rs587782951 — JPH2 Thr161Lys

JPH2 Thr161Lys — When the Heart's Calcium Bridge Breaks

Your heart muscle relies on millisecond-precise calcium pulses to contract and relax in perfect rhythm. The junctophilin-2 protein, encoded by JPH2, is the structural bridge that makes this possible — a molecular tether clamping the plasma membrane and sarcoplasmic reticulum together so that the electrical signal arriving at the cell surface is faithfully translated into a calcium surge inside the cell. rs587782951¹¹ rs587782951
This single-nucleotide variant at chromosome 20q13.12 replaces threonine-161 with lysine, subtly but consequentially altering this coupling machinery. The result is hypertrophic cardiomyopathy (HCM) — the most common inherited cardiac disorder and a leading cause of sudden cardiac death in young people.

The Mechanism

The JPH2 protein contains eight MORN (membrane occupation and recognition nexus) motifs at its N-terminus that anchor it to the plasma membrane, and a C-terminal transmembrane domain that inserts into the sarcoplasmic reticulum²² sarcoplasmic reticulum
the cell's internal calcium store membrane. Between them, the joining region spans the narrow junctional gap between the two membranes, holding them ~10–15 nm apart — a distance perfectly calibrated for calcium channel coupling.

The Thr161Lys substitution falls in the joining region, replacing a neutral polar amino acid (threonine) with a positively charged lysine. Studies in patient-derived iPSC cardiomyocytes show this disrupts protein localization and calcium inactivation kinetics³³ disrupts protein localization and calcium inactivation kinetics, producing a slower time constant of calcium current decay. The consequence: the action potential lingers longer at each beat, calcium handling is dysregulated across millions of cardiomyocytes, and the myocardium hypertrophies — thickening its walls in an attempt to compensate for impaired pump efficiency. Early afterdepolarizations (EADs) arising from this prolonged repolarization create the substrate for lethal arrhythmias.

The Evidence

JPH2 was established as an HCM disease gene in 2007, when Landstrom et al. screened 388 unrelated HCM patients⁴⁴ Landstrom et al. screened 388 unrelated HCM patients
Landstrom AP et al. J Mol Cell Cardiol 2007 and identified three novel JPH2 missense mutations (S101R, Y141H, S165F) absent in 1,000 control alleles. Each mutation caused protein reorganization, disrupted calcium signaling, and increased cardiomyocyte size — establishing a direct causal chain.

The Thr161Lys variant specifically was characterized in a Finnish founder-effect setting. Vanninen et al. 2018⁵⁵ Vanninen et al. 2018
Vanninen SUM et al. PLoS One 2018;13(9):e0203422 followed 26 heterozygous carriers across 9 families and found penetrance of 71% by age 60 and 100% by age 80 — unusually high for a single-gene cardiac variant. Clinical features included left ventricular hypertrophy, third-degree AV block, and end-stage systolic heart failure in a subset of carriers; co-segregation confirmed in 6 of 9 families. This variant is classified as pathogenic in ClinVar (VCV000155800), supported by multiple independent submissions.

The mechanism was further characterized in iPSC-derived cardiomyocytes: Valtonen et al. 2023⁶⁶ Valtonen et al. 2023
Biomedicines 2023;11(6):1558 showed Thr161Lys cardiomyocytes display significantly enlarged cell bodies, disrupted sarcomeric organization, prolonged action potentials at 50% and 90% repolarization, and spontaneous phase-3 early afterdepolarizations — recapitulating the patient arrhythmia phenotype at the cellular level.

Across a broader JPH2 landscape, Parker et al. 2023⁷⁷ Parker et al. 2023
Trends Cardiovasc Med 2023;33(1):1-10 systematically reviewed 61 JPH2 variant carriers and found 47% developed HCM, 18% DCM, and 14% experienced arrhythmia or sudden cardiac death — underscoring the high clinical stakes of JPH2 pathogenic variants as a class.

Practical Implications

Heterozygous carriers of the Thr161Lys variant face near-certain lifetime risk of HCM. The high age-dependent penetrance means that a carrier who is asymptomatic in their 30s may not yet have developed disease, making regular surveillance essential. HCM in JPH2 carriers has a distinctive phenotype: left ventricular hypertrophy combined with risk of high-grade conduction block and arrhythmia, which distinguishes it from sarcomeric HCM caused by MYH7 or MYBPC3 mutations. Early identification enables lifestyle risk reduction (avoiding competitive sports), pharmacologic management of symptoms, and implantable cardioverter-defibrillator consideration in high-risk individuals.

First-degree relatives of a carrier have a 50% probability of inheriting the variant and require cascade genetic testing regardless of current symptoms.

Interactions

JPH2 does not have established interactions with other HCM-associated genes of the type seen with sarcomeric compound heterozygosity (e.g. MYBPC3 + MYH7). However, disease expression may be modulated by variants in genes that regulate calcium handling, including RYR2 (ryanodine receptor) and CASQ2 (calsequestrin), which together with JPH2 form the junctional calcium signaling complex. These interactions are mechanistically plausible but not yet characterized in carriers of Thr161Lys specifically.