GEN1/Yen1 and the SLX4 complex: Solutions to the problem of Holliday junction resolution

Chromosomal double-strand breaks (DSBs) are considered to be among the most deleterious DNA lesions found in eukaryotic cells due to their propensity to promote genome instability. DSBs occur as a result of exogenous or endogenous DNA damage, and also occur during meiotic recombination. DSBs are often repaired through a process called homologous recombination (HR), which employs the sister chromatid in mitotic cells or the homologous chromosome in meiotic cells, as a template for repair. HR frequently involves the formation and resolution of four-way DNA structures referred to as the Holliday junction (HJ). Despite extensive study, the machinery and mechanisms used to process these structures in eukaryotes have remained poorly understood. Recent work has identified XPG and UvrC/GIY domain-containing structure-specific endonucleases that can symmetrically cleave HJs in vitro in a manner that allows for religation without additional processing, properties that are reminiscent of the classical RuvC HJ resolvase in bacteria. Genetic studies reveal potential roles for these HJ resolvases in repair after DNA damage and during meiosis. The stage is now set for a more comprehensive understanding of the specific roles these enzymes play in the response of cells to DSBs, collapsed replication forks, telomere dysfunction, and meiotic recombination.

Mammalian BTBD12/SLX4 assembles a Holliday junction resolvase and is required for DNA repair

Structure-specific endonucleases mediate cleavage of DNA structures formed during repair of collapsed replication forks and double-strand breaks (DSBs). Here, we identify BTBD12 as the human ortholog of the budding yeast DNA repair factor Slx4p and D. melanogaster MUS312. Human SLX4 forms a multiprotein complex with the ERCC4(XPF)-ERCC1, MUS81-EME1, and SLX1 endonucleases and also associates with MSH2/MSH3 mismatch repair complex, telomere binding complex TERF2(TRF2)-TERF2IP(RAP1), the protein kinase PLK1 and the uncharacterized protein C20orf94. Depletion of SLX4 causes sensitivity to mitomycin C and camptothecin and reduces the efficiency of DSB repair in vivo. SLX4 complexes cleave 3' flap, 5' flap, and replication fork structures; yet unlike other endonucleases associated with SLX4, the SLX1-SLX4 module promotes symmetrical cleavage of static and migrating Holliday junctions (HJs), identifying SLX1-SLX4 as a HJ resolvase. Thus, SLX4 assembles a modular toolkit for repair of specific types of DNA lesions and is critical for cellular responses to replication fork failure.

CYP2D6 gene test in psychiatric patients and healthy volunteers

BACKGROUND

Antipsychotic drug therapy meets difficulties in predicting response in psychiatric patients. The medical treatment of these patients may be improved significantly by systematic phamacogenetic diagnosis identifying the drug metabolic capacities of each patient. Genetic polymorphisms in the coding sequence for the drug metabolizing cytochrome P450 enzyme CYP2D6 represent a pharmacogenetic target.

METHODS

A cohort (n = 225) representing psychiatric patients seen during an 18-month trial period was included in the project after the subjects accepted a blood sample being taken to analyse their CYP2D6 allelic composition. To investigate any putative difference in allele frequencies among the psychiatric patients compared to earlier publications on allele frequencies in Caucasian populations, another cohort (n = 122) of local healthy volunteers was likewise included.

RESULTS

Allelic frequencies in the psychiatric patients and healthy volunteers were indistinguishable. Alleles *1 and *2 encoding for normal enzyme activity and alleles *3, *4, *5, *6, *13/*16 representing non-active forms were found as well as alleles *9, *10, *41 encoding for enzymes with decreased activity. Furthermore, examples of the previously described duplications of *1 and *2, which result in enhanced enzyme activity, were also identified.

CONCLUSION

A systematic CYP2D6 gene test of hospitalized psychiatric patients revealed the identification of pharmacogenetically relevant alleles affecting capacity to metabolize antipsychotics. The frequencies of phenotypes in affected patients were 8.4 % intermediate metabolizers (IMs), 8.4 % poor metabolizers (PMs) and 3.1 % ultrafast metabolizers (UMs), whereas 52.4 % were extensive metabolizers (EMs) and 27.6 % heterozygous EMs.