Inbreeding and cancer incidence in human isolates.

This study investigates the incidence of cancer in isolate populations. Thorough anthropological research over the past 3 decades has established island populations in Middle Dalmatia, Croatia, as outstanding examples of genetic isolates. The number of cancer cases on 5 islands (Brac, Hvar, Korcula, Vis, and Lastovo) over a 20-year period (1971-1990) has been extracted from the data of the Croatian Cancer Registry. The population of coastal Dalmatia, characterized by similar environmental factors but a different population genetic structure, was used as a control population. The leading hypothesis was that, if there were genes or gene complexes (especially with recessive inheritance) responsible for genetic susceptibility to certain types of cancer, then the incidence of those cancer types should be greater in reproductively isolated island populations than in a control population because of increased manifestation of such genes or gene complexes caused by inbreeding. Furthermore, the cancer incidence should increase along with greater reproductive isolation (i.e., greater geographic distance of the islands from the mainland). After adjusting the data for sex and age, I confirmed the hypothesis: Island populations have greater total cancer incidence than the control population for both sexes. The excess incidence on the islands shows an almost linear correlation with geographic distance from the mainland. The cancer sites primarily responsible for the excess incidence are bladder cancer in males, and breast, ovarian, brain, and large bowel cancer in females, predominantly in the younger age groups.