Mitochondrial disorders Flashcards
Describe the mitochondrial DNA
- It is a covalently closed DNA molecule located in the mitochondrial matrix with a heavy and kight strand
- It high as high copy number inherited maternally
- It is vulnerable to ROS induced mutation
- Replication, transcription and translation is mediated by nuclear gene products
- There are 2 non-coding regions: The origins of
H-strand (OH) and L-strand (OL) - highly organised with no introns and with no non-coding regions in processed transcripts
What does the mtDNA encode for?
- 13 polypeptide genes that form 4 of 5 complexes (except complex 2)
- 2 rRNAs
- 22tRNAs
Describe the process of mitochondrial replication
- No link at all to cell cycle and can replicate as and when is required
- Mitochondrial DNA is replicated by the DNA polymerase gamma complex (encoded by a nuclear gene)
There are 2 models of replication in contentious debate:
- Asynchronous model: Replication starts in OH - primed by short processed RNA species, in one direction through a substantial part (e.g. two-thirds) of the circular molecule until you expose OL which initiates L-strand synthesis, and then replication of the light strand begins.
- Synchronous model: The more recently reported mode starts at a different origin within the D-loop region and uses coupled-strand replication with simultaneous synthesis of both strands
What is the D-Loop?
A displacement loop that is a non-coding region on mtDNA and acts as the control site
- Strands are transcribed as single polycistrons and have their own promoter (HSP and LSP)
- It is mediated by nuclear encoded proteins: mtRNA polymerase, DNA binding transcription factor and termination factor
Give an example of mtDNA rearrangement disorders
Chronic Progressive External Ophthalmoplegia
It is a type of eye disorder characterised by slowly progressive inability to move the eyes and eyebrows.
- It is often the only feature of mitochondrial disease, in which case the term CPEO may be given as the diagnosis. In other people suffering from mitochondrial disease, CPEO occurs as part of a syndrome involving more than one part of the body
What are the sx of CPEO?
Ptosis, restriction of eye movements
slowly evolving proximal muscle weakness
What is the genetic cause of CPEO?
Can it be inherited? Give examples
What mutation is CPEO associated with that can cause various phenotypic expression?
In most cases, CPEO occurs due to a sporadic deletion or duplication within the mitochondrial DNA.
However, transmission from the mother to the progeny appears only in few cases. Both autosomal dominant and autosomal recessive inheritance can occur, autosomal recessive inheritance being more severe. Dominant and recessive forms of CPEO can be caused by genetic mutations in the ANT1, POLG, POLG2 and PEO1 genes.
One mutation, m.3243A>G is associated with both CPEO, Mitochondrial encephalomyopathy, lactic acidosis, and stroke-like episodes (MELAS) and Maternally-inherited Diabetes and Deafness (MIDD)
What is MELAS?
What are the symptoms?
What episodes happen and how do they present?
Mitochondrial myopathy, encephalomyopathy, lactic acidosis, and stroke-like episodes:
- a maternally transmitted point-mutation MT-ND1 or MT-ND5 that encodes for proteins part of NADH dehydrogenase (complex1)
- Early symptoms may include muscle weakness and pain, recurrent headaches, loss of appetite, vomiting, and seizures (treat with levtoasopam), bowel obstruction and faecal loading. Most affected individuals experience stroke-like episodes beginning before age 40.
- These episodes often involve temporary muscle weakness on one side of the body (hemiparesis), altered consciousness, vision abnormalities, seizures, and severe headaches resembling migraines - and can damage the brain some people get intestinal pseudo-obstruction during stroke like episodes
- can be caused by multiple mutation: e.g mutations in other mt-tRNA genes or protein encoding genes e.g. MTND 1,5,6
What is MIDD?
Maternally inherited diabetes and deafness (MIDD) is a form of diabetes that is often accompanied by hearing loss, especially of high tones.
What are the four genetic rules of mt DNA?
- Maternal inheritance:
- Heteroplasmy
- Threshold effect: many heteroplastic mtDNA mutations are highly recessive, small amounts of wild type can protect against mutation effects. High levels of mutation can be tolerated by cell before it induces a respiratory chain defect. Threshold is different in different tissues and mutations.
- Mitotic segregation: levels of mtDNA mutations can vary enormously between tissues. At mitosis both wild type and mutated mtDHA are randomly segregated to each daughter cell and undergo selection and amplification. This will affect both disease expression and inheritance - consider transmission mtDNA and prenatal testing)
How do we investigate possible mt disorders?
- clinical syndrome? e.g. MELAS etc.
- YES test for mtDNA or nuclear mutations in blood/urine (Can use urinary epithelial cells as it has a close correlation with muscle) if NO, do muscle biopsy
- muscle biopsy: Histochemistry stain COX-SDH, biochemistry - (measurement of chain complex activities and Ubiquinone -CoQ10)
- molecular genetics
What are examples of mt DNA studies?
mtDNA:
Depletion - real time PCR
Rearrangements - long range PCR
Common point mutation - pyrosequencing
Sequencing - Next generation sequencing (to screen for all of the genes at the same time)
Whole exome sequencing more effective at diagnosins mitochondrial mutations
What is the Canonical criteria defining pathogenicity of a mtDNA mutation?
- Mutation absent from healthy controls
- Heteroplasmy
- Higher levels of mutated mtDNA in affected individuals
- proportion of mutated mtDNA higher in affected tissue
- occur ar evolutionary conserved site
- clear segregation of mutation with biochemical effect
How do we manage mt disorders?
Treatment s are very limited and genetic counselling is offered to help make decisions about transferring to offsprings.
Which organs are affected by mitochondria?
Organs with high energy demands:
- Especially neurological organs - Optic atrophy/retinitis pigmentosa, CVA, seizures, developmental delay, deafness, neuropathy
- non neurological organs e.g. Respiratory failure, cardiomyopathy, liver failure etc.
Sometimes neurological and non-neurological combinations of disease in distant organs, give us a clue that mitochondrial disease might be involved e.g. brain and muscle, brain and heart or brain and liver?
What is reversible cox deficiency?
Reversible cox deficiency: Horvath, R. et al, Brain 2009:
Childhood-onset mitochondrial encephalomyopathies are usually severe, relentlessly progressive conditions that have a fatal outcome. However, a puzzling infantile disorder, long known as ‘benign cytochrome c oxidase deficiency myopathy’ is an exception because it shows spontaneous recovery if infants survive the first months of life. Current investigations cannot distinguish those with a good prognosis from those with terminal disease, making it very difficult to decide when to continue intensive supportive care. Here we define the principal molecular basis of the disorder by identifying a maternally inherited, homoplasmic m.14674T>C mt-tRNAGlu mutation in 17 patients from 12 families. Our results provide functional evidence for the pathogenicity of the mutation and show that tissue-specific mechanisms downstream of tRNAGlu may explain the spontaneous recovery. This study provides the rationale for a simple genetic test to identify infants with mitochondrial myopathy and good prognosis.
What is MERF: Myoclonic epilepsy with ragged red fibers?
Get epilepsy, weakness with multiple lipoma
Alpers-Huttenlocher Syndrome
“ Most severe form of POLG disease
“ Epilepsy: Explosive onset, Focal or general, EPC
“ Ataxia
“ Neurodevelopment regression
“ Cortical blindness - as it involves the occipital lobe of the brain
“ Late onset liver failure - Valporate?
“ Occipito-parietal cerebral atrophy
“ Normally don’t live beyond the age of 5 or 6
“ Accelerated neurodegeneration
What is Leigh syndrome?
Leigh Syndrome " Neurodevelopmental regression " Concurrent with a metabolic insult " Spasticity " Dystonia " Cerebellar signs " Hypopnea " Dysphagia " Strabismus " Mortality - respiratory failure " Symptoms? " Leigh Syndrome: One disorder, more than 75 monogenic causes (Paper) " Graph: children neurodevelopment fairly normal till 8/9 months and occur metabolic insult like a viral infection. This poses as a metabolic challenge and they can't cope so they decompensated. Sometimes that in itself results in death. Sometimes they recover.
How do we treat mt disorders?
Vitamin “cocktail”
- Ubiquinone (CoQ10) 3 month trial, antioxidant and electron carrier
o No evidence
- Riboflavin: ACAD9 mutation respond, offered to all with complex 1 deficiency
- Thiamine - cofactor for PDH and therefore respond to supplementation
Eye problems: corrective surgery
Deafness: SALT, audiology testing, cochlear implants
Dysphagia: ?PEG, NG tube? to prevent aspiration
HF: Beta-blocker, ace inhibitor, BiVAD, transplant
