Mitochondria diseases 06/12/22

Question 1

What is inside the mitochondria?

Answer 1

-Outer membrane
-Inner membrane
-Mitochondrial DNA
-Matrix cristae
-Matrix granule
-Ribosome

Question 2

What is the purpose of the mitochondria?

Answer 2

Power house of the cell.

Question 3

What are the cyto-nuclear interactions?

Answer 3

Organelle genomes interact with the nuclear genome during growth and development, and this cooperation is a result of the long co-evolution of nuclear and organelle genomes.

This scheme shows all possible directions of interactions in communication between the nucleus, chloroplast and mitochondrion.

Molecular signalling from organelles to the nucleus is called retrograde signalling. Signalling from the nucleus to organelles is called anterograde signalling.

Question 4

What are cytoplasmic genetics and extrachromosomal inheritance?

Answer 4

The existence of cytoplasmic DNA shapes a different kind of genetics, called cytoplasmic genetics, with specific rules of extrachromosomal inheritance.

1) Uni-parental inheritance
2) Segregation of cytoplasmic organelles (and so their DNA) during cell divisions is random, not precisely controlled. This complicates the correlation of phenotypes with certain changes in cytoplasmic DNA.
3) Heteroplasmy - multiple copies of DNA within each organelle so there is a possibility to have a mix of normal and mutated forms of cytoplasmic DNA in a single organelle or in a cell.

Mutations in cytoplasmic genes could lead to drastic deviations in development, in particular to human diseases, if we are talking about mitochondrial DNA.

Question 5

What is bi-parental inheritance?

Answer 5

Inheritance of traits from both parents.

Question 6

What is uni-parental inheritance?

Answer 6

Inheritance of trails from only one parent. Can be maternal or paternal.

Question 7

What is the size difference between mitochondrial and nuclear genome?

Answer 7

The nuclear genome (3.3x109 bp) is 200,000 times larger in size than the mitochondrial genome (16,569 bp).

Question 8

What is the summary of the mitochondrial human genome?

Answer 8

A multicopy, circular dsDNA molecule
(16.6 kb in humans). It encodes 13 essential polypeptides of the OXPHOS system and the
necessary RNA machinery (2 rRNAs and 22 tRNAs) for their translation within the organelle. The remaining proteins for
respiratory-chain complexes, and for mtDNA maintenance, are nuclear-encoded.

Question 9

What are the transmission and mutability
of cytoplasmic genes?

Answer 9

Transmission of cytoplasmic genes, either mitochondrial or chloroplastic, is different in a few aspects from the transmission of nuclear genes. This is a reflection of the differences between the properties of nuclear and mitochondrial genomes. The mutability of cytoplasmic genes is higher than nuclear genes.

Question 10

What is an example of maternal inheritance?

Answer 10

Maternal inheritance of variegation in reciprocal crosses. Variegation in plants is known as a phenomenon caused by mutations in chloroplast genes that result in producing some cells without chlorophyll.

If a variegated plant is pollinated with pollen of a green plant (the variegated plant is female and the green plant is male) all progenies from seeds of this cross will be variegated.

In the opposite scenario, when a variegated
male plant pollinates a green female plant, all
progenies from seeds will be green. This is
explained by maternal transmission of
chloroplasts and their genes to progenies.

Question 11

Who is affected by mutated mitochondrial DNA?

Answer 11

Mothers can only pass mitochondrial mutations but not fathers. So if the mother has the mutation all children are affected, if the father has the mutation no children are affected.

Question 12

What is the mitochondrial mutation rate compared to nuclear mutation rates in different species?

Answer 12

All species have higher mutation rates in mitochondrial DNA than nuclear ones.

Arachnida and insecta have low mutation rates.

Birds and Squamata (reptiles) have a high mutation rate.

Primates are the most mutable.

Question 13

Why are mutation rates highest in primates?

Answer 13

The reasons for the high mutability of mtDNA could include:

1) Intensive exposure of mitochondria to ROS. 2) Mitochondria might not have such effective repair systems as in the nucleus.

Question 14

Why is downs-syndrome increased with a women’s age?

Answer 14

The spindle formation during metaphase is an energy-driven process. An adequate ATP supply by mitochondria is essential.

In aged oocytes:
-A higher percentage of disrupted spindle
-A higher percentage of abnormal tubulin placement from the metaphase plate during the second meiotic division
-A higher percentage of chromosome
segregation failure in oocytes from advanced-age women when compared to younger counterparts

Due to an insufficient energy supply by the mitochondria in the oocytes of advanced-age women, the regulatory mechanisms responsible for assembly of the meiotic
the spindle is significantly altered, which leads to a higher prevalence of aneuploidy compared to oocytes of younger women.

Question 15

What are the changes in mitochondrial numbers during female germ line development (mitochondrial bottleneck)?

Answer 15

The number of mitochondria per cell and
mtDNA number per mitochondrion are
not constant. They are dependent on
many aspects of development and
physiology. It is also dependent on
energy requirement at a certain stages of
development – since mitochondria are
organelles for producing energy in cells.

The maximum amounts of mitochondria per cell are in mature and fertilized ovicells. The numbers of mitochondria are massively reduced in the blastocyst and primordial follicle, and further reduced in oogonia, with further reduction in primary germ cells where their number is about 10 mitochondria per cell. This reduction to a minimum is called a mitochondrial bottleneck.

Question 16

What is the random sorting of mitochondrial during cell division?

Answer 16

The mitochondrial bottleneck could be defined as a random reduction in the number of certain mitochondrial types. To a large extent, it depends on the rate of mitochondrial replication at a certain stage of development and on the random sorting out
of mitochondria during cell divisions. There are no specialized mechanisms directing
the equal distribution of mitochondria to daughter cells after cell divisions like it happen, for example, in the case of segregation of chromosomes during mitosis and meiosis. Therefore, if a mitochondrion is mutated then the daughter cell will have mutated mitochondria.

Question 17

Summary of the factors that go towards manifesting mitochondrial diseases?

Answer 17

Mitochondrial DNA mutates from time to time; this is a normal process during
the replication of DNA. Mutations in mtDNA and mitochondrial bottleneck cause heteroplasmy. On the other hand, some
mitochondrial mutations are harmful to the energetics in cells.

Depending on the heteroplasmy levels among the normal (not mutated) DNA, the mutated DNA could reach a certain proportion (threshold) which is enough to cause symptoms of mitochondrial disease. The severity of the disease is dependent on the proportions of normal and mutated copies of DNA. The highest levels of heteroplasmy (with the highest proportions of mutated mtDNA) lead to the most severe pathological phenotypes. The lowest heteroplasmy levels determine mild phenotype or no phenotype at all. Even so, in this case, female individuals with low amounts of mutated mtDNA are still carriers for the disease, which could be expressed in future generations from such females.

Question 18

What is mitocondrial fisson and fussion?

Answer 18

Changes within mitochondria are achieved by two core processes - mitochondrial fusion and fission.

-Fusion is joining two mitochondria to make one.
-Fission is the process of splitting one into two.

During these processes the content of mtDNA
per mitochondrion could be changed. For example, in plants fusions of different mitochondria combined experimentally lead to the recombination of mtDNA. In humans,
mtDNA recombination is a very rare event.

Question 19

How many mutations are associated with mitochondrial disease?

Answer 19

Around 150.

Question 20

What are 5 examples of mitochondrial disease?

Answer 20

Kaerns-Sayre syndrome - ophthalmoplegia, pigmentary retinitis, often deafness, cerebellar ataxia, and heart block. Heteroplasmic, sporadic, deletion.

LHON – leber hereditary optic neuropathy.
Heteroplasmic or homoplasmic, maternal, single nucleotide polymorphisms.

MELAS – mitochondrial encephalopathy, lactic acidosis, and stroke-like episodes. Heteroplasmic, maternal, single nucleotide polymorphisms.

MERRF – myoclonic epilepsy and ragged red fiber disease. Heteroplasmic or homoplasmic, maternal, single nucleotide polymorphisms.

Leigh syndrome – loss of mental and movement abilities (death within 2-3 years due to respiratory failure). Heteroplasmic or homoplasmic, sporadic, single nucleotide polymorphisms.

Question 21

Why are mitochondrial diseases called syndromes?

Answer 21

Very often diseases manifest complex sets of multiple symptoms, so they are called syndromes rather than diseases.

Question 22

How are mitochondrial diseases diagnosed?

Answer 22

For diagnosis of mitochondrial diseases,
people often use histochemical analysis of
enzymatic activities associated with
mitochondria. Cells are analyzed for the activity of cytochrome c oxidase (COX). COX-positive cells are brown and COX-deficient cells are blue.

Question 23

What are the guidelines for the recognition of mitochondrial DNA disease?

Answer 23

*Maternal inheritance
* Recognition of classic syndromes
* Recognition of characteristic clinical features. For example, biventricular cardiac hypertrophy
* Involvement of many organ systems. For example, diabetes and deafness
* Specific combinations of symptoms. For example, strokes and migraine, seizures and ataxia
* Abnormal clinical or laboratory investigations. For example, lactic acidaemia in children, characteristic magnetic-resonance-imaging findings, abnormal muscle biopsy that is associated with RAGGED-RED FIBRES, or cytochrome c oxidase-deficient fibers.

Question 24

How is molecular analysis used in mitochondrial disease diagnosis?

Answer 24

Molecular analyses of mtDNA in patients with different mitochondrial diseases discover more and more genetic changes attributed to
particular diseases.

Question 25

How is molecular analysis used in Leigh syndrome?

Answer 25

The mtDNA from fibroblasts of the Leigh
Syndrome proband and his mother
was analyzed by long-range-PCR-based next-generation sequencing. They detected a new variant of the locus m.8993T>G in the proband. They were able to amplify
16.6 kb within this region. The sequencing results showed that proband’s DNA contains 83% of T>G conversion at the m.8993 positions, while the mother’s DNA has T nucleotide there. This is a new SNIP for the Leigh Syndrome.

Question 26

How servere are the results from a single-point mutation on mitochondrial function?

Answer 26

The mutation m.8993T>G leads to altered morphology of mitochondria and abundance of cristae, as it was confirmed by EM of mitochondria from dermal fibroblasts of the proband and his mother. The authors state that fibroblasts of the proband manifested impaired oxidative phosphorylation with decreased respiratory capacity in response to
energy stress. There was also diminished glycolysis and full inhibition of OXPHOS activities. This case shows how severe
metabolic changes could result from a
single nucleotide change in mtDNA.

Question 27

What is MERRF and its symptoms?

Answer 27

MERRF syndrome is an abbreviation for
Myoclonus Epilepsy with Ragged-Red
Fibers. It is a very rare mitochondrial
disease, estimated at 1 in 5,000 people
worldwide. The disease begins in childhood and affects nerves and skeletal muscles

Myoclonus is a sudden, uncontrolled muscle spasm (that’s why it is called epilepsy). Abnormal muscle cells stained with the Gomori trichrome are present in muscles and appear as ragged red fibers under a microscope. They appear due to the accumulation of abnormal mitochondria near the plasma membrane of the muscle fiber.
The other common symptoms could include also short stature, atrophy of the optic nerve,
hearing loss, cardiomyopathy, and abnormal sensation from nerve damage (called peripheral neuropathy). Also impaired coordination of movements (called ataxia), seizures, and development of dementia.

The other name for the disease is Fukuhara
syndrome.

Question 28

What genes are associated with MERRF?

Answer 28

Nucleotide substitutions are found for different regions located far from each other in the mt-genome, but the majority of them are situated in genes for tRNAs. One is in the ND5 gene (which encodes for NADH
dehydrogenase 5). One case is associated with multiple deletions from mtDNA. One mutation associated with MERRF is the POLG gene encoding the catalytic subunit of the
mitochondrial DNA DNA polymerase gamma.

Question 29

What is the common mitochondrial mutation?

Answer 29

The 4977-bp human mtDNA deletion (between nucleotides 8470 and 13447 of the human mitochondrial genome) is also known as a common deletion that eliminates all 5 tRNA genes (L: tRNALeu, S: tRNASer, H: tRNAHis, R:tRNAArg, G: tRNAGly) and 7 genes encoding 4 Complex I subunits 1, complex IV subunit, 2 Complex V subunits. (Yusoff et al., 2019).

Question 30

What is the problem with the treatment of mitochondrial diseases?

Answer 30

The treatment of mitochondrial diseases is a
problematic issue because their mechanisms are quite complex, and they are systemic affecting many systems of the body. This is because mitochondrial mutations reduce the production of energy in the cell, but energy is required for all metabolic and regulatory processes of life. The other problem is maternal inheritance making it impossible to eradicate a disease from the line of a female who acquired this disease once. All progenies of that female will have the disease via mitochondrial lineage. One theoretically justified approach is the replacement of
diseased mitochondria with healthy ones.

Question 31

What is mitochondrial replacement therapy?

Answer 31

It has been legal in Britain since 2015.

1) The nucleus is removed (by
micromanipulation) from a fertilized
mother’s egg containing faulty/diseased
mitochondria.
2) This parent’s nucleus (containing chromosomal DNA from both parents) is kept
under appropriate conditions.
3) The second stage is the removal of the nucleus from a donor’s fertilized egg containing healthy mitochondria. This nucleus should be destroyed.
4) The nucleus-free cytoplasm of the donor egg is used for injection of the parent’s
nucleus isolated at stage 1.
5) Now the reconstructed cell contains the parents’ nucleus and the donor’s cytoplasm with normal (healthy) mitochondria.

Overall, this reconstructed egg combines DNA
from 3 people: nuclear genetic material
from two parents and mtDNA from a
donor. Now such an egg could be implanted into the mother or a surrogate mother to be developed into a healthy child. (Farnezi, 2020).

Question 32

What are the two types of MRT?

Answer 32

-Maternal spindle transfer
-Pronuclear transfer

Question 33

What is the influence of mtDNA heteroplasmy on clinical phenotype?

Answer 33

The authors showed that increasing levels of the mutated mtDNA cause profound changes in the nuclear epigenome. This is confirmed by measuring levels of modification of histones H3 and H4.

Normal mtDNA is associated with the increase of acetyl CoA and histone H4 acetylation.

Pathogenic mtDNA leads to the reduction of acetyl CoA and histone H4 acetylation. At 50% heteroplasmy, there is an increase of alpha-ketoglutarate/succinate levels and the
decrease of methylation of histone H3.

This leads to the different patterns of transcriptional alterations in many nuclear and mt genes and this is due to altered
interactions between nuclear and
mitochondrial genomes. Overall, this
research brings a new concept into
mitochondrial genetics based on the
involvement of epigenetic regulation
linked to nucleo-mitochondrial interactions.

Question 34

How does mitochondrial disease affect gut health?

Answer 34

It is well established now that the
quality of gut microbiota is an important factor in human health. Shifting the gut
microflora could cause a disease.

On the other hand, mitochondrial
mutations also lead to pathologies.
The team of this paper published in a
the reputable journal shows that variations
in mtDNA of mice lead to alterations in
composition of mice gut microflora.
The altered gut microbiota correlated
with the ROS production by the host.
The authors produced an interesting
hypothesis, that the connection
between the gut microflora and
common diseases might be the result of
changes in mitochondrial function. (Yardeni, 2019).

Question 35

How is the gut health linked to other diseases?

Answer 35

Diabetes, autism, and Parkinson’s have associations with changes in the gut microbiome and certain mitochondrial genotypes. Both mtDNA and microbiome communities are inherited from the mother. Therefore, the link between mtDNA and microbiome is possible. this was shown in the study as the mitochondrial genotype of mice correlates with the gut microbiota
diversity. A key factor in mitochondrial regulation of the gut microbiome
is mitochondrial redox status and linked to ROS generation.

Question 36

What DNA is in the mitochondria?

Answer 36

Double stranded circular DNA molecule.