Frataxin is small (100 - 200 amino acids) nuclear-encoded mitochondrial acidic protein conserved from yeast to human, but with no homology with proteins of known functional domains.
■ Sequence conservation of frataxin
Although bacterial frataxins contain only a conserved sequence of approximately 100 to 130 residues, eukaryotic frataxins have an additional N-terminal tail, which contains the mitochondrial import signal. The highly conserved C-terminal region forms a family of sequences that share a sequence identity of approximately 25 but a similarity of 40 to 70 %, indicating that this region is functionally important.
■ Frataxin gene regulation
The human frataxin gene is located on chromosome 9 (q13-q21.1) consisting of seven exons spanning _95 kb of the genomic DNA . It exhibits tissue-specific expression, with its mRNA most abundant in the heart, followed by the liver and skeletal muscle.
■ The structure of frataxins
The fold consists of a globular, slightly elongated domain in which two helices pack against a 5- to 7-residue ß-sheet. The C-terminus, of variable length in the different orthologues, inserts in between the two helices.
The three-dimensional structures of human frataxin have a highly conserved negatively charged surface, similar to the anionic surface involved in the iron-storage mechanism of ferritin, but human frataxin may utilize a second, uncharged surface to assemble with itself and form a negatively charged iron-storage cavity. Conservation residues are essential, either for folding (structural residues, usually buried in the hydrophobic core) or for function (functional residues, usually exposed on the surface).
SYNTHESIS AND TURNOVER
■ Frataxin production
The main form of mature frataxin is a 130 - amino acid protein (aa 81 to 210) after a proteolytic cleavage between Lys80 and Ser81, which is potential MPP cleavage site (mitochondrial processing peptidase).
In human immature frataxin, the structure is compact, with no signature of iron binding or oligomerization in the presence of iron, whereas the mature form assembles in 600-kDa homopolymers and bind 10 atoms of iron per frataxin oligomer.
■ The in vivo functions of frataxin
Frataxin is involved in different aspects of intracellular iron metabolism, from biogenesis of heme and Fe-S clusters, to mitochondrial iron-binding/ storage and iron chaperone activity. It also has a role in controlling cell survival, antioxidant defense, and oxidative phosphorylation/ ATP production. Frataxin-deficient cells are more sensitive to oxidative stress, apoptotic and autophagic cell death.
• Frataxin as a ferritin-like protein: it forms large aggregates of 48 subunits that can bind up to 50 iron atoms per protein monomer, that may function as an iron storage protein. Oligomerisation is non-essential when the protein functions as an iron chaperone during heme and Fe-S cluster assembly.
• Frataxin as an iron chaperone of the Fe-S cluster machinery: it interacts with the desulphurase/scaffold protein complex (isc), when physiological concentrations of ferrous iron are maintained during complex isolation. It also interact with aconitase in a citrate-dependent manner and protects or restores the aconitase activity by converting the inactive [3Fe-4S]1_ form to the active [4Fe-4S]2_ form of aconitase.
• Frataxin as an iron chaperone in heme metabolism: interaction with ferrochelatase, but only in the presence of iron, indicates frataxin involvement in heme synthesis. Frataxin deficiency leads to downregulation of mitochondrial transcripts and to kinetic inhibition of the heme pathway.
• Participation in ROS production control: frataxin reduces the production of reactive oxygen species (ROS) and protects DNA against iron-induced oxidative damage. The protein first binds to Fe2+ and transform it into Fe3+ through ferroxidase chemistry.
■ Disease associated
There is a correlation between the length of the GAA repeat and deficiency of frataxin; this is associated with the onset of Friedreich's ataxia (FRDA) symptoms.
The GAA triplet-repeat expansions in the frataxin gene interferse with frataxin gene transcription because of a self-associated triplex formation at long GAA:TTC repeats (sticky DNA). Hyperexpansion of a GAA trinucleotide repeats (from 100 to 1,000) in the first intron of the frataxin induced oxidative stress and led to cardiomyopathy, cerebellar and sensory ataxia, decreased activities of mitochondrial respiratory chain, decreased aconitase activity, resulting in defects in ISC protein maturation, respiratory deficiency, and mitochondrial iron accumulation.
Targeted disruption of hepatic frataxin expression caused increased oxidative stress, impaired mitochondrial function of respiration and ATP synthesis, along with decreased activity of ISC-containing proteins in liver.
GAA repeats reduce FXN expression by forming non B-DNA structures such as triplex and sticky DNA, that are thought to cause stalling of the RNA polymerase.
Non B-DNA structures may induce HDAC activity that, along with histone methyltransferases (HMTase), generate methylated histones in CpG residues in particular in intron 1, decreasing promoter activity and leading to gene silencing.
1. DNA sequence-specific polyamides and small chemicals: specifically bind to the GAA:TTC repeat and disrupt the triplex formation.
2. histone deacetylase inhibitors (BML-210): reverse frataxin-gene silencing through chromatin relaxation by inducing lysine acetylation of histone H3 and H4.
3. iron chelators (deferoxamine, pyridoxal isonicotinoyl hydrazone and its derivatives): decrease accumulated iron levels in tissues and protect from H2O2-induced cytotoxicity.
4. mitochondriatargeted antioxidants (Idebenone MitoQ and MitoVitE): protect from toxicity caused by glutathione- depletion conditions.
Future work is needed to determine if HDAC inhibitors e DNA binding agents can work synergically to increase frataxin to greater levels than the ones found when these agents are used individually.
Targeting the gene in Friedreich ataxia. Hebert MD.
A Low numbers of GAA repeats in FXN intron 1 result in open chromatin and normal, B-form DNA, leading to optimal frataxin levels.
B High numbers of GAA repeats promote gene silencing and alternative non B-DNA conformations that together decrease frataxin to pathogenic levels.
C HDAC inhibitors have been shown to increase frataxin levels in FRDA cells to around 40% of normal, presumably because heterochromatin has been reversed. However, alternative DNA structures may prevent frataxin expression from reaching 100%.
D DNA binding compounds have been shown to increase frataxin levels to around 30% of normal, possibly because they alleviate transcription insufficiency associated with alternative DNA structures. It is not known if these compounds reduce heterochromatin.
E A combination of HDAC inhibitors and DNA binding compounds may overcome both heterochromatin and alternative DNA barriers, leading to normal frataxin expression.