rs56062135 — SMAD3 SMAD3 intronic variant

SMAD3 rs56062135 — The TGF-beta Vascular Remodeling Switch

Coronary artery disease begins not with blocked arteries but with dysfunctional artery walls — and a key player in that dysfunction is SMAD3, the principal intracellular messenger of TGF-beta (transforming growth factor-beta) signaling¹¹ SMAD3, the principal intracellular messenger of TGF-beta (transforming growth factor-beta) signaling
TGF-beta controls whether vascular smooth muscle cells remain quiescent in a healthy vessel wall or shift into a proliferative, matrix-remodeling state that drives plaque growth. The rs56062135 variant sits within intron 1 of SMAD3 on chromosome 15q22.33, embedded within the haplotype block that controls how strongly an arterial enhancer drives SMAD3 transcription. Carriers of the minor T allele have reduced SMAD3 expression in vascular tissue — and lower coronary artery disease risk.

The Mechanism

rs56062135 is in strong linkage disequilibrium²² strong linkage disequilibrium
LD means two variants are co-inherited so often that tracking one effectively tracks the other with rs17293632 (r²=0.94, D'=0.97), the candidate causal variant at this locus. The C allele at rs17293632 preserves a consensus AP-1 binding site³³ AP-1 binding site
AP-1 (Activator Protein 1) is a transcription factor complex that binds DNA and activates nearby gene transcription in response to cellular stress and growth signals within a SMAD3 intron 1 enhancer, maintaining high enhancer activity in arterial smooth muscle cells. The T allele disrupts this AP-1 site, reducing enhancer output, lowering SMAD3 mRNA and protein levels in both blood and atherosclerotic plaque tissue, and leaving smooth muscle cells less responsive to TGF-beta's proliferative and matrix-remodeling signals.

CRISPRi and lentiMPRA experiments⁴⁴ CRISPRi and lentiMPRA experiments
CRISPRi silences endogenous enhancers; lentiMPRA screens thousands of sequences simultaneously for enhancer activity validated that silencing the rs17293632 enhancer region measurably changes SMAD3 expression in human vascular smooth muscle cells, confirming this is a genuine regulatory eQTL rather than a passive tag. siRNA knockdown of SMAD3 in coronary artery smooth muscle cells increases cell viability⁵⁵ increases cell viability
reflecting reduced antiproliferative TGF-beta signaling, which paradoxically protects against the VSMC phenotype switching that drives plaque progression, providing a mechanistic explanation for how lower SMAD3 expression translates into lower CAD risk.

The broader pathway context is illuminated by the opposing programs of SMAD3 and TCF21⁶⁶ opposing programs of SMAD3 and TCF21
two CAD GWAS genes at distinct loci whose products compete for chromatin access in coronary artery smooth muscle cells. SMAD3 promotes a synthetic, inflammatory smooth muscle phenotype (upregulating ACTA2, TAGLN, and CNN1 while driving proliferation), whereas TCF21 drives fibrous differentiation and plaque stabilization. Higher SMAD3 expression — driven by the common C haplotype — biases smooth muscle cells toward the pro-atherogenic synthetic state.

SMAD3 also regulates extracellular matrix through its interaction with the COL4A1/COL4A2 locus: Turner et al. 2015⁷⁷ Turner et al. 2015 demonstrated that SMAD3 is required for TGF-beta-mediated induction of type IV collagen in vascular smooth muscle cells, and that epistasis across five CAD cohorts reveals a statistical interaction between the SMAD3 and COL4A1/COL4A2 risk loci — meaning the two genetic signals compound on each other.

The Evidence

The genome-wide association evidence for rs56062135 comes from the CARDIoGRAMplusC4D 1000 Genomes meta-analysis⁸⁸ CARDIoGRAMplusC4D 1000 Genomes meta-analysis
the largest CAD GWAS to that date, combining multiple discovery and replication cohorts across European ancestry, which identified the C allele of rs56062135 as associated with coronary artery disease at genome-wide significance (OR 1.07, 95% CI 1.05–1.10; p=4.50×10⁻⁹, additive model). The association is tightly localized between two recombination hotspots flanking the SMAD3 locus, ruling out LD with distant causal variants. An OR of 1.07 per C allele is modest — consistent with most common cardiovascular GWAS hits — but the signal is replicated and biologically grounded, making it more actionable than a purely statistical association.

Population data underscore an important pattern: the T allele is rare in East Asians (~2.8%) but common in Europeans (~24%). This means the protective T haplotype — the one with reduced AP-1 enhancer activity and lower SMAD3 expression — is substantially more prevalent in populations with European ancestry, potentially contributing to differences in the genetic architecture of CAD risk across ancestries.

Practical Actions

For homozygous CC carriers (about 69% of the global population), both copies of the risk allele are present. The common C allele maintains the AP-1 enhancer at full activity, driving higher SMAD3 expression in vascular smooth muscle cells and correspondingly higher TGF-beta-mediated remodeling. Two targeted interventions can modulate this pathway: high-sensitivity CRP monitoring (to detect the downstream inflammatory output of elevated SMAD3/TGF-beta signaling in the vessel wall) and EPA/DHA supplementation (omega-3 fatty acids reduce TGF-beta pathway activation in vascular cells and attenuate the VSMC phenotype switching SMAD3 promotes).

For CT heterozygotes, one T allele provides partial dampening of SMAD3 enhancer activity. The risk is intermediate and the same monitoring approach applies at a lower threshold of urgency.

Interactions

rs56062135 and rs17293632 are in near-complete LD (r²=0.94) and should be interpreted as tagging the same functional haplotype. A second independent signal at the SMAD3 locus — rs17228212 — is not in LD with rs56062135 and may represent a distinct regulatory mechanism at the same gene.

The TCF21 gene⁹⁹ TCF21 gene
a separate CAD GWAS locus whose protein competes with SMAD3 for chromatin access in coronary artery SMCs represents the most biologically coherent interaction partner. Individuals carrying both high-SMAD3 (rs56062135 CC) and low-TCF21 genotypes may experience the strongest pro-atherogenic smooth muscle phenotype shift. The COL4A1/COL4A2 locus (type IV collagen genes) also shows epistatic interaction with SMAD3 variants for CAD association across multiple cohorts.