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 results 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.

Supporting evidence for genetic predisposition includes:

• 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 (Click on What is Psoriasis for a discussion of disease types).

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.

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