The Prognostic Impact of High MEL1 Gene Expression in Pediatric Acute Myeloid Leukemia

Background

Acute myeloid leukemia (AML) is a complex disease caused by mutations, epigenetic modifications, and deregulated expression of genes, leading to increased proliferation and decreased differentiation of hematopoietic progenitor cells. Although, many prognosis-associated gene alterations have been identified in adult AML, genetic alterations responsible for an adverse outcome in most patients with pediatric AML have remained obscure.In our previous study, we have identified NUP98-NSD1 fusion gene in 6 of 124 pediatric AML patients. As a result of gene expression profile using microarray, all these 6 patients have showed high expression of HOXA9, HOXA10, HOXB3, HOXB5, HOXB6, and a zinc finger transcription factor gene MDS1-EVI1-like-1 (MEL1), and clustered a distinct poor prognostic subgroup [4-year-overall survival (OS); 33%]. Furthermore, this subgroup has also formed the bigger poor prognostic subgroup involving the 18 patients without NUP98-NSD1 fusion gene. We defined this subgroup as NUP98-NSD1 signature consisting of 24 of 124 patients (19%), 4-year-OS; 37.5%. MEL1 gene expression represented the specific characteristics of this subgroup.To verify the significance of MEL1 gene expression and the relationship with other poor prognostic markers such as FLT3-ITD, MLL-PTD, and NPM1, we investigated MEL1 expression by real-time PCR in 369 pediatric AML cases other than acute promyelocytic leukemia (APL) and Down syndrome-associated AML, because these subtypes formed genetically different unique entities and treated by other protocols. Interestingly, MEL1 is highly homologous to MDS1/EVI1 gene, which is an alternatively spliced transcript of the EVI1 genes. As EVI1 high expression has been reported as a poor prognostic marker in pediatric AML patients, especially in patients with MLL rearrangement, we also measured and examined the EVI1 gene expression.

Methods

Between 2006 and 2010, 485 de novo pediatric AML patients participated in the Japanese AML-05 study conducted by the Japanese Pediatric Leukemia/Lymphoma Study Group (JPLSG). Among them, 369 samples were available in this study. Quantitative RT-PCR analysis was performed in these patients using the 7900HT Fast Real Time PCR System with TaqMan Gene Expression Master Mix and TaqMan Gene Expression Assay. In addition to EVI1 and MEL1, ABL1 was also evaluated as a control gene. We investigated the correlations between these gene expressions and other genetic alterations, and clarified the prognostic impact of MEL1 gene.

Results

A total of 84 of 369 patients (22.8%) showed high expression of MEL1 gene. The number of patients with MEL1 high expressions were gradually enriched in order of the intermediate risk (IR; 34 of 142 patients, or 23.9%), high risk (HR; 21 of 48 patients, or 43.8%), and non-complete remission (non-CR; 20 of 39 patients, or 51.3%), but were almost all absent in patients with a low risk (LR) cytogenetic profile (4 of 120 patients, or 3.3%) consisting of t(8;21) and inv(16). The absolute values of MEL1 gene expression have also gradually increased in order of LR < IR < HR < non-CR. The overall survival among patients with MEL1 high expressions was significantly lower than that among patients without such gene aberrant expression (50% vs. 82%, P<0.001). Remarkably, MEL1 gene expression is extremely useful to stratify the AML patients with FLT3-ITD (4-year OS; high 30% vs. low 75%, P<0.001) and normal karyotype (4-year OS; high 48% vs. low 84%, P<0.001), whom we could not exactly stratify into the appropriate risk so far. On the other hand, a total of 58 of 369 patients (15.7%) have showed high expression of EVI1 gene. The patients with high expression of EVI1 gene have been mainly found in the IR group, especially in patients with MLL rearrangement. Furthermore, MEL1 high group and EVI1high group have the tendency of mutual exclusive in this study.

Conclusions

MEL1 high expression was highly recurrent in de novo pediatric AML patients with high/intermediate-risk cytogenetic profiles and was independently associated with a poor outcome. Combined with MEL1expression and genetic alterations enable to clarify the genetic background of pediatric AML profiles. The data in this study showed a way in which integrated mutational profiling of a clinical trial cohort can advance our understanding of the biologic characteristics of AML, improve current prognostic models, and inform prospective therapeutic decisions.