Molecular Mechanisms and Therapeutics for the GAA·TTC Expansion Disease Friedreich Ataxia

Abstract

Friedreich ataxia (FRDA), the most common inherited ataxia, is caused by transcriptional silencing of the nuclear FXN gene, encoding the essential mitochondrial protein frataxin. Currently, there is no approved therapy for this fatal disorder. Gene silencing in FRDA is due to hyperexpansion of the triplet repeat sequence GAA·TTC in the first intron of the FXN gene, which results in chromatin histone modifications consistent with heterochromatin formation. Frataxin is involved in mitochondrial iron homeostasis and the assembly and transfer of iron-sulfur clusters to various mitochondrial enzymes and components of the electron transport chain. Frataxin insufficiency leads to progressive spinocerebellar neurodegeneration, causing symptoms of gait and limb ataxia, slurred speech, muscle weakness, sensory loss, and cardiomyopathy in many patients, resulting in death in early adulthood. Numerous approaches are being taken to find a treatment for FRDA, including excision or correction of the repeats by genome engineering methods, gene activation with small molecules or artificial transcription factors, delivery of frataxin to affected cells by protein replacement therapy, gene therapy, or small molecules to increase frataxin protein levels, and therapies aimed at countering the cellular consequences of reduced frataxin. This review will summarize the mechanisms involved in repeat-mediated gene silencing and recent efforts aimed at development of therapeutics.

Keywords: Friedreich ataxia; epigenetics; mitochondrial disease; therapeutics; transcription; trinucleotide repeat expansion.

Fig. 1

Mechanisms of FXN gene silencing. (A) In cells from unaffected individuals, the FXN gene, with short lengths of GAA·TTC repeats, is packaged in open chromatin, allowing RNA polymerase II (pol II) access to the promoter and allowing elongation through the repeats. Nucleosomes bear highly acetylated histone (blue marks on the amino-terminal tails of the histones). (B) In FRDA cells, the FXN gene is packaged in condensed heterochromatin, having particular types of histone methylation marks, such as H3K9me3 (red boxes on the amino-terminal tails of histone H3), which forms the binding site for heterochromatin protein HP1, leading to chromatin condensation. Several models have been proposed to account for heterochromatin formation by long GAA·TTC repeats, including triplexes and/or sticky DNA; R-loops that recruit the heterochromatin machinery; short RNA transcripts that recruit components of the RNA interference machinery; and an antisense transcript called FAST-1, which depletes the chromatin boundary protein CTCF, leading to heterochromatin. All of these mechanisms could lead to promoter silencing and blocking transcription elongation through the repeats

Fig. 2

Cellular dysfunction in FRDA and possible therapeutic opportunities. A cartoon representation of a FRDA patient cell is shown, along with the consequences of the GAA·TTC repeat expansion at the levels of the DNA (1), nuclear chromatin (2), low levels of frataxin in mitochondria (3), and cellular (4) and organismic (5) consequences of reduced frataxin. Possible therapeutic approaches targeting each of these defects are shown in the box at the right

Fig. 3

Therapeutic pipeline for FRDA, including the clinical development stage for each approach as well as the organizations performing research, as of April 2019. Figure provided by J. Farmer of the Friedreich’s Ataxia Research Alliance