Mitochondrial Retinopathies

Abstract

The retina is an exquisite target for defects of oxidative phosphorylation (OXPHOS) associated with mitochondrial impairment. Retinal involvement occurs in two ways, retinal dystrophy (retinitis pigmentosa) and subacute or chronic optic atrophy, which are the most common clinical entities. Both can present as isolated or virtually exclusive conditions, or as part of more complex, frequently multisystem syndromes. In most cases, mutations of mtDNA have been found in association with mitochondrial retinopathy. The main genetic abnormalities of mtDNA include mutations associated with neurogenic muscle weakness, ataxia and retinitis pigmentosa (NARP) sometimes with earlier onset and increased severity (maternally inherited Leigh syndrome, MILS), single large-scale deletions determining Kearns-Sayre syndrome (KSS, of which retinal dystrophy is a cardinal symptom), and mutations, particularly in mtDNA-encoded ND genes, associated with Leber hereditary optic neuropathy (LHON). However, mutations in nuclear genes can also cause mitochondrial retinopathy, including autosomal recessive phenocopies of LHON, and slowly progressive optic atrophy caused by dominant or, more rarely, recessive, mutations in the fusion/mitochondrial shaping protein OPA1, encoded by a nuclear gene on chromosome 3q29.

Keywords: Kearns-Sayre syndrome; Leber’s hereditary optic neuropathy (LHON); ataxia and retinitis pigmentosa (NARP); autosomal dominant optic atrophy (ADOA); mitochondrial DNA; mitochondrial disorders; mtDNA heteroplasmic deletions; neurogenic muscle weakness; optic atrophy; retina; retinitis pigmentosa.

Figure 1

Morbidic map of mtDNA mutations. The scheme of the circular 16.5 kb human mtDNA is depicted, with several clinical and molecular syndromes associated with mutations of the molecule. Genes encoding subunits belonging to the same MRC complex have identical colours. In yellow are indicated the genes encoding the 12S and 18S ribosomal RNAs. Different colours and aminoacids expressed in the single-letter code designate the tRNA-encoding genes. The D-loop control region is in black. LHON is associated predominantly with three-point mutations, usually homoplasmic, in different genes of cI, but other, rare mutations have also been reported. Deletions, associated with KSS and other syndromes including adult-onset progressive external ophthalmoplegia and neonatal Pearson’ syndrome, usually affects the “major” arc occupying approximately two thirds of the mtDNA circle, included between the D-loop and the WANCY cluster where the origin of light strand replication is contained.

Figure 2

Anatomy of the retina and its connections. (A) A section of normal retina. ILM: inner limiting membrane; RNFL: retinal nerve fibres layer; RGCs: retinal ganglion cells; IPL: inner plexiform layer; INKL: inner neuronal layer; OPL: outer plexiform layer; ONL: outer neuronal layer; OLM: outer limiting membrane; PR: photoreceptors (inner segments); RPE: retinal pigmentary epithelium (modified from Maresca A, Carelli V, 2021 [18]). (B) The structure of the eyeball is outlined in the upper left inset and a retinal ganglion cell is outlined with the part of the axon (proximal to the lamina cribrosa, unmyelinated), as well as the initial part of the axon after the emersion from the lamina cribrosa, where it is myelinated by oligodendrocytes of the optic nerve. Notice the high number of mitochondria in the cell body and unmyelinated portion of the axon, often organized in mitochondrial “varicosities” compared with the few mitochondria present in the Ranvier nodes in the myelinated part of the axon (modified from Carelli V et al., 2004 [19]).

Figure 3

NARP/MILS. (A,B). FLAIR T2-weighted sagittal and transverse sequences of a patient with NARP/MILS (modified from Lopez-Gallardo, 2009 [46]). Profound cortical atrophy with enlargement of the ventricular system; severe cerebellar atrophy. (C). Fundus oculi showing retinal dystrophy in a patient with NARP/MILS.

Figure 4

Molecular features of ATP synthase from Pichia angusta (modified from Vinothkumar et al. [47]). (A) Structure of the F-ATPase from P. angusta. The α-, β-, γ-, δ-, and ε-subunits forming the membrane extrinsic catalytic domain are red, yellow, royal blue, green, and magenta, respectively; the inhibitor protein is cyan; and the peripheral stalk subunits OSCP, b, d, and h are sea-green, pink, orange, and purple, respectively. In the membrane domain, the c₁₀-rotor is grey, the resolved region of the associated subunit a is corn-flower blue. Chains Ch1–Ch4 are pale yellow, brick-red, pale cyan, and beige, respectively, and have been assigned as transmembrane α-helices in subunit f and, in ATP8, as aH1 and bH1, respectively. (B) The F-ATPase from P. angusta. The a-subunit (encoded by ATPase6) is corn-flower blue. The c₁₀-ring is grey, the b-subunit (upper part not shown) is pink, and the pale yellow, brick-red, light cyan, and beige segments are transmembrane α-helices, Ch1–Ch4 assigned to subunit f, ATP8, aH1, and bH1, respectively. In the c-ring, I–IV indicate the four transmembrane C-terminal α-helices in contact with subunit a. (C) The a-subunit viewed from outside and looking out from the interface with the c-ring, respectively, with aH1 in pale cyan. Conserved polar residues are yellow; the positions of human mutations associated with pathologies are red. L176 in the cartoon correspond to the L156 (Leu156) mutated in the NARP mutation (either into an Arg or into a Pro). The pink sphere denotes the conserved Arg179 in aH5 that is essential for proton translocation and corresponds to human R159 residue (see text). The lower arrow indicates the inlet pathway for protons that transfer to Glu59 in the C-terminal α–helix-II of the c-ring. They are carried around the ring by anticlockwise rotation, as viewed from above, until they arrive at Arg179 where they enter the exit pathway (upper arrow).

Figure 5

KSS: molecular and clinical synopsis. (A). Cartoon displaying the structure of wild-type and deleted mtDNA molecules, linearized, and subjected to Southern-blotting with audioradiography against purified radiolabeled human mtDNA. The upper band present also in the wild-type mtDNA indicates linearized normal mtDNA (wt-mtDNA). The lower bands present in the other two lanes corresponding to muscle DNA from KSS patients are deleted species (Δ-mtDNA) co-existing with wt mtDNA (heteroplasmy). (B). In the upper set of panels progression of bilateral ptosis in a KSS adolescent. Two FLAIR-T2 transverse brain sequences display multiple lesions. A transverse section of a muscle biopsy shows a ragged-blue fibre (SDH-hyperintense) associated with several COX-negative fibres. The lady depicted in the figure displays bilateral eyelid ptosis (with ophthalmoparesis) and facial weakness. Her ECG shows numerous extrasystolic aberrant impulses. On the left, the fundus oculi shows retinal dystrophy, with irregular defects of both visual fields (bottom).

Figure 6

The retina in LHON. (A) Bilateral optic atrophy in chronic LHON. (B) In LHON the retinal neuronal fibres layer (NFL) and retinal ganglion cells (RGCs) layer are severely atrophic. Modified from from Maresca A and Carelli V, 2021 [18], H&E staining. (C) Loss of nerve fibres in a transverse section of the optic nerve of an LHON patient with the MT-3460 mutation, and in a magnified detail, compared with a control; Courtesy of Alfredo A. Sadun and Fred N. Ross-Cisneros.

Figure 7

Instrumental analysis of optic atrophy. Left panel: Acute LHON-11778. Fundus pictures (upper row) of both eyes reveal an initial loss of fibres in the temporal sector corresponding to the papillomacular bundle, whereas superior, inferior and nasal quadrants present swollen fibres as denoted by the translucid appearance. The loss of temporal fibres is reflected in the visual fields (middle row) displaying enlarged blind spot and central scotoma. Finally, the RNFL thickness, as measured by OCT (lower row), clearly highlights the reduction of thickness in the temporal sector (T, red colour), whereas in the remaining sectors the thickness is at the upper limit, reflecting the swelling of axons (S, I, N, green colour). Central panel: Chronic LHON-11778. Fundus pictures (upper row) of both eyes reveal a completely pale optic disc, denoting the atrophy of the optic nerve. This is reflected by the generalized loss of sensitivity at visual fields (middle row), and reduced RNFL thickness essentially in all quadrants (T, S, N, I, red colour), with the exception of some spared axons in the super-nasal sector (S-N, green colour). Right panel: OPA1-related DOA. Fundus pictures (upper row) of both eyes reveal pale optic discs, denoting the generalized atrophy of the optic nerves. This is reflected by the dense caecocentral scotomas at visual fields (middle row), and reduced RNFL thickness essentially in all quadrants (T, S, N, I, red colour or borderline yellow-green), particularly marked on the temporal sector (T, red colour).

Figure 8

ADOA. (A) immunofluorescence with TMRM, a mitochondrion-specific dye. After 24 h exposure to a medium containing galactose, a carbon source that is energetically exploited only through OXPHOS, the mitochondrial network is similar to that of cells in normal glucose-containing medium in control cells, but appears severely fragmented in OPA1-mutant cells (see text for details). (B) The same experiment investigated the shape and internal structure of mitochondria. In OPA1 mutant cells, mitochondria are sparse and the cristae organization is severely disrupted, compared with a control sample (detail inset modified from supplemental material from Zanna et al., 2008 [120]).