Lecture 28 - Mitochondrial Disease 1 Flashcards
How much ATP do we turn over every day?
65 kg
What is the surface of the mitochondrial inner membrane (all in body combined)?
14,000 m2 (MCG)
What percent of energy does our brain use?
20%
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
Which is the most common mitochondrial disease of childhood?
Leigh disease
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
What clinical consequences are seen in the ear in OXPHOS disorders?
• Deafness
What clinical consequences are seen in the eye in OXPHOS disorders?
- Optical atrophy
- Cataracts
- Retinitis pigmentosa
What clinical consequences are seen in the bone marrow in OXPHOS disorders?
- Myelodysplasia
- Anaemia
- Thrombocytopenia
What clinical consequences are seen in the heart in OXPHOS disorders?
- Hypertrophic cardiomyopathies
* Conduction defects
What clinical consequences are seen in the kidneys in OXPHOS disorders?
- Proximal tubuleopathy
- Glomerulosclerosis
- Nephropathy
What clinical consequences are seen in the GIT in OXPHOS disorders?
- Liver failure
- Chronic diarrhoea
- Villous atrophy
- Pseudo-obstruction
What clinical consequences are seen in the pancreas in OXPHOS disorders?
- Insulin secretion defects
* Exocrine dysfunction
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
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%
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
- Large number of mtDNA molecules in oocyte, one of which is mutant
- Rapid decrease in number: one copy of both mutant and wild-type
- 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
- Clinical
• Leigh syndrome - Metabolic
• Elevated lactate in serum - Histology
- OXPHOS enzymes
• 20% complex 1 i.e. low - 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
- Zygote from couple; nucleus removed
- Zygote form other couple; nucleus removed
- Nucleus from couple put into donor embryo
→ Embryo with parents nuclear DNA and healthy donor mtDNA
