Population genetic studies of psoriasis

Abstract

Psoriasis is a genetically determined, common skin disease affecting about 3 % of the population. The inheritance pattern has earlier been unclear. In collaboration with the Swedish Psoriasis Association, we have collected information on the occurrence of psoriasis in the families of 11,366 members of the Association.Analysis of the answers to a questionnaire showed that 64% of all probands had no parents with psoriasis. The distribution of psoriasis in the parents, the siblings and among the children of probands was compatible with an autosomal recessive inheritance. The cumulative incidence of psoriasis in the elderly can be estimated to 5% and the gene frequency in the population to 25%.Analysis of the ages of onset shows that there is a peak of onset in puberty and that women develop the disease earlier than men. There was a correlation of the onset ages between siblings. Assuming a recessive inheritance, we have calculated the gene frequencies for different onset ages. The earliest onset age, in puberty, is found to have the highest gene frequency, 25%.The risk of acquiring psoriasis depending on the occurrence of psoriasis in the family has been determined empirically. The life-time risk varies from 24%, which is the risk if one sibling already has the disease, to 83%, which is the risk if both parents and one sibling are affected. The risk of getting psoriasis at a certain age is dependent on the age at onset of psoriasis in the affected parent.Family members from 310 families have been examined, altogether 1217 individuals. The body location and extent of psoriasis have been recorded. Remission of the skin disease was found in 13.5% at examination. The accuracy of diagnosis of psoriasis given by the proband was high. We compared concomitant diseases, in the cardiovascular, neurological and endocrine systems, and in joints, as well as iritis and inflammatory bowel disease, in the sibling generation. We found a strong association with joint complaints in persons with psoriasis. For the rest of the disorders studied, we could not find any significant difference. Presence of HLA-Cw6 seemed to protect against diabetes mellitus.