Genetic: Xeroderma Pigmentosum
Genetic: Xeroderma Pigmentosum
Xeroderma pigmentosum, or XP, is a rare condition in which skin cells are not able to repair damage caused by exposure to the ultraviolet portion of sunlight. It was first described in 1874 by two Austrian dermatologists, Ferdinand von Hebra (1816–1880) and Moritz Kaposi (1837–1902). Dr. Kaposi gave the disorder its name, which means “dry discolored skin,” in 1882. XP was not understood to be caused by genetic mutations until 1968, however.
Xeroderma pigmentosum is a rare skin disorder caused by mutations in any of eight different genes that govern the ability of skin cells to repair damage to DNA (the genetic material inside a cell) caused by exposure to sunlight. In a normal person, exposure to the ultraviolet radiation in sunlight causes damage to the DNA of the cells in the upper layer of the skin. This damage is fixed by a mechanism known as nucleotide excision repair, or NER. In patients with XP, however, the enzymes that carry out NER are either reduced in effectiveness or missing entirely. When the damage to the cell's DNA cannot be repaired, the DNA itself may mutate, leading to the death of skin cells or to skin cancer. The average child with XP develops the first signs of skin cancer around age eight.
Xeroderma pigmentosa progresses in three stages:
- Stage 1. The child's skin is healthy at birth but around six months of age the skin develops reddish, scaly patches with heavy freckling or sunburn after even short exposure to sunlight. At first only the face and other areas exposed to direct sun are affected, but gradually the skin changes appear on the neck, lower legs, and even the chest.
- Stage 2. The skin develops irregular patches of lightened or darkened skin with spidery patterns of blood vessels appearing underneath. The skin also becomes much thinner.
- Stage 3. This stage begins when the child is four to five years old. Skin cancers and scaly patches known as solar keratoses appear.
About 80 percent of patients with XP develop eye problems related to sun exposure. The eyes may become bloodshot and painfully sensitive to light. Both cancerous and noncancerous growths may appear near the eyes. In a few cases the eyelids may shrink or even disappear entirely.
Between 20 and 30 percent of patients with XP develop symptoms affecting the nervous system in late childhood or early adolescence. These symptoms range from the loss of tendon reflexes and poor coordination to hearing loss and mental retardation. The child's growth may also slow down. These neurologic symptoms tend to get worse over time.
A Very Special Summer Camp
In 1996 the parents of a child with xeroderma pigmentosum had the idea of creating a nighttime camp for patients with XP that would allow them to enjoy the typical activities of a summer camp experience but without the dangers of exposure to sunlight. By 2005 the couple was able to open a permanent retreat house for Camp Sundown, as they named it, in Craryville, New York. The camp's sessions in recent years include a retreat for family members combined with a medical conference that brings researchers from around the world to the camp to share new information with the families and to offer support for living with the limitations that the disorder imposes on family members. The campers themselves enjoy outdoor sports under artificial lights after dark, including an adventure course.
Camp Sundown provides lodging, meals, and supplies to each camper and one family member completely free of charge. Other family members are charged only nominal fees for staying at the camp. In 2007 the camp ran six sessions through the summer and into October, including one week just for teens. Any person or organization can sponsor a camper for one full session with a donation of $350. That sum offsets the cost of the camper's food, utilities, activities, insurance and transportation.
The frequency of xeroderma pigmentosum varies from country to country. In the United States, it is estimated to affect between one person in 250,000 and one in 1,000,000. In Japan, however, XP is much more common, striking one person in every 22,000. It is also more common in the Middle East and North Africa than in Europe or North America. The reason for these geographical differences is not yet known for certain but is thought to be associated with the high rates of marriages within extended families in some of these countries.
In all countries, XP affects men and women equally. Within the United States, it affects all races and ethnic groups equally.
The cause of xeroderma pigmentosum is a mutation in any of eight genes (on eight different chromosomes) known to affect the production of the enzymes responsible for the repair of DNA in sunlight-damaged skin cells. There are eight subtypes of XP that vary somewhat in the severity of symptoms depending on which of the eight genes has been affected by a mutation. The most common subtype, known as XPA, is also the type most likely to be associated with neurologic symptoms.
A child must inherit a defective gene from both parents in order to develop XP. A person who has only one copy of the gene mutation is called a carrier. When both parents are carriers of a recessive gene mutation, there is a 25 percent chance that a child will inherit two mutations and develop XP.
In addition to the symptoms that have already been described, children with xeroderma pigmentosum may also have:
- Raw patches of broken skin oozing tissue fluid
- Premature aging of the lips, eyes, mouth, and tongue
- Cancer developing on the tip of the tongue
- Limited growth of body hair on the chest and legs
- Increased vulnerability to infectious diseases
The diagnosis of xeroderma pigmentosa is usually made during the baby's first or second year of life, often following a history of severe and long-lasting sunburn following only brief exposure to sunlight, or heavy freckling on the baby's face. There is usually no family history of the disorder. The doctor will, however, ask the parents about a family history of intermarriage among relatives in the extended families on both sides, as kinship marriages are a risk factor for genetic disorders.
The doctor will examine the baby for signs of eye problems, including clouding of the cornea (the transparent covering of the front of the eyeball that admits light) and skin tumors growing near the eyes. The baby's reflexes and coordination may also be tested. The diagnosis of XP can be confirmed by genetic testing or by tests on skin cells that
evaluate whether the cells are unusually sensitive to ultraviolet light. These tests can be performed only in highly specialized laboratories, however.
XP can be diagnosed before birth by amniocentesis or by chorionic villus sampling (CVS). CVS is a test in which the doctor takes a small sample of tissue from the placenta, the organ that forms inside the uterus during pregnancy and links the mother's blood supply with the baby's blood supply. The tissue sample can then be analyzed for evidence of a genetic disorder.
There is no cure for xeroderma pigmentosum. In addition, any damage that occurs to the skin from sun exposure is permanent and irreversible. The main goals of treatment are therefore to protect the patient from sun exposure and monitor the development of symptoms affecting the eyes and nervous system.
Avoiding sun exposure includes:
- Wearing protective clothing (long-sleeved shirts and full-length pants, shirts with collars, tightly-woven fabrics that don't let light through); wide-brimmed hats; and eyewear specifically made to screen out ultraviolet rays.
- Applying sunscreens of SPF 30 or higher to all exposed areas of skin.
- Avoiding going outdoors during daylight hours as much as possible.
- Limiting outdoor activities to nighttime only.
- Avoiding the use of halogen or fluorescent bulbs indoors; these give off enough ultraviolet radiation to affect people with XP.
- Applying special film that blocks ultraviolet rays to house and car windows.
Monitoring patients with xeroderma pigmentosum includes:
- Regular visits to a dermatologist—at least every three to five months—to check for changes in the skin or early signs of skin cancer.
- Yearly testing of the patient's nervous system.
- Frequent visits to an ophthalmologist (eye specialist) to check for clouding of the cornea, tumors near the eye, or other problems.
Skin cancers are usually removed by surgery as soon as they appear. Solar keratoses are treated with a cream containing a drug called 5-fluorouracil.
The prognosis of xeroderma pigmentosum depends on the subtype that the patient belongs to; however, all patients with XP have over a thousand times greater risk of developing skin cancer than people without the disorder. Those with subtype XPA typically develop skin cancer before they are ten years old. Only 40 percent of people with XP live into their twenties. A small percentage of patients with milder symptoms live into middle age. In general, XP reduces a patient's life expectancy by at least thirty years.
Parents of a child with XP have a 25 percent chance of having another child with the disorder in later pregnancies. They should consult a genetic counselor before considering having other children. In addition, people who have a relative with XP should consider having a genetic test before starting a family.
Researchers are trying to better understand the gene mutations responsible for XP and to see whether there are any other genes associated with the condition that have not yet been identified. Treatments being investigated in 2008 included the use of a drug called isotretinoin in preventing skin cancer in patients with XP and a lotion called T4N5 liposome lotion in protecting against skin damage caused by ultraviolet radiation.
SEE ALSO Skin cancer; Sunburn
WORDS TO KNOW
Chorionic villus sampling (CVS): A prenatal test that involves taking a small sample of the placenta, the organ that forms inside the uterus during pregnancy and supplies the baby with oxygen and nutrients carried by the blood.
Dermatologist: A doctor who specializes in diagnosing and treating skin disorders.
Neurologic: Pertaining to the nervous system.
Nucleotide excision repair (NER): A mechanism that allows cells to remove damage caused by ultraviolet light to the cell's DNA.
Solar keratoses (singular, keratosis): Rough scaly patches that appear on sun-damaged skin. They are considered precancerous.
Foderaro, Lisa J. “Hudson Journal: Where Daylight's a Risk, Dark Is a Time to Shine.” New York Times, July 21, 2002. Available online at http://query.nytimes.com/gst/fullpage.html?res=9C01E1D91638F932A15754C0A9649C8B63&sec=&spon=&pagewanted=all (accessed August 7, 2008). This news report is about Camp Sundown, a special outdoor camp in upstate New York for people with XP.
Cancer Net. Xeroderma Pigmentosum. Available online at http://www.cancer.net/patient/Cancer+Types/Xeroderma+Pigmentosa#mainContent%20idmain% Content (updated October 2006; accessed August 7, 2008).
DermNet NZ. Xeroderma Pigmentosum. Available online at http://dermnetnz.org/systemic/xeroderma-pigmentosum.html (updated March 18, 2008; accessed August 7, 2008).
National Organization for Rare Disorders (NORD). Xeroderma Pigmentosum.
Available online at http://www.rarediseases.org/search/rdbdetail_abstract.html?disname=Xeroderma%20Pigmentosum (accessed August 7, 2008).
Sloan Science and Film. XP. Available online at http://www.scienceandfilm.org/films.php?film_id=18 (film made in 2002; accessed August 7, 2008). This 11-minute movie was made by David Barba about a young boy with XP. Barba was moved to make the short movie after reading an article about children with XP.
Xeroderma Pigmentosum Society (XPS). XPS Tips for Students. Available online at http://www.xps.org/student_tips.htm (accessed August 7, 2008).
"Genetic: Xeroderma Pigmentosum." UXL Encyclopedia of Diseases and Disorders. . Encyclopedia.com. (September 21, 2018). http://www.encyclopedia.com/medicine/encyclopedias-almanacs-transcripts-and-maps/genetic-xeroderma-pigmentosum
"Genetic: Xeroderma Pigmentosum." UXL Encyclopedia of Diseases and Disorders. . Retrieved September 21, 2018 from Encyclopedia.com: http://www.encyclopedia.com/medicine/encyclopedias-almanacs-transcripts-and-maps/genetic-xeroderma-pigmentosum