Protocadherin-12 (PCDH12), a cell-adhesion protein belonging to the non-clustered protocadherin family, plays a crucial role in the establishment and regulation of neuronal connections and communication. Bi-allelic loss-of-function (LoF) variants in the PCDH12 gene have been associated with several neurodevelopmental disorders (NDDs) such as diencephalic-mesencephalic junction dysplasia syndrome, cerebral palsy, and cerebellar ataxia, often accompanied by ocular abnormalities. However, genotypes exhibit variable expressivity. Affected individuals sharing the same PCDH12 variant presenting differing phenotypic severities have posed major challenges towards identification of the underlying pathogenic mechanisms. Here, we report three affected individuals from two families, each harbouring non-truncating pathogenic missense variants in PCDH12 . The patients are compound heterozygous, with each individual carrying one extracellular [c.1742T>G (p.Val581Gly) and c.1861_2del/insCA (p.Ile621His)] and one intracellular variant [c.3370C>T (p.Arg1124Cys) and c.3445G>A (p.Asp1149Asn] on each allele. The children present with a range of phenotypes similar to those associated with LoF variants. One child exhibited microcephaly and seizures, while the two siblings displayed developmental delays and severe behavioral disorders. All three children experienced some degree of visual impairment. The missense variants provided new insights into the neurodevelopmental consequences of compromised PCDH12 function by distinguishing the specific consequences associated with dysfunction in the extracellular versus intracellular domains of PCDH12. All identified missense variants are predicted to be deleterious and destabilizing. The expression of PCDH12 in HEK293T and HeLa cells demonstrated that PCDH12 is expressed effectively, regardless of the presence of missense variants. However, the extracellular variants p.Val581Gly and p.Ile621His compromised the stability of PCDH12's homophilic adhesion. Additionally, we found evidence of an interaction between PCDH12 and the extracellular domain of the epilepsy-associated PCDH19 protein. PCDH12 extracellular missense variants also affect PCDH19 stability. Our study provides evidence that PCDH12 mediates both homophilic and heterophilic interactions. Our findings also highlight the importance of stable PCDH12-mediated adhesion, emphasizing the need to further study the functional consequences of PCDH12 missense variants on brain and visual system development.

Diencephalic-mesencephalic junction (DMJ) dysplasia is a rare congenital brain malformation characterized by a poorly defined junction between the diencephalon and mesencephalon, often associated with a butterfly-like contour of the midbrain on magnetic resonance imaging (MR). We report the case of a newborn female diagnosed prenatally with DMJ dysplasia who presented with severe ventriculomegaly, hydrocephalus, and oligohydramnios. Prenatal MRI at 32 weeks revealed a thickened interthalamic adhesion, an elongated midbrain with ventral cleft, aqueductal stenosis, and corpus callosum dysgenesis. Postnatal MRI confirmed these findings, along with the characteristic "butterfly" midbrain morphology. Genetic analysis revealed a pathogenic 11.9 Mb terminal deletion in the 6q25.3q27 region, encompassing candidate neurodevelopmental genes, such as DLL1, and a 3.8 Mb partial duplication in 22q13.31q13.33, of unknown significance. Parental genetic testing revealed a maternal balanced reciprocal translocation between chromosomes 6 and 22 (asymptomatic carrier), which was inherited in an unbalanced form by the proband. A ventriculoperitoneal shunt was placed within the first 48 h of life to manage hydrocephalus, with subsequent adjustments and revisions as needed. This case highlights the importance of advanced prenatal imaging and genetic testing in the diagnosis of complex brain malformations as well as the need for multidisciplinary management of rare congenital anomalies. Further research is essential to elucidate the underlying genetic mechanisms and improve the outcomes in patients with DMJ dysplasia.

Diencephalic-mesencephalic junction dysplasia syndrome is a rare neurogenetic disorder reported to be caused by variants in several genes. Phenotypic presentation is characterized by clinical findings including developmental delay, hypotonia, spasticity, and dyskinetic movements in combination with distinctive imaging features on brain magnetic resonance imaging (MRI). Whole exome sequencing was conducted to unveil the molecular etiology of patients presenting with neurological manifestations from two unrelated families. To the best of our knowledge, here we report the third family affected with diencephalic-mesencephalic junction dysplasia caused by a novel variant in GSX2 and two siblings with a PCDH12 variant exhibiting a less severe phenotype. The siblings with a PCDH12 variant were positioned at the milder end of the phenotypic spectrum. Although both exhibited a clinical phenotype resembling cerebral palsy, one showed partial fusion of the hypothalamus and mesencephalon, whereas MRI was unremarkable in the other. Biallelic GSX2 variants have been implicated in basal ganglia agenesis, and similarly, our patients had basal ganglia hypoplasia along with hypothalamic-mesencephalic fusion. Identifying variants associated with the syndrome in different genes will contribute to genotype-phenotype correlation.