Interhomolog recombination and loss of heterozygosity in wild-type and Bloom syndrome helicase (BLM)-deficient mammalian cells

• Published in: Proceedings of the National Academy of Sciences - PNAS Vol. 108; no. 29; pp. 11971 - 11976
• Main Authors: LaRocque, Jeannine R, Stark, Jeremy M, Oh, Jin, Bojilova, Ekaterina, Yusa, Kosuke, Horie, Kyoji, Takeda, Junji, Jasin, Maria
• Format: Journal Article
• Language: English
• Published: United States National Academy of Sciences 19.07.2011; National Acad Sciences
• Subjects: Animals; Base Sequence; Biological Sciences; Bloom syndrome; Bloom's syndrome; Blotting, Western; Cell Line; Cell lines; Cells; Chromatids; Chromosomal crossover; Chromosomes; Chromosomes, Mammalian - genetics; crossing over; Crossing Over, Genetic - genetics; Cytogenetic Analysis; Daughter cells; DNA Breaks, Double-Stranded; DNA Primers - genetics; Electroporation; embryonic stem cells; Gene conversion; Gene Conversion - genetics; Genes; Genetic recombination; Genetic Vectors - genetics; Genomics; heterozygosity; homologous recombination; hybrids; Loss of heterozygosity; Loss of Heterozygosity - genetics; Mammals; Mice; Mice, Inbred BALB C; mitosis; Molecular Sequence Data; neoplasm cells; Polymorphism, Single Nucleotide - genetics; RecQ Helicases - genetics; Sequence Analysis, DNA; Yeasts
• ISSN: 0027-8424; 1091-6490
• DOI: 10.1073/pnas.1104421108

Genomic integrity often is compromised in tumor cells, as illustrated by genetic alterations leading to loss of heterozygosity (LOH). One mechanism of LOH is mitotic crossover recombination between homologous chromosomes, potentially initiated by a double-strand break (DSB). To examine LOH associated with DSB-induced interhomolog recombination, we analyzed recombination events using a reporter in mouse embryonic stem cells derived from F1 hybrid embryos. In this study, we were able to identify LOH events although they occur only rarely in wild-type cells (≤2.5%). The low frequency of LOH during interhomolog recombination suggests that crossing over is rare in wild-type cells. Candidate factors that may suppress crossovers include the RecQ helicase deficient in Bloom syndrome cells (BLM), which is part of a complex that dissolves recombination intermediates. We analyzed interhomolog recombination in BLM-deficient cells and found that, although interhomolog recombination is slightly decreased in the absence of BLM, LOH is increased by fivefold or more, implying significantly increased interhomolog crossing over. These events frequently are associated with a second homologous recombination event, which may be related to the mitotic bivalent structure and/or the cell-cycle stage at which the initiating DSB occurs.