MicroRNAs Characterize Genetic Diversity and Drug Sensitivity in Pediatric Acute Lymphoblastic Leukemia.

Abstract 89

MicroRNAs (miRNAs) are a new class of small non-coding RNAs that regulate the activity of protein-coding genes, including those involved in cancer. The function of a miRNA depends on the cellular context and hence prominent miRNAs in lymphoma may not be important in acute lymphoblastic leukemia (ALL). To understand which miRNAs may be relevant in pediatric ALL, the expression levels of 397 miRNAs including seven newly cloned miRNAs were measured in seven genetic subtypes of ALL and normal hematopoietic cells. Except for BCR-ABL-positive and B-other ALL all major subtypes, i.e. T-ALL, MLL-rearranged, TEL-AML1-positive, E2A-PBX1-positive and hyperdiploid ALL, have unique miRNA signatures that differ from each other and from those in healthy hematopoietic cells. The expression of miR-383, miR-125b, miR-99a, miR-100 and let-7c was increased by a five to 1700-fold (P < 0.001) in TEL-AML1-positive cases. Hyperdiploid patients demonstrated a three to 24-fold upregulation (P < 0.001) of miR-222/222*, miR-223, miR-511 and miR-660, encoded on either chromosome X or 10 which is often duplicated in hyperdiploid cases. Some ALL subtypes shared similarities in their miRNA expression signature suggesting a common underlying biology e.g. both E2A-PBX1 and T-ALL cases demonstrated a downregulation of eight miRNAs (P ≤ 0.02) and within the TEL-AML1-positive subtype, two distinct groups were identified of which one showed an overlapping miRNA expression profile with hyperdiploid cases. Aberrant miRNA expression may result in dysregulated expression of their targeted proteins. Here we observed that the 70-fold downregulation of let-7b in MLL-rearranged ALL was associated with a 2-fold upregulation of oncoprotein c-Myc (P< 0.0001). Furthermore, a classifier built with a selection of 28 miRNAs predicted the MLL-rearranged, TEL-AML1-positive, E2A-PBX-positive and T-ALL subtypes with 100% sensitivity and specificity. Besides the genetic subtype, cellular drug resistance determines outcome of ALL. In vitro resistance of patients to vincristine, daunorubicin and L-asparginase was characterized by abnormal expression of 27 miRNAs (P < 0.05) whereas no discriminative miRNAs were found for resistance to prednisolone. Most striking was the 14- to 25-fold upregulation (P ≤ 0.002) of miR-125b, miR-99a and miR-100 in cases resistant to vincristine or daunorubicin. In conclusion, genetic subtypes and drug resistant leukemic cells display characteristic miRNA expression levels. Functional studies are indicated for discriminative miRNAs and may provide new insights into leukemogenesis.