H2AXfokukset

H2AX foci, also known as γ‑H2AX foci, are focal accumulations of a modified histone protein that form at sites of DNA double‑strand breaks. They appear when the serine 139 residue of the histone variant H2AX is phosphorylated by at least one member of the PIKK family of kinases, principally ATM, ATR, or DNA‑PKcs. Phosphorylated H2AX, termed γ‑H2AX, undergoes a rapid, large‑scale chromatin response, creating a nucleoprotein platform that recruits additional DNA‑damage response (DDR) factors such as MDC1, 53BP1, and BRCA1. This recruitment is critical for the amplification of the damage signal, the recruitment of repair proteins, and the coordination of repair pathway choice.

The appearance of H2AX foci is typically detected by immunofluorescent microscopy using antibodies directed against the phosphorylated serine 139 epitope. Following cellular fixation and permeabilization, cells are incubated with the primary antibody, then with a fluorophore‑conjugated secondary antibody, and counterstained with a DNA dye such as DAPI. The resulting foci appear as bright nuclear spots whose number and intensity correlate with the extent of DNA damage and the efficiency of repair. Quantification is performed by counting foci per nucleus using image‑analysis software, providing a sensitive assay for measuring radiation damage, chemotherapeutic effects, or endogenous oxidative stress.

Research established the γ‑H2AX focus assay in the late 1990s, and since then it has become a standard tool in cancer biology, toxicology, and clinical diagnostics. In addition to its diagnostic value, γ‑H2AX foci have been employed in high‑throughput screening to identify novel modulation of DNA‑damage pathways and in studies of aging to assess genome instability. The transient nature of foci — forming within minutes and resolving within hours as repair completes — allows kinetic analyses of DDR dynamics. Thus, H2AX foci remain a powerful, quantitative marker for the detection, localization, and study of DNA double‑strand breaks across a wide range of biological contexts.

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