genetic alterations for familial thoracic aortic aneurysms and dissections (TAAD) are missense mutations in vascular smooth muscle (SM) α-actin encoded by We focus here on R258C, a recurrent mutation associated with early onset of TAAD and occlusive moyamoya-like cerebrovascular disease. Recent biochemical results with SM α-actin-R258C predicted that this variant will compromise multiple actin-dependent functions in intact cells and tissues, but a model system to measure R258C-induced effects was lacking. We describe the development of an approach to interrogate functional consequences of actin mutations in affected patient-derived cells. Primary dermal fibroblasts from R258C patients exhibited increased proliferative capacity compared with controls, consistent with inhibition of growth suppression attributed to SM α-actin. Telomerase-immortalized lines of control and R258C human dermal fibroblasts were established and SM α-actin expression induced with adenovirus encoding myocardin-related transcription factor A, a potent coactivator of Two-dimensional Western blotting confirmed induction of both wild-type and mutant SM α-actin in heterozygous R258C cells. Expression of mutant SM α-actin in heterozygous R258C fibroblasts abrogated the significant effects of SM α-actin induction on formation of stress fibers and focal adhesions, filamentous to soluble actin ratio, matrix contraction, and cell migration. These results demonstrate that R258C dominantly disrupts cytoskeletal functions attributed to SM α-actin in fibroblasts and are consistent with deficiencies in multiple cytoskeletal functions. Thus, cellular defects due to this mutation in both aortic smooth muscle cells and adventitial fibroblasts may contribute to development of TAAD and proliferative occlusive vascular disease.