Novel mutation in the PML/RAR alpha chimeric gene exhibits dramatically decreased ligand-binding activity and confers acquired resistance to retinoicacid in acute promyelocytic leukemia

Objective. All-trans retinoic acid (RA) resistance in acute promyelocytic l eukemia (API.) has been a serious clinical problem in differentiation-induc ing therapy. However, the mechanisms underlying acquired PA resistance in A PL patients are not well understood. Materials and Methods. We recently established a spontaneous RA-resistant A PI, cell line (UF-I) from a patient and used this cell line as an excellent in vitro model for KA-resistant clinical situations, We investigated the s tructural and functional abnormalities of chimeric PML/RAR alpha gene in UF -1 cells and preserved materials from the original patient. Results. A novel point mutation was detected in the ligand-binding (E) doma in of the RAR alpha portion of the PML/RAR alpha gene in UF-1 cells. This m utation resulted in amino acid substitution of Arg611 ((C) under bar GG) fo r Trp611 ((T) under bar GG) in the short-form PML/RAR alpha protein, which corresponded to Arg276 in wild-type RAR alpha. Importantly, the same mutati on was also detected in the preserved materials from the original patient. COS-1 cells were transiently transfected with cNA encoding wild- type and m utant PML/RAR alpha constructed by site-directed mutagenesis and performed RA-binding assay. Interestingly, RA-binding activity was dramatically decre ased in the mutant PML/RAR alpha compared with that of the wild-type chimer ic protein, suggesting that this single amino acid substitution is critical for RA binding, Conclusions. These results strongly suggest that a novel point mutation in the ligand-binding domain of the RAR alpha portion (Arg611) of the chimeric PML/RAR alpha gene decreased sensitivity to all-trans RA, We conclude that acquisition of the PML/RAR alpha mutation is one possible mechanism for de velopment of RA resistance in patients with APL in vivo. (C) 2001 Internati onal Society for Experimental Hematology, Published by Elsevier Science Inc .