Circulating microRNAs as minimal residual disease biomarkers in childhood acute lymphoblastic leukemia.

Background: Treatment stratification based on bone marrow minimal residual disease (MRD) at set time points has resulted in considerably improved survival in pediatric acute lymphoblastic leukemia (ALL). Treatment response is assessed using bone marrow samples. MicroRNAs (miRs) easily traffic among fluid spaces and are more stable than most other RNA classes. We examined the role of circulating miRs as putative less invasive MRD biomarkers.

Methods: In an exploratory experiment, expression of 46 preselected miRs was studied in platelet-free blood plasma samples of 15 de novo, 5 relapsed ALL patients and 10 controls by Custom TaqMan Array Advanced MicroRNA Card. Based on their high expression in ALL compared to controls, and on the reduction observed along the induction therapy, four miRs were selected for further analyses: miR-128-3p, -181a-5p, -181b-5p and 222-3p. Their expression was measured by qPCR at 4 time points in 27 de novo ALL patients treated in the ALL IC-BFM 2009 study.

Results: The expression of all 4 miRs significantly decreased over the first week of therapy (miR-128-3p: log fold change - 2.86; adjusted p 3.6 × 10; miR-181b-5p: log fold change - 1.75; adjusted p 1.48 × 10; miR-181a-5p: log fold change -1.33; adjusted p 3.12 × 10; miR-222-3p: log fold change - 1.25; adjusted p 1.66 × 10). However, no significant further reduction in miR expression was found after the 8th day of therapy. Measured drop in expression of 2 miRs at day 8 strongly correlated with day 15 bone marrow flow cytometry MRD results (miR-128-3p: Pearson's r = 0.88, adjusted p = 2.71 × 10; miR-222-3p: r = 0.81, adjusted p = 2.99 × 10).

Conclusion: In conclusion, these circulating miRs might act as biomarkers of residual leukemia. MiR-128-3p and miR-222-3p in blood predict day 15 flow cytometry MRD results 7 days earlier. Although, their sensitivity falls behind that of bone marrow flow cytometry MRD at day 15.