High EVI1 expression Is Associated with MLL rearrangements and Predicts Decreased Survival in Pediatric AML: A Report From the Children's Oncology Group.

The EVI1 gene is highly expressed in a subset of adult acute myeloid leukemia (AML) patients, and has been associated with inferior treatment outcome and specific cytogenetic abnormalities, including 11q23 rearrangements. Although translocations involving the MLL gene on chromosome 11q23 are common in pediatric AML, this group of patients as a whole carries neither favorable or adverse prognosis. We sought to examine the prevalence and clinical associations of high EVI1 expression in pediatric AML.

EVI1 mRNA expression was measured via qRT-PCR in diagnostic specimens obtained from 206 patients enrolled on the pediatric trial COG-AAML03P1; expression levels were normalized against pooled normal blood controls, and correlated with clinical features and outcome. The Kaplan-Meier method was used to estimate overall survival (OS), event-free survival (EFS), and disease-free survival (DFS) in patients with high or low / absent EVI1 expression. Relapse risk (RR) was defined as time from remission at end of course 1 to relapse; estimates of RR were obtained using methods accounting for competing events such as toxic deaths.

EVI1 expression (range: 0 to 6660.88 fold normal peripheral blood, median 0.09) was detectable in the majority of patients (141/206, 68%), although expression higher than normal controls was present in only 57/206 patients (28%). A subset of patients had significant overexpression of EVI1; high EVI1 expression (EVI1^high) was defined as EVI1 expression > 100 fold normal and was detectable in 33/206 (16%) of patients, with a median expression of 388.61 fold normal. By comparison, median expression in the remaining patients with low / absent EVI was 0.035 fold normal. EVI1^high was significantly more common in African-American patients, who accounted for 31% of the EVI1^high cohort as opposed to 13% of the remaining patients (p=0.016). MLL rearrangements were significantly more common in EVI1^high patients (41% of EVI1^high patients had 11q23 abnormalities, as opposed to 16% of patients with low / absent EVI1 expression, p=0.002). FAB class M7 was also enriched in the EVI1^high subset of patients, accounting for 21%. Although EVI1 expression has been linked to 3q26 rearrangements in adult AML, no chromosome 3 abnormalities at the level of conventional cytogenetics were detected in EVI1^high patients in our study. EVI1^high was mutually exclusive of NPM1 mutations, CEBPA mutations, or core-binding factor translocations. Thus, the majority of EVI1^high patients (79%) belonged to the “standard risk” group based on cytogenetic / molecular risk stratification. However, 4 of the 5 patients with high risk cytogenetics (monosomy 7 or del 5q), and 3 additional patients with FLT3/ITD, had high EVI1 expression. EVI1^high patients had significantly lower 5 year OS from study entry (47% vs. 67%, p=0.032) compared to the remaining patients; EFS was also significantly decreased (31% vs. 52%, p=0.007). Corresponding DFS from complete remission was 43% with a RR of 43% for EVI1^high patients, as compared to 59% with a RR of 31% for the patients with low / absent EVI1 expression (DFS: p=0.057, RR: p=0.119). This study was not powered to detect outcome differences based on EVI expression in the 11q23-rearranged subset; for MLL- rearranged patients, 5 year OS for EVI1^high patients (n=12) was 51%, compared to 65% (p=0.705) for patients with low / absent EVI1 expression (n=26).

High EVI1 expression was detectable in 16% of our pediatric AML study population; EVI1^high was significantly associated with decreased OS and EFS. As the majority of these patients lacked additional cytogenetic/molecular prognostic markers, EVI1^high has potential utility in the further risk stratification of “standard risk” patients. A larger cohort of patients from the AAML03P1 successor Phase III trial COG-AAML0531 is currently being evaluated for EVI1 expression, in order to allow for analysis of outcome based on EVI expression in the 11q23-rearranged cohort. As the MLL gene has numerous fusion partners, the study of additional patients will also allow analysis of the correlation between EVI1^high and specific MLL translocations, as some MLL fusions have recently been shown to be individually prognostic and biologically distinct.

No relevant conflicts of interest to declare.