Aetiology of Mitochondrial Disorders Flashcards
what are some mitochondrial functions
mainly the oxphos system
apoptosis
mtDNA replication and expression
protein import and sorting
transport of metabolites
fission and fusion
outline the basic structure of mitochondria
rod shaped organelles
double membrane (folded into cristae)
inside part of inner membrane is mitochondrial matrix where mDNA resides
what is the main function of mitochondria?
providing ATP vis the OXPHOS system
outline the mitochondrial respiratory chain
5 multi subunit complexes
complexes 1-4 = transport electrons across inner membrane
complex 5 = ATP synthase uses electrochemical gradient to power motor and provide ATP
which is the only complex of the mitochondrial respiratory chain that is nuclearly coded (doesnt have subunit made by mitochondrial DNA)?
complex 2
outline mitochondrial DNA
covalently closed DNA molecule
circular
located in mitochondrial matrix
multiple copies per cell - vary from 100 to 10,000 in mature oocytes
what is the only cell that does not contain mitochondrial DNA
erythrocytes
outline the genes in mitochondrial DNA
13 protein-encoding genes = encode hydrophobic OXPHOS components
- 2 rRNAs (mitoribosomes)
- 22 tRNAs - all for mitochondrial translation
what does mitochondrial DNA not have
introns
what are the 2 short non-coding regions of mitochondrial DNA
D-loop (contains ORIH)
ORIL
these regions are for replication
what type of transcription happens in mtDNA?
polycistronic transcription - doesnt make separate mRNA for each gene, read as one long transcript and chopped up afterwards
outline what is meant by tRNA punctuation
in mtDNA, there is a tRNA between each protein coding gene - this allows maturation of nRNA
outline mtDNA in human disease
high mutation rate (10-20x higher than nuclear DNA)
no protective histones
poorly characterised repair mechanisms
vulnerable to nucleolytic attack by ROS in mitochondrial matrix
no introns / little redundancy means mutation often affects coding sequence
what are some non-neurological disease mDNA mutations can cause
cardiomyopathy
liver failure
diabetes
respiratory failure
short stature
what are some neurological diseases mtDNA mutations can cause
CPEO
seizures
developmental delay
deafness
peripheral neuropathy
myopathy
what are the 4 genetic rules of mtDNA
- maternal inheritance
- heteroplasmy
- threshold effect
- mitotic segregation
outline maternal inheritance
inheritance of mtDNA is strictly maternal
paternal mitochondria are eliminated after fertilisation
outline heteroplasmy
a homoplasmic wild type and homoplasmic mutant make heteroplasmic mtDNA
with either high mutant load or low mutant load
outline the threshold effect
many heteroplasmic mtDNA are highly RECESSIVE (need a lot of the mutant to see phenotype of the disease)
- high level of mutation can be tolerated by cell before it induces defect
60-70% of mtDNA needs to mutate to see phenotype
how do you stain mtDNA for defects?
brown stain for complex 4
if stains brown it means lots is present and no defect but light ones dont have much complex 4 and will be defected
outline mitotic segregation
levels of mtDNA mutations can very enourmously between tissues
in mitosis, wild type and mutated mtDNA is randomly segregated to each daughter cell - this affects disease expression and inheritance
what is the mitochondrial genetic bottleneck in mitotic segragation
mature oocytes have highest copy number of mtDNA but when they are split into primary oocytes from primordial germ cell - they increase and replicate
bottle neck = it is 50/50 WT and mutant - all random
what is MELAS
mitochondrial encephalomyopathy , lactic acidosis and stroke-like episodes
it looks like stroke but it isnt - results from lack of ATP and energy NOT a lack of blood flow but similar phenotype
what mutation causes MELAS
mtDNA point mutation
m.3243A>G
not everyone who has the mutation will have MELAS, because of the spread in different tissues
outline where the m3243A>G point mutation is found
highest mutant load in post-mitotic tissues (brain, skeletal muscle) (severe disease)
low/absent in blood - so not picked up and need biopsy
mutation can increase and decrease with time - and disease can get worse
what is CPEO
a mtDNA rearrangement disorder
chronic progressive external opthalmoplegia
affects how eyes move, one more than the other
restricts eye movement
usually a sporadic mtDNA deletion (takes out whole gene)
sporadic - not inherited from parent
