Lecture 10

• Constitutional genome


  • Present since fertilisation → in every cell
  • Less tolerance for variation
• Constitutional genome


  • Trisomy 13 (Patau)
  • Trisomy 18 (Edward)
  • Trisomy 21 (Down syndrome)
• Somatic genome


  • Acquired through age + in individual cells + their progeny
  • More tolerance for variation
• Older cancer drugs


  Highly proliferative cells = targeting DNA synthesis, not tailored — patient response still affected by genetics
• Newer cancer drugs


  Cancer cells only = genomic translocations (Bcr-Abl), over-expressing oncogenes (EGFR-TKIs, B-RAF), phenotypic characteristics (surface protein expression) — constitutional + somatic mutations nullify effect
• 1. Fluorouracil (5-FU)


  • Used in colorectal, stomach, breast + pancreatic — rapidly dividing cells
  • Pyrimidine analogue incorporated into DNA + RNA → cell cycle arrest + apoptosis
  • Inhibits thymidylate synthase
  • Side effects = myelosuppression, diarrhoea + cardiotoxicity (15-30% of patients)
• DPYD genetic variants


  • Reduced activity
  • Null activity
• DPYD variant allele frequencies differ across different populations + dose reduction
• DPYD intermediate metaboliser


  • Decreased DPYD activity at 30-70% of normal population
  • Clinical recommendation: reduce starting dose by 50%
• DPYD poor metaboliser


  • Complete deficiency + increased risk for drug toxicity
  • Clinical recommendation: no 5-FU dose proven safe
• Irinotecan


  • Used in colorectal + lung cancer for >30 years
  • Metabolised to the active form (SN-38) by carboxylesterase
  • Inactivated by conjugation by UGT1A1 + 1A7 to SN-38 glucuronide
  • Inhibition of topoisomerase I
  • Side effects = diarrhoea, neutropenia (15-30% patients)
• Gilbert's syndrome


  Linked to genetic variants in UGT1A1 — give rise to mild hyperbilirubinemia to jaundice
• UGT1A1*28 allele


  • Common in Europeans
  • Additional TA repeat (TA)7 in gene promoter region — affects transcription + protein expression
  • 8-78% of general population are poor metabolisers
  • Genotyping recommended before irinotecan administration
• Death related to tox events during consolidation chemotherapy by UGT1A1*28 phenotype = 14% heterozygotes, 1.9% homozygotes, 0.7% homozygous wt (p<0.001)
• Thiopurines


  • Numerous cancer treatment incl paediatric acute lymphoblastic leukaemia (ALL)
  • Immunosuppressants post-organ transplant + IBD
  • Can cause toxicity
  • Metabolised by thiopurine methyltransferase (TPMT)
  • Direct incorporation into DNA/RNA
  • Inhibit de novo purine synthesis
• • >1% of individuals have variation homozygous
  • 4-12% of population heterozygous
  • Majority base substitution variants
  • Homozygotes have less TPMT activity vs heterozygotes
• TPMT genotype/phenotype


  • Inversely related to thioguanine nucleotides (TGNs) (6-TGN)
  • If given standard doses — deficient patients likely to develop haematopoietic bone marrow toxicity
  • Dose adjustment based on TPMT genotype/phenotype is warrented
• Imatinib


  • Originally developed to treat chronic myeloid leukaemia (CML)
  • Defined by the Ph chromosome — T(9;22) — encodes fusion oncogene BCR-ABL
  • Resistance to imatinib = Mutations of Bcr-Abl kinase domain found in 90% of patients with CML → relapsed after initial response to imatinib
  • Thr315 (T315) = gatekeeper residue within ATP-binding site — prevents H bonding between imatinib + 315 residue critical for drug binding
• Gemtuzumab


  • For CD33+ve acute myeloid leukaemia (AML)
  • CD33 expressed in AML
  • Polymorphisms affect Gemtuzumab efficacy
• rs12459419 — predicts outcome after Gemtuzumab in AML patients
• Trastuzumab (Herceptin)


  • Monoclonal antibody to HER2 expressed in ~20% breast cancers
  • Drives proliferative + confers anti-apoptotic phenotype
  • Over-expression mediated via gene amp.
• BRAFV600E


  • Activating mutations in B-RAF from MAP-kinase pathway (50-60% melanomas, 30-70% thyroid cancers, etc)
  • Activation + unregulated cell proliferation
• Vemurafenib


  Small molecule inhibitor of V600E variant