Identification of Atypical PML-RARA Breakpoint in a Patient with Acute Promyelocytic Leukemia

Abstract

Acute promyelocytic leukemia (APL) of the M3 subtype is characterized by translocation t(15;17) that generates the PML-RARA fusion gene. Depending on the breakpoint position in the PML gene, 3 main fusion transcripts usually result. These breakpoints are bcr1 and bcr3 in introns 6 and 3, respectively, and bcr2 in exon 6. This report describes a rare atypical bcr2 breakpoint in a patient with morphological, cytogenetic and molecular features of APL. The presence of t(15;17) was first revealed by fluorescent in situ hybridization. Molecular analysis by reverse transcription polymerase chain reaction using primers for different PML-RARA junctions showed bands with different sizes compared with those generated from the three classical breakpoints, namely bcr1, bcr2 and bcr3. However, sequence analysis confirmed the presence of a bcr2 transcript with an atypical breakpoint within exon 6. The patient responded well to treatment and is now in complete remission. However, suggesting a favorable prognosis associated with such a rare transcript is difficult as more similar cases are needed to confirm such a conclusion. This article also stresses the importance of sequencing unusual polymerase chain reaction products to confirm their nature.

Acute Promyelocytic Leukemia: A Constellation of Molecular Events around a Single PML-RARA Fusion Gene

“…These are characterized by cDNA deletions of either the distal region of PML exon 6 (from 8 to 146 nucleotides), or the entire exon 6, as a result of a mis-splicing event or a genomic break within PML exon 6 (rarely in PML exon 5). Likewise, atypical bcr2 are frequently associated with insertions of three to 127 extra nucleotides (1 to 42 extra amino acids) of genomic DNA from RARA intron 2 [63,[68][69][70][71][72][73][74]. In most of the cases, V-forms are "private", being observed in single APL patients.…”

“…In most of the cases, V-forms are "private", being observed in single APL patients. However, a few cases share the activation of a novel donor splice site in PML exon 6 [72,73], or express a minor PML-RARA transcript with exon 5 skipped [72,74]. Reported isoforms have been compared with the reference sequences of PML and RARA (GenBank accession numbers: NM_033238.2; NM_000964.3).…”

“…Some reports have shown that bcr3 patients would have a poor prognosis and an aggressive disease course [84], as well as patients harboring PML breakpoints downstream of intron 6 [67,70,75,77]. However, other studies have described cases with a good prognosis, or at least similar to that of APL patients with typical transcripts, even in the presence of atypical isoforms [67,72,78,80]. Therefore, according to the most recent recommendations, standard therapy should not be changed based on the PML-RARA isoforms [85].…”

Identification and monitoring of atypical PML/RARA fusion transcripts in acute promyelocytic leukemia

“…The clinical and biological impact of these atypical isoforms remains unclear. A few reports demonstrated that PML break points downstream intron 6 are potentially associated with poor prognosis and an aggressive disease course, whereas other studies described cases with atypical PML/RARA with clinical outcome similar to that of patients with typical fusion transcripts …”

“…A few reports demonstrated that PML break points downstream intron 6 are potentially associated with poor prognosis and an aggressive disease course, [8][9][10][11] whereas other studies described cases with atypical PML/RARA with clinical outcome similar to that of patients with typical fusion transcripts. 9,[12][13][14][15][16] In this study, we report the characterization of three cases of APL with novel atypical PML/RARA transcripts, which were not clearly detectable using standard molecular procedure.…”