Reply to letter by J. H. Robbins

Abstract

We are grateful to Dr. Robbins for his comments on our recent paper (Johnson et al. 1989), and for bringing to our attention reports of the complete complementation of unscheduled DNA synthesis (UDS) in heterokaryons formed between fibroblasts from xeroderma pigmentosum (XP) complementation group H (XPH) and other XP complementation groups. We regret our previous ignorance of these data (Fujiwara and Satoh 1985; Mamada et al. 1988; Ichihashi et al. 1988) and should have quoted them. As Dr. Robbins points out in his letter, the original report (Moshell et al. 1983) lacked quantitative data on complementation of UDS between XPH and XPD fibroblasts. They did, however, show that complementation for UDS between XPB and XPH cells was less than complete. We cannot explain why the levels of UDS achieved in hybrids between HD2 and different fibroblasts, especially from groups E and F, should appear relatively low, but we state clearly that a degree of complementation is expressed in these cases, as it is when the partner is XPH or XPD. It is also important to reiterate that the UDS levels we see are those in hybrid nuclei, not in the independent nuclei of recent heterokaryon constructs. There is very considerable variation in the capacity for UDS among the different XPD mutations (Robbins et al. 1974; Pawsey et al. 1979; Paterson 1982; Fischer et al. 1982). In no case that we know of is this variation associated with significant removal of photoproducts (Zelle and Lohmann 1979; Paterson 1982; Mitchell 1988). To rely on UDS as a single diagnostic measure of an XPD mutation could be hazardous. The availability of a permanent repair-defective cell line permits backcrosses to be made to the parental and other mutant cells. Hybrids so derived can be tested for the expression of complementation much more extensively than by the shortterm assay of UDS alone. In particular, hybrids allow the restoration of viability to be assessed. At the risk of tedium we reiterate that UDS is a single and possibly misleading assay for competence in nucleotide excision repair, and we suggest that future studies of complementation are likely to include a more wide-ranging assessment of repair restitution in XP crosses. The availability of immortal cell lines from most of the XP complementation groups will make this task possible and probably obligatory. It is always difficult, and rightly so, to provide sufficient evidence to convince sceptics about a negative finding. Direct evidence for the suspected XPD and XPH identity, based on molecular probes for the XPD gene (Arrand et al. 1989), will hopefully soon be available. However, until then, evidence obtained from somatic cell hybrids between XPD and XPH cell lines will have to suffice.