Missense mutations in PML-RARA are critical for the lack of responsiveness to arsenic trioxide treatment

Arsenic trioxide (As₂O₃) is a highly effective treatment for patients with refractory/relapsed acute promyelocytic leukemia (APL), but resistance to As₂O₃ has recently been seen. In the present study, we report the findings that 2 of 15 patients with refractory/relapsed APL treated with As₂O₃ were clinically As₂O₃ resistant. Leukemia cells from these 2 patients harbored missense mutations in promyelocytic leukemia gene-retinoic acid receptor-α gene (PML-RARA) transcripts, resulting in amino acid substitutions of A216V and L218P in the PML B2 domain. When wild-type or mutated PML-RARA (PR-WT and PR-B/L-mut, respectively) were overexpressed in HeLa cells, immunoblotting showed SUMOylated and/or oligomerized protein bands in PR-WT but not in PR-B/L-mut after As₂O₃ treatment. Protein-localization analysis indicated that PR-WT in the soluble fraction was transferred to the insoluble fraction after treatment with As₂O₃, but PR-B/L-mut was stably detected in fractions both with and without As₂O₃. Immunofluorescent microscopy analysis showed PR-WT localization as a microgranular pattern in the cytoplasm without As₂O₃ and as a macrogranular pattern with As₂O₃. PR-B/L-mut was diffusely observed in the cytoplasm with and without As₂O₃. Nearly identical localization patterns were observed in patients' primary cells. Therefore, B2 domain mutations may play an important role in aberrant molecular responses toAs₂O₃ and may be critical for As₂O₃ resistance in APL.