Regulation of the activation of the Fanconi anemia pathway by the p21 cyclin-dependent kinase inhibitor

Abstract

Fanconi Anemia (FA) is a rare disease characterized by congenital defects, progressive bone marrow failure and heightened cancer susceptibility. The FA proteins, BRCA1, and FANCD1/BRCA2, function cooperatively in the FA-BRCA pathway, to repair damaged DNA. Activation of the FA-BRCA pathway occurs via the monoubiquitination of the FANCD2 and FANCI proteins, targeting these proteins to discrete nuclear foci where they function in DNA repair. The cellular regulation of FANCD2/I monoubiquitination, however, remains poorly understood. In this study, we have examined the roles of the p53 tumor suppressor protein, as well as its downstream target the p21Cip1/Waf1 cyclin-dependent kinase inhibitor, in the regulation of the activation of the FA-BRCA pathway. We demonstrate that, in contrast to p53, p21 plays a major role in the regulation of the activation of the FA-BRCA pathway: p21 promotes S-phase and DNA damage-inducible FANCD2/I monoubiquitination and nuclear foci formation. Several lines of evidence establish that this effect is not a consequence of a defective G1-S checkpoint or altered cell cycle progression in the absence of p21. Instead, we demonstrate that p21 is required for the transcriptional repression of the USP1 deubiquitinating enzyme upon exposure to DNA damaging agents. In the absence of p21, persistent USP1 expression precludes the DNA damage-inducible accumulation of monoubiquitinated FANCD2 and FANCI. Consequently, p21−/− cells exhibit increased levels of mitomycin C-inducible complex chromosomal aberrations and elevated γ-H2AX nuclear foci formation. Our results demonstrate that p21 plays a critical role in the regulation of the activation of the FA-BRCA pathway and suggest a broader role for p21 in the orchestration of DNA repair processes following exposure to DNA crosslinking agents.

Fanconi Anemia Signaling and Cancer

“…Clearly, the functions of the FA proteins are a necessary part of S and G2 arrest within the ATM signaling. In addition, the regulation of FA signaling by p21 (a cycling-dependent kinase inhibitor) [58] and p53 [59] further supports the role of FA signaling in the downstream of the ATM signaling for regulating cell proliferation. We believe that FA signaling may be involved in coordinating all major events in the checkpoint systems.…”

Cellular functions of the DUBs

“…Monoubiquitylated FANCD2 binds BRCA1 in chromatin foci to participate in DNA repair and is later recycled by the activity of USP1 (Nijman et al, 2005a). Transcription of USP1 is repressed by P21 on exposure to DNA-damaging agents to permit DNAdamage-induced accumulation of monoubiquitylated FANCD2 (Rego et al, 2011). Stalling of the replication fork can result in mono-or Lys63 poly-ubiquitylated forms of PCNA, which then promote the alternative pathways of TLS or template switching, respectively.…”

The Fanconi anemia ID2 complex: Dueling saxes at the crossroads

“…90 The USP1 gene is also transcriptionally repressed upon cellular exposure to DNA damaging agents, a process that requires the function of the p21 cyclin-dependent kinase inhibitor. 87,91 Both negative regulatory mechanisms facilitate the timely accumulation of monoubiquitinated FANCD2 and FANCI following the induction of DNA damage.…”