A Novel Homozygous Frameshift WDR81 Mutation associated with Microlissencephaly, Corpus Callosum Agenesis, and Pontocerebellar Hypoplasia

The "microcephalic hydrocephalus" paradox as a paradigm of altered neural stem cell biology

Characterized by enlarged brain ventricles, hydrocephalus is a common neurological disorder classically attributed to a primary defect in cerebrospinal fluid (CSF) homeostasis. Microcephaly ("small head") and hydrocephalus are typically viewed as two mutually exclusive phenomenon, since hydrocephalus is thought of as a fluid "plumbing" disorder leading to CSF accumulation, ventricular dilatation, and resultant macrocephaly. However, some cases of hydrocephalus can be associated with microcephaly. Recent work in the genomics of congenital hydrocephalus (CH) and an improved understanding of the tropism of certain viruses such as Zika and cytomegalovirus are beginning to shed light into the paradox "microcephalic hydrocephalus" by defining prenatal neural stem cells (NSC) as the spatiotemporal "scene of the crime." In some forms of CH and viral brain infections, impaired fetal NSC proliferation leads to decreased neurogenesis, cortical hypoplasia and impaired biomechanical interactions at the CSF-brain interface that collectively engender ventriculomegaly despite an overall and often striking decrease in head circumference. The coexistence of microcephaly and hydrocephalus suggests that these two phenotypes may overlap more than previously appreciated. Continued study of both conditions may be unexpectedly fertile ground for providing new insights into human NSC biology and our understanding of neurodevelopmental disorders.

Fetal brain arrest broadens the spectrum of WDR81-related developmental brain malformations

Fetal brain arrest is an extremely rare genetic disorder that was described in few patients and encompasses very unique findings of underdeveloped cerebral hemispheres in association with collapsed skull bones. Based on the recurrence among sibs, an autosomal recessive mode of inheritance was proposed; however, no causative gene was identified so far. Here, we report the identification of biallelic variants in the WDR81 gene in two unrelated families (4 patients) with fetal brain arrest including the originally described family and an additional new family. Two homozygous variants were identified: a new missense (c.1157 T > C, p.Val386Ala) and a previously described frameshift variant, c.4668_4669delAG (p.Gly1557AspfsTer16). We assessed the expression of WDR81 at the protein level by western blot analysis using primary skin fibroblast cultures established from the patient with the missense variant and noticed that WDR81 expression was significantly reduced in comparison to normal control confirming the pathogenicity of this variant. Our findings confirm the involvement of WDR81 in the pathogenesis of fetal brain arrest syndrome and suggest that fetal brain arrest represents the severe end of the spectrum phenotypes caused by pathogenic variants in WDR81. In addition, we reviewed the clinical and molecular data on WDR81-related disorders and phenotype/genotype correlations.