Morphological Subtype According to the FAB Classification Correlates with Molecular Subtype Using Genomic Risk Classification Based on Somatic Mutation Profiles in Normal Karyotype AML

Background: in the 1970s, French-American-British (FAB) classification had been proposed and had been used for the classification of acute myeloid leukemia (AML) until cytogenetic risk classification was introduced. The FAB classification was based largely on morphologic features of leukemic blasts, and was useful to group AML. Morphology of leukemic blast is an endproduct of genetic changes in DNA/RNA levels in leukemic cells. Thus it is plausible that morphologic subtype can correlates with genetic changes in AML. The present study attempted to explore whether a specific somatic mutation correlates with morphologic subtype classified according to the FAB classification using mutation profile data by targeted deep sequencing in normal karyotype (NK) AML.

Patients and Methods: A total of 328 patients were included in the present study who had a diagnosis of NK-AML and treated with conventional induction chemotherapy using a standard protocol. Analysis of genetic mutations were performed using targeted resequencing by Illumina Hiseq 2000 (Sureselect custom probe set targeting entire exon regions of a myeloid panel consisting of 94 genes). Bone marrow aspirate/flow cytometric data have been extensively reviewed retrospectively in accordance with FAB classification.

Results: The median age of the cohorts was 51 years (range, 15-84), and 50% were male. Frequency of subtype by the FAB classification was as follow; M0 (n=13; 4.0%), M1 (n=55; 16.8%), M2 (n=136; 41.5%), M4 (n=60; 18.3%), (M5, n=43; 13.1%), M6 (n=15; 4.6%) and, M7 (n=6;1.8%). The FLT3 mutations were detected in 117 patients (35.7%) and CEBPA double mutations were observed in 52 patients (15.9%). CEBPA double mutations and was more frequently observed in M2 (29.4% vs. 6.3%, p<0.001), but less frequently detected in M1 (5.5% vs. 17.9%, p=0.021) and M4 (5.5% vs. 17.9%, p=0.011) in comparison with rest of the cohort. The FLT3 mutations was more frequently observed in M1 (54.5% vs. 6.3%, p=0.001) and, less frequently detected in M0 (7.7% vs. 36.8%, p=0.032) and M2 (26.4% vs. 42.1%, p=0.003) in comparison with rest of the cohort.

Median follow up duration was 57.1 months. Of 328 patients receiving induction chemotherapy, complete remission (CR) was achieved by 272 (82.9%). The CR rate in the subgroup of M0 and M7 was 53.8% (p=0.004) and 50% (p=0.031), respectively which was signifciantly lower CR rate in relative to rest of the cohort. The 5-year overall survival (OS) in the subgroup of M2 was favorable in comparison with rest cohort [45.3% (95% CI, 35.9-54.7%) vs. 35.5% (95% CI, 28.4-42.6), p=0.008]. In multivariate analysis, age, WBC count at diagnosis, NPM1 mutation, FLT3 mutation and CEBPA double mutations were confirmed to be significant prognostic factors affecting OS independently (all, p<0.05), while M2 morphologic subtype by the FAB classification did not affect OS (p>0.05).

Conclusion: In NK-AML cohort, favorable molecular mutations such as CEBPA double mutations are frequently associated with M2, while FLT3 mutations are less frequent in M2, compared to other FAB subgroups. We could confirm that specific somatic mutation correlates with morphologic subtype classified according to the FAB classification.