Lecture 4

Question 1

Describe the mitochondrial genome

Answer 1

Many of the genes needed for mitochondrial function have moved from the mitochondrion into the nuclear genome

mt genome codes for:
13 of the respiratory chain proteins
2 rRNA
22 tRNA

Other differences:
mt genetic code differs from the normal universal genetic code

tRNA structure differs from nuclear tRNA

Question 2

Explain the reason for the mutation of mtDNA

Answer 2

Respiratory chain is the major producer of reactive oxygen species (ROS)

mt genome suffers the greatest exposure to, and damage by, ROS

mtDNA less effective at correcting mistakes and repairing mt DNA damage

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

Question 3

Name the reactive oxygen species

Answer 3

Superoxide anion (O2.-)
Hydroxyl radical (HO.)
Peroxide ion (O22-)
Hydrogen peroxide (H2O2)
Hypochlorous acid (HOCl)

Question 4

Discuss the efficiency of OXPHOS

Answer 4

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

Defects in OXPHOS:
involve tissues most reliant on OXPHOS
occurs later in life
progressive with age
show progressive enrichment in mutated mtDNA’s

Question 5

Discuss mitochondrial diseases

Answer 5

Diseases arising from defects in mt enzymes and systems e.g. in the TCA cycle and OXPHOS are rare

Often involve CNS and musculoskeletal system

Some tissues / cell types e.g. neurons, myocytes, skeletal muscle cells and the b-cells of the pancreas are less able to tolerate lowered ATP production

Question 6

What are mt myopathies?

Answer 6

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 7

Name the biochemical classification of mt myopathies

Answer 7

Defects of mitochondrial transport systems
Carnitine palmitoyltransferase (CPT I and II) deficiencies

Defects of substrate utilisation
Pyruvate dehydrogenase complex (PDC) deficiency
Fatty acid oxidation defects

Defects of TCA cycle
Fumarase deficiency OR a- ketoglutarate dehydrogenase deficiency

Defects of OXPHOS coupling
Luft’s syndrome

Defects of oxidative phosphorylation
Complexes I / II / III / IV / V deficiencies combined defects of respiratory chain components

Question 8

Explain LHON syndrome

Answer 8

Leber’s hereditary optic neuropathy:
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 9

Explain MERRF syndrome

Answer 9

Myoclonus epilepsy with ragged-red fibre

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 10

Explain MELAS syndrome

Answer 10

Mitochondrial encephalomyopathy

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 11

Explain KSS

Answer 11

Kearns-Sayre syndrome

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 12

What is the current treatment for mitochondrial myopathies?

Answer 12

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

Question 13

Describe mitochondrial gene replacement

Answer 13

1. Patient’s egg with abnormal mitochondria fertilised with patient’s sperm
2. Patient’s zygote with abnormal mitochondria
3. Patient’s pronuclei removed from zygote and transferred to enucleated egg which has normal mitochondria.
4. Cleaving embryo with normal mitochondria and maternal and patronal genome can be transferred to the uterus.

Question 14

Describe the maternal spindle transfer

Answer 14

1. unfertilised patients egg with abnormals mitochondria
2. Spindle and associated chromosomes removed as karyoplast from patient’s egg and fused into ‘enucleated’ donor egg
3. Reconstituted egg is fertilised (by ICSI) with sperm from patient’s partner
4. Cleaving embryo with normal mitochondria and maternal and paternal genome can be transferred to there uterus