Leber’s Hereditary – inherited from mitochondrial DNA Optic – affects the eye Neuropathy – disease/abnormality of nervous system
Introduction Maternally inherited disorder characterized by degeneration characterized by degeneration of the retinal ganglion cells (RGCs) of the retinal ganglion cells (RGCs) and atrophy of the optic nerve and atrophy of the optic nerve Mutations are in the mitochondrial (not nuclear) genome (not nuclear) genome Usually begins between the ages of 25 and 35 (but can occur at any age) and leads to legal blindness More common in males than females It was first described in 1871 by Theodore Leber and is the most common cause of optic atrophy.
Mitochondria Inherited from mother to all offsprings Only the egg contributes mitochondria to the embryo 2 billion mitochondria are made every second throughout a person’s life 250 mitochondria on the average are found in a cell Life span is around 100 days.
Mitochondrial or mtDNA Number varies from cell to cell Circular strand of DNA consisting of 16,569 nucleotide bases Contains 37 genes 2 Encode for rRNA and 22 for tRNA 13 genes encode for proteins required for the biochemical reactions in process that generates ATP high energy molecules process that generates ATP high energy molecules The rest 74 proteins for oxidative phosphorylation are encoded by nuclear DNA
Heteroplasmy Heteroplasmy Heteroplasmy - Some of the mt DNA has mutations - Some of the mt DNA has mutations some does not some does not - 15% of individuals with LHON are - 15% of individuals with LHON are heteroplasmic heteroplasmic - The rest are homoplasmic - The rest are homoplasmic Homoplasmy All of the mtDNA has at least one of the three types of mutations for All of the mtDNA has at least one of the three types of mutations for or no mutations at all!! or no mutations at all!! This determines the type of phenotype and the risk transmission.
There is also a mitochondrial disorder known as LHON (Leber's Hereditary Optic Neuropathy) where the mitochondrial DNA mutations which causes the disease (acquired blindness) are homoplasmic - meaning that all of the mitochondria carry the defect. However, just because a person has one of the LHON mitochondrial DNA mutations does not mean they will become blind, only about 10% will. Confusing? You bet!
Phenotypic threshold effect in mtDNA mutatins Associated with mtDNA heteroplasmy Critical threshold in proportion of mutations in mtDNA must be exceeded before disease appears This is normally about 90% BUT, LHON is generally homoplasmic Interestingly certain homoplasmic mtDNA mutations do not express LHON phenotype at all!!!
85% to 90% of cases of LHON are usually due to LHON are usually due to one of the three mtDNA one of the three mtDNA point mutations point mutations ND4 ND1 and ND6 subunit genes of of the oxidative phosphorylation chain in mitochondria, which is the 1 st step of the oxidative phosphorylation chain in mitochondria, which is the 1 st step of the The rate of mtDNA mutations is 10-times greater than in nuclear DNA
Complex 1 Site effected by the 3 point mutations found in LEBERS
Causes of LHON 3 point mutations at these sites : 1) G11778A 1) G11778A 50-60% LHON population 50-60% LHON population Mutation is located at position Mutation is located at position 11778 change is G to A 11778 change is G to A 2) T14484C 2) T14484C 10% LHON population 10% LHON population 3) G3460A: 3) G3460A: 8-25% LHON population 8-25% LHON population These mutations decrease production of ATP resulting in cell dysfunction and cell death Production of ROS Reactive oxygen species – byproduct of oxidative phosphorylation Reactive oxygen species – byproduct of oxidative phosphorylation Environmental factors
Environmental factors in LHON Not everyone with one of these mutations will develop LHON mutations will develop LHON Additional genetic or environmental factors play an important role to factors play an important role to development of central vision loss development of central vision loss Males with one of these mutations have a 40% lifetime risk to develop symptoms Females have a 10% risk, although the actual risk varies slightly from mutation to mutation
What are the environmental factors? What are the environmental factors? Those factors that can reduce the blood supply to the retina and optic nerve They are suspect to 'trigger' the vision loss in LHON a. Heavy drinking or smoking a. Heavy drinking or smoking b. Exposure to poisonous fumes b. Exposure to poisonous fumes such as carbon monoxide such as carbon monoxide c. High levels of stress c. High levels of stress d. Medications: d. Medications: Ethambutol – Rx for TB Ethambutol – Rx for TB Chloramphenicol – for conjunctivitis Chloramphenicol – for conjunctivitis e. Many other known toxins e. Many other known toxins that may cause blindness that may cause blindness
RGC (retinal ganglial cells) RGC (retinal ganglial cells) These cells depend on oxidative phosphorylation due to their: Huge ATP demand Huge ATP demand Very sensitive to energy supply and mitochondria defects Very sensitive to energy supply and mitochondria defects Limited regeneration abilities Limited regeneration abilities END RESULT = Vision damage from degeneration of optic nerve due to insufficient ATP supply END RESULT = Vision damage from degeneration of optic nerve due to insufficient ATP supply
LHON is found in 80% of young men in their twenties Female carriers have 85 - 90% chance of staying healthy Why? X - chromosome markers have been found which may influence disease outcome in carriers, called protective factors
Asymptomatic until visual blurring develops Acute Phase : Painless, acute onset of central vision loss Peripheral vision (seeing out of the corner of the eye) remains Loss of visual acuity/color Once symptoms appear in one eye, other eye affected within few weeks Sub acute Phase Atrophy of optic disc =
Additional symptoms Cardiac conduction defects Tremors Numbness or weakness in arms or leg Loss of ankle reflexes Symptoms vary by gender and type of mutation present — the most common mutation and usually the most severe vision loss — usually has the best long term prognosis or outcome — has an intermediate presentation
Molecular genetic blood test using polymerase chain reaction (PCR) techniques The test is 100% accurate for LHON when visual loss has already occurred Interestingly, significant number of individuals who are suspected to have LHON do not have one of the three primary mtDNA LHON mutations.
Treatment So far no treatment has been proven effective in controlled trials for LHON BUT…….. Idebenone, a synthetic analogue of coenzymeQ10 has been studies in clinical trials in Canada, Germany and UK Idebenone is a strong antioxidant and may be the key to providing stability to nerve cell, decreasing the likelihood of oxidative damage caused by free radicals released when cells are destroyed by chemical toxins
Possible Rx options Neutralize free radical production by neuronal cells with supplementation of antioxidants Vitamins Natural plant extracts Vitamin E Co-enzyme Q Vitamin C Vitamin A Ginkgo biloba Curcumin
Conclusion Point mutations in complex 1 due play a major role in causing Lebers Decreased production of ATP underlies cell dysfunction and cell death Although there is no treatment at this time, family history along with genetic testing and healthy lifestyle may effect the outcome of this disease as with other disease such as Cancer