DNA Double-stranded Breaks Induce Histone H2AX Phosphorylation on Serine 139

When mammalian cell cultures or mice are exposed to ionizing radiation in survivable or lethal amounts, novel mass components are found in the histone H2A region of two-dimensional gels. Collectively referred to as gamma, these components are formed in vivo by several procedures that introduce double-stranded breaks into DNA. gamma-Components, which appeared to be the only major novel components detected by mass or 32PO4 incorporation on acetic acid-urea-Triton X-100-acetic acid-urea-cetyltrimethylammonium bromide or SDS-acetic acid-urea-cetyltrimethylammonium bromide gels after exposure of cells to ionizing radiation, are shown to be histone H2AX species that have been phosphorylated specifically at serine 139. gamma-H2AX appears rapidly after exposure of cell cultures to ionizing radiation; half-maximal amounts are reached by 1 min and maximal amounts by 10 min. At the maximum, approximately 1% of the H2AX becomes gamma-phosphorylated per gray of ionizing radiation, a finding that indicates that 35 DNA double-stranded breaks, the number introduced by each gray into the 6 x 10(9) base pairs of a mammalian G1 genome, leads to the gamma-phosphorylation of H2AX distributed over 1% of the chromatin. Thus, about 0.03% of the chromatin appears to be involved per DNA double-stranded break. This value, which corresponds to about 2 x 10(6) base pairs of DNA per double-stranded break, indicates that large amounts of chromatin are involved with each DNA double-stranded break. Thus, gamma-H2AX formation is a rapid and sensitive cellular response to the presence of DNA double-stranded breaks, a response that may provide insight into higher order chromatin structures.