ENPP4 and HOXA3 as potential leukaemia stem cell markers in acute myeloid leukaemia.

Introduction: Acute myeloid leukaemia (AML) is a heterogeneous malignant disease with a high degree of treatment failure using chemotherapy. Leukaemia stem cells (LSCs) are CD34+CD38- early progenitors associated with poor prognosis in AML. A unique LSC phenotype that excludes rare normal haematopoietic stem cells (HSC) is still elusive. This study aimed to determine expression of selected potential LSC markers in normal and leukaemic myeloid cells and correlate prognosis in AML patients.

Materials and methods: Flow cytometry and RT-qPCR measured expressions of ALDH, IL3RA/CD123, CLEC12A/CLL-1/CD371, HOXA3 and ENPP4. Normal cord blood (n=3) and blood monocytes (n=5) represented HSC and mature cells, respectively. Myeloid leukaemia cell lines (THP-1, KG-1a, K562 and HL-60) represented progenitor cells at various stages of maturation. AML samples included chemo-resistant (n=8), early relapse (n=2) and late relapse (n=18).

Results: Combining protein/gene expressions, CD34+CD38- was a feature of immature cells seen in cord blood, KG-1a, and K562 but not more mature cells (blood monocytes and HL-60). Normal cells expressed CD371 while mature cells (blood monocytes and HL-60) lacked CD123. ENPP4 was not expressed on normal cells while HOXA3 was expressed only on cord blood and THP-1. In AML, CD123, HOXA3, ENPP4 (but not CD371) were significantly increased in the CD34+CD38- fraction of chemo-resistant patients while ALDH was associated with chemo-resistance.

Conclusion: CD34+CD38- presented an immature phenotype and with ALDH were associated with poor prognosis. CD123, HOXA3 and ENPP4 further enriched the LSC population. ENPP4 has not been reported and has the advantage of not being expressed on HSC and normal monocytes.