Mitochondrial Inheritance & Disease

Question 1

How do mitochondria replicate?

Answer 1

Similar to bacteria–after replicating DNA, when they get too large they divide via fission.

Question 2

What are four functions of mitochondria (besides powering cells)?

Answer 2

1. Contribute to redox
2. Contribute to Ca2+ homeostasis
3. Provide intermediary metabolites
4. Store pro-apoptotic factors

Question 3

What percentage of the mitochondrial genome is made of C’s and G’s?

Answer 3

44%

Question 4

How many base pairs in the mitochondrial genome?

Answer 4

16,569

Question 5

Describe the strands of the mitochondrial genome.

Answer 5

Heavy is G-rich and light is C-rich; plus there is a triple-stranded region of ,121base pairs.

Question 6

Where is mitochondrial DNA located?

Answer 6

Inside nucleoid structures found in the matrix (each may have 2-10 copies of mitochondrial DNA–meaning over 1,000 copies of mtDNA in each cell for mammals/humans).

Question 7

Describe the ribosomes and RNA associated with the mitochondrial genome.

Answer 7

Mitochondria has its own ribosomes and 22 tRNA’s, 2 rRNA’s, and can make 13 mitochondrial proteins.

Question 8

How are most mitochondrial proteins encoded and translated?

Answer 8

95% encoded in nuclear genome, translated in the cytoplasm, and imported into the mitochondria.

Question 9

Proteins targeted for mitochondria usually have what cell “marker” on them?

Answer 9

An amino-terminal leader peptide, which is cleaved after it’s imported into the mitochondria.

Question 10

Describe the five mitochondrial complexes, and their origins.

Answer 10

Complex I: NADH dehydrogenase
Complex II: Succinate dehydrogenase & CoQ reductase
Complex III: Ubiquinol cytochrome C reductase (made with mtDNA as well as nuclear DNA)
Complex IV: Cytochrome C oxidase
Complex V: ATP synthase

Question 11

What three metabolic pathways are mitochondria involved?

Answer 11

1. Beta-oxidation of fatty acids.
2. Biosynthesis of pyrimidines, amino acids, phospholipids, nucleotides, and Haem (?).
3. Oxidative phosphorylation to produce ATP from the reduction of oxygen for energy.

Question 12

Oxidation of 1mol of glucose yields how many ATP?

Answer 12

36 mols

Question 13

What is the Mitochondria Eve theory?

Answer 13

ALL human DNA originated over ~200,000 years ago in Africa from ONE original female (evidence obtained from human polymorphism studies of human populations around the world).

Question 14

What percentage of mitochondrial DNA is inherited from the mother and why?

Answer 14

99.99% because sperm carries ~100 mtDNA in its tail vs. 100,000 in oocyte and as cells develop it’s diluted out.

Question 15

What are three ways in which paternal mitochondrial DNA is diluted out?

Answer 15

1. Sperm mtDNA may be diluted in egg’s mtgenome.
2. Oxidants normally present in cell may destroy sperm mitochondria.
3. Ubiquination marks sperm mitochondria for destruction–meaning that cells may have processes to actively destroy paternal mitochondria

Question 16

During oogenesis, what happens to the mitochondria and its DNA?

Answer 16

Mitochondria:cell ratio increases by 100 fold; mtDNA:mitochondria ratio falls to ~1-2.

Question 17

When does mitochondrial DNA replication most likely resume after oogenesis?

Answer 17

At blastocyst stage.

Question 18

What is one reason why it is beneficial for cells to only inherit mitochondria from one parent?

Answer 18

Reduces chance of mutations accumulated from two different genomes.

Question 19

What is heteroplasmy in cells?

Answer 19

Mixed mitochondria from more than one parent.

Question 20

How likely are males vs. females to inherit abnormal mitochondria from their mothers?

Answer 20

Equally likely, with the exception of Leber’s Hereditary Optic Neuropathy (LHON)in which males are more commonly affected.

Question 21

What is one reason why males may be more commonly affected by LHON?

Answer 21

It is postulated that there is some protective factor in the female body, perhaps a related X chromosome marker, or that female hormones like progesterone offer some protective factor.

Question 22

What are two things to note about females who develop LHON in comparison to males?

Answer 22

1. Women who develop it are affected slightly later in life than males.
2. They are more severely affected.

Question 23

What is LHON?

Answer 23

Leber’s hereditary optic neuropathy is a degeneration of retinal ganglion cells and their axons that lead to an accute or sub-accute loss of central vision (usually due to one of three mtDNA point mutations).

Question 24

Can men with mitochondrial mutations/disorders pass them on to their children?

Answer 24

No.

Question 25

Explain the mitochondrial bottleneck effect.

Answer 25

Maternal mtDNA is abundant and only select few used in primordial germ cells (bottleneck effect) and replicated, which helps guarantee homoplasmy and uniform quantity of these organelles in the oocyte. NOTE: This doesn’t exist in somatic cells.

Question 26

What are three things to remember about ATP production and the result of a mitochondrial disorder?

Answer 26

1. Mitochondrial mutations cause disruption in ATP supply for all cells in body.
2. When process is repeated on a large scale, whole systems can fail and life of individual can be compromised.
3. Cells requiring the most energy = skillet muscles, brain, and heart = most affected.

Question 27

What are three causes of mitochondrial disorder?

Answer 27

1. Loss of coupling between oxidation and ATP production.
2. Dissipation of heat of the metabolic energy.
3. Need for increased metabolism to compensate for impaired ability to generate ATP.

Question 28

When was the first mitochondrial disease described and what was the case/symptoms?

Answer 28

1. 1962
2. 27 year old female with symptoms for 20 years of heavy perspiration, large calorific and fluid intake, weakness, and very high oxygen consumption (180% above normal).

Question 29

What are five general features of mitochondria diseases due to mtDNA mutations?

Answer 29

1. Disease signs manifest especially in tissues with a high energy expenditure (brain, heart, and skeletal muscle).
2. Often causes deficient function in respiratory chain.
3. Abnormal oxidative phosphorylation enzymes.
4. Highly variable both clinically and at the genetic level.
5. Similar clinical syndrome can result from different DNA mutations (same genetic defect can present in different ways).

Question 30

What two types of mtDNA mutations are there and how many adults do they affect?

Answer 30

1. Point mutations and larger rearrangements (including deletions and duplications).
2. 1/15,000 adults.

Question 31

Compare the fixation of mtDNA mutations vs. nuclear DNA mutations in patients with mitochondrial disorder?

Answer 31

10 times higher, respectively.

Question 32

Give two possible explanations for the difference in fixation between mtDNA and nuclear DNA mutations in patients with mitochondrial disorder?

Answer 32

1. Lack of histones = high influx of oxygen radicals in mitochondria are not protected by histones, which makes mtDNA mutations more likely to be fixated.
2. Absence of effective DNA repair systems in mitochondria.

Question 33

Mitochondrial DNA base pair deletions range from what to what size?

Answer 33

~100 - 10,000 base pairs

Question 34

Mitochondrial DNA deletions can be accumulated in what kind of cells?

Answer 34

Aged cells (meaning deletions can be sporadic in nature).

Question 35

What are two clinical presentations of mitochondrial DNA deletions?

Answer 35

1. Chronic Progressive External Opthalmoplegia (CPEO)

2. Kearns-Sayre syndrome

Question 36

What are four symptoms of Kearns-Sayre syndrome?

Answer 36

1. Progressive external ophthalmoplegia
2. Mild skeletal muscle weakness
3. Retinal pigmentation
4. Cardiac conduction defects

Question 37

What are three interesting things to know about mitochondrial DNA duplications?

Answer 37

1. May not be pathological.
2. Triplication reported in normal individual.
3. Inversion found in normal patient.

Question 38

What two things are significant in the “4977 bp deletion” in mtDNA?

Answer 38

1. Characteristic of aging post mitotic cells (heart muscle, for example) although not common and always found in less than 1% of the population except for Kearns-Sayre patients.
2. It’s a deletion of almost a quarter of the entire mt genome that includes coding regions for mitochondrial genes involved in oxidative phosphorylation, which is why it is so heavily related to oxidative damage.

Question 39

How many mitochondrial point mutations have been catalogued to date and what do they mainly code for?

Answer 39

100+; protein-encoding genes but sometimes rRNA or tRNA.

Question 40

What is the most frequent mitochondrial DNA mutation and how often does it occur?

Answer 40

Point mutations, 16/100,000 in adult population.

Question 41

What are three clinical presentations of mitochondrial point DNA mutations in order of severity from highest to lowest?

Answer 41

1. Leigh’s syndrome
2. MELAS (Mitochondrial Encephalomyopathy Lactic Acidosis and Stroke-like episodes)
3. Oligosymptomatic mitochondrial disorders
4. NOTE: Higher levels of mutated DNA = more severe symptoms.

Question 42

What are five symptoms of Leigh’s syndrome?

Answer 42

1. Developmental delay
2. Hypotonia
3. Lactate raised
4. Weakness
5. Respiratory disturbance

Question 43

How is heteroplasmy and homoplasmy relevant to inheritance of mitochondrial DNA-related mitochondria disorders?

Answer 43

1. Cells contain many mitochondria and mtDNA is divided with possibility for both mutated and healthy mtDNA heteroplasmy.

Question 44

What are three important things to note about heteroplasmy and the phenotype of mitochondrial diseases?

Answer 44

1. Highly variable both clinically and genetically
2. Different mutations can cause the same clinical syndrome, which isn’t surprising because they al affect the respiratory chain.
3. Identical genetic mutations can give rise to differences in phenotypes.

Question 45

What are two reasons why identical genetic mutations can give rise to different phenotypes of mitochondrial disorders?

Answer 45

1. Threshold effect (“mutation load”) = Depends on the degree of heteroplasmy and the dependence of tissue in which it occurs upon oxidative metabolism.
2. Intracelluar drift = In post-mitotic, non-dividing cells (like neurons, cardiac tissue, etc.), mitochondrial DNA continues to divide, which can alter heteroplasmy within a cell and cause symptoms to present later in life.

Question 46

How often do clinically affected females with mtDNA deletions pass their mutated DNA to offspring?

Answer 46

Rarely

Question 47

Up to how many mtDNA molecules displaying heteroplasmy may a maternal oocyte contain?

Answer 47

~100,000

Question 48

Compare the frequency of mtDNA deletions present in oocytes vs. embryos.

Answer 48

More common in oocytes than embryos; (NOTE: supposedly affected by age but studies show conflicting results).

Question 49

What region in the mitochondrial DNA is especially risky in the event of a mutation?

Answer 49

Mitochondrial DNA Replication Control Region.

Question 50

Describe one case in which paternal mtDNA was found in an adult, causing a relevant mutation-related disease.

Answer 50

One had mutated paternal mtDNA which was found to be abundant in patient’s skeletal muscle; proves that paternal mtDNA survives in zygote and can carry through to adulthood but doesn’t negate general rule of maternal mitochondrial inheritance.

Question 51

Explain mitochondrial aging’s relevance to oocyte aging.

Answer 51

Decreased oocyte ability to metabolize and produce energy (low ATP levels associated with poor oocyte quality).

Question 52

What are three questions Preimplantation Genetic Diagnosis of mitochondrial disorder aims to answer?

Answer 52

1. Can clinically affected females with mitochondrial DNA point mutations be diagnosed?
2. Can heteroplasmic females with point mutations be predicted to prevent transmission of mutant mitochondrial DNA to different offspring?
3. Will polar body biopsy or embryo biopsy be reliable for mitochondrial preimplantation genetic diagnosis?

Question 53

How can cytoplasmic transfer during ART contribute to mitochondrial disorder?

Answer 53

When donor ooplasm is injected into patient’s oocyte to supplement cytoplasmic factors required for normal development, the population of mitochondria in the oocyte becomes from two different sources.

Question 54

What are six important things to note about experiments with ART and ooplasmic transfer?

Answer 54

1. Normal fertilization
2. 16 babies
3. 1 spontaneous XO miscarriage
4. 30 cycles (27 couples)
5. 3/13 babies exhibited mitochondrial DNA heteroplasmy
6. 1/16 baby with pervasive developmental disorder (PDD).

Question 55

What are four hypothetical reasons why nuclear transfers have been, thus far, inefficient?

Answer 55

1. Differences in centrosome/spindle structure
2. Faulty genomic reprograming
3. Poorly collected,contaminated, or genetically abnormal starting materials
4. Interference in nuclear-mitochondrial communication possibly due to heteroplasmy

Question 56

What are five reasons why there may be a connection between mitochondrial and chromosomal abnormalities?

Answer 56

1. Mitochondrial disorder potentially causes high error rates in female meiosis I which leads to chromosomal abnormalities.
2. Changes on cell cycle checkpoint genes and proteins like cohesins, separins (trigger chromosomal segregation) and securins (which help separins enter nucleus and then stop their activity until cells are ready to divide).
3. Other meiosis specific spindle-associated proteins changed.
4. Telomeres disorder
5. Mitochondrial dysfunction.