Histone Deacetylase Blockers Show Potential in Treating FA, Study Finds

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by Steve Bryson, PhD |

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Compounds that blocked the activity of histone deacetylases — enzymes that suppress gene activity — increased the activity of the FXN gene, which is low in people with Friedreich’s ataxia (FA) and limits frataxin protein levels, a proof-of-concept study demonstrated.

Researchers used nerve cells derived from FA patients to screen for potential medicines and recommend this approach as an “excellent” preclinical model for work on FA therapy development.

The study, “Selected Histone Deacetylase Inhibitors Reverse the Frataxin Transcriptional Defect in a Novel Friedreich’s Ataxia Induced Pluripotent Stem Cell-Derived Neuronal Reporter System,” was published in the journal Frontiers in Neuroscience.

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FA is caused by a mutation in the FXN gene, which encodes frataxin, a protein essential for the proper functioning of energy-producing mitochondria, and is mainly expressed (produced) in tissues with a high metabolic rate.

The mutation effectively deactivates the FXN gene, leading to a severe reduction in frataxin levels. Targeting impaired activation of FXN is a potential therapeutic strategy to enhance frataxin expression and ease symptoms.

One method to identify potential medicines that increase FXN activity is to use human induced pluripotent stem cells (iPSCs) reprogrammed from cells isolated from FA patients. Then, these iPSCs can be further transformed into nerve cells (neurons), and compounds can be tested to determine whether they increase frataxin production.

This was the approach used by researchers based at the University of Alabama at Birmingham and evaluated for proof-of-concept.

Initial work isolated fibroblasts from FA patients, the most common cell type in connective tissue, and reprogrammed the fibroblasts into iPSCs. These cells were modified such that enhanced FXN gene activity and frataxin expression could be easily measured. Then, modified iPSCs were transformed into neuron precursor cells called neural progenitor cells (NPCs), as well as fully mature neurons.

To show that these patient-derived neurons can demonstrate FXN activity, the team exposed them to RG109, a known inducer of the FXN gene that inhibits the activity of histone deacetylases (again, enzymes that suppress gene activity). RG109 increased FXN activity; FXN messenger RNA (mRNA) levels, the molecule that carries the genetic instructions to make frataxin; and frataxin protein levels.

Next, the NPCs were exposed to 281 different compounds known to target processes involved in gene activity regulation. After two independent rounds of screening, 16 compounds were identified as candidates for further evaluation.

Among these compounds, five were shown to increase FXN activity at one or two days: entinostat, mocetinostat, cerdulatinib, CI994, and UF 010, all of which are experimental cancer therapies.

Dose-dependent analysis demonstrated that cerdulatinib, mocetinostat, and entinostat increased FXN activity at low doses, meaning they were more potent, while CI994 and UF 010 required higher concentrations to achieve the same level of activity and so were less potent.

After exposing cells at a high concentration for two days, none of these compounds showed overt toxic effects as measured by the release of LDH, a marker of cell damage.

A separate validation test showed elevated FXN activity in NPCs with a high dose of mocetinostat and entinostat and, to a lesser extent, with CI994 and UF 010 compared to vehicle control. Consistently, there was up to 2.5 times higher expression of mature frataxin protein. Cerdulatinib failed to increase FXN activity protein levels in this evaluation.

The team confirmed these results in two additional NPC cell lines derived from different FA patients.

Lastly, these compounds were tested in fully mature neurons created from FA patients. Here, mocetinostat, entinostat, and CI994 — all blocking the activity of histone deacetylases — increased FXN mRNA up to three times and frataxin protein levels up to 1.75 times.

“A patient cell-based reporter system utilizing endogenous [within cells] FXN expression as a readout represents an excellent model to evaluate any pre-clinical approach aimed at increasing FXN mRNA or protein levels,” the researchers concluded.