High Expression of Myocyte Enhancer Factor 2C (MEF2C) Is Associated with Adverse Risk Features and Poor Outcome in Pediatric Acute Myeloid Leukemia: A Report from the Children's Oncology Group

Background: Myocyte enhancer factor 2C (MEF2C) was initially identified as essential transcription factor for cardiac muscle development. However, subsequent studies have indicated that MEF2C plays a much broader biological role, including in the normal hematopoietic system. Recent studies have now identified MEF2C as cooperating oncogene in acute myeloid leukemia (AML) and suggested a contribution to the aggressive nature of at least some subtypes of AML. These findings raised the possibility that MEF2C could serve as marker of poor-risk disease and, therefore, have prognostic significance in AML. To test this hypothesis, we retrospectively quantified MEF2C expression in participants of the AAML0531 trial and correlated expression levels with disease characteristics and clinical outcome.

Patients and Methods: AAML0531 (NCT00372593) was a multicenter phase 3 study that determined the addition of gemtuzumab ozogamicin to intensive chemotherapy among 1,022 eligible patients aged <30 yearswith newly diagnosed de novo non-APL AML, excluding those with bone marrow failure syndromes, juvenile myelomonocytic leukemia, or Down syndrome (if ≤3 years of age) between 2006 and 2010. Cryopreserved pretreatment ("diagnostic") specimens from patients enrolled on AAML0531 who consented to the biology studies and had bone marrow samples were available were included in this study. Total RNA from unsorted specimens was extracted, quantified, and subjected to quantitative reverse-transcriptase polymerase chain reaction (qRT-PCR) using TaqMan primers to determine expression of MEF2C and, for normalization, the housekeeping gene, β-glucuronidase (GUSB). Patient samples were run in duplicate, and the ΔΔCT method quantified as 2^(-ΔΔCT) was used to determine the expression levels of MEF2C relative to GUSB.

Results: In all 751 available patient specimens, MEF2C mRNA was detectable and varied >3,000-fold relative to GUSB (0.0091-29.1272 [median: 0.7978]). Patients with the highest relative MEF2C expression (4^th quartile) less likely achieved a complete remission after one course of chemotherapy than the other patients (67% vs. 78%, P=0.005). They also had an inferior overall survival (P=0.014; at 5 years: 55±8% vs. 67±4%), inferior event-free survival (P<0.001; at 5 years: 38±7% vs. 54±4%), and higher relapse risk than patients within the lower 3 quartiles of MEF2C expression (P<0.001; at 5 years: 53±9% vs. 35±5%). Of note, exploratory multiple cutpoint analyses for overall and event-free survival indicated that the most statistically significant results were centered around the Q4 cutpoint region, supporting our approach of comparing patients with the highest quartile of relative MEF2C expression with those having lower relative MEF2C expression. Importantly, MEF2C expression was strongly associated with cytogenetic and molecular abnormalities. Specifically, patients with high MEF2C expression less likely had CBF translocations (inv(16): P=0.007, and t(8;21): P<0.001) or normal karyotype AML (P<0.001); conversely, they were more likely to have leukemia with monosomy 7 (P<0.001) and abnormalities involving 11q23 (P<0.001). Furthermore, patients with high MEF2C less likely had a FLT3/ITD (P =0.018) or a mutation in either NPM1 (P=0.010) or CEBPA (P =0.002). Consistently, patients with high MEF2C expression less likely had low-risk disease (16% vs. 46%, P<0.001) and more likely had standard-risk disease (68% vs. 42%, P <0.001) than those with lower MEF2C expression. Indeed, after adjustment for disease risk, age, FAB category, and treatment arm, high MEF2C expression was no longer statistically significantly associated with inferior overall survival (hazard ratio [HR]=0.99 [95% confidence interval: 0.72-1.36], P=0.929), inferior event-free survival (HR: 1.14 [0.86-1.49], P=0.365), or higher relapse risk (HR: 1.32 [0.91-1.92], P=0.137), suggesting that MEF2C cooperates with additional pathogenic abnormalities.

Conclusion: High MEF2C expression identifies a subset of AML patients with adverse-risk disease features and poor outcome. These findings provide the rationale for therapeutic targeting of MEF2C transcriptional activation in AML.