We have previously demonstrated amelioration of Huntington's disease (HD)-related phenotypes in R6/2
transgenic mice in response to treatment with the novel histone deacetylase (HDAC) inhibitor 4b. Here we
have measured the selectivity profiles of 4b and related compounds against class I and class II HDACs and
have tested their ability to restore altered expression of genes related to HD pathology in mice and to rescue
disease effects in cell culture and Drosophila models of HD. R6/2 transgenic and wild-type (wt) mice received
daily injections of HDAC inhibitors for 3 days followed by real-time PCR analysis to detect expression differences
for 13 HD-related genes. We find that HDACi 4b and 136, two compounds showing high potency for
inhibiting HDAC3 were most effective in reversing the expression of genes relevant to HD, including
Ppp1r1b, which encodes DARPP-32, a marker for medium spiny striatal neurons. In contrast, compounds targeting
HDAC1 were less effective at correcting gene expression abnormalities in R6/2 transgenic mice, but did
cause significant increases in the expression of selected genes. An additional panel of 4b-related compounds
was tested in a Drosophila model of HD and in STHdhQ111 striatal cells to further distinguish HDAC selectivity.
Significant improvement in huntingtin-elicited Drosophila eye neurodegeneration in the fly was observed in
response to treatment with compounds targeting human HDAC1 and/or HDAC3. In STHdhQ111 striatal cells,
the ability of HDAC inhibitors to improve huntingtin-elicited metabolic deficits correlated with the potency
at inhibiting HDAC1 and HDAC3, although the IC50 values for HDAC1 inhibition were typically 10-fold higher
than for inhibition of HDAC3. Assessment of HDAC protein localization in brain tissue by Western blot analysis
revealed accumulation of HDAC1 and HDAC3 in the nucleus of HD transgenic mice compared to wt mice,
with a concurrent decrease in cytoplasmic localization, suggesting that these HDACs contribute to a repressive
chromatin environment in HD. No differences were detected in the localization of HDAC2, HDAC4 or
HDAC7. These results suggest that inhibition of HDACs 1 and 3 can relieve HD-like phenotypes in model systems
and that HDAC inhibitors targeting these isotypes might show therapeutic benefit in human HD.