Unexpected heterogeneity of PML/RARα fused mRNA detected by nested polymerase chain reaction in acute promyelocytic leukemia

We have analyzed ten APL patients using reverse transcription polymerase chain reaction (RT-PCR) technique to detect PML/ RARα fused mRNA. All patients in this study had PM/RARα fused mRNA (three cases of the short type and seven cases of the long type), although the chromosomal translocation t(15;17) was not detected in one patient. After ethidium bromide staining, two-thirds of the short type and all cases of the long type were found to have multiple PCR products (192 and 93 base pair (bp) bands in the short type and 666, 522, 263, and 164 bp in the long type). A total of six distinct fused mRNAs were sequenced (P1R1, P1R2, P3R1, P2R1, and P2R2). Southern hybridization analysis showed only one rearranged band in each of the patients. These results suggest that the longest mRNAs in each type are the authentic fused mRNAs and the other smaller mRNAs are generated through splicing events. In RARα, a novel fusion point (R2) was identified within the fourth exon. This uncommon splicing may be caused by the instability of the splicing mechanism of the rearranged PML/RARα gene. Among the ten APL patients, no correlation was observed between the type of fused mRNA and the clinical characteristics examined.