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.