Mutations at proximal cysteine residues in PML impair ATO binding by destabilizing the RBCC domain

Acute promyelocytic leukemia (APL) is characterized by the fusion gene promyelocytic leukemia-retinoic acid receptor-alpha (PML-RARA) and is conventionally treated with arsenic trioxide (ATO). ATO binds directly to the RING finger, B-box, coiled-coil (RBCC) domain of PML and initiates degradation of the fusion oncoprotein PML-RARA. However, the mutational hotspot at C212-S220 disrupts ATO binding, leading to drug resistance in APL. Therefore, structural consequences of these point mutations in PML that remain uncertain require comprehensive analysis. In this study, we investigated the structure-based ensemble properties of the promyelocytic leukemia-RING-B-box-coiled-coil (PML-RBCC) domains and ATO-resistant mutations. Oligomeric studies reveal that PML-RBCC wild-type and mutants C212R, S214L, A216T, L217F, and S220G predominantly form tetramers, whereas mutants C213R, A216V, L218P, and D219H tend to form dimers. The stability of the dimeric mutants was lower, exhibiting a melting temperature (T_m) reduction of 30 °C compared with the tetrameric mutants and wild-type PML protein. Furthermore, the exposed surface of the C213R mutation rendered it more prone to protease digestion than that of the C212R mutation. The spectroscopic analysis highlighted ATO-induced structural alterations in S214L, A216V, and D219H mutants, in contrast to C213R, L217F, and L218P mutations. Moreover, the computational analysis revealed that the ATO-resistant mutations C213R, A216V, L217F, and L218P caused changes in the size, shape, and flexibility of the PML-RBCC wild-type protein. The mutations C213R, A216V, L217F, and L218P destabilize the wild-type protein structure due to the adaptation of distinct conformational changes. In addition, these mutations disrupt several hydrogen bonds, including interactions involving C212, C213, and C215, which are essential for ATO binding. The local and global structural features induced by these mutations provide mechanistic insight into ATO resistance and APL pathogenesis.