203m1b

• Genetic testing


  The analysis of DNA, RNA or chromosomes, proteins or certain metabolites in order to detect alterations related to a heritable disorder
• Types of genetic testing

  • Direct testing- looking for mutations
  • Cytogenetic testing- examining the number and structure of chromosomes
  • Biochemical testing- assaying metabolites
• Uses of genetic testing

  • Diagnostic testing
  • Pre-symptomatic or predictive testing
  • Prenatal Diagnosis
• Diagnostic testing


  Used to help confirm a condition in a patient who already has symptoms
• Pre-symptomatic or predictive testing


  Offered to asymptomatic individuals with an increased risk of a family history of a genetic disorder
• Prenatal Diagnosis


  Chromosomal and genetic analysis of a fetus can be determined by analysis of cultured cells from the amniotic fluid
• Prenatal diagnosis was initially important for testing women of late maternal age for an increased risk of Down syndrome (trisomy 21)
• The ethical issues surrounding prenatal diagnosis and selective termination of pregnancy are both complex and emotive
• Summary of Prenatal Testing

  • First Trimester: First Trimester Combined Screen, Cell-Free DNA (cfDNA) Screen, Chorionic Villus Sampling (CVS)
  • Second Trimester: Integrated Screen, Quad Screen, Maternal Serum AFP, Amniocentesis
• Maternal serum alpha-fetoprotein (AFP)


  A major plasma protein produced by the yolk sac and the liver during fetal life, and is excreted into the urine. Levels are elevated in neural tube defects, and reduces in Down syndrome
• Inhibin A and human Chorionic Gonadotrophin (hCG)

  High in Down syndrome
• Oestriol


  Low in Down syndrome
• Amniocentesis


  1. Amniotic fluid (~20mL) is collected for chromosome studies, enzyme measurements (AFP for neural tube defects) or DNA studies, in combination with ultrasound

  2. The amniotic cells contain fetal cells that need to be cultured

  3. Usually performed at the 14-16th week, but can be performed as early as 10 weeks

  4. Associated with ~0.5-1% risk of inducing miscarriage over the baseline risk of 2-3% of any pregnancy at this stage of gestation
• Chorionic villus sampling


  1. Involves biopsy of tissue from the villous area of the chorion transcervically or transabdominally usually ~10-14 weeks, in combination with ultrasound

  2. Advantage of CVS is getting results at an earlier stage, and allow for an earlier termination

  3. No AFP testing

  4. Increase in fetal loss is ~1% compared to baseline of 2-5% at this time of gestation
• Pre-implantation Genetic Diagnosis (PGD)


  The use of in vitro fertilisation (IVF) and genetic testing to select embryos free of a specific genetic condition for transfer to the uterus
• Molecular analysis can be undertaken by PCR and cytogenetic analysis by FISH, SNP arrays, array CGH
• An unaffected embryo is transferred to the uterus of the mother, affected embryos are discarded, raising ethical issues
• Once a mutation in a gene has been found, need to prove that that mutation is actually involved in causing the disease (a 'driver') rather than being a 'passenger mutation'
• Mutation screening


  • Failure to observe such a mutation in a large sample of normal controls
  • Novel appearance and subsequent co-segregation of the gene mutation and disease phenotype through a family pedigree
  • Previous independent occurrence of the mutation in an unrelated patient with the disease