The SLX4 protein is a tumor suppressor known for its scaffolding role in several genome protection pathways: homologous recombination, repair of DNA interstrand crosslinks, and maintenance of common fragile sites and telomeres. In particular, it coordinates the action of several nucleases. The helicase RTEL1, for its part, is involved in telomere replication and stability. Until now, these two proteins were thought to exert independent, or even antagonistic, functions. The work presented here challenges this view by revealing a direct physical interaction between SLX4 and RTEL1.
To characterize this complex, the team combined proteomic approaches, structural biology through comparative modeling, and cell biology techniques such as coprecipitation, in situ proximity ligation assay, and DNA fiber analysis. These analyses made it possible to localize the SLX4 domain involved in binding to RTEL1 and to show that SLX4 binds to the HD1 harmonin domain of RTEL1. Importantly, the authors identified mutations that abolish complex formation: two cancer-associated SLX4 variants (D614G and L618P, detected in clinical metastatic samples from a prospective trial) and RTEL1 mutations associated with Hoyeraal-Hreidarsson syndrome, a severe form of dyskeratosis congenita accompanied notably by microcephaly, growth retardation, and intellectual disability.
Functionally, the study demonstrates that both SLX4 and RTEL1 are recruited to newly synthesized DNA and that they closely co-localize with active RNA polymerase II. In complex with RTEL1, SLX4 also promotes the co-localization of FANCD2 with RNA polymerase II. Disruption of the SLX4-RTEL1 interaction leads, in unstressed cells, to DNA replication defects revealed by fiber analysis. Crucially, these defects are corrected when transcription is inhibited using agents such as cordycepin or triptolide, directly linking the observed phenotype to replication-transcription conflicts.
Taken together, these data establish that SLX4 and RTEL1 interact to prevent replication-transcription conflicts. The authors further provide evidence that this function is independent of the nuclease-scaffolding role classically attributed to SLX4. This work thus sheds light on a genome protection mechanism distinct from the already known activities of SLX4 and provides a framework for interpreting the mutations identified in cancer and Hoyeraal-Hreidarsson syndrome.