PML mutants resistant to arsenic induced degradation fail to generate the appropriate SUMO and ubiquitin signals required for RNF4 and p97 recruitment
Arsenic is an effective treatment for Acute Promyelocytic Leukaemia as it induces degradation of the Promyelocytic Leukaemia (PML) - retinoic acid receptor alpha (RARA) oncogenic fusion protein. Some patients relapse with arsenic resistant disease because of missense mutations in PML. To determine the mechanistic basis for arsenic resistance we reconstituted PML-/- cells with YFP fusions of wild type (WT) and two mutant forms of PMLV found in patients refractory to arsenic, A216T and L217F. Both mutants formed PML bodies that were larger, but less numerous than WT and neither responded to arsenic by degradation. Analysis of immunoprecipitated PML bodies indicated that while WT PML experiences increased SUMO1, SUMO2/3 and ubiquitin conjugation, A216T PML is almost completely unresponsive and therefore does not recruit the SUMO targeted ubiquitin E3 ligase RNF4. Compared to WT PML, L217F PML was modified to a similar extent by SUMO2 but not SUMO1 and although it recruited RNF4, it failed to develop the appropriate poly-ubiquitin conjugates required to recruit the segregase p97, which is essential for PML degradation.