Missense mutations in the PML/RAR alpha ligand binding domain in ATRA-resistant As2O3 sensitive relapsed acute promyelocytic leukemia

Background and Objectives. Acute promyelocytic leukemia is characterized by the chromosomal translocation t(15;17) which yields the fusion product PML/RAR alpha. Art-trans retinoic acid probably induces differentiation of atypical promyelocytes and clinical remission in APL patients by binding to the ligand binding domain (LBD) of the RAR alpha portion of the PML-RAR alpha chimeric protein. Structural alterations of the LED of the PML/RAR alpha have been revealed in ATRA-resistant APL cell lines and in a few APL patients with acquired clinical resistance to ATRA therapy. Two APL relapsed patients with clinical resistance to ATRA therapy were evaluated for the presence of nucleotide mutations in the LED of PML/RAR alpha gene and then treated with arsenic trioxide (As2O3). Design and Methods. DNA fragments from the LED of the PML/RAR alpha: chimeric transcript were obtained by reverse-transcribed polymerase chain reaction. Direct sequencing was performed by an unambiguous bidirectional automatic analysis. Samples representative of APL onset and relapse were analyzed from both patients. Results. In both patients, at the ATRA-resistant relapse, a missense point mutation in the LED of the PML/RAR alpha gene was found. The mutations, absent at APL onset, led to an Arg272Gln and to an Arg276Trp amino acid substitution, according to the sequence of the RAR alpha protein. Both patients had complete clinical and hematologic remission after treatment with As2O3. Interpretation and Conclusions. LED missense mutations appear to be a significant mechanism of acquired ATRA-resistance in vivo, closely related to clinical APL relapse. The two cases reported here provide the first in vivo evidence of Apt, relapsed patients, who have become ATRA-resistant for molecular reasons, being sensitive to arsenic trioxide.