UV-induced DNA repair synthesis in cells of patients with different forms of xeroderma pigmentosum and of heterozygotes

Five complementation groups in xeroderma pigmentosum

A collaborative study was undertaken to determine the relationship between the three DNA repair complementation groups in xeroderma pigmentosum found at Erasmus University, Rotterdam, and the four groups found at the National Institutes of Health, Bethesda. The results of this study reveal that there are five currently known complementation groups in xeroderma pigmentosum.

The influence of caffeine on cell survival in excision-proficient and excision-deficient xeroderma pigmentosum and normal human cell strains following ultraviolet-light irradiation

A uniform response to UV of four normal cell strains was demonstrated. One excision-proficient xeroderma pigmentosum variant strain (XP7TA) had a wild-type UV response but a second (XP30RO) was more sensitive. An excision-deficient xeroderma pigmentosum strain XP4L0 was substantially more sensitive than wild-type cell strains. A continuous post-irradiation treatment with non-toxic levels of caffeine enhanced the lethal effect of UV light in both xeroderma pigmentosum variant cell strains but not in cells from normal individuals. There was no detectable effect on cells from a xeroderma pigmentosum individual from complementation group A. These results correlate well with observations on the influence of caffeine on post-replication repair in the three classes of cells.

Heterokaryons in the analysis of genes and gene regulation

Cytological and chemical analysis of heterokaryons, the immediate product of cell fusion, offer new possibilities for studying the factors responsible for genetic regulation in eukaryotic cells. In comparison with proliferating cell hybrids the heterokaryon state offers the important advantage that a heterokaryon contains two complete genomes since chromosome loss does not occur, but since segregation and recombination are absent, heterokaryons cannot be used for gene mapping in the same way as proliferating cell hybrids. However, if two cell types carrying different genetic defects are fused the analysis can be used for studies of gene complementation. The biological information obtained with heterokaryons has emphasized the role of the cytoplasm in the control of nuclear activity. When a G1 nucleus is brought into contact with the cytoplasm of an S phase cell the G1 nucleus is stimulated to synthesize DNA. If the nucleus is brought into a mitotic cell, the chromatin of the G1 nucleus is forced to condense into prematurely condensed chromosomes. Inactive nuclei such as the dormant chick erythrocyte nucleus will be stimulated to initiate RNA and DNA synthesis when brought into contact with an active cytoplasm by cell fusion. Specific nuclear proteins have been shown to be responsible for this process of reactivation. Other inactive nuclei such as the nuclei of macrophages and spermatozoa have likewise been shown to be reactivated by fusion with active cells. The degree of activation in all of these cases appears to be determined by the state of the active cell. Inactive nuclei are activated to the same level as the active nucleus but seldom beyond this level. If differentiated cells are fused with undifferentiated cells, usually the differentiated character is lost rapidly after fusion. This observation is in agreement with several studies on proliferating cell hybrids indicating some type of negative control of differentiated properties. In heterokaryons obtained by fusion of cells of a similar type of histotypic differentiation usually coexpression of the differentiated markers is observed.

DNA repair in tumor cells from the variant form of xeroderma pigmentosum

Cells from most patients with xeroderma pigmentosum (XP) can be shown to be defective in repairing ultraviolet (UV) light-induced damage to their DNA, for they have a reduced rate of UV-induced thymidine incorporation. XP variants, however, have clinical manifestations of XP, but all their tissues tested to date have a normal rate of UV-induced 3H-thymidine incorporation. We have now tested tumor cells from an XP variant and from a typical XP patient. The variant's tumor cells, in contrast to those of the typical patient, had no detectable defect in their UV-induced thymidine corporation. We conclude, therefore, that the cells that formed tumors in this XP variant resemble his other cells in DNA repair capacity, and do not represent a minor cell population with the kind of DNA repair defect that is reflected in reduced UV-induced thymidine incorporation.

Postreplication repair of DNA in UV-irradiated mammalian cells

Some current controversies concerning the mechanism of postreplication repair in mammalian cells are discussed. We have found that two xeroderma pigmentosum cell lines normal in excision-repair are abnormal in postreplication repair. The rate of postreplication repair is slower than that in normal cells and is caffeine sensitive. In normal human fibroblasts, caffeine has little effect.

Response of homozygous and heterozygous xeroderma pigmentosum cells to several chemical and viral carcinogens

The topics discussed include the relationship between the carcinogenic property of various compounds and their capacity to elicit DNA damage that results in a DNA repair synthesis; the metabolic activation of precarcinogens; the in vivo use of DNA repair synthesis in the identification of tissues that will give rise to neoplasms; the possible involvement of DNA repair in the sensitivity of human cells to the chromosome-damaging and lethal effect of chemical carcinogens; the different responses of homozygous xeroderma pigmentosum (XP) cells, heterozygous XP cells, and controls to some but not all chemical carcinogens and oncogenic viruses; and the possible link between DNA damage, its repair, and viral-induced neoplastic transformation.

Genetic complementation analysis of xeroderma pigmentosum

To characterize the mutations in different unrelated xeroderma pigmentosum (XP) patients, a complementation test was performed using the technique of somatic cell hybridization. DNA repair following UV exposure was studied in multinuclear cells resulting from fusions between in vitro cultivated cells from different patients. The parental XP cells performed low or negligible levels of repair DNA synthesis, whereas in some combinations binuclear hybrid cells showed repair DNA synthesis as a result of complementation. The results indicate the presence of five different complementation groups in XP.

Xeroderma pigmentosum cells with normal levels of excision repair have a defect in DNA synthesis after UV-irradiation

Cells cultured from most patients suffering from the sunlight-sensitive hereditary disorder xeroderma pigmentosum are defective in the ability to excise ultraviolet light (UV)-induced pyrimidine dimers from their DNA. There is, however, one class of these patients whose cells are completely normal in this excision repair process. We have found that these cells have an abnormality in the manner in which DNA is synthesized after UV-irradiation. The time taken to convert initially low-molecular-weight DNA synthesized in UV-irradiated cells into high-molecular-weight DNA similar in size to that in untreated cells is much greater in these variants than in normal cells. Furthermore, this slow conversion of low to high-molecular-weight newly synthesized DNA is drastically inhibited by caffeine, which has no effect in normal cells. Two cell lines from classes of xeroderma pigmentosum that are defective in excision-repair show intermediate effects, with regard to both the time taken to convert newly synthesized DNA to high molecular weight and the inhibition of this process by caffeine.

Levels of some enzymes acting on DNA in xeroderma pigmentosum

We have determined the levels of DNA polymerase, DNA ligase, a DNase acting on single-stranded DNA, an endonuclease making single-strand breaks in double - stranded DNA and polynucleotide kinase in fibroblasts obtained from nine normal persons and from nine patients with Xeroderma Pigmentosum; the pathological lines belong to the different described clinical forms and to the three different complementation groups described so far. All the enzymes are present in the normal lines and in the Xeroderma lines. The levels are quite variable, but the values obtained in the pathological lines lie within the ones observed in the normal population.