introduction of a normal human chromosome 8 corrects Werner Syndrome cells

Kentaro Ariyoshi and colleagues successfully introduced a normal human chromosome 8 into Werner Syndrome (WS) cells(fibroblasts) immortalized by expressing a human telomere reverse transcriptase subunit (hTERT) gene. In their study (published in the Journal of Radiation Research), they demonstrated that the abnormal WS cellular phenotypes like sensitivity to 4-nitroquinoline-1-oxide (4NQO) and hydroxyurea (HU), and chromosomal radiosensitivity at G(2) phase can be corrected by expression of the WRN gene upon introduction of a chromosome 8 via microcell fusion. Their results provide more evidence that the multiple abnormal WS phenotypes (clinical, cellular, and chromosomal) are derived from a primary, but not secondary, defect in the WRN gene.

WRN and MUS81

There is a Journal of Cell Biology paper by Annapaola Franchitto and colleagues entitled:

Replication fork stalling in WRN-deficient cells is overcome by prompt activation of a MUS81-dependent pathway

Below is tha abstract:

Failure to stabilize and properly process stalled replication forks results in chromosome instability, which is a hallmark of cancer cells and several human genetic conditions that are characterized by cancer predisposition. Loss of WRN, a RecQ-like enzyme mutated in the cancer-prone disease Werner syndrome (WS), leads to rapid accumulation of double-strand breaks (DSBs) and proliferating cell nuclear antigen removal from chromatin upon DNA replication arrest. Knockdown of the MUS81 endonuclease in WRN-deficient cells completely prevents the accumulation of DSBs after fork stalling. Also, MUS81 knockdown in WS cells results in reduced chromatin recruitment of recombination enzymes, decreased yield of sister chromatid exchanges, and reduced survival after replication arrest. Thus, we provide novel evidence that WRN is required to avoid accumulation of DSBs and fork collapse after replication perturbation, and that prompt MUS81-dependent generation of DSBs is instrumental for recovery from hydroxyurea-mediated replication arrest under such pathological conditions.

Published online October 13, 2008
doi:10.1083/jcb.200803173
The Journal of Cell Biology, Vol. 183, No. 2, 241-252

WRN mini-review

In a recent issue of DNA Repair [7 (2008) 1776–1786], Julia Sidorova reviews the role of WRN in preserving DNA integrity during replication and propose that WRN can function in coordinating replication fork progression with replication stress-induced fork remodeling. She further discusses damage tolerance pathways, redundancy, and cooperation with other RecQ helicases.

Fig. 3. Possible scenarios of WRN function in coordinating fork progression with damage repair via control over daughter/daughter duplex expansion and/or half-life. (A) An unproductive daughter/daughter duplex with the 3′ overhang is unwound to redirect damage bypass towards translesion synthesis (TLS). (B) An extension of a daughter/daughter duplex leads to exposure of ssDNA regions of mother strands (for simplicity, only one of the strands is shown coated with RPA). Accumulation of RPA stimulates helicase activity of WRN to limit propagation of daughter/daughter duplex and restore an original fork conformation. (C) Lagging strand synthesis in the presence of a daughter/daughter duplex can lead to formation of long flaps. WRN can prevent their formation by limiting half-life of a daughter/daughter duplex, or stimulate FEN-1 to cleave such flaps once they are formed.