Psoralens and serendipity: aspects of the genetic toxicology of 8-methoxypsoralen

Inactivation of wild-type and rad mutant Caenorhabditis elegans by 8-methoxypsoralen and near ultraviolet radiation

Survival of wild-type and four radiation-sensitive (rad) mutants of the nematode Caenorhabditis elegans was determined after near-UV irradiation in the presence of 8-methoxypsoralen (8-MOP). Three sets of inactivation profiles were generated for each strain by irradiating synchronous populations of either early embryos, late embryos or first-stage larvae (L1s). Late embryos were consistently the most sensitive. Curiously, none of the four rad mutants were even moderately hypersensitive. Split-dose experiments indicated that DNA-DNA crosslinks were primarily responsible for lethality. Crosslink induction and repair were determined using two different assays. In both cases, little if any repair was observed in wild-type. This lack of repair thus explains why the rad mutants were not hypersensitive to 8-MOP photoinactivation. Since early embryos undergo extensive cell cycling, their resistance to 8-MOP photoinactivation suggests that replication is highly refractory to both monoadducts and crosslinks, as has been demonstrated previously for UV radiation-induced photoproducts (Hartman et al., 1991, Mutat. Res., 255, pp. 163-173).

Sunlight-induced cancer: some new aspects and implications of the xeroderma pigmentosum model

Symptoms of xeroderma pigmentosum, a hereditary disease characterized by accelerated light-induced ageing of the skin, are due to deficiencies in the repair of damaged DNA. Following UV irradiation a high incidence of thioguanine-resistant mutations have been observed, which may be a model for the abnormally high incidence of freckling and benign and malignant transformation observed in these patients. Cells from patients with Cockayne's syndrome and trichothiodystrophy have also been shown to be hypermutable by UV radiation with a similar DNA repair defect, but unlike xeroderma pigmentosum patients they do not experience a higher incidence of skin cancer or freckling. An immunological defect may be a further crucial factor determining the dermatological symptoms of xeroderma pigmentosum patients. Much can be learned about the way normal individuals function from a study of aberrant functioning of individuals with defined genetic deficiencies. In the field of senescence of the skin, the disease xeroderma pigmentosum (XP) has been of particular value, as the exposed skin of these sun-sensitive individuals shows at an early age many of the features of aged sun-exposed skin of normal individuals. These features of XP skin include hyperkeratosis, freckling, benign lesions and malignant tumours, including melanomas. A full discussion of this syndrome has been given by Kraemer et al.

Nucleotide excision repair in Escherichia coli

One of the best-studied DNA repair pathways is nucleotide excision repair, a process consisting of DNA damage recognition, incision, excision, repair resynthesis, and DNA ligation. Escherichia coli has served as a model organism for the study of this process. Recently, many of the proteins that mediate E. coli nucleotide excision have been purified to homogeneity; this had led to a molecular description of this repair pathway. One of the key repair enzymes of this pathway is the UvrABC nuclease complex. The individual subunits of this enzyme cooperate in a complex series of partial reactions to bind to and incise the DNA near a damaged nucleotide. The UvrABC complex displays a remarkable substrate diversity. Defining the structural features of DNA lesions that provide the specificity for damage recognition by the UvrABC complex is of great importance, since it represents a unique form of protein-DNA interaction. Using a number of in vitro assays, researchers have been able to elucidate the action mechanism of the UvrABC nuclease complex. Current research is devoted to understanding how these complex events are mediated within the living cell.

Defect in excision repair alters the mutational specificity of PUVA treatment in the lacI gene of Escherichia coli

The sequences of 152 lacI- mutations obtained following exposure of Escherichia coli UvrB- strain NR3951 to ultraviolet light in the presence of 8-methoxypsoralen (PUVA treatment) were compared to the spectrum of mutation induced by PUVA treatment in a Uvr+ strain, NR3835. Mutations recovered following PUVA treatment of the UvrB- strain were quite different from those recovered in the Uvr+ strain. In addition, they occurred at a restricted number of unique sites. For example, A.T----T.A base substitutions at position 141, minus G frameshifts at positions 586/587/588 and deletions of 15 base-pairs from position 78 to 92 accounted for 50% or more of mutations recovered in each of the above mutational classes. This altered mutational specificity was accompanied by a failure to recover mutations frequently identified following PUVA treatment of the Uvr+ strain. These mutations include spontaneous-hotspot frameshifts involving the gain or loss of a tetramer 5'-CTGG-3' repeated three times at position 620 to 631; and minus A.T base-pair frameshifts recovered at potential T-T crosslink sites. These results indicate that while crosslinks may play a substantial role in the induction of mutation in the Uvr+ strain, they do not contribute substantially to mutagenesis in the UvrB- strain. In addition, the data also suggest that excision repair may not always occur in an error-free manner.

Mutagenesis by 8-methoxypsoralen plus near-UV treatment: analysis of specificity in the lacI gene of Escherichia coli

We have studied the specificity of mutation induced by PUVA treatment in the lacI gene of E. coli. Cells were exposed to near UV (approximately 365 nm) in the presence of 8-methoxypsoralen under conditions yielding about 7% survival and a 10-fold increase in mutation frequency. The cloning and sequencing of 131 mutants recovered following PUVA treatment revealed that almost all classes of mutation including base substitutions, frameshifts and deletions were induced. The distribution of mutations was non-random and a region of the lacI gene was found to be virtually silent for all classes of mutation. Intriguingly, the broad spectrum of mutation is accompanied by the recovery of mutation at two spontaneous hotspots. We observed a 7-fold increase at a frameshift hotspot involving the gain or loss of a tetramer tandemly repeated 3 times at this site and a 23-fold increase at an A:T----G:C transition hotspot located at the +6 mutational spectrum recovered following PUVA treatment was unique and a detailed analysis of the different classes of mutations indicates a role for DNA repair of both monoadducts and cross-links in the production of mutation.

Repair in E. coli of transforming plasmid DNA damaged by psoralen plus near-ultraviolet irradiation

Treatment of DNA with psoralen plus near-ultraviolet irradiation gives rise to both monoadducts and cross-links. We have examined the repair of plasmid NTP16 DNA treated in this way in vitro and then used to transform E. coli. Monoadducts are found to be potentially lethal, and can be repaired by uvr-dependent and recA-dependent pathways. The presence of a related resident plasmid in the transformed cells can enhance the survival of the incoming damaged NTP16 DNA. This effect is not recA-dependent, and a similar effect (designated "resident enhanced repair") has been observed previously with UV-irradiated plasmids of this particular incompatibility group. Removal of unbound psoralen from the plasmid DNA and exposure to further NUV is known to increase the ratio of cross-links to monoadducts, and we demonstrate that such cross-linked plasmid DNA is not readily repaired following transformation. However in the presence of homologous DNA (related resident plasmid) there is evidence for the repair, and hence uptake by the cell, of cross-linked DNA.

Uvr-independent repair of 8-methoxypsoralen crosslinks in Escherichia coli: evidence for a recombinational process

On the basis of survival data, repair of 8-methoxypsoralen DNA crosslinks in Escherichia coli strains lacking a functional uvrABC endonuclease, is shown to require the products of the recA, recB, recF and recN genes. Bacteria, grown under conditions where most cells contain only a single genome, show no evidence of crosslink repair. Similarly, bacteriophage lambda shows evidence of crosslink repair only in SOS-induced cells, and only at multiplicities of infection greater than 1. The requirement for rec+ genes may be partly ascribed to the need for a functional SOS response, but taken together, the results suggest a recombinational step involving a homologous region of DNA may occur during uvr-independent crosslink repair.

Further characterization of repair of 8-methoxypsoralen crosslinks in UV-excision-defective Escherichia coli

Evidence was previously presented for a new pathway for the repair of 8-methoxypsoralen DNA crosslinks. The pathway, which is independent of the uvrA gene but deficient in rep mutants, has now been further characterized and shown to be more active in minimal than in nutrient growth media and to be inhibited by acriflavine. Although crosslink repair is much reduced in recA bacteria, some still occurs as judged by the effect of acriflavine. By the same criterion, crosslink repair occurs in bacteria with point mutations in the uvrA and uvrB genes, in bacteria with a deletion covering the uvrB gene, and in polA uvrA bacteria. Bacteria with insertions rather than point mutations in the uvrA gene, although showing evidence of repair, demonstrated minimal inhibition with acriflavine suggesting the possibility that the uvrA gene product, even if enzymically inactive, might be able to interact with DNA lesions in the presence of acriflavine and prevent crosslink repair. Crosslink repair in E. coli WP2 uvrA is associated with base-pair substitution mutagenesis and may be characterized as an error-prone process. Crosslink repair in uvrA bacteria is reduced but not eliminated by a mutation in the umuC gene.