Complex Breakpoints and Template Switching Associated with Non-canonical Termination of Homologous Recombination in Mammalian Cells.
A proportion of homologous recombination (HR) events in mammalian cells resolve by "long tract" gene conversion, reflecting copying of several kilobases from the donor sister chromatid prior to termination. Cells lacking the major hereditary breast/ovarian cancer predisposition genes, BRCA1 or BRCA2, or certain other HR-defective cells, reveal a bias in favor of long tract gene conversion, suggesting that this aberrant HR outcome might be connected with genomic instability. If termination of gene conversion occurs in regions lacking homology with the second end of the break, the normal mechanism of HR termination by annealing (i.e., homologous pairing) is not available and termination must occur by as yet poorly defined non-canonical mechanisms. Here we use a previously described HR reporter to analyze mechanisms of non-canonical termination of long tract gene conversion in mammalian cells. We find that non-canonical HR termination can occur in the absence of the classical non-homologous end joining gene XRCC4. We observe obligatory use of microhomology (MH)-mediated end joining and/or nucleotide addition during rejoining with the second end of the break. Notably, non-canonical HR termination is associated with complex breakpoints. We identify roles for homology-mediated template switching and, potentially, MH-mediated template switching/microhomology-mediated break-induced replication, in the formation of complex breakpoints at sites of non-canonical HR termination. This work identifies non-canonical HR termination as a potential contributor to genomic instability and to the formation of complex breakpoints in cancer.
Department of Medicine, Beth Israel Deaconess Medical Center and Harvard Medical School, Boston, Massachusetts, United States of America.
Andrea J
AJ
Hartlerode
Department of Medicine, Beth Israel Deaconess Medical Center and Harvard Medical School, Boston, Massachusetts, United States of America.
Nicholas A
http://orcid.org/0000-0003-2091-5462
NA
Willis
Department of Medicine, Beth Israel Deaconess Medical Center and Harvard Medical School, Boston, Massachusetts, United States of America.
Anbazhagan
A
Rajendran
Department of Pathology, Boston Children's Hospital and Harvard Medical School, Boston, Massachusetts, United States of America.
John P
JP
Manis
Department of Medicine, Beth Israel Deaconess Medical Center and Harvard Medical School, Boston, Massachusetts, United States of America.
Ralph
http://orcid.org/0000-0002-5064-0175
R
Scully
D000818:Animals; D019313:BRCA1 Protein / Q000235:genetics; D024682:BRCA2 Protein / Q000235:genetics; D001943:Breast Neoplasms / Q000235:genetics* / Q000473:pathology; D002842:Chromatids / Q000235:genetics; D059766:DNA End-Joining Repair / Q000235:genetics; D004268:DNA-Binding Proteins / Q000235:genetics*; D005260:Female; D005785:Gene Conversion / Q000235:genetics; D042822:Genomic Instability / Q000235:genetics; D059765:Homologous Recombination / Q000235:genetics*; D006801:Humans; D051379:Mice; D000066450:Mouse Embryonic Stem Cells; D010051:Ovarian Neoplasms / Q000235:genetics* / Q000473:pathology
D019313:BRCA1 Protein; D024682:BRCA2 Protein; C551750:BRCA2 protein, human; D004268:DNA-Binding Proteins; C511788:XRCC4 protein, mouse