IF a phosphorothioated locked nucleic acid (LNA) gapmer-14 targeting FANCM (40 mg/kg intraperitoneal, twice weekly) is administered for a discrete 72-hour "damage-loading" window to ATRX-deficient, C-circle-positive LiSa-2 liposarcoma xenografts in NOD-SCID mice, followed sequentially by low-dose olaparib (25 mg/kg oral, daily),

THEN the sequential combination will produce a ≥40% improvement in median progression-free survival (time to tumor volume ≥1500 mm³) compared to either monotherapy or concurrent combination, accompanied at interim analysis (day 30) by ≥2-fold elevation of ex vivo C-circle levels and a ≥3-fold increase in γH2AX/telomere-FISH co-localizing foci (TIFs) relative to vehicle controls,

BECAUSE the following step-by-step causal chain operates:

1. FANCM normally resolves TERRA R-loops at ALT telomeres to suppress replication fork stalling. In ALT-positive cells, FANCM disruption leads to massive C-circle accumulation (a direct readout of uncontrolled telomeric recombination) and elevated telomeric ssDNA, confirming that FANCM restrains hyper-ALT activity to sub-lethal levels. (FANCM suppresses ALT via TERRA R-loop disruption)[https://doi.org/10.1038/s41598-019-55537-5]

2. LNA gapmer-14 delivery induces RNase H-mediated FANCM mRNA degradation, removing this restraint. Within 72 hours post-ASO, TERRA R-loops accumulate at stalled telomeric replication forks, driving uncontrolled BTR (BLM-TOP3A-RMI) and BRCA1 recruitment, converting telomeres into high-density ssDNA substrates decorated with RPA. (FANCM, BRCA1, and BLM cooperatively resolve ALT telomeric stress)[https://doi.org/10.1073/pnas.1708065114]

3. The 72-hour window is specifically calibrated to allow PARP1 to be maximally recruited to these ssDNA-rich telomeric lesions before inhibition. PARP1 is the first responder at stalled replication forks containing ssDNA gaps; in FANCM-deficient ALT cells, PARP1 is required to stabilize the BRCA1-dependent resection intermediates and facilitate PARylation-dependent HDR. This creates a "loaded" state in which PARP1 is densely concentrated at telomeric fork-collapse sites. (BRCA1 promotes end resection in FANCM-deficient ALT cells)[https://doi.org/10.1073/pnas.1708065114] [SPECULATIVE: PARP1 recruitment kinetics to TERRA R-loop-derived ssDNA specifically within the 72-hour post-ASO window have not been directly measured in LiSa-2.]

4. Sequential olaparib administration then traps PARP1 in a catalytically dead, DNA-bound conformation at these telomere-specific sites. This is mechanistically distinct from concurrent administration: concurrent delivery would suppress PARP1 before ssDNA accumulates, allowing alternative repair pathways to compensate. The sequential "load-then-trap" model ensures that olaparib acts on a pre-enriched, telomere-concentrated PARP1 pool, converting the drug from a mild HDR suppressor into a catastrophic telomeric genome-destabilizer. (FANCM-deficient ALT cells become hypersensitive to...

SENS category: OncoSENS

Key references: • doi.org/10.1038/s41598-019-55537-5] • doi.org/10.1073/pnas.1708065114] • doi.org/10.1038/s41598-019-55537-5]. • doi.org/10.1073/pnas.1708065114].