Mitochindrial Disorders Flashcards
How is the mitochondrial genome inherited?
Maternally - sperm mitochondrial DNA are thought to be removed by ubiquitination
Do the genes in mitochondrial DNA contain:
a) Exons only
b) Introns only
c) Both exons and introns
a) Exons only - none of the mitochondrial genes contain introns
How many genes make up the mitochondrial genome?
- 37 genes
- encode components of respiratory complex, mitochondrial tRNAs and rRNAs
What tissues do mitochondrial diseases affect?
mitochondrial rich with high energy demand:
- Skeletal and cardiac muscle
- CNS
- Inner ear
- Kidney
- Pancreas
How many people carry pathogenic mitochondrial DNA mutations and what is the birth prevalence?
Around 1 in 250 ppl carry them but birth prevalence is approx 1 in 5000
What is the difference between disease caused by mutations in nuclear genes vs. Mitochondrial DNA?
- Mutations in nuclear genes usually cause disease in children
- mitochondrial DNA mutations usually cause disease in adults
What is the key diagnostic test for mitochondrial disorders?
Muscle biopsy
What are secondary mitochondrial DNA disorders also known as?
Mitochondrial DNA maintenance disorders
What does the common mitochondrial DNA point mutation screen involve?
- MELAS
- MIDD
- MERRF
- NARP
- LHON
What is the paradigm mutation for maternally inherited mitochondrial DNA disease?
- m.3243A>G
- commonest cause of MELAS
- commonest presentation is MIDD (maternally inherited diabetes and deafness)
What does MELAS stand for in the context of mitochondrial disorders?
Mitochondrial Encephalomyopathy, Lactic Acidosis and Stroke-like episodes
What are the genetic causes of MELAS syndrome?
- Mitochondrial cause = 80% have A3243G mutation
- Autosomal recessive cause = rare, POLG
What are the risks of offspring of women carrying the A3243G mutation based on blood levels?
- Blood level of 30% mutant mitochondrial DNA at 20yrs old carries a risk of 55%
- Blood level drops to 15% mutant mitochondrial DNA at 38yrs old - risk is 25%
What does MERRF stand for in the context of mitochondrial disorders?
Myoclonic epilepsy with ragged-red fibres
What does NARP stand for in the context of mitochondrial disorders?
Neurogenic muscle weakness, Ataxia, and Retinitis Pigmentosa
What does LHON stand for in the context of mitochondrial disorders?
Leber hereditary optic neuropathy
Is LHON usually homoplasmic or heteroplasmic?
Homoplasmic - with little correlation between level of mutant and severity
What are three possible reproductive strategies for women with mitochondrial mutations?
- Oocyte donation
- PGD
- Nuclear transfer into donated oocytes
Within the context of LHON is your recurrence higher if:
- you’re Male or Female?
- your mother is affected or unaffected?
- recurrence risk higher in males
- recurrence risk higher if mother is affected
What is unique about autosomal disorders of mitochondrial DNA maintenance?
Involve defects in both genomes within human cells
- Primary nuclear gene defect in gene affecting mitochondrial DNA replication/maintenance
- Secondary mitochondrial defect = tissue specific
Name a disorder of mitochondrial DNA maintenance that can be reversible
Respiratory chain deficiency
Due to the variation in heteroplasmy levels between tissues for some mitochondrial DNA mutations, where are ideal samples taken from?
- Post-mitotic, clinically affected tissues (e.g. Muscle, liver)
- However, these are invasive, unpleasant for patient and and costly to sample so in practice alternative samples are often taken
- Blood (most common), urine and mouthwash/saliva/buccal
What are the common testing methods for known mitochondrial DNA mutations?
- Pyrosequencing
- Real time PCR
- Digital PCR, including droplet digital PCR
- fluorescent restriction digest PCR
- best for lower level of heteroplasmy = digital PCR
What are the common testing methods for mutations in the whole mitochondrial DNA?
- Sanger sequencing (lower limit of heteroplasmy = 30%)
- NGS (much better)
What disorder is associated with a depletion of mitochondrial DNA in the liver?
Alpers syndrome
What are the different types of mitochondria in eukaryotes?
- Aerobic and anaerobic mitochondria
- Hydrogenosomes
- Mitosomes
What are the three main components of mitochondria?
- Outer membrane
- Inner membrane
- Cristae
What is the major role of mitochondria in the cell?
- Energy production
- Produce 90% of a typical cells ATP
By what process do mitochondria produce ATP in the cell?
Oxidative phosphorylation via the electron transport chain
What are the two processes involved with maintenance of mitochondrial morphology, size and number?
Fusion and fission
How is mitochondrial fusion implicated in disease?
- MFN2 is associated with CMT2A
- OPA1 is associated with Dominant Optic Atrophy
What is heteroplasmy in the context of mitochondrial disease?
- presence of more than one type of organellar genome within a cell or individual
- because most eukaryotic cells contain many hundreds of mitochondria with hundreds of copies of mitochondrial DNA, it is common for mutations to only affect some mitochondria, leaving some unaffected
What is homoplasmy in the context of mitochondrial disease?
- describes a eukaryotic cell whose copies of mitochondrial DNA are all identical
- homoplasmic mitochondrial DNA copies may be normal or mutated (but most mutations are heteroplasmic)
What is the threshold effect in the context of mitochondria?
- For any given mutation there is a critical threshold level of heteroplasmy above which mitochondrion/cell/tissue function will be impaired
- threshold differs for different mutations, different tissue/cell types but is typically 80-90% mutant at cellular level
What is the mitochondrial bottleneck?
- During production of primary oocytes, selected number of mitochondrial DNA molecules are transferred into each oocyte
- Oocyte maturation is associated with rapid replication of this mitochondrial DNA population
- This restriction-amplification event can lead to random shift of mitochondrial DNA mutational load between generations and is responsible for the variable levels of mutated DNA observed in affected offspring from mothers with pathogenic mitochondrial DNA mutations
Why do mitochondrial DNA heteroplasmy levels often vary between tissues in an individual?
- At mitosis, mitochondrial DNA molecules segregate randomly to the daughter cells
- Pathogenic mitochondrial DNA mutations can be selected against in certain rapidly proliferating tissues (e.g. Haematopoietic stem cells) leading to reduced heteroplasmy levels with increasing age for certain mutations in blood DNA
- Pathogenic mitochondrial DNA mutations can also accumulate in post-mitotic tissues as a consequence of mitochondrial proliferation triggered by the cell in an attempt to improve mitochondrial function
