MODULATION OF MICRORNA EXPRESSION IN HUMAN T CELL DEVELOPMENT: TARGETING OF NOTCH3 BY MIR-150

Recent work shows that some microRNAs (miRs) are dynamically regulated in their expression during hematopoietic development, inflammation and leukemogenesis. However, at present little is known about the possible role of miRs during the physiologic development of human T cells, nor their possible significance in T cell-derived malignancies. In order to identify miRs involved in the differentiation and neoplastic transformation of T lymphocytes, we investigated miRs involved in human thymocyte differentiation by comparing the miR expression profiles of thymocytes at the Double-Positive (DP), Single-Positive CD4+ (SP CD4+) and Single-Positive CD8+ (SP CD8+) maturation stages. Microarray analysis showed that each thymocyte population displays a distinct miR expression profile that reflects their developmental relationships. Moreover, analysis of small-RNA libraries generated from human unsorted and DP thymocytes and from mature peripheral CD4+ and CD8+ T lymphocytes, together with the microarray data, indicated a trend toward up-regulation of miR expression during T cell maturation after the DP stage and revealed a group of miRs regulated during normal T cell development, including miR-150, which is strongly up-regulated as maturation progresses. In addition, we showed that Notch3, a member of the Notch receptor family that plays important roles both in T cell differentiation and leukemogenesis, is a relevant target of miR-150 in the T cell system. Our data indicated that miR-150 was expressed at very low levels in a panel of T-ALL cell lines, as well as in leukemia xenografts derived from T-ALL patients compared to normal human thymocytes. This suggested that miR-150 down-regulation could provide a survival advantage to T-ALL cells. Indeed, forced expression of miR-150 reduces Notch3 levels in T-ALL cell lines and has adverse effects on their proliferation and survival. The apoptosis induction in T-ALL cells upon miR-150 over-expression is at least in part dependent on its suppressive effect of Notch3. Overall, these findings suggest that control of the Notch pathway through miR-150 may have an important impact on T cell development and leukemogenesis.