Evolution of the Ikaros family transcription factors: From a deuterostome ancestor to humans

Ikaros family proteins (Ikaros, Helios, Aiolos, Eos) are zinc finger transcription factors essential for the development and function of the adaptive immune system. They also control developmental events in neurons and other cell types, suggesting that they possess crucial functions across disparate cell types. These functions are likely shared among the organisms in which these factors exist, and it is thus important to obtain a view of their distribution and conservation across organisms. How this family evolved remains poorly understood. Here we mined protein, mRNA and DNA databases to identify proteins with DNA-binding domains homologous to that of Ikaros. We show that Ikaros-related proteins exist in organisms from all four deuterostome phyla (chordates, echinoderms, hemichordates, xenacoelomorpha), but not in more distant groups. While most non-vertebrates have a single family member, this family grew to six members in the acoel worm Hofstenia miamia, three in jawless and four in jawed vertebrates. Most residues involved in DNA contact from zinc fingers 2 to 4 were identical across the Ikaros family, suggesting conserved mechanisms for target sequence recognition. Further, we identified a novel KRKxxxPxK/R motif that inhibits DNA binding in vitro which was conserved across the deuterostome phyla. We also identified a EψψxxxψM(D/E)QAIxxAIxYLGA(D/E)xL motif conserved among human Ikaros, Aiolos, Helios and subsets of chordate proteins, and motifs that are specific to subsets of vertebrate family members. Some of these motifs are targets of mutations in human patients. Finally we show that the atypical family member Pegasus emerged only in vertebrates, which is consistent with its function in bone. Our data provide a novel evolutionary perspective for Ikaros family proteins and suggest that they have conserved regulatory functions across deuterostomes.

Treatment of Lymphoid and Myeloid Malignancies by Immunomodulatory Drugs

Thalidomide and its derivatives (lenalidomide, pomalidomide, avadomide, iberdomide hydrochoride, CC-885 and CC-90009) form the family of immunomodulatory drugs (IMiDs). Lenalidomide (CC5013, Revlimid®) was approved by the US FDA and the EMA for the treatment of multiple myeloma (MM) patients, low or intermediate-1 risk transfusion-dependent myelodysplastic syndrome (MDS) with chromosome 5q deletion [del(5q)] and relapsed and/or refractory mantle cell lymphoma following bortezomib. Lenalidomide has also been studied in clinical trials and has shown promising activity in chronic lymphocytic leukemia (CLL) and non-Hodgkin lymphoma (NHL). Lenalidomide has anti-inflammatory effects and inhibits angiogenesis. Pomalidomide (CC4047, Imnovid® [EU], Pomalyst® [USA]) was approved for advanced MM insensitive to bortezomib and lenalidomide. Other IMiDs are in phases 1 and 2 of clinical trials. Cereblon (CRBN) seems to have an important role in IMiDs action in both lymphoid and myeloid hematological malignancies. Cereblon acts as the substrate receptor of a cullin-4 really interesting new gene (RING) E3 ubiquitin ligase CRL4CRBN. This E3 ubiquitin ligase in the absence of lenalidomide ubiquitinates CRBN itself and the other components of CRL4CRBN complex. Presence of lenalidomide changes specificity of CRL4CRBN which ubiquitinates two transcription factors, IKZF1 (Ikaros) and IKZF3 (Aiolos), and casein kinase 1α (CK1α) and marks them for degradation in proteasomes. Both these transcription factors (IKZF1 and IKZF3) stimulate proliferation of MM cells and inhibit T cells. Low CRBN level was connected with insensitivity of MM cells to lenalidomide. Lenalidomide decreases expression of protein argonaute-2, which binds to cereblon. Argonaute-2 seems to be an important drug target against IMiDs resistance in MM cells. Lenalidomide decreases also basigin and monocarboxylate transporter 1 in MM cells. MM cells with low expression of Ikaros, Aiolos and basigin are more sensitive to lenalidomide treatment. The CK1α gene (CSNK1A1) is located on 5q32 in commonly deleted region (CDR) in del(5q) MDS. Inhibition of CK1α sensitizes del(5q) MDS cells to lenalidomide. CK1α mediates also survival of malignant plasma cells in MM. Though, inhibition of CK1α is a potential novel therapy not only in del(5q) MDS but also in MM. High level of full length CRBN mRNA in mononuclear cells of bone marrow and of peripheral blood seems to be necessary for successful therapy of del(5q) MDS with lenalidomide. While transfusion independence (TI) after lenalidomide treatment is more than 60% in MDS patients with del(5q), only 25% TI and substantially shorter duration of response with occurrence of neutropenia and thrombocytopenia were achieved in lower risk MDS patients with normal karyotype treated with lenalidomide. Shortage of the biomarkers for lenalidomide response in these MDS patients is the main problem up to now.

The Role of the Ikaros Family of Transcription Factors in Regulatory T cell Development and Function

Regulatory T (Treg) cells are a subset of immune cells that maintain homeostasis by promoting immune tolerance and suppressing the immune response via a variety of mechanisms such as secreting cytokines, killing reactive immune cells, and inducing anergy. Dysfunction of Treg cells has been implicated in inflammatory diseases such as autoimmunity and transplant rejection. Conversely, too many or hyperresponsive Treg cells has been observed in cancer and chronic infections. Treg cells have proven to be difficult to study as there are no definitive Treg surface markers. Additionally, Tregs can gain pro-inflammatory phenotype depending on stimuli. In this commentary, we discuss the expression and function of members of the Ikaros family of transcription factors during Treg cell development and activation.

The Ikaros family of zinc-finger proteins

Ikaros represents a zinc-finger protein family important for lymphocyte development and certain other physiological processes. The number of family members is large, with alternative splicing producing various additional isoforms from each of the five homologous genes in the family. The functional forms of Ikaros proteins could be even more diverse due to protein–protein interactions readily established between family members. Emerging evidence suggests that targeting Ikaros proteins is feasible and effective in therapeutic applications, although the exact roles of Ikaros proteins remain elusive within the intricate regulatory networks in which they are involved. In this review we collect existing knowledge as to the functions, regulatory pathways, and molecular mechanisms of this family of proteins in an attempt to gain a better understanding through the comparison of activities and interactions among family members.