Xeroderma pigmentosum (XP) was first described in 1874 by Hebra and Kaposi. In 1882, Kaposi coined the term xeroderma pigmentosum for the condition, referring to its characteristic dry, pigmented skin. Xeroderma pigmentosum is a rare disorder transmitted in an autosomal recessive manner.[1] It is characterized by photosensitivity, pigmentary changes, premature skin aging, and malignant tumor development.[2] These manifestations are due to a cellular hypersensitivity to ultraviolet (UV) radiation resulting from a defect in DNA repair.[3]
The basic defect in xeroderma pigmentosum is in nucleotide excision repair (NER), leading to deficient repair of DNA damaged by UV radiation.[4] This extensively studied process consists of the removal and the replacement of damaged DNA with new DNA. Two types of NER exist: global genome (GG-NER) and transcription coupled (TC-NER).[5] The last decade has seen the cloning of the key elements of NER, and the process has been reconstituted in vitro.
Seven xeroderma pigmentosum repair genes, XPA through XPG, have been identified. These genes play key roles in GG-NER and TC-NER. Both forms of NER include a damage-sensing phase, performed in GG-NER by the product of the XPC gene complexed to another factor. In addition, the XPA gene product has been reported to have an affinity for damaged DNA. Therefore, XPA likely also plays a role in the damage-sensing phase.
Following detection of DNA damage, an open complex is formed. The XPG gene product is required for the open complex formation. The XPB and XPD gene products are part of a 9-subunit protein complex (TFIIH) that is also needed for the open complex formation. Subsequently, the damaged DNA is removed. The XPG and XPF genes encode endonucleases; however, the XPF gene product functions as an endonuclease when complexed to another protein. The resulting gap is filled in with new DNA by the action of polymerases.
A xeroderma pigmentosum variant has also been described. The defect in this condition is not in NER, but is instead in postreplication repair. In the xeroderma pigmentosum variant, a mutation occurs in DNA polymerase η.[6, 7]
Seven complementation groups, XPA through XPG, corresponding to defects in the corresponding gene products of XPA through XPG genes, have been described. These entities occur with different frequencies (eg, XPA is relatively common, whereas XPE is fairly rare), and they differ with respect to disease severity (eg, XPG is severe, whereas XPF is mild) and clinical features. Cockayne syndrome can rarely occur with XPB, XPD, and XPG.[8]
The continued presence of repair proteins at sites of DNA damage may also contribute to the pathogenesis of cutaneous cancer, as has been shown in XPD.[9]
In addition to the defects in the repair genes, UV-B radiation also has immunosuppressive effects that may be involved in the pathogenesis of xeroderma pigmentosum.[10] Although typical symptoms of immune deficiency, such as multiple infections, are not usually observed in patients with xeroderma pigmentosum, several immunologic abnormalities have been described in the skin of patients with xeroderma pigmentosum. Clinical studies of the skin of patients with xeroderma pigmentosum indicate prominent depletion of Langerhans cells induced by UV radiation. Various other defects in cell-mediated immunity have been reported in xeroderma pigmentosum. These defects include impaired cutaneous responses to recall antigens, decreased ratio of circulating T-helper cells to suppressor cells, impaired lymphocyte proliferative responses to mitogen, impaired production of interferon in lymphocytes, and reduced natural killer cell activity.
In addition to their role in DNA repair, xeroderma pigmentosum proteins also have additional functions. For example, Fréchet et al[11] have shown that matrix metalloproteinase 1 is overexpressed in dermal fibroblasts from patients with XPC.[12] They also demonstrated accumulation of reactive oxygen species in these fibroblasts in the absence of exposure to UV. They concluded that the XPC protein has roles in addition to NER. Matrix metalloproteinase 1 overexpression has been shown to occur in both aging of skin and carcinogenesis.
XPG has been shown to form a stable complex with the transcription factor TFIIH, as mentioned above. Some manifestations of XPG/Cockayne syndrome in patients may therefore be due to abnormal transcription.[13]
With respect to neurodegeneration seen in some cases of xeroderma pigmentosum, it may be associated with TC-NER rather than GG-NER.[14]
The frequency in the United States is approximately 1 case per 250,000 population. Group XPC is the most common form in the United States.
International
The frequency in Europe is approximately 1 case per 250,000 population. In Japan, it is higher, 1 case per 40,000 population. Groups XPA and XPC are the most common. Group XPA is the most common form in Japan.[15]
Race
Cases of xeroderma pigmentosum are reported in persons of all races.
Sex
An equal prevalence has been reported in males and females.
Less than 40% of patients survive beyond age 20 years. Individuals with milder disease may survive beyond middle age. Individuals with this disease develop multiple cutaneous neoplasms at a young age. Two important causes of mortality are metastatic malignant melanoma and squamous cell carcinoma.[16] Patients younger than 20 years have a 1000-fold increase in the incidence of nonmelanoma skin cancer and melanoma. The mean patient age of skin cancer is 8 years in patients with xeroderma pigmentosum, compared with 60 years in the healthy population. Actinic damage occurs between ages 1 and 2 years.
Constant education of the patient is the most important objective in the management of xeroderma pigmentosum. The need for adequate solar protection cannot be overemphasized and should be reinforced at every visit.
Sunblocks should be used, even in winter months and during evening and early morning hours. The exposed surfaces of the skin should be shielded with protective, double-layered clothing and broad-brimmed hats. The eyes should be shielded with UV-absorbing sunglasses with side shields.
Even unlikely sources of illumination can prove hazardous and should be pointed out to patients; for example, fluorescent lights that emit radiation below 320 nm can be dangerous.
The Xeroderma Pigmentosum Society provides information for individuals who are affected, their families, and the public. It also provides peer support for patients and their families.
A history of severe persistent sunburn can be found in many patients. The history should focus on the relationship of the eruption to sun exposure, with a careful determination of its time course and morphology.[17]
As with most autosomal recessive disorders, usually no family history is present; the parents, being heterozygotes, are healthy. Additionally, a history of consanguinity may be elicited.
The disease typically passes through 3 stages.[18] The skin is healthy at birth. Typically, the first stage appears after age 6 months. This stage is characterized by diffuse erythema, scaling, and frecklelike areas of increased pigmentation (see following image). These findings, as would be expected from the pathophysiologic basis for the disease, are seen over light-exposed areas, appearing initially on the face. With progression of the disease, the skin changes appear on the lower legs, the neck, and even the trunk in extreme cases. While these features tend to diminish during the winter months with decreased sun exposure, as time passes, these findings become permanent.
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Face of a toddler with xeroderma pigmentosum, representative of an early stage of the disease. Note the freckling and the scaling. Courtesy of Neil S.....
The second stage is characterized by poikiloderma. Poikiloderma consists of skin atrophy, telangiectasias, and mottled hyperpigmentation and hypopigmentation, giving rise to an appearance similar to that of chronic radiodermatitis (see following image). Although telangiectasias also occur in the sun-exposed areas, they have been reported to arise in unexposed skin and even buccal mucosa.
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Back of an adolescent with xeroderma pigmentosum, representing a later stage of the disease. Note the mottled hyperpigmentation and atrophy. Courtesy ....
The third stage is heralded by the appearance of numerous malignancies, including squamous cell carcinomas, malignant melanoma, basal cell carcinoma,[19] and fibrosarcoma. These malignancies may occur as early as age 4-5 years and are more prevalent in sun-exposed areas.
Photosensitivity should be suspected and evaluated in any patient with intermittent or persistent abnormalities on light-exposed areas. Photosensitivity in xeroderma pigmentosum is variable, but it generally occurs in the range of 290-320 nm. The minimal erythema dose is lower than normal at most wavelengths. In xeroderma pigmentosum, the photosensitivity is acute in nature. The action spectrum for elicitation of the photosensitivity may be suggested by the seasonal or diurnal variability of the eruption and by any protective effect of window glass or sunscreens.
Ocular problems[20] occur in nearly 80% of individuals with xeroderma pigmentosum. The initial problems include photophobia and conjunctivitis. Eyelid solar lentigines occur during the first decade of life, and they might transform into malignant melanoma. Ectropion, symblepharon with ulceration, repeated conjunctival inflammation, infections, and scarring might develop in these patients. In addition, vascular pterygia; fibrovascular pannus of the cornea; and epitheliomas of the lids, the conjunctivae, and the cornea can occur. Finally, the propensity for malignancies, such as squamous cell carcinoma, basal cell carcinoma, sebaceous cell carcinoma, and fibrosarcoma, can also involve the eyes of patients with xeroderma pigmentosum.
Neurologic problems[20] are seen in nearly 20% of patients with xeroderma pigmentosum, more commonly in groups XPA and XPD. The severity of these problems is proportional to the sensitivity of xeroderma pigmentosum fibroblasts to UV radiation. The problems include microcephaly, spasticity, hyporeflexia or areflexia, ataxia, chorea, motor neuron signs or segmental demyelination, sensorineural deafness, supranuclear ophthalmoplegia, and mental retardation. The neurologic problems might overshadow the cutaneous manifestations in some patients with xeroderma pigmentosum. De Sanctis-Cacchione syndrome refers to the combination of xeroderma pigmentosum and neurologic abnormalities (including mental retardation and cerebellar ataxia), hypogonadism, and dwarfism.
The Medscape Reference article Neurologic Manifestations of Xeroderma Pigmentosum may be of interest.
Multiple cutaneous neoplasms develop at a young age in persons with xeroderma pigmentosum. Death is usually caused by metastatic malignant melanoma or squamous cell carcinoma.
Patients with xeroderma pigmentosum are also susceptible to infection and, in some subtypes, neurologic complications.
No consistent routine laboratory abnormalities are present in xeroderma pigmentosum patients. The diagnosis of xeroderma pigmentosum can be established with studies performed in specialized laboratories. These studies include cellular hypersensitivity to UV radiation and chromosomal breakage studies, complementation studies, and gene sequencing to identify the specific gene complementation group.
In the cellular hypersensitivity to UV radiation and chromosomal breakage studies, the xeroderma pigmentosum fibroblasts are stressed with different doses of UV radiation. Then, chromosomal breakage is evaluated in at least 100-200 cells, with at least 2 replicates for each dose. The cells from the patient are compared with those from the patient's parents (if possible, as they are obligate heterozygotes for xeroderma pigmentosum). Cells from unrelated healthy individuals are used as controls. To eliminate subjectivity, the person evaluating the chromosomal abnormalities is not informed as to which group the slides being examined belong. Prenatal diagnosis of xeroderma pigmentosum can be accomplished using similar chromosomal breakage studies on amniocytes from at-risk fetuses.
The xeroderma pigmentosum complementation groups can be determined using cell-fusion techniques followed by assessment of DNA repair or by gene sequencing.
Prenatal diagnosis is possible by amniocentesis or chorionic villi sampling. Unscheduled DNA synthesis is the classic method for diagnosis. A modified technique using cultivation of both patient and control cells at the same time has also been described.[21] A faster technique is the alkaline comet assay (single-cell gel electrophoresis assay). This method, in addition to being faster, requires fewer cells and does not require radioactivity.[22]
The histologic findings of the first stage of the disease include hyperkeratosis and increased melanin pigment (this corresponds to the clinical freckling) in the basal cell layer (not necessarily accompanied by an increase in the numbers of melanocytes). Some rete ridges may be elongated, whereas other rete ridges may be atrophic. These findings may be accompanied by a chronic inflammatory infiltrate in the upper dermis.
In the second stage, atrophy ensues, and the hyperkeratosis and the hyperpigmentation are more marked. Telangiectasia may be prominent. These findings correspond to poikiloderma. In addition, the epidermis may exhibit architectural disorder and atypia, and the dermis may be elastotic. Therefore, the histologic picture might be indistinguishable from that of actinic keratosis (see following image). The histologic appearances of the various tumors that complicate xeroderma pigmentosum are seen in the third stage of xeroderma pigmentosum.
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Histologic features of actinic keratosis in an individual with xeroderma pigmentosum. Note the atypia of the keratinocytes and the parakeratosis.
The goal of treatment is to protect the patient from sunlight. To this end, regular visits to the dermatologist might be necessary for the purposes of patient education and early detection and treatment of any malignancies.
The use of sunscreens in conjunction with other sun-avoidance methods (eg, protective clothing, hats, eyewear) can minimize UV-induced damage in patients with xeroderma pigmentosum. Sunscreens should be applied to all exposed surfaces (including the hands, the back of the neck, the ears, the lower lips, and the anterior part of the chest) whenever UV exposure is expected. The 2 basic types of sunscreens are physical and chemical.
Physical sunscreens scatter and reflect radiation. They contain large particles, such as titanium dioxide, zinc oxide, red ferric oxide, talc, and kaolin. Physical sunscreens block UV rays, infrared rays, and visible light. Their main disadvantage is that most are opaque, making them less cosmetically acceptable. New advances in physical sunscreens include microfine particles of titanium dioxide or zinc oxide, which are transparent.
Chemical sunscreens absorb UV radiation. Para-amino benzoic acid (PABA) was the first agent developed, but its potential to cause allergic reactions has limited its use. Some agents, such as benzophenones, mainly block UV-A, but they are weak UV-B photoprotectors. Avobenzone (Parsol 1789) has been introduced commercially in the United States. It is a much stronger UV-A photoprotector. Other agents, such as PABA esters, salicylates, and cinnamates, mainly block UV-B. Broad-spectrum chemical sunscreens include a combination of ingredients designed to block both UV-B and UV-A. Many of the new preparations are also designed to be water resistant.
No matter which sunscreen is used, the degree of protection is only partial. The effectiveness of sunscreens is expressed as a sun protection factor (SPF). The SPF is the ratio of the least amount of UV radiation required to produce a minimum erythema reaction with a sunscreen to the amount of the energy required to produce the same erythema without any sunscreen. Usually, sunscreens with a SPF of 15 or greater are recommended.
The Medscape Reference article Sunscreens and Photoprotection provides a detailed discussion of these agents.
Oral retinoids have been shown to decrease the incidence of skin cancer in patients with xeroderma pigmentosum.[23] This therapy is limited by dose-related irreversible calcification of ligaments and tendons.
Chemical therapy with 5-fluorouracil may be useful for actinic keratoses. Giannotti et al[24] suggested in a case report that topical treatment with imiquimod and acitretin is an alternative to surgery. They prescribed imiquimod 5% cream to be applied 3 times weekly in combination with oral acitretin (20 mg/d) for 4-6 weeks. No adverse events were reported during treatment, and the tumors had resolved at the 6-month follow-up visit.
A new approach to photoprotection is to repair DNA damage after UV exposure. This can be accomplished by delivery of a DNA repair enzyme into the skin by means of specially engineered liposomes. T4 endonuclease V has been shown to repair cyclobutane pyrimidine dimers resulting from DNA damage.[10]
Yarosh et al studied in a randomized fashion the effect of topically applied T4 endonuclease V in liposomes in xeroderma pigmentosum patients. Thirty patients were enrolled in this prospective double-blinded study. The annualized rate of new actinic keratoses was 8.2% among the patients assigned T4N5 liposome lotion and 25.9% among those assigned placebo. For basal cell carcinoma, the annualized rates of new lesions were 3.8% in the treatment group and 5.4% in the placebo group. No significant adverse effects were found among any of the patients. The topical application of DNA repair enzymes to the sun-damaged skin of patients with xeroderma pigmentosum lowered the rate of development of 2 forms of these lesions during 1 year of treatment.
Gene therapy for xeroderma pigmentosum is still in a theoretical and experimental stage. Various methods of correcting the defects in xeroderma pigmentosum have been attempted in vitro and in animal studies using viral vectors (adenoviruses and retroviruses) carrying the gene replacement products. Ex vivo skin gene therapy, which refers to grafting skin that has the genetic defect corrected, may be useful in xeroderma pigmentosum in the future.
Consultation with an ophthalmologist is recommended because of the ocular problems associated with xeroderma pigmentosum. The use of UV-absorbing sunglasses should be included as part of the ocular management in patients with this disease. Artificial tears might be used. If corneal opacities supervene, corneal transplants can be performed.
Consultation with a neurologist is also recommended because neurologic abnormalities are seen in 20% of patients with xeroderma pigmentosum.
Patients should receive follow-up care every 3 months. Follow-up care should be focused on educating the patient and the patient's parents about effective sun protection and early recognition of skin cancer.
Genetic counseling should be offered for families at risk. Antenatal diagnosis is possible by amniocentesis or chorionic villi sampling.
Clinical Context:
Isotretinoin is a synthetic 13-cis isomer of the naturally occurring tretinoin (trans- retinoic acid). It is used orally for the treatment of serious dermatologic conditions.
These agents may prevent some of the neoplasms in xeroderma pigmentosum. They decrease the cohesiveness of abnormal hyperproliferative keratinocytes, and they may reduce the potential for malignant degeneration. They modulate keratinocyte differentiation. They have been shown to reduce the risk of skin cancer formation in patients who have undergone renal transplantation.
Linda J Fromm, MD, MA, FAAD, Private Practice, Fromm Dermatology at Health Concepts, Rapid City, South Dakota
Disclosure: Nothing to disclose.
Specialty Editors
Richard P Vinson, MD, Assistant Clinical Professor, Department of Dermatology, Texas Tech University Health Sciences Center, Paul L Foster School of Medicine; Consulting Staff, Mountain View Dermatology, PA
Disclosure: Nothing to disclose.
Jeffrey J Miller, MD, Associate Professor of Dermatology, Pennsylvania State University College of Medicine; Staff Dermatologist, Pennsylvania State Milton S Hershey Medical Center
Disclosure: Nothing to disclose.
Chief Editor
William D James, MD, Paul R Gross Professor of Dermatology, Vice-Chairman, Residency Program Director, Department of Dermatology, University of Pennsylvania School of Medicine
Disclosure: Received income in an amount equal to or greater than $250 from: Elsevier; WebMD.
Additional Contributors
A Hafeez Diwan, MD, PhD, Associate Professor, Department of Pathology, University of Texas MD Anderson Cancer Center
Disclosure: Nothing to disclose.
Craig A Elmets, MD, Professor and Chair, Department of Dermatology, Director, Chemoprevention Program Director, Comprehensive Cancer Center, UAB Skin Diseases Research Center, University of Alabama at Birmingham School of Medicine
Disclosure: Serve(d) as a director, officer, partner, employee, advisor, consultant or trustee for: University of Alabama at Birmingham; University of Alabama Health Services Foundation<br/>Serve(d) as a speaker or a member of a speakers bureau for: Ferndale Laboratories<br/>Received research grant from: NIH, Veterans Administration, California Grape Assn<br/>Received consulting fee from Astellas for review panel membership; Received salary from Massachusetts Medical Society for employment; Received salary from UpToDate for employment. for: Astellas.
Acknowledgements
The authors and editors of Medscape Reference gratefully acknowledge the contributions of previous author Marcelo Horenstein, MD, to the development and writing of this article.
Face of a toddler with xeroderma pigmentosum, representative of an early stage of the disease. Note the freckling and the scaling. Courtesy of Neil S. Prose, MD, Duke University Medical Center, Durham, North Carolina.
Back of an adolescent with xeroderma pigmentosum, representing a later stage of the disease. Note the mottled hyperpigmentation and atrophy. Courtesy of Neil S. Prose, MD, Duke University Medical Center, Durham, North Carolina.
Histologic features of actinic keratosis in an individual with xeroderma pigmentosum. Note the atypia of the keratinocytes and the parakeratosis.
Face of a toddler with xeroderma pigmentosum, representative of an early stage of the disease. Note the freckling and the scaling. Courtesy of Neil S. Prose, MD, Duke University Medical Center, Durham, North Carolina.
Back of an adolescent with xeroderma pigmentosum, representing a later stage of the disease. Note the mottled hyperpigmentation and atrophy. Courtesy of Neil S. Prose, MD, Duke University Medical Center, Durham, North Carolina.
Histologic features of actinic keratosis in an individual with xeroderma pigmentosum. Note the atypia of the keratinocytes and the parakeratosis.
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