Unraveling the Mechanisms of Valvular Heart Disease to Identify Medical Therapy Targets

Authors, Journal, Affiliations, Type, DOI

• Authors: Aeron M. Small (Chair), Katherine E. Yutzey, Bryce A. Binstadt, Kaitlin Voigts Key, Nabila Bouatia-Naji, David Milan, Elena Aikawa, Catherine M. Otto, Cynthia St. Hilaire (Vice Chair)
• Journal: Circulation. 2024;150:e109–e128
• Affiliations: Brigham and Women's Hospital, Harvard Medical School; University of Pittsburgh; University of Minnesota; INSERM Paris; Massachusetts General Hospital; University of Washington; University of Kentucky; Cincinnati Children's Hospital
• Type: AHA Scientific Statement
• DOI: 10.1161/CIR.0000000000001254

Overview

This AHA Scientific Statement reviews the molecular and cellular mechanisms underlying the four major primary valvular heart diseases (VHDs): calcific aortic valve disease (CAVD), bicuspid aortic valve (BAV), mitral valve prolapse (MVP), and rheumatic heart disease (RHD). The central and clinically urgent finding is that despite robust mechanistic discovery, no effective medical therapy exists for any VHD — and cardiovascular drugs proven in atherosclerosis (notably statins) have failed in AS. The statement identifies numerous molecular targets for drug development, outlines ongoing clinical trials, and maps major knowledge gaps that should guide future research priorities.

Keywords

AHA Scientific Statements; aortic valve stenosis; bicuspid aortic valve disease; cellular structures; heart valve diseases; mitral valve prolapse; rheumatic heart disease

Key Takeaways

Cardiac Valve Structure and Developmental Origins

• All cardiac valves have three layers: collagen-rich fibrosa, proteoglycan-rich spongiosa, and elastin-rich ventricularis (semilunar) or atrialis (atrioventricular)
• Valvular endothelial cells (VECs): contiguous with myocardial/aortic endothelium; derived from embryonic cardiac cushion endothelial cells
• Valve interstitial cells (VICs): fibroblast-like cells interspersed throughout all three layers; originate from endocardial cushions, neural crest, and epicardium; normally quiescent but become activated in disease
• Single-cell analysis identifies specific disease-driver VIC subpopulations capable of osteogenic differentiation in CAVD

Calcific Aortic Stenosis (CAVD)

Epidemiology:

• Most common VHD; age-standardized prevalence 116.3/100,000 (GBD 2019); global prevalence increased 124% between 1990–2017
• Risk factors: older age, male sex, hypertension, T2DM, dyslipidemia (especially elevated Lp(a)), smoking, elevated BMI, ESRD
• Supplemental calcium may increase mortality in mild-to-moderate AS
• ~25% of individuals ≥65 years have aortic sclerosis; ~1.8% progress to AS annually; most never develop hemodynamically significant AS

Molecular Mechanisms:

• Initial trigger: shear stress + mechanical stress → disruption of VEC layer → inflammation, lipid infiltration, oxidative stress
• Osteogenic transcriptional programs activated in VICs → fibrocalcific remodeling; ECM remodeling → stiffening
• Calcification starts in the fibrosa, typically the noncoronary cusp first
• Lipids: LDL-C, Lp(a), and remnant cholesterol have independent causal roles (Mendelian randomization); Lp(a) promotes disease via oxidised phospholipids → osteogenic transformation of VICs; autotaxin (ATX, metabolises Lp(a)-associated lysophosphatidylcholine) is overexpressed in CAVD VICs
• Inflammation: IL-6 (GWAS-implicated); NLRP3 inflammasome; clonal hematopoiesis of indeterminate potential (CHIP) amplifies NLRP3-mediated gene programs; interferonopathies (Singleton-Merten syndrome, ADAR-related) → extreme early valve calcification
• Extracellular vesicles (EVs): sortilin loads calcification enzyme TNAP (ALPL gene) into EVs → trapped in ECM → hydroxyapatite microcalcification nucleation → progressive macrocalcification

Statin Paradox:

• Epidemiological/genetic evidence strongly supports lipid involvement in CAVD initiation, yet multiple clinical trials of statins (SALTIRE trial) showed no benefit in attenuating AS progression
• Suggests disease initiation and progression are distinct processes with different therapeutic windows

Therapeutic Targets and Ongoing Trials:

• Lp(a)-lowering: pelacarsen (anti-sense oligonucleotide) — NCT05646381 (n=502, primary endpoint: aortic valve calcium score + peak AV velocity)
• Inflammation: colchicine (CHIANTI trial, n=150; COPAS pilot, n=24)
• LDL-C: PCSK9 inhibitor (NCT04968509, n=160)
• Endothelial dysfunction: evogliptin (DPP4 inhibitor) — EVOID-AS (n=867)
• Renin-angiotensin system: ARBs (ARBAS, n=144)
• PPAR-γ: pioglitazone (n=100, 3-year mortality primary endpoint)
• Additional pathways: NOTCH signaling, mineral nucleation, RAAS, dipeptidyl peptidase 4

Bicuspid Aortic Valve (BAV)

• Most common valve malformation; prevalence ~1.5%; unicuspid is rare
• BAV results from fusion or absence of valve leaflet progenitors; two leaflets classified by orientation
• Almost all BAV patients require valve intervention in their lifetime; ~50% of all patients undergoing AVR for AS have BAV
• Genetics: heritability up to 89%; most genetic basis unidentified (complex pathology); monogenic variants in NOTCH1, GATA4/5/6, SMAD6, primary cilia genes, endothelial mesenchymal transition genes; linked to Turner, Marfan, Loeys-Dietz syndromes
• Male predominance: 7.1/1000 vs 1.9/1000 at birth; X-chromosome inactivation hypothesised as contributing mechanism
• BAV causes abnormal flow patterns and stress → accelerated calcification vs trileaflet aortic valves
• No medically accessible molecular targets for congenital valve malformations at present

Mitral Valve Prolapse (MVP)

Epidemiology:

• Prevalence 1 in 40 individuals; most never develop significant MR or require intervention
• Up to 25% develop significant MR; prevalence equal between sexes (Framingham); women have different anatomical features (less posterior prolapse, less flail, more thickening) and worse outcomes for severe MR

Two forms:

• Barlow disease: myxomatous degeneration; thick valves; excess connective tissue; proteoglycan-rich; fragmented elastin; chordae tendineae also affected
• Fibroelastic deficiency: thin leaflets; reduced proteoglycans, collagen, elastin

Molecular Mechanisms:

• Genetic triggers partially determine pathogenesis; mechanical stress and aging (decreased cellularity, disoriented collagen, increased elastin fibres) contribute
• Syndromic forms: Marfan (FBN1) → increased MMPs → collagen/elastin fragmentation; Loeys-Dietz (TGFBR genes) → central TGF-β pathway activation; vascular Ehlers-Danlos (COL3A1) → collagen fragmentation
• Familial forms: FLNA (X-linked) → actin-binding protein defect; DCHS1 → altered VIC migration and cell polarity; DZIP1 (primary cilia gene) → altered ECM deposition + myxomatous degeneration
• GWAS: TNS1 (focal adhesion/cytoskeleton); LMCD1 (calcineurin/NFAT pathway, GATA6 repressor); TGFβ2, LTBP2, β-spectrin → TGF-β scaffolding; cardiomyopathy genes ALPK3, BAG3, RBM20 as potential causal loci
• Serotonin link: polymorphisms in SLC6A4 (serotonin transporter SERT) → decreased SERT activity → accelerated mitral valve remodeling and earlier surgery

Therapeutic Targets:

• TGF-β pathway is central but pleiotropic (cancer, fibrosis, development) — tissue-specific targeting required
• Calcineurin/NFAT via LMCD1: reduced LMCD1 expression increases MVP risk; calcineurin inhibitors approved but activators unavailable clinically
• Tensin-1 (TNS1) and transcription factor GLIS1 as potential but less-characterized targets

Rheumatic Heart Disease (RHD)

Epidemiology:

• Highest prevalent VHD in low-income countries; ~2.8 million incident cases and >40 million prevalent cases globally (GBD 2019)
• 80% of cases are women; left-sided valves disproportionately affected (especially mitral)
• Remains prevalent in regions with limited access to diagnostic resources and penicillin

Molecular Mechanisms:

• Group A streptococcus (pharyngitis) → molecular mimicry: CD4+ T-cell and B-cell reactivity to streptococcal antigens (M protein, N-acetyl-b-D-glucosamine) → autoantibodies cross-reactive with valve endothelium proteins (laminin, cardiac myosin)
• Autoantibodies → ↑VCAM-1 → T-cell and macrophage infiltration into valve interstitium
• Early phase: proinflammatory cytokines (IFN-γ, IL-17, TNF-α)
• Chronic phase: TGF-β → fibrosis
• Complement: ficolin (FCN1-3) and mannose-binding lectin (MBL2) pathway activation triggered by group A streptococcal cell wall → susceptibility loci in GWAS
• Genetics: HLA class II MHC risk alleles (population-specific); CTLA4, FCGR2A, IL10, IL1RN, MBL2, TLR2, TNF, TGFB1; novel locus on chromosome 11 exclusive to Black African individuals
• ProTα (prothymosin-α): newly identified in RHD pathogenesis; CD8+ T-cell cytotoxicity via estrogen receptor α — possible mechanism for female sex predilection

Therapeutic Targets:

• Primary prevention: penicillin for acute group A streptococcal infection + secondary prophylaxis
• Biologics targeting T-cell activation, proinflammatory cytokines, cell adhesion molecules, complement — theoretically applicable but cost/distribution barriers in resource-limited settings
• Dapagliflozin trial in severe mitral stenosis (Dapa-Rhemis, NCT05618223, n=36)

Right-Sided VHD

• Majority of right-sided VHD in adults is secondary (left-sided disease, pulmonary disease)
• Mild TR: up to 80% of healthy individuals; more severe in older adults with AF, HF, or mitral valve disease
• Primary tricuspid disease: congenital (Ebstein anomaly, 1/200,000 live births; NKX2.5, MYH7 variants) or acquired (RHD, endocarditis, carcinoid)
• Pulmonary valve stenosis/regurgitation: almost always congenital (tetralogy of Fallot, Noonan syndrome)
• Why pulmonary valve does not develop calcific stenosis (unlike aortic) despite similar semilunar structure remains unknown

Limitations of the Document

• AHA Scientific Statement (expert consensus), not a systematic review with formal GRADE evidence rating
• Predominantly reviews molecular/basic science evidence; clinical trial data limited due to absence of approved therapies
• Most large CAVD population studies and GWASs focus on White individuals; sex-stratified analysis often underpowered
• Murine CAVD models do not faithfully recapitulate human valve calcification (melanocytes in mouse VICs confound von Kossa staining)
• Knowledge gaps for BAV, MVP (especially fibroelastic deficiency form), and RHD mechanisms remain substantial
• No current medical therapy; ongoing trials are phase 1/2 with surrogate imaging endpoints (CT calcium score, AV velocity)

Key Concepts Mentioned

• concepts/CAVD-Mechanisms — molecular mechanisms of calcific aortic valve disease; osteogenic transformation; EVs; Lp(a)
• concepts/Aortic-Stenosis — clinical AS management framework; grading, intervention thresholds
• concepts/Valvular-Heart-Disease — overview of VHD types and general management principles

Key Entities Mentioned

• entities/Bicuspid-Aortic-Valve — genetic basis and accelerated calcification
• entities/Mitral-Valve-Prolapse — TGF-β/ECM/serotonin molecular mechanisms
• entities/Rheumatic-Heart-Disease — molecular mimicry, complement, sex predilection
• concepts/Lipoprotein-a — independent causal role in CAVD via autotaxin/oxidised phospholipids

Wiki Pages Updated

• Created: wiki/sources/vhd-mechanism-aha-2024.md
• Created: wiki/concepts/CAVD-Mechanisms.md
• Created: wiki/entities/Mitral-Valve-Prolapse.md
• Created: wiki/entities/Bicuspid-Aortic-Valve.md
• Created: wiki/entities/Rheumatic-Heart-Disease.md
• Updated: wiki/concepts/Aortic-Stenosis.md
• Updated: wiki/concepts/Lipoprotein-a.md
• Updated: wiki/wikiindex.md
• Updated: wiki/sourceindex.md