University of Michigan scientists have found a common genetic variation in an immune system gene that makes people much more likely to develop psoriasis - a disfiguring inflammatory skin disease.
Named PSORS1 (SORE-ESS-1), for psoriasis susceptibility 1, the gene is the first genetic determinant of psoriasis to be definitively identified in a large clinical study. Its discovery could lead to new, more effective treatments for psoriasis without the risks and side-effects of current therapies.
The gene's causative role in psoriasis was demonstrated in a University of Michigan Medical School study of 2,723 people from 678 families in which at least one family member had the disease.
Results of the U-M study - the most comprehensive analysis of a psoriasis gene to date - will be published in the May 2006 issue of the American Journal of Human Genetics.
Psoriasis is a chronic disease that affects about 2 percent of the U.S. population. People with psoriasis develop thick, flaky white patches on their skin and scalp. The disease is disfiguring and can have a negative effect on quality of life. About 25 percent of people with psoriasis eventually develop psoriatic arthritis, which can be severe.
Unlike diseases caused by a mutation in just one gene, psoriasis is what scientists call a multi-factorial disease. This means that people must inherit several disease-related genes, plus be exposed to one or more environmental triggers, in order to get psoriasis.
"For every individual with psoriasis who carries the PSORS1 gene, there are 10 other people with the gene who don't get psoriasis," says study director James T. Elder, M.D., Ph.D., a professor of dermatology and of radiation oncology in the U-M Medical School and the Ann Arbor VA Healthcare System.
"It's as if you are pushing a shopping cart down the aisle at the grocery store and putting genes in your cart," Elder adds. "There are several different brands of each gene on the shelf and one of them is bad for you. If you pull down enough bad ones, then you can get sick.
"But even if you get all the bad genes, you still need a trigger from the environment to develop the disease," explains Elder. "In psoriasis, strep throat is a very common initial trigger. It activates the immune system to attack the strep bacteria. But once the strep infection is cleared, the immune system starts attacking the patient's own skin cells.
About half the time, strep-induced psoriasis goes away and never comes back. But for the other 50 percent of young people who get it, psoriasis progresses to become a chronic life-long disease."
The PSORS1 gene is actually one of over 20 different varieties (scientists call them alleles) of a gene called HLA-C. "In terms of our grocery store analogy, think of PSORS1 as one of 20 'brands' of HLA-C on the shelf," Elder says.
Located on human chromosome 6, HLA-C is one of several genes in the major histocompatibility complex (MHC) that regulate how the immune system fights off infection. MHC genes carry DNA-coded instructions for proteins whose job it is to distinguish between what belongs in the body and what doesn't.
"There is a great deal of genetic variation in the MHC, because it's on the front lines of dealing with pathogens and cancer," Elder explains. "It's an area where it's good to be different. If everybody were the same, we'd be like hybrid corn. A plague could come along and wipe us all out."
Scientists have been searching for genes associated with psoriasis for more than 30 years, but until now studies have been inconclusive, according to Rajan P. Nair, Ph.D., the study's first author and a U-M assistant research professor in dermatology.
"Researchers have identified 19 candidate loci, or areas on chromosomes, that may be genetically linked to psoriasis," Nair says. "Many studies confirmed a strong association with the MHC, but no one could determine which gene in the MHC was involved in psoriasis."
In a previous study, Nair and his U-M colleagues narrowed the search for the PSORS1 gene down to a 300,000-base-pair segment of chromosome 6 that included HLA-C and at least 10 other genes.