Immunophenotypically Aberrant Stem Cells in Myelodysplastic Syndromes: A Biomarker for Leukemic Progression

Introduction: Myelodysplastic syndromes (MDS) are generally referred to as a spectrum of myeloid malignancies that are notorious for their heterogeneity. Subsequently, the question arises as to what is the best method to dissect the heterogeneity of these syndromes, which may range from a normal life expectancy and MDS-unrelated dead to rapid progression to acute myeloid leukemia (AML). Most effort is still being put into the development of morphological classification and prognostic models like the classification of the World Health Organization (WHO) and the revised International Prognostic Scoring System (IPSS-R). However, the need for a more refined classification based on new insights into MDS pathophysiology is emerging.

Methods: Hitherto we propose a new approach to predict leukemic progression among MDS patients using hematopoietic stem cell immunoprofiles. In this study, leukemic stem cell associated antigens - CD7, CD11b, CD22, CD33, CD44, CD45RA, CD56, CD123, CD366, CD371 - were analyzed on CD34⁺CD38^- hematopoietic stem cells (HSCs) derived of 130 patients and controls at time of diagnosis.

Results: Aberrant expression of these antigens was found in 33% (24/72) and in 20% (1/5) of the MDS and chronic myelomonocytic leukemia (CMML) patients, respectively. Even in patients diagnosed with idiopathic cytopenia of undetermined significance (ICUS) based on inexplicable cytopenia but less than 10% dysplasia, expression of aberrant antigens was found in 50% (4/8). In contrast to MDS, CMML, and ICUS patients, aberrant antigen expression was absent on HSCs of healthy, age-matched controls (0/9) and pathological controls (0/36), i.e. patients with non-myeloid clonal disorders. In MDS patients, the presence of aberrant immunophenotypic HSCs (AIP-HSCs) at diagnosis was negatively associated with the leukemia free and overall survival time (Figure 1a-b, p<.001), and increased the likelihood of leukemic progression and death by 99.5% (HR=0.005, 95%CI: 0-10.0) and 87.2% (HR=0.128, 95%CI: 0.04-0.41). The robust performance of flow cytometric detection of AIP-HSCs as diagnostic test to identify MDS patients prone to AML was illustrated by a high sensitivity of 88% and specificity of 69% at a follow-up time of six months. Although a slide trend toward increased AIP-HSCs prevalence for higher risk categories, MDS patients with AIP-HSCs were found across almost all WHO 2016 and IPSS-R categories (Figure 2a-b). Interestingly, multivariate regression analyses, including AIP-HSC percentage as well as the IPSS-R variables cytogenetic abnormalities, bone marrow blasts, hemoglobin, absolute neutrophil count, and platelets, revealed AIP-HSC percentage to be the only determinant with a significant effect on leukemia free survival. In predicting overall survival, apart from AIP-HSCs, cytogenetic abnormalities rendered statistical significant value.

Conclusion: Flow cytometric detection of AIP-HSCs can be used as a biomarker to predict which MDS patients will finally develop AML. As a result, the presence of AIP-HSCs may improve currently used classification models and substitute the 20% blast cut-off to discriminate between MDS and AML. Addition of molecular features will be needed to validate the malignant nature of these AIP-HSCs. In the near feature, we anticipate that flow cytometric detection of AIP-HSCs will not only contribute to the dissection of MDS heterogeneity and an increased understanding of MDS stem cell biology, but also to therapy decision making, disease monitoring, and the clinical outcome of MDS patients.

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