The impact of Down syndrome-specific non-malignant hematopoietic regeneration in the bone marrow on the detection of leukemic measurable residual disease

Background: Detection of measurable residual disease detection (MRD) by flow cytometry after the first course of chemotherapy is a standard measure of early response in patients with acute myeloid leukemia (AML). Myeloid leukemia associated with Down Syndrome (ML-DS) is a distinct form of AML. Differences in steady-state and regenerating hematopoiesis between patients with or without DS are not well understood. This understanding is essential to accurately determine the presence of residual leukemia in patients with ML-DS.

Methods: A standardized antibody panel defined quantitative antigen expression in 115 follow-up bone marrow (BM) aspirates from 45 patients following chemotherapy for ML-DS or DS precursor B-cell acute lymphoblastic leukemia (B-ALL-DS) with the "difference from normal (ΔN)" technique. When possible, FISH and SNP/CGH microarray studies were performed on sorted cell fractions.

Results: 93% of BM specimens submitted post chemotherapy had a clearly identifiable CD34⁺ CD56⁺ population present between 0.06% and 2.6% of total non-erythroid cells. An overlapping CD34⁺ HLA-DR^{heterogeneous} population was observed among 92% of patients at a lower frequency (0.04%-0.8% of total non-erythroid cells). In B-ALL-DS patients, the same CD34⁺ CD56⁺ HLA-DR^{heterogeneous} expression was observed. FACS-FISH/Array studies demonstrated no residual genetic clones in the DS-specific myeloid progenitor cells.

Conclusions: Non-malignant myeloid progenitors in the regenerating BM of patients who have undergone chemotherapy for either ML-DS or B-ALL-DS express an immunophenotype that is different from normal BM of non-DS patients. Awareness of this DS-specific non-malignant myeloid progenitor is essential to the interpretation of MRD by flow cytometry in patients with ML-DS.