1. ^*Department of Radiation Oncology, University of Utah Health Sciences Center, Salt Lake City, Utah, USA
2. ^†Department of Dermatology, University of Utah Health Sciences Center, Salt Lake City, Utah, USA

Correspondence: Dr Raymond L. Warters, Department of Radiation Oncology, University of Utah Health Sciences Center, Salt Lake City, Utah 85132, USA. Email: ray.warters@hsc.utah.edu

Received 16 November 2003; Revised 27 May 2004; Accepted 26 July 2004.

Abstract

When human cells sustain a DNA double-strand break (dsb), histone H2AX in chromatin surrounding the DNA break is phosphorylated, marking repair foci. The number of phosphorylated histone H2AX (H2AX) foci approximates the number of dsb present in the cell's nuclear DNA. We observed 0.4 H2AX foci per nucleus in primary human melanocytes. In contrast, in four melanoma cell lines, we detected 7–17 H2AX foci per nucleus, a 17–42 times increase in the basal level of H2AX foci in melanoma cells relative to melanocytes (MC). Thus, untreated melanoma cells express significantly greater numbers of H2AX foci than do untreated MC. Detection and rejoining of ionizing radiation-induced DNA dsb proceeded as rapidly in melanoma cells as in MC. Melanoma cells, however, reduced the number of radiation-induced H2AX foci down only to pre-irradiation levels. Co-localization of the majority of H2AX foci with ataxia telangiectasia mutated, BRCA1, 53BP1, and Nbs1 foci in untreated melanoma cells indicated that the additional foci in melanoma cells were associated with a DNA change that the cells interpret as DNA dsb. Co-localization of H2AX foci with the telomere replication factor 1 protein in untreated melanoma cells indicates that the additional foci in untreated melanoma cells are associated with dysfunctional telomeres that induce a DNA damage stress response.