Regulation of p53 by the mitotic surveillance/stopwatch pathway: implications in neurodevelopment and cancer

Abstract

The transcription factor p53 (encoded by TP53) plays diverse roles in human development and disease. While best known for its role in tumor suppression, p53 signaling also influences mammalian development by triggering cell fate decisions in response to a wide variety of stresses. After over 4 decades of study, a new pathway that triggers p53 activation in response to mitotic delays was recently identified. Termed the mitotic surveillance or mitotic stopwatch pathway, the USP28 and 53BP1 proteins activate p53 in response to delayed mitotic progression to control cell fate and promote genomic stability. In this Minireview, I discuss its identification, potential roles in neurodevelopmental disorders and cancer, as well as explore outstanding questions about its function, regulation and potential use as a biomarker for anti-mitotic therapies.

Keywords: 53BP1; USP28; apoptosis; c-MYC; cancer; microcephaly; neurodevelopment; p53.

FIGURE 1

The MSP complex. (A) Schematic p53, USP28 and 53BP1 protein domains. ATM and CHK2 kinase phosphorylation sites, transcriptional activation domain (TAD), proline rich domain (PRD), oligomerization domain (OD) and C-terminal domain (CTD) are shown for p53 with amino acids indicated. The ability of the CTD to negatively regulate the DBD is indicated (red lines). The Ubiquitin Associated domain (UBA), ubiquitin interacting motif (UIM), SUMO interacting motif (SIM) and ubiquitin specific protease (USP) domains are shown for USP28. The LC8/DYNLL1 binding domain (LC8), oligomerization domain (OD), glycine-arginine rich (GAR) domain, tandem Tudor domain (TTD), ubiquitin-dependent recruitment motif (UDR), nuclear localization signal (NLS), BRCA C-terminal domains (BRCT) and intrinsically disordered loop (IDL) are indicated for 53BP1. Dotted lines indicate 53BP1 domains implicated in interactions with USP28 and p53 in the MSP complex (pink) or following TIRR depletion (green). The box illustrates the pathways (blue) in which specific domains have been implicated with the domains involved in the MSP highlighted in pink (Meitinger et al., 2024; Fong et al., 2016; Rass et al., 2022; Cuella-Martin et al., 2016; Cuella-Martin et al., 2021; Parnandi et al., 2021; Lo et al., 2005; Picart-Armada et al., 2017). TIRR depletion is indicated as TIRRdep. (B) Roles of the MSP complex. The MSP (left) is triggered during prolonged mitosis by PLK1 phosphorylation of the 53BP1 IDL (Meitinger et al., 2024). Subsequent formation of inherited MSP complexes with USP28 binding to the 53BP1-TTD and p53 binding to the 53BP1-BRCT domain leads to activation of p53 target genes in G1 to control cell fate decisions. DNA damage-mediated activation of p53 (right) involves its phosphorylation by ATM and CHK2, as well as inhibition of negative regulators MDM2/MDM4 and WIP1 (not shown). The LC8 interaction domain has been implicated in p53 nuclear localization after damage but its role in transcription remains undefined (Lo et al., 2005). Stabilized, phosphorylated p53 interacts with the 53BP1 BRCT domain. N3 inhibition of MDM2 activates p53 via a similar mechanism but appears to bypass the need for DDR kinase activity. Finally, depletion of TIRR activates p53 in a manner requiring interactions between the 53BP1-TTD and p53-dimethylated on K382 (bottom right) that require USP28 in a yet to be defined role (Parnandi et al., 2021).

FIGURE 2

The MSP complex in neurodevelopment and cancer. Left panel: Delayed mitosis, DNA damage and DNA replication stress (red text), resulting from mutations in different genes involved in human neurodevelopmental disorders, activate p53 through distinct pathways, leading to cell death or arrest in mouse models (Marjanovic et al., 2015; Foster et al., 2012; Shull et al., 2009; O’Brien et al., 2023). The loss of p53 rescues NPCs and suppresses microcephaly phenotypes in many mouse models of human syndromes, including the 3 examples shown here (black bold text at the top of each column) (Bowen and Attardi, 2019; Marjanovic et al., 2015). MSP components USP28 and 53BP1 (pink) were selectively required for p53-activation and microcephaly in response to centrosome defects and delayed mitosis, as their absence failed to rescue DNA damage dependent microcephaly that was dependent on ATM/CHK2 DDR kinase signaling in other models (blue) (Atkins et al., 2020; Phan et al., 2021; Lee et al., 2000; Foster et al., 2012). Right panel: In cancer, frequent loss of USP28 is observed prior to treatment, including in some cancers with defective p53-transcriptional signatures, suggesting that there is a selective pressure for attenuating this component of the MSP (Fito-Lopez et al., 2023). Selection against MSP components could potentially result from aneuploidy, telomere crisis or metabolic defects that perturb mitotic progression, although it remains possible that MSP-independent functions of these proteins drive the selection for their loss (see text for discussion). MSP status predicts the response to antimitotic agents in vitro and likely should be considered in their clinical use (Meitinger et al., 2024). ATM and CHK2 loss predispose to cancer, likely due in large part to their role in activating p53 in response to DNA damage and replication stress. This allows tumors to tolerate higher levels of genomic instability that can drive the acquisition of aggressive traits and promote treatment resistance (middle and right column).