Molecular mechanisms that cause cell death in LHON

Abstract

Mitochondrial impairments, caused by mtDNA mutations, lead to RGC death and optic atrophy causing blindness in LHON. It was once thought that this was a disease of impaired bioenergetics. More recent thinking, considering the tempo of damage, studies in cybrids and a faithful mouse genetic model of LHON, has led to the realization that the problem is more that of the pathological generation of Reactive Oxygen Species (ROS). For example, synaptosome analysis from the brains of LHON mice show normal levels of ATP, but the blockage of Complex I activity was associated with very high levels of ROS. We hypothesized that the mtDNA mutation, specifically for 3460 in ND1, changed the atomic structures around COQ10 such that ROS were generated. In order to test that hypothesis, we conducted a series of experiments with statistical ensembles, built with Monte Carlo and Boltzmann sampling for simulations in computational chemistry. We began with the single amino acid mutation in ND1 of Complex I that switches hydrophobic alanine to hydrophilic threonine, at the entry of the Coenzyme Q10 binding channel. We explicated the mechanistic consequences of this mutation, using the tools of Molecular Dynamics and Free Energy Perturbation simulations. We showed that the mutant remained capable of binding Coenzyme Q10. Indeed, the mutated channel was even more proficient than the wild type in reducing ubiquinone to ubiquinol due to the closer average proximity of the Coenzyme Q10 headgroup to the chain of iron–sulfur clusters, from which it received electrons via quantum mechanical tunnelling. However, the mutation kinetically hindered the exit of the reduced (ubiquinol) form of Q10 from the ND1 channel, slowing the passage of its headgroup due to unfavourable electrostatics. This effect lead to overproduction of ROS via re‐oxidation of Coenzyme Q10 as electrons were sent back up the Iron–sulfur cluster chain, where the ROS production occurs. Another line of studies has shown that ROS lead to the death of RGCs affecting the smallest axons first. In those areas of the optic nerve in which larger axons are preponderate, the ROS are absorbed by the larger axons and little more happens. However, there is a dearth of large axons inferior temporally where the papillomacular bundle enters the optic nerve. Then, the ROS begins a reiterative and vicious cycle of cell death and the production of more ROS. This explains both the very peculiar spatial and temporal features of the optic neuropathy in LHON.