Pml Nuclear Body Disruption Cooperates in APL Pathogenesis, Impacting DNA Damage Repair Pathways

Acute promyelocytic leukemia (APL) is driven by the oncogene PML-RARA which is generated by fusion of the promyelocytic leukemia (PML) and retinoic acid receptor alpha (RARA) genes, and which strongly interferes with downstream signalling and the architecture of multiprotein structures known as PML nuclear bodies (NBs). NB disruption is a diagnostic hallmark of APL, yet the significance of this phenomenon to disease pathogenesis and treatment response remains poorly understood. The majority of APL patients can now be cured with combination therapy with arsenic trioxide (ATO) and ATRA (All Trans-Retinoic Acid), which synergize promoting re-formation of disrupted Pml NBs. To date, the importance of NB disruption has only been studied in vitro. To address this, we generated a knock-in mouse model with targeted NB disruption achieved through mutation of key zinc-binding cysteine residues (C62A/C65A) in the RING domain of Pml. Homozygous PmlC62A/C65A mice are viable, and developmentally normal. At a cellular level, Pml NB disruption was confirmed and treatment with ATO was associated with defective Pml SUMOylation and degradation. A key feature of APL fusion proteins is the capacity to homodimerise (mediated by the fusion partner e.g. PML), which is not a feature of wild-type RARα. This forced homodimerisation of RARα has been shown to be critical for APL pathogenesis. We investigated whether Pml NB disruption could cooperate in vivo with forced RARα homodimerisation (mediated artificially by linking RARα to the dimerisation domain of the NFκB p50 subunit). While no leukemias arose in PmlC62A/C65A mice, p50-RARα mice expressing PmlC62A/C65A presented a doubling in the rate of leukemia development (p<0.0001) compared to PmlWT-p50-RARα, leading to a penetrance comparable to that observed in previously published PML-RARα transgenic models. Moreover, the latency period to onset of leukemia was significantly reduced in the context of NB disruption (p=0.008). ATRA treatment significantly improved the survival of mice transplanted with PmlWT-p50-RARα or Pml-RARα leukemic blasts, but not with PmlC62A/C65A-p50-RARα. These data reveal not only the key role of PML-RARα expression-induced NB disruption in APL development, but also the importance of re-formation of NBs for an effective response to differentiating drug.