Cell Cycle

The p53-binding protein 1 (53BP1)-dependent checkpoint pathway is triggered by IR and other genotoxic events that lead to DSBs. Phosphorylation of histone H2AX to ?-H2AX by ATM occurs at or near the site of DSBs and is required for subsequent phosphorylation of 53BP1 by ATM and localization of 53BP1 into nuclear foci. Additionally, 53BP1 function is critical for coupling ATM to many of its downstream targets, including the tumor suppressor p53 and structural maintenance of chromosomes protein 1 (SMC-1). In the case of the Chk2 kinase, the coupling mechanism to ATM appears to be independent of 53BP1.

G 1 and S Phases of the Cell Cycle

In proliferating cells, the cell cycle consists of four phases. Gap 1 (G 1 ) is the interval between mitosis and DNA replication that is characterized by cell growth. The transition that occurs at the restriction point (R) in G 1 commits the cell to the proliferative cycle. If the conditions that signal this transition are not present, the cell exits the cell cycle and enters G 0 , a nonproliferative phase during which growth, differentiation and apoptosis occur. Replication of DNA occurs during the synthesis (S) phase, which is followed by a second gap phase (G 2 ) during which growth and preparation for cell division occurs. Mitosis and the production of two daughter cells occur in M phase.

Passage through the four phases of the cell cycle is regulated by a family of cyclins that act as regulatory subunits for cyclin-dependent kinases (cdks). The activity of the various cyclin/cdk complexes that regulate the progression through G 1 -S-G 2 phases of the cell cycle is controlled by the synthesis of the appropriate cyclins during a specific phase of the cell cycle. The cyclin/cdk complex is then activated by the sequential phosphorylation and dephosphorylation of the key residues of the complex, located principally on the cdk subunits.

The cyclin cdk complex of early G 1 is either cdk2, cdk4, or cdk6 bound to a cyclin D isoform. There are several proteins that can inhibit the cell cycle in G 1 . If DNA damage has occurred, p53 accumulates in the cell and induces the p21-mediated inhibition of cyclin D/cdk. Mdm2, by facilitating the nuclear export/inactivation of p53, becomes part of an inhibitory feedback loop that inactivates p21-mediated G 1 arrest. Similarly, activation of TGF-b receptors induces the inhibition of cyclin D/cdk by p15, while cyclic AMP inhibits the cyclin D/cdk complex via p27. If the cyclin D/cdk complex is inhibited, retinoblastoma protein (Rb) is in a state of low phosphorylation and is tightly bound to the transcription factor E2F, inhibiting its activity.