Functional characterization of PTCHD1, a risk gene for autism spectrum disorders and intellectual disabilities

Autism spectrum disorders (ASD) are lifelong severe disabling conditions affecting around 1-2% of the global population (Elsabbagh et al., 2012; Baxter et al., 2015; Christensen et al., 2016). In the past decade, improvements in whole genome sequencing methods allowed the identification of increasing numbers of risk genes associated with ASD. Studies suggest that risk genes converge in several common molecular, metabolic, or circuit pathways (de la Torre-Ubieta et al., 2016). Despite these recent advances in the biological understanding of ASD, no efficient treatments have been found yet. Therefore, identifying the function of individual genes leading to converging alterations is a major challenge to define mechanism based stratification of the disorder.
The goal of my study was to explore the functions of the Patched domain containing protein 1, a poorly understood risk gene strongly associated with ASD and intellectual disability (ID) (Noor et al., 2010; Chaudhry et al., 2015). PTCHD1 appears to be a highly penetrant mutation with around 45% of affected individuals having ASD and/or ID (Chaudhry et al., 2015). It is estimated that mutations in the PTCHD1 locus occur in approximately 1% of individuals with ASD (Noor et al., 2010). Thus, compared to other genetic risk factors, PTCHD1 mutation appear to be a common and very penetrant alteration associated with ASD. To date the Ptchd1 protein has remained largely uncharacterized, therefore, there is a lack of insights into how PTCHD1 mutations may lead to ASD related phenotypes. The gene encodes for a transmembrane protein that shares a sterol-sensing domain with the Sonic hedgehog (Shh) receptor Patched (Ptch1) and Niemann-Pick disease, type C1 (NPC1), a cholesterol transport protein. Based on this sequence similarity and transcriptional reporter assays in cell lines, it has been hypothesized that Ptchd1 may contribute to Shh signaling (Noor et al., 2010).
Here I report, using a mouse knock-out model and biochemical assays, that Ptchd1 removal has no effect on Shh dependent neuronal proliferation, in addition, Shh does not show any binding to Ptchd1 in vitro. In an unbiased search for Ptchd1 interacting proteins, I recovered three components of the retromer complex involved in regulating dendritic protein trafficking between endosomal compartments and the plasma membrane (Choy et al., 2014): Sorting-nexin 27, VPS26B and VPS35. In addition, several postsynaptic density proteins were recovered: Dlg1,2,3,4 Magi1,3, Lin7. Considering a potential synaptic role for Ptchd1, we performed electrophysiological recordings on Ptchd1 knock-out animals in granule cells of the dentate gyrus, a cell population where Ptchd1 is highly enriched. We observed that loss of Ptchd1 results in a disruption of excitatory/inhibitory balance in the mouse hippocampus. Finally, Ptchd1 KO animals also showed deficits in hippocampus-related behavioral tasks.
Thereby, my study provides evidence that Ptchd1 loss-of-function experiments do not support a role in sonic-hedgehog-dependent signaling but reveal a profound disruption of synaptic transmission in the mouse dentate gyrus and support an association of Ptchd1 with dendritic trafficking complexes and synaptic scaffolding proteins.