Chromosomal abnormalities and telomerase activity of NB4 cells treated with arsenic. (a) A representative entire metaphase from NB4 cells treated with arsenic at 0.75 μM for 3 weeks. The arrows indicate dicentric fusion chromosomes. (b) Representative dicentric chromosomal fusions as well as other abnormal fused chromosomes and fragments from cells treated with arsenic at 0.75 μM for 3 weeks. (c) Southern blot of digested genomic DNA from NB4 (left) and HeLa cells (right) showed decreased telomere length after arsenic exposure. Lane 1: untreated NB4 cells; lanes 2−4: 0.25 μM arsenic for 4, 5, and 6 weeks, respectively. Lane 5: untreated HeLa cells; lanes 6 and 7: 1 μM arsenic for 3 and 4 weeks, respectively. (d) Dicentric chromosome after FISH with a telomere-specific probe. Arrows indicate centromeres. FISH signals are clearly visible at both ends of the fusion chromosome. Among 16 fusions studied, no hybridization is seen between the centromeres where fusion occurred. In contrast, 95−97% of chromosome ends of a normal lymphocyte control display intense signal (data not shown). (e) Telomerase activity of NB4 cells treated with arsenic. Pooled NB4 cells and independently isolated subclones showed dramatically decreased telomerase activity after 8 days of 0.75 μM arsenic. The internal control represents a band that should appear in every TRAP assay to ensure a reliable PCR reaction. Because of competition of components, higher telomerase activities result in fainter internal control bands.