Recent findings intriguingly place DNA double-strand break repair proteins at chromosome ends in yeast, where they help maintain normal telomere length and structure. In the present study, an essential telomere function, the ability to cap and thereby protect chromosomes from end-to-end fusions, was assessed in repairdeficient mouse cell lines. By using fluorescence in situ hybridization with a probe to telomeric DNA, spontaneously occurring chromosome aberrations were examined for telomere signal at the points of fusion, a clear indication of impaired end-capping. Telomeric fusions were not observed in any of the repair-proficient controls and occurred only rarely in a p53 null mutant. In striking contrast, chromosomal end fusions that retained telomeric sequence were observed in nontransformed DNA-PKcs-deficient cells, where they were a major source of chromosomal instability. Metacentric chromosomes created by telomeric fusion became even more abundant in these cells after spontaneous immortalization. Restoration of repair proficiency through transfection with a functional cDNA copy of the human DNA-PKcs gene reduced the number of fusions compared with a negative transfection control. Virally transformed cells derived from Ku70 and Ku80 knockout mice also displayed end-to-end fusions. These studies demonstrate that DNA double-strand break repair genes play a dual role in maintaining chromosomal stability in mammalian cells, the known role in repairing incidental DNA damage, as well as a new protective role in telomeric end-capping.

Dosimetry parameters such as deposition, clearance, retention, and translocation and dissolution of inhaled particles in and to different lung compartments may be important for the persistence of particles in the lung and may correlate with adverse pulmonary effects. We investigated such correlations using a model involving TiO2 particles of two particle sizes (20 nm diameter, ultrafine; 250 nm diameter, fine) of the same crystalline structure (anatase). A 12-week inhalation experiment in rats resulted in a similar mass deposition of the two particle types in the lower respiratory tract. The ultrafine particles elicited a persistently high inflammatory reaction in the lungs of the animals compared to the larger-sized particles. In the postexposure period (up to 1 year) retention in the alveolar space per se was not different between fine and ultrafine TiO2. However, the following differences between the particle types were noted: a significantly different total pulmonary retention, both quantitatively (significantly prolonged retention of the ultrafine TiO2) and qualitatively (increased translocation to the pulmonary interstitium and persistence there of the ultrafine TiO2); greater epithelial effects (Type II cell proliferation; occlusion of pores of Kohn) and the beginning of interstitial fibrotic foci with ultrafine TiO2; significant sustained impairment of alveolar macrophage function after ultrafine TiO2 exposure as measured by the clearance of test particles. A correlation between particle surface area and effects was observed. A comparison of the adverse reactions with dosimetric parameters of TiO2 in different lung compartments in the postexposure period showed a correlation of the persistence of effects in both the alveolar and interstitial space with the persistence of particles in the respective compartment.

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