Freire R, d'Adda di Fagagna F, Wu L, Pedrazzie G, Stagljar I, Hickson ID and Jackson SP. (2001) Cleavage of the Bloom's syndrome gene product during apoptosis by caspase-3 results in an impaired interaction with topoisomerase III alpha. Nucleic Acids Res. 29:3172-3180.

In higher eukaryotes, the integration of signals triggered in response to certain types of stress can result in programmed cell death. Central to these events is the sequential activation of a cascade of proteinases known as caspases. The final activated effector caspases of this cascade digest a number of cellular proteins, in some cases increasing their enzymatic activity, in others destroying their function. Of the proteins shown to be targets for caspase-mediated proteolysis, a surprisingly large proportion are proteins involved in the signalling or repair of DNA damage. Here, we investigate whether BLM, the product of the gene mutated in Bloom's syndrome, a human autosomal disease characterised by cancer predisposition and sunlight sensitivity, is cleaved during apoptosis. BLM interacts with topoisomerase III alpha and has been proposed to play a role in maintaining genomic integrity through its roles in DNA repair and replication. We show that BLM is cleaved during apoptosis by caspase-3 and reveal that the main cleavage site is located at the junction between the N-terminal and central helicase domains of BLM. Proteolytic cleavage by caspase-3 produces a 120kDa fragment, which contains the intact helicase domain, and three smaller fragments, the relative amounts of which depend on time of incubation with caspase-3. The 120kDa fragment retains the helicase activity of the intact BLM protein. However, its interaction with topoisomerase III alpha is severely impaired. Since the BLM-topoisomerase interaction is believed to be necessary for many of the replication and recombination functions of BLM, we suggest that caspase-3 cleavage of BLM could alter the localisation and/or function of BLM and that these changes may be important in the process of apoptosis.