Mitochondrial myopathies

Question 1

Endosymbiotic theory

Answer 1

Theory that explains the origin of the mitochondria.

The mtDNA is theorised to be an evolutionary descendant of a prokaryote.
- Contains same sized ribosome (70s) and other similar features

Mitochondrion is thought to have developed an endosymbiotic relationship with an ancestral eukaryotic cell.

Question 2

Mitochondrial DNA (mtDNA)

Answer 2

Circular double stranded DNA.

Mt genome is 16.5 Kb is size and there is 5-10 copies in each mitochondrion.

More than 900 different Mt proteins are encoded in nDNA and transported to the mt.

Question 3

Differences between nDNA and mtDNA

• Size
• Number of DNA per cell
• Genes
• Introns
• Coding DNA
• Coding
• Associated proteins
• Mode of inheritance

Answer 3

Size: nDNA much larger.

Number of DNA molecules per cell:
nDNA= diploid, haploid
mtDNA= polyploidy

Genes: nDNA has way more genes

Introns: mtDNA contains no introns, nDNA does.

Coding DNA: Most of mtDNA is coding, whereas only around 3% of nDNA is coding.

Coding: mtDNA does not use the normal universal genetic code.

Associated proteins: mtDNA has no histones, whilst nDNA does.

Mode of inheritance: mtDNA is maternal, nDNA uses mendelian and paternal inheritance.

Question 4

Maternal inheritance of mtDNA

Answer 4

During fertilsation, none of the sperm’s mitochondria is donated to the embryo
- All mitochondria is derived from the egg.

Question 5

Mitochondrial genome

- What does it code for

Answer 5

13 of the genes codes for respiratory chain proteins.

2 codes for rRNA.

22 codes for tRNA- which differs from nuclear tRNA.

Question 6

Mutation of mtDNA

• What causes it
• How does it differ from nDNA mutations

Answer 6

The respiratory chain is very close to mtDNA
- The chain produces ROS which damages mtDNA

mtDNA is less effective than nDNA in repairing DNA damage.
- mtDNA mutates x10 fold more rapidly than nDNA

With age, mtDNA accumulates.

Question 7

Reduction of oxygen

Answer 7

O2–[4e-]–> H2O
- This occurs at the complex IV in the respiratory chain.

Oxygen can be partially reduced at other complexes to form ROS.

1. O2–> O2-
2. O2—> H2O2
3. H2O2—> .OH + OH-

Question 8

Reactive oxygen species

Answer 8

Chemically reactive species with oxygen.
- This is produced in the respiratory chain and can cause mtDNA damage.

Examples:
Superoxide anion: O2-

Hydroxyl anion: .OH

Periode ion O2(2-)

Hydrogen peroxide (H2O2)

Hypochlorous acid (HOCl)

Question 9

Resolving ROS

Answer 9

There are antioxidant enzymes in the mitochondrion that convert ROS, into safer molecules.

H2O2:
Catalase
GPA
Prx3

Superoxide O2-:
MnSOD

Question 10

Antioxidant that converts O2-

Answer 10

MnSOD
- manganese superoxide dismutase

Converts O2- into H2O2

Question 11

Diseases that show OXPHOS enzymes strongly implicated [3]

Answer 11

Alzheimer’s

Parkinson’s

TII diabetes

Question 12

Mitochondrial myopathies

Answer 12

Neuromuscular diseases that result from mt mutation
- Leads to decrease ATP production in a cell.

Especially affects cells that are less tolerable to low ATP:

• Neurones
• Myocytes
• Skeletal muscle cells
• Pancreatic beta-cells

Question 13

Symptoms of Mt myopathies

Answer 13

Begins with exercise intolerance/ muscle weakness

Heart failure/ arrhythmias

Dementia

Deafness

Blindness

Seizures

Question 14

Clinical presentation of mt myopathies

Answer 14

Presentation is very variable due to heteroplasmy [threshold effect] and genetic bottleneck.

Question 15

Threshold effect

Answer 15

This describes the proportional of defective mtDNA in a cell required to cause disease in a person.
- usually 70%

The proportion is unpredictable due to the nature of cell division:
Mitochondria are unequally divided during cell division so a cell can be heteroplasmic or homoplasmic.

Question 16

Mt genetic bottleneck

Answer 16

In primordial germ cells, there can be mutant mtDNA which are randomly selected in cell division to form a primary oocyte.

The primary oocyte have their mitochondria multiply and the population of mt is different from the original primordial germ cell.

This can either cause a very high or low level of mutation in the cell before fertilisation.

Question 17

Biochemical classification of Mt myopathies

Answer 17

Defects in mitochondrial transport systems.

Defects of substrate utilisation.

Defects of TCA cycle.

Defects of OXPHOS coupling.

Defects in oxidative phosphorylation

Question 18

LHON syndrome

Answer 18

Leber’s hereditary optic neuropathy.

A Mt myopathy caused by Single base pair chain in ND4 gene.
- Affects Complex I

Defective e- transport from NADH to Ubiquinone

               OR A single base change in the mt gene for cyt b. - Affects complex III

Results:
Optic nerve damage= blindness

Question 19

Why can those with the LHON syndrome still survive despite a OXPHOS mutation?

Answer 19

The mutation affects either complex I, thus electron transport between NADH and UQ.

Electrons can still reach UQ via succinate, through succinate dehydrogenase.

This can support some metabolism in cells that are not very active.

But it cannot support the active metabolism of neurones

Question 20

MERRF syndrome

Answer 20

Myoclonus epilepsy with ragged-red fibre.

Mitochondrial myopathy caused by a point mutation in the gene for lysine tRNA.

Affects:
Protein synthesis for oxidative phosphorylation.

Characterised by abnormally shaped mitochondria seen as ‘ragged-red fibres’ in staining..

Question 21

Most common mutation in MERRF syndrome

Answer 21

Point mutation at position 8344 in the mt genome.

Question 22

Ragged red fibres

Answer 22

Clumps of defective mitochondria that appear red after staining with Gomori modified Trichrome.

These clumps accumulate in aerobic skeletal muscle fibres.

Question 23

MELAS syndrome

Answer 23

Mitochondrial encephalomyopathy

Mt mutation in genes:
ND5 (complex I)
TH, TL1, TV (tRNA)

Symptoms:
Lactic acidosis
Stroke like episodes
Seizures w/ loss of vision
Myoclonus
Dementia

Question 24

KSS

Answer 24

Kerns-Sayre syndrome

Mutation: 5kb deletion of mt genome

Onset: occurs before age 20.

Symptoms/ presentation:
Short stature
Endocrinopathies

Dementia
Retinitis pigmentosa
Lactic acidosis
Heart conduction defects
Raised protein content in CSF

Question 25

Treatments for mitochondrial myopathies

Answer 25

Occupation/ physical therapy:
- Extend range of muscle movement

Vitamin therapies:
Riboflavin, creatine, CoQ etc to improve function (co enzymes)

Possible mt genome manipulation

Treatment’s effectiveness depends on patient’s metabolism.

Question 26

Prevention of mitochondrial myopathies.

Answer 26

IVF strategy: used to replace defective mitochondrial inherited from mother

• Pro-nuclear stage mitochondrial gene replacement
• Maternal spindle transfer

Question 27

Pro-nuclear mitochondrial gene replacement

Answer 27

Method used in IVF to replace defected mitochondria inherited from mother.

1. Following eggs are fertilised with chosen father’s sperm:
   - Mother’s egg with abnormal mitochondria.
   - Donor’s egg with normal mitochondria.
2. Both eggs form a zygote. Donor’s zygote is taken from it egg and replaced with patient’s zygote.
3. Zygote not grows in egg with normal mitochondria with selected maternal and paternal nuclear genome.
4. Cleaving embryo is now transferred into the uterus of the chosen mother.

Question 28

Maternal spindle transfer

Answer 28

Method used in IVF to replaced defective mitochondria inherited from mother.

1. Both patient and donor’s eggs are unfertilised.
2. Karyoplast from patient’s egg that contain spindle and associated chromosomes is fused into enucleated donor’s egg.
   - Karyoplast from donor’s egg is removed and discarded.
3. Reconstituted egg is fertilised with spermout from patient’s father.
4. Cleaving embryo with normal mitochondria and maternal and paternal genome is transferred into uterus.