Lecture 28 - Mitochondrial Disease 1 Flashcards by Eleanor Nicklason

How much ATP do we turn over every day?

65 kg

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What is the surface of the mitochondrial inner membrane (all in body combined)?

14,000 m2 (MCG)

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What percent of energy does our brain use?

20%

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What is the generalised outcome of mitochondrial disease?

Any symptom, any organ or tissue, any age, any mode of inheritance

This is because mitochondria are important for every tissue in the body

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Which is the most common mitochondrial disease of childhood?

Leigh disease

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Describe Leigh disease
• Symptoms
• Age of onset
• Prognosis

Progressive, episodic neurodegenerative disorder

Symptoms:

Motor and / or intellectual regression

Spongiform lesions in CNS
• Demyelination
• Gliosis

Age of onset:
• 6 months of age

Prognosis:
• Death by about 3 years of age

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What clinical consequences are seen in the ear in OXPHOS disorders?

• Deafness

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What clinical consequences are seen in the eye in OXPHOS disorders?

Optical atrophy
Cataracts
Retinitis pigmentosa

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What clinical consequences are seen in the bone marrow in OXPHOS disorders?

Myelodysplasia
Anaemia
Thrombocytopenia

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What clinical consequences are seen in the heart in OXPHOS disorders?

Hypertrophic cardiomyopathies

* Conduction defects

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What clinical consequences are seen in the kidneys in OXPHOS disorders?

Proximal tubuleopathy
Glomerulosclerosis
Nephropathy

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What clinical consequences are seen in the GIT in OXPHOS disorders?

Liver failure
Chronic diarrhoea
Villous atrophy
Pseudo-obstruction

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What clinical consequences are seen in the pancreas in OXPHOS disorders?

Insulin secretion defects

* Exocrine dysfunction

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Give an overview of structure and function of mitochondria

Two membranes:
• Inner: protein complexes
• Outer

Function:
• Many, but most importantly energy production
• Sugar and O2 in
• ATP out

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Indicate % of cell volume that mitochondria contribute in the following tissues:
• Fast twitch muscle fibres
• Slow twitch
• Liver

Fast twitch muscle fibres: 2%

Slow twitch: 8%

Liver: 20%

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Describe the theory of the origin of mitochondria

Endosymbiosis hypothesis
(Lynn Margulis)

Primitive aerobic bacteria was phagocytksed by a fermentating eukaryotic cell
Symbiosis

Evidence:
• Lipid composition in mitochondria resembles prokaryotic membrane
• Own DNA
• Encode own ribosomes
• rRNAs resemble bacterial rRNAs (ribosomal-RNAs)
• rRNAs sensitive to chloramphenicol

How many genes do we have in the nucleus?

20,000

Describe mtDNA

Single, ds, circular chromosome

37 genes
• Small subset of those that mitochondria actually use
• Many of the proteins are encoded by genes in the nucleus

Inherited from mother only

Encodes:
• 2 rRNA proteins
• 22tRNA proteins
• ETC complex subunits

Which percent of proteins used by mitochondria come from mitochondrial genes?

5-10%

The rest come from nuclear DNA

Compare proportion of mito disorders that are maternally inherited in:
• Child-onset
• Adult-onset

What is the mode of inheritance in these cases?

Child onset:
• 30%
• Mostly autosomal recessive
• Some sporadic, X-linked, autosomal dominant

Adult onset:
• 70%
• Autosomal dominant, sporadic, autosomal recessive

Describe the genes that encode the subunits for the inner mitochondrial membrane complexes

Some encoded by mtDNA
Some encoded by nuclear DNA

Complex 1:
7 of 44 encoded by mtDNA

Complex 2:
All nuclear

Complex 3:
1 encoded by mtDNA

Complex 4:
3 encoded by mtDNA

Complex 5:
Couple of subunits encoded by mtDNA

Outline the functions of the various complexes in the inner mitochondrial membrane

Complex 1:
Transfer of electrons to Co-enzyme Q

Complex 3 & 4:
Transfer of electrons to Co-enzyme Q to cytochrome c and then to oxygen

Complex 5:
Proton movement through pore, generation of ATP

How many of the mitochondrial genes can have pathogenic point mutations?

Give some examples

35 of the 37 genes

MELAS:
• Mitochondrial encephalomyopathy, lactic acidosis and stroke like episodes

LHON:
• Leber’s hereditary optic neuropathy

Describe why maternal inheritance is so unusual

Family line of women for 9 generations:

Nuclear gene alles:
1/256th shared

mtDNA genes:
Identical

Why is there maternal inheritance?

Sperm don't contain much mitochondria | Those that they do contain are consumed by the egg

How many copies of the mitochondrial DNA per mitochondrion? | Thus, how many copies per cell?

2-10 copies per mitochondrion 50-200,000 copies per cell

Characterise the mutation rate of mtDNA

High mutation rate • 10-fold higher than nuclear DNA • This can be exploited to track human migration

Describe heteroplasmy

This is the presence of more than one type of a genome Heteroplasmy is observed for mitochondria, due to their high mutation rate → mutant and wild type mtDNA co-exist

Describe the threshold of heteroplasmy

At some point, the amount of mutant mtDNA becomes such that the tissue is dysfunctional This is called the threshold < 45% mutant: 95% chance of dysfunction NB not linear

What is the mtDNA bottleneck? What is the implication of this?

In early oogenesis, a small number of genomes are selected to repopulate the oocyte This allows rapid shifts in heteroplasmy: e.g. • Small percentage of mutant mtDNA → large percentage of mutant 1. Large number of mtDNA molecules in oocyte, one of which is mutant 2. Rapid decrease in number: one copy of both mutant and wild-type 3. Repopulation from the surviving mtDNA genomes: equal proportions of mutant and wild type genomes Thus, the oocyte went from having a small percent of mutant mtDNA to a great percent of mutant mtDNA

Describe tissue specific segregation / selection What gives rise to this? Give an example

Variation in mtDNA between tissue due to: * After birth: selection for or against mutations * Mutation load in one cell lineage increased / decreased during embryogenesis e.g. Kearns-Sayre Syndrome • Liver cells: 60% mutant mtDNA • Muscle cells: 30% • Fibroblasts: 1-2%

Describe the process of diagnosis in individuals with mitochondrial disease

1. Clinical • Leigh syndrome 2. Metabolic • Elevated lactate in serum 3. Histology 4. OXPHOS enzymes • 20% complex 1 i.e. low 5. DNA • Point mutation in mtDNA in a gene that is well known to cause disease All these things can confirm the diagnosis However, all the tests aren't definitive

Describe pronuclear transfer in humans. | When might this be used?

Used when mother has a mitochondrial disease or there is a high mutant mtDNA load 1. Zygote from couple; nucleus removed 2. Zygote form other couple; nucleus removed 3. Nucleus from couple put into donor embryo → Embryo with parents nuclear DNA and healthy donor mtDNA