Chromosome-breakage syndromes: different genes, different treatments, different cancers.

Comparison of the strikingly different distributions of types of cancer that occur in the genetic disorders that feature chromosome instability raises several interesting points. (a) Bloom's syndrome: the distribution suggests that many of the cancers that occur with regularity in the general population just occur more commonly and at an earlier age. (b) Ataxia telangiectasia: cancers of many types are increased in frequency, but lymphoreticular cancers are exceptionally common, the case also in several other genetically determined immunodeficiency disorders. Both Bloom's syndrome and ataxia telangiectasia share defective immunity as a major clinical feature, but the respective roles, if any, of it and of chromosome instability in producing the cancer predispositions are unknown. (c) Fanconi's anemia: cancer apparently has become common only recently. The types and distribution which occur are unusual. Fanconi's anemia cells have been shown to be hypertransformable by oncogenic virus and to be defective in handling certain types of DNA damage (as well as to manifest chromosome instability) so that the recent increase in cancer incidence is both surprising and unexplained. The degree of cancer proneness of Fanconi's anemia per se, untreated by modern methods, must at present be considered unknown. (d) Xeroderma pigmentosum: the cancer predisposition apparently extends only to cells which receive solar damage, i.e., to skin and eye. This would not have been predicted in view of the fact that the cellular mechanism is defective for repairing DNA damage produced not just by sunlight but also by certain classes of chemical carcinogens.