Psoriasis concordance low in cohort from California Twin Program

Researchers at the University of Southern California School of Medicine are using that institution's California Twin Program (CTP) to gain insight into the epidemiology and genetics of psoriasis.

In a poster presented at American Academy of Dermatology's Academy '06, investigators reported that the concordance of psoriasis among twin pairs was lower than that found in previous studies.

"Although several familial studies of psoriasis have been performed and, more recently, genetic polymorphism and allele clustering analyses have been undertaken, collectively, the findings of those studies primarily reinforce that psoriasis is a complex disorder," Theodore Alkousakis, M.D., says. He is the lead author of the poster on this research.

The project is being directed by David Peng, M.D., M.P.H., visiting assistant professor of clinical dermatology, University of Southern California (USC), Los Angeles. Dr. Alkousakis became involved in it when he was a fourth-year medical student at USC. He is now a dermatology resident at the University of Iowa, Iowa City.

Between 1908 and 1982, 256,000 twins were born in California and eligible for enrollment in the CTP, which was initiated in 1992 by Thomas Mack, M.D., professor of preventive medicine, University of California, Los Angeles. Medical history information is available for about 52,000 of those individuals and was searched to identify individuals with psoriasis.

"Previous twins studies have been limited by the low numbers of individuals with psoriasis in a given population. Due to its large size, the CTP database offers a unique opportunity to revisit psoriasis in a twin study," Dr. Alkousakis says.

156 PAIRS STUDIED

Approximately 1,000 self-identified psoriatics were found in the CTP Database.

The researchers sent out psoriasis-specific questionnaires to a subset of those individuals and received the surveys back from 209 twin pairs. Selecting only those with usable responses, researchers found there were 156 twin pairs available for the analyses. Those sets of twins consisted of about one-fourth monozygotic pairs and three-fourths dizygotic pairs.

The questionnaire asked the subjects to identify whether they or their sibling had psoriasis, and if the answer was yes, if the diagnosis was made by a physician and what type of physician (dermatologist or other). Only subjects with a physician diagnosis were considered as having psoriasis for the analysis. Rates of concordance were 32 percent for the monozygotic twins and 18 percent for the dizygotic twins.

"The prevalence of psoriatics in the CTP database corresponds with the generally accepted prevalence rate of 1 to 2 percent in the general population, and the distribution of monozygotic and dizygotic twins within our study group is similar to the general population occurrence as well. However, the psoriasis concordance rates were only about half of those seen in previous papers, including the landmark psoriasis twin study by Farber et al. published in 1974," Dr. Alkousakis observes.

The latter study was based on 61 twin pairs that were identified by requesting dermatology offices to provide information on twins with psoriasis in their care. As Farber et al. noted in their own discussion, that method of ascertainment might result in an over-representation of twins with both siblings affected, Dr. Alkousakis notes.

"Our study is also based on a larger number of twin pairs compared with the Farber paper and several others combined. Considering our methodology and database size, I feel more confident about the accuracy of our results," he adds.

Further investigating heritability, the analysis determined what percentage of the offspring of subjects with psoriasis also had the disease. The psoriatic females in the twin pairs had an average of 1.6 offspring, of which 21 percent had psoriasis. The offspring with psoriasis were equally divided among males and females. The psoriatic males in the twin pairs had an average of two children, of which only 3 percent were affected with psoriasis. There was a predominance of males among the latter offspring.