TLE Corepressors as Novel Candidate AML Tumor Suppressor Genes That Regulate Wnt Signaling, Cell Cycle Progression, and Myeloid Differentiation.

Deletion of the long arm of chromosome 9, del(9q), is one of the most common mutations associated with t(8;21) AML. Up to 50% of del(9q) AML is seen in association with t(8;21). The Runx1-ETO (AML1-ETO) fusion gene produced by t(8;21) is insufficient for leukemogenesis and loss of a critical gene(s) on chr9q apparently cooperates in leukemogenesis. We recently identified two members of the Groucho family of co-repressors, Transducin-like enhancer of split (TLE) 1 and TLE4, as candidate tumor suppressor genes, based on our mapping of the commonly deleted region in del(9q) AML. These proteins are known to inhibit Wnt signaling which has been implicated in hematopoietic stem cell renewal and they interact with hematopoietic transcription factors such as Runx1 and Pu.1. We have demonstrated that the expression of these two genes is specifically repressed in both del(9q) and t(8;21) AML samples. Knockdown of TLE1 or TLE4 mRNA using specific small interfering RNAs (siRNA) activates Wnt signaling in 293T cells as measured by TOPFLASH activity, while forced expression of TLE1 or 4 inhibits TOPFLASH. We show that siRNA against TLE4 using a specific siRNA increased cell cycle progression and cell division, while over-expression of TLE1 or TLE4 slowed cell cycle progression and lead to a pronounced growth disadvantage in THP-1 and HL60 myeloid cell lines. In addition to these effects on cell proliferation, these genes also affected myeloid cell differentiation. Over-expression of either TLE1 or TLE4 in monocytic THP-1 and promyelocytic HL-60 cell lines initiated myeloid differentiation as monitored by CD11b expression. These two genes have different effects on more terminal myeloid differentiation as TLE4, but not TLE1, was able to induce more terminal differentiation of THP-1 cells into monocytes as measured by CD14 expression, while TLE1 was able to induce the granulocytic marker CD15 in HL-60 cells. Furthermore, lentiviral delivery of siRNAs for either TLE1 or TLE4 to HL-60 cells inhibited induction of granulocytic and monocytic differentiation with all-trans retinoic acid (ATRA) or 1,25-dihydroxycholechalciferol (Vit.D3). TLE1 siRNA inhibited ATRA and Vit.D3 induced CD11b expression by more than 75%. While TLE4 siRNA decreased the induction of monocytic CD14 after Vit.D3 by 40%, TLE1 siRNA almost completely abrogated induction of CD14 with Vit.D3 or granulocytic CD15 with ATRA in HL-60 cells. Ex vivo culture of siRNA infected CD34 sorted human cord blood (CB) cells in semi-liquid media with differentiation inducing cytokines for seven days exhibited a decrease in the percentage of early CD33+ myeloid cells by 15% and 70% relative to controls in TLE1 and TLE4 siRNA infected CB cells, respectively. These results indicate that inhibition of TLE activity can promote cell cycle progression and inhibit myeloid differentiation. This study is the first to demonstrate a potential role for the TLEs in leukemogenesis and also indicates the TLEs may have non-redundant functions in myeloid differentiation. Understanding how various mutations work cooperatively to produce a malignant phenotype is one of the great challenges in oncology. The TLEs may represent an important cooperating mutation with Runx1-ETO in AML that links Wnt signaling and core binding protein transcription factors. Studies are currently underway to demonstrate this cooperativity.