Human Molecular Genetics Advance Access published online on March 18, 2008
Human Molecular Genetics, doi:10.1093/hmg/ddn090
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DNA double strand break repair in parental chromatin of mouse zygotes, the first cell cycle as an origin of de novo mutation
1 Department of Obstetrics and Gynaecology, Radboud University Nijmegen Medical Centre, PO Box 9101, 6500 HB Nijmegen, The Netherlands 2 Department of Radiation Oncology, Radboud University Nijmegen Medical Centre, PO Box 9101, 6500 HB Nijmegen, The Netherlands 3 Current address: Department of Neuroscience and Pharmacology, Rudolf Magnus Institute of Neuroscience, University Medical Center Utrecht, Universiteitweg 100, 3584 CG Utrecht, The Netherlands 4 Current address: Carnegie Institution of Washington, Department of Embryology, 3520 San Martin Drive, Baltimore, MD 21218, USA 5 Current address: Department of Orthopaedics, Radboud University Nijmegen Medical Centre, PO Box 9101, 6500 HB Nijmegen, The Netherlands 6 Department of Radiotherapy, University Medical Center Utrecht, Heidelberglaan 100, 3584 CX Utrecht, The Netherlands
* Correspondence: Dr. P de Boer Department of Obstetrics and Gynaecology Radboud University Nijmegen Medical Centre PO Box 9101, 6500 HB Nijmegen The Netherlands P.deboer{at}obgyn.umcn.nl Tel: +31-243610869 Fax: +31-24-3668597
Received January 29, 2008; Revised March 16, 2008; Accepted March 16, 2008
In the human, the contribution of the sexes to the genetic load is dissimilar. Especially for point mutations, expanded simple tandem repeats and structural chromosome mutations, the contribution of the male germline is dominant. Far less is known about the male germ cell stage(s) that are most vulnerable to mutation contraction. For the understanding of de novo mutation induction in the germline, mechanistic insight of DNA repair in the zygote is mandatory. At the onset of embryonic development, the parental chromatin sets occupy one pronucleus each and DNA repair can be regarded as a maternal trait, depending on proteins and mRNAs provided by the oocyte.
Repair of DNA double strand breaks (DSBs) is executed by non homologous end joining (NHEJ) and homologous recombination (HR). Differentiated somatic cells often resolve DSBs by NHEJ, whereas embryonic stem cells preferably use HR. We show NHEJ and HR to be both functional during the zygotic cell cycle. NHEJ is already active during replacement of sperm protamines by nucleosomes. The kinetics of G1 repair is influenced by DNA-PKcs hypomorphic activity. HR and NHEJ are both operative in S-phase, HR being more active in the male pronucleus. DNA-PKcs deficiency upregulates HR activity. Both after sperm remodeling and at first mitosis, spontaneous levels of 
H2AX foci (marker for DSBs) are high. All immunoflurescent indices of DNA damage and DNA repair point at greater spontaneous damage and induced repair activity in paternal chromatin in the zygote.
# Both authors contributed equally
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