Types

The classification of Xeroderma pigmentosum was developed from complementation groups. For this purpose, connective tissue cells (fibroblasts) from different XP patients were combined. If the DNA repair defect persisted after the fibroblast fusion, the patients belonged to the same XP type.

However, if the DNA repair defect no longer existed, the patients suffered from different types of disease. This classification was later confirmed by genetic analyses. In some types of XP, the genetic defect can also be diagnosed by direct gene transfer.

Currently, this routine genetic analysis is only available for the XPA gene, for the remaining types we are working on the development. The types (A-G) differ in age, frequency, severity of the disease and type of tumors caused by UV radiation. Some types (A,B,F and G) may also be associated with neurological disorders.

Type A: early age of disease; very high photosensitivity to light (photosensitivity); skin tumor: spinocellular carcinoma; function of the defective gene: detection of damaged DNA; common in Japan, associated with DeSanctis cacchione syndrome
Type B: very high photosensitivity; function of the defective gene: separation of the DNA double strand into single strands (enzyme=helicase); transition syndrome of Xeroderma pigmentosum and Cockayne syndrome
Type C: high to very high photosensitivity; skin tumor: spinocellular carcinoma, basal cell carcinoma; function of the defective gene: detection of damaged DNA
Type D: high photosensitivity; skin tumor: malignant melanoma; function of the defective gene: helicase; transitional syndrome of XP and Cockayne syndrome, trichothiodystrophy
Type E: late disease age, increased photosensitivity; skin tumor: basal cell carcinoma; function of the defective gene: detection of damaged DNA
Type F: high photosensitivity; function of the defective gene: DNA cleavage (endonuclease)
Type G: high photosensitivity; function of the defective gene: endonuclease, transitional syndrome of Xeroderma pigmentosum and Cockayne syndrome
Variant: late age of the disease, increased photosensitivity; skin tumor: basal cell carcinoma, function of the defective gene: structure of the DNA (DNA polymerase), better course than the other types