Chromosomal instability in cell lines derived from patients with xeroderma pigmentosum

Regulation of chromatin structure via histone post-translational modification and the link to carcinogenesis

The loss of genome integrity contributes to the development of tumors. Although genome instability is associated with virtually all tumor types including both solid and liquid tumors, the aberrant molecular origins that drive this instability are poorly understood. It is now becoming clear that epigenetics and specific histone post-translational modifications (PTMs) have essential roles in maintaining genome stability under normal conditions. A strong relationship exists between aberrant histone PTMs, genome instability, and tumorigenesis. Changes in the genomic location of specific histone PTMs or alterations in the steady-state levels of the PTM are the consequence of imbalances in the enzymes and their activities catalyzing the addition of PTMs ("writers") or removal of PTMs ("erasers"). This review focuses on the misregulation of three specific types of histone PTMs: histone H3 phosphorylation at serines 10 and 28, H4 mono-methylation at lysine 20, and H2B ubiquitination at lysine 120. We discuss the normal regulation of these PTMs by the respective "writers" and "erasers" and the impact of their misregulation on genome stability.

DNA polymerase stalling, sister chromatid recombination and the BRCA genes

Heritable predisposition to breast and/or ovarian cancer is determined, in part, by germline mutation affecting one of two tumor suppressor genes, BRCA1 and BRCA2 (Miki et al., 1994; Wooster et al., 1995). These genes are required for the maintenance of genomic integrity and for control of homologous recombination in somatic and meiotic cells. Here, we explore the hypothesis that a major role of the BRCA gene products in the somatic DNA damage response centers upon the control of recombination between sister chromatids during S phase. By analogy with model organisms, we suggest that stalling of a mammalian DNA polymerase complex by its encounter with abnormal DNA structure calls forth a series of responses that collaborate to enforce appropriate recombinational outcomes, and to suppress inappropriate or 'illegitimate' recombination.

UvrA and UvrB suppress illegitimate recombination: synergistic action with RecQ helicase

Illegitimate recombination is a major cause of genetic instability in prokaryotes as well as in eukaryotes. This recombination usually occurs at a low frequency, but it is greatly enhanced by UV irradiation or other environmental stresses. DNA damages produced by these environmental stresses are thought to induce DNA double-strand breaks, leading to illegitimate recombination. In this paper we show that UV-induced illegitimate recombination is enhanced by mutations of nucleotide excision repair genes, uvrA or uvrB, and partially by uvrC mutation, but not by uvrD mutation. Unexpectedly, the recombination was enhanced by the uvrA uvrB double mutation even without UV irradiation, but the uvrB uvrC double mutation has not shown this effect, suggesting that illegitimate recombination is mostly suppressed by UvrA and UvrB. Moreover, illegitimate recombination was synergistically enhanced by the recQ uvrA double mutation. In addition, overproduction of the UvrA protein suppressed the hyperrecombination phenotype of the recQ or uvrB mutant, but it did not affect the UV-sensitive phenotype of the uvrB mutant. We concluded that the UvrAB complex suppresses illegitimate recombination in a pathway shared with RecQ helicase. In addition, UvrA protein alone can suppress illegitimate recombination in the pathway, in which RecQ helicase and UvrAB complex work. Possible functions of the proteins involved in these pathways are also discussed.

Mutations in RAD3, MSH2, and RAD52 affect the rate of gene amplification in the yeast Saccharomyces cerevisiae

We report here the use of the ADH4:CUP1 amplification detection system to identify five high amplification rate (HAR) strains of Saccharomyces cerevisiae that display 40- to 600-fold higher amplification rates than those of parental strains. We have identified a mutation in RAD3 DNA repair helicase gene in HAR strain B9-40 that results in a 40-fold increase in amplification rate. RAD3 is the functional homolog of the human XPD gene, suggesting that this model system will provide important candidates for genes that affect gene amplification in human cells. Isolation of the HAR strains has allowed us to test whether RAD52, which is essential for recombinational repair of DNA double-strand breaks, is also essential for amplification. Deletion of RAD52 in HAR strains B3-10 and B11-60 decreases amplification approximately 100-fold. In contrast, deletion of MSH2, which increases recombination between sequences with limited similarity, increases the amplification rate about 10-fold. These results suggest that recombination is an important step in amplification.

Cytogenetic studies of a male with sporadic intestinal lymphangiectasia: 45,X/46,XY mosaicism with pseudo- and hyperdiploid subpopulations in cultured tissues

45,X/46,XY mosaicism was found in peripheral blood, bone marrow, and tissue cultures of an adult male with intestinal lymphangiectasia (IL). Turner phenotype was not present; his meiotic metaphase analysis was normak, and his dermatoglyphics resembled those of his family. Ten separate tissue culture lines from three biopsies of skin and thyroid gland contained 45,X cells (14.8 to 78.3%). Autosomal aneuploidy, resulting in pseudo- or hyperdiploidy, was also present in 4.3 to 41.6% of the cells. A hyperdiploid clone with a 47,X,+10,+18 karyotype was found in 22.6% of cells in one line. A second hyperdiploid clone with a 48,X,+2,+18,+18 karyotype occurred in 7.6% of cells from another line containing a total of 41.6% pseudo- and hyperdiploid cells. Such clonal abnormalities were not typical of tissue cultures from other patients done in our laboratory. Growth of our patient's tissue cultures was subnormal, and none proliferated beyond the fourth subculture. The significance of this observation remains to be determined. Our results do not allow us to conclude whether our patient's mosaicism of somatic tissues arose during embryogenesis, or whether it originated post-natally. The secondary immunodeficiency which occurs in IL may explain persistence of cells with unusual combinations of autosomal aneuploidy in our patient's tissues.