Analysis of the Genetic Background in Patients with Myeloid Malignancies and Divergent Findings Regarding Blasts As Determined By Cytomorphology and Flow Cytometry

Background: Diagnostics of hematologic malignancies is based on an integrated application of various methods. Different methods targeting the same structures, e.g. cytomorphology and flow cytometry both applied to identify and quantify pathologic cell populations, are aiming at revealing consistent results. While in the majority of cases achievement of such consistency is straight forward and in other cases both methods complement each other the alignment between cytomorphologically defined blasts and flow cytometrically defined myeloid progenitor cells (MPC) sometimes is challenging, in particular in myelodysplastic syndromes (MDS), acute myeloid leukemia (AML) and related diseases (thresholds defined for cytomorphology in WHO classification). We therefore identified such cases with higher percentages of cytomorphologically defined blasts than flow cytometrically defined MPCs and analyzed their genetic background in comparison to MDS cases with consistently low percentages by both methods and to acute myeloid leukemia (AML) cases with consistently high percentages by both methods.

Aim: To clarify divergent findings between cytomorphology and flow cytometry on blasts and MPCs in patients with MDS and related diseases by analyzing the genetic background.

Patients and methods: We identified 49 cases analyzed for myeloid malignancies, which were found to have higher percentages of cytomorphologically defined blasts than flow cytometrically defined MPCs (group 1). For comparison 83 patients with AML (group 2) and 53 cases with MDS (group 3) were selected in which percentages of cytomorphologically defined blasts (%B) were matching flow cytometrically defined percentages of MPCs (%MPC). All samples analyzed were bone marrow aspirates. Patients´ ages ranged from 40 to 60 (median 74), 23 to 89 (69) and 46 to 93 (76) years, respectively, in groups 1, 2 and 3 and sex distribution was 26/23 (f/m), 35/48 and 20/33, respectively. The median %B were 24% (range 11 to 62), 46% (20 to 98) and 5% (0 to 18), respectively, for groups 1, 2 and 3. The respective figures for %MPC were 9% (0.1 to 18), 52% (20 to 94) and 3% (0.3 to 16), respectively. The three groups were compared regarding cytogenetics based on chromosome banding analysis as well as regarding NGS data on mutations in 34 genes associated with myeloid malignancies.

Results: Normal karyotypes were present in 37/49 (76%), 42/83 (51%) and 34/53 (64%) cases, respectively in groups 1, 2 and 3 (p=0.02). The respective figure for complex karyotypes was 5/49 (10%), 15/83 (18%) and 5/53 (9%, n.s.). Other chromosomal aberrations occurred less frequently and did not differ significantly between groups. Thus, karyotypes observed in group 1 were more similar to MDS than to AML.

Regarding mutational profiling group 1 was characterized by a mutation spectrum similar to MDS developing into secondary AML while AML cases displayed a different spectrum as anticipated. Thus, in group 1, as compared to groups 2 and 3, the following gene were found more frequently mutated: ASXL1 (45% vs. 16% vs. 30%, p=0.001), SRSF2 (43% vs. 24% vs. 23%, p=0.042), TET2 (36% vs. 13% vs. 29%, p=0.014), RUNX1 (27% vs. 15% vs. 17%, n.s.). Interestingly, these genes were mutated at the lowest frequencies in group 2 most probably reflecting the low proportion of AML evolved as secondary disease after MDS. In contrast, SF3B1, which confers a favorable prognosis in MDS, was present at low frequencies in both groups 1 and 2 and at higher frequency in group 3 reflecting the low rate of AML progression in MDS with SF3B1 mutations (8% vs. 4% vs. 21%, p=0.005). Matching these findings mutations in genes typically associated with de novo AML were found most frequently in group 2 and only at low frequencies in groups 1 and 3: NPM1 (6% vs. 22% vs. 0%, p<0.001), FLT3-ITD (5% vs. 16% vs. 0%, p=0.001). No significant differences between the three groups were observed for mutations in the remaining genes.

Conclusions: The genetic findings in the present series point to a similarity between cases with MDS and cases which display a lower level of %MPC and a higher level of %B at the same time. Therefore, while the latter cases based on WHO classification are defined as MDS-EB2 and AML, respectively, the genetic profile is matching better to the lower %PMC values. This may be used as a basis to more extensively integrate mutation data into the classification of these myeloid neoplasms.