The apoptotic ring: a novel entity with phosphorylated histones H2AX and H2B and activated DNA damage response kinases.

We recently showed that histone H2AX phosphorylated on serine 139 (gamma-H2AX), a hallmark of DNA damage response (DDR), also forms early during apoptosis induced by death receptor activation. Here, we extend and discuss our findings on apoptotic gamma-H2AX, which differs from the well-established DDR with nuclear foci. During apoptosis induced by death receptors agonists (TRAIL and FasL) and staurosporine, gamma-H2AX is initiated in the nuclear periphery immediately inside the nuclear envelope while total H2AX remains distributed throughout the nucleus. This process is readily detectable by immunofluorescence microscopy and we refer to it as the "gamma-H2AX ring". It is conserved both in cancer and normal cells. The gamma-H2AX ring contains the activated checkpoints kinases, ATM, Chk2 and DNA-PK; the latter being the main effector for the apoptotic gamma-H2AX phosphorylation. Notably, we show here that the gamma-H2AX ring coincides with phosphorylated H2B on serine 14 (P(S14)-H2B), another histone modification associated with apoptosis. The coordinated phosphorylations of H2AX and H2B suggest a previously unrecognized histone phosphorylation signature for apoptosis consisting of gamma-H2AX together with P(S14)-H2B and possibly P(Y142)-H2AX. This signature ("phosphohistone 2 code") together with the gamma-H2AX ring provides a new feature to monitor and study apoptosis.