Thursday 20 November 2003
Werner syndrome is an autosomal recessive genetic disease caused by mutation of the WRN gene. Patients with this disease show many symptoms of premature ageing, including hair graying and loss, cateracts, atherosclerosis and osteoporosis. They also display some characteristics not directly associated with ageing, including reduced fertility and a predisposition to sarcomas, a rare tumour type.
The main clinical feature of the Werner syndrome (WS) is premature aging, but many other features (such as short stature, neoplasia, hyperrecombination, cells with defects on DNA initiation and chain elongation) resemble those of BS. Cells from patients with Werner syndrome are genomically unstable and have a reduced division potential.
The WRN gene encodes a protein with 3’ to 5’ DNA helicase activity but as yet its precise celluar function is not known.
germline mutations in the RECQL2 gene (MIM.604611)
- RECQL2 encodes a homolog of the E. coli RecQ DNA helicase
germline mutations in the LMNA gene (MIM.150330), encoding nuclear lamin A/C
- ’atypical Werner syndrome’ with a more severe phenotype
Mutations in the RECQL2 gene give rise to WS, which is associated at a relatively early age with many, but not all, of the features of the normal ageing process.
Hence, WS individuals show many age-related disorders that develop from puberty, including greying and thinning of the hair, cataracts, type II diabetes mellitus, osteoporosis and atherosclerosis. Moreover, WS individuals are also cancer-prone, although to a more limited extent than is seen in BS individuals, in particular displaying an elevated incidence of sarcomas.
RECQL2 has been shown to form complexes with proteins involved both in cellular responses to DNA damage and in DNA replication. The identification of a functional interaction between RECQL2 and the p53 tumour suppressor protein serves to emphasize the role of the RecQ family (RECQLs) in the maintenance of genomic stability.
RECQL2 and TP53
Mutations in TP53, encoding p53, which has been dubbed the ’guardian of the genome’, are seen in >50% of all sporadic cancers in humans. p53 functions in a highly dynamic and controlled manner; induction of p53 leads to cell cycle arrest in G1 and/or G2, allowing time for DNA repair to take place, but may additionally lead to apoptotic cell death. Moreover, the loss of p53 results in genomic instability. In WS cells, p53-mediated apoptosis is attenuated, while ectopic expression of RECQL2 in these cells can rescue this deficiency.
Overexpression of RECQL2 results in elevated p53-dependent transcriptional activity and induction of CDKN1A (p21Waf1). The interaction between p53 and RECQL2 takes place via their respective C-terminal domains. Interestingly, this is the region where the majority of missense mutations found in WS patients are located.
Without detailed knowledge of the cellular functions of RECQL2, any discussion about the potential roles for the p53- RECQL2 complex is necessarily speculative.
However, there is increasing evidence to suggest that WRN acts at DNA replication origins or at sites of blocked replication forks. Hence loss of the p53/ RECQL2 interaction could result in RECQL2 being unable to recruit p53 to replication origins/forks in response to DNA damage.
In normal cells, the p53-RECQL2 complex may recognize abnormal DNA structures, such as stalled replication forks, leading either to a coordination of cell cycle and DNA repair events or to the induction of apoptosis.
There are two possible routes for repair at this stage with a requirement for RECQL2. RECQL2 could be involved directly in restoration of DNA replication by displacing (perhaps abnormal?) Okazaki fragments on the lagging strand and degrading the displaced DNA. RECQL2 exonuclease catalyses also structure-dependent degradation of DNA, suggesting that WRN resolves abnormal DNA structures via both its helicase and its exonuclease activities.
Consistent with this notion, there is also a role for the WRN exonuclease activity during repair processes. It was shown the Ku70 and Ku86 DNA end-binding complex directly interacts with WRN, and stimulates its 3’ 5’ exonuclease activity.
A second possible role for RECQL2 in replication fork repair would be after removal of the damaged DNA strand at blocked forks. For example, it is possible that RAD51 (the human RecA homologue) could stabilize the replication fork at this stage, allowing the continuation of DNA synthesis without a need for re-initiation of replication.
Alternatively, RECQL2 could be involved in repair at blocked forks via homologous recombination; the role of RECQL2 in this case could be to promote translocation of Holliday junctions, and prevent aberrant recombination events at sites of stalled replication forks by dissociating recombination intermediates. Indeed, there is now considerable evidence supporting a role for RECQL2 in cellular response to DNA damage, and its presence at sites of DNA replication.
The interaction of RECQL2 with DNA polymerase provides a direct biochemical link between RECQL2 and DNA synthesis. It was shown that WRN increases the rate of nucleotide incorporation by DNA polymerase in the absence of proliferating cell nuclear antigen (PCNA); however, RECQL2 has no significant stimulatory effect on the DNA polymerase holoenzyme (polymerase delta-PCNA complex).
Therefore, RECQL2 is unlikely to function in normal processive DNA synthesis, which requires the polymerase delat-PCNA complex. Rather, they speculated that RECQL2 may function in a replication restart pathway at sites where damaged DNA/unusual DNA secondary structures have blocked DNA replication and where the DNA replication machinery has detached from the DNA.
It has also been shown that overexpressed RECQL2 is able to recruit DNA polymerase to the nucleolus, suggesting that RECQL2 could be involved in regulating the initiation and progression of DNA replication by recruiting polymerase to particular sites of DNA synthesis.
RECQL2 and SUMO1
One potential way of regulating and coordinating the many roles of RECQL2 could be by small ubiquitin-related modifier (SUMO1) modification. Addition of SUMO1 to target proteins can change their localization or their interaction with other proteins.
RECQL2 has been shown to be covalently attached with SUMO1 via the conjugating enzyme UBE21. UBE21 has been shown to play a role in the degradation of certain proteins, including S and M phase cyclins.
As discussed in more detail below in the section on BLM-interacting proteins, SUMO1 modification is linked in some cases to regulation of intranuclear localization. SUMO1 modification is also involved in regulating p53 function, through interactions involving the Mdm2 protein and changes in the half-life of p53.
Potentially, SUMO1 modification of p53 and RECQL2 could play a critical role in orchestrating the cross-talk between these proteins and hence regulate pathways for the maintenance of genome integrity.
Franchitto A, Pichierri P. Protecting genomic integrity during DNA replication: correlation between Werner’s and Bloom’s syndrome gene products and the MRE11 complex.
Hum Mol Genet. 2002 Oct 1;11(20):2447-53. PMID: 12351580
Lombard, D. B. and Guarente, L. (1996). Cloning the gene for Werner syndrome: a disease with many symptoms of premature aging. TIG 12, 283-286.
Pennisi (1996) Premaure aging gene discovered. Science 272, 193-194.