Mitochondrial myopathies

Question 1

Describe mitochondrial DNA

Answer 1

Closed circular double stranded molecule

Question 2

What is an endosymbiotic origin?

Answer 2

Eukaryotic cells evolved from prokaryotic cells

Both cell types benefited from this relationship

Question 3

How many copies of the genome are in each mitochondrion?

Answer 3

5-10

Question 4

How many kb is the human mitochondrial genome

Answer 4

16.5

Question 5

How are mitochondrial proteins made?

Answer 5

> 900 different mt proteins are encoded by nDNA on cytosolic ribosomes, imported and then assembled in the mitochondrion

Question 6

Describe why the mitochondria are inherited from the mother

Answer 6

The embryo essentially derives all its mitochondria from the egg
Most sperm mitochondria are in the tail - not absorbed on fertilisation
Any paternal mitochondria that do enter egg are destroyed

Question 7

What does the mitochondrial genome code for?

Answer 7

13 of the respiratory chain proteins
2 rRNA
22 tRNA

Question 8

What is the major producer of reactive oxygen species?

Answer 8

Respiratory chain

Question 9

Why are ROS dangerous?

Answer 9

Can disrupt function of mitochondria

Question 10

Mitochondrial genome suffers the greatest exposure and damage to…

Answer 10

ROS

Question 11

How effective is mitochondrial DNA at correcting mistakes and repairing DNA damage?

Answer 11

Less effective

Question 12

Describe what happens to defects in mitochondrial DNA over time

Answer 12

Consequently defects in mtDNA accumulate with age and mtDNA mutates more rapidly (x 10-fold ) than nDNA

Question 13

Give the equation for the full reduction of oxygen

Answer 13

O2 + 4e- —–> H2O

Question 14

What can also occur in terms of reduction of oxygen?

Answer 14

Partial and subsequent reduction of oxygen generating ROS

Question 15

Describe the efficiency in OXPHOS

Answer 15

Efficiency declines with age as a result of the accumulation of mutations to mtDNA caused by ROS
OXPHOS enzyme defects strongly implicated in Alzheimer’s/Parkinson’s and type II diabetes

Question 16

Describe the defects in OXPHOS

Answer 16

Involve tissues most reliant on OXPHOS
Occurs later in life
Progressive with age
Show progressive enrichment in mutated mtDNA’s

Question 17

Describe some mitochondrial diseases

Answer 17

Diseases arising from defects in mt enzymes and systems e.g. in the TCA cycle and OXPHOS are rare
Major defects are incompatible with life and affected embryos very rarely survive
Nevertheless there are over 150 different mitochondrial diseases and some are linked to mtDNA
Often involve CNS and musculoskeletal system

Question 18

Why do most mitochondrial diseases affect the CNS and skeletal muscle?

Answer 18

Have highest numbers of mitochondria

Question 19

Describe mitochondrial myopathies

Answer 19

A number of human diseases are attributed to mutations in mt genes in mtDNA that reduce the capacity of cells to produce ATP
Some tissues / cell types e.g. neurons, myocytes, skeletal muscle cells and the -cells of the pancreas are less able to tolerate lowered ATP production

Group of neuromuscular diseases
Most occur before the age of 20, often beginning with exercise intolerance or muscle weakness
Other symptoms include heart failure / rhythm disturbances, dementia, deafness, blindness and seizures

Question 20

Onset of clinical symptoms, phenotypic variability, and variable penetrance of mt diseases are governed by:

Answer 20

Homoplasmy and heteroplasmy of mt - threshold effect

Mt genetic bottleneck

Question 21

Describe the threshold effect

Answer 21

Within a cell you may have normal and mutated mitochondria.
When the cell divides the proportion of mutated to normal may change therefore changes the severity.
Many diseases are caused by mutant mt DNA - a cell is termed heteroplasmic if it has both normal and mutant mt DNA present. Heteroplasmy is the threshold value of expression (varies) and this determines if a disease is manifested.
When the level of mutant mitochondria exceeds a certain threshold, the cell expresses dysfunction

Question 22

Describe the mitochondrial genetic bottle neck

Answer 22

Randomly selected mitochondria goes into each early egg

Question 23

Give some examples of mitochondrial myopathies

Answer 23

LHON- Lebers hereditary optic neuropathy
MERRF- myoclonus epilepsy with ragged red fibre
MELAS- mitochondrial encephalomyopathy, lactic acidosis and stroke-like episodes
KSS- Kearns-Sayre syndrome

Question 24

Describe Leber’s hereditary optic neuropathy

Answer 24

Single base change in the mt gene ND4, (from Arg to His) in a polypeptide of Complex I
Mitochondria partially defective in e- transport from NADH to UQ
Some ATP produced by e- transport from succinate, but not enough to support the very active metabolism of neurons
Results in damage to the optic nerve and leads to blindness
A single base change in the mt gene for cyt b in complex III also produces LHON

Question 25

Describe Myoclonus epilepsy with ragged-red fibre

Answer 25

Caused by a point mutation in the mt gene encoding a tRNA specific for lysine (tRNALys)
Disrupts synthesis of proteins essential for oxidative phosphorylation (ATP synthesis)
MERRF syndrome is caused by a mutation at position 8344 in the mt genome in over 80% of cases
Many other genes are involved and include:
mt – TK, mt - TL1, mt – TH, mt – TF
Skeletal muscle fibres of MERRF patients have abnormally shaped mitochondria

Question 26

What are ragged red fibres?

Answer 26

Clumps of defective mitochondria accumulate in aerobic skeletal muscle fibres (appear red after staining with Gomori modified Trichrome)

Associated with mitochondrial disease

Question 27

Describe Mitochondrial encephalomyopathy

Answer 27

Mt myopathy affecting primarily the brain and skeletal muscle
mt gene dysfunction involving mt ND5 (complex I) and mt-TH, mt-TL1 and mt-TV (all involved with tRNA)
Symptoms appear in childhood and include: a build up of lactic acid (lactic acidosis), stroke-like episodes with muscle weakness, seizures leading to loss of vision, movement difficulties (incl. involuntary muscle spasms (myoclonus) and dementia

Question 28

Describe Kearns-Sayre syndrome

Answer 28

Results from a 5kb deletion of the mt genome
Onset before age 20
Affected patients have short stature and often have multiple endocrinopathies including diabetes
Symptoms include dementia & retinitis pigmentosa
Other symptoms include lactic acidosis, heart conduction defects and raised cerebrospinal fluid protein content

Question 29

What is the prognosis for patients with mitochondrial myopathies

Answer 29

Variable and dependent on the type of disease and the patient’s metabolism (varies greatly between individuals)

Question 30

Give some examples of the treatments used

Answer 30

Occupational / physical therapy may extend the range of muscle movement. Vitamin therapies such as riboflavin, creatine, CoQ, C, K and carnitine may improve function for some
No specific treatments and attention is now turning to the development of genetic strategies for manipulating the mt genome

Question 31

How can mitochondrial myopathies be prevented?

Answer 31

IVF strategy designed to replace defective mitochondria inherited from a mother
Strategy involves merging DNA from two fertilised eggs - one from the mother with defective mitochondria, the other from a healthy donor with functioning mitochondria
Malfunctioning mitochondria are replaced by the donor healthy ones

Question 32

List the two procedures used to prevent mitochondrial myopathies

Answer 32

Mitochondrial gene replacement (pronuclear stage)

Maternal spindle transfer

Question 33

Describe mitochondrial gene replacement (pronuclear stage)

Answer 33

Patients egg with abnormal mitochondria fertilised with patients sperm
Patients zygote with abnormal mitochondria
Patients pronuclei removed from the zygote and transferred to enucleated egg which has normal mitochondria
Cleaving embryo with maternal and parental DNA and normal mitochondria is transferred back to the mothers uterus

Question 34

Describe the process of maternal spindle transfer

Answer 34

Unfertilised patients egg with abnormal mitochondria
Spindle and associated chromosomes removed as karyoplast from the patients egg and then fused with the enucleated donor egg
Reconstituted egg is fertilised with the patients sperm
Cleaving embryo with parental DNA and normal mitochondria can be transferred into the uterus