Week 5 - Mitochondrial Disease

Question 1

What is the function of the nucleus?

Answer 1

The nucleus contains chromosomes and the genetic material consisting of DNA and proteins organised into chromatin structures. Human DNA encodes for approximately 25,000 genes which are expressed into proteins and located within the cytoplasm. Mitochondria and chloroplasts have their own DNA (mtDNA and cpDNA)

Question 2

What are cyto-nuclear interactions?

Answer 2

Organelle genomes and interact with nuclear genome during growth and development, which are modulated by environmental factors.

Question 3

Mendelian Genetics

Answer 3

1. Cytoplasmic genes are typically transmitted to progenies only from one parent (usually female) which is called uniparental inheritance as opposed to biparental inheritance,
2. Segregation of cytoplasmic DNA during cell division is random.
3. Due to multiple copies of DNA within each organelle, there is a possibility to have a mix of both mutated and normal forms of DNA which is known as heteroplasmy as opposed to homoplasmy which implies only one type of DNA per organelle.

Question 4

Biparental and uniparental inheritance

Answer 4

• Biparental inheritance is the inheritance of traits from both parents
• Uniparental inheritance is the inheritance of traits from one parent
• In most eukaryotes, cpDNA and mtDNA is inherited maternally

Question 5

Variation of mitochondrial DNA and nuclear DNA mutations

Answer 5

• Generally, mitochondrial mutations are higher than nuclear mutations
• The reasons for this could be the extensive exposure of mitochondria to reactive oxygen species (ROS). Secondly, mitochondria may not have such effective repair mechanisms as the nucleus.

Question 6

Spindle formation in metaphase II oocytes of young and advanced age women

Answer 6

The spindle formation during metaphase is an energy drive process so ATP is essential. There is a higher percentage of disrupted spindle function and higher percentage of chromosome segregation failure in oocytes from advanced age women in comparison to younger women due to an insufficient energy supply. The regulatory mechanism responsible for the assembly of the mitotic spindle are significantly altered which leads to a higher prevalence of aneuploidy when compared to younger women.

Question 7

The mitochondrial bottleneck

Answer 7

• Defined as the random reduction in numbers of certain mitochondrial types.
• To a large extent, it depends on the rate of mitochondrial replication at certain stages of development and random sorting out of mitochondria during cell division. There are no specialised mechanisms that direct equal distribution of mitochondria to daughter cells.
• High proportion of mutated mtDNA could be enough for determining a strong expression of pathological phenotype if the mutation causes a disease. Low levels of the mutation could be below the threshold of mutated mitochondria sufficient for expression of disease, so symptoms would not be expressed in a future individual derived from this oocyte. A moderate load could be associated with a mild pathology.
• The prediction is uncertain as asymptomatic females with low loads can potentially give rise to highly pathological offspring in the next generation due to future bottlenecks.

Question 8

Mitochondrial fusion and fission

Answer 8

1. Fusion - joining two mitochondria to make one
2. Fission - splitting one mitochondria into two

Question 9

Clinical disorders

Answer 9

Mitochondrial diseases often affect functions of different muscles and nerves so they are called myopathies and neuropathies respectively. ‘Pathy’ means weakness of muscle and nerves. Some diseases cause weakness, deafness, diabetes, fatigue, loss of vision, epilepsy. Some diseases are caused by large deletions in mtDNA, some by SNPs. Most diseases are expressed in heteroplasmic state, while a few of them manifest homoplasmy. The inheritance of the majority of mitochondrial diseases is strictly maternal with some cases of sporadic inheritance.

Question 10

Diagnostic approaches

Answer 10

• Often use histochemical analysis of enzymatic activities associated with mitochondria. Cells can be analysed for the activity of cytochrome C oxidase (COX). COX positive cells are brown and COX negative cells are blue.
• Analysis of Leigh Syndrome can be used by magnetic resonance imaging. mtDNA form fibroblasts of the Leigh Syndrome proband and his mother was analysed by long range PCR based next generation sequencing. A new variant was detected which led o the altered morphology of the mitochondria and the abundance of cristae. Scientists state that fibroblasts of the proband manifested impaired oxidative phosphorylation with decreased respiratory capacity in response to energy stress. There were also diminished glycolysis and full inhibition of OXPHOS activities.

Question 11

Treatment of mitochondrial diseases

Answer 11

• Treatment is problematic, because their mechanisms are complex as mt-mutations reduced the production of energy within the cell but energy is required for all metabolic and regulatory processes of life. The other problem is maternal inheritance makes it impossible to eradicate a disease from the line of a female who acquired the disease.

Question 12

Mitochondrial replacement therapy (MRT)

Answer 12

1. The nucleus is removed by micromanipulation from a fertilised mother’s egg containing healthy mitochondria. This parent’s nucleus (containing chromosomal DNA) is kept under appropriate conditions.
2. The second stage is the removal of the nucleus from a donor’s egg containing healthy mitochondria. The nucleus is destroyed
3. At stage 3, the nucleus-free cytoplasm of a donor egg is used for infection of the parent’s nucleus isolated at stage 1.