Growing evidence in mice, primary human tumors, and mammalian cell culture models indicate that CHFR may function as a potent tumor suppressor. CHFR functions as part of an early G2\/M checkpoint, more specifically in antephase. Antephase refers to late G2 when chromosome condensation starts. This early mitotic checkpoint causes a delay in chromosome condensation in response to mitotic stresses. The human CHFR gene was originally identified during a search for novel cell cycle checkpoint proteins that have fork-head associated domains. Initial analysis indicated that the CHFR-associated G2\/M checkpoint was inactivated in a subset of cancers as demonstrated by high mitotic indices (a high percentage of cells that have condensed chromosomes) in response to exposure to the microtubule poison, nocodazole, due to lack of CHFR expression or CHFR mutations in various cancers. Many other studies showed promoter hypermethylation leading to low\/no expression of CHFR.
In vitro studies suggest that the RING finger domain in CHFR also facilitates ubiquitin ligase function and that it is essential for checkpoint function of CHFR (Chaturved et al., 2002).. In vitro Xenopus extract experiments suggested that CHFR specifically targets PLK1 (polo-like kinase 1) for degradation when extracts are supplemented with high ubiquitin concentrations (Kang, 2002). Thus, according to this in vitro model, CHFR is able to halt cell cycle progression early in mitosis by degrading PLK1, a major player for the activation of mitosis promoting factor. In addition, AURORA A is known to phosphorylate and activate PLK1 as well as CDC25B eventually driving CYCLIN B/CDK1 activation. Interestingly, CHFR was also found to bind via its cysteine rich C-region and ubiquitinate AURORA A, leading to its degradation (Yu et al., 2005). The auto-ubiquitylation ability of CHFR at G2 Phase was proposed to be required for the accumulation of Plk1 and mitotic entry in mammalian cells (Kim et al., 2011). Earlier, Oh et al., showed deubiquitination of Chfr, by USP7/HAUSP (deubiquitinating enzyme) also to regulate its own stability and activity (Oh et al,. 2007).
On the contrary, Summers et al. suggested PLK1 and AURORA A levels not to change when CHFR was expressed in HCT116 cells treated with Nocodazole (Summers et al., 2005).
More recently, other proteins including TOPK and PTEN have been shown to play a role in the CHFR related mitotic spindle checkpoing (Shinde et al. 2013)
Furthermore, Bothos et al., showed that CHFR was able to activate the p38 stress kinase pathway, which reverses chromosome condensation and induces a mitotic arrest and suggested that the ubiquitin ligase function of CHFR may be different than the current in vitro model and that instead of Lys48 ubiquitination, CHFR may link ubiquitin to target protein or proteins via Ly63 due to its interaction with the heteromeric ubiquitin conjugating enzyme complex, Ubc13-Mms2 (Bothos et al., 2003). In the canonical ubiquitin/proteasome pathway, Lys48 is a signal for degradation of target proteins whereas Lys63 ubiquitination functions as a non-proteolytic tag for protein targets. Lys63 ubiquitination is thought to be involved in DNA repair mechanisms. Indeed, CHFR appears to have important roles in DNA damage response (Shtivelman et al., 2003). CHFR and RNF8 (A ubiquin ligase) ubiquitinate histones (H2A and H2B) upon ioinizing radiation (Al-Hakim et al., 2010; Wu et al., 2011). These ubiquitinations seem to be important for the eventual activation of the key DNA damage checkpoint effector, ATM (Derks et al., 2006; Lavin and Kozlov, 2007). Recently, CHFR was reported to interact with MAD2, an important component of the spindle assembly checkpoint. CHFR knockdown resulted in mislocalization of MAD2 and disruption of the MAD2/CDC20 interaction. The cysteine-rich region of CHFR appears to be the essential domain for the CHFR/MAD2 interaction and for promoting interaction between MAD2 and CDC20 to inhibit the anaphase-promoting complex (Privette et al., 2008; Keller and Petty, 2011).
Ayse E Erson-Bensan ; Hesna Begum Akman ; Elizabeth M Petty
CHFR (Checkpoint with fork-head associated and ring finger)
Atlas Genet Cytogenet Oncol Haematol. 2014-12-01
Online version: http://atlasgeneticsoncology.org/gene/526/chfr-(checkpoint-with-fork-head-associated-and-ring-finger)
2004-06-01 CHFR (Checkpoint with fork-head associated and ring finger) by Ayse E Erson,Elizabeth M Petty