Ikaros induces quiescence and T-cell differentiation in a leukemia cell line.

Cancer is a disease of uncontrolled cell growth. It is precipitated by genetic mutations that affect the regulation of cell cycle progression and genomic integrity. Tumor suppressor genes are a group of genes whose function is lost through mutation, leading to cancer (16). Studies of mouse model systems have identified the nuclear factor Ikaros as a novel candidate tumor suppressor involved in the regulation of T-cell proliferation (2, 56, 58). It is hypothesized that Ikaros functions through its ability to target chromatin remodeling complexes and their associated histone-modifying enzymes to specific genetic loci (24). A decrease of Ikaros activity in mice, either by a complete knockout or expression of a dominant negative (DN) Ikaros isoform, results in T-cell leukemogenesis with 100% penetrance (56, 58). Data suggest that deregulation of T-cell receptor (TCR) signaling pathways may underlie T-cell transformation. Without exception, leukemic cells express a receptor linked to TCR signaling pathways (TCRαβ, TCRγδ, or pre-TCR), and this expression is essential to the leukemogenic process (57). When the Ikaros mutation was backcrossed onto a Rag 1−/− genetic background, which prevents somatic rearrangement of TCR genes, the resulting mice were completely protected from development of leukemia. Leukemogenesis again occurred with 100% penetrance when TCR expression was restored through expression of a TCR transgene in these mice. In support of Ikaros's role as a regulator of TCR signaling pathways, prior to becoming transformed, Ikaros mutant T cells display augmented proliferative responses when stimulated via TCR complex engagement (2, 56, 58). This hyperresponsive phenotype is increasingly more dramatic as the levels of Ikaros activity are reduced (2), suggesting that Ikaros functions as a rheostat for TCR activation signals. Moreover, Ikaros mutant T cells require less TCR signal to drive them into cell cycle from the quiescent or G0 state and display a shortened G1 stage and premature entry into S phase (2). These data imply that Ikaros functions as a regulator of cell cycle progression and proliferation in T cells. This function is likely due to Ikaros's role as a component of at least two chromatin remodeling complexes that are linked to histone acetylase (HAT) or histone deacetylase (HDAC) activity, thereby affecting chromatin structure and gene expression (24). It has been hypothesized that Ikaros's role in these complexes is to target them to specific genetic loci via its sequence-specific DNA binding ability. Significantly, it has been demonstrated that components of chromatin remodeling complexes are mutated in several forms of human cancer, including acute lymphocytic leukemia and acute myelogenous leukemia (41). The high degree of conservation of Ikaros in mice and humans (95% at the amino acid level) strongly suggests that Ikaros may also play a tumor suppressive role in human cells (31). In support of this, it has been shown that a high percentage of analyzed infant and childhood acute lymphocytic leukemias display defects in Ikaros gene expression that would result in a significant decrease in Ikaros activity (48, 49). Consequently, understanding the mechanism by which Ikaros mutations lead to T-cell leukemia in mice may contribute toward understanding how functionally similar events occur in humans. Tumor suppressor genes are genes whose loss of function results in tumorigenesis. Moreover, reintroduction of their function into deficient cancer cells usually results in growth arrest (5, 14, 15, 21). In this report, we provide strong evidence that Ikaros fulfills this definition. Reintroduction of Ikaros activity into an Ikaros null T leukemia cell line arrests its uncontrolled proliferation. We show that growth arrest is accompanied by up-regulation of p27kip1 expression and initiation of a T-cell developmental program similar to that observed in developing T cells responding to TCR-mediated cues. Dramatically, these changes are accompanied by global changes in histone H3 acetylation status. These studies establish Ikaros as a tumor suppressor gene for the T-cell lineage and provide insight into the mechanism of Ikaros's tumor suppressive activity.