Lecture 19 - Molecular Aetiology of Mitochondrial Disorders

Question 1

What is the main function of mitochondria?

Answer 1

Provides the vast majority of ATP for cells.

Question 2

What are some other functions of mitochondria?

Answer 2

Protein import and sorting
Protein and organelle quality control
Fission and fusion
Mitophagy
Haem and FeS biosynthesis
mtDNA replication and expression
OXPHOS assembly
Mitochondrial translation
Apoptosis
Transport of metabolites

Question 3

Describe the structure of mitochondria.

Answer 3

Rod shaped organelles with 2 membranes.
Inner membrane is folded (cristae) and the inner part is the mitochondrial matrix where DNA resides.
Mitochondria are spread throughout a cell and are always moving.

Question 4

Describe the mitochondrial respiratory chain.

Answer 4

Main function of mitochondria.
Contains 5 subunit complexes.
Electron transport chain is composed of complexes 1-4.
Complex 5 is ATP synthase that uses the electrochemical gradient to power the motor that facilitates production of ATP.

Question 5

Describe mitochondrial DNA.

Answer 5

mtDNA.
Covalently closed DNA molecule located in the mitochondrial matrix and multiple copies per cell.
Number of copies per cell depends on cell type.
13 protein encoding genes that all encode for hydrophobic OXPHOS components - explains why they aren’t lost to the nucleus as can’t cross the cytosol.
No introns and highly organised.
2 rRNAs (mitroribosomes)
22 tRNAs

Question 6

Why is there a high mutation rates in mtDNA?

Answer 6

The structure and location means it is not protected.
Not many histones.
Bad repair mechanisms.
Vulnerable to being attacked by ROS.
Few non-coding regions means most mutations affect the coding sequence.

Question 7

What cell type are particularly affected by mitochondrial disease and why?

Answer 7

Neurons because they require lots of energy so have many mitochondria.

Question 8

What are some examples of neurological mitochondria disease?

Answer 8

Optic atrophy
Retinitis pigmentosa
CPEO, CVA
Seizures
Developmental delay
Deafness
Peripheral neuropathy
Myopathy

Question 9

What mtDNA point mutation causes mitochondrial encephalomyopathy lactic acidosis and stroke-like episodes (MELAS)?

Answer 9

m.3243A>G.
Most common mtDNA point mutation.
Wide range of clinical symptoms.
Higher mutant loads in post-mitotic tissues means more likely to get disease as they can accumulate and aren’t undergoing mitosis and separating.

Question 10

Describe CPEO.

Answer 10

Chronic progressive external ophthalmoplegia
mtDNA rearrangment disorder.
Affects the eyes usually because of its fast motions.
Usually a deletion - whole genes can be removed, different patients have different sizes of deletions.
Sporadic.

Question 11

Describe maternal inheritance in terms of mtDNA genetic rules.

Answer 11

Inheritance of mtDNA is strictly maternal, paternal mitochondrial eliminated after fertilisation.

Question 12

Describe heteroplasty in terms of mtDNA genetic rules.

Answer 12

So many copies of mtDNA.
If you have a variant you can have any % of that variant leading to high or low mutant loads.

Question 13

Describe the threshold effect in terms of mtDNA genetic rules.

Answer 13

Many heteroplastic mtDNA mutations are highly recessive (a lot of the mutant is needed before a phenotype of a disease is presented).
High levels of mutation can be tolerated by the cell before it induces respiratory chain defects.
Different tissues and mutations have different thresholds.

Question 14

Describe mitotic segregation in terms of mtDNA genetic rules.

Answer 14

Levels of mtDNA mutations can vary enormously between tissues, at mitosis both wt and mutated mtDNA are randomly segregated to each daughter cell.
This affects both disease expression and inheritance.

Question 15

How can muscle histology and histochemistry be used to investigate suspected mt disease?

Answer 15

COX staining where brown cells mean good.
COX staining then SDH.
SDH completely nuclear encoded so if there is a problem with mtDNA there won’t be a problem with SDH and will always stain.

Question 16

What are some clinical cues of mt disease?

Answer 16

Typical phenotype, combination of systems involved, family history.
In children - developmental delay, hypotonia, lactic acidosis, abnormal MRI.
In adults - ptosis, muscle weakness, seizures, deafness, fatigue, stroke-like episodes, neuropathy.

Question 17

What is some treatment for mt disease?

Answer 17

Management of symptoms - surgery to correct ptosis, cochlear implant.
Pharmacological/metabolic - CoQ10.
Genetic strategies - target DNA to mitochondria, selectively degrade mutant mtDNA molecules.
Exercise.
Prevention of mtDNA disease transmission.

Question 18

Describe prevention of transmission.

Answer 18

mt disease is hard to treat.
Bets treatment is to prevent the transmission in the first place.
Done through prenatal testing, preimplantation genetic diagnosis etc.
Selecting embryos with low levels of mutations to then be implanted via IVF. However, sometimes not always an option as some others only have embryos with high mutant loads.

Question 19

Describe giving a donor zygote in hopes of preventing mt disease.

Answer 19

Some mothers only have embryos with high mutant loads.
Can take a donor zygote and the recipient zygote and remove both nuclei.
Put the donor nuclei into the recipient zygote to have children that at a nuclear level are related to the parents but have mitochondrial DNA from another woman - usually a female from the fathers side.

Question 20

Describe CPEO+

Answer 20

CPEO with additional symptoms.
Autosomal recessive or autosomal dominant so a nuclear mutation NOT an mtDNA mutation.
Mutation in mtDNA Pol PolG provides a proof reading error so introduces mutations to the mtDNA that are secondary to the mtDNA pol mutation but presents the same as mitochondrial disease due to the issues with mtDNA.