Genomic instability often underlies cancer. Analyses of proteins implicated in a cancer-predisposing condition called Bloom's syndrome illustrate the intricacies of protein interactions that ensure genomic stability.
Bloom's syndrome, which is characterized by severe growth retardation, immunodeficiency, anaemia, reduced fertility and predisposition to cancer1, is caused by mutations in the gene BLM. At the cellular level, the hallmark of this genetic disorder is a high rate of sister-chromatid exchange — the swapping of homologous stretches of DNA between a chromosome and its identical copy generated during DNA replication2. Understanding how mutations in BLM lead to this chromosomal abnormality has been of considerable interest to both scientists and clinicians. So the latest clue to solving the mystery of Bloom's syndrome, which Xu et al.3 and Singh et al.4 report in Genes & Development, is a welcome advance.
The product of BLM, the enzyme BLM helicase, functions as part of a protein complex of the same name, which is involved in both suppression of sister-chromatid exchange and maintenance of genomic stability. Besides this helicase, which unwinds complementary DNA double strands, this complex was thought to contain three other protein components: a topoisomerase (Topo 3α) enzyme, which unknots the two DNA strands by introducing transient nicks; RPA, which binds tightly to single-stranded DNA; and RMI1, which binds directly to BLM and Topo 3α, and perhaps less tightly to other factors.
Through its combined nicking and unwinding activities, the BLM complex catalyses the splitting (resolution) of a double-cross-shaped DNA structure called a double Holliday junction that arises from reciprocal exchanges of single strands of DNA between homologous double-stranded sequences during the process of homologous recombination (Fig. 1). Thus, the BLM complex prevents the formation of hybrid recombinant DNA molecules called crossovers that could lead to a phenomenon known as loss of heterozygosity, which significantly contributes to cancer.
Xu et al.3 and Singh et al.4 independently identify a fifth component of the BLM complex: a small protein designated RMI2. Strikingly, both teams find that RMI2 is required to maintain the stability of the BLM complex, and that its deficiency in vertebrate cells results in chromosomal instability. RMI2 and BLM seem to function in the same pathway to suppress sister-chromatid exchange3. Moreover, RMI2-depleted cells are sensitive to DNA damage that stalls the process of replication, and this protein is essential for efficient targeting of BLM to chromatin (complexes of DNA and histone proteins) and to nuclear foci during replicational stress4.
How does RMI2 stabilize and orchestrate the activity of the BLM complex? The answer is not simple and awaits further investigation. For starters, the current studies3,4 suggest that RMI2 functions by interacting directly with RMI1, which, in turn, binds to BLM and Topo 3α. It is possible, however, that — in addition to the members of the BLM complex — RMI2 also binds less tightly to other protein complexes in the cell that survey and repair the genome, or to peculiar DNA configurations that arise at stalled or converging replication forks (structures formed by the separation of two complementary DNA strands during replication).
As for the significance of RMI2 to the BLM complex, for analogy let's imagine a mousetrap. It contains several components, including a spring, a platform, a hammer, a hold-down bar and a catch5. Omit certain components of the trap, and the device may still operate, albeit less efficiently. With all of the components in place — including those with primarily structural roles such as the hold-down bar and the platform — the trap is most likely to catch the mouse. Returning to the BLM complex: through its interaction with RMI1, RMI2 allows the 'BLM–Topo-3α device' to assume optimal stability and configuration so that it can efficiently catalyse the splitting of the double Holliday junction, and so prevent the escape of deleterious DNA structures that would lead to crossovers (Fig. 1). RMI2 therefore seems to have an integral structural role in the BLM–Topo-3α device by orchestrating its action.
What are the implications of these observations? Mutations in the gene encoding RMI2 are likely to occur in hereditary diseases characterized by chromosomal instability and cancer. It is provocative, therefore, that there is a connection between the BLM complex and proteins mutated in another genetic disorder called Fanconi's anaemia, which also carries a high risk of cancer.
The core protein complex affected in Fanconi's anaemia (FA) and the BLM complex associate together in a supercomplex known as BRAFT (BLM, RPA, FA and Topo 3α)6. Singh and colleagues4 identify FA proteins in complexes containing RMI2 and BLM, consistent with previous findings that BLM and its associated factors collaborate with FA proteins in response to replicational stress7,8. So an emerging theme from these studies is that a complex network of proteins that work through overlapping and interacting pathways confers genomic integrity. To understand the functional mechanism of these protein complexes, every 'mousetrap device' must be dissected one by one. Understanding how the trap catches the mouse will allow us to grasp the consequences of chromosomal instability associated with cancer and other diseases.
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