Page 176 - Read Online

Page 404 Saneto. J Transl Genet Genom 2020;4:384-428 I http://dx.doi.org/10.20517/jtgg.2020.40

mtDNA nucleoid maintenance), and protein translation. Dominant variants in this gene express global
developmental delay, hypotonia, optic atrophy, axonal neuropathy, and cardiomyopathy and isolated
hereditary spastic paraplegia [227,228] .

Iron-sulfur cluster biosynthesis and mitochondrial iron homeostasis
A strict balance of iron (Fe) and sulfide ion (S) concentrations is maintained to prevent their damaging
oxidative properties when present in excess. Complex biosynthetic systems have evolved to “protect” the
cell, while incorporating Fe/S clusters into specific apoproteins for cellular functioning. To date, there
are at least 18 known Fe/S cluster assembly proteins involved in proper biogenesis and trafficking of Fe/S
containing clusters within mitochondria. A full review of this process is beyond the scope of this article but
can be found elsewhere [229-231] .

The incorporation of Fe/S clusters into mitochondrial proteins involves initial assembly into 2Fe/2S
complexes within the mitochondrion on a specific scaffold protein complex. Fe and S are complexed
together by specific transfer proteins into the 2Fe/2S complex, subsequently converted into the 4Fe/4S and
3Fe/4S complexes, and then inserted into Complexes I, II, and III. The 2Fe/2S clusters are trafficked to the
late-acting machinery for 4Fe/4S cluster synthesis. During the late synthetic process, there is one 3Fe/4S
cluster that is inserted into Complex II. Once fully synthesized, the 4Fe/4S clusters are inserted into the
ETC Complexes I and II. Complex III only contains a single 2Fe/2S complex.

The assembly of iron and sulfur complexes is a highly regulated process and the mechanisms involved are
not completely understood. Intracellular iron homeostasis is controlled post-transcriptionally by iron-
regulatory protein 1 (IRP1) and 2 (IRP2). The IRPs bind to RNA stem-loops known as iron-responsive
elements of mRNAs involved in iron uptake and sequestration, transferrin receptor expression, and
ferritin levels. IRP2 is intimately involved in iron homeostasis via an oxygen-responsive iron-sulfur cluster.
Although both IRPs are members of the aconitase family of proteins, IRP2 cannot form a 4Fe/4S cluster and
does not express aconitase function. Mouse models of IRP2 deficiency leads to reduced transferrin receptor
expression and reduced iron acquisition [232] . The connection to mitochondrial function comes from mouse
models and rare patients with pathological variants, which confirm a mitochondrial dysfunction [232,233] .

Ferredoxins are iron-sulfur proteins involved in the early cluster assembly. Ferredoxin reductase (FDXR)
donates electrons from NADPH to homologous ferredoxins, FDX1 and FDX2, and initiates the 2Fe/2S
scaffold complex. Variants in FDXR diminished iron-sulfur cluster assembly and induce mitochondrial
iron overload [234] . The diminished cluster assembly phenotypically gives rise to optic atrophy and sensory
neuropathy [235,236] . Recessive mutations in FDX2 induces a complex phenotype consisting of optic atrophy,
reversible leukoencephalopathy, myopathy, and axonal polyneuropathy [233,236] . The scaffold protein iron-
sulfur cluster assembly enzyme ISCU serves as the initial step of the 2Fe/2S complex formation. Sulfur,
arising from cysteine, is shuttled to the ISCU by the cysteine desulfurase, NFS1. The ISCU complex consists
of at least the NFS1, ISD11, ISCU, and Frataxin (FXN) proteins [229,230] . Recessive pathological variants have
been described in all of the early scaffold assembly complex genes. A recessive infantile onset disorder
with ETC Complex II and III deficiency, multisystem organ failure, and hypotonia has been described with
variants in NFS1 [237] . LYRM4 encodes the ISD11 protein, which forms a complex with, as well as stabilizes,
the sulfur donor NFS1. Recessive mutations in the LYRM4 induces deficiency of Complexes I-III in muscle
and liver [238] . Another component of the initial iron-sulfur scaffold, variants in ISCU induce a myopathy and
exercise intolerance. Recessive variants in ISCU in muscle have been demonstrated to induce myopathy [239] .
Friedreich ataxia, the most common autosomal recessive ataxia, is due to homozygous expansion of a
GAA trinucleotide repeat in intron 1 of FXN [240] . Deficiency in the FXN encoded FXN protein leads to a
progressive spinocerebellar neurodegeneration associated with gait and limb ataxia, dysarthria, muscle
weakness, cardiomyopathy, and diabetes.