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Summaries and Questions & Answers
The following is a totally fictitious case study constructed for educational purposes. The journal, the gene, the exposures and all the numbers are all made up.
A recent case-control study published in the Journal of 21 st Century Medicine reported that 80 out of 100 patients with rheumatoid arthritis seen at a university rheumatology referral center were found to carry the common variant C707T of the rheumatoid arthritis (RA) gene (point mutation at position at 707). This variant is associated with an enhanced autoimmune inflammatory response in several animal species. All 100 patients were interviewed to look for risk factors and it turns out that 80 patients had been using for a year or more a herbal preparation (called ImmuneBlast) purported to contain immune function enhancers. 70 patients had the C707T variant and used ImmuneBlast. The controls were 100 subjects without rheumatoid arthritis seen at the same clinic and frequency matched to cases by gender, age and race. It turns out that 40 controls had the C707T variant, 50 had used ImmuneBlast and 20 had both. The authors concluded that they found a new cause of rheumatoid arthritis that is due to gene-environment interaction and strongly recommended that people should be tested for the RA gene before taking ImmuneBlast.
Question 1: Based on this study, what is the magnitude of the overall association between C677T and rheumatoid arthritis? between ImmuneBlast and rheumatoid arthritis? Show calculations in terms of relative risks and population attributable fractions.
Question 2: Is there evidence of gene-environment interaction in this case-control study? Compute the effects of the exposure alone, the genotype alone and the combination. Show calculations in terms of relative risks and population attributable fractions.
Question 3: Discuss the etiologic inference of the authors in terms of causal criteria including comment on study design and analysis.
Question 4: If the authors did not have a control group, what kind of inference on gene-environment interaction could they obtain from a case only analysis? Is this a valid result? Discuss pros and cons of this approach compared to a standard case-control analysis.
Question 5: Can you comment on the recommendations of the authors to test the general population for the RA gene before taking ImmuneBlast?
**The above case study was totally fictitious scenario and had been constructed for educational purposes only. The journal, the gene, the exposures and all the numbers were all made up.**
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