Normalizing complementary DNA by quantitative reverse transcriptase- polymerase chain reaction of β₂-microglobulin: Molecular monitoring of minimal residual disease in acute promyelocytic leukemia

Reverse transcription (RT)-polymerase chain reaction (PCR) raises unique methodological matters that may hamper the reliability of the procedure, especially when results should direct therapeutic decisions. One of these matters is represented by the RT step. The present study shows that differences in complementary DNA (cDNA) preparations purposely containing increasing amounts of retrotranscribed RNA were not disclosed by nonquantitative RT-PCR by two different housekeeping genes, leading to fictitious results when the expression of a given gene was quantitatively assessed. To overcome this problem, the following are proposed: 1) to evaluate the efficiency of RT step through the quantification, by competitive RT-PCR, of the expression levels of the housekeeping gene β₂-microglobulin (β₂M); 2) to normalize each cDNA preparation to be comprised within I standard deviation of the mean value of β₂M absolute level (3.14 ± 1.14 attomoles/μg RNA) found by analyzing 33 cell lines of hematopoietic origin. To validate this strategy in a clinical setting, serial cDNA samples from patients were checked by conventional and quantitative RT-PCR for β₂M. Again, only a quantitative evaluation of β₂M levels was allowed to unveil significant differences, otherwise undetected, in the efficiency of RT reactions among these cDNA samples. Normalization of samples to obtain cDNA preparations containing comparable β₂M levels, eventually led to an increased sensitivity in the detection of PML-RARα fusion transcripts. This approach seems of great value for the monitoring of minimal residual disease in serial patient samples when a tumor-specific marker is available.