Research

I am actively involved in the following projects in understanding different aspects of DNA damage reponse (DDR).

The Yeast project - Regulation of ribonucleotide reductase (RNR) by a Cullin4-based ubiquitin ligase (with Prof Anthony Carr)The synthesis of a balanced dNTP pool is important for high-fidelity DNA replication and genome stability. Multilayer control mechanisms have been implicated in regulating the function of the key enzyme RNR. We have firstly reported that subcellular compartmentation of the small subunit of RNR (Suc22) is a conserved mechanism to constrain the RNR activity. During S phase and in response to DNA damage, Suc22 is exported from nucleus to cytoplasm and subsequently associated with the large subunit (Cdc22) to form an active tetrameric complex. This nuclear export event is triggered by the timely destruction of a small protein, Spd1, which presumably is the molecular switch for the nuclear anchor of Suc22. Deletion of Spd1 therefore results in constitutive cytoplasmic localisation of Suc22. Antagonistically, a cullin-RING ubiquitin ligase (CSN-Pcu4) promotes the degradation of Spd1 in a ubiquitin/proteasome-dependent manner and therefore releases Suc22 into cytoplasm. The activation of the CSN-Pcu4 enzyme is largely dependent on the transactivation of its WD repeat substrate adaptor, Cdt2. However, it still remains elusive how the degradation of Spd1 triggers the repression of the speculated anchoring mechanism of Suc22 and we are currently concentrating on the missing link of this regulatory circuit.

From the study of Spd1 degradation, we also proposed that the COP9 signalosome (CSN) is a polyubiquitin-promoting factor for targeted substrates by the cullin-RING ubiquitin ligase and the function of Csn1 and Csn2 in regulating the Spd1-RNR pathway is separated from the neddylation activity of Csn5/JAMM1.

The Mouse project - modelling the role of ATR signalling pathway in cell cycle and brain development (with Prof Penelope Jeggo and Prof Anthony Carr)ATR (AT-Rad3-related) is a PIKK kinase that specifically activates the downstream kinase Chk1. The ATR-Chk1 axis, together with co-factors such as Rad17, 9-1-1 complex, BRCA1, Claspin and TopBP1, is pivotal in cellular response to replication stress and DSB when cells are challenged with genotoxic reagents. This pathway is also essential for unpurturbed mitotic cell cycle, probably by monitoring the status of the replication fork and therefore stabilise it. Several clinical and genetic studies have linked defective ATR with Seckel I syndrome, an inherited disorder characterised by microcephaly and developmental retardation. Seckel I patients have not been found to be strongly predisposed to cancer.However, the molecular mechanisms of the checkpoint and cell cycle regulation by the ATR pathway is still poorly understood. In this project, we are going to combine the capacities of mouse genetics, ES cell and embryonic transfer technology, and biochemical tools to unravel the multi-faceted functions of ATR.

(To be continued)

^{conditional knock-out/in: Nagy's lab in Samuel Lunenfeld}

^{Ret-ET system}

Dr MK Maconochie our ace collaborator expertised inmouse genetics!!!