The Principles of Clinical Cytogenetics - Extra Materials - Springer

Chromosome Instability 359
Fig. 8. Top: lesions of the face in an XP patient. Note multiple scars of carcinomas and an aged aspect of
the skin. Bottom: multiple basocellular carcinomas on the face of an XP patient. The thick arrow points to a
recent lesion and the thin arrow points to a scar of an old lesion. (Reprinted from ref. 56; Image courtesy of
Daniel Wallach, and used with permission of the Atlas of Genetics and Cytogenetics in Oncology and
Haematology.)
All XP genes have been cloned. With the exception of XP-V, the products of the XP genes are all
involved in different steps of the NER system, a major cellular defense against the carcinogenic
effects of UV exposure (59). Cockayne syndrome and the photosensitive form of trichothiodystrophy,
two other NER-deficiency syndromes, should be considered in differential diagnosis because of the
common feature of extreme sensitive to sunlight shared by these disorders.
Successful treatment for XP using a topical DNA repair enzyme has recently been reported (60).