PED3014 L5 Genetics of non-CYP phase 1 metabolism

flavin linked monooxygenases

• microsomal enzymes with FAD as a prosthetic group

• especially important in oxidation reactions at nitrogen, sulfur and phosphorous centres

• At least 5 different forms but FMO3 is the major hepatic isoform in humans

FMO3 deficiency - trimethylamine is an unpleasant smelling compound that is a breakdown product of a number of precursors (such as L-carnitine) present in a range of foods…

gut bacteria are involved in TMA formation

Trimethylamine normally undergoes conversion to

Trimethylamine N-oxide (TMAO) by FMO3

some individuals lack FMO3 and suffer from

fish odour syndrome

• the ability to convert TMA to TMAO is lost

Pro153Leu is an amino acid substitution which results in…

complete loss of enzyme activity

what are the FMO3 polymorphisms E158K/E308G associated with

decreased activity which may produce mild fish odor syndrome in children

FMO3 metabolises sulindac

• NSAID used in chemoprevention of colon cancer for individuals at high risk of disease

• better outcome in those with polymorphisms due to slower metabolism

  • sulindac is a prodrug but is activated by gut flora prior to absorption

Esterases

• Arylesterases - paraoxonase

• butyrylchilinesterase - high levels in plasma

• carboxyesterases - CES1 in liver, CES2 intestine

paraoxonase

arylesterase which hydrolyses organophosphates in the liver and serum

• referred to as PON1 as there are 2 related genes which may have different functions

what is PON1 associated with

high density lipoprotein (HDL) in serum

• protects against atherosclerosis - hydrolyses oxidised LDL-associated cholesterol, with potential protective effects

50% of Europeans have low activity for paraoxon hydrolysis in serum

due to a polymorphism at codon 192 with a Gln to Arg amino acid substitution (Q192R)

PON1 effect on polymorphism is substrate-dependent

• paraoxon - low activity with Gln form

• Phenylacetate - no effect

• Diazoxon - high activity with Gln form

variation in PON1 could be relevant due to…

use of and choice of pesticides but phenotype-genotype relationships in PON1 are complex

what is cholinesterase

a plasma esterase

• hydrolyses the muscle relaxant succinylcholine (suxamethonium)

• approx 5% population are heterozygous for alleles associated with poor hydrolysis of this compound

• several alleles identified

can detect individuals with cholinesterase deficiency…

phenotypically

• by inhibition patterns of benzoylcholine hydrolysis with either dibucaine or fluoride

what is the genotypic basis of cholinesterase deficiency

• atypical form associated with abnormal dibucaine inhibition

• fluoride-sensitive forms

• silent forms

relevance of cholinesterase deficiency

• patients that are homozygous for defective alleles need prolongued ventilation after use of succinylcholine (due to apnoea)

• could avoid this by giving lower doses to these individuals

• individuals with the deficiency could have sensitivity to pesticides

CES1 oseltamavir (Tamiflu)

polymorphism of functional significance

CES2- aspirin, irinotecan

no functionally important polymorphisms

Dihydropyrimidine dehydrogenase

• metabolic enzyme that converts uracil and thymine to dihydrometabolites facilitating further metabolism to amino acids

• 5-fluorouracil is an important anticancer drug and undergoes metabolism by DPYD

• rare inborn error of metabolism described where patients completely lack DYPD and suffer epilepsy and mental retardation

molecular basis of DYPD deficiency

• Deficiency is a recessive trait but heterozygotes (2 to 3% of Europeans) show lower levels of enzyme than normal

• Most common variant allele associated with deficiency has G to A base change in intron 14 (position 1) resulting in skipping of exon 14 and truncated protein

• Other variants also identified

• Individuals heterozygous for DPYD mutations are at risk of toxicity if given 5FU treatment

• Can genotype for mutations

aldehyde dehydrogenase

conversion of acetaldehyde to acetate following ethanol consumption

• 2 main isoforms in aldehyde dehydrogenase, ALDH1 and ALDH2

• ALDH2 is a mitochondrial enzyme and has a lower Km than cytosolic ALDH1

East asians flush and suffer nausea following ethanol consumption due to

accumulation of acetaldehyde in the blood

what is the phenotype that causes east asians to flush

aminoacid substitution in ALDH2 of GLU487Lys at C-terminal end of protein

• results in unstable protein being produced

• ALDH2 deficiency

heterozygotes for ALDH2 mutation suffer a flushing response as well as homozygous mutants so the disorder is dominant

dominant inheritance due to active enzyme consisting of 4 identical subunits

• proteins that contain just a single ALDH2×2 subunit are catalytically inactive