Mitochondrial genetics

Question 1

Mitochondrial function

Answer 1

1. ATP production via OXPHOS
2. Apoptosis-programmed cell death
3. Calcium signalling
4. Other:
   - Kreb’s cycle
   - beta oxidation
   - heat production
   - calcium storage
   - cell signalling (via ROS)
   - regulation of membrane potential.
   - regulation of cellular metabolism
   - haem/ steroid synthesis
   - urea cycle (hepatocytes)

Question 2

Mitochondria: hub for energy metabolism and signalling

Answer 2

Amyloid beta
- quality control/ mitophagy
- protein import
- [Ca++m] and other cations
- heat
- ATP

Question 3

mtDNA- related mitochondrial diseases

Answer 3

> 250 pathogenic mutations

Examples:
- L1 : PEO, LHON, MELAS, myopathy, cardiomyopathy, diabetes and deafness

• 12s : Parkinsonism, aminoglycoside-
  induced deafness

Question 4

Inheritance of mtDNA

Answer 4

1. Maternal inheritance of mtDNA
2. Mitochondrial DNA heteroplasmy
3. The threshold effect
4. Mitochondrial genetic bottleneck
5. Mitotic segregation distinguished
   mitochondrial from Mendelian genetics
   (Replicative segregation)

• age, age-adjusted blood heteroplasmy
  levels and sex are poor predictors of
  phenotypic severity.

Question 6

Mitochondrial genetic heterogeneity

Answer 6

Example:
Chronic progressive external ophthalmoplegia (CPEO)

• gradually progressive reduction in eye
  movements with ptosis
• genetically heterogenous: single/ multiple
  mtDNA rearrangements; mtDNA point
  mutations
• usually sporadic, as with single-large scale
  mtDNA deletions
• inheritance maternal or Mendelian

CPEO
1. Single mitochondrial DNA deletion - 2%
2. Point mutation mtDNA - 44%
3. Nuclear DNA defects - 52%

1. Colonal deletions of mtDNA detected in muscle, sporadic, size/location selection varies, purely myopathic (add features; dysphasia, proximal limb weakness, exercise intolerance)
2. Mitochondrial encephalopathy, lactic acid and stroke like episodes (MELAS), >30mtDNA mutations reported, maternally transmitted, carrier rate 1:400 (progressive decline with acute neurological deterioration; stroke 2-40yrs old, encephalopathy with seizures +/- dementia, age of death 10-35yrs (up 6th decade)
   Others:
   - materially inherited diabetes and deafness (MIDD)
   - CPEO plus
   - Leigh syndrome (subacute necrotising encephalomyelopathy)
   (Add features; isolated myopathy, cardiomyopathy, seizures, migraine, ataxia, cognitive impairment, bowel dismotility, short stature)
3. CPEO plus
   - POLG most common followed by C10orf2, RRM22B and RNASHEH1

Question 7

Mitochondrial clinical heterogeneity

Answer 7

Multi- system involvement

Single system involvement

Clinical variability

Question 8

Diagnosing mitochondrial disease

Answer 8

Any age
Any course
Any organ
Any mode of interference

Question 9

Age of disease

Answer 9

Rare genetic disorders that affect any organ at any age:
- paediatric disorders (early onset, often
very severe —> 25% mtDNA)
- adult disorders (generally minor, more benign outcomes —> 75% mtDNA)

Neonate
(Congenital, lactic, acidosis)

Infant
(LS, HCM, Alpers)

Child/ Adolescent
(KSS, MELAS)

Adolescent/ Adult
(CPEO, MERRF, MELAS, NARP, exercise intolerance)

Elderly
(Myopathy)

Question 10

Investigating mitochondrial diseases

Answer 10

• mutations in mtDNA or nDNA
• heteroplasmy
• adult prevalence 1 in 4300
• wide variety of manifestations

Investigating complex:
- mtDNA in multiple tissues
- nDNA separately

Adults with suggestive but non specific clinical phenotype:

DNA from blood
- screen for common mtDNA point
mutations
- next generation sequencing of mtDNA
- nuclear gene tests or gene panels
- whole exome sequencing

Muscle biopsy
- histopathology
- respiratory chain enzyme analysis
- rerun mtDNA sequencing from muscle
- rearrangement analysis (e.g LR-PCR followed by southern blotting)