U.v. induces long-lived DNA breaks in Cockayne's Syndrome and cells from an immunodeficient individual (46BR): defects and disturbance in post incision steps of excision repair

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Abstract
In normal cells exposed to low u.v. doses the several enzymic steps of the excision repair process are closely coupled with the result that DNA gaps are transient and present at such low frequency that it is very difficult to detect them. Cells from a u.v.-sensitive human genetic disorder, Cockayne's Syndrome (CS) and from an immunodeficient individual 46BR, have been examined with respect to their incision capacity after u.v. in the presence and absence of inhibitors of DNA synthesis. We have measured the initial rates of DNA break accumulation in the presence of hydroxyurea and 1-β-D arabinofuranosylcytosine and find that in both these groups the rate is only slightly lower than in normal cells. However, there is a marked difference between u.v. sensitive cells and normal in the accumulation of long-lived DNA breaks in the absence of inhibitors. While in normal cells practically no breaks could be detected, the u.v. sensitive cells accumulated significant numbers of DNA breaks within 15 min of incubation; 46BR cells showed almost the same level of DNA breaks without the inhibitors as with them. In CS break accumulation can be detected in the absence of inhibitors for only a short time after irradiation (∼ 30 min), but less so when deoxyribonucleosides are provided. The spontaneous break accumulation is related to the time elapsed since proteolytic detachment of the cells from monolayer; 24 h after replating CS breaks no longer accumulate in response to u.v. 46BR cells, on the other hand, accumulate breaks even 1 day after replating and express unligated gaps 2 h after irradiation with a relatively low u.v. dose such as 4 Jm −2 . Provision of DNA precursors does not greatly reduce break accumulation. The extremely slow rate of gap sealing in 46BR cells is consistent with the hypothesis that a ligase defect is expressed in these cells. In the absence of inhibitors break accumulation in CS cells often fluctuates between experiments for unaccounted reasons; though 11961 fluctuates less than other CS strains this phenotypic trait helps to confirm its assignation as a Cockayne Syndrome. Spontaneous conditional break accumulation is not restricted to CS cells; fibroblasts from a normal individual also express similar behaviour though their ability to seal repair sites is considerably greater.