A critical window of CAG repeat-length correlates with phenotype severity in the R6/2 mouse model of Huntington's disease

Cummings DM, Alaghband Y, Hickey MA, Joshi PR, Hong SC, Zhu C, Ando TK, Andre VM, Cepeda C, Watson JB, Levine MS. A critical window of CAG repeat-length correlates with phenotype severity in the R6/2 mouse model of Huntington's disease. J Neurophysiol 107: 677-691, 2012. First published November 9, 2011; doi:10.1152/jn.00762.2011.-The R6/2 mouse is the most frequently used model for experimental and preclinical drug trials in Huntington's disease (HD). When the R6/2 mouse was first developed, it carried exon 1 of the huntingtin gene with similar to 150 cytosine-adenine-guanine (CAG) repeats. The model presented with a rapid and aggressive phenotype that shared many features with the human condition and was particularly similar to juvenile HD. However, instability in the CAG repeat length due to different breeding practices has led to both decreases and increases in average CAG repeat lengths among colonies. Given the inverse relationship in human HD between CAG repeat length and age at onset and to a degree, the direct relationship with severity of disease, we have investigated the effect of altered CAG repeat length. Four lines, carrying similar to 110, similar to 160, similar to 210, and similar to 310 CAG repeats, were examined using a battery of tests designed to assess the basic R6/2 phenotype. These included electrophysiological properties of striatal medium-sized spiny neurons, motor activity, inclusion formation, and protein expression. The results showed an unpredicted, inverted "U-shaped" relationship between CAG repeat length and phenotype; increasing the CAG repeat length from 110 to 160 exacerbated the R6/2 phenotype, whereas further increases to 210 and 310 CAG repeats greatly ameliorated the phenotype. These findings demonstrate that the expected relationship between CAG repeat length and disease severity observed in humans is lost in the R6/2 mouse model and highlight the importance of CAG repeat-length determination in preclinical drug trials that use this model.