Structural and single-molecule insights into the core human mitochondrial DNA replisome¶

The finding¶

This review integrates recent cryo-EM and single-molecule studies to build a quantitative mechanistic framework for how the core human mitochondrial replisome—POLG, Twinkle, and mtSSB—coordinates replication of the light and heavy strands of mtDNA. The authors synthesize structural data on replisome architecture with dynamic measurements of fork progression, describing how initiation, elongation, and regulation are achieved at the mitochondrial replication fork, and identify remaining gaps toward a complete mechanistic model.

Where it fits¶

This paper speaks directly to Loop B (Mitochondrial/NLRP3) in the SAMHD1 A565T model—specifically, the proposed mechanism by which dNTP pool expansion causes POLG replisome stress. The model posits that excess cytosolic dNTPs flood mitochondrial carriers, perturb the matrix pool, and stall POLG, leading to oxidized mtDNA (8-OHdG) and VDAC1 destabilization, which licenses the NLRP3 inflammasome. This review provides the structural and kinetic context for why POLG is vulnerable to nucleotide imbalances: the replisome must coordinate leading- and lagging-strand synthesis with a tightly regulated dNTP supply, and asymmetric pools—exactly what a partial loss-of-function SAMHD1 variant would produce—could plausibly stall the polymerase and generate the replication stress that feeds the downstream inflammatory cascade. The single-molecule approaches highlighted here are particularly relevant, as they can directly probe how POLG kinetics respond to altered dNTP concentrations and ratios.

Caveats¶

This is a review, not a primary study; the mechanistic claims are synthesized from other groups' work, not newly demonstrated here.
The paper does not address SAMHD1 or any disease variant—it describes the wild-type replisome under normal conditions, not the perturbed state induced by dNTP pool expansion.
Structural and single-molecule data come from reconstituted in vitro systems, which may not fully capture the mitochondrial environment (e.g., membrane tethering, local nucleotide compartmentalization) relevant to the A565T pathology.

What to watch¶

The next key question is whether single-molecule POLG assays can be run under asymmetric dNTP pools mimicking the SAMHD1 A565T state—if POLG stalling is directly observable under those conditions, it would provide a mechanistic bridge from the enzyme defect to the oxidized mtDNA that primes NLRP3.

Source: Structural and single-molecule insights into the core human mitochondrial DNA replisome — 2026.