Smooth muscle cells (SMCs) play a critical role in atherosclerosis and coronary artery disease (CAD) risk.
During disease, a small subset of SMCs proliferate extensively, undergo cell fate change(s) and migrate into
the lesion in a process termed phenotypic modulation. Using single cell RNA sequencing (scRNAseq), we
have shown that this cell state transition is a continuum in which these SMCs first assume a fibroblast-like
phenotype and ultimately transition to a calcific phenotype. We have also shown that top CAD-associated
genes modify disease risk by altering the process of SMC phenotypic modulation. Our subsequent preliminary
scRNAseq data suggest that different modulated SMC phenotypes might arise as the result of discrete cell fate
decisions resulting in different transcriptional paths during disease. This suggests that causal CAD genes
function by altering the clonal proliferation and/or cell fate determination of SMCs during disease, but these
processes cannot be directly observed with traditional scRNAseq approaches. We have recently shown that
the transcription factor TWIST1 is the causal gene at 7p22.1, a genomic locus in human GWAS that is
associated with multiple vascular diseases including CAD, stroke, peripheral artery disease and Moyamoya
disease. Our preliminary data show that SMC-specific Twist1 knockdown: i) decreases lesion size in the ApoE-
/- atherosclerotic mouse model, consistent with the directionality predicted by human GWAS, ii) results in a
>60% reduction in the number of SMC-derived cells within the lesion and iii) specifically results in depletion of
the calcific modulated SMC phenotype in scRNAseq data. In addition, siTwist1 knockdown in human coronary
artery SMCs (HCASMCs) reveals a strong reduction in cellular proliferation/viability and migration. Taken
together, these data lead to our central hypothesis that the 7p22.1 locus acts through TWIST1 to drive vascular
disease risk by promoting SMC proliferation and altering cell fate decisions during phenotypic modulation,
which will be explored through three Specific Aims. Aim 1 will use a novel scRNAseq-compatible cell
barcoding mouse model to track individual SMCs as they proliferate and undergo cell fate determination during
atherosclerosis, with and without Twist1 knockout. In Aim 2, we will use a SMC-specific myc-tagged Twist1
overexpression mouse model to simulate the human risk allele, and determine whether increased Twist1
expression in SMCs is sufficient to worsen atherosclerosis. We will also leverage the expression of myc-Twist1
to query genome-wide Twist1 DNA binding in SMCs, identifying the direct molecular targets of Twist1 in vivo
during atherosclerosis. Aim 3 will determine the cellular pathways and transcription factors that act on the
causal SNP at 7p21.1 to regulate TWIST1 expression. Completion of these studies will result in a detailed map
of the SMC proliferation and lineage determination landscape during atherosclerosis, which will be a major
resource for the field. We will determine how Twist1 alters this landscape, and elucidate the detailed molecular
mechanisms of how 7p21.1 and TWIST1 affects the risk for multiple vascular diseases in humans.