Pathogenic variants affecting alpha-2 smooth muscle actin (ACTA2) account for approximately 20% of non-syndromic familial thoracic aortic aneurysms (TAA) and confer a high risk of dissection; however, the cell-type-specific transcriptional mechanisms underlying□ACTA2-associated TAA remain poorly defined, particularly for variants of uncertain significance. In this study, we investigated the transcriptional and cellular effects of a novel ACTA2 p.Met49Thr mutation identified in a young male in his early twenties who developed a dissected ascending aortic aneurysm without traditional risk factors. Using the clinically archived formalin-fixed paraffin-embedded (FFPE) aortic tissue, we isolated intact nuclei and performed single-nucleus RNA sequencing (snRNA-seq) to generate 17,938 transcriptomes. Relative to non-genetic hypertensive TAA control, The ACTA2-p.Met49Thr sample displayed marked expansion of vascular smooth muscle cells (VSMCs) relative to TAA (70.6% vs. 39.7%), accompanied by upregulation of proliferation-associated transcripts including FOSB, FOS, JUN, and DEPP1. Lineage tracing via trajectory analysis revealed a transcriptional progression from quiescent to pro-proliferative VSMC states enriching for human loci associated with aortic strain and diameter. Histological evaluation corroborated these findings, demonstrating medial hypercellularity, elastic fiber fragmentation, and adventitial fibrosis enriched within the ACTA2-p.Met49Thr specimen. Taken together, these findings implicate a novel pathogenic ACTA2 variant that drives transcriptional reprogramming and proliferative VSMC remodeling, supporting that ACTA2-associated aortopathy progression occurs via functional cell state transitions. Additionally, this work demonstrates the feasibility of FFPE-compatible snRNA-seq as a useful tool for clinical variant annotation.

The authors have declared no competing interest.

This work was funded by National Institutes of Health Grants U01-HL166060, R01-HL164811, DP2-HL152423, and the Eugene Braunwald Junior Faculty Scholar Award (R.M.G.). M.E.P. received funding support for sequencing through a junior investigator award from the Brigham and Womens Hospital Department of Medicine.

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This study was approved by the Institutional Review Board of Brigham and Womens Hospital (IRB protocol #2021P001077) and conducted in accordance with the Declaration of Helsinki. The requirement for written informed consent was waived due to the use of anonymized data.

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