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.

Basic Facts About Nongenetic Factors in Psoriasis
Genetic predisposition to psoriasis is not necessarily a genetic fate. Genetic predisposition establishes a set of conditions that may be necessary to cause psoriasis, but are not in and of themselves sufficient to cause psoriasis. Psoriasis may result, in some persons, only when a genetic predisposition is "triggered" by an environmental event.

Triggers for Psoriasis
Environmental triggers may be:

• The provocation for the initial appearance of psoriasis;
• Factors that cause new outbreaks of psoriatic lesions while the disease is in an active phase; or,
• Factors that worsen psoriasis while the disease is in an active phase.

Environmental triggers that have been identified range from climate to physical injury (trauma) to infection to psychological stress:

Climate and Weather
Epidemiologic studies indicate that cold weather may be a predisposing effect or trigger for psoriasis, in contradistinction to hot and sunny climate that appears to be beneficial.

Physical Trauma and Other Skin Conditions
In the 19^th Century, a Dr. Koebner described a patient with psoriasis who developed new lesions at spots where he was bitten by his horse. The relationship between physical trauma and psoriatic lesions was subsequently documented in many more patients and was given the name "Koebner’s phenomenon". Analysis of patient records has indicated that up to 50 percent of persons with psoriasis have had a "Koebner’s phenomenon" experience—that is, have had a psoriatic lesion develop at the site of an injury or skin condition. In perhaps about 10 percent of psoriasis patients every injury or skin condition causes a new psoriatic lesion to develop at the site of injury. The likelihood of Koebner’s phenomenon is probably increased during an active phase of the disease.

A broad range of injuries and skin conditions has been linked with Koebner’s phenomenon:

Infections
Both dermatologic and systemic infections have been known to trigger the onset of psoriasis or a worsening of psoriasis. The incidence of infectious triggering has varied from study to study, but the triggering effect of infections is well documented. Systemic infections that have been associated with triggering include viral upper respiratory disease, streptococcal pharyngitis ("strep throat"), and human immunodeficiency virus (HIV). Staphylococcal skin infections (boils) have been a trigger.

Psychological Stress
It is difficult to separate job and family-related stress from the psychological stress of living with psoriasis. One cause of stress probably reinforces the others. Nevertheless, the perception of patients that psychological stress can worsen psoriasis has been supported in clinical studies. Patients may be tempted to reduce stress by abusing drugs or alcohol—activities that can actually increase stress. A stress-depression, stress-aggression, or stress-obsession pattern may be accompanied by increased pruritus (itching). A psoriasis patient who feels overwhelmingly stressed should ask the dermatologist to arrange for psychological counseling, which may include members of the family.