Editor,—The editors of theJournal of Epidemiology and Community Healthshould be congratulated on their far reaching audience. The comment entitled “Genes as causes: scientific fact or simplistic thinking?” is a good excuse for transdisciplinary discussion.1 The response to such comment could indeed be entitled “Genes as causes: simplistic thinking? No, scientific facts”. The “genes as causes” tenet is more than an opinion and there is ample evidence that, for certain severe diseases, “genes are sufficient causes for human suffering” as long as their effects are not so detrimental that they are incompatible with life, in which case proof of causation will be even more difficult to obtain. The issues raised by Porta and colleagues1 are legitimate but, in our opinion, they are extreme.

As epidemiologists well know, there are few human illnesses that can be attributed to a single factor. When so, this requires exposure—here used in a very broad term—to factors exerting very strong effects. You can refer to genes as “exposures”, as we are inevitably “exposed” to our genetic background and cannot in fact effectively prevent exposure to genes. Only very severe mutations, fortunately uncommon, are necessary and sufficient causes of disease: it is not worth providing a detailed list of gene mutations leading to the lack of a protein product or to protein devoid of function, such information can readily be found in classic genetics textbooks.2 For the majority of medical conditions, genes are only “one of the pool of exposures” leading to disease, therefore often being neither absolutely required nor sufficient for disease development. Even in the case of a severe illness, such as cystic fibrosis, mutations in the gene encoding the cystic fibrosis transmembrane regulator (CFTR) lead to a complex constellation of phenotypes ranging from classic cystic fibrosis to mild …

Professor Porta (mporta{at}imim.es)

Professor Porta (mporta{at}imim.es)