Failure of cerebrovascular reserve is a fundamental determinant of ischemic vulnerability, yet the mechanisms by which vascular smooth muscle dysfunction compromises reserve and predisposes the brain to injury remain incompletely defined. We therefore tested whether a pathogenic smooth muscle mutation produces a baseline failure of cerebrovascular reserve sufficient to render the brain vulnerable to hypoperfusion, even in the absence of fixed arterial occlusion. We examined cerebrovascular structure, hemodynamics, and reserve in a genetically defined mouse model of ACTA2-associated multisystemic smooth muscle dysfunction syndrome with systemic or brain-restricted expression of the mutant allele. Cerebral artery morphology was assessed using magnetic resonance angiography and black ink angiography. Vascular smooth muscle phenotype was evaluated by immunohistochemistry and proliferation assays. Blood pressure reactivity and cerebral blood flow (CBF) were measured simultaneously using femoral arterial catheterization and laser speckle flowmetry during vasoactive challenges and controlled hypotension. Cerebrovascular stress responses were tested using unilateral common carotid artery occlusion. Downstream brain effects were assessed by histology, resting state functional connectivity imaging, and behavioral testing. Impaired smooth muscle contractility drove rectification and narrowing of major cerebral arteries, downregulation of contractile markers, and increased vascular cell proliferation. These structural changes produced a distinct physiological phenotype: mutant mice exhibited blunted vasoreactivity, diminished spontaneous vasodynamic activity, and a downward shift in the blood pressure-CBF relationship across a wide range of arterial pressures, consistent with loss of cerebrovascular reserve. As a result, CBF was reduced at baseline and could not be maintained during hypotension or acute vascular stress. During carotid occlusion, mutant mice showed impaired compensatory perfusion, greater physiological instability, and worse behavioral outcomes. Chronic reserve failure coincided with white matter loss, reduced neuronal density, disrupted large-scale functional connectivity, and deficits in locomotion, anxiety-related behavior, and working memory. Pathogenic smooth muscle dysfunction caused by ACTA2 mutation produces a baseline failure of cerebrovascular reserve that renders the brain vulnerable to hypoperfusion and stress-induced ischemic injury. These findings establish cerebrovascular reserve failure as a central physiological mechanism linking vascular dysfunction to end-organ brain injury and identify reserve preservation as a critical, potentially actionable determinant of brain health in hypotension-prone vascular disease.

Multisystemic smooth muscle dysfunction syndrome (MSMDS) is a rare disorder caused by ACTA2 mutations, including the R179H variant, which alters actin filament stability and dynamics and smooth muscle contractility. Cardiovascular complications dominate its clinical presentation, but gastrointestinal (GI) dysfunction significantly affects quality of life. To investigate the structural, functional, and cellular basis of gut dysmotility in MSMDS, we reviewed clinical data from 24 patients with MSMDS and studied the ACTA2 R179H mouse model. Patients exhibited severe gut dysmotility, with 75% requiring medication for chronic constipation. ACTA2 mutant mice displayed cecal and colonic dilatation, reduced intestinal length, and disrupted colonic migrating motor complexes. Delayed whole-gut transit and impaired contractile responses to electrical and pharmacological stimulation were observed. Transcriptomic analysis revealed significant actin cytoskeleton-related gene changes in smooth muscle cells, and immune profiling identified increased lymphocytic infiltration. Despite functional abnormalities, there were no obvious changes in the enteric nervous system. These findings establish ACTA2 mice as a robust model for studying GI pathology in MSMDS, elucidating the role of smooth muscle dysfunction in gut dysmotility. This model provides a foundation for developing targeted therapies aimed at restoring intestinal motility by directly addressing actin cytoskeletal disruptions in smooth muscle cells.

Pathogenic missense mutations in the alpha actin isotype 2 (ACTA2) gene cause multisystemic smooth muscle dysfunction syndrome (MSMDS), a genetic vasculopathy that is associated with stroke, aortic dissection and death in childhood. Here we perform mutation-specific protein engineering to develop a bespoke CRISPR-Cas9 enzyme with enhanced on-target activity against the most common MSMDS-causative mutation ACTA2 R179H. To directly correct the R179H mutation, we screened dozens of configurations of base editors to develop a highly precise corrective A-to-G edit with minimal deleterious bystander editing that is otherwise prevalent when using wild-type SpCas9 base editors. We create a murine model of MSMDS that shows phenotypes consistent with human patients, including vasculopathy and premature death, to explore the in vivo therapeutic potential of this strategy. Delivery of the customized base editor via an engineered smooth muscle-tropic adeno-associated virus (AAV-PR) vector substantially prolongs survival and rescues systemic phenotypes across the lifespan of MSMDS mice, including in the vasculature, aorta and brain. Our results highlight how bespoke mutant-specific CRISPR-Cas9 enzymes can improve mutation correction with base editors.

Vascular smooth muscle cells (SMCs), the predominant cell type in the aortic wall, play a crucial role in maintaining aortic integrity, blood pressure, and cardiovascular function. Vascular SMC contractility and function depend on ACTA2 (smooth muscle α-actin 2). The pathogenic variant ACTA2 c.536G>A (p.R179H) causes multisystemic smooth muscle dysfunction syndrome, a severe disorder marked by widespread smooth muscle abnormalities, resulting in life-threatening aortic disease and high risk of early death from aneurysms or stroke. No effective treatments exist for multisystemic smooth muscle dysfunction syndrome. To develop a comprehensive therapy for multisystemic smooth muscle dysfunction syndrome, we used CRISPR (clustered regularly interspaced short palindromic repeats)-Cas9 (CRISPR-associated protein 9) adenine base editing to correct the ACTA2 R179H sequence variant. We generated isogenic human induced pluripotent stem cell lines and humanized mice carrying this pathogenic missense sequence variant. Induced pluripotent stem cell-derived SMCs were evaluated for key functional characteristics, including proliferation, migration, and contractility. The adenine base editor ABE8e-SpCas9-VRQR under control of either an SMC-specific promoter or a cytomegalovirus promoter, and an optimized single guide RNA under control of a U6 promoter were delivered intravenously to humanized R179H mice using adeno-associated virus serotype 9 and phenotypic outcomes were evaluated. The R179H sequence variant causes a dramatic phenotypic switch in human induced pluripotent stem cell-derived SMCs from a contractile to a synthetic state, a transition associated with aneurysm formation. Base editing prevented this pathogenic phenotypic switch and restored normal SMC function. In humanized mice, the ACTA2R179H/+ sequence variant caused widespread smooth muscle dysfunction, manifesting as decreased blood pressure, aortic dilation and dissection, bladder enlargement, gut dilation, and hydronephrosis. In vivo base editing rescued these pathological abnormalities, normalizing smooth muscle function. This study demonstrates the effectiveness of adenine base editing to treat multisystemic smooth muscle dysfunction syndrome and restore aortic smooth muscle function. By correcting the ACTA2 R179H sequence variant, the pathogenic phenotypic shift in SMCs was prevented, key aortic smooth muscle functions were restored, and life-threatening aortic dilation and dissection were mitigated in humanized mice. These findings underscore the promise of gene-editing therapies in addressing the underlying genetic causes of smooth muscle disorders and offer a potential transformative treatment for patients facing severe vascular complications.