Molecular epidemiology: biomarkers
Biomarkers are biochemical, cellular, genetic or molecular indicators, measurable in the individual using non-invasive techniques. They might reflect:
- Exposure to a disease-causing agent
- Exposure to a protective agent
- Disease risk
- Genetic factors
- Individual susceptibility
They can also, if well chosen, give information on molecular mechanisms.
Distinguishing biomarkers of exposure and risk is difficult but important.
Figure The closer a biomarker is to the initiating event, the more likely it is to reflect exposure. The more distal biomarkers may correlate better with cancer risk.
DNA damage is a marker of exposure. But already it is affected by metabolic activation/detoxification, and antioxidants. DNA repair and apoptosis further modulate the effect of exposure. Chromosome aberrations and mutations indicate effect, and are closer to reflecting disease risk.
- DNA damage: (adducts, strand breaks, altered bases)
- Mutations: (in HPRT or glycophorin A)
- Chromosome aberrations
- DNA repair
Genetic changes in tumours
The mutations that accumulate in tumours can be analysed by looking at specific genes (oncogenes, tumour suppressor genes, or 'neutral' genes like HPRT) in the excised tumour.
It would be very useful to be able to detect mutations at an early stage of tumour development. Mutated genes can be detected in DNA recovered from faeces of colon cancer patients. But so far the sensitivity is not great enough to detect mutations before the tumour is at an advanced clinical stage.