The Ikaros family proteins: molecular targets and downstream events in lymphocyte development – Ikaros-in-Lymphocytes

We study these questions in the mouse, using mice in which these factors have been inactivated. Studying the defects of these mice allow us to understand the functions of these factors

Our results indicate that these factors are important to regulate biological pathways involved in leukemia development in humans, as well as in auto-immune disorders

Our work should provide a better understanding of the mechanisms involved in pathogenesis of leukemia and autoimmunity.

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Lymphocyte development is a highly regulated process that involves multiple stages and the coordinated activation and repression of thousands of genes. Gene expression programs like that of the Notch pathway must be efficiently regulated in progenitor cells as they can be oncogenic when misexpressed. The Ikaros family of transcription factors plays an essential role in lymphocyte differentiation. This family of 4 related members is expressed throughout T and B cell differentiation, at all stages (Ikaros) or in a more restricted pattern (Aiolos, Helios, Eos). Ikaros is the most well studied and is clearly important. Ikaros-deficient mice show a developmental block in early B cells, and deregulated T cell differentiation that results in leukemia development. The latter phenotype has been linked to a role for Ikaros in silencing Notch pathway genes in competition with RBPJ, the transcriptional mediator of Notch signaling, and was the subject of our ANR grant in 2006. Aiolos is associated with lymphoma development in B cells but is less important in T cells, as is Helios, suggesting functional redundancy. Eos is required for Treg function but its role in other lymphocyte subsets is unknown. Family members have also been proposed to function antagonistically (Ikaros/Aiolos vs. Helios in B cells), suggesting that their coordinated actions may be more complex than just functional redundancy. At the molecular level, it is currently unclear how the Ikaros family proteins regulate gene expression. All members associate with NURD histone deacetylase complexes in vitro, and are thus thought to initiate epigenetic changes at target genes. Surprisingly, we find that Ikaros deficiency is associated with a loss of the Polycomb-dependent repressive mark H3K27me3.
We hypothesize that the Ikaros family is critical for coordinating the gene expression programs in lymphocyte development. These factors operate by competing with other regulators for binding to target sequences. Binding recruits NURD, and possibly other complexes, and initiates chromatin remodeling events that ultimately lead to stable epigenetic changes such as H3K27 tri-methylation.
Our specific aims:
1) Determine how the affinity of Ikaros for Notch target sequences shapes the sensitivity of these genes to Notch signaling in T cells. These results will serve as a paradigm to understand how Ikaros family proteins interfere with other transcriptional programs.
2) Determine how Ikaros binding induces the deposition of Polycomb-dependent chromatin marks. We will show that Ikaros recruits NURD and possibly other factors (ie. TIF1b) to target genes. We will determine if targeting of these co-factors is required for subsequent Polycomb recruitment. These studies will highlight the events downstream of DNA binding that are likely to be relevant for the function of all family members.
3) Dissect the specific and redundant functions of the Ikaros family proteins in lymphocyte differentiation through genetic and molecular analyses. We will generate and compare the phenotypes of single and compound mutations of all family members. We will address specificity by determining if Aiolos, Helios or Eos can replace Ikaros in cellular systems. We will determine if specific subsets of member-specific target genes contribute to functional specificity.

Together, these studies should significantly advance our understanding of how the Ikaros family controls lymphocyte differentiation.