Recurrent De Novo Mutations Disturbing the GTP/GDP Binding Pocket of RAB11B Cause Intellectual Disability and a Distinctive Brain Phenotype.

The Rab GTPase family comprises ∼70 GTP-binding proteins, functioning in vesicle formation, transport and fusion. They are activated by a conformational change induced by GTP-binding, allowing interactions with downstream effectors. Here, we report five individuals with two recurrent de novo missense mutations in RAB11B; c.64G>A; p.Val22Met in three individuals and c.202G>A; p.Ala68Thr in two individuals. An overlapping neurodevelopmental phenotype, including severe intellectual disability with absent speech, epilepsy, and hypotonia was observed in all affected individuals. Additionally, visual problems, musculoskeletal abnormalities, and microcephaly were present in the majority of cases. Re-evaluation of brain MRI images of four individuals showed a shared distinct brain phenotype, consisting of abnormal white matter (severely decreased volume and abnormal signal), thin corpus callosum, cerebellar vermis hypoplasia, optic nerve hypoplasia and mild ventriculomegaly. To compare the effects of both variants with known inactive GDP- and active GTP-bound RAB11B mutants, we modeled the variants on the three-dimensional protein structure and performed subcellular localization studies. We predicted that both variants alter the GTP/GDP binding pocket and show that they both have localization patterns similar to inactive RAB11B. Evaluation of their influence on the affinity of RAB11B to a series of binary interactors, both effectors and guanine nucleotide exchange factors (GEFs), showed induction of RAB11B binding to the GEF SH3BP5, again similar to inactive RAB11B. In conclusion, we report two recurrent dominant mutations in RAB11B leading to a neurodevelopmental syndrome, likely caused by altered GDP/GTP binding that inactivate the protein and induce GEF binding and protein mislocalization.

MPP1 links the Usher protein network and the Crumbs protein complex in the retina.

The highly ordered distribution of neurons is an essential feature of a functional mammalian retina. Disruptions in the apico-basal polarity complexes at the outer limiting membrane (OLM) of the retina are associated with retinal patterning defects in vertebrates. We have analyzed the binding repertoire of MPP5/Pals1, a key member of the apico-basal Crumbs polarity complex, that has functionally conserved counterparts in zebrafish (nagie oko) and Drosophila (Stardust). We show that MPP5 interacts with its MAGUK family member MPP1/p55 at the OLM. Mechanistically, this interaction involves heterodimerization of both MAGUK modules in a directional fashion. MPP1 expression in the retina throughout development resembles the expression of whirlin, a multi-PDZ scaffold protein and an important organizer in the Usher protein network. We demonstrate that both proteins interact strongly by both a classical PDZ domain-to-PDZ binding motif (PBM) mechanism, and a mechanism involving internal epitopes. MPP1 and whirlin colocalize in the retina at the OLM, at the outer synaptic layer and at the basal bodies and the ciliary axoneme. In view of the known roles of the Crumbs and Usher protein networks, our findings suggest a novel link of the core developmental processes of actin polymerization and establishment/maintenance of apico-basal cell polarity through MPP1. These processes, essential in neural development and patterning of the retina, may be disrupted in eye disorders that are associated with defects in these protein networks.