Psoriasis in Australian twins.

BACKGROUND Occurrence of psoriasis has been found to be strongly genetically controlled in Northern European and U.S. twin and family studies. OBJECTIVE Our purpose was to assess cumulative incidence and heritability of psoriasis in the Australian population. METHODS Australian twins reporting psoriasis on a screening questionnaire received from 3808 pairs were mailed a detailed instrument designed to validate the diagnosis, supplemented by telephone interview and examination of medical records. RESULTS Only 94 of 160 subjects who screened positive were confirmed to have psoriasis. The cumulative incidence of confirmed psoriasis was 2% in 30- to 60-year-old subjects. The monozygotic twin casewise concordance for confirmed psoriasis was 35% (12 of 34 pairs), and the dizygotic twin concordance 12% (5 of 43 pairs), giving an estimated heritability of 80%, was similar to that found in a genetic reanalysis of three previous twin studies. A case-control analysis of psoriasis-discordant twin pairs found no evidence for influences of alcohol or coffee intake, overweight, birth weight, or personality in the origin of psoriasis. CONCLUSION Occurrence of psoriasis in the Australian population is highly heritable, but identical twins are often discordant; the factor responsible for the onset of disease in one twin and not the other is unclear.