Clonal chromosome rearrangements in a fibroblast strain from a patient affected by xeroderma pigmentosum (complementation group C)

Chromosomal fragility in the cancer-prone disease xeroderma pigmentosum preferentially involves bands relevant for cutaneous carcinogenesis

Spontaneous and folate-induced chromosomal fragility was analyzed in peripheral blood lymphocytes from 6 patients affected by the cancer-prone disease xeroderma pigmentosum (XP), from the parents of 4 of the patients, and from 12 normal subjects. All XP patients were defective in nucleotide-excision repair; 4 belonged to group C and 1 each to groups A and D. A tendency toward increased spontaneous chromosomal fragility was observed in the XP family members, and lesions indicating substantial chromosomal damage, which were not observed in any healthy donors, were frequently found. The spontaneous lesion sites in lymphocytes from homozygous and heterozygous carriers of XP defects appeared to be significantly associated with those observed in normal skin fibroblasts from the same subjects. These XP spontaneous fragility sites showed a statistically significant association with the rearrangement breakpoints reported in skin pre-tumoral and tumoral lesions from normal and unrelated XP donors. Under conditions of folate deprivation, the chromosomal fragility level, the pattern and the frequency of expression of fragile sites in XP patients and in their parents were similar to normal. However, XP patients generally showed a higher susceptibility to breakage at sites described as mutagen and carcinogen targets.

Chromosomal instability and telomere length variations during the life span of human fibroblast clones

Growth characteristics, karyotype changes, and telomere length variations were analyzed during the life span of 12 anchorage-independent clones isolated from a xeroderma pigmentosum fibroblast strain. After an initial period of comparable active growth, all the clones showed a decline in the growth rate and finally entered a phase of replicative senescence; however, the number of population doublings and the time required to enter senescence varied among the clones. Repeated cytogenetic analyses during culture propagation showed the appearance of chromosome anomalies, mainly telomeric association (tas) and unbalanced translocations. In all the clones the percentage of abnormal mitoses increased with culture passage, but reached different levels (from less than 10% to about 100%). This finding indicates that the replicative block may be associated with differently altered cytogenetic patterns. Specific chromosome arms (5p, 16q, 19q, and 20q) were preferentially involved in tas, suggesting that alterations in chromosome ends may occur which predispose to fusion. In some clones it was possible to demonstrate the origin of marker chromosomes from the evolution of tas. Telomere length analysis by Southern blotting on DNA samples prepared from 7 clones and from the parental cell lines showed that the terminal restriction fragment (TRF) profiles were homogeneous in senescent parental cells and in the clones during the last part of their life in culture, regardless of the degree of karyotype abnormalities. The homogeneity of the TRF profiles supports the hypothesis of a critical telomere length at senescence.

Late induction of human DNA ligase I after UV-C irradiation

We have studied the regulation of DNA ligase I gene expression in UV-C irradiated human primary fibroblasts. An increase of approximately 6-fold both in DNA ligase I messenger and activity levels was observed 24 h after UV treatment, when nucleotide excision repair (NER) is no longer operating. DNA ligase I induction is serum-independent and is controlled mainly by the steady-state level of its mRNA. The activation is a function of the UV dose and occurs at lower doses in cells showing UV hypersensitivity. No increase in replicative DNA polymerase alpha activity was found, indicating that UV induction of DNA ligase I occurs through a pathway that differs from the one causing activation of the replication machinery. These data suggest that DNA ligase I induction could be linked to the repair of DNA damage not removed by NER.

Structural instability of a transmissible end-to-end dicentric chromosome in a xeroderma pigmentosum fibroblast clone

Cytogenetic analysis was performed in a fibroblast clone (XP9UV25) selected for anchorage-independent growth from an XP strain of normal origin and characterized by the presence of clonal chromosome rearrangements. A dicentric chromosome involving the 5p and 16q telomeric regions was observed in XP9UV25 cells at the fifth passage from colony isolation and at successive passages. The specific anomaly was present with increasing frequency (from 22 to 60% of mitoses) during culture propagation, undergoing rearrangements that gave rise to: 1) (5;16) dicentrics with deletions or duplications of the intercentromeric region; 2) homodicentrics for chromosomes 5 or 16, either end-to-end associations or rearranged; and 3) derivative 5p+ and 16q+ monocentric chromosomes. The frequency of other anomalies involving other chromosomes was negligible. These findings represent the first demonstration that a telomeric association leads to a variety of balanced and unbalanced chromosome rearrangements. These rearrangements may result from asymmetric interchanges between sister chromatids, "bridge-breakage-fusion" events during cell division, breakage and reunion of isochromatids, and breakage followed by healing of the ends. The type of anomaly and the sequence of karyotypic changes we observed in the XP9UV25 clone and their mechanisms of origin may be the same as those occurring during transformation from diploidy to aneuploidy in neoplastic cells.

Chromosome rearrangements in normal fibroblasts from xeroderma pigmentosum homozygotes and heterozygotes

Chromosome analysis was carried out in cultured fibroblasts from unaffected skin of five unrelated xeroderma pigmentosum (XP) patients and nine family members. Structural chromosome changes were observed in cultures from all examined individuals. Furthermore, in one XPD patient and in one XPC patient and his parents, cytogenetically abnormal clones were detected. Some of these clones were present starting from the primary explant. This cytogenetic pattern is similar to that observed in an XPC patient previously studied by us. The analysis of breakpoint distribution from clonal and non-clonal chromosome rearrangements showed that some breakpoints were more frequent and common to different families or to different family members although definite evidence of preferential involvement of chromosome bands was not obtained. This investigation indicates that there is a consistent tendency toward chromosome instability in XP mutation carriers. The instability could be related to the multiple chromosome anomalies characterizing skin tumors in XP subjects.

Inducible cellular responses to ultraviolet light irradiation and other mediators of DNA damage in mammalian cells

Both naturally occurring and carcinogen-induced tumors display not only point mutations in cellular oncogenes but also more complex changes in cellular oncogenes and other cellular genes. For this and other reasons, it seems likely that DNA damage in mammalian cells can induce alterations in gene expression that may have both short and long term consequences in the target cell. The purpose of this review is to summarize current available information on inducible responses to UV-irradiation and other mediators of DNA damage in mammalian cells, and to provide some working hypotheses. We have divided these responses into three time frames, immediate (0-12 hours), early (12-48) and late (beyond 48 hours). Immediate responses include the action of DNA repair enzymes, some of which are induced as a consequence of DNA damage, and transient inhibition of DNA synthesis. Within the past few years considerable evidence has accumulated that during this immediate period there is increased expression of certain cellular oncogenes, proteases and proteins whose functions remain to be identified. It is of interest that the expression of some of these genes is also induced by certain growth factors, tumor promoters and heat shock. Alterations in gene expression during the subsequent "early" period (12-48 hrs.) have not been studied in detail, but it is during this period that one can detect increased replication of several types of viruses in cells that harbor these viruses. We have examined in detail the induction of asynchronous polyoma DNA replication (APR) in a rat fibroblast cell line carrying integrated copies of this DNA. We have obtained evidence that UV-irradiation of these cells leads to the synthesis of a 40 kd protein, within the first 1-24 hrs after irradiation, that binds to a specific sequence TGACAACA in the regulatory region of polyoma DNA. We suggest that this protein acts together with other proteins to induce APR and that this serves as a useful model for understanding the mechanisms responsible for amplification of cellular genes, a phenomenon often seen in malignant tumors. Finally, we discuss how the events occurring during the immediate and early periods following DNA damage might lead to late effects in the target cell that are stable and contribute to the genotype and phenotype of some of the progeny of these cells that are destined to become tumor cells.