A circRNA-based uricase replacement therapy for sustained treatment of hyperuricemia

This study demonstrates that a lipid nanoparticle-delivered circRNA encoding an engineered human-like uricase effectively and safely lowers serum urate levels for extended periods in a mouse model of hyperuricemia, offering a promising long-term therapeutic strategy for the condition.

The Gist

The Big Problem: A Clogged Drain

Imagine your body is a house, and uric acid is like a sticky, hard-to-dissolve sludge that builds up in your pipes. In most animals, there is a special "cleaning crew" (an enzyme called uricase) that breaks this sludge down into harmless water and soap, which flows right out the drain.

But humans lost this cleaning crew millions of years ago. Now, when we eat certain foods, that sludge builds up. If it gets too high, it turns into sharp, needle-like crystals. These crystals can:

• Jam the joints: Causing the excruciating pain of gout.
• Clog the kidneys: Leading to kidney failure.
• Cause other issues: Like high blood pressure and heart trouble.

The Current Fix: Turning Off the Faucet vs. Cleaning the Pipe

Right now, doctors treat high uric acid in two main ways, but both have flaws:

1. Turning off the faucet (Allopurinol/Febuxostat): These drugs stop your body from making new sludge. But they can't clean up the sludge that's already there. It's like turning off the water to a flooded basement, but the water is still sitting on the floor. Also, these drugs can cause nasty rashes or heart issues in some people.
2. Sending in a cleaning crew (Pegloticase): This is an injection of the missing enzyme. It works fast! But it's like hiring a cleaning crew that the house's security system (your immune system) attacks. The body builds antibodies against the drug, rendering it useless after a while. Plus, it's very expensive and needs to be given often.

The New Solution: A "Self-Replicating" Instruction Manual

This paper introduces a brand-new idea: circRNA therapy.

Think of your cells as a factory. Usually, if you want the factory to make a product (like uricase), you send them a paper instruction manual (mRNA). But paper instructions tear easily and get thrown away quickly.

Circular RNA (circRNA) is like a laminated, circular instruction manual. Because it has no loose ends, it's incredibly tough. It can't be easily shredded by the factory's "trash collectors" (enzymes).

• The Strategy: The scientists created a circular instruction manual that tells the liver cells to build their own "cleaning crew" (uricase).
• The Delivery: They wrapped these instructions in a tiny, invisible bubble made of fat (Lipid Nanoparticles). This bubble acts like a Trojan Horse, sneaking the instructions right into the liver cells without setting off the immune system's alarms.

What Happened in the Experiments?

The researchers tested this on mice that were genetically engineered to be like humans: they had no uricase and were suffering from severe "sludge" buildup (hyperuricemia).

1. The Quick Fix: They gave the mice a single injection. Within 24 hours, the "cleaning crew" started working, and the uric acid levels dropped like a stone. The effect lasted for 10 days from just one shot.
2. The Long-Term Fix: They gave the mice injections once a week for 10 weeks.
   • Result: The uric acid stayed low the whole time.
   • Kidney Protection: The untreated mice had swollen, damaged kidneys (like clogged pipes). The treated mice had healthy, normal-looking kidneys.
   • Safety: The mice didn't get sick, their livers didn't get damaged, and they didn't develop antibodies against the treatment.

Why Is This a Big Deal?

Imagine you have a leaky roof.

• Old drugs are like putting a bucket under the leak. It catches the water, but the roof is still broken, and you have to keep emptying the bucket every day.
• This new therapy is like sending a repair crew up to the roof to fix the shingles. Once they are there, they keep fixing the roof for a long time.

The Key Takeaways:

• Speed: It works faster than current drugs because it destroys existing sludge, not just stops new sludge.
• Durability: Because the instructions are circular and tough, they last longer in the body, meaning fewer shots are needed.
• Safety: It doesn't seem to trigger the immune system like older protein injections do.
• Future: This could be a game-changer for people with severe gout or kidney disease who can't take current medications. It might even be used for other diseases where the body is missing a specific protein.

In short, the scientists found a way to teach the body to make its own missing "cleaning crew" using a super-durable instruction manual, offering hope for a long-term cure for high uric acid.

Technical Summary

1. Problem Statement

• Clinical Challenge: Hyperuricemia (elevated serum urate) is a major risk factor for gout, chronic kidney disease (CKD), and cardiovascular/metabolic disorders. It arises from the evolutionary loss of the uricase (UOX) enzyme in humans, leaving uric acid as the end-product of purine metabolism.
• Limitations of Current Therapies:
  • Xanthine Oxidase Inhibitors (e.g., Allopurinol, Febuxostat): These inhibit urate production but cannot rapidly degrade pre-existing urate (limiting utility in Tumor Lysis Syndrome). They also carry risks of severe cutaneous adverse reactions (Allopurinol) and cardiovascular toxicity (Febuxostat).
  • Uricosurics (e.g., Benzbromarone): Associated with hepatotoxicity and nephrotoxicity.
  • Recombinant Uricases (e.g., Pegloticase): While effective at degrading urate, they suffer from high immunogenicity (anti-drug antibodies), short half-lives, and infusion reactions, often leading to loss of efficacy.
• Unmet Need: There is a critical need for a therapy that offers rapid urate reduction, sustained long-term efficacy, low immunogenicity, and favorable biosafety for chronic management.

2. Methodology

The study utilized a multi-stage approach combining molecular engineering, in vitro screening, and in vivo validation in mouse models.

• Molecular Design:

  • Candidate Selection: Three engineered human-like uricase variants were synthesized as circular RNAs (circRNAs): rHU (reconstructed from human pseudogene), PBC (pig-baboon chimera), and An96.
  • circRNA Architecture: The coding sequences were inserted into a circRNA backbone containing:
    • CVB3 IRES: To drive cap-independent translation.
    • N-terminal FLAG tag: For detection (removed in long-term studies).
    • C-terminal Peroxisomal Targeting Signal (PTS): To ensure correct subcellular localization.
    • Self-splicing introns: Used for in vitro circularization.
  • Delivery System: The circRNAs were encapsulated in Lipid Nanoparticles (LNPs) using the ALC-0315 formulation (clinically approved for mRNA vaccines) to target the liver.

• In Vitro Screening:

  • Transfection of human 293T and HepG2 cells to assess protein expression, enzymatic activity, and peroxisomal localization (via GFP/mCherry co-localization).
  • Selection: The rHU variant was selected due to its superior enzymatic activity in cell lysates.

• In Vivo Models:

  • Wild-Type (WT) Mice: Used to evaluate pharmacokinetics, biodistribution, and biosafety (liver/kidney toxicity) of single-dose circUOX-LNP.
  • Uricase-Knockout (Uox⁻/⁻) Mice: A model of severe hyperuricemia and early-onset renal injury. Used to test therapeutic efficacy.
  • Dosing Regimens:
    • Single-dose (0.3–1.2 mg/kg) to determine duration of action.
    • Repeated dosing (every 4 days for 3 doses) to compare with Allopurinol.
    • Long-term dosing (every 8 days for 69 days) to simulate chronic therapy.

• Analytical Techniques:

  • RT-qPCR and Western Blot (using anti-FLAG immunoprecipitation) for RNA/protein quantification.
  • Serum biochemical assays (Uric acid, ALT, AST, Creatinine, Urea).
  • Histopathology (H&E staining) of major organs.
  • RNA-seq to analyze transcriptomic changes in kidney tissue.

3. Key Contributions

• First Proof-of-Concept: Demonstrates the first application of circRNA-based uricase replacement for treating hyperuricemia.
• Superior Stability: Leverages the covalently closed structure of circRNA to resist RNase degradation, enabling sustained protein expression without the need for nucleoside modifications (often required for linear mRNA to reduce immunogenicity).
• Engineered Enzyme: Successfully expressed a humanized uricase (rHU) that localizes correctly to peroxisomes and retains high catalytic activity in vivo.
• Therapeutic Window: Established that a single injection provides urate-lowering effects for up to 10 days, and repeated dosing maintains efficacy for 10 weeks.

4. Key Results

• Efficient Expression:
  • In human cells, rHU-circRNA showed the highest enzymatic activity among the variants.
  • In WT mice, hepatic expression of uricase was dose-dependent and detectable up to 5 days post-injection.
• Biosafety:
  • No significant abnormalities in liver enzymes (ALT, AST, ALP) or kidney function markers (Creatinine, Urea) in WT mice at doses up to 1.2 mg/kg.
  • Histological examination of major organs (heart, liver, spleen, lung, kidney) showed no pathological changes.
• Therapeutic Efficacy in Uox⁻/⁻ Mice:
  • Rapid Reduction: A single injection reduced serum urate below the saturation threshold (~400 µM) within 24 hours, faster than Allopurinol (which took 3–5 days).
  • Sustained Effect: The urate-lowering effect lasted ~10 days per dose.
  • Renal Protection: Repeated dosing (3 doses) significantly reduced serum urea and creatinine levels and mitigated hydronephrosis and renal parenchymal loss compared to untreated controls.
  • Long-Term Study: Weekly administration for 69 days maintained sustained urate reduction. Treated mice showed no mortality, whereas control mice suffered severe kidney damage and death.
• Transcriptomic Restoration:
  • RNA-seq revealed that circUOX treatment partially restored the kidney transcriptome toward a healthy state.
  • Specifically, it downregulated pro-inflammatory pathways (NF-κB, IL-17, TNF signaling) and chemokines (CCL2, CCL4, IL1B), confirming the alleviation of inflammation-driven renal injury.

5. Significance and Future Outlook

• Clinical Translation: This approach addresses the limitations of current therapies by offering a protein replacement strategy that degrades existing urate rapidly (crucial for Tumor Lysis Syndrome) and can be administered less frequently than daily oral pills.
• Chronic Management: The ability to maintain therapeutic protein levels for weeks with repeated dosing makes it a viable candidate for lifelong management of refractory hyperuricemia.
• Platform Potential: The success of repeated circRNA administration for sustained liver expression suggests this platform could be extended to other protein-deficiency disorders.
• Next Steps: Future work should focus on optimizing IRES elements and LNP delivery to further extend the expression half-life, potentially reducing dosing frequency, and conducting toxicology studies in larger animal models to assess long-term human safety.

Conclusion: The study presents a robust, safe, and effective circRNA-based therapy that restores uricase function, rapidly lowers serum urate, and prevents renal damage in a severe hyperuricemia model, offering a promising new avenue for treating gout and related metabolic diseases.