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Leber's Optic Neuropathy
C H A P T E R
11 Leber’s Optic Neuropathy The German ophthalmologist Theodore Leber published a description in 1871 of a peculiar form of blindness that occurred suddenly in young adults (Leber, 1871). Within just a few weeks, there was major loss of central vision in one eye, followed within a few more weeks by loss in the other. He noted that the disorder clustered in certain families and males were more often affected than females. He also discerned that the disorder could be transmitted to a son by a mother who did not show symptoms herself. Other investigators compiled pedigrees of families hosting Leber’s form of blindness, and it became apparent that the disease was always transmitted via the mother (Imai & Moriwaki, 1936) and probably involved some feature of the cytoplasm of the egg. A man with Leber’s disease never had sons or daughters with the same disorder. The pattern resembled that of an X-linked gene like the androgen receptor, wherein XY persons always obtain it from the mother, but some facts did not fit with X-linkage. The frequency of affected daughters was quite high in some families, and in large pedigrees, almost all daughters passed the defect to some of their offspring. The proportions of affected sons and daughters differed greatly among families but did not seem to fit the expected proportions for a single gene on the X. One author observed that the disease was a “clinical and genetic puzzle” (Waardenburg, 1969). The disorder became known as Leber’s hereditary optic neuropathy (LHON). The H term was added to distinguish it from other forms of blindness, but that really was not necessary. Using Leber’s name clearly meant a form of blindness transmitted via the mother. Huntington disease, AIS (see Chapter 10 on Androgen Insensitivity Syndrome), and Down syndrome never include the H term. Nevertheless, traditional usage prevails today. Some authors refer to LHON as Leber’s neuritis rather than neuropathy. Neuritis indicates inflammation of the optic nerve, whereas neuropathy is a more general term that can involve a significant deterioration and even death of tissue, something that occurs in many but not all cases. Discovery of a genetic cause clarified these quandaries of diagnosis and naming.
Genes, Brain Function, and Behavior https://doi.org/10.1016/B978-0-12-812832-9.00011-7
While scientists were struggling to solve the puzzle, a new phenomenon was discovered among sheep in England where a disease called scrapie was shown to be passed from mother to offspring by an infective agent, a strange kind of virus resident in the cell cytoplasm that worked very slowly to degrade the nervous system (Dickinson, Young, Stamp, & Renwick, 1965). A similar disease sometimes affected cattle—bovine spongiform encephalopathy (BSE) or mad cow disease. It appeared that Creutzfeld-Jakob disease (CJD), a rare but fatal neurological disorder in humans, also arose from a virus that was similar to BSE. In a few cases, the infective agent was passed to unrelated persons via surgical instruments used in brain surgery on CJD patients. It was then proposed that Leber’s optic neuropathy is transmitted by an infective agent that resides in the cytoplasm (Erickson, 1972; Wallace, 1970), which might explain why it is transmitted via the mother but does not seem to occur in definite ratios of affected and unaffected offspring the way a classical Mendelian gene would behave. A lively controversy ensued. One author proposed that the infective agent is a protein called a “prion” (Prusiner, 1982), while others argued forcefully that transmission across generations must involve the nucleic acids RNA and DNA (Taubes, 1986). The reality of a novel form of protein that can transmit CJD and BSE has been confirmed (Collinge, 2001), but doubts linger. The precise structure of the infectious agent has not yet been elucidated (Requena & Wille, 2014). The official fact sheet about CJD by the National Institute for Neurological Diseases and Stroke in the United States in 2017 designates the prion as “the leading scientific theory” about CJD rather than established fact. Further study of LHON, on the other hand, showed definitively that it is not related to CJD or BSE and is not caused by an infectious protein. Instead, it arises from genetic mutations of a conventional kind, except that the DNA is located in an unusual place.
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11. LEBER’S OPTIC NEUROPATHY
THE PUZZLE SOLVED: MITOCHONDRIA New discoveries pointed to genes in a small organelle in the cytoplasm, the mitochondrion. It occurs in all cells and has its own DNA that is transmitted to both male and female children via the maternal cytoplasm (Borst, 1977). It even has its own special variant of the genetic code (Table 2.1) in which the triplet UGA codes for tryptophan instead of STOP and AUA codes for methionine instead of isoleucine. The mitochondrial genome is much smaller than that of the chromosomes in the cell nucleus, and it lacks introns. A major effort was made to determine its DNA sequence. This was achieved (Anderson et al., 1981) more than 25 years before the immense chromosomal genome sequence was finished in 2006. The mtDNA has precisely 16,569 base pairs arranged in a circle (Fig. 11.1), and it contains 37 genes, 13 of which code (12s rRNA)
RNR1
F
PT
D-loop
V
CYB
OH
E
(16s rRNA)
15257
RNR2
ND6
14484 14459
L1
3460
ND1 I Q M
mtDNA 16,569 bp
13051
13 proteins 2 rRNA 22 tRNA
ND2 W AN C Y
ND5
L2 S2 H
11778
ND4
10663
OL
R
G
CO1 S1 D
CO2
FIG. 11.1
ND4L
ND3
CO3 K
ATP6 ATP8
Thirty-seven genes comprising the circular DNA of mitochondria. Thirteen code for proteins that act as enzymes (ATP6 and ATP8; CO1, CO2, and CO3; CYB; and ND1 to ND6), two code for ribosomal RNAs (RNR1 and RNR2), and 22 code for tRNA (A to Y). Full gene symbols and names are given in Table 11.2. The DNA is arranged into two circular chains termed light and heavy that are superimposed in this diagram. Sites where transcription is started for the light and heavy chains are shown as OL and OH. Locations of mutations known to give rise to LHON are shown inside the circle at the base number where a base substitution occurs. The three most common mutations listed in Table 11.1 are shown by *. Sources: http://www.mitomap.org 2017 “Morbid map of the human mtDNA genome”; Anderson, S., Bankier, A. T., Barrell, B. G., de Bruijn, M. H., Coulson, A. R., Drouin, J., et al. (1981). Sequence and organization of the human mitochondrial genome. Nature, 290, 457–465; Chinnery, P. F., & Hudson, G. (2013). Mitochondrial genetics. British Medical Bulletin, 106, 135–159; Taanman, J. W. (1999). The mitochondrial genome: structure, transcription, translation and replication. Biochimica et Biophysica Acta (BBA) – Bioenergetics, 1410, 103–123; Tuppen, H. A., Blakely, E. L., Turnbull, D. M., & Taylor, R. W. (2010). Mitochondrial DNA mutations and human disease. Biochimica et Biophysica Acta (BBA) – Bioenergetics, 1797, 113–128.
for enzymes with critical roles in metabolism and energy production in all cells. Two genes code for an essential part of the mechanism for synthesis of all proteins in every kind of cell in the body—the RNA involved in making a ribosome (rRNA)—and 22 others specify the structures of the small transfer RNAs (tRNA) that help to assemble a new protein (Fig. 3.1). One segment of mtDNA known as the D loop contains no genes and is a hotspot for mutations because they have no phenotypic effects and therefore are not pruned from a family tree. Knowing the normal DNA sequence of human mitochondria, organelles that are always transmitted via the mother, researchers assessed several pedigrees of families in which there were several cases of LHON. Wallace et al. (1988) examined 33 people, 23 of them blind, from nine families and found that all of those with LHON had the A nucleotide substituted for G at position 11778 that resulted in the amino acid arginine instead of histidine at position 340 in the protein encoded by the MT-ND4 gene. This was the first point mutation in any gene that was shown to cause a neurological disorder. But in two pedigrees, that mutation was not the cause of the blindness; instead, mutations elsewhere in the mtDNA gave rise to almost identical clinical phenotypes of blindness. Furthermore, there was a wide range of severity of symptoms in those with the same mutation. Thus, discovery of the 11778G > A mutation was a big step forward but did not explain everything about LHON. Medical geneticists around the world then tested many other cases of LHON and found even more mutations in mitochondrial genes associated with the maternally transmitted factor causing blindness. Table 11.1 summarizes data for 12 mutations involving six different mitochondrial genes that cause LHON in different families. The MitoMaps database now lists 18 mutations known to cause LHON in several members of different families and an additional 17 mutations identified in only a single family, sometimes involving just one individual case. The MalaCards database lists 43 different mitochondrial mutations, most of which have been observed in just one family. The overwhelming majority of all cases worldwide involve just one of three mutations, those at positions 3460, 11778, and 14484 in genes MT-ND1, MT-ND4, and MT-ND6, respectively. The 11778 mutation is most common, especially in Asia where it accounts for more than 90% of LHON cases. All of the mitochondrial genes coding for enzymes or parts of them are intimately involved in a biochemical process that enables all cells to derive energy from food. It is clear from the example of LHON that the same abnormal phenotype, a clinical syndrome, can be caused by different mutations in different genes.
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INCOMPLETE PENETRANCE, SEX, SMOKING, RECOVERY
INCOMPLETE PENETRANCE, SEX, SMOKING, RECOVERY Having an unambiguous biochemical means to identify mitochondrial mutations, researchers soon learned that many people in affected families carry a mutation but never show vision loss at any age. This phenomenon denotes incomplete penetrance in the parlance of genetics. The term was coined at a time when many believed that a gene codes for some interesting trait or character such as vision or blindness. When every person having the mutation shows the abnormal phenotype, it is said that the information in the gene penetrates to the observable level of a phenotype; it is fully penetrant. If only some with the abnormal genotype show the neurological defect, penetrance is said to be incomplete. Penetrance is a matter of gene expression, not gene transmission. That is why a mother can pass a mutation causing LHON to her offspring while she herself has normal vision. The data in Table 11.1 show the percentages of all male family members with a specific mitochondrial mutation who also show the symptoms of LHON. None of the better known mutations has a penetrance close to 100% among people in middle age. This fact generates hope for the future. If the reasons for some people being symptom-free can be discovered, they may point to things that can be done to prevent others from developing LHON. An even more potent source of incomplete penetrance is genetic sex. Females are far less likely to show LHON when they possess a mutation listed in Table 11.1. In situations in which about half of males with a mutation show LHON, 10% or fewer females usually show vision
TABLE 11.1
loss when they carry the same mutation. The reason for this dramatic sex difference is uncertain. Another possible source of low versus high penetrance might be the features of the environment. Many studies have found hints and suggestions that environmental toxins contribute to LHON. It is conceivable that a particular toxin has little harmful effect on vision in most people, whereas, among those carrying a mitochondrial gene mutation, the same toxin could harm cells in the retina. In a large family in Brazil with many cases of LHON extending over seven generations, all were descended from just one immigrant woman from Verona, Italy, and every one of 265 cases showing LHON carried the 11778 mutation (Sadun et al., 2003). The decline in penetrance over generations was striking (Fig. 11.2). In the second and third generations living in Brazil, 50%–70% of LHON cases were male, whereas in generations four to six, all were male. In the Brazilian family, the rate of smoking tobacco was 70% among those with LHON but only 16% among people carrying the 11778 mutation who did not show vision loss. LHON was also more common among those who consumed alcohol. The effect of smoking was similar to an earlier study (Tsao, Aitken, & Johns, 1999) that found more than 90% penetrance among male smokers versus <50% penetrance among nonsmokers in families showing LHON. A large study of LHON in the United Kingdom corroborated the large sex difference and also noted that penetrance in male smokers exceeded 90% if they lived long enough. The verdict on smoking and LHON is not yet final, because several research groups have not detected the same pattern in their study populations (e.g., Kerrison et al., 2000).
Mutations Causing LHON
Mutation
Gene
Percentage of all cases
Penetrance in males
Recovery rate
Vision loss
3460G>A
MT-ND1
13%
40%–80%
22%
Can see some light
11778G>A
MT-ND4
69%
82%
4%
Can see no light
14484T>C
MT-ND6
14%
68%
37%–65%
Can count fingers
15257G>A
MT-CYB
Rare
72%
28%
Can see hand motion
3635G>A
MT-ND1
Rare
54%
Low
?
3733G>A
MT-ND1
Rare
36%–44%
Yes
?
4171C>A
MT-ND1
Rare
47%
Yes
?
10663T>C
MT-ND4L
Rare
60%
?
?
13051G>A
MT-ND5
Rare
63%
?
?
14482C>A
MT-ND6
Rare
89%
Yes
?
14459G>A
MT-ND6
Rare
10%
No
Severe loss
14502T>C
MT-ND6
Rare
11%
?
?
? ¼ unknown or not specified. Sources: MitoMaps, OMIM.
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11. LEBER’S OPTIC NEUROPATHY
% of Carriers with LHON
50%–70% male 70 60 50 All male
40 30 20 10 2
5 4 3 Generation
6
FIG. 11.2
The percentage of LHON cases among confirmed carriers of a mutation in a mitochondrial gene that is associated with LHON. Data from a large pedigree of 265 members of a multigeneration family in Brazil who were all descended from a single female immigrant born in Italy in 1861. All possessed the same 11778 mutation in the same gene (ND4). Overall penetrance declined substantially over generations, and the proportion of male LHON cases rose to 100%. Among carriers with LHON, 70% were smokers, while smoking occurred in only 16% of carriers who did not show LHON. Adapted from Sadun, A. A., Carelli, V., Salomao, S. R., Berezovsky, A., Quiros, P. A., Sadun, F., et al. (2003). Extensive investigation of a large Brazilian pedigree of 11778/haplogroup in Leber’s hereditary optic neuropathy. American Journal of Ophthalmology, 136, 231–238, Copyright 2003, with permission from Elsevier.
The typical course of LHON begins with mild blurring of vision in the central part of the visual field of one eye and then progresses over several weeks to severe loss of vision in both eyes. Most notably affected are the retinal ganglion cells that gather information from across the retina and send it along the optic nerve. In many cases of LHON, large numbers of those cells die and degenerate, followed by the degeneration of the myelin sheaths and axons in the optic nerve, similar to what is seen in multiple sclerosis (Chalmers & Schapira, 1999; Tran, Bhargava, & MacDonald, 2001). Loss of vision is severe when many cells and axons perish. Nevertheless, for reasons that are not well understood, many cases experience gradual recovery of visual capacity after reaching a nadir, especially for the 14484T > C mutation. Recovery further reduces penetrance.
been variously estimated at 1 per 8500 in England (Man et al., 2003) and 1 per 25,000 to 1 in 50,000 in Northern Europe (Genetics Home Reference; Man, Turnbull, & Chinnery, 2002). The population frequency of the mutations listed in Table 11.1 is likely to be considerably higher in entire populations but is currently unknown. In Finland, about 1 per 50,000 people shows LHON, whereas roughly 1 per 9000 carries one of the three major mutations (Puomila et al., 2007). Much of the discrepancy arises from incomplete penetrance and the sex difference. Mutation frequencies in local populations in the Americas can deviate greatly from those in their countries of origin in Europe because of the specific individuals who first immigrated. When most people in a modern population descend from one or a few immigrant women, the mutation frequency will tend to reflect the genotypes of the founders. For example, researchers in Montreal, Canada, examined mtDNA of 42 patients diagnosed as LHON and their extended families, testing for the three most common mutations listed in Table 11.1. Complete data were obtained from 31 index cases and 91 relatives for a total sample of 122 individuals (Macmillan et al., 1998). The large majority (89%) of index cases and relatives carried the 14484T > C mutation, whereas in Europe, the 11778G > A mutation is by far the most common (about 70%) and in Asia it accounts for more than 90% of LHON cases. Analysis of eight other mtDNA mutations in the D-loop zone that lacks genes revealed that all individuals in the Quebec sample with the 14484T > C mutation had nearly identical sets of mutations at those eight sites, none of which is related to LHON. This made it highly likely that all cases of LHON in the study sample descended from just one immigrant woman who moved to Quebec more than 200 years ago. She was the founder of a large group of people with a common ancestry on the female side of the pedigree. The founder effect thus generated a local population with a high frequency of the 14484T > C mutation embedded in a surrounding population with mainly 11778G > A mutations. The large discrepancy in allele prevalence rate was attributed to a founder effect.
WHAT THE GENES DO PREVALENCE AND FOUNDER EFFECTS Because of the rather low penetrance for several mutations, the large sex difference in expression, and the wide range of age of onset of LHON, it is difficult to give a precise estimate of how many people in a population carry a specific mutation in the mtDNA. No population surveys of mtDNA mutations have yet been published. Almost all data have been collected in families having one or more index cases who experienced some kind of medical problem and sought help. The frequency of LHON itself has
The small size of the mitochondrial genome with relatively few genes involved in common processes should make it easier to understand what those individual genes do and how the knowledge might aid the search for a means of curing or preventing LHON. Mitochondria are present in every cell in the body and perform two kinds of functions that are absolutely essential in all kinds of cells. Because of their central place in the biochemistry of life, mitochondrial genes have been studied extensively and are relatively well understood (Chinnery &
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WHAT THE GENES DO
Hudson, 2013; Tuppen, Blakely, Turnbull, & Taylor, 2010). The first kind of function is protein synthesis. As described in Chapter 3, mRNA is translated into protein at the ribosome in the cytoplasm. The ribosome is made of several subunits, including two kinds of RNA that are encoded in the MT-RNR1 and MT-RNR2 genes, and the translation of each set of three nucleotides in mRNA into an amino acid is accomplished via small tRNA molecules whose structures are encoded in mitochondrial genes (Table 11.2). Any major mutation in any one of these genes will usually be fatal. No known variants have any involvement in LHON. The second function is the production of energy for use in all kinds of metabolic processes throughout the body (Lodish, Berk, & Zipursky, 2000). The key molecule in this process is adenosine triphosphate or ATP. It is manufactured from adenosine diphosphate or ADP in the mitochondria and then transported to the cytoplasm of the cell. One might imagine a world in which just one enzyme could convert ADP to ATP, similar to the way phenylalanine is converted to tyrosine in the liver by the enzyme phenylalanine hydroxylase (Fig. 8.1). But the world we inhabit has devised a more intricate mechanism that depends on numerous proteins arranged in a series of complexes—the respiratory chain—and is encoded by multiple genes, some of which are part of the mitochondrial genome. Through this process, the digestion of dietary sugar and other nutrients enables the cell to create an abundance of ATP. It has been estimated that one molecule of glucose, when fully metabolized to CO2 and water (H2O), enables the synthesis of 30 molecules of ATP (Berg, Tymoczko, & Stryer, 2002). Mutations of the subunits of the large protein complexes can impair the efficiency of this process and cause local shortages of energy to power cellular processes. Certain of these mutations can cause LHON. TABLE 11.2
Five kinds of protein complexes are arranged in a series along the inner membrane of the mitochondrion. Certain of the complexes are tightly linked to other processes that metabolize carbohydrates, fats, and proteins. The term “complex” is well chosen, because the structures are both marvelous and intimidating. Why so many components are needed to make a single complex work properly is an unanswered question. Mutations in the protein subunits prove beyond doubt that things must be arranged in this way if the whole is to work properly. The net effect is that hydrogen ions or protons (H+) are generated at each step and cause a cascade of changes that deliver three such ions to complex V where ADP is converted to ATP. Two special features of this process are relevant to the discussion of LHON. First, it is well established that the molecular complexes I–V (or 1–5) utilize proteins that are mostly encoded in genes residing in the cell nucleus (Table 11.2) and then transported into the mitochondria where they work hand in hand with proteins derived from mtDNA. This division of tasks has evolved over many hundreds of millions of years. It is apparent that mitochondria originated as bacteria that were then incorporated into larger cells having additional genes (Martin, Garg, & Zimorski, 2015), although specifics of just how this happened long ago are lacking. Gradually, the two life forms merged into one integral whole. Second, the genes and proteins involved in mitochondrial function are ubiquitous (present and necessary in all cells) and have nothing to do in any specific way with nervous system function or vision. Nevertheless, mutations at certain sites in the mtDNA molecule can impair the functioning of the cascade of energy along the complex I–V chain in a way that impairs the functioning of retinal ganglion cells more than most other kinds of cells. Certain specific mitochondrial mutations result in what is now called LHON.
Genes Specifying Parts of Mitochondrial Complexes That Are Encoded in Mitochondrial and Nuclear DNA
Complex
MT genes
Genes in nucleus
MT gene symbols
MT gene names
1
7
38
MT-ND1, MT-ND2, MT-ND3, MT-ND4, MT-ND4L, MT-ND5, MT-ND6
NADH-ubiquinone oxidoreductase subunits 1, 2, 3, 4, 4L, 5, 6
2
0
4
None
None
3
1
9
MT-CYB
Cytochrome b subunit III
4
3
16
MT-CO1, MT-CO2, MT-CO3
Cytochrome c oxidase subunit 1, 2, 3
5
2
17
MT-ATP6, MT-ATP8
Adenosine triphosphate ATP synthase 6, 8
Ribosome
2
>150
MT-RNR1, MT-RNR2
Ribosomal RNA 12S, 16S
tRNA
22
0
MT-TA to TY
22 Transfer RNAs, coding for alanine (A) to tyrosine (Y)
Sources: Human Gene Nomenclature, Mitochondrial respiratory chain complex, HGNC; Gene Cards; OMIM; Anderson, S., Bankier, A. T., Barrell, B. G., de Bruijn, M. H., Coulson, A. R., Drouin, J., et al. (1981). Sequence and organization of the human mitochondrial genome. Nature, 290, 457–465; Wallace, D. C. (1994). Mitochondrial DNA mutations in diseases of energy metabolism. Journal of Bioenergetics and Biomembranes, 26, 241–250.
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11. LEBER’S OPTIC NEUROPATHY
Are the multiple proteins arranged into intricate complexes unique to mitochondrial metabolism? Probably not. The five complexes appear to have many more subunits than, for example, the sarcomere in striped muscles shown in Fig. 4.8, but that diagram is a simplification of an organelle that involves many more kinds of proteins and genes that are not shown. The apparent complexity of a structure reflects to a large degree the knowledge we have about it based on extensive research. Recall the evidence cited in Chapter 3 on gene expression that indicates hundreds and even thousands of genes are expressed in many kinds of tissues and cells. Those diverse proteins in a cell are not just floating randomly in a cytoplasmic soup. They must be arranged into structures wherein many kinds of proteins adhere to each other in order to make things work properly. Discerning the nature of those structures is not an easy task. Each mitochondrial complex is itself the focus of many talented scientists working in teams and exchanging information daily (e.g., www.complexi.org and www.atpsynthase. info). The mtDNA sequence was decoded in 1981, and LHON was ascribed to a specific mutation in the MT-ND4 gene in 1988. Thirty years later, it is easy enough to point to the place in a mitochondrial protein complex where a mutation can cause LHON, but it is still not understood just how the altered protein impairs functioning of retinal ganglion cells in particular or how some people can carry the identical mutation but not show any symptoms at all.
TREATMENT AND CURE There is no generally accepted treatment for LHON, according to a review of the medical literature (Yu-WaiMan, Griffiths, Hudson, & Chinnery, 2009) and the 2016 edition of the EyeWiki website of the American Academy of Ophthalmology. The Genetic and Rare Disease Information Center (GARD) website in 2017 notes that there is no cure, but ongoing studies aim to find a treatment. One option is oral idebenone, a synthetic analog of ubiquinone that is involved in the cascade that generates ATP in mitochondria. Idebenone, originally intended to treat dementia and now marketed as Raxone, is said to be the only clinically proved treatment option for LHON (Gueven, 2016). There are facts to support the claim about efficacy of idebenone, yet the picture is not so clear. The European Medicines Agency in 2013 refused to license it on the grounds that the clinical benefit appeared to be too small to warrant approval (Refusal of the marketing authorisation for Raxone (idebenone), 2013). In view of this continuing uncertainty about idebenone, it is illuminating to examine the principal evidence of its benefits. A study reported by Klopstock et al. (2011)
involved 85 patients diagnosed with LHON and carrying one of the three most common mtDNA mutations (Table 11.1). They were randomly assigned to receive either a placebo or idebenone for 24 weeks. Group assignment was constrained to give equal proportions of men and women and the three genotypes in the two treatment conditions. Tests of visual acuity were given before and after 4, 12, and 24 weeks of treatment by people who did not know which patients were receiving the drug. Because neither the patients nor those giving the tests knew who was getting idebenone, it was a well-controlled doubleblind study. The researchers also declared in advance that a particular outcome measure would serve as the primary end point of the study: best recovery score. The recovery of vision score was the difference between baseline acuity measures at the outset and after the 24-week period when using the Early Treatment Diabetic Retinopathy Study (ETDRS) chart (Fig. 6.1). Patients in both conditions scored better on average on the 24-week test than on the initial baseline test. Average recovery scores were higher for the idebenone-treated patients than those receiving placebo, but the difference was not very large. As the authors remarked, “the difference between groups did not reach statistical significance” (p. 2680). The authors of the study were very forthright about the implications of their findings: “considerable advances in our understanding of the molecular and biochemical basis of mitochondrial DNA-associated diseases have not yet translated into treatments of proven efficacy.” Being a drug with “antioxidant” effects on mitochondrial metabolism, idebenone has been advocated or at least mentioned on several websites (e.g., WebMD, HBC, and SDFT) as a treatment for a large number of conditions: dementia, Alzheimer’s disease, Friedreich’s ataxia, Parkinson’s disease, multiple sclerosis, liver and heart disease, and LHON. The drug is moving through the approval process in the United States as an agent to combat Duchenne muscular dystrophy. Even though it has not been formally approved by governments to treat these conditions, at the present time, idebenone is readily available via the Internet as a “dietary supplement” and as an “antiaging” skin wrinkle therapy. The widespread publicity and easy availability of idebenone create further challenges for researchers seeking conclusive evidence that it really works. Being a relatively rare disease, LHON requires a major effort to find enough patients willing to take part in a clinical trial. Not just willing but drug-free patients are needed. Authors of the double-blind study lamented that “patients often find the prospect of taking placebo unacceptable and self-medicate, using Internetbased suppliers of … unapproved medication” (Klopstock et al., 2011). Those who have already been taking idebenone on their own will probably not be suitable subjects for a well-controlled clinical trial.
REFERENCES
Prevention of LHON is not yet possible. A person can carry an undetected mutation in mtDNA for years before symptoms appear in young adults. If there is a family history of LHON, genetic testing may uncover that same mutation in other relatives, some of whom may still be asymptomatic. If only there were an effective treatment for those who already show symptoms of LHON, that treatment might be worth a try as a preventive medicine, provided side effects are minimal. Indeed, it might be more effective at prevention than as a treatment for a retina that has already suffered major damage. Several medical authorities recommend that those at risk for LHON because of a family history refrain from smoking tobacco and drinking large amounts of alcohol, which is good advice to all of us to prevent disease.
HIGHLIGHTS • The mitochondria, minute organelles in the cytoplasm with their own DNA (mtDNA with 16,569 base pairs encoding 37 genes), are transmitted from mother to both sons and daughters. • Mutations in several mitochondrial genes can lead to a form of blindness known as LHON that is much more prevalent in males than females. • Most mutations causing LHON show incomplete penetrance, whereby many people who carry the mutation show no symptoms at all. A woman with no symptoms can nevertheless transmit an mtDNA mutation to her children. • Specific mutations sometimes show a striking founder effect in which dozens of people descended from the same immigrant woman carry the identical mutation. • Unknown environmental factors clearly influence the degree of penetrance. Smoking appears to increase the risk of LHON in males. • The mitochondrial genes involved in LHON are part of large molecular complexes consisting of proteins derived from dozens of genes in both the nucleus and the mitochondria that combine into complexes to derive energy from glucose to power many cell processes.
References Anderson, S., Bankier, A. T., Barrell, B. G., de Bruijn, M. H., Coulson, A. R., Drouin, J., et al. (1981). Sequence and organization of the human mitochondrial genome. Nature, 290, 457–465. Berg, J. M., Tymoczko, J. L., & Stryer, L. (2002). Section 18.3, The respiratory chain consists of four complexes: three proton pumps and a physical link to the citric acid cycle. In Biochemistry (5th ed.). New York: W.H. Freeman. Borst, P. (1977). Structure and function of mitochondrial DNA. Trends in Biochemical Sciences, 2, 31–34.
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Chalmers, R. M., & Schapira, A. H. (1999). Clinical, biochemical and molecular genetic features of Leber’s hereditary optic neuropathy. Biochimica et Biophysica Acta (BBA)—Bioenergetics, 1410, 147–158. Chinnery, P. F., & Hudson, G. (2013). Mitochondrial genetics. British Medical Bulletin, 106, 135–159. Collinge, J. (2001). Prion diseases of humans and animals: their causes and molecular basis. Annual Review of Neuroscience, 24, 519–550. Dickinson, A. G., Young, G. B., Stamp, J. T., & Renwick, C. C. (1965). An analysis of natural scrapie in Suffolk sheep. Heredity, 20, 485–503. Erickson, R. P. (1972). Leber’s optic atrophy, a possible example of maternal inheritance. American Journal of Human Genetics, 24, 348–349. Gueven, N. (2016). Idebenone for Leber’s hereditary optic neuropathy. Drugs Today, 52, 173–181. Imai, Y., & Moriwaki, D. (1936). A probable case of cytoplasmic inheritance in man: a critique of Leber’s disease. Journal of Genetics, 33, 163–167. Kerrison, J. B., Miller, N. R., Hsu, F., Beaty, T. H., Maumenee, I. H., Smith, K. H., et al. (2000). A case-control study of tobacco and alcohol consumption in Leber hereditary optic neuropathy. American Journal of Ophthalmology, 130, 803–812. Klopstock, T., Yu-Wai-Man, P., Dimitriadis, K., Rouleau, J., Heck, S., Bailie, M., et al. (2011). A randomized placebo-controlled trial of idebenone in Leber’s hereditary optic neuropathy. Brain, 134, 2677–2686. € Leber, T. (1871). Uber heredit€are und congenital-angelegte Sehnervenleiden. Graefe’s Archive for Clinical and Experimental Ophthalmology, 17, 249–291. Lodish, H., Berk, A., & Zipursky, S. L. (2000). Molecular cell biology (4 ed.). New York: W. H. Freeman & Company. Macmillan, C., Kirkham, T., Fu, K., Allison, V., Andermann, E., Chitayat, D., et al. (1998). Pedigree analysis of French Canadian families with T14484C Leber’s hereditary optic neuropathy. Neurology, 50, 417–422. Man, P., Griffiths, P., Brown, D., Howell, N., Turnbull, D., & Chinnery, P. (2003). The epidemiology of Leber hereditary optic neuropathy in the North East of England. The American Journal of Human Genetics, 72, 333–339. Man, P. Y. W., Turnbull, D., & Chinnery, P. (2002). Leber hereditary optic neuropathy. Journal of Medical Genetics, 39, 162–169. Martin, W. F., Garg, S., & Zimorski, V. (2015). Endosymbiotic theories for eukaryote origin. Philosophical Transactions of the Royal Society of London Series B: Biological Sciences, 370, 20140330. Prusiner, S. B. (1982). Novel proteinaceous infectious particles cause scrapie. Science, 216, 136–144. Puomila, A., H€am€al€ainen, P., Kivioja, S., Savontaus, M.-L., Koivum€aki, S., Huoponen, K., & Nikoskelainen, E. (2007). Epidemiology and penetrance of Leber hereditary optic neuropathy in Finland. European Journal of Human Genetics, 15, 1079–1089. Refusal of the marketing authorisation for Raxone (idebenone) (2013). European Medicines Agency (pp. 1–2). . Requena, J. R., & Wille, H. (2014). The structure of the infectious prion protein: experimental data and molecular models. Prion, 8, 60–66. Sadun, A. A., Carelli, V., Salomao, S. R., Berezovsky, A., Quiros, P. A., Sadun, F., et al. (2003). Extensive investigation of a large Brazilian pedigree of 11778/haplogroup J Leber hereditary optic neuropathy. American Journal of Ophthalmology, 136, 231–238. Taubes, G. (1986). The game of the name is fame. But is it science? Discover, 7, 28–52. Tran, M., Bhargava, R., & MacDonald, I. M. (2001). Leber hereditary optic neuropathy, progressive visual loss, and multiple-sclerosis-like symptoms. American Journal of Ophthalmology, 132, 591–593. Tsao, K., Aitken, P. A., & Johns, D. R. (1999). Smoking as an aetiological factor in a pedigree with Leber’s hereditary optic neuropathy. British Journal of Ophthalmology, 83, 577–581. Tuppen, H. A., Blakely, E. L., Turnbull, D. M., & Taylor, R. W. (2010). Mitochondrial DNA mutations and human disease. Biochimica et Biophysica Acta (BBA) – Bioenergetics, 1797, 113–128.
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Waardenburg, P. J. (1969). Some remarks on the clinical and genetic puzzle of Leber’s optic neuritis. Journal de Genetique Humaine, 17, 479–495. Wallace, D. (1970). A new manifestation of Leber’s disease and a new explanation for the agency responsible for its unusual pattern of inheritance. Brain, 93, 121–132. Wallace, D. C., Singh, G., Lott, M. T., Hodge, J. A., Schurr, T. G., Lezza, A. M. S., et al. (1988). ). Mitochondrial DNA mutation associated with Leber’s hereditary optic neuropathy. Science, 242, 1427–1430. Yu-Wai-Man, P., Griffiths, P. G., Hudson, G., & Chinnery, P. F. (2009). Inherited mitochondrial optic neuropathies. Journal of Medical Genetics, 46, 145–158.
Further Reading Taanman, J. W. (1999). The mitochondrial genome: structure, transcription, translation and replication. Biochimica et Biophysica Acta (BBA) – Bioenergetics, 1410, 103–123. Wallace, D. C. (1994). Mitochondrial DNA mutations in diseases of energy metabolism. Journal of Bioenergetics and Biomembranes, 26, 241–250.
11 Leber’s Optic Neuropathy The German ophthalmologist Theodore Leber published a description in 1871 of a peculiar form of blindness that occurred suddenly in young adults (Leber, 1871). Within just a few weeks, there was major loss of central vision in one eye, followed within a few more weeks by loss in the other. He noted that the disorder clustered in certain families and males were more often affected than females. He also discerned that the disorder could be transmitted to a son by a mother who did not show symptoms herself. Other investigators compiled pedigrees of families hosting Leber’s form of blindness, and it became apparent that the disease was always transmitted via the mother (Imai & Moriwaki, 1936) and probably involved some feature of the cytoplasm of the egg. A man with Leber’s disease never had sons or daughters with the same disorder. The pattern resembled that of an X-linked gene like the androgen receptor, wherein XY persons always obtain it from the mother, but some facts did not fit with X-linkage. The frequency of affected daughters was quite high in some families, and in large pedigrees, almost all daughters passed the defect to some of their offspring. The proportions of affected sons and daughters differed greatly among families but did not seem to fit the expected proportions for a single gene on the X. One author observed that the disease was a “clinical and genetic puzzle” (Waardenburg, 1969). The disorder became known as Leber’s hereditary optic neuropathy (LHON). The H term was added to distinguish it from other forms of blindness, but that really was not necessary. Using Leber’s name clearly meant a form of blindness transmitted via the mother. Huntington disease, AIS (see Chapter 10 on Androgen Insensitivity Syndrome), and Down syndrome never include the H term. Nevertheless, traditional usage prevails today. Some authors refer to LHON as Leber’s neuritis rather than neuropathy. Neuritis indicates inflammation of the optic nerve, whereas neuropathy is a more general term that can involve a significant deterioration and even death of tissue, something that occurs in many but not all cases. Discovery of a genetic cause clarified these quandaries of diagnosis and naming.
Genes, Brain Function, and Behavior https://doi.org/10.1016/B978-0-12-812832-9.00011-7
While scientists were struggling to solve the puzzle, a new phenomenon was discovered among sheep in England where a disease called scrapie was shown to be passed from mother to offspring by an infective agent, a strange kind of virus resident in the cell cytoplasm that worked very slowly to degrade the nervous system (Dickinson, Young, Stamp, & Renwick, 1965). A similar disease sometimes affected cattle—bovine spongiform encephalopathy (BSE) or mad cow disease. It appeared that Creutzfeld-Jakob disease (CJD), a rare but fatal neurological disorder in humans, also arose from a virus that was similar to BSE. In a few cases, the infective agent was passed to unrelated persons via surgical instruments used in brain surgery on CJD patients. It was then proposed that Leber’s optic neuropathy is transmitted by an infective agent that resides in the cytoplasm (Erickson, 1972; Wallace, 1970), which might explain why it is transmitted via the mother but does not seem to occur in definite ratios of affected and unaffected offspring the way a classical Mendelian gene would behave. A lively controversy ensued. One author proposed that the infective agent is a protein called a “prion” (Prusiner, 1982), while others argued forcefully that transmission across generations must involve the nucleic acids RNA and DNA (Taubes, 1986). The reality of a novel form of protein that can transmit CJD and BSE has been confirmed (Collinge, 2001), but doubts linger. The precise structure of the infectious agent has not yet been elucidated (Requena & Wille, 2014). The official fact sheet about CJD by the National Institute for Neurological Diseases and Stroke in the United States in 2017 designates the prion as “the leading scientific theory” about CJD rather than established fact. Further study of LHON, on the other hand, showed definitively that it is not related to CJD or BSE and is not caused by an infectious protein. Instead, it arises from genetic mutations of a conventional kind, except that the DNA is located in an unusual place.
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11. LEBER’S OPTIC NEUROPATHY
THE PUZZLE SOLVED: MITOCHONDRIA New discoveries pointed to genes in a small organelle in the cytoplasm, the mitochondrion. It occurs in all cells and has its own DNA that is transmitted to both male and female children via the maternal cytoplasm (Borst, 1977). It even has its own special variant of the genetic code (Table 2.1) in which the triplet UGA codes for tryptophan instead of STOP and AUA codes for methionine instead of isoleucine. The mitochondrial genome is much smaller than that of the chromosomes in the cell nucleus, and it lacks introns. A major effort was made to determine its DNA sequence. This was achieved (Anderson et al., 1981) more than 25 years before the immense chromosomal genome sequence was finished in 2006. The mtDNA has precisely 16,569 base pairs arranged in a circle (Fig. 11.1), and it contains 37 genes, 13 of which code (12s rRNA)
RNR1
F
PT
D-loop
V
CYB
OH
E
(16s rRNA)
15257
RNR2
ND6
14484 14459
L1
3460
ND1 I Q M
mtDNA 16,569 bp
13051
13 proteins 2 rRNA 22 tRNA
ND2 W AN C Y
ND5
L2 S2 H
11778
ND4
10663
OL
R
G
CO1 S1 D
CO2
FIG. 11.1
ND4L
ND3
CO3 K
ATP6 ATP8
Thirty-seven genes comprising the circular DNA of mitochondria. Thirteen code for proteins that act as enzymes (ATP6 and ATP8; CO1, CO2, and CO3; CYB; and ND1 to ND6), two code for ribosomal RNAs (RNR1 and RNR2), and 22 code for tRNA (A to Y). Full gene symbols and names are given in Table 11.2. The DNA is arranged into two circular chains termed light and heavy that are superimposed in this diagram. Sites where transcription is started for the light and heavy chains are shown as OL and OH. Locations of mutations known to give rise to LHON are shown inside the circle at the base number where a base substitution occurs. The three most common mutations listed in Table 11.1 are shown by *. Sources: http://www.mitomap.org 2017 “Morbid map of the human mtDNA genome”; Anderson, S., Bankier, A. T., Barrell, B. G., de Bruijn, M. H., Coulson, A. R., Drouin, J., et al. (1981). Sequence and organization of the human mitochondrial genome. Nature, 290, 457–465; Chinnery, P. F., & Hudson, G. (2013). Mitochondrial genetics. British Medical Bulletin, 106, 135–159; Taanman, J. W. (1999). The mitochondrial genome: structure, transcription, translation and replication. Biochimica et Biophysica Acta (BBA) – Bioenergetics, 1410, 103–123; Tuppen, H. A., Blakely, E. L., Turnbull, D. M., & Taylor, R. W. (2010). Mitochondrial DNA mutations and human disease. Biochimica et Biophysica Acta (BBA) – Bioenergetics, 1797, 113–128.
for enzymes with critical roles in metabolism and energy production in all cells. Two genes code for an essential part of the mechanism for synthesis of all proteins in every kind of cell in the body—the RNA involved in making a ribosome (rRNA)—and 22 others specify the structures of the small transfer RNAs (tRNA) that help to assemble a new protein (Fig. 3.1). One segment of mtDNA known as the D loop contains no genes and is a hotspot for mutations because they have no phenotypic effects and therefore are not pruned from a family tree. Knowing the normal DNA sequence of human mitochondria, organelles that are always transmitted via the mother, researchers assessed several pedigrees of families in which there were several cases of LHON. Wallace et al. (1988) examined 33 people, 23 of them blind, from nine families and found that all of those with LHON had the A nucleotide substituted for G at position 11778 that resulted in the amino acid arginine instead of histidine at position 340 in the protein encoded by the MT-ND4 gene. This was the first point mutation in any gene that was shown to cause a neurological disorder. But in two pedigrees, that mutation was not the cause of the blindness; instead, mutations elsewhere in the mtDNA gave rise to almost identical clinical phenotypes of blindness. Furthermore, there was a wide range of severity of symptoms in those with the same mutation. Thus, discovery of the 11778G > A mutation was a big step forward but did not explain everything about LHON. Medical geneticists around the world then tested many other cases of LHON and found even more mutations in mitochondrial genes associated with the maternally transmitted factor causing blindness. Table 11.1 summarizes data for 12 mutations involving six different mitochondrial genes that cause LHON in different families. The MitoMaps database now lists 18 mutations known to cause LHON in several members of different families and an additional 17 mutations identified in only a single family, sometimes involving just one individual case. The MalaCards database lists 43 different mitochondrial mutations, most of which have been observed in just one family. The overwhelming majority of all cases worldwide involve just one of three mutations, those at positions 3460, 11778, and 14484 in genes MT-ND1, MT-ND4, and MT-ND6, respectively. The 11778 mutation is most common, especially in Asia where it accounts for more than 90% of LHON cases. All of the mitochondrial genes coding for enzymes or parts of them are intimately involved in a biochemical process that enables all cells to derive energy from food. It is clear from the example of LHON that the same abnormal phenotype, a clinical syndrome, can be caused by different mutations in different genes.
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INCOMPLETE PENETRANCE, SEX, SMOKING, RECOVERY
INCOMPLETE PENETRANCE, SEX, SMOKING, RECOVERY Having an unambiguous biochemical means to identify mitochondrial mutations, researchers soon learned that many people in affected families carry a mutation but never show vision loss at any age. This phenomenon denotes incomplete penetrance in the parlance of genetics. The term was coined at a time when many believed that a gene codes for some interesting trait or character such as vision or blindness. When every person having the mutation shows the abnormal phenotype, it is said that the information in the gene penetrates to the observable level of a phenotype; it is fully penetrant. If only some with the abnormal genotype show the neurological defect, penetrance is said to be incomplete. Penetrance is a matter of gene expression, not gene transmission. That is why a mother can pass a mutation causing LHON to her offspring while she herself has normal vision. The data in Table 11.1 show the percentages of all male family members with a specific mitochondrial mutation who also show the symptoms of LHON. None of the better known mutations has a penetrance close to 100% among people in middle age. This fact generates hope for the future. If the reasons for some people being symptom-free can be discovered, they may point to things that can be done to prevent others from developing LHON. An even more potent source of incomplete penetrance is genetic sex. Females are far less likely to show LHON when they possess a mutation listed in Table 11.1. In situations in which about half of males with a mutation show LHON, 10% or fewer females usually show vision
TABLE 11.1
loss when they carry the same mutation. The reason for this dramatic sex difference is uncertain. Another possible source of low versus high penetrance might be the features of the environment. Many studies have found hints and suggestions that environmental toxins contribute to LHON. It is conceivable that a particular toxin has little harmful effect on vision in most people, whereas, among those carrying a mitochondrial gene mutation, the same toxin could harm cells in the retina. In a large family in Brazil with many cases of LHON extending over seven generations, all were descended from just one immigrant woman from Verona, Italy, and every one of 265 cases showing LHON carried the 11778 mutation (Sadun et al., 2003). The decline in penetrance over generations was striking (Fig. 11.2). In the second and third generations living in Brazil, 50%–70% of LHON cases were male, whereas in generations four to six, all were male. In the Brazilian family, the rate of smoking tobacco was 70% among those with LHON but only 16% among people carrying the 11778 mutation who did not show vision loss. LHON was also more common among those who consumed alcohol. The effect of smoking was similar to an earlier study (Tsao, Aitken, & Johns, 1999) that found more than 90% penetrance among male smokers versus <50% penetrance among nonsmokers in families showing LHON. A large study of LHON in the United Kingdom corroborated the large sex difference and also noted that penetrance in male smokers exceeded 90% if they lived long enough. The verdict on smoking and LHON is not yet final, because several research groups have not detected the same pattern in their study populations (e.g., Kerrison et al., 2000).
Mutations Causing LHON
Mutation
Gene
Percentage of all cases
Penetrance in males
Recovery rate
Vision loss
3460G>A
MT-ND1
13%
40%–80%
22%
Can see some light
11778G>A
MT-ND4
69%
82%
4%
Can see no light
14484T>C
MT-ND6
14%
68%
37%–65%
Can count fingers
15257G>A
MT-CYB
Rare
72%
28%
Can see hand motion
3635G>A
MT-ND1
Rare
54%
Low
?
3733G>A
MT-ND1
Rare
36%–44%
Yes
?
4171C>A
MT-ND1
Rare
47%
Yes
?
10663T>C
MT-ND4L
Rare
60%
?
?
13051G>A
MT-ND5
Rare
63%
?
?
14482C>A
MT-ND6
Rare
89%
Yes
?
14459G>A
MT-ND6
Rare
10%
No
Severe loss
14502T>C
MT-ND6
Rare
11%
?
?
? ¼ unknown or not specified. Sources: MitoMaps, OMIM.
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11. LEBER’S OPTIC NEUROPATHY
% of Carriers with LHON
50%–70% male 70 60 50 All male
40 30 20 10 2
5 4 3 Generation
6
FIG. 11.2
The percentage of LHON cases among confirmed carriers of a mutation in a mitochondrial gene that is associated with LHON. Data from a large pedigree of 265 members of a multigeneration family in Brazil who were all descended from a single female immigrant born in Italy in 1861. All possessed the same 11778 mutation in the same gene (ND4). Overall penetrance declined substantially over generations, and the proportion of male LHON cases rose to 100%. Among carriers with LHON, 70% were smokers, while smoking occurred in only 16% of carriers who did not show LHON. Adapted from Sadun, A. A., Carelli, V., Salomao, S. R., Berezovsky, A., Quiros, P. A., Sadun, F., et al. (2003). Extensive investigation of a large Brazilian pedigree of 11778/haplogroup in Leber’s hereditary optic neuropathy. American Journal of Ophthalmology, 136, 231–238, Copyright 2003, with permission from Elsevier.
The typical course of LHON begins with mild blurring of vision in the central part of the visual field of one eye and then progresses over several weeks to severe loss of vision in both eyes. Most notably affected are the retinal ganglion cells that gather information from across the retina and send it along the optic nerve. In many cases of LHON, large numbers of those cells die and degenerate, followed by the degeneration of the myelin sheaths and axons in the optic nerve, similar to what is seen in multiple sclerosis (Chalmers & Schapira, 1999; Tran, Bhargava, & MacDonald, 2001). Loss of vision is severe when many cells and axons perish. Nevertheless, for reasons that are not well understood, many cases experience gradual recovery of visual capacity after reaching a nadir, especially for the 14484T > C mutation. Recovery further reduces penetrance.
been variously estimated at 1 per 8500 in England (Man et al., 2003) and 1 per 25,000 to 1 in 50,000 in Northern Europe (Genetics Home Reference; Man, Turnbull, & Chinnery, 2002). The population frequency of the mutations listed in Table 11.1 is likely to be considerably higher in entire populations but is currently unknown. In Finland, about 1 per 50,000 people shows LHON, whereas roughly 1 per 9000 carries one of the three major mutations (Puomila et al., 2007). Much of the discrepancy arises from incomplete penetrance and the sex difference. Mutation frequencies in local populations in the Americas can deviate greatly from those in their countries of origin in Europe because of the specific individuals who first immigrated. When most people in a modern population descend from one or a few immigrant women, the mutation frequency will tend to reflect the genotypes of the founders. For example, researchers in Montreal, Canada, examined mtDNA of 42 patients diagnosed as LHON and their extended families, testing for the three most common mutations listed in Table 11.1. Complete data were obtained from 31 index cases and 91 relatives for a total sample of 122 individuals (Macmillan et al., 1998). The large majority (89%) of index cases and relatives carried the 14484T > C mutation, whereas in Europe, the 11778G > A mutation is by far the most common (about 70%) and in Asia it accounts for more than 90% of LHON cases. Analysis of eight other mtDNA mutations in the D-loop zone that lacks genes revealed that all individuals in the Quebec sample with the 14484T > C mutation had nearly identical sets of mutations at those eight sites, none of which is related to LHON. This made it highly likely that all cases of LHON in the study sample descended from just one immigrant woman who moved to Quebec more than 200 years ago. She was the founder of a large group of people with a common ancestry on the female side of the pedigree. The founder effect thus generated a local population with a high frequency of the 14484T > C mutation embedded in a surrounding population with mainly 11778G > A mutations. The large discrepancy in allele prevalence rate was attributed to a founder effect.
WHAT THE GENES DO PREVALENCE AND FOUNDER EFFECTS Because of the rather low penetrance for several mutations, the large sex difference in expression, and the wide range of age of onset of LHON, it is difficult to give a precise estimate of how many people in a population carry a specific mutation in the mtDNA. No population surveys of mtDNA mutations have yet been published. Almost all data have been collected in families having one or more index cases who experienced some kind of medical problem and sought help. The frequency of LHON itself has
The small size of the mitochondrial genome with relatively few genes involved in common processes should make it easier to understand what those individual genes do and how the knowledge might aid the search for a means of curing or preventing LHON. Mitochondria are present in every cell in the body and perform two kinds of functions that are absolutely essential in all kinds of cells. Because of their central place in the biochemistry of life, mitochondrial genes have been studied extensively and are relatively well understood (Chinnery &
133
WHAT THE GENES DO
Hudson, 2013; Tuppen, Blakely, Turnbull, & Taylor, 2010). The first kind of function is protein synthesis. As described in Chapter 3, mRNA is translated into protein at the ribosome in the cytoplasm. The ribosome is made of several subunits, including two kinds of RNA that are encoded in the MT-RNR1 and MT-RNR2 genes, and the translation of each set of three nucleotides in mRNA into an amino acid is accomplished via small tRNA molecules whose structures are encoded in mitochondrial genes (Table 11.2). Any major mutation in any one of these genes will usually be fatal. No known variants have any involvement in LHON. The second function is the production of energy for use in all kinds of metabolic processes throughout the body (Lodish, Berk, & Zipursky, 2000). The key molecule in this process is adenosine triphosphate or ATP. It is manufactured from adenosine diphosphate or ADP in the mitochondria and then transported to the cytoplasm of the cell. One might imagine a world in which just one enzyme could convert ADP to ATP, similar to the way phenylalanine is converted to tyrosine in the liver by the enzyme phenylalanine hydroxylase (Fig. 8.1). But the world we inhabit has devised a more intricate mechanism that depends on numerous proteins arranged in a series of complexes—the respiratory chain—and is encoded by multiple genes, some of which are part of the mitochondrial genome. Through this process, the digestion of dietary sugar and other nutrients enables the cell to create an abundance of ATP. It has been estimated that one molecule of glucose, when fully metabolized to CO2 and water (H2O), enables the synthesis of 30 molecules of ATP (Berg, Tymoczko, & Stryer, 2002). Mutations of the subunits of the large protein complexes can impair the efficiency of this process and cause local shortages of energy to power cellular processes. Certain of these mutations can cause LHON. TABLE 11.2
Five kinds of protein complexes are arranged in a series along the inner membrane of the mitochondrion. Certain of the complexes are tightly linked to other processes that metabolize carbohydrates, fats, and proteins. The term “complex” is well chosen, because the structures are both marvelous and intimidating. Why so many components are needed to make a single complex work properly is an unanswered question. Mutations in the protein subunits prove beyond doubt that things must be arranged in this way if the whole is to work properly. The net effect is that hydrogen ions or protons (H+) are generated at each step and cause a cascade of changes that deliver three such ions to complex V where ADP is converted to ATP. Two special features of this process are relevant to the discussion of LHON. First, it is well established that the molecular complexes I–V (or 1–5) utilize proteins that are mostly encoded in genes residing in the cell nucleus (Table 11.2) and then transported into the mitochondria where they work hand in hand with proteins derived from mtDNA. This division of tasks has evolved over many hundreds of millions of years. It is apparent that mitochondria originated as bacteria that were then incorporated into larger cells having additional genes (Martin, Garg, & Zimorski, 2015), although specifics of just how this happened long ago are lacking. Gradually, the two life forms merged into one integral whole. Second, the genes and proteins involved in mitochondrial function are ubiquitous (present and necessary in all cells) and have nothing to do in any specific way with nervous system function or vision. Nevertheless, mutations at certain sites in the mtDNA molecule can impair the functioning of the cascade of energy along the complex I–V chain in a way that impairs the functioning of retinal ganglion cells more than most other kinds of cells. Certain specific mitochondrial mutations result in what is now called LHON.
Genes Specifying Parts of Mitochondrial Complexes That Are Encoded in Mitochondrial and Nuclear DNA
Complex
MT genes
Genes in nucleus
MT gene symbols
MT gene names
1
7
38
MT-ND1, MT-ND2, MT-ND3, MT-ND4, MT-ND4L, MT-ND5, MT-ND6
NADH-ubiquinone oxidoreductase subunits 1, 2, 3, 4, 4L, 5, 6
2
0
4
None
None
3
1
9
MT-CYB
Cytochrome b subunit III
4
3
16
MT-CO1, MT-CO2, MT-CO3
Cytochrome c oxidase subunit 1, 2, 3
5
2
17
MT-ATP6, MT-ATP8
Adenosine triphosphate ATP synthase 6, 8
Ribosome
2
>150
MT-RNR1, MT-RNR2
Ribosomal RNA 12S, 16S
tRNA
22
0
MT-TA to TY
22 Transfer RNAs, coding for alanine (A) to tyrosine (Y)
Sources: Human Gene Nomenclature, Mitochondrial respiratory chain complex, HGNC; Gene Cards; OMIM; Anderson, S., Bankier, A. T., Barrell, B. G., de Bruijn, M. H., Coulson, A. R., Drouin, J., et al. (1981). Sequence and organization of the human mitochondrial genome. Nature, 290, 457–465; Wallace, D. C. (1994). Mitochondrial DNA mutations in diseases of energy metabolism. Journal of Bioenergetics and Biomembranes, 26, 241–250.
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11. LEBER’S OPTIC NEUROPATHY
Are the multiple proteins arranged into intricate complexes unique to mitochondrial metabolism? Probably not. The five complexes appear to have many more subunits than, for example, the sarcomere in striped muscles shown in Fig. 4.8, but that diagram is a simplification of an organelle that involves many more kinds of proteins and genes that are not shown. The apparent complexity of a structure reflects to a large degree the knowledge we have about it based on extensive research. Recall the evidence cited in Chapter 3 on gene expression that indicates hundreds and even thousands of genes are expressed in many kinds of tissues and cells. Those diverse proteins in a cell are not just floating randomly in a cytoplasmic soup. They must be arranged into structures wherein many kinds of proteins adhere to each other in order to make things work properly. Discerning the nature of those structures is not an easy task. Each mitochondrial complex is itself the focus of many talented scientists working in teams and exchanging information daily (e.g., www.complexi.org and www.atpsynthase. info). The mtDNA sequence was decoded in 1981, and LHON was ascribed to a specific mutation in the MT-ND4 gene in 1988. Thirty years later, it is easy enough to point to the place in a mitochondrial protein complex where a mutation can cause LHON, but it is still not understood just how the altered protein impairs functioning of retinal ganglion cells in particular or how some people can carry the identical mutation but not show any symptoms at all.
TREATMENT AND CURE There is no generally accepted treatment for LHON, according to a review of the medical literature (Yu-WaiMan, Griffiths, Hudson, & Chinnery, 2009) and the 2016 edition of the EyeWiki website of the American Academy of Ophthalmology. The Genetic and Rare Disease Information Center (GARD) website in 2017 notes that there is no cure, but ongoing studies aim to find a treatment. One option is oral idebenone, a synthetic analog of ubiquinone that is involved in the cascade that generates ATP in mitochondria. Idebenone, originally intended to treat dementia and now marketed as Raxone, is said to be the only clinically proved treatment option for LHON (Gueven, 2016). There are facts to support the claim about efficacy of idebenone, yet the picture is not so clear. The European Medicines Agency in 2013 refused to license it on the grounds that the clinical benefit appeared to be too small to warrant approval (Refusal of the marketing authorisation for Raxone (idebenone), 2013). In view of this continuing uncertainty about idebenone, it is illuminating to examine the principal evidence of its benefits. A study reported by Klopstock et al. (2011)
involved 85 patients diagnosed with LHON and carrying one of the three most common mtDNA mutations (Table 11.1). They were randomly assigned to receive either a placebo or idebenone for 24 weeks. Group assignment was constrained to give equal proportions of men and women and the three genotypes in the two treatment conditions. Tests of visual acuity were given before and after 4, 12, and 24 weeks of treatment by people who did not know which patients were receiving the drug. Because neither the patients nor those giving the tests knew who was getting idebenone, it was a well-controlled doubleblind study. The researchers also declared in advance that a particular outcome measure would serve as the primary end point of the study: best recovery score. The recovery of vision score was the difference between baseline acuity measures at the outset and after the 24-week period when using the Early Treatment Diabetic Retinopathy Study (ETDRS) chart (Fig. 6.1). Patients in both conditions scored better on average on the 24-week test than on the initial baseline test. Average recovery scores were higher for the idebenone-treated patients than those receiving placebo, but the difference was not very large. As the authors remarked, “the difference between groups did not reach statistical significance” (p. 2680). The authors of the study were very forthright about the implications of their findings: “considerable advances in our understanding of the molecular and biochemical basis of mitochondrial DNA-associated diseases have not yet translated into treatments of proven efficacy.” Being a drug with “antioxidant” effects on mitochondrial metabolism, idebenone has been advocated or at least mentioned on several websites (e.g., WebMD, HBC, and SDFT) as a treatment for a large number of conditions: dementia, Alzheimer’s disease, Friedreich’s ataxia, Parkinson’s disease, multiple sclerosis, liver and heart disease, and LHON. The drug is moving through the approval process in the United States as an agent to combat Duchenne muscular dystrophy. Even though it has not been formally approved by governments to treat these conditions, at the present time, idebenone is readily available via the Internet as a “dietary supplement” and as an “antiaging” skin wrinkle therapy. The widespread publicity and easy availability of idebenone create further challenges for researchers seeking conclusive evidence that it really works. Being a relatively rare disease, LHON requires a major effort to find enough patients willing to take part in a clinical trial. Not just willing but drug-free patients are needed. Authors of the double-blind study lamented that “patients often find the prospect of taking placebo unacceptable and self-medicate, using Internetbased suppliers of … unapproved medication” (Klopstock et al., 2011). Those who have already been taking idebenone on their own will probably not be suitable subjects for a well-controlled clinical trial.
REFERENCES
Prevention of LHON is not yet possible. A person can carry an undetected mutation in mtDNA for years before symptoms appear in young adults. If there is a family history of LHON, genetic testing may uncover that same mutation in other relatives, some of whom may still be asymptomatic. If only there were an effective treatment for those who already show symptoms of LHON, that treatment might be worth a try as a preventive medicine, provided side effects are minimal. Indeed, it might be more effective at prevention than as a treatment for a retina that has already suffered major damage. Several medical authorities recommend that those at risk for LHON because of a family history refrain from smoking tobacco and drinking large amounts of alcohol, which is good advice to all of us to prevent disease.
HIGHLIGHTS • The mitochondria, minute organelles in the cytoplasm with their own DNA (mtDNA with 16,569 base pairs encoding 37 genes), are transmitted from mother to both sons and daughters. • Mutations in several mitochondrial genes can lead to a form of blindness known as LHON that is much more prevalent in males than females. • Most mutations causing LHON show incomplete penetrance, whereby many people who carry the mutation show no symptoms at all. A woman with no symptoms can nevertheless transmit an mtDNA mutation to her children. • Specific mutations sometimes show a striking founder effect in which dozens of people descended from the same immigrant woman carry the identical mutation. • Unknown environmental factors clearly influence the degree of penetrance. Smoking appears to increase the risk of LHON in males. • The mitochondrial genes involved in LHON are part of large molecular complexes consisting of proteins derived from dozens of genes in both the nucleus and the mitochondria that combine into complexes to derive energy from glucose to power many cell processes.
References Anderson, S., Bankier, A. T., Barrell, B. G., de Bruijn, M. H., Coulson, A. R., Drouin, J., et al. (1981). Sequence and organization of the human mitochondrial genome. Nature, 290, 457–465. Berg, J. M., Tymoczko, J. L., & Stryer, L. (2002). Section 18.3, The respiratory chain consists of four complexes: three proton pumps and a physical link to the citric acid cycle. In Biochemistry (5th ed.). New York: W.H. Freeman. Borst, P. (1977). Structure and function of mitochondrial DNA. Trends in Biochemical Sciences, 2, 31–34.
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Chalmers, R. M., & Schapira, A. H. (1999). Clinical, biochemical and molecular genetic features of Leber’s hereditary optic neuropathy. Biochimica et Biophysica Acta (BBA)—Bioenergetics, 1410, 147–158. Chinnery, P. F., & Hudson, G. (2013). Mitochondrial genetics. British Medical Bulletin, 106, 135–159. Collinge, J. (2001). Prion diseases of humans and animals: their causes and molecular basis. Annual Review of Neuroscience, 24, 519–550. Dickinson, A. G., Young, G. B., Stamp, J. T., & Renwick, C. C. (1965). An analysis of natural scrapie in Suffolk sheep. Heredity, 20, 485–503. Erickson, R. P. (1972). Leber’s optic atrophy, a possible example of maternal inheritance. American Journal of Human Genetics, 24, 348–349. Gueven, N. (2016). Idebenone for Leber’s hereditary optic neuropathy. Drugs Today, 52, 173–181. Imai, Y., & Moriwaki, D. (1936). A probable case of cytoplasmic inheritance in man: a critique of Leber’s disease. Journal of Genetics, 33, 163–167. Kerrison, J. B., Miller, N. R., Hsu, F., Beaty, T. H., Maumenee, I. H., Smith, K. H., et al. (2000). A case-control study of tobacco and alcohol consumption in Leber hereditary optic neuropathy. American Journal of Ophthalmology, 130, 803–812. Klopstock, T., Yu-Wai-Man, P., Dimitriadis, K., Rouleau, J., Heck, S., Bailie, M., et al. (2011). A randomized placebo-controlled trial of idebenone in Leber’s hereditary optic neuropathy. Brain, 134, 2677–2686. € Leber, T. (1871). Uber heredit€are und congenital-angelegte Sehnervenleiden. Graefe’s Archive for Clinical and Experimental Ophthalmology, 17, 249–291. Lodish, H., Berk, A., & Zipursky, S. L. (2000). Molecular cell biology (4 ed.). New York: W. H. Freeman & Company. Macmillan, C., Kirkham, T., Fu, K., Allison, V., Andermann, E., Chitayat, D., et al. (1998). Pedigree analysis of French Canadian families with T14484C Leber’s hereditary optic neuropathy. Neurology, 50, 417–422. Man, P., Griffiths, P., Brown, D., Howell, N., Turnbull, D., & Chinnery, P. (2003). The epidemiology of Leber hereditary optic neuropathy in the North East of England. The American Journal of Human Genetics, 72, 333–339. Man, P. Y. W., Turnbull, D., & Chinnery, P. (2002). Leber hereditary optic neuropathy. Journal of Medical Genetics, 39, 162–169. Martin, W. F., Garg, S., & Zimorski, V. (2015). Endosymbiotic theories for eukaryote origin. Philosophical Transactions of the Royal Society of London Series B: Biological Sciences, 370, 20140330. Prusiner, S. B. (1982). Novel proteinaceous infectious particles cause scrapie. Science, 216, 136–144. Puomila, A., H€am€al€ainen, P., Kivioja, S., Savontaus, M.-L., Koivum€aki, S., Huoponen, K., & Nikoskelainen, E. (2007). Epidemiology and penetrance of Leber hereditary optic neuropathy in Finland. European Journal of Human Genetics, 15, 1079–1089. Refusal of the marketing authorisation for Raxone (idebenone) (2013). European Medicines Agency (pp. 1–2). . Requena, J. R., & Wille, H. (2014). The structure of the infectious prion protein: experimental data and molecular models. Prion, 8, 60–66. Sadun, A. A., Carelli, V., Salomao, S. R., Berezovsky, A., Quiros, P. A., Sadun, F., et al. (2003). Extensive investigation of a large Brazilian pedigree of 11778/haplogroup J Leber hereditary optic neuropathy. American Journal of Ophthalmology, 136, 231–238. Taubes, G. (1986). The game of the name is fame. But is it science? Discover, 7, 28–52. Tran, M., Bhargava, R., & MacDonald, I. M. (2001). Leber hereditary optic neuropathy, progressive visual loss, and multiple-sclerosis-like symptoms. American Journal of Ophthalmology, 132, 591–593. Tsao, K., Aitken, P. A., & Johns, D. R. (1999). Smoking as an aetiological factor in a pedigree with Leber’s hereditary optic neuropathy. British Journal of Ophthalmology, 83, 577–581. Tuppen, H. A., Blakely, E. L., Turnbull, D. M., & Taylor, R. W. (2010). Mitochondrial DNA mutations and human disease. Biochimica et Biophysica Acta (BBA) – Bioenergetics, 1797, 113–128.
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Waardenburg, P. J. (1969). Some remarks on the clinical and genetic puzzle of Leber’s optic neuritis. Journal de Genetique Humaine, 17, 479–495. Wallace, D. (1970). A new manifestation of Leber’s disease and a new explanation for the agency responsible for its unusual pattern of inheritance. Brain, 93, 121–132. Wallace, D. C., Singh, G., Lott, M. T., Hodge, J. A., Schurr, T. G., Lezza, A. M. S., et al. (1988). ). Mitochondrial DNA mutation associated with Leber’s hereditary optic neuropathy. Science, 242, 1427–1430. Yu-Wai-Man, P., Griffiths, P. G., Hudson, G., & Chinnery, P. F. (2009). Inherited mitochondrial optic neuropathies. Journal of Medical Genetics, 46, 145–158.
Further Reading Taanman, J. W. (1999). The mitochondrial genome: structure, transcription, translation and replication. Biochimica et Biophysica Acta (BBA) – Bioenergetics, 1410, 103–123. Wallace, D. C. (1994). Mitochondrial DNA mutations in diseases of energy metabolism. Journal of Bioenergetics and Biomembranes, 26, 241–250.