Mitochondrial Genetics

Question 1

What are mitochondria?

Answer 1

• organelles found in nucleated cells (not RBCs)
• regulate energy production, calcium homeostasis, apoptosis, radical species generation and scavenging, steroid biosynthesis, orchestrating metabolism
• move and distribute within cells via cellular cytoskeleton
  undergo fission and fusion

Question 2

Mitochondrial DNA

Answer 2

• ~16,500bp
• double-stranded circle
• 37 genes and no introns
• genes encode for protein structures involved in oxidative phosphorylation, rRNAs, and tRNAs
• no homologous recombination or meiosis
• replication continuous and not synchronized with cell cycle
• more prone to errors and mutations in replication than nuclear DNA
• multiple copies per cell depending on energy needs
• maternally inherited; mtDNA haplogroups can influence clinical spectrum of disease

Question 3

Heteroplasmy vs Homoplasmy

Answer 3

• cells can have different proportions of mutated and normal/wild-type mtDNA
• heteroplasmy means not all mtDNA in the cell is the same
• homoplasmy means all of the mtDNA is the same (whether 100% normal or 100% mutated)

Question 4

Threshold Effect

Answer 4

• for an individual to express symptoms of disease, the amount of mutated mtDNA must exceed a certain threshold
• tissues like brain and muscle have lower thresholds because of higher energy requirements
• amount of mutated mtDNA present can influence disease severity
• a lot of mutated mtDNA may mean severe disease, moderate level of mtDNA means more mild manifestation

Question 5

Bottleneck Effect

Answer 5

• if maternal oocyte has mutated mtDNA, some may end up in egg cell that goes on to be fertilized and form embryo
• mtDNA that ends up in embryo is a random sample of all maternal mtDNA
• as embryo divides and more cells generated, random sampling of mtDNA also happens and some tissues may end up with higher concentrations of mutated mtDNA than others

Question 6

Mitochondrial Disease Recurrence Risks

Answer 6

• mtDNA point mutations: 1-4% for full sibs when no symptoms in mom; up to 50% for sons/daughters if mom affected, no risk from affected male to children
• mtDNA del/dups: up to 50% for full sibs
  and children whether or not mom affected, no risk from affected male to children

Question 7

Mitochondrial Disease Red Flags

Answer 7

• males affected and linked through related females in pedigree
• neurological: encephalomyopathy, cognitive decline in young person, basal ganglia lesions, SNHL, stroke-like event w/o vascular pattern, exercise intolerance, epilepsia partialis continua (EPC), myoclonus, ataxia
• ophthalmologic: ptsosis, PEO, pigmentary retinopathy, night blindness, optic atrophy, color vision loss
• cardiac: cardiomyopathy, cardiac conduction block, WPW
• blood: Pearson’s syndrome
• GI: hepatic steatosis, dysphagia, dysmotility, pseudo-obstruction
• endocrine: diabetes
• renal: Fanconi syndrome
• other: short/thin habitus, hypotonia, FTT, lactic acidosis, anesthesia sensitivity, valproate sensitivity

Question 8

Testing for Mitochondrial Diseases

Answer 8

• biochemical: lactate levels (high), plasma/CSF amino acids (high alanine), urine organic acids (TCA intermediates), plasma acylcarnitine (low carnitine, elevated acyl)
• imaging studies: brain MRI (Leigh syndrome), brain MRS (high lactate, low NAA)
• tissue biopsy: skeletal muscle (ragged red fibers, COX-deficient fibers, high number of mitochondria, ETC activity, CoQ10, mtDNA analysis); liver (ETC acitivity, mtDNA analysis); skin (ETC activity, mtDNA analysis, fibroblast cell line)
• molecular testing: mtDNA sequencing (NGS detects heteroplasmy >1.2%; Sanger detects heteroplasmy >50%), nuclear gene sequencing (symptoms can mimic other conditions), exome (diagnosis in 50% of patients)

Question 9

Treatment for Mitochondrial Diseases

Answer 9

• no FDA approved therapies
• supplemental cocktails (CoQ10, cofactors, arginine, enzyme activators, antioxidants)
• avoid drugs with mitochondrial toxicity
• exercise increases non-mutated mtDNA
• clinical trials

Question 10

Management for Mitochondrial Diseases

Answer 10

• treat symptoms
• minimize intercurrent illnesses (get vaccines)
• avoid fasting
• be careful with anesthesia
• arginine and citrulline therapy for neurologic change
• mitochondrial replacement therapy (spindle or pronuclear transfer; not approved in US)

Question 11

Of the 5 protein complexes involved in oxidative phosphorylation, which one only contains proteins made from nDNA?

Answer 11

Complex II

Question 12

Mitochondrial diseases with immediate onset after birth are most likely to be caused by…

Answer 12

AR nDNA mutations

Question 13

Mitochondrial disease with later onset are most likely caused by…

Answer 13

mtDNA mutations