Bloom syndrome protein

Bloom syndrome protein is a protein that in humans is encoded by the BLM gene and is not expressed in Bloom syndrome.

The Bloom syndrome gene product is related to the RecQ subset of DExH box-containing DNA helicases and has both DNA-stimulated ATPase and ATP-dependent DNA helicase activities. Mutations causing Bloom syndrome delete or alter helicase motifs and may disable the 3' → 5' helicase activity. The normal protein may act to suppress inappropriate homologous recombination.

Meiosis
Recombination during meiosis is often initiated by a DNA double-strand break (DSB). During recombination, sections of DNA at the 5' ends of the break are cut away in a process called resection. In the strand invasion step that follows, an overhanging 3' end of the broken DNA molecule then "invades" the DNA of an homologous chromosome that is not broken. After strand invasion, the further sequence of events may follow either of two main pathways leading to a crossover (CO) or a non-crossover (NCO) recombinant (see Genetic recombination and bottom of Figure in this section).

The budding yeast Saccharomyces cerevisiae encodes an ortholog of the Bloom syndrome (BLM) protein that is designated Sgs1 (Small growth suppressor 1). Sgs1(BLM) is a helicase that functions in homologous recombinational repair of DSBs. The Sgs1(BLM) helicase appears to be a central regulator of most of the recombination events that occur during S. cerevisiae meiosis. During normal meiosis Sgs1(BLM) is responsible for directing recombination towards the alternate formation of either early NCOs or Holliday junction joint molecules, the latter being subsequently resolved as COs.

In the plant Arabidopsis thaliana, homologs of the Sgs1(BLM) helicase act as major barriers to meiotic CO formation. These helicases are thought to displace the invading strand allowing its annealing with the other 3’overhang end of the DSB, leading to NCO recombinant formation by a process called synthesis dependent strand annealing (SDSA) (see Genetic recombination and Figure in this section). It is estimated that only about 4% of DSBs are repaired by CO recombination. Sequela-Arnaud et al. suggested that CO numbers are restricted because of the long-term costs of CO recombination, that is, the breaking up of favorable genetic combinations of alleles built up by past natural selection.

DNA repair and apoptosis
Bloom syndrome protein facilitates DNA repair when cells are stressed by agents that cause DNA damage, specifically when DNA replication forks are stalled. Damage present during S phase of the cell cycle causes Bloom syndrome protein to rapidly form foci with gamma H2AFX protein at replication forks that develop DNA breaks. These BLM foci then recruit repair complexes composed of BRCA1 and NBS1 proteins to the stalled replication forks. In addition to its role in repairing DNA damages, Bloom syndrome protein facilitates apoptosis (programmed cell death), a process dependent on p53 protein when cells are stressed by agents that cause unrepairable DNA damage, particularly damage that causes stalled DNA replication forks.

Both DNA repair and apoptosis are enzymatic processes necessary for maintaining genome integrity in humans. Cells that are deficient in DNA repair tend to accumulate DNA damages, and when such cells are also defective in apoptosis they tend to survive even though excessive DNA damage is present. Replication of DNA in such cells tends to lead to mutations and such mutations may cause cancer. Thus Bloom syndrome protein appears to have two roles related to the prevention of cancer, where the first role is to promote repair of a specific class of damages and the second role is to induce apoptosis if the level of such DNA damage is beyond the cell's repair capability

Interactions
Bloom syndrome protein has been shown to interact with:


 * ATM,
 * CHAF1A,
 * CHEK1,
 * FANCM,
 * FEN1,
 * H2AFX,
 * MCM6
 * MLH1
 * P53,
 * RAD51L3,
 * RAD51,
 * RPA1,
 * TOP3A,
 * TP53BP1,
 * WRN, and
 * XRCC2.