Mutator Phenotype and DNA Double-Strand Break Repair in BLM Helicase-Deficient Human Cells.
Bloom syndrome (BS), an autosomal recessive disorder of the BLM gene, predisposes sufferers to various cancers. To investigate the mutator phenotype and genetic consequences of DNA double-strand breaks (DSBs) in BS cells, we developed BLM helicase-deficient human cells by disrupting the BLM gene. Cells with a loss of heterozygosity (LOH) due to homologous recombination (HR) or nonhomologous end joining (NHEJ) can be restored with or without site-directed DSB induction. BLM cells exhibited a high frequency of spontaneous interallelic HR with crossover, but noncrossover events with long-tract gene conversions also occurred. Despite the highly interallelic HR events, BLM cells predominantly produced hemizygous LOH by spontaneous deletion. These phenotypes manifested during repair of DSBs. Both NHEJ and HR appropriately repaired DSBs in BLM cells, resulting in hemizygous and homozygous LOHs, respectively. However, the magnitude of the LOH was exacerbated in BLM cells, as evidenced by large deletions and long-tract gene conversions with crossover. BLM helicase suppresses the elongation of branch migration and crossover of double Holliday junctions (HJs) during HR repair, and a deficiency in this enzyme causes collapse, abnormal elongation, and/or preferable resolution to crossover of double HJs, resulting in a large-scale LOH. This mechanism underlies the predisposition for cancer in BS.
Division of Genetics and Mutagenesis, National Institute of Health Sciences, Setagaya-ku, Tokyo, Japan.
Tetsuya
T
Suzuki
Division of Genetics and Mutagenesis, National Institute of Health Sciences, Setagaya-ku, Tokyo, Japan.
Manabu
M
Yasui
Division of Genetics and Mutagenesis, National Institute of Health Sciences, Setagaya-ku, Tokyo, Japan honma@nihs.go.jp.
Masamitsu
M
Honma
D001816:Bloom Syndrome / Q000235:genetics*; D002460:Cell Line; D053903:DNA Breaks, Double-Stranded; D059766:DNA End-Joining Repair; D004260:DNA Repair; D005785:Gene Conversion; D042822:Genomic Instability; D059765:Homologous Recombination; D006801:Humans; D019656:Loss of Heterozygosity; D053484:RecQ Helicases / Q000172:deficiency*
C097151:Bloom syndrome protein; D053484:RecQ Helicases