Necessary and sufficient causes

1. Introduction: causal reasoning in epidemiology Paolo VineisImperial College LondonVenice, HuGE workshop, 9-10 november 2006

2. Three theoretical issues:- necessary, sufficient causes, or neither?- how to interpret evidence (BH guidelines?)- linear combination of genes and environment?

3. Necessary and sufficient causes Necessary YesNo YesChr 21 Guillotine Down syndr. Cut neck Sufficient NoMycobacterium Tobacco Pulmonary TB Lung cancer (R Saracci, 2005)

4. In multifactorial diseases Rothman’s pies derive from Mackie’s model of Insufficient Non-redundant component of an Unnecessary Sufficient Complex (INUS)

5. Why did the house burn?(a) wood was burning in the fireplace (b) there was a strong wind(c) the alarm did not work(d) the house is woodenNone of these is a single necessary cause, and none of them is sufficient, however …

6. … the whole of these conditions is a sufficient cause (in fact it was effective in burning the house)Each component is called INUSInsufficient Non-redundant component of an Unnecessary Sufficient Complex Main limitation of this model: it is static, does not take time into account (Vineis and Kriebel, Environ Health 2006)

7. Bradford-Hill “guidelines”: how do they apply to association studies in genetics (low-penetrant gene variants)?

8. 1. Strength of association – Penetrance of a gene variant depends on events such as the interaction with external exposures, the internal environment, or other genes (BRCA1-2 and hormones). The strength of association is a relative and not absolute concept and requires the study of interactions.

9. 2. Consistency in genetic studies is usually poor, with few associations confirmed in more than 1 study. However, the intrepretation of the reasons is far from being clear. One possibility is that studies are biased, e.g. for an incorrect choice of control subjects, and cannot be replicated. Another explanation is that the role of genes is strongly dependent on the surrounding circumstances (environment, other genes, gene expression)(effect modification).

10. 3. Specificity of the association –The association is more likely to be causal if it involves for example a single histological type (angiosarcoma of the liver) and a very particular exposure (vinyl chloride). In the case of genes this criterion should be met more easily, considering the functional knowledge that is expected to support the causal reasoning. For example, 5-HTT and bipolar disorder through serotonin metabolism, NAT2 and bladder cancer through arylamines.

11. 4. Temporality – The time relationship between exposure and the onset of disease should be biologically meaningful. This criterion does not apply to genetics, since the genotype is stable over time and is present at birth (with exceptions)5. Biological gradient – BH referred with this criterion for example to dose-response relationships in environmental epidemiology. Only if the genetic model is codominant a dose-response can be observed. However, a different kind of dose-response is observable if we consider the cumulative effect of multiple genes or SNP.

13. 6. Biological plausibility – This is one of the most elusive among BH’s criteria, because it can easily be met a posteriori. For example, even if a direct role of a gene in a disease seems unlikely, the gene variant can still act indirectly, for example in some lateral metabolic pathway. This criterion is partly overlapping with the following.

14. 7. Coherence with previous knowledge – Guidelines for assessment of genetic evidence: (a) linkage to a particular region of the human genome (LOD>3); (b) one or more independent mutations that are perfectly concordant with disease status in affected families; (c) defects that lead to macrochanges in the protein; (d) putative mutations that are not present in a sample from a control population; (e) the presence of some other line of biological evidence (Glazier et al, 2002).

15. 8. Experimental evidence – Many discoveries on genes came from experimental studies in animals (for example on genes involved in hypertension or the metabolic syndrome). 9. Reasoning by analogy – It is often used, also in genetics, but can be fallacious. For example, the fact that perhaps the 5-HTT gene variants are involved in the bipolar syndrome has led to hypothesize that they could also be involved in anorexia nervosa. This is, however, an unwarranted inference, because it rests on the questionable assumption that there are similarities between the two diseases, or that they are caused by the same mechanism.

16. Example of NAT 2 and bladder cancer:Association stronger in people exposed to arylamines (ORs in the order of >3)Association consistent and apparently unbiased, replicated in populations from different geographic/ethnical backgroundsSpecificity and biological plausibility if we refer to bladder cancer (role of arylamines)(but also involved in lung and colon cancer and in mesothelioma)Arylamines in black- vs. blond tobacco (former more strongly associated with bladder cancer)Work in non-human systems (Mutagenicity of 4-aminobiphenyl and 4-acetylaminobiphenyl in Salmonella typhimurium strains expressing different levels of N-acetyltransferase. Toxicol Appl Pharmacol. 1999; Rodent models of the human isoniazid-acetylator polymorphism. Drug Metab Dispos. 1979)

17. Means and Ses of 4-aminobiphenyl hemoglobin adducts in black and blond tobacco smokers Acetylation phenotype slow fastnon-smokers 31.7 (3.8) 19.4 (4.9)blond tobacco 118 (13) 86.4 (14.5)black tobacco 175 (11) 117.5 (13.7)(Vineis et al, Nature 369: 154-156, 1994)

18. Is the additive model for the cumulative effect of genes and environment valid?There is at least one example of strong gene-gene interaction:in Bardet-Biedl syndrome (historically interpreted as a recessive trait) three interacting mutations at two loci and possibly with a third locus have been described (Badano et al, Nature 2006)