rs104894502 — TPM1 E180G

TPM1 E180G — When the Heart's Safety Catch Breaks

Your heart contracts and relaxes roughly 100,000 times a day. Each cycle depends on a molecular off-switch: a protein called tropomyosin¹¹ tropomyosin
Tropomyosin is a long, rod-shaped protein that wraps around actin filaments in muscle cells. At rest it physically blocks the sites where myosin (the motor protein) can grab actin, preventing contraction until calcium signals it to step aside that sits like a lid on actin filaments, blocking the cardiac motor machinery until calcium says "go." The TPM1 E180G variant — a single amino-acid substitution replacing glutamic acid with glycine at position 180 of cardiac alpha-tropomyosin — weakens that lid. The result is a heart that can't fully disengage its own throttle, pushing toward hypertrophic cardiomyopathy (HCM): progressive thickening of the left ventricular wall that stiffens the pump and, in some carriers, triggers dangerous arrhythmias.

E180G was first identified in the landmark 1994 Cell paper by Thierfelder and colleagues²² landmark 1994 Cell paper by Thierfelder and colleagues
Thierfelder L et al., Cell 77:701–712, 1994 — the paper that established familial HCM as "a disease of the sarcomere" by showing mutations in multiple distinct sarcomeric proteins produce the same cardiac phenotype, which catalogued the first alpha-tropomyosin mutations causing familial HCM on chromosome 15q2. It is classified as ClinVar Pathogenic (VCV000012455), supported by multiple independent submitters without conflicts, and listed as the first allelic variant in OMIM entry 191010.

The Mechanism

Normally, tropomyosin exists in a tightly coiled double-helix that shifts between three positions — blocked, closed, and open — depending on calcium levels. The Glu180Gly substitution removes a charged glutamic acid residue and replaces it with tiny glycine, which has almost no side chain. This dramatically increases the local and global flexibility³³ local and global flexibility
Measured as persistence length — a quantitative index of a polymer's resistance to bending. Wild-type tropomyosin has a persistence length of approximately 150 nm; E180G reduces this, making the filament more prone to curving and bending of the tropomyosin filament by approximately 35% compared to wild-type protein.

The consequence is mechanical: excess flexibility impedes normal propagation of the "blocked → open" activation signal along the thin filament, and requires a smaller calcium signal to trigger contraction. Atomic force microscopy⁴⁴ Atomic force microscopy
A technique that traces the physical contour of a single protein molecule along a surface, allowing direct measurement of bending at nanometer scale and molecular dynamics simulations⁵⁵ molecular dynamics simulations
Computer models that simulate how individual atoms in a protein move over time, revealing conformational changes too fast to observe experimentally both confirm this increased flexibility, which destabilizes the low-calcium "off" state of cardiac muscle. The result: higher resting actin-myosin interaction, enhanced contractility, and — critically — failure of the muscle to fully relax between beats (diastolic dysfunction).

The Evidence

The molecular evidence for E180G's pathogenic mechanism is strong and consistent across independent methods:

Structural mechanics: Li et al., 2012⁶⁶ Li et al., 2012
Li XE et al. Biochem Biophys Res Commun 2012 — examined both D175N and E180G using electron microscopy of isolated tropomyosin molecules; persistence length reductions were statistically significant across >200 molecules per condition showed increased bending flexibility leads to excess Ca²⁺-activation and shifts regulatory equilibrium toward the "on" state even at diastolic calcium concentrations.

Calcium sensitivity and kinetics: Sewanan et al., 2016⁷⁷ Sewanan et al., 2016
Sewanan LR et al. Front Physiol 2016 — computational myofilament model incorporating E180G-specific stiffness and duty-cycle changes; validated against published in vitro motility data predicted E180G increases both maximum and diastolic force generation, and slows the time from peak tension to 50% relaxation — a signature of HCM diastolic dysfunction.

Signaling cascades: Robinson et al., 2018⁸⁸ Robinson et al., 2018
Robinson P et al. J Biol Chem 2018 — guinea pig cardiomyocyte model expressing thin-filament HCM mutations including the closely related D175N demonstrated that increased myofilament calcium buffering from these mutations elevates diastolic calcium and activates CaMKII and calcineurin/NFAT⁹⁹ CaMKII and calcineurin/NFAT
Two calcium-sensitive kinases that, when chronically activated, trigger gene programs causing hypertrophy, fibrosis, and arrhythmia — the cardinal features of HCM signaling cascades — providing a mechanism linking the sarcomeric defect to macroscopic cardiac remodeling.

Actin-myosin interaction: Kopylova et al., 2019¹⁰¹⁰ Kopylova et al., 2019
Kopylova GV et al. J Muscle Res Cell Motil 2019 — combined single-molecule optical trap and ensemble in vitro motility assay; both HCM mutations E180G and D175N increased calcium sensitivity in ensemble measurements, whereas dilated cardiomyopathy mutations showed opposite effects confirmed E180G increases thin-filament sliding velocity and calcium sensitivity in reconstituted assays, in the same direction as D175N — and in the opposite direction from dilated cardiomyopathy TPM1 mutations, validating the disease-specific mechanism.

E180G is ultra-rare globally (absent from gnomAD population databases), consistent with its being a disease-causing variant with strong negative selection pressure. The variant is classified Pathogenic with a 3-star ClinVar review status reflecting consistent classification across multiple independent submitters.

Practical Actions

Identifying an E180G carrier changes clinical management in ways that directly reduce morbidity and mortality. HCM is one of the most common causes of sudden cardiac death in individuals under 35. For carriers, the priorities are: (1) confirm diagnosis and baseline LV morphology, (2) identify high-risk features that warrant ICD implantation or septal reduction therapy, and (3) extend family testing to first-degree relatives. The autosomal dominant inheritance means each biological child of a carrier has a 50% chance of inheriting the variant.

Avoidance of competitive athletics and extreme exertion is recommended pending full clinical evaluation — sudden cardiac death events in HCM are disproportionately exercise-associated.

Interactions

E180G's closely related neighbor on chromosome 15, TPM1 D175N (rs104894503), shares nearly identical functional properties: both increase tropomyosin flexibility and calcium sensitivity, both were identified in the same 1994 paper, and both are classified Pathogenic for HCM. D175N is a Finnish founder mutation (accounting for 6.5% of Finnish HCM cases in a cohort of 306 patients) but is not known to interact with E180G as a compound heterozygote — these are independent dominant mutations in the same gene affecting adjacent codons.

Other sarcomeric protein HCM genes — MYBPC3 (rs36211723), MYH7, TNNT2 — can produce overlapping clinical phenotypes. Patients with multiple sarcomeric variants ("double positive" HCM) tend to have more severe hypertrophy and earlier onset, though compound heterozygosity specifically for E180G has not been studied in published literature.