Acute promyelocytic leukemia with insertion of PML exon 7a and partial deletion of exon 3 of RARA: a novel variant transcript related to aggressive course and not detected with real-time polymerase chain reaction analysis

Abstract

We report the case of a 38-year-old man with acute promyelocytic leukemia (APL) showing a distinct breakpoint cluster region 2 (bcr2) variant transcript. Findings from bone marrow, cytogenetic, fluorescence in situ hybridization, and qualitative reverse transcriptase-polymerase chain reaction (RT-PCR) analyses were consistent with the diagnosis of APL. Although PCR products of size 841 bp and 984 bp were amplified from bone marrow specimen, the quantitative PCR (RQ-PCR) findings were negative. Given the discrepancy in PCR results, sequencing of PCR products was performed to determine the detailed composition of these fusion transcripts. By cloning and sequencing, we discovered that these two bands were isoforms, in which one exon (exon 5, 144 bp) of the PML gene was spliced out of the smaller products (minor PCR products); one sequence (G) insertion and one base substitution (T→C) of PML exon 4 generate a stop codon in the smaller fusion transcript. In addition, a search of the Ensembl database revealed that these variant PML/RARA fusion transcripts were composed of exon 7a insertion of the PML gene and partial deletion (46 bp) of exon 3 of the RARA gene, in addition to inserted sequences of intron 7 of PML and genomic sequence ATCT of unknown origin at the fusion junction site. Although the biological significance of most atypical transcripts remains unclear, sequence analysis of these atypical products should be performed, to reveal the composition of such a fusion transcript and elucidate the molecular mechanism.