Mitochondrial Diseases

Question 1

Mitochondrial Genome

Answer 1

• 5–10 copies of mtDNA per mitochondrion
• circular, double-stranded DNA encodes for 37 genes
• Limited DNA repair
• Maternally inherited

Question 2

Define Replicative segregation in Mitochondrial disorders

Answer 2

Each cell mitochondria replicate and distribute randomly between two daughter cells = causes significant variability in manifestations of mitochondrial disorders among different tissues and/or patients

Question 3

Differentiate Homoplasmy vs heteroplasmy in Mitochondrial genome

Answer 3

NOTE: Random distribution of mutant and normal mtDNA into daughter cells = Different proportion of wild-type and mutant
mitochondrial genomes

Homoplasmy: Purely normal OR mutated mtDNA
Heteroplasmy: Mixture of mutant AND wild-type mtDNA

Question 4

Describe Mitochondrial genome bottleneck

Answer 4

• number of mtDNA within developing oocytes is reduced before being amplified
• mother with high proportion of mutant mtDNA are more likely to produce eggs with a higher proportion of mutant mtDNA = more likely to have affected offspring

Question 5

Describe Clonal expansions and changes of Heteroplasmy with Age

Answer 5

Mutated mtDNA is favoured during replication, and becomes more abundant than normal mtDNA = clonal expansion and heteroplasmy

Question 6

Mitochondrial Proteome

Answer 6

~1,130 proteins
- Majority encoded by nuclear DNA and imported into mitochondria after cytosolic translation
- 13 polypeptides encoded by mtDNA

Question 7

T or F: mitochondrial diseases are very common

Answer 7

TRUE; 1/5000 individuals with Mitochondrial disease is VERY COMMON

Question 8

Which organs/ system can mitochondrial disease affect ?

Answer 8

ANY organ & system

Question 9

Which inheritance pattern is most common for mitochondrial diseases ?

Answer 9

Autosomal RECESSIVE

NOTE: Mitochondrial disease can have the same phenotype = different genotypes; same genotype = different phenotypes

Question 10

Phenotypes in Mitochondrial Disease

Answer 10

• (MELAS) = Mitochondrial encephelalomyopathy, Lactic Acidosis, and Stroke-like episodes
• Leigh syndrome (bilateral brain-stem changes)
• cataracts/ pigmentary retinopathy (white dots)

Question 11

Characteristics of Mitochondrial inheritance

Answer 11

• mothers will pass off mutation to all offspring
• fathers cannot pass mutation to offspring
• heteroplasmic deletions are not inherited

Question 12

Molecular Genetic Investigation for Mitochondrial disease (4)

Answer 12

1. Targeted Next Generation Sequencing: Multigene panels for specific phenotype OR neuroimaging features
2. Exome sequencing (1.5% of entire genome = exons): can only account for 30% of Mitochondrial disease
3. Genome sequencing - accounts for 50%
4. Mitochondrial genome sequencing
   - muscle biopsy is ideal but invasive
   - sample type is important: mutated mtDNA in blood is undetectable

Question 13

Other tests to investigate Mitochondrial disease (4)

Answer 13

1. Lactate: Pyruvate ratio>20 is abnormal
2. Cardiac Assessments
3. CT or brain MRI
4. Tissue biopsy ie. muscle
5. Neurophysiological studies (EEG, EMG)

Question 14

Positive tissue biopsy stain

Answer 14

Gomori Trichrome = red

Question 15

Sample type to measure oxidative phosphorylation enzyme activity in Mitochondria

Answer 15

Frozen sample

Question 16

Why is Prenatal investigation for Mitochondrial disease challenging ?

Answer 16

• due to mtDNA heteroplasmy, %mutated mtDNA in chorionic villus biopsy* may not reflect mutated mtDNA in other tissues
• %mutated mtDNA changes during development and life

*NOTE: tissue from placenta

Question 17

Which mitochondrial pathogenic variants are more appropriate for prenatal testing ?

Answer 17

m.8993T>G and m.8993T>C mtDNA pathogenic variants:
- More even tissue distribution between chorionic villus biopsy and other fetal tissues
- %mutated mtDNA level of these two variants does not appear to change significantly over time