Quantitative assessment of the CD26+ leukemic stem cell compartment in chronic myeloid leukemia: patient-subgroups, prognostic impact, and technical aspects

// Martin Culen 1, * , Marek Borsky 1, 2, * , Veronika Nemethova 2, 3 , Filip Razga 2, 3 , Jiri Smejkal 2 , Tomas Jurcek 1, 2 , Dana Dvorakova 1, 2 , Daniela Zackova 1, 2 , Barbora Weinbergerova 2 , Lukas Semerad 1, 2 , Irina Sadovnik 4 , Gregor Eisenwort 4, 5 , Harald Herrmann 4, 5 , Peter Valent 4, 5 , Jiri Mayer 1, 2, 6 , Zdenek Racil 1, 2, 6 1 Department of Internal Medicine, Hematology and Oncology, Faculty of Medicine, Masaryk University, Brno, Czech Republic 2 Department of Internal Medicine, Hematology and Oncology, University Hospital Brno, Brno, Czech Republic 3 Polymer Institute of the Slovak Academy of Sciences, Bratislava, Slovak Republic 4 Department of Internal Medicine I, Division of Haematology and Hemostaseology, Medical University of Vienna, Vienna, Austria 5 Ludwig Boltzmann Cluster Oncology, Medical University of Vienna, Vienna, Austria 6 Central European Institute of Technology (CEITEC), Masaryk University, Brno, Czech Republic * These authors contributed equally to this work Correspondence to: Zdenek Racil, email: zracil@fnbrno.cz Keywords: CML, LSC, DPPIV/CD26, FISH, FACS Received: December 17, 2015 Accepted: April 10, 2016 Published: April 29, 2016 ABSTRACT Little is known about the function and phenotype of leukemic stem cells (LSCs) in chronic myeloid leukemia (CML) or about specific markers that discriminate LSCs from normal hematopoietic stem cells (HSCs). CD26 has recently been described as a specific marker of CML LSCs. In the current study, we investigated this marker in a cohort of 31 unselected CML patients. BCR/ABL1 positivity was analyzed in highly enriched stem cell fractions using fluorescence in situ hybridization (FISH) and reverse transcription PCR (RT-PCR). The proportion of CD26 + LSCs and CD26 – HSCs varied considerably among the patients analyzed, and the percentage of CD26 + cells correlated with leukocyte count. The CD26 expression robustly discriminated LSCs from HSCs. This required a strict gating of the stem cell compartment. Thus, in patients with very low LSC or HSC numbers, only the highly sensitive RT-PCR method discriminated between clonal and non-clonal cells, while a robust FISH analysis required larger numbers of cells in both compartments. Finally, our data show that the numbers of CD26 + CML LSCs correlate with responses to treatment with BCR-ABL1 inhibitors.