Importance of molecular testing in dominant optic atrophy

Structural model of the OPA1 GTPase domain may explain the molecular consequences of a novel mutation in a family with autosomal dominant optic atrophy

Autosomal dominant optic atrophy (ADOA) is the most frequent hereditary optic neuropathy. Three loci have been reported for ADOA: a major locus, harboring all identified mutations to date, maps to 3q28 (OPA1), a second locus is linked to 18q12.2-q12.3 (OPA4) and a third locus on 22q12.1-q13.1 (OPA5) has been reported recently. We describe a six-generation Iranian family in which optic atrophy runs as an autosomal dominant trait with an age of onset at 14-15years. We performed linkage analysis with markers mapping to 3q28 and 18q12.2-q12.3 and found linkage to 3q28. Subsequent sequencing of OPA1 identified a novel heterozygous missense mutation (c.1313A>G) replacing aspartic acid by glycine (p.D438G) in the GTPase domain of OPA1. Interestingly, another missense mutation at the same position (c.1313A>T, D438V) has been reported before in two unrelated German families, indicating a possible mutation hot spot. Further evidence supporting the importance of D438 is its conservation from human to acoelomata. OPA1 is believed to be the human orthologue of yeast MGM1, a dynamin-related protein required for the integrity of mitochondrial DNA. Homology modeling of the OPA1 GTPase domain revealed extensive structural similarity to the Dictyostelium dynamin A GTPase domain and showed that D438 may interact with residues of the G1 and the G4 motifs, which are crucial in coordinating GTP. Based on this analysis, we propose a mechanism which explains the gradual decline of vision in ADOA patients with OPA1 mutations at position 438.

Novel mutations in the OPA1 gene and associated clinical features in Japanese patients with optic atrophy

PURPOSE

Autosomal dominant optic atrophy (ADOA) is characterized by symmetrical bilateral optic atrophy associated with reduced corrected visual acuity (VA), central or centrocecal scotoma, and color vision disturbances. The disease is genetically heterogeneous, and the OPA1 gene has been identified as the only causative gene. The aims of this study were to identify and report mutations in the OPA1 gene in Japanese patients with ADOA and to describe the clinical features associated with the mutations.

DESIGN

Molecular genetic study and observational case reports.

PARTICIPANTS

Nine unrelated Japanese families with optic atrophy and 8 isolated cases of optic atrophy.

METHODS

Genomic DNA was extracted from peripheral leukocytes, and all exons containing the open reading frame of the OPA1 gene and the flanking intron splice sites were sequenced directly. Complete ophthalmologic examinations were performed.

MAIN OUTCOME MEASURES

Direct sequencing of the OPA1 gene and clinical evaluations including VA, visual field, color vision, and disc appearance.

RESULTS

Ten different heterozygous mutations, including 6 novel mutations, were detected in the OPA1 gene. The identified mutations included 5 deletions/insertions (c.2061delA, c.2098_2103delCTTAAA, c.2538insT, c.2591insC, and c.2708_2711delTTAG), 4 nonsense mutations (c.112C>T [p.R38X], c.181C>T [p.Q61X], c.946A>T [p.R316X], and c.2713C>T [p.R905X]), and 1 missense mutation (c.1635C>A [p.S545R]). The most common mutation in Caucasians (c.2708_2711delTTAG) was found in 3 unrelated families, suggesting that it is a mutational hot spot. We detected an OPA1 mutation in 8 of 9 familial cases of optic atrophy and in 4 of 8 cases that were initially considered to be sporadic from the patients' family histories. Examinations of family members of 2 sporadic probands revealed the existence of other family members with the OPA1 mutations whose phenotype was very mild or within normal limits. This indicates that patients with ADOA sometimes seem to be sporadic because of the extensive variation in the phenotype or, alternatively, a low penetrance of ADOA.

CONCLUSIONS

OPA1 gene mutations are causative in most familial cases of ADOA in Japanese. Sporadic cases of optic atrophy frequently may be caused by OPA1 mutations in the Japanese population. Molecular genetic examinations are useful in determining the hereditary patterns in some cases of optic atrophy.