Amplification of EVI1 on cytogenetically cryptic double minutes as new mechanism for increased expression of EVI1

In acute myeloid leukemia (AML) and myelodysplastic syndromes (MDS), increased expression of EVI1 (ecotropic virus integration site 1) was found to be associated with adverse prognosis. Although increased expression of the EVI1 gene is mainly caused by chromosomal rearrangements involving chromosome band 3q26, where EVI1 is located, it can also be identified in cases without these rearrangements. The mechanisms that cause increased EVI1 expression in the absence of 3q26 rearrangements remain unclear. Here, we present four cases with increased EVI1 expression due to EVI1 amplification on cytogenetically cryptic double minutes (dmin). The dmin are small acentric chromosome structures and were observed in about 1% of AML and MDS. We investigated the four cases by conventional cytogenetics, fluorescence in situ hybridization, and array comparative genomic hybridization. Furthermore, EVI1 expression was measured by quantitative reverse transcriptase-PCR. By conventional chromosome analysis, the EVI1 dmin cannot be detected, due to the small size of the amplicons of 0.49-0.78 Mbp. Median % EVI1/ABL expression was 88.9% and therefore comparable to the median % EVI1/ABL expression of patients with EVI1 rearrangements. In conclusion, EVI1 amplification on cytogenetically cryptic dmin causes increased expression of EVI1 and is a new mechanism that causes increased EVI1 expression without a 3q26 rearrangement.