National Vitiligo Foundation
Grant Report - 1998

Vitiligo is a pigment disorder of the skin and hair follicles, which is seen on patients as depigmented lesions of the skin that gradually enlarge. Vitiligo is a common condition, affecting 1-2% of all racial groups worldwide, and predisposes affected persons to skin damage by sunburn, with an increased risk of melanoma. Vitiligo is also associated with an increased risk for other autoimmune diseases, such as thyroid conditions. The current treatments for vitiligo are difficult, expensive, and often disappointing. Topical steroids and phototherapy with psoralens and ultraviolet light (PUVA) are the current standard treatments, although re-pigmentation may be patchy and slow, and the lesions may even worsen during treatment. Surgical techniques such as dermabrasion and re-epithelialization have recently been used to attain re-pigmentation.

The cause of vitiligo is unknown, but is thought to involve defects both in the pigment-producing cells in the skin (melanocytes) and in cells of the immune system (lymphocytes). Vitiligo sometimes runs in families, and one study found that 20% of the relatives of vitiligo patients also have vitiligo, with the highest risks for children and siblings of patients. This suggests that some people are born with genes that make them more susceptible to developing vitiligo. However, there are probably still environmental factors, such as traumatic skin injury, sunburn, stress, etc., that influence whether or not an individual with those "susceptibility genes" will develop vitiligo during their lifetime.

Why study "susceptibility genes"? By identifying disease susceptibility genes, we can learn more about the cause of vitiligo. Depending on which genes are involved, what their normal function is, and what genetic changes are found, it might be possible to design new treatments based on understanding those genes. Knowing which genes are important may also allow genetic testing for potentially susceptible individuals, so that environmental risk factors can be more carefully avoided. Theoretically, it is possible that future advances in gene therapy might also help susceptible or affected persons.

Studies of disease susceptibility genes can be very complicated. Let’s start with a simple one-gene example many people are familiar with – cystic fibrosis. Cystic fibrosis (CF) is inherited as a recessive condition caused by mutations (genetic changes) in a single gene. This means that a person with cystic fibrosis must have inherited mutations in both copies of their CF gene, the copy inherited from their mother and the copy inherited from their father. Researchers are now finding that several genes (maybe as many as 6-10) are involved for many other conditions, especially for autoimmune diseases such as systemic lupus erythematosus and type 1 diabetes. The same is now thought to be true for vitiligo susceptibility, and these genes may be important for normal melanocyte function or regulation of the immune response, or both.

Let’s assume there are 10 "vitiligo susceptibility genes". One person with vitiligo might have genetic changes in both copies of any 5 out of the 10 genes. In another person with vitiligo, the changes might be in a different 5 out of the 10 genes, or an overlapping set of 5 genes. So it could take mutations in both copies in ANY 5 or more of the 10 genes involved to cause vitiligo. In genetics research, this is called a "quantitative trait" because it involves many genes, and a "threshold trait", because it requires genetic changes in at least a certain number of these genes.

Keep in mind that all of us carry recessive changes in many genes, and it is only by chance that combinations of changes are inherited together that results in diseases that have a genetic component. It is nobody’s fault when a person is affected with a genetic disease. In the case of vitiligo, we believe that both parents may carry recessive changes in some small number of vitiligo susceptibility genes, but the number might be below the "threshold" number required for the parents to have vitiligo themselves. It is only the random chance of gene combinations that happen to be inherited by a child that results in susceptibility to vitiligo.

We are using "case/control association" to try to identify vitiligo susceptibility genes. Case/control studies compare the variations of a given candidate gene. A gene variant or change will be considered to be positively associated with vitiligo when it is found more often in vitiligo patients than in controls. It will be said to be negatively associated (protective) when it occurs at a significantly lower frequency in patients than in controls. Large numbers of association tests must be performed for the most reliable statistical results, and repeating the studies in different populations or ethnic groups is extremely useful to confirm real associations.

We are also using "family-based association", which requires DNA samples family members of the vitiligo patient. Families with one or more affected children can be analyzed, and the statistics test whether a particular change in a vitiligo susceptibility gene is passed from parents to affected children more frequently than expected due to random chance. Advantages of family-based association studies include the use of families with any number of affected children, and the combining of information from multiple ethnic groups and/or geographical populations.

Our goals during 1998 were to continue genetic studies of vitiligo susceptibility that we had recently started in human patients and the Smyth line chicken animal model. First, we wanted to expand our collection of vitiligo patient and family DNA samples. Second, we wanted to establish the lab procedures and begin screening human candidate vitiligo genes by association analyses. And third, we wanted to determine the usefulness of a chicken model for testing of vitiligo susceptibility genes.

We have been collecting samples from affected and unaffected family members of vitiligo patients for the family-based genetic association analyses. We have obtained samples from 59 vitiligo patients without family members, 56 patients with only one or two first-degree relatives, and 31 patients with three or more relatives. It is the last group that is the most useful for family-based studies, and our emphasis is now to continue collecting family samples consisting of the patient, both parents, and at least one sibling. We are also attempting to collect families with multiple affected members and/or multiple generations affected by vitiligo.

Our second goal was to begin case/control association studies of candidate vitiligo susceptibility genes. The time it has taken to collect enough numbers of samples to begin these studies was unexpectedly long. However, we have begun the testing for twelve candidate genes, including six genes important in melanocytes and six genes involved in regulating the immune system. Based on our preliminary results, eight of the genes tested demonstrate no significant association with vitiligo in the Caucasian patients tested so far. However, the results for four of these genes are more promising, and more patients need to be tested before we rule them out. We also have many more candidate genes to analyze (and that work is in progress), and we have accomplished the most important goal of the first year of this project, which was to establish the patient sample collection and set up all of the lab procedures.

The third goal of these studies was to determine the usefulness of genetic studies in the Smyth line chicken, which has symptoms nearly identical to human vitiligo. During this past year, Smyth line and control chickens were bred together, and the offspring and parents were tested for genetic variability. Although other experiments in our laboratory had previously suggested that these chickens were genetically variable using a different type of genetic marker, our results during this past year suggest that the gene mapping approach is not feasible, and the chicken genetic studies are not being continued.