GENOMIC VARIABILITY AND ALTERNATIVE SPLICING GENERATE MULTIPLE PML/RAR-ALPHA TRANSCRIPTS THAT ENCODE ABERRANT PML PROTEINS AND PML/RAR-ALPHA ISOFORMS IN ACUTE PROMYELOCYTIC LEUKEMIA

The acute promyelocytic leukaemia (APL) 15;17 translocation generates a PML/RAR-alpha chimeric gene which is transcribed as a fusion PML/RAR-alpha mRNA. Molecular studies on a large series of APLs revealed great heterogeneity of the PML/RAR-alpha transcripts due to: (i) variable breaking of chromosome 15 within three PML breakpoint cluster regions (bcr1, bcr2 and bcr3), (ii) alternative splicings of the PML portion and (iii) alternative usage of two RAR-alpha polyadenylation sites. Nucleotide sequence analysis predicted two types of proteins: multiple PML/RAR-alpha and aberrant PML. The PML/RAR-alpha proteins varied among bcr1, 2 and 3 APL cases and within single cases. The fusion proteins contained variable portions of the PML N terminus joined to the B-F RAR-alpha domains; the only PML region retained was the putative DNA binding domain. The aberrant PML proteins lacked the C terminus, which had been replaced by from two to ten amino acid residues from the RAR-alpha sequence. Multiple PML/RAR-alpha isoforms and aberrant PML proteins were found to coexist in all APLs. These findings indicate that two potential oncogenic proteins are generated by the t(15;17) and suggest that the PML activation pathway is altered in APLs.