Curation HPE5 ZIC2

Screened 509 unrelated HPE patients with normal chromosomes; 16 affected individuals from 15 unrelated families had ZIC2 mutations. Seven mutations were frameshift/null alleles, and a recurrent C-terminal alanine-tract expansion occurred in seven patients from six families, including de novo and paternal mosaic cases.

Compilation of 83 HPE-specific ZIC2 variants absent from controls showed recurrent predicted loss-of-function, including frequent frameshift/truncating variants (>55%), zinc-finger missense clustering at conserved DNA-binding residues, and COOH-terminal/polyalanine variants predicted or shown to impair ZIC2 function.

In vitro ZIC2 reporter assays showed that HPE-associated mutations altered transcriptional activation; the naturally occurring 25-alanine expansion reduced apoE promoter transactivation to ~5% of wild-type activity.

ChIP, in vitro DNA‑binding, and reporter assays showed that ZIC2 directly binds cis‑regulatory elements upstream of TGIF1 (another HPE gene) and activates its transcription, with higher affinity than GLI proteins. This identifies TGIF1 as a direct downstream target of ZIC2, linking ZIC2‑mediated transcriptional regulation to forebrain development and HPE.

C-terminal deletion/truncation and alanine-tract length changes altered ZIC2-mediated transcriptional activation in a promoter-specific manner; the C-terminal contains activation/repression domains and the alanine tract modulates DNA binding.

In non‑patient cell models, genome‑edited mouse embryonic stem cells differentiated toward neural progenitors showed that ZIC2 directly regulates neural patterning genes by binding and activating distal enhancers. Loss of ZIC2 disrupted chromatin accessibility and WNT‑related regulatory programs, demonstrating a cell‑autonomous regulatory role for ZIC2 during early neural differentiation.

Gene-level evidence: A mouse model with reduced Zic2 expression shows delayed neurulation, leading to holoprosencephaly and spina bifida, along with impaired roof plate and neural crest development. These findings demonstrate that proper Zic2 dosage is critical for early neural tube patterning and timing, providing a mechanistic link to human HPE.

The study used homozygous Zic2^Ku missense mutant mice, which lack ZIC2 DNA‑binding and transcriptional activity, and showed that Zic2 loss causes holoprosencephaly through an early organizer defect during mid‑gastrulation. This leads to impaired prechordal plate development and forebrain midline defects independent of Sonic hedgehog signaling.