Critical interaction domains between bloom syndrome protein and RAD51

A Structural Guide to the Bloom Syndrome Complex

The Bloom syndrome complex is a DNA damage repair machine. It consists of several protein components which are functional in isolation, but interdependent in cells for the maintenance of accurate homologous recombination. Mutations to any of the genes encoding these proteins cause numerous physical and developmental markers as well as phenotypes of genome instability, infertility, and cancer predisposition. Here we review the published structural and biochemical data on each of the components of the complex: the helicase BLM, the type IA topoisomerase TOP3A, and the OB-fold-containing RMI and RPA subunits. We describe how each component contributes to function, interacts with each other, and the DNA that it manipulates/repairs.

Accumulation and Phosphorylation of RecQ-Mediated Genome Instability Protein 1 (RMI1) at Serine 284 and Serine 292 during Mitosis

Chromosome instability usually leads to tumorigenesis. Bloom syndrome (BS) is a genetic disease associated with chromosome instability. The BS gene product, BLM, has been reported to function in the spindle assembly checkpoint (SAC) to prevent chromosome instability. BTR complex, composed of BLM, topoisomerase IIIα (Topo IIIα), RMI1 (RecQ-mediated genome instability protein 1, BLAP75) and RMI2 (RecQ-mediated genome instability protein 2, BLAP18), is crucial for maintaining genome stability. Recent work has demonstrated that RMI2 also plays critical role in SAC. However, little is know about RMI1 regulation during the cell cycle. Here we present that RMI1 protein level does not change through G1, S and G2 phases, but significantly increases in M phase. Moreover, phosphorylation of RMI1 occurs in mitosis. Upon microtubule-disturbing agent, RMI1 is phosphorylated primarily at the sites of Serine 284 and Serine 292, which does not interfere with the formation of BTR complex. Additionally, this phosphorylation is partially reversed by roscovitine treatment, implying cycling-dependent kinase 1 (CDK1) might be one of the upstream kinases.

Crystal structure of the Bloom's syndrome helicase indicates a role for the HRDC domain in conformational changes

Bloom's syndrome helicase (BLM) is a member of the RecQ family of DNA helicases, which play key roles in the maintenance of genome integrity in all organism groups. We describe crystal structures of the BLM helicase domain in complex with DNA and with an antibody fragment, as well as SAXS and domain association studies in solution. We show an unexpected nucleotide-dependent interaction of the core helicase domain with the conserved, poorly characterized HRDC domain. The BLM–DNA complex shows an unusual base-flipping mechanism with unique positioning of the DNA duplex relative to the helicase core domains. Comparison with other crystal structures of RecQ helicases permits the definition of structural transitions underlying ATP-driven helicase action, and the identification of a nucleotide-regulated tunnel that may play a role in interactions with complex DNA substrates.

BLM and RAD51 genes polymorphism and susceptibility to breast cancer

DNA repair by homologous recombination is one of the main processes of DNA double strand breaks repair. In the present work we performed a case-control study (304 cases and 319 controls) to check an association between the genotypes of the c.-61 G>T and the g.38922 C>G polymorphisms of the RAD51 gene and the g.96267 A>C and the g.85394 A>G polymorphisms of the BLM gene and breast cancer occurrence. Genotypes were determined in DNA from peripheral blood by PCR-RLFP and by PCR-CTPP. We observed an association between breast cancer occurrence and the T/G genotype (OR 4.41) of the c.-61 G>T-RAD51 polymorphism, the A/A genotype (OR 1.69) of the g.85394 A>G-BLM polymorphism and the A/A genotype (OR 2.49) of the g.96267 A>C-BLM polymorphism. Moreover, we demonstrated a correlation between intra- and intergenes genotypes combinations and breast cancer occurrence. We found a correlation between progesterone receptor expression and the T/G genotype (OR 0.57) of the c.-61 G>T- RAD51 polymorphism. We also found a correlation between the T/G genotype (OR 1.86) and the T/T genotype (OR 0.56) of the c.-61 G>T- RAD51 polymorphism and the lymph node metastasis. We showed an association between the A/A genotype (OR 2.45) and the A/C genotype (OR 0.41) of the g.96267 A>C-BLM polymorphism and G3 grade of tumor. Our results suggest that the variability of the RAD51 and BLM genes may play a role in breast cancer occurrence. This role may be underlined by a common interaction between these genes.

RecQ Helicases: Conserved Guardians of Genomic Integrity

The RecQ family of DNA helicases is highly conserved throughout -evolution, and is important for the maintenance of genome stability. In humans, five RecQ family members have been identified: BLM, WRN, RECQ4, RECQ1 and RECQ5. Defects in three of these give rise to Bloom's syndrome (BLM), Werner's syndrome (WRN) and Rothmund-Thomson/RAPADILINO/Baller-Gerold (RECQ4) syndromes. These syndromes are characterised by cancer predisposition and/or premature ageing. In this review, we focus on the roles of BLM and its S. cerevisiae homologue, Sgs1, in genome maintenance. BLM/Sgs1 has been shown to play a critical role in homologous recombination at multiple steps, including end-resection, displacement loop formation, branch migration and double Holliday junction dissolution. In addition, recent evidence has revealed a role for BLM/Sgs1 in the stabilisation and repair of replication forks damaged during a perturbed S-phase. Finally BLM also plays a role in the suppression and/or resolution of ultra-fine anaphase DNA bridges that form between sister-chromatids during mitosis.

Complex activities of the human Bloom's syndrome helicase are encoded in a core region comprising the RecA and Zn-binding domains

Bloom's syndrome DNA helicase (BLM), a member of the RecQ family, is a key player in homologous recombination (HR)-based error-free DNA repair processes. During HR, BLM exerts various biochemical activities including single-stranded (ss) DNA translocation, separation and annealing of complementary DNA strands, disruption of complex DNA structures (e.g. displacement loops) and contributes to quality control of HR via clearance of Rad51 nucleoprotein filaments. We performed a quantitative mechanistic analysis of truncated BLM constructs that are shorter than the previously identified minimal functional module. Surprisingly, we found that a BLM construct comprising only the two conserved RecA domains and the Zn²⁺-binding domain (residues 642–1077) can efficiently perform all mentioned HR-related activities. The results demonstrate that the Zn²⁺-binding domain is necessary for functional interaction with DNA. We show that the extensions of this core, including the winged-helix domain and the strand separation hairpin identified therein in other RecQ-family helicases, are not required for mechanochemical activity per se and may instead play modulatory roles and mediate protein–protein interactions.