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

Abstract

The physiologic actions of retinoic acids (RAs) are mediated through RA receptors (RARs) and retinoid X receptors (RXRs). The RAR␣ 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, RAR␣ is fused to the PML gene as a result of the t(15;17) translocation. Three distinct types of PML-RAR␣ 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 RAR␣ 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 RAR␣. Of note, all previously reported ''spacer'' sequences in V-type transcripts were found in intron 2 of the RAR␣ 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-RAR␣ 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-RAR␣ fusion transcript, some consensus motifs were identified at the hotspots of the chromosome 17q breakpoints within intron 2 of RAR␣, strengthening the importance of this intron in the molecular pathogenesis of APL. R etinoic acids (RAs) play a wide variety of physiological roles during embryogenesis and adult life. The effects of RAs are mediated through two families of nuclear receptors, RA receptors (RARs) and retinoid X receptors (RXRs), each including three members, ␣, ␤, and ␥. Whereas RXRs can form homodimers, RARs need to heterodimerize with RXRs to constitute a functional receptor. These receptors bind to DNA motifs known as RAREs (RA response elements), located in the regulatory sequences of RA target genes (1). The transregulatory properties of RAR͞RXR are determined, in a ligand-dependent manner, by their interaction with two protein complexes, nuclear receptor corepressor (CoR) and coactivator (2). Each member of the RAR and RXR families is encoded by a unique gene, which may generate a number of isoforms with distinct N-terminal sequences through alternative use of 5Ј exons (3). The combinatorial expression of both gene families, as well as their isoforms, varies across different tissue͞cell types at distinct stages of development, contributing to the biological complexity of RA signaling (1-4).Over the last decade, the biomedical importance of RAR␣ has been highlight...

Mutations of the PML tumor suppressor gene in acute promyelocytic leukemia

“…16 The patient carrying the IVS3 Ϫ 1G 3 A variant ( Figure 1A, right panel; Figure 1B, Mut 2) was a 19-year-old male affected by APL expressing the V form of PML-RAR␣ with a relatively large deletion of PML exon 6 and insertion of a long overcompensatory sequence from RAR␣ intron 2 in the fusion gene. 20 Notably, this patient had evidence of decreased sensitivity to ATRA even before beginning therapy by nitroblue tetrazolium (NBT) test 21 (NBT positivity lower than 20% at 100 nM RA), but not by the CD11b marker. He received only 3 weeks of ATRA owing to development of a fungal infection.…”

MicroRNAs as Potential Biomarkers in Acute Promyelocytic Leukaemia

“…The breakpoints on chromosome 17 are consistently located within the second intron of the RAR gene, but, on chromosome 15, there are different breakpoint cluster regions, namely bcr1, bcr2, and bcr3 located in intron 6, exon 6, and intron 3, respectively, of the PML gene [2,3]. The location of bcr1, bcr2, and bcr3 produces fusion transcripts of varying lengths referred to as the long, variant, and short forms, respectively [3].…”

A large exon 6 break in V-form acute promyelocytic leukemia: relevance to clinical management

“…Patients with short bcr2 transcripts as a consequence of exon 6 PML deletions are referred as V-form PML-RARa. [2][3][4] In this work, we report an unusually large exon 6 PML deletion (161 bp) associated with a short intron capture (5 bp), giving rise to a net 156 bp deletion (52 amino acids). The outcome was good.…”

“…This recruitment would be mediated by an intronic spacer sequence flanked by gt splice donor sites. 3,4 Given that the patient under discussion had an unusually large deletion, we performed direct sequencing to rule out a large intron capture produced by a mechanism of illegitimate recombination. Furthermore, we were interested in learning how this unusual transcript could affect the currently used minimal residual disease studies using real-time PCR.…”