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Dual-chassis EcN PDB + uricase additive SUA prediction (comp-031)

Question

Does engineered EcN expressing the 2,8-dioxopurine PDB cluster (CBT2.0, Li 2025) co-administered with a PULSE-style luminal uricase deliver additive ΔSUA beyond either arm alone? Does PDB-derived butyrate compound the gut-lumen sink via PPARγ/HDAC ABCG2 modulation? See chassis-pending-interventions.md §M1.

Verdict

YELLOW (provisional) — combined > either arm but well below the naive sum. The two urate-consumption arms compete for scarce luminal urate substrate (per comp-019 substrate-limited regime); the PDB pathway adds an INDEPENDENT mechanism axis via butyrate → PPARγ ABCG2 induction (WT alleles) + butyrate → HDAC Q141K trafficking rescue. Combined ΔSUA: −1.8 to −1.9 mg/dL across genotypes (90% CI roughly −2.2 to −1.3). Additive bump over PDB-alone: ~−0.1 to −0.2 mg/dL, genotype-stratified (largest in Q141K-hom).

"Provisional" reflects: (a) DOPDH kinetic Km is mechanistic extrapolation from sister Mo-Se hydroxylases, (b) mouse-to-human translation 0.5× point-estimate (range 0.3–0.7), © in-vivo crypt butyrate concentration empirically open per purine-degrading-bacteria.md §Concentration-gap.

Key results

Genotype Uricase alone PDB alone (med, 90% CI) Combined (med, 90% CI)
WT/WT male gout −0.83 mg/dL −1.74 (−2.18, −1.24) −1.87 (−2.28, −1.37)
Q141K het male gout −0.66 mg/dL −1.62 (−2.03, −1.15) −1.76 (−2.14, −1.30)
Q141K hom male gout −0.49 mg/dL −1.55 (−1.94, −1.10) −1.82 (−2.19, −1.37)

Combined ΔSUA ~25–30% larger than PDB-alone (NOT 2–3× as naive addition predicts). Q141K rescue fraction ~0.43 (Basseville 2012 Hill activation at modeled crypt butyrate); rescue bump in Q141K-hom is the genotype-stratified differentiator that uricase alone cannot deliver.

Compositional finding

The two arms compete on urate consumption (combined ≈ dominant + ~10% of minor; substrate-competition factor 0.06–0.14 per comp-019 capacity-vs-supply ratios) but compose additively on the butyrate-mediated ABCG2 axis (uricase produces no butyrate). Different mechanism, different math.

Method summary

Literature-anchored kinetic + flux model, Python stdlib only, n=15000 Monte Carlo. Per-arm ΔSUA (uricase inherits comp-019; PDB anchored on Li 2025 with mouse-to-human attenuation + intact-kidney renal-compensation correction); substrate-competition (combined ≈ dominant + small residual capture); butyrate-mediated additive axis (PPARγ on WT alleles, Hill-function HDAC rescue on Q141K). Sensitivity over CBT2.0 colonization 10⁹–10¹¹ CFU/g, mouse-to-human 0.3–0.7, butyrate yield 0.3–0.7 mol/mol, crypt attenuation 0.05–0.5, colonic urate 50–500 μM. Full provenance + reproduction at ./etc/experiments/comp-031-dual-chassis-ecn-pdb-uricase-additive-sua/.

Limitations

DOPDH kinetic parameters mechanistic extrapolation from sister enzymes; CBT2.0 anchor single-paper Uox⁻/⁻ mouse; in-vivo crypt butyrate empirically open (validation-experiments.md §1.14 butyrate dose-response is the wet-lab resolution); butyrate stoichiometry 0.5 mol/mol pathway-extrapolated; PPARγ WT-induction magnitude approximated; no human RCT of CBT2.0-class PDB; first-order steady-state (no circadian / food-effect / CKD-grade modeling).

Impact on experimental priorities

Reframes the M1 multi-chassis stack engineering question from "should we build it?" to "build it as TWO strains, not one dual-cassette." Substrate-competition means a single dual-cassette EcN (PDB 8-gene cluster + uricase + selenium-cofactor coordination) gains ~nothing relative to two separate strains co-administered. The butyrate synergy operates from gut lumen on enterocyte, not at bacterial-cell scale — co-administration captures it. Avoids the regulatory and stability complexity of an 8-gene selenoprotein cluster + uricase coordinated in one chassis.

Does NOT change prioritization of either single arm — PULSE uricase remains independently valuable (~−0.7 mg/dL); PDB-EcN remains the higher-magnitude single intervention (~−1.6 mg/dL). The stack is YELLOW for additivity (modest but real) and a clear NO for single-chassis dual-cassette.

Cross-references