Surgical excision is the best therapeutic option for tumours in the retrorectal space. Classically, surgery in this area required an abdominal or posterior approach, or a combination of the two methods. We report the use of transanal endoscopic microsurgery for the treatment of retrorectal tumours as an alternative to classical procedures.

The use of TEM/TEO in atypical indications may benefit patients by avoiding surgical procedures associated with greater morbidity.

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The linear ubiquitin chain assembly complex (LUBAC) is the only known ubiquitin ligase that generates linear/Met1-linked ubiquitin chains. One of the LUBAC components, HOIL-1L, was recently shown to catalyse oxyester bond formation between the C-terminus of ubiquitin and some substrates. However, oxyester bond formation by LUBAC has not been directly observed. We present the first low-resolution 3D structure of LUBAC obtained by electron microscopy and report its generation of heterotypic ubiquitin chains containing linear linkages with oxyester-linked branches. We found that addition of the oxyester-bound branches depends on HOIL-1L catalytic activity. We suggest a coordinated ubiquitin relay mechanism between the two ligases supported by cross-linking mass spectrometry data, which show proximity between the catalytic RBR domains of HOIP and HOIL-1L, Mutations in the linear ubiquitin chain-binding NZF domain of HOIL-1L reduces chain branching suggesting its role in the process. In cells, these heterotypic chains were induced by TNF. In conclusion, we demonstrate that LUBAC assembles heterotypic ubiquitin chains with linear and oxyester-linked branches by the concerted action of HOIP and HOIL-1L.Keywords: E3 ubiquitin ligase/ Ester-bond linkage/ HOIL-1L/ RBR/ Ubiquitin