Optic neuropathy and cerebellar ataxia associated with a rare missense variation (A14510G) of mitochondrial DNA

Leber's hereditary optic neuropathy with the 3434, 9011 mitochondrial DNA point mutation

BACKGROUND

Leber's hereditary optic neuropathy (LHON) contains several well-known mitochondrial DNA (mtDNA) point mutations. We report a case with characteristic clinical manifestations of LHON involving a possible new LHON point mutation.

CASE

A 34-year-old man was diagnosed with LHON. The patient exhibited (1) sudden onset of bilateral visual loss, (2) normal light reflex, and (3) swelling of the peripapillary nerve fiber layer. After subsequent development of bilateral optic disc pallor, we concluded that the patient had LHON. mtDNA analysis was conducted using non-radioisotopic single-strand conformational polymorphism followed by direct sequencing. There was no change in the patient's visual acuity during the 26-month follow-up period.

OBSERVATIONS

The mtDNA point mutations were found at T3434C, G3483A, and V9011A. The confirmed mtDNA substitutions included (1) A-G at nucleotide position 3434, (2) G-A at nucleotide position 3483, and (3) C-T at nucleotide position 9011. The amino acid code at the nucleotide positions 3434 and 9011 was phylogenetically highly conserved.

CONCLUSION

The 3434 and 9011 mtDNA point mutations are candidates for a new LHON mutation.

Mitochondrial DNA Mutations with Leber's Hereditary Optic Neuropathy in Japanese Patients with Open-Angle Glaucoma

PURPOSE

Abnormal optic disc excavations are found in patients with Leber's hereditary optic neuropathy (LHON). The purpose of this study was to determine whether heteroplasmy for the major three LHON mutations or for the rare LHON mutations are risk factors for open-angle glaucoma.

METHODS

Blood samples from 835 Japanese subjects were screened with the Invader assay for ten LHON-associated mutations: three major mutations (G3460A, G11778A, T14484C) and seven rare mutations (T9101C, G9804A, C14482A, C14482G, G14459A, T14498C, and A14510G). Of the 835 subjects, 241 were patients with primary open-angle glaucoma (POAG), 310 were patients with normal-tension glaucoma (NTG), and 284 were healthy controls.

RESULTS

Five POAG patients and three NTG patients had one of five mutations, C9099A, T9101G, T9101C, G9804A, or G11778A, but none of these patients had LHON. The C9099A (Ile191Met) and T9101G (Ile192Ser) mutations were novel and identified within the probes by lack of signal in the assay. Two patients with the G11778A mutation showed heteroplasmy, with 15% mutant mtDNA in the male patient and 80% in the female patient. The remaining LHON-associated mutations were not detected in any of the subjects. A case-control study did not show a significant difference (P = 0.099): eight potentially disease-associated variants in 551 patients versus zero variants in the 284 controls.

CONCLUSIONS

Rare LHON-associated mitochondrial DNA mutations were found in Japanese patients with open-angle glaucoma (OAG). However, whether mitochondrial DNA mutations are risk factors for OAG is still open to question.

Mitochondrial dysfunction as a cause of optic neuropathies

Mitochondria are increasingly recognized as central players in the life and death of cells and especially of neurons. The energy-dependence of retinal ganglion cells (RGC) and their axons, which form the optic nerve, is singularly skewed. In fact, while mitochondria are very abundant in the initial, unmyelinated part of the axons anterior to the lamina cribrosa, their number suddenly decreases as the myelin sheath begins more posteriorly. The vascular system also presents different blood-brain barrier properties anterior and posterior to the lamina, possibly reflecting the different metabolic needs of the optic nerve head (unmyelinated) and of the retrobulbar optic nerve (myelinated). Mitochondrial biogenesis occurs within the cellular somata of RGC in the retina. It needs the coordinated interaction of nuclear and mitochondrial genomes. Mitochondria are then transported down the axons and distributed where they are needed. These locations are along the unmyelinated portion of the nerve, under the nodes of Ranvier in the retrobulbar nerve, and at the synaptic terminals. Efficient transportation of mitochondria depends on multiple factors, including their own energy production, the integrity of the cytoskeleton and its protein components (tubulin, etc.), and adequate myelination of the axons. Any dysfunction of these systems may be of pathological relevance for optic neuropathies with primary or secondary involvement of mitochondria. Leber's hereditary optic neuropathy (LHON) is the paradigm of mitochondrial optic neuropathies where a primary role for mitochondrial dysfunction is certified by maternal inheritance and association with specific mutations in the mitochondrial DNA (mtDNA). Clinical phenocopies of this pathology are represented by the wide array of optic neuropathies associated with vitamin depletion, toxic exposures, alcohol and tobacco abuse, and use of certain drugs. Moreover, the recent identification of mutations in the nuclear gene OPA1 as the causative factor in dominant optic atrophy (DOA, Kjer's type) brought the unexpected finding that this gene encodes for a mitochondrial protein, suggesting that DOA and LHON may be linked by similar pathogenesis. Polymorphisms in this very same gene may be associated with normal tension glaucoma (NTG), which might be considered a genetically determined optic neuropathy that again shows similarities with both LHON and DOA. Exciting new developments come from first examples of mitochondrial optic neuropathies in animal models that are genetically determined or are the result of ingenious engineering of mitochondrial gene expression, or from biochemical manipulations of the respiratory complexes. Even more exciting is the first successful attempt to correct the LHON-related complex I dysfunction by the allotopic nuclear expression of the recoded mitochondrial gene. There is hope that the genetic complexities, biochemical dysfunctions, and integrated anatomical-physiological cellular relationships will soon be precisely delineated and that promising therapeutic and prophylactic strategies will be proposed.