Genetics of Mitochondria

Question 1

What are characteristics of mitochondrial diseases?

Answer 1

Monogenic disorders

Phenotypically heterogenous and can affect multiple organ systems, particularly organs that require much energy such as the brain and heart

Can affect adults and children

Can begin at any age

No cure or treatment available

Hallmarks of mitochondrial disease include hydrotropic cardiomyopathy

Question 2

What are most mitochondrial disorders caused by?

Answer 2

Mutations in genes that either directly affect subunits of oxidated phosphorylation complexes, genes required for synthesis of those complexes, genes involved in metabolism or different substrates which ultimately used to produce ATP

Question 3

The Mitochondrial genome encode/required to produce what?

Answer 3

Mitochondria are required to produce 13 proteins of OXPHOS. OXPHOS consists of five enzyme complexes CI-CV, in the inner membrane of mitochondria.

The first four complexes are the respiratory chain (RC) complexes (CI-CIV) and CV is the ATP synthase enzyme.

Three of the RC complexes pump protons across the membrane generating electrochemical potential across the membrane. This potential is then utilized by CV to produce ATP. This is known as chemiosmosis and was proposed by Peter Mitchell in the 1960s for which he won a Nobel Prize.

Note that only 13 proteins required for OXPHOS are encoded by mtDNA. More than 100 are needed!

Question 4

How can mitochondrial disease be inherited?

Answer 4

Primary mtDNA mutations

Nuclear mutations

Autosomal recessive

Autosomal dominant

Question 5

What are heteroplastic mutations?

Answer 5

More than 1 type of mitchondrial DNA haplotype/variant. E.g., there may be some unaffected indviduals with very low levels of the mutation, with those affected harbouring high levels of mutant. Heteroplasmy is important in understanding level of mutation needed to manifest phenotype.

Inheritance of mutation load is random

Question 6

What are homoplasmic mutations?

Answer 6

Single mitochondrial DNA variant. Each human cell contains thousands of copies of mtDNA that are usually identical

Females carrying homoplasmic mutation transmit to all children

Question 7

Most mitochondrial variants are what?

Answer 7

Heteroplasmic

Question 8

What defects are associated with mitochondrial disease?

Answer 8

Monogenic

> 300 mitochondrial disease genes

Question 9

Humans produce and consume approximately…

Answer 9

their own body weight in ATP (~65 kg) each day

Question 10

The heart accounts for 0.5% of body weight but is responsible for what percentage of ATP consumption?

Answer 10

8%

Question 11

How is mitochondria in the adult heart organised?

Answer 11

Peri-nuclear (PN) mitochondria which are freely arranged in an area adjacent to the nucleus; interfibrillar (IF) mitochondria which are arranged along the myofibrils alongside the sarcomere; subsarcolemmal mitochondria (SSM) which are freely arranged in an area located just beneath the subsarcolemma.

Mitochondria account for approximately 30% ofcardiomyocytes- this is the highest content for any tissue

This demonstrates requirement for high amounts of ATP by cardiac cells for contract and relaxation process

Approximately 90% of ATP generated in cardiac cells is used for contraction and relaxation

Question 12

Demand for ATP is increased during exercise.

Why is this important?

Answer 12

In mitochondrial diseases as for several mitochondrial disorders, especially myopathies, exercise intolerance is common

Question 13

If mitochondria are dysfunctional and ATP production is reduced this can lead to what?

Answer 13

Increased oxidative stress and production of reactive oxygen species which are considered to be secondary events occurring within cardiac cells which can contribute to overall pathology that develops in heart when there is an issue with ATP production.

Question 14

What is cardiomyopathy and what can it lead to?

Answer 14

Cardiomyopathy: Disease of heart muscle that create difficulty for heart to pump blood to rest of body and eventually this can lead to heart failure

Question 15

Mutations in how many genes cause mitochondrial cardiomyopathy?

Answer 15

> 100 genes

Question 16

Describe the spectrum by which Mitochondrial cardiomyopathy occurs?

Answer 16

Variable spectrum

Asymptomatic to sudden cardiac death

Question 17

Mitochondrial cardiomyopathy is typically manifested as what?

But what can occur?

Answer 17

Hypertrophic cardiomyopathy but dilated and left ventricular non-compaction occur

Question 18

Mitochondrial cardiomyopathy occurs in approximately…

Answer 18

20-40% of children with mitochondrial disease

• 17/101 of children with mitochondrial disease present with cardiomyopathy (Eur Heart J. 2003;24(3):280-288)
• Onset birth to 10 years
• Diagnosis birth to 27 years
• All non-obstructive hypertrophic type

Question 19

Cardiac anomalies in adults and this is probably under recognized.

In what forms do these occur?

Answer 19

Hypertrophic or dilated

Heart block common in Kearns-Sayre Syndrome

Question 20

Abnormal mitochondria and COX (CIV) deficiency in endomyocardial biopsy is found in what?

Answer 20

Childhood cardiomyopathy

Question 21

Cardiac function routinely monitored in patients with mitochondrial disease with what?

Answer 21

Electrocardiography and echocardiography

Question 22

What patients are excluded for heart transplantation?

Answer 22

Patients with multisystem disease.

Question 23

What is an example of an experimental treatment options for Mitochondrial DNA depletion syndrome (MDDS) syndrome?

Answer 23

Nucleoside therapy

Question 24

Treatments available when specific genetic defect is known

True or false

Answer 24

True

Question 25

Mitochondrial replacement therapy (Three-parent babies) is used for what?

Answer 25

mtDNA mutations

Question 26

Mitochondrial diseases with cardiac manifestation can be classified into which 6 biochemical pathways/functions?

Answer 26

1. Disorders of oxidative phosphorylation (OXPHOS) complex subunits and their assembly factors 2. Defects of mitochondrial DNA, RNA, and protein synthesis 3. Defects in the substrate-generating upstream reactions of OXPHOS 4. Defects in cofactor metabolism 5. Defects in mitochondrial homeostasis 6. Defects in other mitochondrial pathways

Question 27

Disorders of oxidative phosphorylation (OXPHOS) complex subunits and their assembly factors involve what?

Answer 27

5 complexes For all complexes there is one gene linked to cardiomyopathy, although for complex three it is not a main feature ACAD9- a protein thought to be involved in a substrate metabolism but has shown to be an assembly factor for complex 1, for this gene cardiomyopathy is a main presenting feature of the disorder. One of the very first genes identified with WES PPA2- function not as well described. Exquisite sensitivity to alcohol manifested in carriers, for some resulting in sudden cardiac death ACAD9 is one of the most common

Question 28

Defects of mitochondrial DNA, RNA, and protein synthesis involve what?

Answer 28

Genes encoding proteins required for replication of mitochondrial DNA,RNA and protein synthesis All can be classed as gene expression POLG - one of the most common mitochondrial disease genes, usually affecting brain, muscle and liver and cardiomyopathy present.

Question 29

Defects in the substrate-generating upstream reactions of OXPHOS involve what?

Answer 29

Includes beta-oxidation disorders involved in fatty acid metabolism Heart highly dependent on fatty acids for ATP synthesis CPT2- one of the more common causes

Question 30

Defects in cofactor metabolism involve what?

Answer 30

Involves cofactors for enzyme including vitamins, small metals present in respiratory chain enzyme complexes such as iron Fredreichs ataxia caused by mutations in frataxin (FXN), sensory ataxia main feature however some patients present with cardiomyopathy COQ2- one electron carrier in respiratory chain, transferring electrons to complex three and mutations in these genes essentially affect energy transfer in mitochondria

Question 31

Defects in mitochondrial homeostasis involve what?

Answer 31

Barth syndrome- one of the most common cardiomyopathies, x linked, caused by mutation in Taz Cardiolipin- major lipid in membranes of mitochondria. If this is absent, issue in maintaining integrity of mitochondrial membranes

Question 32

What mitochondrial DNA syndromes manifest with cardiomyopathy?

Answer 32

Kearns-Sayre MELAS MERRF NARP/mtDNA-associated LS

Question 33

What are new genetic causes of mitochondrial cardiomyopathies?

Answer 33

1. Genetic diagnosis of mitochondrial cardiomyopathy in kidCMP population- KidCMP are a cohort of children with cardiomyopathy investigated in single centre in Helsinki University Hospital. NGS performed and mutations in genes with mitochondrial functions explain significant amount of total cardiomyopathies (DNAJC19, HADHB, AARS2, MRPL44, PPA2, TAZ, BAG3). 2. Nuclear genetic causes of OXPHOS diseases- Many of the new genetic causes linked mitochondrial disease and cardiomyopathy, found to be those in the class of gene expression, mainly because several genes involved in this pathway, hundreds of proteins needed to replicate mitochondrial DNA. Many new disease genes found in mitochondrial ribosome 3. Recessive mutations in mitoribosomal subunits cause mitochondrial disorders 4. Mitoribosomal subunit disease with cardiomyopathy 5. Two studies discovered MRPL2 and MRPL44, Heterogeneity

Question 34

How do missense mutations in mitochondrial ribosome proteins lead to cardiomyopathy?

Answer 34

1. Protein misfolding / instability of mutated subunit 2. Destabilisation of the mitochondrial ribosome structure 3. Reduced synthesis of 13 mtDNA encoded OXPHOS subunits

Question 35

There are specific treatments and no cures True or false

Answer 35

True

Question 36

Genetic diagnosis leads to what?

Answer 36

Management and genetic counselling More disease genes to be discovered Clinical trials of novel therapies