FUNCTIONAL ANALYSIS OF THE AUTISM AND INTELLECTUAL DISABILITY GENE PTCHD1 REVEALS HEDGEHOG RECEPTOR-LIKE FUNCTIONS AND PDZ-BINDING DOMAIN-SPECIFIC REGULATION OF CNTNAP1 AND NLGN1

Background Genomic copy number variants (CNVs) have identified genes associated with autism spectrum disorder (ASD) and intellectual disability (ID). Recently, we reported on 23 individuals in 17 families with loss CNVs or frame-shifting indels disrupting PTCHD1. However, little is known for the function of PTCHD1. Objectives PTCHD1 , an X-linked gene for ASD and/or (ID), encodes a protein with homology to the sonic hedgehog (Shh) receptors PTCH1 and 2, and has been reported to show similar Gli repression activity. However, little else is known about the function of PTCHD1 or the pathway (s) that are perturbed by PTCHD1 mutations leading to ASD/ID. Design/method We looked at the effect of over-expression of PTCHD1 on transcription levels of neuronal and synapse-related genes. We also included a truncated construct lacking the C-terminal four amino acids, Ile-Thr-Thr-Val (ITTV) of PTCHD1 , which is predicted to interact with the PDZ domains of proteins. We tested the transcription of Shh and its putative receptor, Ptchd1 , and Smoothened (Smo) , from mouse embryonic and postnatal brains. We also analysed the expression of PTCHD1 and Smo in primary cilia. Results Transcriptional analysis showed Shh, Ptch1, and Smo expressed in developing mouse embryonic and postnatal hippocampus, but with gradually decreasing levels, whereas Ptchd1 levels steadily increase, suggesting that Ptchd1 may be more relevant in later embryonic and postnatal neurodevelopment. Preliminary results indicate that PTCHD1 is able to localise to the cilia. A significant increase in level of transcription of NLGN1 and CNTNAP1 that is dependent on the C-terminal four amino acids of PTCHD1 suggests either a regulatory or a downstream effect of PTCHD1 on these genes via a PDZ-domain containing protein. Positive PTCHD1 immunolabeling was visible in the hippocampal neurons. Preliminary results also suggest localization of PTCHD1 in cilia. Conclusions We report here a potential role for the PDZ binding domain in PTCHD1 in regulating expression of Nlgn1 and Cntnap1. PDZ domains are found in many eukaryotic and bacterial proteins, and in particular many proteins that are important for synaptic functioning. We speculate that PTCHD1 localization to primary cilia in hippocampal neurons could inhibit the Hh pathway by excluding SMO, allowing cilia to function as Shh sensors, in a manner similar to that of PTCH1, during neurodevelopment. The PTCHD1 gene has a role in a neurobiological pathway that delivers information to cells during brain development and CNVs and mutations may disrupt crucial developmental processes, contributing to the onset of autism.