Peroxisomal Inborn Errors of Metabolism

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Peroxisomal Inborn Errors of Metabolism

• Mylai Le mylai.le_at_utoronto.ca
• Li Cao li.cao_at_utoronto.ca
• Mai Le maithi.le_at_utoronto.ca
• Benson Kua benson.kua_at_utoronto.ca
• March 18, 2009

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What are peroxisomes?

• Single-membraned organelles in eukaryotic cells
• Lots in liver and kidney (for detoxification)
• Contain various enzymes (catalase and various
  oxidases)
• Enzymes are transported into peroxisomes from the
  cytosol

Rat liver peroxisome internal dark areas are
urate oxidase crystals
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Enzyme Uptake in Peroxisomes

• Enzymes have a Ser-Lys-Leu (SKL) sequence on
  carboxyl terminus which signals for peroxisome
  uptake
• SKL on enzyme is recognized by a cytosolic
  carrier protein
• Enzyme-carrier protein complex can then bind to a
  membrane receptor for internalization

Uptake of Catalase into Peroxisome
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What do peroxisomes do?

• Peroxisomal enzymes carry out oxidative reactions
  
• Oxidases catalyze the reaction
• RH2 O2 --gt R H2O2
• Catalase can use H2O2 for further oxidation
• H2O2 RH2 --gt R 2H2O
• Catalase also detoxifies excess H2O2 generated
  from oxidase reactions
• Some oxidase substrates are phenols, formic acid,
  formaldehyde, and alcohol
• Also involved in other pathways like biosynthesis
  of phospholipids and cholesterol

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Major Role of Peroxisomes

• Involved in lipid metabolism
• Example ß-oxidation of fatty acid molecules
• Usually handles VLCFA (very long chain fatty
  acids)
• Alkyl chain of fatty acid shortened sequentially
  by 2 carbons at a time (stops at octanyl)
• Cleaved carbons transformed to acetyl-CoA
• Can also have a-oxidation (shortening by 1
  carbon)

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What can go wrong?

• Peroxisomal Biogenesis Disorder - peroxisomes are
  not made
• Deficiency/defect in one enzyme
• Deficiency/defect in multiple enzymes

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Zellweger Syndrome
(aka Cerebrohepatorenal Syndrome)

• Inherited human disease
• Usually fatal within first year of life
• Severe peroxisomal deficiency due to defect in
  importing proteins into peroxisomes
• Leads to empty peroxisomes (no enzymes to get rid
  of toxic substances)
• Individuals have severe abnormalities in the
  brain, liver, kidneys
• Common features enlarged liver, high levels of
  iron and copper in bloodstream, and vision
  disturbances
• Symptoms at birth lack of muscle tone, inability
  to move, and glaucoma
• Other symptoms unusual facial characteristics,
  mental retardation, seizures, and inability to
  suck/swallow

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Zellweger Syndrome

• Transport of many or all proteins into
  peroxisomal matrix is impaired
• Newly synthesized peroxisomal enzymes remain in
  the cytosol and are eventually degraded
• Remarkable feature is that cells contain empty
  peroxisomes that have normal complement of
  peroxisomal membrane proteins
• Thus Zellweger mutations cause defects in a
  peroxisomal receptor or transport protein for
  peroxisomal matrix proteins but not for membrane
  proteins
• Enzymes/proteins used for lipid oxidation cannot
  be imported into the peroxisome so lipid
  peroxidation cannot occur

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Classic Refsums Disease Biochemistry

• Rare, autosomal recessive disorder on chromosome
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• Caused by impaired a-oxidation of phytanic acid
  (a fatty acid found in food products such as
  dairy products, certain meats, and fish)
• The defective enzyme is phytanoyl-coenzyme A
  hydroxylase, a peroxisomal enzyme responsible for
  catalyzing the second step of the metabolism of
  phytanic to pristanic acid. This results in an
  accumulation of phytanic acid in the brain,
  tissues and blood
• Mechanism of phytanic acid toxicity is unclear
  but it may get incorporated into tissue lipids
  and impair myelin formation or it may affect the
  metabolism of fat-soluble vitamins

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Classic Refsums Disease

• Impaired metabolism of phytanic acid
• Refsums Disease--gt defective phytanoyl CoA
  hydroxylase

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Classic Refsums Disease Clinical Presentation

• Infants generally appear normal at birth
• Onset of symptoms is usually late childhood or
  adolescence
• Progressive with periods of remission
• Initial presentation is ataxia and/or failing
  vision
• Clinical manifestations peripheral neuropathy,
  impaired vision from retinitis pigmentosa,
  hearing deficit, anosmia, cardiomyopathy and
  conduction defects, and ichythyosis

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Classic Refsums Disease Prognosis

• Phytanic acid restricted diet (lt 10 mg/day)
  delays progression of disease
• However, vision and hearing problems may persist
  and sense of smell may not return
• If left untreated, premature death usually occurs
  due to heartbeat abnormalities

Characteristic shortening of fourth toe in
Refsums disease
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Treatments and Therapies

• No standard course of therapy, mainly supportive
  (emotional, physical and psychological therapies)
  to alleviate symptoms and complications
• Dietary supplementation with
• - Antioxidant vitamins
• - Limitation of intake of fatty acids,
  especially
• VLCFAs (very long chain fatty acids)
• - Pure DHA (an essential omega 3 fatty
  acid),early in
• life, in conjunction with a
  well-balanced diet
• Dietary changes should be monitored closely

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Hepatocyte Transplantation

• Re-population with normal hepatocytes containing
  fully functional peroxisomal enzymes
• Experiments done in mice?
• Host cells are ablated to provide growth
  advantage to donor cells

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Gene Therapy

• Amplification of the cDNA of the peroxisomal gene
  of interest, followed by liposomal transfection
  techniques to selectively deliver into cultured
  human hepatocytes ?
• In vitro, high efficiency of transfection and
  appropriate intracellular localization to
  peroxisomes

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Summary

• Peroxisomes are organelles that contain a lot of
  enzymes used for detoxification (mainly oxidases)
• Enzymes are transported from the cytosol into the
  peroxisomal matrix using carrier/transport
  proteins and membrane proteins
• Major function is for lipid metabolism Example
  B-oxidation of VLCFA (very long chain fatty
  acids) cuts fatty acid sequentially by 2 carbons
  at a time a-oxidation can also occur (cuts fatty
  acids by 1 carbon at a time)
• Peroxisomal errors peroxisomal biogenesis
  disorder, defect/deficiency in one or multiple
  enzymes
• Zellwegers Syndrome --gt transport of enzymes
  into peroxisome is impaired so peroxisome does
  not have normal detoxification function
  (deficient in peroxisomal matrix enzyme)
• Classic Refsums Disease --gt an enzyme for
  a-oxidation of phytanic acid is impaired (defect
  in peroxisomal enzyme)
• No standard treatment, mainly supportive and
  dietary restrictions to alleviate symptoms and
  complications

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References

• Alberts et al. Molecular Biology of the Cell.
  Garland Publishing, 1994.
• Stryer et al. Biochemistry 5th Ed. W.H. Freeman
  Company, 2002.
• Lodish et al. Molecular Cell Biology. W.H.
  Freeman Company, 2000.
• Wills et al. Refsums Disease. Q J Med, 94,
  403-406, 2001.
• National Institute of Neurological Disorders and
  Stroke, Accessed on March 10, 2009
  http//www.ninds.nih.gov/disorders/refsum/refsum.h
  tm
• Wanders et al. The inborn errors of peroxisomal
  B-oxidation A review. J Inh Met Disease, 13,
  4-36, 1990.
• Accessed on March 17, 2009 - http//www.cartage.o
  rg.lb/en/themes/sciences/zoology/AnimalPhysiology/
  Anatomy/AnimalCellStructure/Peroxisomes/Peroxisome
  s.htm