Mitochondrial Disorders - BioChem Genetics

Mitochondria

• Site of oxidation phosphorylation: via the electron transport chain embedded in the inner mito membrane

  • Produce ATP

• The other biochemical processes occur in the Mito:

  • Pyruvate oxidation

  • Krebs Cycle

  • Fatty Acid Beta-Oxidation

Oxidative Phosphorylation

The electron transport chain

• Inner membrane has 5 distinct protein complexes embedded: some encoded by nuclear DNA, some encoded by Mito DNA

• Use NADH + FADH coming form Krebs cycle break down of Actyl-CoA

  • Fatty Acids (generated via F.A. B-Oxidation)

  • Pyruvate (generated via glycolysis)

• Electron transport down the chain of complexes: creates gradient by pumping IN H+ ions

• Complex V uses gradient to Generate ATP as H+ ions move OUT

Mitochondrial Disorders

Oxidative Phosphorylation/Electron Transport chain dysfunction

Two varieties:

• Secondary Mitochondrial dysfunction: Non-genetic conditions

  • Hypoxemia (inadequate Oxygen for Oxidative Phosphorylation)

  • Medication: valproic acid, HIV meds

  • Toxins: cyanide, rotenone

• Primary Mitochondrial Disease

  • mitochondrial DNA itself or nuclear DNA mutations

Mitochondrial Genome

The Mitochondrial Chromosome: encodes 37 genes

• only 3% of Mito. proteins are encoded by mito DNA

• 97% are encoded by nuclear DNA and imported into mitochondria

Complex 1

46 total proteins

MtDNA encoded: 7

nuDNA: 39

• Leigh Syndrome

• Leukodystrophy

Complex 2

4 proteins: ALL nuDNA ENCDOED

• Leigh Syndrome

• Paraganglioma

• Pheochromocytoma

Complex 3

11 proteins

MtDNA: 1

nuDNA: 10

• Leigh syndrome

• GRACILE syndrome

Complex 4

mtDNa: 3

nuDNA: 10

• Leigh Syndrome

• Hepatopathy

• Cardioencephalomyopathy

• Leukodystrophy/tubulopathy

Complex V

mtDNA: 2

nuDNA: 14

Maternal Inheritance

mtDNA mutations can only be inherited through the mother

• all mito provided by the ovum

• no mito contriubted by the sperm

Heteroplasmy

Mito genomes can differ between mitochondria in a given cell and % of mutant mtDNA can vary in an individual from cell-to-cell and tissue-to-tissue

• Each cell has up to 1000 mitochondria, each with their own copy of the mito genome

• mtDNA mutation rate is 10-20x nuclear DNA mutation rate

Threshold Effect.

• energy requirements vary between tissues

• mtDNA mutation burden varies tissue to tissue (heteroplasmy)

• Tissue specific % mutant mtDNA threshold for disease

• Phenotypic variability results

Example: Brain and Muscle have a lower threshold than Skin and Kidney

Mitochondrial Disorders: Presentation

Can present in almost any way and vary from person to person, but 3 general categories

• “Classic” Mitochondrial diseases: reproducible, multi-organ pattern

• Unexplained multi-organ dysfunction:

  • Hearing loss short stature

  • Diabetes + hypertrophic cardio myopathy

  • ophthalmoplegia +ptosis

• Unexplained single organ syndrome: just hearing loss, epilepsy, GI

Often elevated Lactic acid in Blood or CNA and Mitochondrial proliferation in muscle

Myopathy, Encephalopathy,, Lactic Acidosis, and Stroke-like Episodes (MELAS)

• Age of Onset: before 40yo (average 5-15)

• Clinical

  • Stroke like episodes + Epilepsy, Dementia

  • Muscle weakness (myopathy), Cardiomyopathy, Lactic Acidosis

  • Hearing-Loss, Retinopathy, Diabetes

• CT/MRI: Infarcts→ but not seen in vasuclar regions: infarct occurs due to region engery insufficney from Mitocondrial

• Etiology: heterogeneous mtDNA mutations (Often mt-t RNA) → VERY dependent on Heteroplasmy with individual

Myoclonic Epilepsy with Ragged Red Fibers (MERRF)

• Adolescent onset

• Clinical manifestations

  • Epilepsy (myoclonic)

  • Muscle weakness (myopathy), Lactic acidosis, Ataxia

  • Encephalopathy, Hearing Loss

• EMG

• EEG:

• Muscle Biopsy: (if done on affected muscle) will show ‘ragged red fibers’ caused by mitochondria proliferation

• Etiology: Single mtDNA-tRNA mutation 80 to 90%

Leber’s Hereditary Optic Neuropathy (LHON)

• Age of onset 20-24 yo

• Clinical:

  • Acute or sub-acute bilateral central vision loss→ Rapid progression to blindness (usually confined to optic nerve)

  • Rarely: heart block, dystonia, MS-like symptoms

• Fundoscopy: early tortuous retinal arteries, followed by optic atrophy

• Etiology: 95% mtDNA “ND” (electron transport subunit) gene mutations MATERNAL INHERITANCE

  • ****4:1 M:F ration → X-linked modifier genes that make females less affected****

Ophthalmoplegia, Ptosis and Myopath

Chronic Progressive Ophthalmoplegia (CPEO)

• External ophthalmoplegia, bilateral ptosis, mild myopathy

• Onset after 20yo (slowly progressive)

Kerns-Sayre Syndrome (KSS)

• Retinitis Pigmentosa, Cardiac conduction defects, ataxia, CSF protein >100

• Also: myopathy, dysphagia, Sensory hearing loss, dementia, diabetes

Etiology:

• Mainly mtDNA deletions→ can be smaller or larger chunks of mtDNA (smaller =CPEO, larger=KSS)

• Majority are SPONTEOUS

Subacute Necrotizing Encephalopathy (Leigh Syndrome)

• 6-12 months - 3-5 years (25% have later onset or slower forms)

• Clinical: (often abrupt decompensations/regression with infection/fever)

  • Developmental stagnation and regression

  • Seizures, Ataxia, Hypotonia, spasticity

  • ophthalmoplegia, nystagmus, optic atrophy

• Diagnostic Testing

  • Elevated CSF blood, MRspect: LacticAcid peaks, MR Basal.G lucciences

  • Deficiencies in Complex I (20%), IV(20%), PDH/PC (10%)

Leigh Etiology

Genetic Heterogeneity

• 10-30% mitochondrial DNA mutations → maternal inheritance

• 90-70% nuclear DNA mutations → Classic Mendelian

  • HETEROPLASMY AFFECT: If a lower # of mito. in a cell have these mutations = Later onset Neuropathy, Ataxia, Retinitis Pigmentosa (NARP)

Pyruvate Dehydrogenase Complex (PDHC) Deficiency: Clinical + Testing

Failure to convert Pyruvate to Actyl-CoA (via PDH)

Lactic Acid levels elevated (PDHC most common cause of Lactic Acidosis)

Clinical Features: Progressive intermittent neurologic deterioration

• hypotonia, seizures, ataxia, ophthalmoplegia, dystonia

• Presents similar to mitochondrial dysfunction

Suggestive Abnormal Tests

• Plasma: increased Lactic Acid + Pyruvate, but normal Pyr : L.A ratio

  • ***MITOCONDIRLA DISORDERS: increased L.A but norm pyruvate***

• Cerebral Spinal Fluid: increased Lactic Acid

Pyruvate Dehydrogenase Complex (PDHC) Deficiency: Metabolism +Etiology

• Failure to convert Pyruvate to Actyl-CoA (via PDH)

• Lactic Acid levels elevated (PDHC most common cause of Lactic Acidosis)

Etiology: PDHC is a multisubunit complex

• Catalytic components: E1, E2, E3

• Regulatory component: PDH Phosphatase

Confirmation:

• PDHC enzyme activity assay

• Sequencing of

  • E1 → PDHA1 : MOST COMMON , X-Linked (males only)

  • E2 → DLAT, Recessive

Mitodoncrial Diease: Work Up

• Serum levels: increased anion gap + metabolic acidosis

  • Lactic Acid: Pyruvate ratios (>30 Mito. Dis ; <10 PDHC Def.)

• Imaging: brain MRI, Spectroscopy ( LA peaks over brain regions( BasalGang)

  • Basal Ganglia hypodensities: generalized atrphy

  • Hypoplastic corpus callosum if fetal lactic acidosis

• Muscle Biopsy

• Genetic Testing

Mitochondrial Disease: Muscle Biopsy

Allows for:

• Detecting ragged red fibers (mito. proliferation)

• Abnormal mitochondria proliferation

• Detecting enzyme activity of the chain-genes

• Mutational analysis of mitoDNA

Pitfalls:

• need 1 gram of flesh (large amount)

• biopsy of moderately affected muscle

• may not distinguish exact genetic mechanisms

Genetic Testing

mtDNA:

• Leigh Syndrome

• LHON

• MERRF (blood/muscle)

• MELAS (blood/muscle)

• NARP (blood/muscle)

• KSS/CPEO (muscle)

nDNA (all in blood)

• Leigh syndrome

• MNGIE

• Mohr-Tranebjaerg

• Friedreich’s Ataxia

• AR spastic paraparesis

• AD PEO

Mitochondrial Disorders: Treatments

Less evidence for specific treatments that actually improve outcomes

• Trials with Vitamins that optimize Electron Transport chain function:

  • Carnitine

  • Biotin

  • thiamine

  • Riboflavin

• High Fat/ Low Carb diet: low carb→ less glycolysis→ less LA

• Avoid Mito toxic meds

• Reduce LA, control acidosis (dialysis/vent)