MYC-containing double minutes in hematologic malignancies: evidence in favor of the episome model and exclusion of MYC as the target gene

Double minutes (dmin) - circular, extrachromosomal amplifications of specific acentric DNA fragments - are relatively frequent in malignant disorders, particularly in solid tumors. In acute myeloid leukemia (AML) and myelodysplastic syndromes (MDS), dmin are observed in approximately 1% of the cases. Most of them consist of an amplified segment from chromosome band 8q24, always including the MYC gene. Besides that, little is known about their internal structure. We have characterized in detail the genomic organization of 32 AML and 2 MDS cases with MYC-containing dmin. The minimally amplified region was shown to be 4.26 Mb in size, harboring 5 known genes, with the proximal and the distal amplicon breakpoints clustering in two regions of approximately 500 and 600 kb, respectively. Interestingly, in 23 (68%) of the studied cases, the amplified region was deleted in one of the chromosome 8 homologs at 8q24, suggesting excision of a DNA segment from the original chromosomal location according to the "episome model". In one case, sequencing of both the dmin and del(8q) junctions was achieved and provided definitive evidence in favor of the episome model for the formation of dmin. Expression status of the TRIB1 and MYC genes, encompassed by the minimally amplified region, was assessed by Northern blot analysis. The TRIB1 gene was found overexpressed in only a subset of the AML/MDS cases, while MYC, contrary to expectations, was always silent. The present study, therefore, strongly suggests that MYC is not the target gene of the 8q24 amplifications.