Among DNA lesions, double-strand breaks (DSBs) are particularly serious because they can initiate apoptosis and inaccurately repaired DSBs lead to mutations and gross chromosomal rearrangements. There are two main pathways to repair DSBs: Firstly, non-homologous end-joining (NHEJ) is described as an error-prone mechanism in which broken DNA ends are processed and then rejoined. Deletions or insertions happen frequently in the course of this process. Secondly, homologous recombination (HR) is error-free as it uses homologous DNA segments to restore damaged information. Besides DSB repair, HR is also involved in rescuing stalled replication forks and in generating genetic diversity during mitosis and meiosis. In principle, two homologous DNA duplexes pair with each other, DNA ends are processed in a Rad52-dependent manner, Rad51 facilitates the exchange of strands and thus a DNA four-way junction or Holliday junction (HJ) is formed. HJs are mobile connections between homologous strands that can move along DNA and create new segments of heteroduplex DNA (branch migration). HJ-resolving enzymes (X-resolvases) terminate the recombination process by cleaving and separating connected strands. Subsequently, DNA ligases seal the breaks in the DNA backbones.

Simplified scheme of double-strand break repair by homologous recombination and DNA four-way junction resolution.

HJ resolution is a critical step and as such of particular interest. Prokaryotic or phage X-resolvases employ a symmetrical cleavage pattern in which corresponding positions are cut in homologous strands. In contrast, recent studies in eukaryotic cells revealed a more complex picture of HJ processing. HJs can be cleaved either in a symmetrical or in an asymmetrical manner. Depending on cleavage direction, this can result in crossover or non-crossover products. Asymmetrical cleavage products can contain flaps and gaps that need further processing. Furthermore, the BLM complex can “dissolve” HJs by migrating and decatenating the branch point in a combined helicase and topoisomerase activity. The original gene organization is restored. We are currently focusing on the processing of DNA junctions in higher organisms. In particular, we would like to find out how branch points are recognized and what determines the fate of the DNA during DSB repair. Ultimately, we would like to learn why junction processing is more complex compared to the prokaryotic system and how it is connected to the rescue of replication forks and other recombination events. Interestingly, many factors are involved in different DNA repair pathways. In a long-term approach we are trying to understand the modular network of DNA repair factors, their regulation and interconnection with other cellular functions. We think that a combined structural and functional approach will be the best method to address this goal. Moreover, we hypothesize that a comprehensive understanding of DNA repair and oncogenesis on a molecular level is required to develop prophylactic measures or new therapies against cancer and related syndromes.

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