High BAALC Expression in Cytogenetically Normal Acute Myeloid Leukemia Strongly Correlates with Adverse Markers Such As RUNX1mut, MLL-PTD and FLT3-ITD and Is Useful for Disease Monitoring
Authors: Weber Simone , Alpermann Tamara , Eder Christiane , Dicker Frank , Jeromin Sabine , Kohlmann Alexander , Fasan Annette , Haferlach Claudia , Kern Wolfgang , Haferlach Torsten , Schnittger Susanne 
Published: October 2019

Abstract
Abstract 1376

Introduction:
BAALC expression is thoroughly explored in cytogenetically normal AML (CN-AML) and has been shown to be associated with an adverse outcome. Nevertheless, its prognostic importance in relation to other molecular markers and its relevance for detection of minimal residual disease (MRD) remains to be defined. The objective of this study was to evaluate the prognostic impact of BAALC expression on clinical outcome in the context of other relevant molecular prognosticators and to examine its utility as a marker for detection of MRD in CN-AML.

Patients:
BAALC mRNA expression was analyzed in a cohort of 332 patients with de novo CN-AML. The cohort was composed of 169 females (50.9%) and 163 males (49.1%). Age ranged from 18.3 to 64.8 years (median: 52.9). Data on other molecular markers was available in: FLT3-ITD: n=332, NPM1: n=332, RUNX1: n=330, CEBPA: n=332, MLL-PTD: n=332, ASXL1: n=330, FLT3-TKD: n=331, IDH1G105: n=229, IDH1R132: n=253, IDH2: n=232, NRAS: n=254, WT1 mut: n=247, TET2: n=74 and TP53: n=182. In addition, BAALC expression was assessed in 25 follow-up samples of 8 patients in comparison with an established MRD marker.

Methods:
To assess BAALC mRNA expression levels RQ-PCR was performed by the use of the Applied Biosystems 7500 Fast Real Time PCR System. BAALC expression was normalized against the expression of the control gene ABL1. The median ratio BAALC/ABL1 was used to separate low from high BAALC expressers. Expression data of diagnostic samples was correlated to clinical outcomes and to the presence of molecular mutations. BAALC expression ratios of follow-up samples were also correlated to the status of other molecular markers available in the same patients.

Results:
1) Evaluation of prognostic relevance: Expression ratios of BAALC/ABL1 represented a continuum ranging from 0.001 to 80.199 (median: 0.321). In agreement with previous studies, patients with high BAALC expression had shorter overall survival (OS at 3 years: 51.4% vs 73.0%, p=0.046) and event free survival (EFS at 3 years: 38.5% vs 50.6%, p=0.021) as compared to low BAALC expressers. Though, associations of BAALC expression to other molecular markers were found. In high BAALC expressers, as compared to low BAALC expressers, the following mutations were more frequent: RUNX1 mut (32/164, 19.5% vs 2/166, 1.2%, p<0.001), MLL-PTD (22/166, 13.3% vs 5/166, 3.0%, p=0.001) and FLT3-ITDmut/wt ratio>0.5 (51/166, 30.7% vs 24/166, 14.5%, p=0.001), whereas NPM1 mut were less frequent (72/166, 43.3% vs 138/166, 83.1%, p<0.001). In univariable Cox regression analyses shorter OS and EFS was associated with higher age, higher WBC count, high BAALC expression and the presence of at least one of the established adverse markers RUNX1 mut, MLL-PTD, or FLT3-ITDmut/wt ratio>0.5 grouped together as one parameter (OS, p<0.001, <0.001, 0.0048, <0.001, respectively; EFS, p=0.002, <0.001, 0.022, <0.001, respectively). In multivariable models, BAALC expression had an independent impact on EFS (p=0.042) but not on OS. Further, Kaplan Meier analysis within the subgroup with adverse markers (RUNX1 mut, MLL-PTD, or FLT3-ITDmut/wt ratio>0.5, n=98) revealed no additional impact of BAALC expression on OS or EFS. Also within the prognostically favorable subgroup with NPM1 mut/FLT3-ITDmut/wt ratio<0.5 high BAALC expression had no impact on OS or EFS.

2) Validation as follow-up marker:
To evaluate the impact of BAALC expression for disease monitoring during follow-up, the BAALC/ABL1 ratio of high BAALC expressers was compared to the %MLL-PTD/ABL1 or %RUNX1 mut levels in patients who had at least one of these aberrations in parallel. In total, 33 diagnostic and follow-up samples of 8 patients were analyzed. Comparison of BAALC/ABL1 ratios to mutational status of MLL-PTD and/or RUNX1 revealed a significant correlation (r=0.577, p<0.001). Additionally, in one case with paired samples at diagnosis and relapse, BAALC expression levels at diagnosis and at relapse were in the same range (BAALC/ABL1: 10.870 vs 17.600).

Conclusion:
1) BAALC expression in CN-AML predicts an adverse clinical outcome, but is associated with other well established prognostic markers. Thus, in our study BAALC expression had no independent prognostic impact on OS when analyzed together with RUNX1, MLL-PTD, FLT3-ITD and NPM1. 2) Correlation of BAALC expression with MLL-PTD and RUNX1 during follow up indicates that BAALC expression is a potential target for MRD monitoring.

Disclosures:
Weber: MLL Munich Leukemia Laboratory: Employment. Alpermann:MLL Munich Leukemia Laboratory: Employment. Eder:MLL Munich Leukemia Laboratory: Employment. Dicker:MLL Munich Leukemia Laboratory: Employment. Jeromin:MLL Munich Leukemia Laboratory: Employment. Kohlmann:MLL Munich Leukemia Laboratory: Employment. Fasan:MLL Munich Leukemia Laboratory: Employment. Haferlach:MLL Munich Leukemia Laboratory: Equity Ownership. Kern:MLL Munich Leukemia Laboratory: Equity Ownership. Haferlach:MLL Munich Leukemia Laboratory: Equity Ownership. Schnittger:MLL Munich Leukemia Laboratory: Equity Ownership.