Schematic representation of PML breaksites and PML-RARα fusion mRNAs.

(A) The 3 PML breakpoint cluster regions (bcr-1, -2, and -3) are indicated by vertical arrowheads, and the resultant L, V, and S PML-RARα isoforms are diagrammed. PML exons are indicated by numbered, shaded rectangles. The positions of primers used to amplify PML-RARα mRNA are indicated below their respective exons. (B) PCR products were electrophoresed and visualized by staining with ethidium bromide and then transferred to nylon membranes and hybridized with junctional probe PR. In L-form APL, as well as in the NB4 cell line, the PML breaksite is in intron 6 (bcr-1), and full-length PML exon 6 is fused in-frame to exon 3 (Ex 3) of RARα. In such cases, primers P2 and R2 generate a 283-bp chimeric PCR product that is detected by hybridization to the junctional oligonucleotide probe, designated PR (lanes L1, L2, NB4). V-form cases, by definition, lack variable amounts of terminal exon 6 sequence and give variable-sized bands with primer set P2/R2 (panel B, lanes V1-V5). In most cases, due to loss of exon 6 sequence, the amplicon is smaller than 283 bp (ie, V2, V3, V5); however, due to insertion of genomic DNA from RARα intron 2, the amplified fragment can be larger (case V1) or essentially the same size (case V4) as the 283-bp L-form fragment. These latter V-form cases are distinguished from L-form cases by lack of hybridization to the PR probe, and/or by sequencing the P2/R2 amplicon.