Epidemiology: A Very Short Introduction by Rodolfo Saracci

Author:Rodolfo Saracci [Saracci, Rodolfo]
Language: eng
Format: epub, mobi, pdf
Published: 2010-03-13T02:09:00+00:00

A wide range of specific studies is being conducted within the EPIC cohort and findings of major interest have already emerged. Nearly 15,000 deaths from any cause have been recorded and it has been shown that the distribution of fat in the body, in particular an increased deposit of fat in the abdomen translating into a large abdominal girth, predicts the risk of death. Another study found that the risk of cancer of the intestine (colon and rectum) is associated with high consumption of red and processed meat. In a third study, the risk of breast cancer occurring after menopause was shown to be related to the level of both female and male hormones in the blood, a caution against the use of male hormones (like testosterone) that had been proposed for prevention of bone fragility in post-menopausal women.

Each of the blood specimens stored in the EPIC depository contains different fractions: serum and plasma, in which many biochemical compounds can be analysed; envelopes of red cells, in which some substances like fatty acid molecules can be assayed; and most important, white blood cells that provide DNA for genetic studies. Genes are embodied as sequences of variable length of elementary molecules (nucleotides or bases, coupled as base-pairs) in the long molecules of DNA. These are in turn packed within the chromosomes included in a cell nucleus. Every human being receives 23 chromosomes from the father and 23 from the mother, each of these two sets containing more than three billion base-pairs. About 99% of these are common to all humans, but this leaves more than ten million nucleotides that can vary from one person to another. In these variations are hidden the differences in individual susceptibility to disease, a universe that has become accessible to direct exploration only recently, since the revolution in molecular biology and technology has made it possible first to measure and then to test the differences in the structure of individual nucleotides on a very large scale. Today, it is feasible to test a million nucleotides at once for variations in structure (form), in studies of `single nucleotide polymorphisms' (SNPs) that cover all chromosomes, i.e. the whole `genome'.

Within EPIC and in a fast-expanding number of other studies, associations are now sought between these genetic variations and disease, in the same way as in the past associations of smoking with disease were investigated. Testing hundreds of thousands or millions of associations, and understanding whether and in what way a gene variant causes a disease, involves three major challenges. First, it requires a very large number of cases of a disease, even beyond the numbers achievable in a project like EPIC: hence data from similar, albeit smaller, studies are combined with those of EPIC for `consortium' analyses. Second, testing a million associations increases the number of them that turn out to be statistically significant at the commonly adopted levels of 5% or 1% probability of error. A 5% level implies that 50,000 associations will appear as statistically significant merely

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