Mitochondrial Disorder
-
Upload
mahesh-kakanale -
Category
Education
-
view
659 -
download
2
description
Transcript of Mitochondrial Disorder
MITOCHONDRIAL DISEASES
Introduction
• Mitochondria are crucial to the flow of energy in
cells.
• Mitochondria presumably originated as parasites
that formed a symbiotic relationship with
eukaryotic cells more than 2 billion years ago, in
response to an increase in atmospheric oxygen.
Primary cellular functions of mitochondria
Supply energy to cell in form of ATP
Generate and regulate reactive oxygen
species
Buffer cytosolic calcium ions
Regulate apoptosis through the
mitochondrial permeability transition pore
Serves as a cellular site for the following
metabolic pathways –
Electron transport chain
Tricarboxylic acid cycle or Krebs cycle
Beta oxidation of fatty acids
Gluconeogenesis
Urea Synthesis
MITOCHONDRIA LIFE CYCLE :FUSION, FISSION AND AUTOPHAGY
• Mitochondria can’t be synthesized
de novo, so new mitochondria must arise from
existing mitochondria.
• At any point of time, mitochondria are in a
dynamic flux between fission and fusion.
Unravelling the genetics of
mitochondria
MITOCHONDRIAL DNA
• Circular, double stranded, and composed of
heavy and light chains or strands
• Contains 16,569 bp
• Encodes 13 proteins
22 tRNA
2rRNA
MITOCHONDRIAL VS NUCLEARGENOME
• Mitochondrial genome has
. smaller number of genes
. higher copy number
. less effective repair mechanisms
. higher mutation rates
Peculiarities of Mitochondrial Genetics
Maternal inheritance
high copy number
heteroplasmy
bottleneck and segregation
threshold
MATERNAL INHERITANCE
MATERNAL MODE OF INHERITANCE
HETEROPLASMY
• Cells contain hundreds of mitochondria , and each
mitochondria contains hundreds of mtDNA. So cells
contain thousands of copies of mtDNA.
• For the most parts ,their sequence will be
identical (homoplasmy )
Mutation arises in mtDNA
Mixed population of wild- type and mutant mtDNA within a single cell ( heteroplasmy )
Heteroplasmic cells divide and the mtDNA is distributed randomly to daughter cells resulting in skewed populations of wild type or mutant mtDNA
Random mitotic segregation of mtDNA causes varying proportions of
mutant mtDNA In daughter cells
Degree of heteroplasmy determines clinical phenotype
BOTTLENECK AND SEGREGATION
• Of the 1,50,000 mtDNA molecules in human
oocytes ,only a small proportion of mtDNA is transmitted
during oogenesis and subsequently to embryo.
• Important implications in
high intrafamilial clinical variation
changing phenotype over time
THRESHOLD
• For heteroplasmic mtDNA mutations
• Cell can compensate for reduced wild-type mtDNA until
a certain threshold is met
- function of cell become compromised
• Disease occurs when enough cells in a tissue are affected
• Threshold depends on specific mutation and cell types
Ex : neurons have a lower threshold for disease state
PATHOPHYSIOLOGY
• Primary mitochondrial disease:
Diseases involving defects of oxidative phosphorylation.
• Tissues with high aerobic demands such as
brain tissue,
heart muscle ,
skeletal muscle
usually more severely affected.
• Mitochondrial disease can arise through :
1. defect in mtDNA
2. defect in nuclear-encoded mitochondrial
protein
mtDNA and disease
• Mutation creates two distinct classes of mtDNA variants :
- single base pair variants - mtDNA rearrangements (deletions and
insertions)
CLINICAL SYNDROMES OF mtDNA mutations
mtDNA vs Nuclear DNA mutations
Feature mtDNA mutations Nuclear DNA mutations
Mode of inheritance
Maternal Mendelian
Age of onset Adults Infancy / childhood
Severity of disease
Less More
Lactic acidosis More common Not seen
APPROACH TO MITOCHONDRIAL DISORDERS
• Idiopathic, chronic, intermittent or progressive
illness involving at least two different high-energy
requiring tissues
– Neuron (brain, esp. basal ganglia, special senses
and autonomic neuron)
– Muscle (skeletal, cardiac, or smooth)
– Endocrine gland
– Renal tubule
• Examples
– Mental retardation and diabetes mellitus
– Migraine and hypotonia
– Gastrointestinal dysmotility and stroke
– Hypothyroidism and cardiomyopathy
– Dysautonomia and deafness
– Depression and renal tubular acidosis
• Family history, intermittent disease, biochemical data
(lactic acidosis, elevated Krebs cycle intermediates)
can all increase suspicion of mitochondiral disease
• Mitochondrial disease affects tissues most
highly dependent on ATP production
– Nerves
– Muscles
– Endocrine
– Kidney
• Low energy-requiring tissues are rarely directly
affected, but may be involved secondarily
– Lung
– Connective tissue
• Symptoms can be intermittent
– Increased energy demand (illness, exercise)
– Decreased energy supply (fasting)
SYSTEM CLINICAL MANIFESTATIONS
Cardiovascular heart failure arrhythmias sudden death left ventricular myocardial noncompaction
Pulmonary dyspnea orthopnea respiratory failure respiratory acidosis
neurologic encephalopathy ataxias movement disorders seizure disorder mental retardation stroke like episodes migraine
endocrine diabetes mellitus diabetes insipidus hypothyroidism hypoparathyroidism ACTH deficiency
Ocular optic atrophy external opthalmoplegia ptosis retinitis pigmentosa cataract
Musculoskeletal myopathy•Skeletal muscle : ocular>axial/proximal>bulbar>distal•Smooth muscle : dysphagia•Cardiac : cardiomyopathy myalgias
Renal renal tubular defects benign renal cysts focal segmental glomerulosclerosis nephritic syndrome
Hematological anemia leukopenia thrombocytopenia eosinophilia
Gastrointestinal malabsorption villous atrophy pseudo-obstruction
LABORATORY EVALUATION
– Serum CK level: mildly elevated in mitochondrial myopathies
but are often normal,High-CPEO and ptosis;Very high in limb
weakness
– Lactate level: fasting blood lactate conc >3mm/l support the
diagnosis
– CSF lactate: fasting conce>1.5mm/l
• Normal level can be seen in NARP
• Elevated with short exercise
• Electrocardiography and echocardiography– cardiac involvement – (cardiomyopathy or atrioventricular conduction defects).
• Neuroimaging : – suspected CNS disease.
• CT: basal ganglia calcification +/ diffuse atrophy
• MRI: focal atrophy of the cortex / cerebellumhigh signal change on T2WI, particularly
occipital generalized leukoencephalopathy.
Cerebellar atrophy (pediatrics)
• Neurophysiologic studies:
– indicated in individuals with limb weakness,
sensory symptoms, or areflexia.
– Electromyography (EMG) is often normal but may
show myopathic features.
– Nerve conduction velocity (NCV)
may be normal or may show a predominantly
axonal sensorimotor polyneuropathy
• Electroencephalography (EEG)
– Indicated in suspected encephalopathy / seizures.
Encephalopathy: generalized slow wave activity on
the EEG.
Seizures : Generalized or focal spike and wave discharges
may be seen
MUSCLE BIOPSY
– More specific test of mitochondrial myopathies
– analyzed for histologic or histochemical evidence of mitochondrial disease.
– Respiratory chain complex studies are carried out on skeletal muscle or skin fibroblasts.
– Ragged red fibers (RRFs) are seen on muscle biopsy.
– Presence of more than 2% RRFs in skeletal muscle biopsy is taken as one of the criteria for the diagnosis of mitochondrial disease.
Distinctive features of muscle biopsy in mithochondrial myopathies :• Succinate dehydrogenase (SDH) stain: Increased staining of muscle fibers Most sensitive & specific stain for mitochondrial proliferation in muscle fibers
• Cytochrome oxidase (COX) stain: – Absent or reduced staining of muscle fibers: Reduced COX activity. – May be diffuse or in scattered fibers.
IMMUNOHISTOCHEMISTRY
– Abnormal protein accumulation in ragged red fibers:
• Desmin
• αβcrystallin,
• Heat shock proteins,
• Dysferlin,
• Emerin,
• Caveolin.
• ELECTRON MICROSCOPY:
– Usually not specific or sensitive in adults with non-
diagnostic histochemistry results ,
– Ultrastructure may be only evidence of
mitochondrial pathology in 6%
MOLECULAR GENETICS
• Testing carried out on genomic DNA
– Blood (suspected nuclear DNA mutations and
some mtDNA mutations)
– Muscle(suspected mtDNA mutations)
– Southern blot analysis may reveal a pathogenic
mtDNA rearrangement. The deletion or
duplication breakpoint may then be mapped by
mtDNA sequencing.
– If a recognized point mutation is not identified,
the entire mitochondrial genome may be
sequenced.
PRINCIPLES OF TREATMENT
Treat Underlying Neurologic Issues
– Seizures(antiepileptic drugs)
– Spasticity- baclofen, botulinium toxin
– Dystonia- diazepam, botulinium toxin, trihexyphenidyl
– headache –
acute: nonsteroidal anti-inflammatory drugs and
acetaminophen; avoid aspirin and triptans in MELAS,
chronic: amitriptyline, calcium blockers, riboflavin, coenzyme
Q10,
• Nutritional:
– Identify and treat deficiencies in vitamins (vitamins A, B12,
E, D, folate for red blood cells), minerals (iron, zinc,
selenium, calcium, magnesium), and protein calorie
(albumin).
Avoid Metabolic Stressors
• Extremes of heat and cold are not well tolerated. Fever
should be treated with acetaminophen (10 mg/kg
every 4 hours to 15 mg/kg every 4 hours). Shivering is
metabolically expensive and should be avoided.
• Avoid unaccustomed strenuous exercise, especially in
the fasting state or with a concomitant illness.
• Avoid prolonged (greater than 12 hours) fasting.
MITOCHONDRIAL GENETIC DISORDERS
REARRANGEMENTS POINT MUTATIONS
CPEO MELAS
Kearns-Sayre syndrome MERRF
Pearson marrow pancreas syndrome CPEO
Diabetes and deafness Myopathy
Cardiomyopathy
NARP
LHON
Nuclear genetic disorder
• Autosomal dominant progressive ophthalmoplegia• Mitochondrial neurogastrointestinal
enecephalomyopathy• Leigh syndrome• Cardioenecephalomyopathy• Optic atrophy and ataxia• Tubulopathy, encephalopathy and liver failure
MELAS(Mitochondrial myopathy, Encephalopathy, Lactic Acidosis and Stroke
like episodes)
– Most common mitochondrial encephalomyopathy
– Maternally inherited point mutation
– A3243G point mutation in tRNA-80%
– Onset in majority patients is before the age of 20 yrs
– Seizures: partial or generalized, may be first sign
– Stroke like episodes, do not conform to a vascular
distribution
– Hemiparesis, hemianopia and cortical blindness
– Associated condition, hearing loss, diabetes mellitus,
growth hormone deficiency
– Fatal outcome
Diagnosis of MELAS
CSF protein Increased but <100mg/dl
Muscle biopsy •Ragged red fibres•SDH positive fibres•COX positive fibres
CSF lactate Increased
Imaging •Grey and white matter involvement•Basal ganglia calcification•Focal lesion which mimic infraction are present in occipito-parietal
Genetics 80% have A3243G mutation in tRNA leucine
Normal: Mild SDH staining of amedium sized perimysial vessel.
Increased SDH staining of amedium sized perimysial vesselin a MELAS patient.
MUSCLE BIOPSY – SDH STAIN
Scattered "ragged red" muscle fibers: Gomori trichrome
Scattered abnormal, vacuolated fibers with clear rim: H & E
KEARNS-SAYRE SYNDROME (KSS)
– Multiorgan disorder
– Triad-onset before 20yrs,CPEO ,pigmentary
retinopathy
– Plus one or more of following: complete heart
block, cerebellar ataxia, or increased CSF protein
100mg/dl
– Common 5-kb mtDNA deletion,
deletion/duplications, A3243G
– KSS/CPEO-like phenotype can be caused by
nuclear mutations in genes for mtDNA
maintenance (ANT1, Twinkle and POLG)
MERRF(Myoclonic Epilepsy with
Ragged Red Fibres)
– Onset : childhood to middle adult
– Point mutation A8344G of tRNA lysine
– Characteristic : myoclonic epilepsy
cerebellar ataxia
progressive muscle weakness
– Others: dementia, peripheral neuropathy, optic
atrophy, hearing loss and diabetes mellitus
– Lipomas-cervical, symmetrical
DIAGNOSISSerum CPK Normal or increased
Lactate(serum and CSF)
Elevated
EMG Myopathic
EEG May be abnormal, non specific
Muscle biopsy •Ragged red fibres•SDH positive fibres•COX negative fibres
Genetics •A8344G mutation•Base pair substution-T8356C, G8363A
Leber’s hereditary optic neuropathy (LHON)
– Onset in early 20s– Maternally inherited– Three mutation all are located within mtDNA
complex I genes• G11778A mutation in ND4• G3460A mutation• T14484C mutation in ND6
– Characterized by acute and subacute bilateral painless visual loss
– Visual loss is severe and permanent– Dystonia or striatal degeneration
LEIGH’S SYNDROME
– Subacute necrotising encephalomyopathy
– Onset: infancy and early childhood
– Most commonly caused by high mutant loads
(>95%) of T8993G/C
– Point mutations in ATP synthase gene, affects
complex V
– Other causes include complex I def (NDUFV1),
complex IV def (SURF1), PDHC defenciency
– Progressive psychomotor deterioration,
respiratory failure
– MRI leukodystropy, changes in basal ganglia and
brain stem
Neuropathy, Ataxia, Retinitis Pigmentosa(NARP)
– Onset :childhood
– Moderate heteroplasmy for T8993G/C in ATPase
6gene (same as Leigh but lower mutant load)
– Polyneuropathy, cerebellar ataxia, retinitis
pigmentosa
– Muscle biopsy -normal
Mitochondrial, Neurogastrointestinal encephalomyopathy(MNGIE)
– Adolescent
– Autosomal recessive
– Mutation in thymidine phosphorylase in Ch 22
– Thymidine phosphorylase activity is reduced and
plasma thymidine levels are elevated
– Peripheral neuropathy,CPEO,gastrointestinal
dysmotility
Toxin induced MtDNA myopathy
– Exogenous cause of mtDNA abnormalities is HIV infection
and antiretroviral therapy
– Zidovudine induced myopathy patient presents with
myalagia,weakness , atrophy of thigh and calf muscle
– S.CK- raised
– EMG-myopathic
– Muscle biopsy-ragged red fibres with minimal
inflammation
• Association with neurodegenerative disorders
– Parkinson disease
– Alzheimer disease
– Huntington disease
– Friedreich ataxia
THANK YOU