Loss of Tgif Function Causes Holoprosencephaly by Disrupting the Shh Signaling Pathway

Abstract

Holoprosencephaly (HPE) is a severe human genetic disease affecting craniofacial development, with an incidence of up to 1/250 human conceptions and 1.3 per 10,000 live births. Mutations in the Sonic Hedgehog (SHH) gene result in HPE in humans and mice, and the Shh pathway is targeted by other mutations that cause HPE. However, at least 12 loci are associated with HPE in humans, suggesting that defects in other pathways contribute to this disease. Although the TGIF1 (TG-interacting factor) gene maps to the HPE4 locus, and heterozygous loss of function TGIF1 mutations are associated with HPE, mouse models have not yet explained how loss of Tgif1 causes HPE. Using a conditional Tgif1 allele, we show that mouse embryos lacking both Tgif1 and the related Tgif2 have HPE-like phenotypes reminiscent of Shh null embryos. Eye and nasal field separation is defective, and forebrain patterning is disrupted in embryos lacking both Tgifs. Early anterior patterning is relatively normal, but expression of Shh is reduced in the forebrain, and Gli3 expression is up-regulated throughout the neural tube. Gli3 acts primarily as an antagonist of Shh function, and the introduction of a heterozygous Gli3 mutation into embryos lacking both Tgif genes partially rescues Shh signaling, nasal field separation, and HPE. Tgif1 and Tgif2 are transcriptional repressors that limit Transforming Growth Factor β/Nodal signaling, and we show that reducing Nodal signaling in embryos lacking both Tgifs reduces the severity of HPE and partially restores the output of Shh signaling. Together, these results support a model in which Tgif function limits Nodal signaling to maintain the appropriate output of the Shh pathway in the forebrain. These data show for the first time that Tgif1 mutation in mouse contributes to HPE pathogenesis and provide evidence that this is due to disruption of the Shh pathway.

Tgif1 Regulates Quiescence and Self-Renewal of Hematopoietic Stem Cells

“…It has previously been hypothesized that the closely related homeobox gene Tgif2 might compensate for the reduced activity of Tgif1 in certain tissues or developmental contexts, since it shares many of the functional attributes of Tgif1 (12). In addition, Tgif1 and Tgif2 clearly perform overlapping functions during early embryonic development in mice (12,13). Furthermore, our data also show that SLAM cells from Tgif1 Ϫ/Ϫ mice express significantly higher levels of Tgif2 than SLAM cells from wild-type mice (see Fig.…”

“…The majority of these mutations would cause a loss of protein function and are hypothesized to alter signaling by TGF-␤-related ligands (9)(10)(11). In mice, loss of both Tgif1 and Tgif2 is lethal, but epiblast-specific deletion of Tgif1 in combination with a null mutation in Tgif2 results in HPE, which is at least partly due to deregulation of Nodal signaling, suggesting that human TGIF1 mutations may cause HPE by affecting TGF-␤ signaling (12,13).…”

Retinoic acid signaling regulates development of the dorsal forebrain midline and the choroid plexus in the chick

“…This is achieved through the expression of specific signaling molecules in the dorsal forebrain, such as FGFs, BMPs and Wnts (Furuta et al, 1997;Grove et al, 1998;Crossley et al, 2001). In addition, transcription factors such as Six3, Tgif, Gli3 and Zic2, which are expressed by the RP, have also been implicated as regulators of dorsal forebrain development (Theil et al, 1999;Wallis et al, 1999;Warr et al, 2008;Taniguchi et al, 2012). Perturbation in the activity of these signaling molecules and transcription factors adversely affect the development of the dorso-medial forebrain-derived structures and may also lead to HPE.…”

Chronic Otitis Media: Microbial Environment During Viral Infection and Genetics of Host Susceptibility

“…275 OM mouse models have provided evidence of defects in the regulation of TGFβ signaling in the middle ear 71,73 , and prior studies have identified a link between HH signaling and the TGFβ pathway in holoprocencephaly and cancer. 263,264 Because of the connections between TGFβ signaling pathway and OM 71,73,95,96 and TGFβ and HH signaling pathways 263,264 , we investigated whether TGFβ regulated KIF7 expression. We also measured expression of other HH signaling pathway members, both upstream (PTCH1 and SUFU) and downstream (GLI1) of KIF7 activity.…”

“…KIF7 lies within the Sonic hedgehog (SHH) pathway, which has been shown to interact with the TGFβ pathway. 263,264 Because of these connections, we sought to investigate whether the TGFβ pathway could regulate KIF7.…”