Mutation of the PTCHD1 Gene, Which Encodes a Transmembrane Protein Expressed in Postsynaptic Dendritic Spines, Associated with Autism

Background: The PTCHD1gene (Patched Homology domain 1), located at Xp22.1, has been recently involved in studies describing genomic deletions and missense mutations in patients with autism and/or non-syndromic intellectual disability (ID).

Objectives: Considering that this gene represent a strong candidate for these neurodevelopmental disorders, we searched for mutations on genomic DNA of patients and families collected from the EuroMRX consortium cohort.  

Methods: The PTCHD1 gene was sequenced in 400 families with autism and/or ID. We also performed neurodevelopmental expression and neuronal subcellular localization studies in mouse brain:the neurodevelopmental expression profile of the mouse Ptchd1 gene was assessed in brain tissues at different developmental stages (in vivo analysis) and in primary neuronal cultures (in vitro). The neuronal subcellular localzation was performed by immunocytochemistry analyses on mature primary neuronal cultures using a custom PTCHD1 antibody, and by transfection experiments using GFP tagged PTCHD1.  

Results: We characterized a point deletion in the PTCHD1 gene coding sequence (c.2128delC, p.L710CfsX12), which would lead to the expression of a truncated form of the PTCHD1 protein lacking the last 180 amino-acids, in a French family with non-syndromic ID and autism. Little is known regarding the neurodevelopmental expression of the PTCHD1gene, as well as the neuronal function of the encoded protein. Immunocytochemistry analyses on mature primary neuronal cultures using a custom PTCHD1 antibody revealed that PTCHD1 transmembrane protein co-localized with postsynaptic inhibitory (Gephyrin) and excitatory (SHANK3) markers. We also showed that the C-terminal intracellular tail of PTCHD1 was essential for dendrite targeting. Interestingly, the protein sequence indicate that the last 4 amino-acids (ITTV) fit with a PDZ binding domain consensus sequence that is found in various postsynaptic proteins (NLGN, NMDAR subunits) for linking PDZ-domain containing proteins.     

Conclusions: We confirm the contribution of PTCHD1 gene mutations in X-linked ID and autism, therefore suggesting that this gene would has a crucial role in cognition and communication processes during the development of the central nervous system. We also indicate for the first time that PTCHD1 protein is present in both postsynaptic inhibitory and excitatory sites.