Stable low molecular weight DNA in xeroderma pigmentosum cells

Xeroderma pigmentosum (XP) cells from several complementation groups contained more low molecular weight DNA upon alkaline sucrose gradient centrifugation than did other human cells examined. Under conditions in which only 5% of the DNA in normal cells sedimented at 16 S or less, 20% of the DNA in XP cells from complementation group A sedimented at 16 S or less. Because cells were layered directly onto the gradients for lysing of cells and denaturing of the DNA, it appears that this low molecular weight material is due to naturally existing gaps or alkali-sensitive sites, or both, in the cellular DNA. The increase in low molecular weight DNA seen in XP complementation group A cells also is seen in complementation groups C, D, and E. When prelabeled cells were incubated for increasing times after removal of the radioactive label, the amount of low molecular weight material remained constant over a 3-hr period. The introduction of the DNA-damaging agent, bleomycin, to prelabeled XP cells produced a surprising effect. The normal response of human cells to bleomycin is an increase in low molecular weight DNA, dependent on the dose of the drug and time of treatment. In XP cells the reverse was observed. That is, the low molecular weight DNA observed in untreated XP cells disappeared upon addition of the drug. The process responsible for the unusual response of XP cells to bleomycin is unknown, but these results are compatible with an inducible repair process.