ABSENCE OF DISEASE PHENOTYPE AND INTERGENERATIONAL STABILITY OF THE CAG REPEAT IN TRANSGENIC MICE EXPRESSING THE HUMAN HUNTINGTON DISEASE TRANSCRIPT

The mutation underlying Huntington disease (HD) is CAG expansion in th e first exon of the HD gene, In order to investigate the role of CAG e xpansion in the pathogenesis of HD, we have produced transgenic mice c ontaining the full length human HD cDNA with 44 CAG repeats, By 1 year , these mice have no behavioral abnormalities and morphometric analysi s at 6 (one animal) and 9 (two animals) months age revealed no changes , Despite high levels of mRNA expression, there was no evidence of the HD gene product in any of these transgenic mice. In vitro transfectio n studies indicated that the inclusion of 120 bp of the 5' UTR in the cDNA construct and the presence of a frameshift mutation at nucleotide 2349 prevented expression of the HD cDNA. These findings suggest that the pathogenesis of I-ID is not mediated through DNA-protein interact ion and that presence of the RNA transcript with an expanded CAG repea t is insufficient to cause the disease, Rather, translation of the CAG is crucial for the pathogenesis of HD. In contrast to that seen in hu mans, the CAG repeat in these mice was remarkably stable in 97 meioses , This suggests that genomic sequences may play a critical role in inf luencing repeat instability.