Olivia Mitchel

Background: Smooth muscle cell (SMC) phenotypic reprogramming toward a mixed synthetic-proteolytic state is a central feature of aortic root aneurysm in Marfan syndrome (MFS). Previous work identified Klf4 as a potential mediator of SMC plasticity in MFS. Methods: MFS ( Fbn1 C1041G/+ ) mouse strains with an inducible vascular SMC fluorescent reporter ( MFS SMC ) with or without SMC-specific deletion of Klf4 exons 2 to 3 ( MFS SMC-Klf4Δ ) were generated. Simultaneous SMC tracing and Klf4 loss-of-function ( Klf4Δ mice) was induced at 6 weeks of age. Aneurysm growth was assessed via serial echocardiography (4–24 weeks). Twenty-four-week-old mice were assessed via histology, RNA in situ hybridization, and aortic single-cell RNA sequencing. Results: MFS mice demonstrated progressive aortic root dilatation compared with control (WT SMC ) mice regardless of Klf4 genotype ( P <0.001), but there was no difference in aneurysm growth in MFS SMC-Klf4Δ versus MFS SMC ( P =0.884). Efficient SMC Klf4 deletion was confirmed via lineage-stratified genotyping, RNA in situ hybridization, and immunohistochemistry. Single-cell RNA sequencing of traced SMCs revealed a highly similar pattern of phenotype modulation marked by loss of contractile markers (eg, Myh11, Cnn1 ) and heightened expression of matrix genes (eg, Col1a1, Fn1 ) between Klf4 genotypes. Pseudotemporal quantitation of SMC dedifferentiation confirmed that Klf4 deletion did not alter the global extent of phenotype modulation, but reduced expression of 23 genes during this phenotype transition in MFS SMC-Klf4Δ mice, including multiple chondrogenic genes expressed by only the most severely dedifferentiated SMCs (eg, Cytl1, Tnfrsf11b ). Conclusions: Klf4 is not required to initiate SMC phenotype modulation in MFS aneurysm but may exer