Hypothesis

Hepatic zone 3 hepatocytes accumulate a critical threshold of mtDNA D-loop heteroplasmy that triggers a bistable switch from adaptive lipid oxidation to maladaptive fibrogenesis, and selective elimination of this heteroplasmy reverses NAFLD progression.

Mechanistic Rationale

• The D-loop, lacking histone protection, is a hotspot for oxidative lesions D-loop is particularly susceptible to oxidative damage. Oxidative damage stalls mtDNA replication, promoting deletion mutants and point mutations that increase heteroplasmy.
• In zone 3, low O2 and high fatty‑acid oxidation raise ROS production, exacerbating D-loop damage while hypoxic signaling suppresses mitophagy Impaired mitophagy drives NAFLD progression, allowing mutant genomes to persist.
• Once heteroplasmy exceeds a zone‑specific threshold (≈30% mutant load), transcriptional reprogramming shifts from PPARα‑driven β‑oxidation to TGF‑β‑driven stellate cell activation, producing an abrupt phenotypic transition Progressive heteroplasmy increases cause abrupt transcriptional reprogramming.
• Nuclear‑encoded regulators of mtDNA copy number cannot compensate because the lesion resides in the replication control region Nuclear genome regulates mtDNA copy number and heteroplasmy.

Experimental Design

1. Spatial mapping – Laser‑capture microdissection of zones 1, 2, 3 from human NASH livers and mouse models (high‑fat diet + streptozotocin). Perform duplex sequencing to quantify D-loop heteroplasmy per zone.
2. Threshold determination – Correlate heteroplasmy levels with zone‑specific transcriptomic signatures (β‑oxidation vs fibrosis genes) and histologic scores.
3. Targeted mtZFN delivery – Construct an AAV9 vector carrying a hypoxia‑responsive element (HRE) driving a mitochondrially targeted ZFN pair specific for the prevalent D-loop mutant. Inject intravenously; verify zone‑3 preferential expression via mito‑GFP reporter.
4. Outcome measures – After 8 weeks, assess: (a) D-loop heteroplasmy reduction (qPCR‑based mutant/wild‑type ratio), (b) ROS levels (MitoSOX), (c) mitophagy flux (LC3‑II/p62 with mitochondrial marker), (d) fibrosis (Sirius Red, α‑SMA), and (e) metabolic function (glucose tolerance, serum ALT).

Predictions & Falsifiability

• Prediction 1: Zone 3 will show significantly higher D-loop heteroplasmy than zones 1‑2 (p<0.01). Falsification: no zonation difference.
• Prediction 2: Heteroplasmy >30% in zone 3 correlates with fibrosis stage ≥F2. Falsification: weak or absent correlation.
• Prediction 3: HRE‑mtZFN reduces zone‑3 heteroplasmy by ≥50% and improves fibrosis metrics without affecting zones 1‑2. Falsification: no heteroplasmy shift or no histologic improvement.
• Prediction 4: Rescue is dependent on mitophagy restoration; co‑administration of a mitophagy inhibitor abolishes benefit. Falsification: benefit persists despite mitophagy blockade.

If any prediction fails, the hypothesis that zone‑specific mtDNA D-loop heteroplasmy drives NAFLD progression would be refuted, redirecting focus to nuclear or extracellular mechanisms.