Mitochondria Lesson 4 PDF

Mitochondria MD210 – GGE – Genetics Lesson 4 1 Essential Learning Outcomes By the end of this lesson you should be able to: • Discuss the penetrance of inherited disorders of mitochondria with reference to Leber’s Hereditary Optic Neuropathy • Understand how mtDNA sequences can be used for phylog...

Mitochondria MD210 – GGE – Genetics Lesson 4 1 Essential Learning Outcomes By the end of this lesson you should be able to: • Discuss the penetrance of inherited disorders of mitochondria with reference to Leber’s Hereditary Optic Neuropathy • Understand how mtDNA sequences can be used for phylogenetic studies of humans (Haplogroups) • Discuss the correlation between mtDNA Haplogroups and disease risk • Describe how nuclear DNA mutations can be associated with mitochondrial disease and how this affects inheritance, with reference to Leigh Syndrome 2 Case • Frank - 22 Year Old Male • Noticed blurred vision 1 week ago • Colours seemed pale • Optician noticed abnormality of retina Normal Frank Ophthalmic Atlas Images by EyeRounds.org, The University of Iowa are licensed under a Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License. 4 Case – Frank Family History • Older sister aged 24 - healthy • Mother and Father – healthy - aged 49 and 53 respectively • Grandparents RIP – heart disease and breast cancer • Uncle is blind since age 26 • Aunt is blind since age 45 • The Missing Piece of Information ? †Breast Cancer 77 †CHD 68 54 49 24 Frank 22 28 24 50 30 • It is the maternal uncle and aunt (mothers brother and sister that are affected) 5 LHON: Leber Hereditary Optic Neuropathy • Bilateral, painless, subacute visual failure that develops during young adult life. • Males are approximately four times more likely to be affected than females. 6 LHON • Entirely asymptomatic until onset in one eye • Similar symptoms appear in the other eye an average of eight weeks later • In about 25% of cases, visual loss is bilateral at onset • In a small percentage of cases, central vision gradually improves – but recovery is incomplete • For most central vision loss is profound and permanent Courtesy of Optolec • Some individuals with LHON, usually women, may also have a multiple sclerosis (MS)-like illness – termed ‘LHON-plus’. 7 LHON: Genotypes & Phenotype Approximately 95% of individuals with LHON have one of three point mutations of mitochondrial DNA (mtDNA): • m.3460G>A in MT-ND1 • Severe phenotype, little chance of recovery • m.11778G>A in MT-ND4 • Generally most common in most Asians/Caucasians • Intermediate phenotype • m.14484T>C in MT-ND6 • (French Canadians – Founder Effect) • Mild, best long-term prognosis 8 9 LHON Heteroplasmy • Heteroplasmy in Polymorphonuclear Leukocytes occurs in circa 10 to 15 % of those with a pathogenic LHON causing mtDNA mutation • Individuals with a m.11778G>A pathogenic variant load of less than 75% in their leukocytes may be unaffected (Smith 1993) • Heteroplasmy in cone cells may influence risk of developing visual impairment • However in general heteroplasmy of <75% pathogenic variant load is rare • – majority of LHON carriers are homoplasmic 10 LHON Management Supportive • Avoid smoking! • Smokers more likely to be symptomatic • Limit alcohol use • Avoid other environmental toxins • Diet Sx = symptomatic NSx = Non-symptomatic DA= Designated age Source: Tsao et al 1999 http://dx.doi.org/10.1136/bjo.83.5.577 11 LHON Genetic counselling • Note gender- and age-dependent penetrance †Breast Cancer 77 †CHD 68 • 95th centile for age of onset = 50 years 49 • Mother of a proband usually has the mtDNA mutation and may or may not have symptoms • Up to 40% of cases are simplex (i.e., occur in a single individual in a family). 24 Frank 22 54 28 24 50 30 13 LHON Genetic counselling • A male (affected or unaffected) with a primary LHON-causing mtDNA mutation cannot transmit the mutation to any of his offspring • A female (affected or unaffected) with a primary LHON-causing mtDNA mutation transmits the mutation to all of her offspring but to varying degrees (heteroplasmy) 14 LHON: Leber Hereditary Optic Neuropathy Prenatal diagnosis? • the mtDNA mutational load in amniocytes and chorionic villi is unlikely to correspond to that of other fetal or adult tissues • the presence of the mtDNA mutation does not reliably predict the occurrence, age of onset, severity, or rate of disease progression 15 LHON Penetrance • Approximately 50% of males • Approximately 10% of females who harbor a primary LHON-causing mtDNA mutation become affected. • A compensatory mechanism of activated mitochondrial biogenesis and increased mtDNA copy number may explain why some carriers are unaffected • This is promoted by estrogens in females, which may partially explain the gender bias Factors influencing incomplete penetrance in mitochondrial optic neuropathies Source: Caporali et al, Mitochondrion (36) 2017. 16 Menti question The pedigree shows the inheritance of LHON in 4 generations of a family 1. Individual 1 is more likely to be homoplasmic for the mtDNA LHON mutation than individual 2, true or false? 2. 4. 3. 17 Menti questions • What is the most likely explanation for the fact that individual 3, but not his sister manifests the phenotype? • What non-genetic factors may have contributed to individual 4 not expressing the LHON phenotype? 1. 2. 4. 3. 18 Haplogroups Describes individual branches – or closely related groups of branches – on the genetic family tree of all humans. All members of a haplogroup can trace their ancestry back to a single individual. Haplogroups arose through mutation and migration. 20 Mitochondrial DNA Haplogroups • mtDNA sequence polymorphism variations that have occurred over more than 150 000 years and correlate with the geographic origins of populations traced through the maternal lineages • Mitochondrial Eve --- ATGCGTGCCGAATCCCGTAG---- 4 children: • 2 male (mtDNA dead ends) • 2 female (Cinderella and Snow White) • and all get this sequence • Eve and her children are a “haplogroup” E C S 21 Mitochondrial Haplogroups • Cinderella has 2 daughters: E • X: --- ATGCGTGCCGAATCCCGTAG---• Z: --- ATGAGTGCCGAATCCCGTAG---• Z has a random (silent) change All descendants of Z – (in perpetuity) will be different from descendants of X* C X S Z No meiosis “crossing over” Linear Inheritance (matrilineal) 22 Mitochondrial Haplogroups Some Generations Later – Look at 8 individuals – Group Them J---ATGCGTGCGGAATCCCGTAG---Q---ATGCGTGCCGAATCCCGTAG----(0) K---ATGCGTGCCGAGTCCCTTAG---J---ATGCGTGCGGAATCCCGTAG----(1) L---ATGAGTGCGGAATCTCGTAG---M---ATGAGTGCCGAATCCCGTAG----(1) M---ATGAGTGCCGAATCCCGTAG---N---ATGCGTGCCGAATCCCTTAG----(1) N---ATGCGTGCCGAATCCCTTAG---P---ATGAGTGCGGAATCCCGTAG----(2) O---ATGCGTGCGGAATACCGTAG---K---ATGCGTGCCGAGTCCCTTAG----(2) P---ATGAGTGCGGAATCCCGTAG---O---ATGCGTGCGGAATACCGTAG----(2) Q---ATGCGTGCCGAATCCCGTAG---L---ATGAGTGCGGAATCTCGTAG----(3) 23 Mitochondrial Haplogroups • By aligning mtDNA sequences we can identify groups of individuals that appear to have a more/less recent common ancestor • Members of a “haplogroup” have a MRCA http://www.eupedia.com/europe/european_mtdna_haplogroups_frequ ency.shtml • Subclades within haplogroups with an even MRCA 24 Why is that significant? • Members of an mtDNA haplogroup have more similar total mtDNA to other members of the same haplogroup • Have similar mutations/polymorphisms in other mtDNA genes • As most mtDNA codes for proteins, mitochondrial polymorphisms can influence energy metabolism and contribute to the expression of the overall clinical phenotype 25 Mitochondrial Haplogroups & LHON • A meta-analysis of 159 European LHON pedigrees indicated that the risk of visual loss for the three primary LHON-associated mutations is influenced by the mtDNA haplogroup [Hudson et al 2007] • Greater risk when the m.11778G>A and m.14484T>C mutations arose on haplogroup J background • Greater risk when the m.3460G>A mutation arose on haplogroup K background • Lower risk when the m.11778G>A mutation arose on haplogroup H • Southeast Asian LHON pedigrees show no association between mtDNA haplogroups and the risk of visual loss [Tharaphan et al 2006]. 26 mt Haplogroups & Other Disease • Parkinson Disease has been found to be higher among mtDNA haplogroup H, but lower for haplogroups J and K • Haplogroup H associated with protection from sepsis • I, J and T also associated with increased longevity • Ethical considerations? 27 Haplogroups Is there an equivalent for the male line ? Yes – the Y Chromosome (except for the pseudoautosomal region) Y – Chromosomal Adam By Maulucioni - Own work, CC BY 3.0, https://commons.wikimedia.org/w/index.php?curid=12087218 29 Is mtDNA inheritance exclusively matrilineal? Biparental inheritance of mitochondrial DNA in humans “In this paper, we have uncovered multiple instances of biparental inheritance of mtDNA spanning three unrelated multiple generation families…. Surprisingly, this pattern of inheritance appears to be determined in an autosomal dominant like manner… Our results suggest that, although the central dogma of maternal inheritance of mtDNA remains valid, there are some exceptional cases where paternal mtDNA could be passed to the offspring..” 30 Case 2 • 2 year old boy, Kevin • Presents with cough, cold, fever and difficulty breathing for 2 weeks prior to admission • Mechanically ventilated for acute bronchopneumonia • Asymptomatic until 9 mo - developed fever and seizures • Slow to obtain developmental milestones and difficulty speaking (dysarthria) and swallowing (dysphagia) • Neurological examination showed dilated, unreactive left pupil • Hypertonic and involuntary movements • Palpable liver of 2cm below costal margin 31 Case 2 Family History • 2 normal sisters • Mother, Jean, had 2 early abortions previously • Jean’s brother died unexpectedly as an infant Jean Sp. Ab. 2 mo Unexplained 1 mo Sp. Ab. 1 mo Kevin 32 Investigations • Elevated lactic acid ( blood & CSF) • MRI – Bilateral lesions in the thalami, posterior parietal and temporal lobes • Positive on metabolic screening • Diagnosis? 33 Leigh Syndrome • A progressive neurometabolic disorder first described in 1951 • Mortality 50% per year from diagnosis. • Mitochondrial dysfunction caused by a hereditary genetic defect • Characterized by MRI necrotizing lesions in the midbrain and brainstem • Bilateral symmetrical degeneration of the brain stem, cerebellum and basal ganglia • Typical onset in infancy or childhood, often after a viral infection (metabolic challenge), but may begin later • Estimated incidence at least 1 in 40,000 births 34 Leigh-like Syndrome • Criteria for LS only partially met despite overall clinical picture indicative of LS • Often peripheral nervous system involvement, including polyneuropathy or myopathy • Often non neurologic abnormalities – dysmorphic features, cardiac, endocrine and GI • Respiratory impairment due to brainstem involvement usually absent 35 Leigh Syndrome: Broad Genetic Heterogeneity • Mutations in at least 75 genes have been identified • Mitochondrial disorder with broadest genetic (and clinical) heterogeneity • Pyruvate dehydrogenase (PDHC) deficiency (citric acid cycle) • Respiratory chain enzyme defects Complexes I, II, IV, and V • The NARP mutation (mtDNA) • The MERRF mutation (mt DNA) 36 Leigh’s Syndrome – Different Patterns of Inheritance • • • • Matrilineal (mitochondrial) ~20% De novo mutation X-linked recessive Autosomal recessive • Audience question - why so many different inheritance patterns with Leigh Syndrome? 37 Mitochondrial disorders – inheritance patterns • Remember - most mitochondrial proteins are encoded on nuclear genome • Synthesised in cytoplasm and transported to mitochondria • Therefore, most inherited disorders of mitochondria are related to changes in nuclear DNA rather than mtDNA • These mutations are inherited according to the classic Mendelian rules, not mitochondrial (matrilineal) inheritance 38 Menti problem – Breakout groups • What is the most likely pattern of inheritance of Kevin’s Leigh’s Syndrome? Jean Sp. Ab. 2 mo Unexplained 1 mo Sp. Ab. 1 mo Kevin 39 What if you could prevent passing on your hereditary mitochondrial disease? • Mitochondrial transplantation/donation • Can be done before or after fertilisation • 3-parent embryo • UK parliamentary debate 2015 41 Things To Remember 1. An oocyte has many mitochondria and the precise sequence of mtDNA may not be the same in all of those mitochondria (heteroplasmy). Predicting phenotype from mtDNA genotype is particularly difficult because of heteroplasmy 2. A male affected by a disease related to mutation of mtDNA, such as LHON, can not transmit 3. mt Haplogroups refer to groups of individuals with related mtDNA sequence (thus assumed to have more recent common ancestry). 4. Haplogroups and subclades can influence mt (and other) disease penetrance and severity because they are defined by polymorphisms in mtDNA, most of which codes for proteins involved in energy metabolism. 5. If the mutation is on the nuclear genome, for e.g. 80% of Leigh Syndrome, then the inheritance pattern of the mitochondrial disease will usually be Mendelian 43