M104 T2 L5 Flashcards
What is mitochondrial DNA like?
a closed circular double stranded molecule
How many base pairs are in the human mitochondrial genome?
16.5 kb - 16,500 base pairs
How many copies of the human mitochondrial genome are in each mitochondrion?
5 -10 copies
How many mitochondria are in each cell of the body?
2 - 2000 mitochondria
How many different mitochondrial proteins are encoded by nDNA?
over 900 mt
What happens to mitochondrial proteins encoded by nDNA?
translated on cytosolic ribosomes, imported, then assembled in the mitochondrion
What is the idea of endosymbiotic origin?
that mitochondria are very likely the descendants of a prokaryote that developed an endosymbiotic relationship with ancestral eukaryotic cells back in the mists of evolutionary time
What is the relationship between human mitochondrial DNA and prokaryotic or bacterial mitochondrial DNA?
they have many features in common
From where does the embryo derives all its mitochondria from?
from the egg
none from the sperm
Why doesn’t the embryo recieve any mitochondria from the sperm?
most sperm mitochondria are in the tail - they aren’t absorbed on fertilisation
any paternal mitochondria that do enter egg are destroyed
What is the difference between human nuclear and mitochondrial genome?
their genes are much more dense (more common per a certain no of base pairs)
have no introns
different codon usage - don’t use the universal genetic code
inheritance is exclusively maternal rather than Mendelian
Where are the genes needed for mitochondrial function located?
many have moved from the mitochondrion into the nuclear genome over evolutionary time
What does the mitochondrial genome code for?
respiratory chain proteins x13
rRNA x2
tRNA x22 (different to nuclear tRNAs)
Which respiratory chain proteins does the mitochondrial genome code for?
Complex I: 7 subunits
Complex III: 1 subunit
Complex IV: 3 subunits
ATP synthase: 2 subunits
What is the relationship between mitochondrial efficiency and age?
it declines with age
this is partly caused by the accumulation of damage and mutations to mitochondrial DNA caused by reactive oxygen species
What are defects in oxidative phosphorylation associated with?
involve tissues most reliant on OXPHOS - that really can’t tolerate any deficiency in ATP levels
What are examples of conditions that defects in oxidative phosphorylation are associated with?
Alzheimer’s/Parkinson’s
type II diabetes
When do conditions associated with defects in oxidative phosphorylation occur?
later in life - and they are progressive with age
they show progressive enrichment in mutated mitochondrial DNA’s
What is the overall equation for the complete reduction of oxygen?
O2 (+4e-) —> H2O
What are the equations for the partial reduction of oxygen?
O2 (+e-) —> O2-
O2- (+e-) —> H2O2
H2O2 (+e-) —> OH + OH-
OH + OH- (+e-) —> H2O
What are examples of Reactive Oxygen Species?
Superoxide anion (O2.-) Hydroxyl radical (HO.) Peroxide ion (O2,2-) Hydrogen peroxide (H2O2) Hypochlorous acid (HOCl)
How are Reactive Oxygen Species generated in the mitochondrion?
at various points in the respiratory chain, electrons can be lost from the respiratory chain and can partially reduce oxygen, generating reactive oxygen species
What is the role of superoxide dismutases?
to remove Reactive Oxygen Species from the mitochondria
How do superoxide dismutases work?
they convert ROSes into h2o2
this can be further detoxified by catalase, glutathione peroxidases and peroxiredoxins
turn h2o2 into h2o and molecular oxygen
Why are ROS’s a problem if they’re removed by superoxide dismutases?
bc they are so reactive and toxic that inevitably some biological molecules such as DNA, lipids and proteins will be damaged before the cell has had a chance to remove them fully
What is a major producer of reactive oxygen species?
the respiratory chain
What structure in the cell is most affected by ROS’s and how?
the mitochondrial genome suffers the greatest exposure to, and damage by, ROS due to proximity
What is mitochondrial DNA’s repairative qualities like?
probably given is prokaryotic ancestry, it is relatively not very effective at correcting mistakes and repairing mitochondrial DNA damage
Where are defects leading to mitochondrial diseases rare?
in the TCA cycle
oxidative phosphorylation
What happens to embryos with mitochondrial diseases?
they very rarely survive
bc major defects are incompatible with life
How many different mitochondrial diseases are there?
over 150
What areas do mitochondrial diseases usually affect?
usually the CNS and musculoskeletal system
(mitochondrial myopathies)
due to the very high E demands of these tissues
How are mitochondrial diseases classified?
according to the biochemical defect
What are the different defects in the biochemical classification of mitochondrial diseases?
mitochondrial transport systems substrate utilisation TCA cycle OXPHOS coupling oxidative phosphorylation
What is an example of a defect to mitochondrial transport systems?
CPT I and II deficiencies
What is an example of a defect to substrate utilisation?
PDC deficiency
FA oxidation defects
What is an example of a defect to the TCA cycle?
Fumarase deficiency OR a- ketoglutarate dehydrogenase deficiency
What is an example of a defect to OXPHOS coupling?
Luft’s syndrome
What is an example of a defect to oxidative phosphorylation?
Complex I / II / III / IV / V deficiencies (defects of respiratory chain components)
What is the effect of human diseases attributed to mutations in mitochondrial genes?
they reduce the capacity of cells to produce ATP
What are examples of tissues / cell types that are less able to tolerate lowered ATP production?
neurons, myocytes, skeletal muscle cells and the b-cells of the pancreas
When do most mitochondrial myopathies occur?
under the age of 20, often beginning with exercise intolerance or muscle weakness
What are symptoms of mitochondrial myopathies?
heart failure / rhythm disturbances
dementia and seizures
deafness and blindness
Why is the clinical presentation of mitochondrial diseases so varied?
the biology of the mitochondria gives rise to a threshold effect and a mitochondrial genetic bottleneck
What is the threshold effect caused by?
heteroplasmy
the presence of both normal and mutated mtDNA in a large amount of mitochondria
What is the threshold of defective mitochondria in a cell that will cause mitochondrial disease?
70% +
What type of cells could be produced by a progenitor cell showing heteroplasmy of mitochondria?
a cell that has completely normal mtDNA
a cell that has mutated mtDNA
a cell that is heteroplasmic but is still normal (under the threshold)
a cell that is heteroplasmic and has mitochondrial disease (over the threshold)
What type of primary oocytes could be produced from the primordial germ cell with a low heteroplasmy?
cells with varied levels of heteroplasmy
bc when primary oocytes are formed, they receive a random sampling of the mitochondria from the primordial germ cell
What are the common mitochondrial myopathies?
LHON, MERRF - myoclonus epilepsy with ragged-red fibre
MELAS, KSS
What causes KSS?
a 5kb deletion where a whole chunk of the mitochondrial genome can be lost in some cases
What is the main genetic cause of LHON syndrome?
a single base change in the mt gene ND4 in a PPT of Complex I
What amacs are changed in LHON syndrome?
changes from Arganine to Histidine
What are the effects of the main genetic mutation involved in LHON syndrome?
the mitochondria are partially defective in e- transport from NADH dehydrogenase to ubiquinone
Some ATP is produced by e- transport from succinate, but not enough to support the very active metabolism of neurons
What is the effect of a lack of ATP in LHON syndrome?
damage to the optic nerve, leads to blindness
What is a secondary genetic cause of LHON syndrome?
a single base change in the mt gene for cyt b in complex III
What is a genetic cause of MERRF syndrome?
a point mutation in the mt gene encoding a tRNA specific for lysine (tRNALys)
What is the effect of the genetic mutation involved in MERRF syndrome?
Disrupts the synthesis of proteins essential for oxidative phosphorylation (ATP synthesis)
Where is the genetic mutation leading to MERRF syndrome in over 80% of cases?
at position 8344 in the mt genome
What are genes that code for tRNA in mtDNA that can experience mutations leading to MERRF in rare cases?
mt – TK
mt - TL1
mt – TH
mt – TF
What is unusual about the skeletal muscle fibres of MERRF patients?
they have abnormally shaped mitochondria
When do ragged red fibers appear red?
after staining with Gomori modified Trichrome
Where do ragged red fibers accumulate?
in aerobic skeletal muscle fibres
What are ragged red fibers indicators of?
mitochondrial disease, can be used for diagnosis of
What type of condition is MELAS syndrome?
Mt myopathy affecting primarily the brain and skeletal muscle
What is a genetic cause of MELAS syndrome?
mt gene dysfunction involving mt ND5 (complex I) and mt-TH, mt-TL1 and mt-TV (all involved with tRNA)
When do symptoms of MELAS syndrome first appear?
in childhood
What are the symptoms of MELAS syndrome?
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)
dementia
When is the onset of KSS?
before age 20
What are the symptoms of KSS?
they have short stature often have multiple endocrinopathies including diabetes and dementia retinitis pigmentosa lactic acidosis heart conduction defects raised CSF protein content
How are mt myopathies diagnosed?
combination of biochemical tests, histology & genetic testing
What is the prognosis of mt myopathies?
they are variable and dependent on the type of disease and the patient’s metabolism (varies greatly between individuals)
How can mt myopathies be managed?
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
How are mt myopathies treated?
No specific treatments
Development of genetic strategies for manipulating the mt genome
How are mt myopathies prevented?
IVF strategy designed to replace defective mitochondria inherited from a mother
What is the principe of the IVF strategy designed to replace defective mitochondria inherited from a mother?
Malfunctioning mitochondria are replaced by the donor healthy ones
How does the IVF strategy designed to replace defective mitochondria inherited from a mother work?
Mitochondrial Gene Replacement
involves merging DNA from two eggs - one from the mother with defective mitochondria, the other from a healthy donor with functioning mitochondria
At what stage of development is Mitochondrial Gene Replacement performed?
in the pronuclear stage
How does Mitochondrial Gene Replacement work?
two different zygotes are developed, one from the patient’s egg and their partner’s sperm (has abnormal mitochondria) and the other from a donor that is normal
the zygotic cells are transferred from the patient’s egg to the donor’s egg, which has its zygote removed
What are the two different ways in which mt myopathies can be prevented?
Mitochondrial Gene Replacement
Maternal Spindle Transfer
How does Maternal Spindle Transfer work?
two unfertilised eggs, one from the patient (abnormal mitochondria) and one from the donor (normal)
the spindle and the associated chromosomes are removed from the patient’s egg and are fused into the donor egg which has been enucleated
What are the two different types of myoclonus?
positive myoclonus - muscle contractions
negative myoclonus - muscle relaxation
What are the classical characterisations of Kearns-Sayre syndrome?
progressive limitation of eye movements until there is complete immobility
eyelid droop
What are the additional symptoms of Kearns-Sayre syndrome?
mild skeletal muscle weakness
heart block (a cardiac conduction defect)
short stature
hearing loss, impaired cognitive function
ataxia
diabetes
What conditions are included in Samter’s Triad? (ASP)
Asthma
Sinus inflammation w/ recurring nasal Polyps
What medications should people with Samter’s Triad NOT be prescribed?
aspirin or a similar drug
Why should people with Samter’s Triad not be prescribed aspirin?
bc they can have a severe reaction with both upper and lower respiratory symptoms
What happens to levels of MnSOD in certain diseases?
they are reduced
What are examples of diseases in which MnSOD levels decrease?
neurodegenerative diseases
cancer
psoriasis
When do symptoms of CPT I deficiency usually appear?
during early childhood
What are CPT deficiencies caused by?
mutations to CPT I and II enzymes
What is an effect of a CPT deficiency?
the body is prevented from using certain fats for E, particularly during fasting
