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Mitochondrial Disorders: An Update
I Smuts
Department of Paediatrics and Child Health
University of Pretoria
Steve Biko Academic Hospital
Mitochondrial structure
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1: Outer membrane i. Porins 5000 daltons molecules diffuse ii. Translocase of the OM larger molecules iii. Mitochondria-associated ER membrane
2: Intermembrane space i. High concentration ions and sugar ii. Cytochrome c
3: Inner membrane i. OXPHOS ii. ATP production iii. Transport proteins regulating metabolite passage (TIM) iv. Protein import v. Fusion and fission proteins
4: Cristae i. Surface area ii. Chemiosmotic
5: Matrix i. ATP production ii. Oxidation of pyruvate and fatty acids iii. Krebs cycle iv. RNA and protein production v. mtDNA
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1 i. Porins 5000 daltons molecules diffuse ii. Translocase of the OM larger molecules iii. Mitochondria-associated ER membrane (MAM)
Importance of MAM • Signalling and trafficking hub • Ca2+ signalling • Critical in intrinsic apoptosis • Lipid biosynthesis
Fission and fusion Interorganelle traficking for
Phosholipid biosynthesis Ceramide Cholestrol Glycosphingolipids
Functions in summary
• Energy conversion
• Heat production
• Storage of calcium
• Signalling through ROS
• Apoptosis
• Regulation of cell metabolism
• Steroid synthesis
• Hormonal signalling
Dual genetic control
1 500 genes involved in energy metabolism
16.5kb circular double stranded molecule 37 genes encoding:
13 structural proteins 2 rRNA 22 tRNA
Definition of mitochondrial disease
Two definitions
Lombѐs et al. Biochimie 100:1171-176, 2014.
1. Alteration to the OXPHOS pathway
2. All the diseases due to an alteration of a mitochondrial component
Primary mitochondrial disorder
Diseases associated with defects in OXPHOS function attributable to genetic differences in mtDNA or nDNA
UMDF/NIH workgroup. Mitochondrion 13:945-952, 2013.
Secondary mitochondrial disorder
Degenerative conditions with significant mitochondrial dysfunction
– Autism
– Alzheimer’s disease
– Parkinsonism
– ALS
– Muscle disorders
– Etc…..
How do mitochondrial disorders present?
• Any age
• Any organ or tissue
• Enormous variation Muscle Exercise intolerance Ocular myopathy Axial/limb weakness Myolysis
Brain Cognitive impairment Epilepsy Movement disorders Cerebellar syndrome Pseudo-strokes Leukodsytrophy
Peripheral nerves Sensory neuropathy Demyelinating neuropathy Axonal motor neuropathy
Heart Cardiomyopathy Conduction defects
Sensory organs Pigment retinopathy Optic atrophy Deafness
Organs Liver (Failure, cholestasis) Kidney (Proximal tubulopathy, glomerulopathy) Endocrine (Diabetes, hypogonadism, growth hormone defect…)
Bone marrow Sideroblastic anaemia Pancytopaenia
Lombѐs et al. Biochimie 100:1171-176, 2014.
Phenotypic diversity
• Heteroplasmy
• Restricted tissue expression of an OXPHOS defect
• Modifying factors
– Nuclear genes encoding mtDNA replication factors
– mtDNA variation/Haplogroups • Haplogroup L0: G263A C1048T C3516a T5442C T6185C
C9042T A9347G G10589A G12007A A12720G
• L0a1a1: T2759C
Lombѐs et al. Biochimie 100:1171-176, 2014.
mtDNA variation
• mtDNA accumulated variants over time
• Ancient variants extant over time in populations
• Haplogroups are defined
– Geographically
– Ethnically
http:www.mitomap.org
Human mtDNA migrations
http:www.mitomap.org
Simplified mt Lineages
Example
A10398G
• MTND3 (Complex I)
• Homoplastic on haplogroup J and K
Effects
• Reduces CI activity
• Reduces cytosolic calcium levels
• Reduces mitochondrial membrane potential
• Reduces level of ROS
Result • Reduces the risk of Parkinson’s disease
Genetic basis of mitochondrial diseases
mt-DNA defects (37 genes) – Point mutations (maternally inherited)
• tRNA
• Subunits
• rRNA
– Rearrangements • Deletions
• Duplications
DiMauro et al., 2013
Reported mutations on Mitomap 2015 864
• Apoptosis 50 • Metabolism 400 • Proteases 25 • aa-tRNA synthesis 23 • Chaperone 25 • Dynamics 12 • Nucleases 5 • Transporters 75 • Cytochromes 15 • Replication 1 • RNA modification 5 • Protein modification 15 • Cytoskeletal 10 • Transcription 30 • Translation 15
• Protein import: TIMMs and TOMMs 30 • DNA repair 12 • Regulation 10 • MRPLs 55 • RC complex and assembly 150 • Fe–S cluster 5 • Porins 4 • MRPSsc 36 • RNA modification/processing 15 • Immune 3 • ROS 15 • Others 150 • Unknown 150
Wong. Mitochondrion 13:379-387, 2013.
Genetic basis of mitochondrial disorders nDNA~1500 nuclear genes involved in mitochondrial function
Genetic basis of mitochondrial disease
117 nuclear genes
Nuclear encoded subunits
27/80 genes
Import, processing assembly
38 genes
mtDNA replication 5
genes
Nucleotide transport synthesis
9 genes
Nucleotide transport, synthesis
9 genes
mtDNA expression 24 genes
95: autosomal recessive 10:autosomal dominant
5: recessive or dominant 7: X-linked
Ohtake et al. Biochimica et Biophysica Acta 1840:1355-13596, 2014.
? Parents insist on answers
Money
Availability of facilities
Clinicians
Just write a script!!!
Clinicians
Explain
Epilepsy
Autism
Neuromuscular disease
Patients
Mitochondrial dysfunction in autism
• Biochemical marker suggestive of abnormal mt function • ETC deficiencies
– 14 different studies (2000-2012) • 131 patients • 100 ETC deficiencies
• mtDNA mutations – 8 different studies (1999-2013)
• 139 patients • 49 mtDNA mutations (355)
• nDNA mutations – 6 different studies – Gene expression in post mortem brains
Legido et al. Semin Pediatr Neurol 20:163-175, 2013.
Mitochondrial dysfunction in autism
• Prevalence of autism: 2:100
• Prevalence of MD: 1:2000
• Then expected:
– 1:110 pt with MD will have autism
– 1:2000 pt with ASD would have MD
Legido et al. Semin Pediatr Neurol 20:163-175, 2013.
Mitochondrial dysfunction in autism
Legido et al. Semin Pediatr Neurol 20:163-175, 2013.
Conclusion
• Genetic and biochemical evidence for mt role in ASD of a subset of children
• Mt dysfunction may still be secondary
• Further studies required
Mitochondrial dysfunction in neuromuscular disorders
Muscular dystrophies
Congenital muscular
dystrophies (CMD)
Duchene muscular
dystrophy (DMD)
Limb girdle muscular dystrophy
(LGMD)
Myotonic dystrophy (MD)
Congenital myopathies
Centronuclear myopathies
Core myopathies
Inflammatory myopathies
Inclusion body myositis (IBM)
Polymyositis
Spinal muscular atrophy –Motor neuron disease
Spinal muscular atrophy (SMA)
Amyotrophic lateral sclerosis
(ALS)
Peripheral neuropathies
Hereditary peripheral
neuropathies
Friedreich’s ataxia
Diabetic neuropathy
Drug induced neuropathies
Muscular dystrophies
Congenital muscular
dystrophies (CMD)
Duchene muscular
dystrophy (DMD)
Limb girdle muscular dystrophy
(LGMD)
Myotonic dystrophy (MD) • Disrupted Ca2+ homeostasis + impaired response to
oxidative stress underlie muscle cell death • Permeability transition pore (PTP) dysregulation • Defective autophagy
• CsA • Cyclophilin D
inhibitors
PTP
Congenital myopathies
Centronuclear myopathies
Core myopathies
Ryanodine receptor 1 (RYR1) is a skeletal muscle calcium release channel
Inflammatory myopathies
Inclusion body myositis
(IBM)
Polymyositis
• Ragged red fibres in IBM
• COX-deficient fibres
• Multiple mtDNA deletions
• Mt abnormalities
• Mt changes triggered by immune-mediated mechanisms of cellular injury
Spinal muscular atrophy –Motor neuron disease
Spinal muscular
atrophy (SMA)
Amyotrophic lateral
sclerosis (ALS)
SMA
– CI-IV deficiencies
– mtDNA in muscle decreased
– Decreased SDH activity
– Diminution of COX subunits
?
Peripheral neuropathies
Hereditary peripheral
neuropathies
Friedreich’s ataxia
Diabetic neuropathy
Drug induced neuropathies
• Abnormal fission and fusion and mt movement
– e.g CMT2A
• Mt dysfunction in Schwann cells contribute to the pathogenesis of demyelination and axonal degeneration
• Hyperglycaemia induce abn neuronal Ca2+ handling and mt dysfunction. ATP exhausted and then axonal degeneration
• NRTI anti-retrovirals different mechanisms
Mitochondrial dysfunction in epilepsy
MD vs Epilepsy
Seizures vs
Mt function
AED vs Mt function
Relationship of mitochondria and epilepsy
Energy depletion
Homeostasis affected
Tissue excitability
Seizures
ROS
Mt dysfunction
Khurana et al. Semin Pediatr Neurol 20:163-175176-187, 2013.
Anti-epileptic drugs and mitochondrial function
AED (Examples) Mt impairment
Valproic acid CI+IV activity Inhibits O2 consumption ATP synthesis Inhibits ß-oxidation Impairs structural organization of the inner membrane
Carbamazepine Inhibits ATPase Impairs respiration Impairs calcium uptake or release
Lamotrigine CI activity CII, CII+III and CIV activities
Vigabatrin Induces apoptosis in the developing brain
Topiramate Zonisamide
Inhibits isoenzyme human carbonic anhydrase V
Khurana et al. Semin Pediatr Neurol 20:163-175176-187, 2013.
Anti-epileptic drugs and mitochondrial function
AED (Examples) Beneficial effects
Valproic acid Rotenone-induced toxicity Inhibits histone deacetylases
Carbamazepine Protects against CI inhibitor rotenone
Lamotrigine Neuro-protective Increases ATP production
Topiramate Inhibits PTP Increase CI activity Neuro-protective
Levetiracetam Anti-oxidant effect Neuro-protective
Khurana et al. Semin Pediatr Neurol 20:163-175176-187, 2013.
Other potential treatment modalities
• Antioxidant supplementations
– Vitamin E
– Melatonin
– Resveratrol
– Lipoic acid
– N-acetyl-cystein
– CoQ10
– Vitamin C
? Evidence
Khurana et al. Semin Pediatr Neurol 20:163-175176-187, 2013.
Conclusion