Heterozygous GLI2 loss of function mutations in humans have been reported in holoprosencephaly (HPE), HPE-like phenotypes associated with pituitary anomalies and combined pituitary hormone deficiency with or without other extra-pituitary findings.
Our study provides structural insights of the probable pathogenesis mechanism of two TGIF1-related HPE cases, and evidences for the roles of P192 and R219 in HD folding.
Genetic testing included chromosomal microarray, Sanger sequencing for SHH, ZIC2, SIX3, and TGIF, and whole-exome sequencing (WES) of 10 trios.ResultsSemilobar HPE was the most common subtype of HPE, seen in 50% of the participants.
Zic2-binding to the cis-regulatory element near the Tgif1 promoter may be involved in the mechanism underlying forebrain development and incidences of HPE.
Although, some developmental processes that are regulated by TGIFs may be Nodal-dependent, it appears that the forebrain patterning defects and HPE in Tgif mutant mouse embryos is primarily due to altered signaling via the Shh pathway.
Three of the remaining families had (likely) pathogenic variants in the JBTS gene C5orf42, and one patient had a novel de novo frameshift variant in SHH known to cause autosomal dominant holoprosencephaly.
TGIF1 is a transcriptional repressor that limits the output of the Transforming Growth Factor ß (TGFß)/Nodal signaling pathway, and HPE in patients with TGIF1 variants has been suggested to be due to increased Nodal signaling.
Although normally indistinguishable from wild-type littermates, we demonstrate that mice with single-allele Gli2 mutations exhibit increased penetrance and severity of HPE in response to low-dose teratogen exposure.
Sonic Hedgehog (SHH) and GLI2, an obligatory mediator of SHH signal transduction, are holoprosencephaly (HPE)-associated genes essential in pituitary formation.
However, the family presented here is unique as none of the three identified individuals with a GLI2 deletion showed any typical signs of holoprosencephaly, whereas all patients reported so far were referred for genetic testing because at least one member exhibited holoprosencephaly and related features.
Although mutations in the sonic hedgehog gene SHH and more than 10 other genes are known to cause holoprosencephaly, many patients remain without a molecular diagnosis.
Pituitary insufficiency other than isolated diabetes insipidus is a rare finding of HPE, and occurs most frequently in patients with GLI2 mutations (the phenotype of which typically does not include frank neuroanatomic anomalies such as HPE); ours is the only described patient with a ZIC2 mutation and both anterior and posterior pituitary dysfunction.
Genomic GLI2 aberrations that mainly result in truncated proteins have been reported to cause holoprosencephaly or holoprosencephaly-like features, sometimes associated with hypopituitarism.
Heterozygous nonsense GLI2 mutations have been reported in patients with isolated or combined pituitary hormone deficiency (CPHD), with or without holoprosencephaly.
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.