Epigenetic approaches and methods in developmental toxicology: role of HDAC inhibition in teratogenic events

From transient transcriptome responses to disturbed neurodevelopment: role of histone acetylation and methylation as epigenetic switch between reversible and irreversible drug effects

The superordinate principles governing the transcriptome response of differentiating cells exposed to drugs are still unclear. Often, it is assumed that toxicogenomics data reflect the immediate mode of action (MoA) of drugs. Alternatively, transcriptome changes could describe altered differentiation states as indirect consequence of drug exposure. We used here the developmental toxicants valproate and trichostatin A to address this question. Neurally differentiating human embryonic stem cells were treated for 6 days. Histone acetylation (primary MoA) increased quickly and returned to baseline after 48 h. Histone H3 lysine methylation at the promoter of the neurodevelopmental regulators PAX6 or OTX2 was increasingly altered over time. Methylation changes remained persistent and correlated with neurodevelopmental defects and with effects on PAX6 gene expression, also when the drug was washed out after 3-4 days. We hypothesized that drug exposures altering only acetylation would lead to reversible transcriptome changes (indicating MoA), and challenges that altered methylation would lead to irreversible developmental disturbances. Data from pulse-chase experiments corroborated this assumption. Short drug treatment triggered reversible transcriptome changes; longer exposure disrupted neurodevelopment. The disturbed differentiation was reflected by an altered transcriptome pattern, and the observed changes were similar when the drug was washed out during the last 48 h. We conclude that transcriptome data after prolonged chemical stress of differentiating cells mainly reflect the altered developmental stage of the model system and not the drug MoA. We suggest that brief exposures, followed by immediate analysis, are more suitable for information on immediate drug responses and the toxicity MoA.

Inhibition of histone deacetylases 1 and 3 protects injured retinal ganglion cells

PURPOSE

Thy-1 is a marker of retinal ganglion cell (RGC) differentiation. Optic nerve injury triggers reduction of Thy-1 promoter activation followed by retinal ganglion cell (RGC) death. This study determined whether MS-275, an inhibitor of the histone deacetylases 1 and 3, can inhibit these changes.

METHODS

Mice expressing cyan fluorescent protein (CFP) under control of the Thy-1 promoter received MS-275 (subcutaneous) or vehicle three times per week starting 1 week before optic nerve crush and continuing for 6 weeks. The same retinal area was imaged using the blue-light confocal scanning laser ophthalmoscope before and after optic nerve crush every week, and fluorescent spots were counted manually. The eyes were then processed for histopathologic analysis.

RESULTS

The mean proportions of fluorescent retinal neurons remaining in the vehicle group following optic nerve crush were 36 ± 8, 18 ± 6, 13 ± 10, 12 ± 4, 13 ± 5, and 13 ± 5% at weeks 1 through 6, respectively (n = 6). In contrast, the mean proportions of fluorescent retinal neurons remaining in the group treated with MS-275 were 59 ± 19, 39 ± 11, 34 ± 12, 33 ± 15, 32 ± 13, and 27 ± 15% at weeks 1 through 6, respectively (n = 7, P < 0.05 at weeks 1 through 5). Rate analysis showed that MS-275 slowed the rate of loss during the first 2 weeks by 23% (P < 0.05) and subsequently was similar. Histopathologic analysis revealed 27 ± 13% greater ganglion cell layer (GCL) neurons in the eyes from mice that received MS-275 treatment (P < 0.02).

CONCLUSIONS

These results indicate that treatment with MS-275 protects against the loss of RGC differentiation and promotes RGC survival following optic nerve injury.