Quantitation of minimal residual disease in acute promyelocytic leukemia patients with t(15;17) translocation using real-time RT-PCR.

We took advantage of a recently developed system allowing performance of real-time quantitation of polymerase chain reaction to develop a quantitative method of measurement of PML-RARalpha transcripts which are hallmarks of acute promyelocytic leukemia (APL) with t(15;17) translocation. Indeed, although quantitation of minimal residual disease has proved to be useful in predicting clinical outcome in other leukemias such as chronic myeloid leukemia or acute lymphoblastic leukemia, no quantitative data have been provided in the case of APL. We present here a method for quantitation of the most frequent subtypes of t(15;17) transcripts (namely bcr1 and bcr3). One specific forward primer is used for each subtype in order to keep amplicon length under 200 bp. The expression of PML-RARalpha transcripts is normalized using the housekeeping porphobilinogen deaminase (PBGD) gene. This technique allows detection of 10 copies of PML-RARalpha or PBGD plasmids, and quantitation was efficient up to 100 copies. One t(15;17)-positive NB4 cell could be detected among 106 HL60 cells, although quantitation was efficient up to one cell among 105. Repeatability and reproducibility of the method were satisfying as intra- and inter-assay variation coefficients were not higher than 15%. The efficiency of the method was finally tested in patient samples, showing a decrease of the PML-RARalpha copy number during therapy, and an increase at the time of relapse.