Clonal Hematopoiesis and Its Implications for Flow Cytometric Assessment of Measurable Residual Disease in Patients with NPM1­-mutated Acute Myeloid Leukemia

INTRODUCTION: NPM1 mutations (NPM1^mut) occur in ∼30% of acute myeloid leukemia (AML) and frequently co-occur with mutations in other genes including those attributed to clonal hematopoiesis (CH) including DNMT3A and TET2, among others. CH mutations may persist beyond attaining NPM1^mut-negative remission. Persistent CH may be associated with immunophenotypic alterations in myeloid progenitors detected by flow cytometry (FC) which poses an interpretive challenge in assessment of measurable residual disease (MRD) by FC. The aim of this study was to characterize the immunophenotypic alterations associated with persistent CH in the setting of NPM1^mut clearance and to determine their possible clinical or biologic significance.

METHODS: The cohort included 67 consecutive patients (pts) with NPM1^mut AML treated at our institution between 01/2017 and 11/2019. FC assessment for MRD was performed an eight-color panel using FACSCanto II instruments (BD Biosciences, San Diego, CA) with a sensitivity of 10^-3 to 10^-4. Whole bone marrow (BM) DNA was interrogated for mutations with an 81-gene myeloid next-generation sequencing (NGS) panel using an Illumina MiSeq sequencer (Illumina, Inc., San Diego, CA, USA) with a sensitivity: 1% variant allelic frequency (VAF).

RESULTS: Pts included 26 men and 41 women with a median age of 64 (range, 19-84) years with newly diagnosed NPM1^mut AML. AML blasts had the following immunophenotype at baseline: promyelocytic-like phenotype (CD34-, CD117+, HLA-DR-) in 18 (27%), aberrant myeloid CD34-/CD117+/HLA-DR+ in 15 (22%), aberrant myeloid CD34+ in 13 (19%), myelomonocytic in 11 (16%), and monocytic in 10 (15%) cases. All pts had additional co-mutations at baseline (Fig 1). The most frequently co-mutated genes were DNMT3A (58%) FLT3 (51%), TET2 (27%), IDH2 (24%), PTPN11 (19%), IDH1 (18%), NRAS (16%), and SRSF2 (12%). Pts were treated with intensive (35;52%) and non-intensive induction regimens (32; 48%) (Fig 1); 22 (33%) received an allogeneic hematopoietic stem cell transplant as post-remission consolidation. We compared FC and NGS results in follow-up samples in pts achieving NPM1^mut negative remission with adequate data available for comparison (n=50). 13 (26%) pts cleared all mutations whereas 37 (74%) had persistent CH. The most common mutations in the setting of residual CH involved DNMT3A (70%), TET2 (27%), IDH2 (19%) and IDH1 (11%). Among 37 pts with residual CH, 19 (51%) had no phenotypic alterations detected by FC while 17 (49%) had myeloid progenitors with alterations in intensity of antigen expression (increased CD13, CD123, CD117 and/or decreased CD38) or deviation from normal maturation but not diagnostic for AML MRD (herein referred to as pre-leukemic (PL) phenotype); 1 sample was MRD+ by FC. Mutation VAF of ≥5% was significantly more common (p=0.008) in cases with FC PL+ (100%) vs cases with normal FC phenotype (63%). IDH2 and SRSF2 mutation were exclusively observed in PL+CH+ cases with the former being statistically significant when compared with the FC-normal group (p=0.016). PL phenotype by FC did not correlate with intensity of induction therapy (41% treated with intensive regimens vs 59% non-intensive). The CH+/PL+ cohort had more pts ≥60 yrs old (67%) but the difference was not significant. There was no significant association between PL+ and residual mutation count. Presence of PL+ phenotype was not associated with a shorter relapse-free survival (RFS) (median not reached for both groups).

CONCLUSIONS: Post-remission clonal hematopoiesis in the setting of NPM1^mut clearance is common, and may result in immunophenotypic changes in myeloid progenitors, posing interpretive challenges for MRD assessment by FC. These alterations may be attributable to specific CH characteristics, such as IDH2 and SRSF2 mutations and VAF, but are not associated with a shorter RFS and thus should not be interpreted as residual AML or considered a high-risk attribute. Additional studies in other AML subtypes are warranted to further delineate these changes and their clinical significance.