Sarcomere Variants of Uncertain Significance identify an Intermediate Clinical Risk Profile in Hypertrophic Cardiomyopathy

This multicenter retrospective study demonstrates that sarcomere variants of uncertain significance (VUS) in hypertrophic cardiomyopathy patients represent an intermediate clinical risk profile with distinct phenotypic features, suggesting they are not entirely clinically neutral but rather part of a continuum of sarcomere dysfunction between pathogenic variants and genotype-negative cases.

The Gist

Imagine your heart is a high-performance race car. The muscles that make the engine run are called sarcomeres. In a condition called Hypertrophic Cardiomyopathy (HCM), the engine gets built with too many parts, making the walls of the heart thick and stiff, which can be dangerous.

To understand why a specific person's heart is built this way, doctors look at the "blueprints" (DNA). They are looking for typos in the instructions.

This paper is about a very specific, confusing type of typo called a VUS (Variant of Uncertain Significance).

The Three Types of Typos

Think of the genetic test results as three different categories of instructions found in the blueprint:

1. The "Definite Crash" Typos (P/LP Variants):
   These are clear, obvious errors. The instructions say "Build a wall here," but the typo says "Build a wall 10 feet high." We know for a fact these cause the engine to malfunction. Doctors call these Pathogenic or Likely Pathogenic.

   • The Risk: High. These patients are like race cars with a known, broken part. They need close monitoring and often extra safety gear (like an ICD, which is a tiny defibrillator implanted in the chest).

2. The "Clean Bill of Health" Typos (Benign Variants):
   These are typos that don't actually change the meaning of the sentence. Maybe a comma was moved, or a letter was swapped for a synonym that means the exact same thing. The engine works perfectly fine.

   • The Risk: None. These patients are treated as if they have no genetic cause for their heart condition.

3. The "Mystery Note" Typos (VUS):
   This is the star of the story. The blueprint has a weird symbol or a word we've never seen before. We don't know if it's a harmless glitch or a hidden bomb.

   • Current Rule: Because we aren't sure, doctors usually treat these like the "Clean Bill of Health." They say, "We don't know, so we'll ignore it for now."
   • The Problem: This study asks, "What if we're ignoring a ticking time bomb?"

The Big Discovery: The "Middle Ground"

The researchers looked at 438 patients with HCM and split them into three groups: those with "Definite Crash" typos, those with "Mystery Note" typos (VUS), and those with no typos at all.

They found that the "Mystery Note" group wasn't actually in the middle of the road; they were in the middle of the danger zone.

• The "Definite Crash" group had the worst hearts and the highest risk of sudden failure.
• The "No Typos" group had the best hearts and the lowest risk.
• The "Mystery Note" (VUS) group? They were right in the middle.

The Analogy:
Imagine a scale of heart health.

• Left side: Perfectly healthy engine.
• Right side: Exploding engine.
• The "Definite Crash" group is on the far right.
• The "No Typos" group is on the far left.
• The "Mystery Note" group is standing exactly halfway between them. They aren't as bad as the exploding engines, but they are definitely worse than the perfect ones.

The "Ladder" Effect

The study found something even more interesting. It wasn't just a flat middle ground. It was a ladder.

• If you have zero mystery notes, your risk is low.
• If you have one mystery note, your risk goes up a little.
• If you have two or three mystery notes, your risk goes up even more.

It's like having a few loose screws in your car. One loose screw might not stop the car, but three loose screws make it much more likely to break down. The more "Mystery Notes" a patient had, the higher their risk of heart failure or sudden cardiac death.

Where are these "Mystery Notes" hiding?

The researchers also looked at where these typos were located in the blueprint.

• The "Definite Crash" typos were mostly in the main engine parts (the thick and thin filaments).
• The "Mystery Notes" were mostly found in the scaffolding and support beams (Z-discs and giant proteins).

Think of it like a building. The main beams (thick filaments) are critical; if they break, the building falls. The support beams and scaffolding (Z-discs) hold everything together. If the scaffolding is slightly wobbly (a VUS), the building might still stand, but it's not as stable as a building with perfect scaffolding. It's an "intermediate" state of instability.

What Does This Mean for You?

The Old Way: If you got a "Mystery Note" result, doctors would say, "Don't worry, it's probably nothing. We can't do anything about it."

The New Way (Based on this paper): "Mystery Notes" might not be nothing. They might be a sign that your heart is in an intermediate risk zone.

• Don't Panic: This doesn't mean everyone with a VUS will have a heart attack.
• Do Pay Attention: Doctors should probably treat these patients with a bit more caution than those with "no typos." They might need more frequent check-ups or closer monitoring, even if they don't have a "Definite Crash" typo yet.

The Bottom Line

This paper suggests that the world of genetics isn't just black and white (Safe vs. Dangerous). There is a gray area in the middle.

The "Mystery Notes" (VUS) are like a yellow warning light on your car dashboard. It's not a red "Stop Now" light, but it's not a green "Go" light either. It's telling you, "Hey, something is a little off here. Keep an eye on it."

By recognizing that these "Mystery Notes" carry a real, intermediate risk, doctors can better protect patients who fall into this middle ground, potentially preventing tragedies before they happen.

Technical Summary

1. Problem Statement

Hypertrophic Cardiomyopathy (HCM) is a common inherited heart disorder where genetic testing is standard for diagnosis and family screening. However, the clinical interpretation of Variants of Uncertain Significance (VUS) remains a major challenge.

• Current Consensus: Under current guidelines (ACMG/AMP), VUS are considered clinically non-actionable and are treated as equivalent to genotype-negative status.
• The Gap: Emerging data suggests VUS carriers may not be benign but rather represent an intermediate risk group. With the expansion of Next-Generation Sequencing (NGS) panels, the number of reported VUS is increasing, creating uncertainty in clinical decision-making regarding risk stratification, family screening, and implantable cardioverter-defibrillator (ICD) placement.
• Objective: This study aimed to evaluate the genotype–phenotype and genotype–outcome associations in HCM patients to determine if sarcomere VUS represent a distinct, intermediate clinical risk profile compared to Pathogenic/Likely Pathogenic (P/LP) variants and genotype-negative patients.

2. Methodology

• Study Design: Multicenter, retrospective cohort study.
• Population: 438 patients with a confirmed diagnosis of HCM from two tertiary hospitals in South Korea (Seoul National University Hospital and Seoul National University Bundang Hospital).
  • Inclusion: Patients $\ge$15 years old with unexplained left ventricular hypertrophy ($\ge$15mm, or $\ge$13mm with family history) and no phenocopies (e.g., Fabry disease, amyloidosis).
  • Exclusion: Patients without imaging data or those diagnosed with HCM phenocopies.
• Genetic Testing:
  • NGS-based panels targeting HCM-associated genes.
  • Variants were classified as P/LP, VUS, or Benign/Likely Benign based on 2015 ACMG/AMP guidelines with ClinGen Cardiomyopathy Expert Panel specifications.
  • Grouping: Patients were stratified into three groups:
    1. P/LP carriers (n=171, 39.0%).
    2. VUS carriers (n=159, 36.3%; defined as having VUS but no P/LP variants).
    3. Genotype-negative (no sarcomere mutations or only benign variants; n=108, 24.7%).
• Endpoints:
  • Primary Endpoint: A composite of cardiovascular death, aborted sudden cardiac death (SCD), appropriate ICD therapy, and heart transplantation.
  • Secondary Endpoint: SCD-equivalent composite (SCD, aborted SCD, appropriate ICD therapy).
• Statistical Analysis:
  • Time-to-event analysis using Kaplan-Meier curves and log-rank tests.
  • Cox Proportional Hazards Models: Used Firth's penalized partial likelihood approach to handle small sample sizes and rare events.
  • Models were adjusted for age and sex, stratified by institution.
  • Ordinal Analysis: Variant classification was modeled as an ordinal variable (Genotype-negative $\to$ VUS $\to$ P/LP) to test for a dose-response relationship.

3. Key Contributions

• Reframing VUS: The study challenges the binary view of VUS as "benign," proposing instead that they represent an intermediate category along a continuum of sarcomere dysfunction.
• Structural Enrichment: The study identified a specific biological pattern where VUS are preferentially enriched in Z-disc and giant sarcomere scaffolding proteins (e.g., FLNC, TTN, OBSCN), whereas P/LP variants are concentrated in thick and thin filament proteins.
• Dose-Response Evidence: It provides statistical evidence of a graded risk increase correlating with the "pathogenicity" of the genetic status, even when VUS alone do not reach statistical significance in categorical multivariable models.

4. Key Results

• Phenotypic Differences:
  • P/LP Group: Younger age at testing, higher prevalence of septal hypertrophy, greater maximal wall thickness, higher HCM-risk-SCD scores, and higher rates of non-sustained VT.
  • VUS Group: Demonstrated intermediate phenotypic features between P/LP carriers and genotype-negative patients. They had higher wall thickness and risk scores than genotype-negative patients but lower than P/LP carriers.
  • Genotype-Negative Group: Oldest age, more apical/diffuse hypertrophy patterns, and lowest risk scores.
• Clinical Outcomes:
  • Primary Endpoint Incidence: 6.6% overall.
    • P/LP: Highest event rate.
    • VUS: Intermediate event rate.
    • Genotype-negative: Lowest event rate.
  • Survival Analysis: Kaplan-Meier analysis showed a significant difference in event-free survival across the three groups (Log-rank p = 0.0026).
  • Hazard Ratios (HR):
    • When modeled as categorical variables, only P/LP was independently associated with adverse outcomes compared to genotype-negative (HR 4.57, p=0.034). VUS was not statistically significant in this specific comparison.
    • Crucial Finding: When modeled as an ordinal variable (increasing pathogenicity), there was a significant stepwise increase in risk (HR 2.05 per step up in classification, 95% CI 1.11–4.16, p=0.019).
  • VUS Burden: Among VUS-only carriers, the incidence of primary endpoints increased with the number of VUS (4.1% for 1 VUS vs. 9.1% for $\ge$3 VUS), though this did not reach statistical significance due to sample size.
• Gene Distribution:
  • Thick/Thin filament variants were predominantly P/LP.
  • Z-disc and giant scaffolding protein variants were almost exclusively classified as VUS.

5. Significance and Implications

• Clinical Management: The findings suggest that sarcomere VUS should not be dismissed as entirely neutral. Patients with VUS may warrant closer monitoring than genotype-negative patients, as they exhibit an intermediate risk profile.
• Risk Stratification: The study supports a continuous risk model over a binary (pathogenic vs. benign) model. The "dose-response" relationship implies that the accumulation of VUS or their presence in specific structural domains (Z-disc/scaffolding) contributes to disease expression.
• Guideline Evolution: Current guidelines do not recommend cascade screening or ICD consideration based solely on VUS. This study argues for re-evaluating these thresholds, suggesting that VUS status should be integrated into broader phenotypic and structural contexts for risk assessment.
• Future Directions: The authors call for large-scale prospective studies and improved reclassification methodologies (functional assays, RNA studies) to resolve the uncertainty of VUS, particularly those in non-canonical sarcomere genes.

Conclusion: Sarcomere VUS in HCM patients represent a distinct, intermediate clinical risk profile characterized by specific phenotypic features and a graded increase in adverse outcomes, challenging the notion that they are clinically neutral.