Researchers are also investigating whether changes in additional genes, particularly genes on the X chromosome, contribute to the development of signs and symptoms. The degree of heteroplasmy, the percentage of mitochondria that have mutant alleles, may play a role. Patterns of mitochondrial alleles called haplogroup may also affect expression of mutations. History
LHON was first described by the German ophthalmologist Theodor Leber (1840–1917) in 1871. In a paper, Leber described four families in which a number of young men had abrupt loss of vision in both eyes either simultaneously or sequentially. This disease was initially thought to be X-linked but was subsequently shown to be mitochondrial. The nature of the causative mutation was first identified in 1988 by Wallace et al. who discovered the guanine (G) to adenosine (A) mutation at nucleotide position 11778 in nine families. This mutation converts a highly conserved arginine to histidine at codon 340 in the NADH dehydrogenase subunit 4 of complex I of the mitochondrial respiratory chain. The other two mutations known to cause this condition were identified in 1991 (G to A point mutation at nucleotide position 3460) and 1992 (thymidine (T) to cytosine (C) mutation at nucleotide 14484). These three mutations account for over 95% of cases: the 11778 mutation accounts for 50-70% of cases, the 14484 mutation for 10-15% and the 3460 mutation for 8-25%. Research
Human clinical trials are underway at GenSight Biologics (ClinicalTrials.gov # NCT02064569) and the University of Miami (ClinicalTrials.gov # NCT02161380) to examine the safety and efficacy of mitochondrial gene therapy in LHON.
Lebers hereditary optic neuropathy (LHON) is a mitochondrially inherited (transmitted from mother to offspring) degeneration of retinal ganglion cells (RGCs) and their axons that leads to an acute or subacute loss of central vision; it predominantly affects young adult males. LHON is  transmitted only through the mother, as it is primarily due to mutations in the mitochondrial (not nuclear) genome, and only the egg contributes mitochondria to the embryo. LHON is usually due to one of three pathogenic mitochondrial DNA (mtDNA) point mutations. These mutations are at nucleotide positions 11778 G to A, 3460 G to A and 14484 T to C, respectively in the ND4, ND1 and ND6 subunit genes of complex I of the oxidative phosphorylation chain in mitochondria. Men cannot pass on the disease to their offspring. Signs and symptoms
Clinically, there is an acute onset of visual loss, first in one eye, and then a few weeks to months later in the other. Onset is usually young adulthood, but age range at onset from 7-75 is reported. The age of onset is slightly higher in females (range 19–55 years: mean 31.3 years) than males (range 15–53 years: mean 24.3). The male-to-female ratio varies between mutations: 3:1 for 3460 G>A, 6:1 for 11778 G>A and 8:1 for 14484 T>C.This typically evolves to very severe optic atrophy and a permanent decrease of visual acuity. Both eyes become affected either simultaneously (25% of cases) or sequentially (75% of cases) with a median inter-eye delay of 8 weeks. Rarely, only one eye is affected.