The genetic epidemiology of cancer: interpreting family and twin studies and their implications for molecular genetic approaches.

The recent completion of a rough draft of the human genome sequence has ushered in a new era of molecular genetics research into the inherited basis of a number of complex diseases such as cancer. At the same time, recent twin studies have suggested a limited role of genetic susceptibility to many neoplasms. A reappraisal of family and twin studies for many cancer sites suggests the following general conclusions: (a) all cancers are familial to approximately the same degree, with only a few exceptions (both high and low); (b) early age of diagnosis is generally associated with increased familiality; (c) familiality does not decrease with decreasing prevalence of the tumor-in fact, the trend is toward increasing familiality with decreasing prevalence; (d) a multifactorial (polygenic) threshold model fits the twin data for most cancers less well than single gene or genetic heterogeneity-type models; (e) recessive inheritance is less likely generally than dominant or additive models; (f) heritability decreases for rarer tumors only in the context of the polygenic model but not in the context of single-locus or heterogeneity models; (g) although the family and twin data do not account for gene-environment interactions or confounding, they are still consistent with genes contributing high attributable risks for most cancer sites. These results support continued search for genetic and environmental factors in cancer susceptibility for all tumor types. Suggestions are given for optimal study designs depending on the underlying architecture of genetic predisposition.