The zinc finger of the cerebellum gene (ZIC) discovered in Drosophila melanogaster (odd-paired) has five homologs in Xenopus, chicken, mice, and humans, and seven in zebrafish. This pattern of gene copy expansion is accompanied by a divergence in gene and protein structure, suggesting that Zic family members share some, but not all, functions. ZIC genes are implicated in neuroectodermal development and neural crest cell induction. All share conserved regions encoding zinc finger domains, however their heterogeneity and specification remain unexplained. In this review, the evolution, structure, and expression patterns of the ZIC homologs are described; specific functions attributable to individual family members are supported. A review of data from functional studies in Xenopus and murine models suggest that ZIC genes encode multifunctional proteins operating in a context-specific manner to drive critical events during embryogenesis. The identification of ZIC mutations in congenital syndromes highlights the relevance of these genes in human development.

The Zic transcription factors play critical roles during embryonic development. Mutations in the ZIC2 gene are associated with human holoprosencephaly, but the etiology is still unclear. Here, we report a novel function for ZIC2 as a regulator of ␤-catenin⅐TCF4-mediated transcription. We show that ZIC2 can bind directly to the DNA-binding high mobility group box of TCF4 via its zinc finger domain and inhibit the transcriptional activity of the ␤-catenin⅐TCF4 complex. However, the binding of TCF4 to DNA was not affected by ZIC2. Zic2 RNA injection completely inhibited ␤-catenin-induced axis duplication in Xenopus embryos and strongly blocked the ability of ␤-catenin to induce expression of known Wnt targets in animal caps. Moreover, Zic2 knockdown in transgenic Xenopus Wnt reporter embryos led to ectopic Wnt signaling activity mainly at the midbrain-hindbrain boundary. Together, our results demonstrate a previously unknown role for ZIC2 as a transcriptional regulator of the ␤-catenin⅐TCF4 complex.

The ZIC2 transcription factor is one of the genes most commonly mutated in Holoprosencephaly (HPE) probands. Studies in cultured cell lines and mice have shown a loss of ZIC2 function is the pathogenic mechanism but the molecular details of this ZIC2 requirement remain elusive. HPE arises when signals that direct morphological and fate changes in the developing brain and facial primordia are not sent or received. One critical signal is sent from the prechordal plate (PrCP) which develops beneath the ventral forebrain. An intact NODAL signal transduction pathway and functional ZIC2 are both required for PrCP establishment. We now show that ZIC2 acts downstream of the NODAL signal during PrCP development. ZIC2 physically interacts with SMAD2 and SMAD3, the receptor activated proteins that control transcription in a NODAL dependent manner. Together SMAD3 and ZIC2 regulate FOXA2 transcription in cultured cells and Zic2 also controls the foxA2 expression during Xenopus development. Variant forms of the ZIC2 protein, associated with HPE in man or mouse, are deficient in their ability to influence SMAD-dependent transcription. These findings reveal a new mechanism of NODAL signal transduction in the mammalian node and provide the first molecular explanation of how ZIC2 loss-of-function precipitates HPE.

In the last decade, the amount of investigations on the involvement of ZIC genes in the cancer field have exponentially expanded. In most cancer types, promoter methylation leads to silenced ZIC family members, but specific subsets of patients clearly show increased expression of one or head-to-head located ZIC genes in the respective tumor tissue. It is unclear at this stage how these transcription factors contribute to tumorigenesis, but the potential implications in pathways that are most frequently mutated in cancer such as the canonical Wnt, TGF-beta, and STAT-3 pathway are evident. By exploring well-established developmental models, researchers were able to position ZIC genes not only as classical transcription factors but also as cofactors of chromatin remodeling complexes that are crucial for maintenance of the cell but also during differentiation and maturation of ZIC-expressing tissues in vivo. The translation of this obtained evidence to the cancer field will be challenging but will indisputably lead to a better understanding how the factors can contribute to the tumor development in the given subsets of patients.