The Role of Different Genetic Subtypes In CEBPA Mutated AML
Authors: Schnittger Susanne , Alpermann Tamara , Eder Christiane , Schindela Sonja , Grossmann Vera , Kern Wolfgang , Haferlach Torsten , Haferlach Claudia 
Published: October 2019

Abstract
Abstract 752
To evaluate the role of CEBPA mutations (CEBPAmut) in the context of other molecular mutations and cytogenetic aberrations we have analyzed 1567 AML cases for CEBPAmut. The patients were selected according to cytogenetics excluding the following karyotypes: t(15;17)/PML-RARA, t(8;21)/AML1-ETO, inv(16)/t(16;16)/CBFB-MYH11, inv(3)/t(3;3)/EVI1, t(6;9)/DEK-CAN and 11(q23)/MLL, and complex aberrations. The cohort was composed of 697 females and 870 males. Age ranges from 16.7 to 88.3 years (y) (median: 71.0 y). CEBPAmut were detected in 126/1567 cases (8.0%). The biologic characteristics of the CEBPAmut patients (age range 16.7 to 87.6 y, median: 64.8 y) were further investigated. Three different CEBPAmut patterns were observed: 1) in 50/126 cases (39.7%) one mutation and one wildtype allele were detected (monoallelic pattern), 2) 61 cases (48.4%) had two different mutations (biallelic pattern), 3) 15 cases (11.9%) had one mutation without detectable wildtype allele due to loss of heterozygosity (LOH). Overall we found 186 different mutations of following types: 1) 108 led to a premature N-terminal stop of the protein (6 due to a nonsense and 102 due to a frameshift mutation), 2) 60 were inframe mutations in the b-ZIP region, 3) 8 were frameshifts in the b-ZIP region and, 4) 2 were frameshifts 3`of the b-ZIP region, and 8 were C-terminal point mutations. Correlation to cytogenetics shows a normal karyotype (NK) in 86 (68.3%) of the 126 CEBPAmut patients whereas in 40 pts (31.7%) at least one cytogenetic aberration was detected (-7: n=7; +8: n=7, 9q-: n=2; 11q-: n=3, other trisomies: n=11; other non recurrent translocations: n=4, all others: n=6). Cytogenetic aberrations were more frequent in the monoallelic group (55%) compared to the biallelic (35%) (p=0.001) and to cases with LOH (10%) (p=0.047). Interestingly in the total cohort of 13 pts with monosomy 7 seven pts (53.8%) were CEBPA mutated (53.8%) and of these 6 were in the monoallelic group. Additional mutations were detected in 48 cases (RUNX1: n=11, NPM1: n=10, FLT3-ITD: n=20, FLT3-TKD: n=3, MLL-PTD: n=5, NRAS: n=9, IDH1: n=2, IDH2: n=7; 15 pts showed 2 and 2 pts 3 of these mutations). Similar to the cytogenetic aberrations the molecular mutations were more frequent in the monoallelic group (61.9%) compared to the biallelic (31.0%) and the LOH group (7.1%) (p=0.001). NPM1 mutations were mutually exclusive of biallelic CEBPAmut. As previously described we also detected a significantly higher expression of CD7 in the CEBPAmut compared to the CEBPAwt group (71.2% vs. 18.9%, p<0.001). Furthermore, CD7 was higher expressed in biallelic cases as compared to the monoallelic ones (86.2% vs. 43.8%, p=0.011). It was similar in the LOH group (71.4%) compared to the biallelic group. There was no influence of cytogenetic aberrations or any additional mutation on EFS and OS. Solely the presence of high FLT3-ITD load (>0.5 FLT3-ITD/FLT3wt) was correlated with a shorter EFS (EFS at 2 y: 20.3% vs. 44.8%; p=0.020) and OS (OS at 2 y 54% vs 68%, p=0.047) when compared to the combined group of FLT3wt and those with an FLT3-ITD load of <0.5. Regarding the different CEBPA groups the biallelic cases had a slightly better OS compared to monoallelic cases (OS at 2 y: 75.0% vs. 60.8%; n.s.). The 2-year OS in the LOH group was significantly lower (33.8%; p=0.023, compared to the biallelic group; and p=0.043 compared to 69.7% in the combined biallelic + monoallelic group). In addition, the different functional mutation types were analyzed. Out of frame mutations in b-ZIP had no specific impact on survival within the CEBPAmut cohort. N-terminal stop mutations and in frame mutations in b-ZIP were associated with favourable outcome (OS at 2 y: 72.1% vs. 43.9% all others mutations, p=0.007 and 2 y OS at 2 y: 76.3% vs. 46.9%; p=0.043, respectively), whereas all other mutations were extremely unfavourable (OS at 2 y: 0% vs. 70.5% compared to N-terminal and b-ZIP mutations, p<0.001). In summary, 1) the biology and prognostic impact varies depending on distinct CEBPAmut patterns. 2) Cytogenetic and molecular alterations had no prognostic impact with the exception of FLT3-ITD with a mutation load of >0.5 FLT3-ITD/FLT3wt. 3) Our data for the first time demonstrate that CEPBAmut with LOH are associated with an even inferior outcome than monoallelic mutations. In conclusion, these data show that CEBPA should be analyzed in detail in all NPM1wt NK AML and in those with unfavourable but non-complex karyotypes.

Disclosures:
Schnittger: MLL Munich Leukemia Laboratory: Employment, Equity Ownership. Alpermann:MLL Munich Leukemia Laboratory: Employment. Eder:MLL Munich Leukemia Laboratory: Employment. Schindela:MLL Munich Leukemia Laboratory: Employment. Grossmann:MLL Munich Leukemia Laboratory: Employment. Kern:MLL Munich Leukemia Laboratory: Employment, Equity Ownership. Haferlach:MLL Munich Leukemia Laboratory: Employment, Equity Ownership. Haferlach:MLL Munich Leukemia Laboratory: Employment, Equity Ownership.