S. RAMBOZ1, K.R. WALKER1, K.COX1, M.BANSAL1, M. KWAN1, J.BELTRAN1, A.GHAVAMI1, L.PARK2, S.KWAK3, D.HOWLAN3
1PsychoGenics, Inc., Paramus, NJ, USA; 2CHDI Foundation, Los Angeles, CA, USA;
3CHDI Foundation, Princeton, NJ, USA;
The z_Q175 knock-in mouse line (CHDI-81003003) with an expanded CAG repeat size of approximately 190 has been the subject of extensive biochemical and behavioral analysis. In recent years, a new knock-in mouse line denoted z_Q175DN (CHDI-81003019) has been generated by CHDI by the genetic excision of the neomycin resistance cassette that was present in the 5’ sequence flanking the HTT locus of the z_Q175 line. Molecular studies at PsychoGenics and other organizations have demonstrated that the excision of the neomycin resistance cassette results in a modest but significant increase in mutant HTT mRNA levels, and a trend towards an increase in soluble mHTT protein and extranuclear inclusions. The effect of these changes in steady state mHTT mRNA on the behavioral phenotype has to date not been evaluated.
Utilizing PsychoGenics proprietary cubes platform technologies: PhenoCube®, Smart Cube® and Neurocube® as well as standard behavioral testing paradigms including Open Field and Tapered Balance Beam, the effect of excision of the neomycin resistance cassette on behavioral phenotype in heterozygous z_Q175 and z_Q175DN mice was examined at 2, 4, 6 and 10 months of age.
In comparison to the z_Q175 line, a significant decrease in bodyweight was detected in the heterozygous z_Q175DN mice compared to their wild-type littermates from 38 weeks of age onwards. Smart Cube® and Neurocube® testing did not reveal any significant effect of the excision of the neomycin resistance cassette at any age analyzed. However, differences in behavioral features between z_Q175 and z_Q175DN mice were detected as early as 2mths of age utilizing the PhenoCube® platform. As expected, activity was significantly reduced in both the heterozygous z_Q175 and z_Q175DN mice compared to their wild-type littermates beginning at 4months of age, however, no difference was observed between the two lines at any age. Tapered balance beam testing revealed deficits in coordination at 43 weeks of age in z_Q175 mice that were absent in the z_Q175DN line.