De novo variants in DENND5B cause a neurodevelopmental disorder.

The Rab family of guanosine triphosphatases (GTPases) includes key regulators of intracellular transport and membrane trafficking targeting specific steps in exocytic, endocytic, and recycling pathways. DENND5B (Rab6-interacting Protein 1B-like protein, R6IP1B) is the longest isoform of DENND5, an evolutionarily conserved DENN domain-containing guanine nucleotide exchange factor (GEF) that is highly expressed in the brain. Through exome sequencing and international matchmaking platforms, we identified five de novo variants in DENND5B in a cohort of five unrelated individuals with neurodevelopmental phenotypes featuring cognitive impairment, dysmorphism, abnormal behavior, variable epilepsy, white matter abnormalities, and cortical gyration defects. We used biochemical assays and confocal microscopy to assess the impact of DENND5B variants on protein accumulation and distribution. Then, exploiting fluorescent lipid cargoes coupled to high-content imaging and analysis in living cells, we investigated whether DENND5B variants affected the dynamics of vesicle-mediated intracellular transport of specific cargoes. We further generated an in silico model to investigate the consequences of DENND5B variants on the DENND5B-RAB39A interaction. Biochemical analysis showed decreased protein levels of DENND5B mutants in various cell types. Functional investigation of DENND5B variants revealed defective intracellular vesicle trafficking, with significant impairment of lipid uptake and distribution. Although none of the variants affected the DENND5B-RAB39A interface, all were predicted to disrupt protein folding. Overall, our findings indicate that DENND5B variants perturb intracellular membrane trafficking pathways and cause a complex neurodevelopmental syndrome with variable epilepsy and white matter involvement.

Co-occurring Usher syndrome Type 1 and Renal Failure.

PURPOSE: To describe a patient with a rare co-occurrence of Usher syndrome type 1C (USH1C) and renal disease, suspected to be secondary to Alport syndrome. METHOD: Case report and literature review of cases with Usher syndrome and renal failure. Clinical examination, color fundus photography, visual field tests, electroretinography and whole exome sequencing were used to diagnose and document the patient's clinical presentation. RESULTS: An 18-year-old female with known history of congenital hearing loss and chronic renal failure, presents with progressive night and peripheral visual impairment suspicious for an inherited retinal disease. Visual field testing, fundus exam and electroretinography findings supported the diagnosis of Usher syndrome. Whole exome sequencing (WES) identified a novel homozygous frameshift variant (c.238del) in USH1C. WES also identified a homozygous COL4A3 variant of unknown significance, which may be responsible for concomitant Alport syndrome. CONCLUSION: By presenting this rare case of co-occurring Usher syndrome Type 1 and renal failure, we highlight the importance of conducting further investigations that could reveal an additional underlying etiology when these entities are present.