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Genes are essential for the body to perform the functions necessary for life. These are coded for by 3 billion units of genetic material that make up "deoxyribonucleic acid" (DNA). DNA provides the information to direct the "synthesis," or making, of proteins. Proteins are large molecules that perform a wide variety of very specific functions in the body. Researchers now know that we have approximately 30,000 to 40,000 genes.

Among the benefits expected to derive from the Human Genome Project is the identification of specific genes associated with specific diseases. While there is always hope that identification will be as simple as "one gene, one disease", this is unlikely to be the case for most major diseases that have a genetic component. For most diseases, including psoriasis, there are probably multiple genes involved in producing the sequence of events that result in the expression of disease. Complicating the picture still further is the probability that genes found to be associated with psoriasis may or may not cause psoriasis in an individual person, depending on the activity of the genes in relation to one another. The activity of genes in relation to one another is often determined by the pattern in which they were inherited from parents.

Is Psoriasis a Disease with Genetic Predisposition?
Numerous studies over many years support the finding that genetic predisposition has a major role in the pathogenesis (development of the disease processes) of psoriasis. Genetic predisposition means an inherited tendency to develop the disease. Genetic predisposition does not mean a 100 percent guarantee that the disease will appear; other initiating factors such as injury of infection may act together with genetic predisposition to set the disease process in motion.

There is a higher-than-average incidence of psoriasis in relatives of people with psoriasis, indicating "familial tendency" to develop the disease; however, in some people with psoriasis no family history is evident.
There is an increased incidence of psoriasis in children when one or both parents has psoriasis.
In studies of identical and nonindentical twins, psoriasis is much more likely to appear in both identical twins than in both nonidentical twins, a finding that also confirms that more than one gene must be inherited to establish genetic predisposition for psoriasis.
There is a higher than expected frequency of certain white cell antigens (Class I human leucocyte antigens or HLAs) on cells of people with psoriasis and their close relatives; this finding also supports psoriasis inheritability and also suggests that the gene(s) involved in psoriasis may be on the same chromosome that holds the genes for HLA. There are many types of HLA in the HLA complex, and studies have shown that HLA type may be associated in some degree with timing of disease onset, type of psoriasis, and disease severity.

While the genetics of psoriasis remain to be worked out in detail, much is already known about patterns of inheritability. Persons with psoriasis who are worried about "passing it on" to children may wish to ask their dermatologist for genetic counseling.

Genes hold the key to understanding how the immune system becomes activated in people who have psoriasis or psoriatic arthritis. When a specific gene is found to be linked to psoriasis, researchers work to determine what the gene does under "normal" conditions. Then, they will have to determine how the gene is behaving differently in psoriasis. This is a very difficult and time-consuming process, because researchers believe that more than one gene is responsible for causing psoriasis.

Genes control everything from height to eye and hair color. Differences between people result from different versions of the gene. Genes determine how cells act. When genes are working normally, the body and its cells function normally. When a misstep occurs in the way a gene works, a genetic disease may result. Even psoriasis is considered a genetic disease.

Scientists now believe that at least 10 percent of the general population inherits one or more of the genes that create a predisposition to psoriasis. However, only 2 percent to 3 percent of the population develops the disease.
This is thought to be because only 2 percent to 3 percent of people encounter the "right" mix of genetics and triggers that lead to the development of psoriasis. Those who have a genetic disease but don't have a family history of it, for example, may have "inherited" two genes from their father and two from their mother–neither of whom had all four and therefore never developed the disease.

Research into the genetics of psoriasis didn't begin until the early 1970s. Recently, it has exploded in scope, thanks to improvements in medical and genetic technology, National Psoriasis Foundation encouragement and increased funding.
The National Psoriasis Tissue Bank remains the first and only public source of genetic material for researchers investigating psoriasis. Just weeks after it opened in 1994, a research team from the University of Texas Southwestern and Baylor University Medical Center in Dallas announced the first genetic marker for psoriasis on chromosome 17. A marker is a section of DNA that can track with a disease.
If a marker exists in families with a genetic disease, the marker and the disease gene are said to be "linked," meaning they are located quite close to each other in the genetic sequence. Because each chromosome contains thousands of genes, with 10 to 100 of them close enough to the marker to be "linked" to it, the identification of linkage only sets the stage for the identification of the disease gene. However, linkage does greatly narrow down the list of possible candidate genes. Markers have been identified on at least 11 chromosomes (1, 3, 4, 6, 8, 10, 16, 17, 18, 19 and 20).

Because of recent progress in understanding human genes and the completion of the Human Genome Project, researchers at Washington University in St. Louis recently reported three genes on chromosome 17 that are associated with the development of psoriasis. The findings were published in the December 2003 issue of Nature Genetics. Each of the three genes, which in turn regulate other genes and the proteins they make, is associated with the immune system and the skin.