br The HDC knockout mouse
The HDC knockout mouse as a pathophysiological model of TS The implication of a hypomorphic allele of the Hdc gene as a rare cause of TS (Ercan-Sencicek et al., 2010) created a rare opportunity for the generation of a pathophysiologically grounded model of the disorder. Modeling pathophysiology of TS, like other genetically complex neuropsychiatric diseases, has been challenging (Pittenger, 2014). Even the largest GWAS studies reported to date have produced no confirmed disease-associated JANEX-1 (Fernandez et al., 2015, Scharf et al, 2013); and common disease-associated alleles tend to be of small effect size and therefore of limited utility in grounding informative animal models. Though it is rare, the Hdc mutation is of large effect size; every known carrier has TS. Furthermore, it has a well-defined biochemical effect – abrogation of the biosynthetic capacity of the enzyme – that can readily be assayed in vitro or in vivo. Capitalizing on these characteristics, our laboratory has focused on the Hdc knockout mouse as a potentially informative model of TS with unique etiologic validity (Castellan Baldan et al., 2014). This knockout mouse was developed over a decade ago (Ohtsu et al., 2001) and has been studied in a variety of contexts (Ohtsu et al, 2001, Abe et al., 2004, Acevedo et al., 2006a, Acevedo et al., 2006b, Anaclet et al, 2009, Brabant et al, 2007, Chepkova et al, 2012, Dere et al, 2004, Dere et al., 2003, Fitzsimons and et al, 2001, Gong et al, 2010, Gong et al., 2010, Kubota et al, 2002, Lin et al, 2008, Liu et al, 2007, Parmentier et al, 2002, Schneider et al., 2014), but its potential as a model of TS did not become clear until the genetic discoveries described above (Ercan-Sencicek et al., 2010, Fernandez et al., 2012). Hdc knockout mice, backcrossed extensively onto C57Bl/6, have undetectable brain HA (Castellan Baldan et al., 2014). Past studies have found no marked alterations in whole-tissue DA or its metabolites in these mice (Dere et al., 2003, Kubota et al, 2002); however, they have elevated DA turnover (Dere et al., 2003), and in vivo microdialysis reveals modestly but significantly elevated extracellular dopamine (DA) in the striatum (Castellan Baldan et al, 2014, Rapanelli et al, 2014), the primary input nucleus of the basal ganglia and a structure repeatedly implicated in the pathophysiology of TS (Williams et al., 2013). The knockout mice have been shown to be more active after either acute or chronic treatment with the psychostimulant methamphetamine (Kubota et al., 2002). HDC knockout mice do not exhibit detectable tic-like movements at baseline. However, when challenged with either high-dose psychostimulants or acute stress they develop repetitive purposeless movements – stereotypy or excess grooming – that may recapitulate the tics that are pathognomonic of TS (Castellan Baldan et al, 2014, Xu et al., 2015). Knockout mice also have a deficit in prepulse inhibition, a measure of sensorimotor gating that is impaired in patients with tics – including, importantly, the carriers of the Hdc mutation (Castellan Baldan et al., 2014). No animal model can recapitulate all aspects of the complex phenomenology of TS (Pittenger, 2014) – the lack of spontaneous tics is a particular point of divergence from what is observed in patients. Nevertheless, these behavioral phenotypes suggest that the Hdc knockout mouse is successfully reproducing key aspects of the disorder and is therefore a valid model for further investigations into pathophysiology. Abnormalities of dopaminergic regulation of the basal ganglia have long been implicated in TS (Williams et al., 2013, Denys et al, 2013); as noted above, the Hdc knockout mouse has elevated extracellular DA, as measured by microdialysis, in the striatum (Castellan Baldan et al, 2014, Rapanelli et al, 2014). This motivated an investigation of dopamine receptors throughout the basal ganglia in TS patients who carry the Hdc mutation, using PET imaging with the D2R/D3R agonist ligand 11C–PHNO. Receptor levels were not detectably altered in the striatum, where PHNO binding primarily reflects D2R levels, but they were elevated in the substantia nigra, where D3R predominates. Importantly, this increased receptor binding is also seen in the Hdc knockout mouse, assayed ex vivo using the D2R/D3R ligand raclopride (Castellan Baldan et al., 2014). Molecular signaling pathways known to be regulated by DA in the striatum are complexly dysregulated in the Hdc KO mice (Rapanelli et al., 2014). Further characterization of these abnormalities has the potential to identify new candidate mechanisms for TS and new molecular targets for intervention.