Background and objective: Acute promyelocytic leukemia is characterized by the chromosomal translocation t(15;17) which yields the fusion product PML/RARa. All-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 RARa portion of the PML-RARa chimeric protein. Structural alterations of the LBD of the PML/RARa 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 LBD of PML/RARa gene and then treated with arsenic trioxide (As2O3).
Design and methods: DNA fragments from the LBD of the PML/RARa chimeric transcript were obtained by reverse-transcribed polymerase chain reaction. Direct sequencing was performed by an unambiguous bi-directional 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 LBD of the PML/RARa 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 RARa protein. Both patients had complete clinical and hematologic remission after treatment with As2O3.
Interpretation and conclusions: LBD 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 APL relapsed patients, who have become ATRA-resistant for molecular reasons, being sensitive to arsenic trioxide.