Lecture 3

• Functionally important polymorphisms for metabolism


  • CYP450s
  • Flavin-linked monooxygenase (FMOs)
  • Dehydrogenases
  • Esterases
  • Acetyltransferases
  • Glutathione S-transferases
  • UDP-glucuronosyltransferases
  • Sulfotransferases
• CYP450s


  • 1A1
  • 1A2
  • 2A6
  • 2C9
  • 2C19
  • 2D6
  • 3A4/5
• Flavin-linked monooxygenase (FMOs)


  • FMO3
• Dehydrogenases


  • Aldehyde dehydrogenase 2
  • Alcohol dehydrogenase 2+3
• Esterases


  • Cholinesterase
  • Paraoxonase
• Acetyltransferases


  • NAT2
• Glutathione S-transferases


  • GSTM1
  • GSTT1
• P450s are involved in the metabolism of 70% of prescribed drugs, with the majority being CYP3A4 (over 50%)
• Highly polymorphic P450s with absence of activity


  • CYP2D6
  • CYP2C19
  • 2A6 (relatively rare)
  • 3A5 (relatively common)
• Highly polymorphic P450s with decreased activity


  • 2C9 ('classic')
  • 3A4
  • 1A2
  • 2D6 (relatively common)
  • 2C19 (relatively common)
• Highly polymorphic P450s with increased activity


  • CYP2D6
  • 1A1
  • 2E1
  • 2C19
• CYP2D6


  Typical substrates have a basic nitrogen and a hydrophobic region, with the position of oxidation 5-7Å from the basic nitrogen. 5-10% lack this enzyme due to polymorphisms. Substrates include cardiovascular agents, antipsychotics and antidepressants. Hydroxylation reactions are common.
• Debrisoquine polymorphism


  Some patients suffer hypotension when administered debrisoquine, which is metabolised by CYP2D6 to 4-OHD. Polymorphisms in CYP2D6 lead to altered metabolism.
• CYP2D6 alleles with loss of activity


  • 2D6*3 (A2549 deletion → frameshift)
  • 2D6*4 (G1846A → splicing defect)
  • 2D6*5 (CYP2D6 deleted)
• CYP2D6 alleles with reduced/increased activity


  • Reduced: 2D6*9 (A2613-A2615 → K281 deleted), 2D6*10 (C100T → P34S), 2D6*17 (C1023T;C2850T → T107I; R296C)
  • Increased: Amplification (n= 2,3,4,5 or 13) of 2D6
• In the wild-type CYP2D6 gene, the distance between Xba1 restriction sites is 29kb, which increases with gene amplification
• Common CYP2D6 variants


  • CYP2D6*4 (intron 3/exon 4 - truncated protein, premature stop codon)
  • CYP2D6*5 (Entire CYP2D6 gene deleted - no protein synthesised)
  • CYP2D6*3 (exon 5 - frameshift, premature stop codon)
  • CYP2D6*6 (exon 3 - frameshift, non-functional protein)
• Variations in CYP2D6 allele frequencies between populations

  Europe: *3, *4, *5 and *6 common

  China: *5 and *10 (reduced activity) common

  Africa: *4, *5 and *17 (reduced activity) common

  Amplifications: More common in southern Europe + Africa
• CYP2C19


  Relatively low concentrations in the liver. Some clinically important substrates include omeprazole, diazepam, proguanil and clopidogrel. Also known as mephenytoin hydroxylase. 3% of Europeans and 20% of East Asians lack CYP2C19 activity.
• Mephenytoin metabolism


  EM - S enantiomer rapidly metabolised (t1/2 <1 day), R enantiomer slower (t1/2 = 6 days). PM - both enantiomers metabolised slowly.
• CYP2C19 alleles with no activity


  • 2C19*2 (Splicing defect)
  • 2C19*3 (premature stop codon)
  • 2C19*4 (initiation codon - no transcription)
• CYP2C19*8


  Reduced activity (T358C → W120R)
• CYP2C19*17


  Increased activity due to polymorphisms in the upstream control sequence (C-806T and C-3402T) resulting in higher gene transcription and faster metabolism of substrates like omeprazole and mephenytoin. 42-fold more frequent in Mediterranean-Southern Europeans than in East Asians.
• CYP2D6*3


  A2549del -> frameshift
• CYP2D6*4


  Splicing defect -> truncated protein (intron 3/exon 4)
• CYP2D6*5


  CYP2D6 deleted
• CYP2D6*9


  K281 deleted
• CYP2D6*10


  P34S
• CYP2D6*17


  R296C
• Increased CYP2D6


  Amplification
• 2C19*2

  Splicing defect
• CYP2C19*3


  Premature stop codon
• CYP2C19*4


  GTG initiation codon - no transcription initiated
• CYP2C19*8


  W120R
• CYP2C19*17


  C-806T and C-3402T - polymorphism in upstream regulatory region, affects transcription factor binding