Stable low molecular weight DNA in xeroderma pigmentosum cells

Environmentally induced ribosomal DNA (rDNA) instability in human cells and populations exposed to hexavalent chromium [Cr (VI)]

Hexavalent Chromium [Cr (VI)] is an established toxicant, carcinogen, and a significant source of public health concern. The multicopy ribosomal DNA (rDNA) array is mechanistically linked to aging and cancer, is the most evolutionarily conserved segment of the human genome, and gives origin to nucleolus, a nuclear organelle where ribosomes are assembled. Here we show that exposure to Cr (VI) induces instability in the rDNA, triggering cycles of rapid, specific, and transient amplification and contraction of the array in human cells. The dynamic of environmentally responsive rDNA copy number (CN) amplification and contraction occurs at doses to which millions of individuals are regularly exposed. Finally, analyses of human populations occupationally exposed to Cr (VI) indicate that environmental exposure history and drinking habits but not age shape extensive naturally occurring rDNA copy number variation. Our observations identify a novel pathway of response to hexavalent chromium exposure and raise the prospect that a suite of environmental determinants of rDNA copy number remain to be discovered.

Analysis of nucleotide excision repair by detection of single-stranded DNA transients

Nucleotide excision repair (NER) removes bulky DNA lesions and is thus crucial for the protection against environmental carcinogens and UV light exposure. Deficiencies in NER cause increased mutation rates and chromosomal aberrations. Current methods for studying NER are mostly based on either quantitation of lesion removal or detection of repair DNA synthesis. Both have their limitations: lesion removal is inaccurate at very short times post-lesion, where the fraction of removal is low. Repair synthesis is difficult to apply to normally cycling cells due to the need to discriminate repair from replicative DNA synthesis. To overcome these problems we developed a method for analysis of NER based on detection of transient single-stranded (ss) DNA stretches generated at the nucleotide excision step. Cells are metabolically labelled with BrdU, exposed to UV-irradiation and the ssDNA transients generated during excision repair are detected using an anti-BrdU antibody. The method allows single-cell microscopic analysis of the distribution of DNA repair sites as well as kinetic analysis of the DNA repair response. Studies using various DNA repair-deficient cell lines indicate that the detection method integrates a number of pre-synthesis nucleotide excision repair stages. Thus, assembled repair sites can be detected even when they may not lead to complete resolution of the DNA lesion. Using this approach, we show that repair helicase-deficient cells differ from endonuclease-deficient cells.