2020 lecture 11

What symptoms can mitochondrial diseases cause?

• fatigue

• muscle weakness

• seizures

• developmental delays

Treatments availabel to maage symptoms, not disease.

What is the definition of mitochondiral diseases?

Mutations in mitochondrial DNA (mtDNA) or nuclear DNA (nDNA), mitochondria function depends on both genomes.

Can be inherited or acquired.

What are the classifications of mitochondrial disease?

• Primary mitochondrial disease (PMD)

• Secondary mitochondrial disease / dysfunction (SMD)

What is the definition of Primary mitochondrial diseas (PMD)?

Germline mutations in mtDNA or nDNA inpacting the electron transport chain. Only inherited.

What is the definition of Secondary mitochondrial diseas / dysfunction (SMD)?

Any abnormal mitochondrial function - OTHER than PMD. Are often acquired secondary to environmental stress that can cause oxidative stress.

Can also be inherited.

What is the variation of symptoms in PMD?

Some individuals only experience mild symptoms. Others may experience severe muscle weakness, respiratory and cardiac complications and neuroloical symptoms.

What is the inheritance pattern for PMD?

Can be autosomal dominant, autosomal recessive or mtDNA inheritance.

Is heterogeneous group of disorders.

What are the occurance factors of PMD?

1 in 10,000. Occurs in all ages, but often presents in childhood or early adulthood.

What are some examples of PMD’s?

• Myoclonic epilepsy with ragged red fibres (MERRF)

• Kearns-Sayre syndrom (KSS)

• Mitochondrial encephalopathy, lactic acidosis, and stroke-like episodes (MELAS)

• Mitochondrial DNA depletion syndrome (Alpers syndrome)

• Leigh syndrome

How many sections in a mitochondiral genome?

Protein-coding regions = 13 reigions

tRNA genes = 22 regions

Total genome length = 16,569 base pairs

What sare the two unusual features of mitochondrial inheritance?

• Matrilineal inheritance (inheritence from only the mothers lineage)

• Frequent heteroplasmy (two distinct mtDNA molecules per cell — one healthy wild-type, and the other containing a pathogenic mutation)

Do sperm contribute mitochondria to zygote?

No

Do cells posses completly healthy genomes?

No, they posses a mixed population of normal and mutant genomes.

What are the symptoms of MERRF?

Most common = myoclonus (involuntary muscle twitching).

Epilepsy, ataxia (lack of coordination, muscle weakness and hearing loss.

What is the appearance of MERRF?

The muscle fibers have abnormal microscopic appearance, with “ragged red” fibres.

What mutations cause MERRF?

majority of cases associated with 4 point mutations within mitochondrial genome.

80% = m.A8344G mutation (tRNA-Lysine gene - 70 nucleotides)

10% = m.T8356C, m.G8361A & m.G8363A

10% = unknown

What are the symptoms of MELAS?

Most common = stroke-like episodes accompanied by weakness, numbness and difficulty speaking/ understanding speech.

Also effects eyes, heart and kidneys.

What are the occurance factors of MELAS?

Around 1 in 40,000 - 1 in 100,000.

More common in males than females.

What mutation causes MELAS?

80% = m.3243A>G point mutation in tRNA for amino acid leucine (75 nucleotides)

Induces alterations in structure, stability and codon recognition.

Carriers of MELAS

>75% of carriers manifest maternally inherited diabetes and deafness (MIDD)

Under 20% of m.3243A>G carriers develop MELAS

What is the MELAS m.3243A>G mutation responsible for?

The mutation effects the production of proteins involved in mitochondira fucntion and energy production, leading to mitochondrial dysfunction and cellular energy deficiency.

NADH dehydrogenase (complex I)

Large protein complex in inner mitochondiral membrane. Involved in oxidative phosphorylation - transfers electrons from NADH to coenzyme Q (CoQ), a lipid soluble molecule important in ETC.

NADH dehydrogenase mutations

• Mutations in MT-ND1 & MT-ND5 associated with MELAS

• Mutations in MT-ND1, 2, 3, 4, 5 and 6 (mtDNA) and NDUFA10, 11, 12 (nDNA) associated with Leigh syndrome

Mitochondrial DNA depletion syndrome (MDDS)

A group of rare genetic disorders characterised by significant reduction in mtDNA content in affected tissues.

Effects multiple organ systems including liver, muscle and brain.

Mutations in MDDS?

Disease is cause by mutated genes involved in mtDNA replication and maintenance:

• POLG

• TK2

• SUCLA2

• DGOK

POLG in MDDS

A gene encoding for the catalytic subunit of the mitochondrial DNA polymerase, which is responsible for mtDNA replication and repair.

Mutation causes MDDS.

TK2 in MDDS

A gene encoding for thymidine kinase 2, an enzyme involved in the synthesis of deoxynucleoside triphosphates required for mtDNA replication.

Mutation causes MDDS.

Nucleoside therapy shows sign of improvement in TK2-deficient MDDS.

SUCLA2 in MDDS

A gene encoding for the beta subunit of the succinyl-CoA synthetase, which is involved in the tricarboxylic acid (TCA) cycle and energy production.

Mutation causes MDDS.

DGUOK in MDDS

A gene encoding deoxyguanosine kinase, an enzyme involved in the salvage pathway for deoxynucleotide synthesis.

Mutation causes MDDS.

What is nucleoside therapy?

Experimental treatment. The administration of a cocktail of deoxynucleoside analogs (DNA building blocks).

Aim: to bypass defective mtDNA replication machinery in MDDS patients and provide functional mtDNA.

How to prevent inherited mitochondrial disease?

Three-person in vitro fertilisation (IVF) is technique used to prevent transmission of mitochondrial diseases from mother to child. Involves combining genetic material from mother, father and female embryo donor.

aka mitochondrial replacement therapy (MRT)

Mitochondrial replacement therapy (MRT) / Three-person in vitro fertilisation (IVF) background

John Zhang - created first three person baby in mexico in 2016. Mother had Leigh syndrome.

2017 follow up revealed child still had 1-2% maternal mitochondria (less than the 20% threshold for Leigh syndrome.

Secondary mitochondrial disease (SMD) affiliations

Can contribute to diseases other than inherited metabolic conditions:

• Alzheimers disease

• Parkinsons disease

• Cardiovascular disease

What is the process of Mitophagy?

Removal of damaged mitochondria though autophagy-like process.

Mutations in E3 ubiquitin ligase Parkin are major risk factor of early-onset parkinson’s, mitophagy removes defective mito through ubiquitination in response to PINK1 accumulation.

Motor Neuron Disease (MND)

A progressive neurodegenerative disorder effecting the motor neurons in brain and spinal cord. Leads to weakness, muscle wasting and eventually paralysis.

Most common form = amyotrophic lateral sclerosis (ALS) ~70% cases

Believed to also be caused by environmental stress (oxidative stress)

What is the charecterisation of Motor Neron Disease (MND)?

The degeneration of upper motor neurons (UMNs - in the brain) and lower motor neurons (LMNs - spinal cord and brainstem).

What happens when theres mitochondrial dysfunction in MND?

Mitochondrial dysfuction in UMNs and LMSs leads to decreased energy production and increased ROS production which damages neurons and causes cell death.

What is Superoxide dismutase 1 (SOD1)?

An enzyme that helps protect cells from oxidative stress. Localised in cytoplasm and mitochondrial intermembrane space.

Superoxide dismutase 1 (SOD1) mechanism

SOD1 converts superoxide radicals (which are highly reactive and damage cells) into hydrogen peroxide (is less reactive, can be broken down further).

Superoxide dismutase 1 (SOD1) mutation affiliations

Mutations in SOD1 gene (over 180 found) have been linked to familial amytrophic lateral sclerosis (fALS) ( accounts for 10% of all ALS)

Superoxide dismutase 1 (SOD1) mutation mechanism

Mutation leads to production of abnormal SOD1 proteins prone to misfolding and aggregation (dominant-negative), leading to mitochondrial dysfunction and motor neuron cell death.

Prognosis for MND

Individuals with ALS survive 2-5 years after diagnosis as disease progresses rapidly. Currently no cure, only treatment.

Whats the treatment for MND?

• Riluzole and Edaravone to slow progression of disease

• Physical therapies, occupational therapy and speech therapy to maintain muscle function

• Assistive devices e.g. wheelchairs & breathing machines

What happens when we age?

Mitochondrial function declines, leads to decrease in energy production. mtDNA damage and mutaitons lead to the production of reactive oxygen species (ROS) which damage other cellular components.

What can Mitochondrial dysfucntion lead to?

Apoptosis, contribution to age-related diseases.