Aetiology of Mitochondrial Disorders

Question 1

what are some mitochondrial functions

Answer 1

mainly the oxphos system
apoptosis
mtDNA replication and expression
protein import and sorting
transport of metabolites
fission and fusion

Question 2

outline the basic structure of mitochondria

Answer 2

rod shaped organelles
double membrane (folded into cristae)
inside part of inner membrane is mitochondrial matrix where mDNA resides

Question 3

what is the main function of mitochondria?

Answer 3

providing ATP vis the OXPHOS system

Question 4

outline the mitochondrial respiratory chain

Answer 4

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

Question 5

which is the only complex of the mitochondrial respiratory chain that is nuclearly coded (doesnt have subunit made by mitochondrial DNA)?

Answer 5

complex 2

Question 6

outline mitochondrial DNA

Answer 6

covalently closed DNA molecule
circular
located in mitochondrial matrix
multiple copies per cell - vary from 100 to 10,000 in mature oocytes

Question 7

what is the only cell that does not contain mitochondrial DNA

Answer 7

erythrocytes

Question 8

outline the genes in mitochondrial DNA

Answer 8

13 protein-encoding genes = encode hydrophobic OXPHOS components
- 2 rRNAs (mitoribosomes)
- 22 tRNAs - all for mitochondrial translation

Question 9

what does mitochondrial DNA not have

Answer 9

introns

Question 10

what are the 2 short non-coding regions of mitochondrial DNA

Answer 10

D-loop (contains ORIH)
ORIL
these regions are for replication

Question 11

what type of transcription happens in mtDNA?

Answer 11

polycistronic transcription - doesnt make separate mRNA for each gene, read as one long transcript and chopped up afterwards

Question 12

outline what is meant by tRNA punctuation

Answer 12

in mtDNA, there is a tRNA between each protein coding gene - this allows maturation of nRNA

Question 13

outline mtDNA in human disease

Answer 13

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

Question 14

what are some non-neurological disease mDNA mutations can cause

Answer 14

cardiomyopathy
liver failure
diabetes
respiratory failure
short stature

Question 15

what are some neurological diseases mtDNA mutations can cause

Answer 15

CPEO
seizures
developmental delay
deafness
peripheral neuropathy
myopathy

Question 16

what are the 4 genetic rules of mtDNA

Answer 16

• maternal inheritance
• heteroplasmy
• threshold effect
• mitotic segregation

Question 17

outline maternal inheritance

Answer 17

inheritance of mtDNA is strictly maternal
paternal mitochondria are eliminated after fertilisation

Question 18

outline heteroplasmy

Answer 18

a homoplasmic wild type and homoplasmic mutant make heteroplasmic mtDNA
with either high mutant load or low mutant load

Question 19

outline the threshold effect

Answer 19

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

Question 20

how do you stain mtDNA for defects?

Answer 20

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

Question 21

outline mitotic segregation

Answer 21

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

Question 22

what is the mitochondrial genetic bottleneck in mitotic segragation

Answer 22

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

Question 23

what is MELAS

Answer 23

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

Question 24

what mutation causes MELAS

Answer 24

mtDNA point mutation
m.3243A>G
not everyone who has the mutation will have MELAS, because of the spread in different tissues

Question 25

outline where the m3243A>G point mutation is found

Answer 25

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

Question 26

what is CPEO

Answer 26

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