Variant-type PML-RARα fusion transcript in acute promyelocytic leukemia: Use of a cryptic coding sequence from intron 2 of the RARα gene and identification of a new clinical subtype resistant to retinoic acid therapy

The physiologic actions of retinoic acids (RAs) are mediated through RA receptors (RARs) and retinoid X receptors (RXRs). The RARα gene has drawn particular attention because it is the common target in all chromosomal translocations in acute promyelocytic leukemia (APL), a unique model in cancer research that responds to the effect of RA. In the great majority of patients with APL, RARα is fused to the PML gene as a result of the t(15;17) translocation. Three distinct types of PML-RARα transcripts, long (L), short (S), and variant (V), were identified. The V-type is characterized by truncation of exon 6 of PML and in some cases by the insertion of a variable “spacer” sequence between the truncated PML and RARα mRNA fusion partners, although the precise mechanisms underlying formation of the V-type transcript remain unclear. To get further insights into the molecular basis of the t(15;17), we sequenced the entire genomic DNA region of RARα. Of note, all previously reported “spacer” sequences in V-type transcripts were found in intron 2 of the RARα gene and most of these sequences were flanked by gt splice donor sites. In most cases, these “cryptic” coding sequences maintained the ORF of the chimeric transcript. Interestingly, two cases with a relatively long spacer sequence showed APL cellular and clinical resistance to RA treatment. In these cases, the aberrant V-type PML-RARα protein displayed increased affinity to the nuclear corepressor protein SMRT, providing further evidence that RA exerts the therapeutic effect on APL through modulation of the RAR–corepressor interaction. Finally, among patients with the L- or S-type PML-RARα fusion transcript, some consensus motifs were identified at the hotspots of the chromosome 17q breakpoints within intron 2 of RARα, strengthening the importance of this intron in the molecular pathogenesis of APL.