EVI1 Overexpression in Pediatric Acute Myeloid Leukemia Associated with Unfavorable Subtypes.

The EVI1 (ecotropic virus integration-1) gene plays an important role in hematopoiesis especially in megakaryocyte development. The MDS1 gene is located upstream of EVI1, and its function is currently unknown. Normally the MDS1/EVI1 intergenic splice variant is co-expressed with EVI1. In adult acute myeloid leukemia (AML) overexpression of EVI1 (EVI1+) can be found in patients with chromosome 3q26-rearrangements. Often, these patients do not co-express MDS1/EVI1. Recently high EVI1 expression was also discovered in a separate subgroup of patients that did not have 3q26-rearrangements. Occasionally, they did not show overexpression of MDS1/EVI1. In these patients cryptic inversions of chromosome 3 were identified with fluorescence in situ hybridization (FISH). Of interest, EVI1+ was found to be an independent poor prognostic marker in adult AML (Lugthart et al, Blood 2008). In pediatric AML, 3q26-rearrangements are rare and the role of EVI1 is unknown. In this study, we investigated the frequency and clinical relevance of EVI1+ in pediatric AML. EVI1 expression was analyzed in 233 pediatric AML patients, of whom microarray gene expression profiling data were available. EVI1+ was found in 25 pediatric AML patients (11%), and confirmed with real-time quantitative PCR. This included 13/49 (26%) patients with MLL-rearranged AML: 5/22 (23%) cases with t(9;11); and all (n=4) cases with t(6;11). Moreover, EVI1+ was found in 4/7 (57%) cases with AML M7; in 2/3 (66%) cases with AML M6; in both cases with monosomy 7; in 1/43 (2%) cases with normal cytogenetics; in 2 patients with random cytogenetics, and in 1 patient with a cytogenetic failure. EVI1+ was not found in the t(8;21), inv(16) and t(15;17) subgroups. 3/25 EVI1+ patients lacked the MDS/EVI1 transcript, but no cryptic 3q26-rearrangements were detected with FISH. Molecular analysis showed that one patient had a CEBPα mutation; one patient had an FLT3-ITD; and 3 patients showed a mutation in the RAS oncogene. EVI+ was not correlated with sex or white blood cell count. However, the frequency in children younger than 10 years old was twice as high when compared to older children (14% vs 7%, p=0.12). Survival analysis was restricted to the subset of patients who were treated using uniform DCOG and BFM treatment protocols (n=204). In this cohort, EVI1+ patients had a worse 5-years event-free survival (pEFS) compared to patients without EVI1+ (30 vs. 43%, p=0.02). However, multivariate analysis, including cytogenetics (favorable [t(8;21, inv(16), t(15;17)] vs. other), FLT3-ITD, age and WBC, showed that EVI1+ was not an independent prognostic factor for survival. Moreover, within the unfavorable/normal cytogenetic subgroup, there was no difference in outcome between patients with and without EVI1+. We conclude that EVI1+ is found in ~10% of pediatric AML, and highly correlated with specific unfavorable cytogenetic (MLL-rearrangements) and morphologic (FAB M6/7) subtypes. In contrast to adult AML, no 3q26-rearrangements or cryptic inversions were found, and EVI1+ was not an independent prognostic factor. This difference in prognostic relevance may be due to differences in treatment. Alternatively, these results may indicate that EVI1 plays a different role in disease biology between adult and pediatric AML. This is at least suggested by the lack of 3q26 aberrations in pediatric AML.