The Identification of Increased DYRK1A Protein Levels in TS65DN Mice Guides the Targeted Adminstration of the Novel DYRK1A Inhibitor CX-4945

Down syndrome (DS) is caused by three copies of human chromosome 21 (Hsa21) and results in phenotypes including intellectual disability. Ts65Dn mice, the most extensively studied DS model, have three copies of ~50% of the genes on Hsa21 and display many phenotypes associated with DS, including cognitive deficits. DYRK1A is a dosage-sensitive gene found in three copies in humans with Trisomy 21 and in Ts65Dn mice and is involved in CNS development. Overexpression of DYRK1A is hypothesized to cause many of the cognitive and developmental deficits observed in DS and has been touted as a target for drug development in DS. Definitive evidence that excessive expression/activity of Dyrk1a directly contributes to specific phenotypes in DS mouse models is limited, and there is no direct evidence that verified pharmacological inhibition of Dyrk1a in vivo causes enduring improvement in DS cognitive phenotypes. In part, this reflects the remarkably limited knowledge of the temporal regulation of Dyrk1a expression and activity in different brain regions across development in DS mouse models. To establish the therapeutic potential of Dyrk1a inhibitors, the first aim of this study was to determine when and in what brain regions excessive Dyrk1a is evident and to identify developmental periods when elevated expression of Dyrk1a may be contributing to enduring aberrant functional development. This aim provided systematic quantification of Dyrk1a protein level at key postnatal (P) ages (P12, P15, P18, P24 P30, P42) in Ts65Dn mice, at ages of translational relevance to clinical applications in humans (birth, early adolescence, late adolescence, young adult). Western blot analysis showed that significant elevation of Dyrk1a with the largest effect sizes occurred in trisomic mice on P15. The second aim of this study was to test whether treating Ts65Dn with a novel Dyrk1a inhibitor (CX-4945) during the time of Dyrk1a elevation would rescue the behavioral and structural abnormalities observed. From P14-P18, Ts65Dn and euploid mice were treated with 75mg/kg CX-4945 or DMSO (vehicle) and tested on a homing task and locomotor activity in a novel arena on P17-P18, counterbalanced for order. At the cessation of treatment, hippocampal cell proliferation was assessed. While there was a lack of statistically significant improvements with CX-4945 treatment, there were modest effect sizes. In addition, several of the behavioral studies were significantly underpowered, making it difficult to conclusively ascertain the efficacy of CX-4945 on specific phenotypes. Nevertheless, this study demonstrates that Dyrk1a is dynamically expressed across development in mice, and suggests that consideration of the spatial and temporal expression of Dyrk1a may well be critical in the development of therapeutics for DS