Mitochondrial Disorder


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Mitochondrial Disorder

  2. 2. 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.
  3. 3. 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
  4. 4. 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
  6. 6. • 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.
  7. 7. Unravelling the genetics of mitochondria
  8. 8. MITOCHONDRIAL DNA • Circular, double stranded, and composed of heavy and light chains or strands • Contains 16,569 bp • Encodes 13 proteins 22 tRNA 2rRNA
  9. 9. MITOCHONDRIAL VS NUCLEAR GENOME • Mitochondrial genome has . smaller number of genes . higher copy number . less effective repair mechanisms . higher mutation rates
  10. 10. Peculiarities of Mitochondrial Genetics Maternal inheritance  high copy number  heteroplasmy  bottleneck and segregation  threshold
  13. 13. 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 )
  14. 14. 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
  15. 15. 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
  16. 16. 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
  17. 17. 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.
  18. 18. • Mitochondrial disease can arise through : 1. defect in mtDNA 2. defect in nuclear-encoded mitochondrial protein
  19. 19. mtDNA and disease • Mutation creates two distinct classes of mtDNA variants : - single base pair variants - mtDNA rearrangements (deletions and insertions)
  20. 20. CLINICAL SYNDROMES OF mtDNA mutations
  21. 21. 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
  23. 23. • 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
  24. 24. • Examples – Mental retardation and diabetes mellitus – Migraine and hypotonia – Gastrointestinal dysmotility and stroke – Hypothyroidism and cardiomyopathy – Dysautonomia and deafness – Depression and renal tubular acidosis
  25. 25. • Family history, intermittent disease, biochemical data (lactic acidosis, elevated Krebs cycle intermediates) can all increase suspicion of mitochondiral disease
  26. 26. • Mitochondrial disease affects tissues most highly dependent on ATP production – Nerves – Muscles – Endocrine – Kidney
  27. 27. • 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)
  28. 28. 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
  29. 29. Ocular Musculoskeletal Renal  optic atrophy  external opthalmoplegia  ptosis  retinitis pigmentosa  cataract  myopathy •Skeletal muscle : ocular>axial/proximal>bulbar>distal •Smooth muscle : dysphagia •Cardiac : cardiomyopathy  myalgias  renal tubular defects  benign renal cysts  focal segmental glomerulosclerosis  nephritic syndrome Hematological  anemia  leukopenia  thrombocytopenia  eosinophilia Gastrointestinal  malabsorption  villous atrophy  pseudo-obstruction
  31. 31. – 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
  32. 32. • 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 / cerebellum high signal change on T2WI, particularly occipital generalized leukoencephalopathy. Cerebellar atrophy (pediatrics)
  33. 33. • 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
  34. 34. • 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
  36. 36. – 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.
  37. 37.  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.
  39. 39. – Abnormal protein accumulation in ragged red fibers: • Desmin • αβcrystallin, • Heat shock proteins, • Dysferlin, • Emerin, • Caveolin.
  40. 40. • ELECTRON MICROSCOPY: – Usually not specific or sensitive in adults with nondiagnostic histochemistry results , – Ultrastructure may be only evidence of mitochondrial pathology in 6%
  42. 42. • 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.
  43. 43. – If a recognized point mutation is not identified, the entire mitochondrial genome may be sequenced.
  45. 45. 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,
  46. 46. • 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).
  47. 47. 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.
  49. 49. REARRANGEMENTS POINT MUTATIONS CPEO MELAS Kearns-Sayre syndrome MERRF Pearson marrow pancreas syndrome CPEO Diabetes and deafness Myopathy Cardiomyopathy NARP LHON
  50. 50. Nuclear genetic disorder • Autosomal dominant progressive ophthalmoplegia • Mitochondrial neurogastrointestinal enecephalomyopathy • Leigh syndrome • Cardioenecephalomyopathy • Optic atrophy and ataxia • Tubulopathy, encephalopathy and liver failure
  51. 51. MELAS (Mitochondrial myopathy, Encephalopathy, Lactic Acidosis and Stroke like episodes)
  52. 52. – 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
  53. 53. – 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
  54. 54. 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
  55. 55. MUSCLE BIOPSY – SDH STAIN Normal: Mild SDH staining of a medium sized perimysial vessel. Increased SDH staining of a medium sized perimysial vessel in a MELAS patient.
  56. 56. Scattered abnormal, vacuolated fibers with clear rim: H & E Scattered "ragged red" muscle fibers: Gomori trichrome
  58. 58. – 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
  59. 59. – 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)
  60. 60. MERRF (Myoclonic Epilepsy with Ragged Red Fibres)
  61. 61. – 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
  62. 62. Serum 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
  63. 63. Leber’s hereditary optic neuropathy (LHON)
  64. 64. – 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
  66. 66. – 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
  67. 67. – Progressive psychomotor deterioration, respiratory failure – MRI leukodystropy, changes in basal ganglia and brain stem
  68. 68. Neuropathy, Ataxia, Retinitis Pigmentosa(NARP)
  69. 69. – 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
  70. 70. Mitochondrial, Neurogastrointestinal encephalomyopathy(MNGIE)
  71. 71. – 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
  72. 72.  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
  73. 73. • Association with neurodegenerative disorders – Parkinson disease – Alzheimer disease – Huntington disease – Friedreich ataxia
  74. 74. THANK YOU