PTCHD1 interacts with the SNARE-associated protein SNAPIN in vitro via its first exoplasmic loop

Patched domain-containing 1 ( PTCHD1 ) is a susceptibility gene for autism spectrum disorder (ASD) and intellectual disability (ID). PTCHD1 is predicted to encode a multi-pass protein consisting of 12 transmembrane domains, two large exoplasmic loops, and a C-terminal PDZ-binding domain. PTCHD1 contains sequence homology with the Hedgehog signaling pathway repressors Patched domain-containing-1 (PTCH1) and -2 (PTCH2), as well as the cholesterol trafficking protein Niemann-Pick disease type C1 (NPC1). Despite this homology, the functional role of PTCHD1 in brain development remains elusive. Studies have sought to characterize the function of PTCHD1 by elucidating its neural network of interacting proteins. Using a yeast two-hybrid assay, we have identified a novel interaction between the first exoplasmic loop of PTCHD1 and the SNARE-associated protein SNAPIN, which is implicated in synaptic vesicle exocytosis. Clinically-associated missense variants within this region of PTCHD1 did not disrupt SNAPIN binding, indicating that the pathoetiology of these variants is unrelated to this interaction. However, several of these missense variants exhibited pronounced retention within the endoplasmic reticulum, and impaired trafficking to the plasma membrane, indicating a possible mechanism of pathogenicity. These data yield insights regarding the possible role of PTCHD1 in neurodevelopment and neurotransmission, as well as suggest a pathoetiological mechanism for numerous clinical missense variants, providing a platform for subsequent diagnostic assays, and an avenue for possible therapeutic intervention.