Xeroderma pigmentosum is a rare inherited genetic disease. People with this condition develop skin and eye cancers at young ages because their DNA is extremely susceptible to damage caused by ultraviolet radiation. Xeroderma (dry, scaly skin) and pigmentosum (freckling and abnormal skin coloring) refer to changes that occur after exposure to sunlight or other ultraviolet radiation.
Xeroderma pigmentosum refers to a group of similar conditions. Each subgroup is designated by a letter or a roman numeral. Xeroderma pigmentosum is also often abbreviated XP. XP A and XP I are the same, as are XP B and XP II, XP C and XP III, etc. There are seven types of xeroderma pigmentosum designated A–G or I–VII. An eighth type of XP is called the "variant" type. XP VIII/XP H was once a separate subgroup; now it known to be part of XP D/XP IV.
Each of the eight types of xeroderma pigmentosum has its own DNA defect. However, each section of DNA affected is involved in the same process. These defects affect the body's ability to repair DNA damage, especially DNA damage to the skin caused by exposure to ultraviolet radiation. Sunlight is the most common source of ultraviolet radiation. Everyone's DNA is damaged when it is exposed to sunlight. However, the body has complex and very effective methods to repair the DNA damage. This repair mechanism does not work properly in people with xeroderma pigmentosum. They quickly accumulate damage to their DNA if they are exposed to ultraviolet radiation. Cumulative DNA damage leads to cancer, especially of the skin and the eyes.
DeSanctis-Cacchione syndrome refers to the combination of xeroderma pigmentosum along with mental retardation, short stature, and other symptoms. Trichothiodystrophy (TTD) is sometimes caused by the same DNA change that causes XP D, and rarely XP B. People with TTD also have brittle hair and nails, and physical and mental retardation.