REVERSE TRANSCRIPTION POLYMERASE CHAIN-REACTION FOR THE REARRANGED RETINOIC ACID RECEPTOR-ALPHA CLARIFIES DIAGNOSIS AND DETECTS MINIMAL RESIDUAL DISEASE IN ACUTE PROMYELOCYTIC LEUKEMIA

The characteristic t(15;17) of acute promyelocytic leukemia (APL) fuse s the retinoic acid receptor alpha (RAR-alpha) gene on chromosome 17 t o a gene on chromosome 15 called PML, a putative transcription factor. This distinct translocation results in a fusion mRNA detected by Nort hern analysis. Two cDNAs have been isolated that differ in the extent of 3' PML nucleic acid sequence contained. This study describes a reve rse transcription polymerase chain reaction (RT-PCR) assay for the PML /RAR-alpha fusion transcript, which amplifies PML/RAR-alpha mRNA from APL cells with either reported breakpoint. DNA sequencing of the predo minant RT-PCR products from 6 patients showed identical RAR-alpha exon ic breakpoints and two PML breakpoints. This RT-PCR assay was positive in leukemic cells from 30/30 APL patients with the molecular rearrang ement confirmed by cytogenetics or Northern analysis. In leukemic cell s of patients with a morphologic diagnosis of APL lacking the t(15;17) by routine cytogenetics, a positive RT-PCR assay predicted clinical r esponse to all-trans-retinoic acid (RA) therapy. Dilutional studies wi th leukemic cells that express (NB4) or do not express (HL-60) a PML/R AR-alpha fusion mRNA reveal that this RT-PCR assay detects the transcr ipt from as little as 50 pg of total RNA. In APL cells from 5/6 patien ts treated with RA alone, a complete response by clinical and cytogene tic criteria accompanied a persistently positive RT-PCR assay. This pr eceded relapse by 1-6 months. RT-PCR for PML/RAR-alpha mRNA provides a more-sensitive test for the t(15;17) than routine cytogenetics or Nor thern analysis. This molecular rearrangement detected by RT-PCR best d efines this RA-responsive malignancy. The RT-PCR assay for the PML/RAR -alpha transcript yields important diagnostic and prognostic informati on in the management of APL patients.