Holoprosencephaly and ZIC2 microdeletions: novel clinical and epidemiological specificities delineated

Holoprosencephaly (HPE), the most common malformation of the human brain results from abnormal cleavage of the forebrain during the early embryonic developmental stages. The spectrum of malformations in HPE is wide, ranging from the classical cyclopia/proboscis to fairly asymptomatic forms [i.e. a single maxillary central incisor (SMCI)]. HPE may be caused by environmental or genetic factors. ZIC2 (13q32) was the second gene identified in which mutations cause HPE and recently a specific phenotype was ascribed to ZIC2-mutation HPE. Earlier, we reported a boy presenting HPE and deafness. Cytogenetic analyses were normal. Using array-comparative genomic hybridization (aCGH), we found a de novo 129 kb del(13)(q32) encompassing ZIC2 and ZIC5. There is no evidence for the involvement of ZIC5 in human diseases. We reviewed the literature for ZIC2-ZIC5 deletions and their involvement in neural tube defects (NTDs). Interestingly, we found evidence for a specific facial phenotype for ZIC2 gene deletion patients distinct from those with point mutations. In addition, based on the clinical data together with pathology, imaging and functional studies, we suggest an outline for a model explaining the genetic heterogeneity of ZIC2-ZIC5-associated NTDs and propose further studies for validation.