Mitochondrial inheritance and disease Flashcards
what is a primary mitochondrial disease?
Disease due to a mtDNA mutation- affects the mitochondrial respiratory chain and OXPHOS
what is the pathogenesis of mitochondrial disease?
Disease is due to
- low energy production
- ROS generation
- lactic acidosis
what is the incidence and age of onset of mitochondrial disease?
onset at age
- 5 per 100,000 in adults
- 7 per 100,000 in children
What is the phenotype of mitochondrial disease?
can affect any tissue
most affected are those with the highest energy demand e.g. brain and muscle
CNS encephalopathy seizures dementia stroke-like episode ataxia depression
EYE opthalmoplegia cataracts ptsosis optic atrophy
CARDIAC
hypertrophic cardiomyopathy
dilated cardiomyopathy
GI dysphagia psuedo obstruction constipation hepatic failure
ENDOCRINE
diabetes
hypothyrpidism
gonadal failure
PNS
myopathy
neuropathy
HEARING
sensironeural deafness
describe the mitochondrial genome
replicates independently of nDNA
circular- 16.6Kb long and codes 37 genes- 13 mitochondrial peptides, 22tRNAs and 2rRNAs
no introns and so splicing is not a mechanism of disease.
maternally transmitted
each cell has 100-100,000’s of copeis of the MT genome
can be heterplasmic or homoplasmic
The level of mutant mtDA can vary within individuals from the same family and between tissues in the same individual due to the mitochondrial bottleneck where WT and Mut mt is randonly distributed to daughter cells - this can be uneven resultin in cell lines with higher or lower levels of heteroplasmy
why may a mt variant not be detected in blood?
some variants are lost from blood due to rapid mitotic division.
e. g. m.3243A>G
- most common mtDNA mut and found in MELAS and MIDD
- this makes genetic testing difficult and need to carefully consider the choice of tissue
what is the function of the mitochondria?
Produces energy for the cell via the oxidative phopshorylation pathway (OXPHOS)
also involved in ca2+ signalling, cellular metabolism, haem and steroid synthesis
why is there a high mutation rate in mitochondria?
- inefficient damage repair
- proximity of the DNA to OXPHOS resulting in damage from ROS
- high replicative rate
what is OXPHOS?
the OXPHOS oathway transfers electrons through e- transport carriers to create an electrochemical gradient across the MM which is used to drive the production of ATP from ADP
What is the heteroplasmy threshold fro mitochondrial disease.
For mtDNA variants to manifest with a phenotype there is a threshold level of mutant that needs to be present
~40-60% for mtDNA deletions
~90 for point mutations in mtDNA tRNAs
what are the 3 mitochondrial deletion diseases and there characteristics?
- de novo
- heteroplasmic
- several genes deleted
Pearsons- anemia, pancytopenia, lacic acidosis, pancreatic failure (onset in infancy)
Kearns-Sayre- myopathy deafness, opthalmoplegia, cardiomyopathy (adult onset)
CPEO- opthalmoplegia, ptosis, imparied eye movement
what are secondary mitochondrial disease?
due to nDNA defects and involves genes that have a direct effect on the function of the mitchondria
Give an example of nDNA mutation affecting mtDNA maintenance and expression
direct effects e.g. POLG, POL2 (specific mtDNA polymerase and TWNK
indrect effect e.g. SLC25A4 and RRM2B- affect nucleoside transport, synthesis and salvage
- mtDNA needs a supply of dNTPs for replication, im mitotically active tissue this is provided by import from the cytoplasm. In non-mitotically active tissue the dNTPS are obtained from the salvage pathway- these differences result in tissue specific phenotype
Give an example of nDNA mutation resulting in mt dysfunction without affecting mtDNA maintenance or function?
Mutations in the assembly factors of the respiratory chain e.g SURF mutations in MELAS
mutations in genes involved in mt dynamics e.g. fusion and fission which in turn perturbs the number and distribution of mitochondria
e.g. MFN2 in CMT (fusion)
OPA1 in optic atrophy (fission)
what are the mtDNA point mutation diseases?
maternally inherited (except AID- sporadic) heteroplasmic
most common mut is m.3243A>G in MEALS and MIID
MELAS- myopahty, encephalopathy, acidosis and stroke like episodes
MERRF- myoclonic epilepsy and ragged red fibres
NARP- neurogenic weakness, ataxia and retinitis pigmentosa
MIDD- maternally inherited diabetes and deafness
AID - amnio glycoside induced deafness
LHON- leber hereditary optic neuropathy
How is mt disease associated with neurodegenerative disorders?
Parkinsons- mtDNA deletions are observed in neurons of parkinsons patients and mutations in genes involved in mitochondrial function are mutated in early onset familal parkinsons
alzheimers - respiratory chain dysfunction due to mtDNA mut found in neurons of patients
How can mt be used as a cancer biomarker?
WARBURG EFFECT
tumours preferentially use glycolysis to produce ATP and injury of the respiratory chain is a key event in carcinogenesis
mt genome could be used as an early biomarker of cancer as mutation does not appear to be restricted to certain cancer types.
What approach is used for diagnosis of mitochondrial disease?
considers phenotype and family history and often requires several approaches including biochemistry, immunohistochemistry, nuclear and mitochondrial DNA testing
what are the requirements and usage of histochemistry in mitochondrial disease diagnosis?
Needs muscle biopsy
can stain for ragged red fibres using a gromi trichome stain (sub sarcolemma collection of MT)
can test for the activity of specific mitochondrial enzymes
- SDH (succinate dehydrogenase) loss is indicative of a complex II deficiency
- COX (oxidase) subunitss encoded by both nuclear and mitchondrial DNA
Normal IHC does not exclude a diagnosis and can see age related mitchondrial defects so need to consider this in older patients
what are the requirements and usage of biochemistry in mitochondrial disease diagnosis?
requires muscle biospy- want a sample enriched for mt
can measure rates of flux, substrate oxidation and AT|P generation and can measure the level of activity of each XOPHOS substrate seperately
Need a lot of sample (50-100mg) may not detect subtle OXPHOS deficiencies especially when mosaic and only a few muscle fibres are affected
How are mt DNA rearrangements detected?
- includes sinlge dels and dups and multiple mt DAN deletions
- long range PCR is the primary method for detection and may also detect age related mt deletions
the presence of multiple mtDNA deletions may indicate that there is a deletion in a nDNA gene involved in mtDNA maintenance e.g. POLG, ANT1, TWNK
How are mt point mutations detected?
common mt mutations can be detected by sequencing
- technique need to be sensitive to detect low heteroplasmy (mosaicism)
What is mutation searching for mitchondrial disease?
real time PCR can be used to detect mt copy number- an increase in mtDNA CN suggests that the moelcualr effect is likely to be located in the mitochondrial genome, hence whole mtDNA genome sequencing may be considered
need to consider heteroplasmy and use a technique that is sensitive to low level mutation
85-90% of patients with a suspected primary mitochondrial disease do not have a pathogenic mutation detected in the mdt genome highlighting the importance of considering an nDNA mutation- screen genes involved in OXPHOS or mt genome maintenance
NGS is proving pivital in mt disease diagnosis and >250 nDNA genes have now been shown to cause mitochondrial disease
What are the considerations for mt disease testing (sample type)?
Muscle is best but may not be practical or desired by the pateint
- however failure make a diagnosis in blood does not mean the patient does not have mt disease, just that the mutation is not detectable in blood
- many labs include caveat on their reports to this affect and request a muscle sample if a diagnosis is still strongly suspected.
Urine can be used for some mutations e.g. m.3243A>G in MELAS as levels in urine correspond with the level in blood.
