NPPA (Atrial Natriuretic Peptide Precursor) — Atrial Dilated Cardiomyopathy
Details
NPPA (1p36.22) encodes prepro-ANP, the precursor of atrial natriuretic peptide. Homozygous loss-of-function variants cause atrial dilated cardiomyopathy (ADCM) — a primary atrial myocardial disease with biatrial dilatation, progressive atrial tachyarrhythmia, and evolution to complete atrial standstill, with preserved left ventricular systolic function.
It is the second of the three molecular routes to atrial standstill and the most important conceptually: SCN5A was read as primary electrical disease, whereas NPPA demonstrated that atrial standstill can be a fibrotic myocardial disease with normal ventricular myocardium. (sources/atrial-standstill-hrs-2026, rating: medium)
Key Facts
The Founder Mutation
- p.Arg150Gln, homozygous missense, identified in a 37-year follow-up cohort from Northeast Italy: LOD score 3.38 (P=.002); every affected individual homozygous; heterozygotes asymptomatic with normal ANP. (sources/atrial-standstill-hrs-2026, rating: medium)
- MR-pro-ANP in homozygotes is 14.5 pmol/L against a normal 18–120 — severely reduced, confirming loss of ANP signalling rather than a toxic gain of function.
- SCN5A, GJA5, LMNA and EMD were all sequenced and excluded in the original families, establishing NPPA as an independent pathway rather than a modifier.
- Allele frequency in NHLBI ESP 0.015%. All genetically confirmed cases descend from a single founder population (6 families); prevalence elsewhere is unknown.
Phenotype
- Adult onset, autosomal recessive, age-dependent penetrance beginning after age 20.
- Biatrial dilatation to giant dimensions — right atrial volume reported up to 744 mL. On CT: complete standstill LA 253 mL/m², RA 443 mL/m²; partial standstill LA 72–105, RA 98–195 mL/m². (sources/atrial-standstill-hrs-2026, rating: medium)
- Tachy–brady course: early supraventricular arrhythmias and palpitations (~54%) → progressive atrial voltage reduction → complete atrial standstill.
- LGE-MRI shows biatrial wall enhancement — RA fibrosis 36–70% and LA fibrosis 63–82% in partial standstill (Utah stage IV), predominantly septal and extending to the RA free wall. Ventricular LGE is absent. (sources/atrial-standstill-hrs-2026)
- LV function is preserved long-term — LVEF 54–74% over 37 years, NYHA I–II. Heart failure therapy is rarely needed.
- Prognosis is nevertheless poor: thromboembolism in 54% (7 of 13) and mortality 38% (5 of 13 — 1 stroke, 4 sudden deaths).
Mechanism
- Loss of ANP function produces progressive atrial interstitial fibrosis and myocyte loss, consistent with Nppa⁻/⁻ mouse models. This is the opposite of the SCN5A "primary electrical disease" model — hence EHRAS class II rather than class I.
- Heterozygous NPPA frameshift mutations behave differently: they raise ANP, shorten atrial action potential duration and cause familial atrial fibrillation without severe atrial dilatation or standstill. Zygosity, not merely gene, determines phenotype.
- A dual-mechanism model — loss of ANP-mediated signalling plus proteotoxic endoplasmic reticulum stress from mutant pro-ANP misfolding — has been proposed but not experimentally resolved.
Management Implications
- Anticoagulate at diagnosis, regardless of conventional risk scores — the 54% thromboembolic rate is the strongest single-etiology estimate in the atrial standstill literature. Warfarin INR 2–3 or a NOAC. (sources/atrial-standstill-hrs-2026, rating: medium)
- Pacing for symptomatic bradycardia; complete standstill means VVI is often the only option.
- No ICD recommendation can be made. Four of the five deaths were sudden, with no documented ventricular arrhythmia and no autopsy to separate arrhythmic from thromboembolic causes.
- Cascade screening: siblings carry a 25% recurrence risk; carrier screening is reasonable in the founder population; ECG, Holter and echo every 1–2 years from age 20. Prenatal and preimplantation genetic diagnosis should be discussed.
Contradictions / Open Questions
- The global prevalence and genetic spectrum are unknown. Every clinical statement about NPPA rests on 13 patients from 6 families in one Italian community. Whether other populations carry different variants with different phenotypes has not been established, and the 54% thromboembolic rate may be a founder-specific estimate.
- Why patients die suddenly is unresolved. Four out-of-hospital sudden deaths without documented arrhythmia, and without autopsy the arrhythmic and thromboembolic explanations cannot be separated — so the central device question for this genotype has no answer.
- Loss of function versus proteotoxicity. Whether the phenotype is caused by absent ANP signalling, by endoplasmic reticulum stress from misfolded mutant pro-ANP, or both, is experimentally open, and it determines whether a therapeutic target exists.
- The relationship to the CORIN pathway is unexplained. CORIN cleaves pro-ANP to active ANP, so complete CORIN loss also abolishes ANP — yet the CORIN phenotype is isolated left atrial fibrosis with hypertrophic cardiomyopathy and resistant hypertension, while NPPA produces biatrial giant dilatation with normal blood pressure. Two lesions in one pathway, two different diseases. (entities/CORIN)
- Penetrance begins after age 20, which raises the question of whether predictive testing in minors is appropriate when childhood surveillance would not change management.
Connections
- The syndrome it causes: concepts/Atrial-Standstill — the recessive form, with the highest reported thromboembolic rate
- The parent spectrum: concepts/Atrial-Cardiomyopathy — natriuretic peptide pathway, EHRAS class II interstitial fibrosis
- The neighbouring gene in the same axis: entities/CORIN — also abolishes ANP, with an LA-predominant phenotype
- The other two pathways: entities/SCN5A · entities/EMD · entities/LMNA
- How it is imaged: concepts/Late-Gadolinium-Enhancement — biatrial fibrosis with a clean ventricle
- What it causes: concepts/Atrial-Failure — atrial standstill as the end-stage
- Family management: concepts/Cascade-Family-Screening — 25% sibling risk, autosomal recessive