Biallelic WDR91 variants cause a neurodevelopmental disorder through impaired endosomal maturation and autophagy dysregulation

This study identifies biallelic WDR91 variants as the cause of a severe neurodevelopmental disorder characterized by microcephaly and epilepsy, demonstrating that these mutations lead to disease through impaired endosomal maturation and autophagy dysregulation.

The Gist

The Big Picture: A Broken Delivery System in the Brain

Imagine your body's cells as bustling cities. Inside these cities, there is a complex delivery and recycling system. Packages (nutrients, signals, and waste) need to be picked up, sorted, delivered to the right address, and sometimes recycled or thrown away.

This paper is about a specific "delivery manager" protein called WDR91. The researchers found that when this manager is broken, the city's delivery system collapses. This causes a severe brain disorder in a young child, leading to a shrinking brain, seizures, and developmental delays.

The Mystery Patient: A City in Crisis

The story starts with a young boy. From birth, his head was much smaller than average (microcephaly), and it kept getting smaller as he grew. He had severe seizures and couldn't sit up or speak by age five.

When doctors looked at his blood under a microscope, they saw something strange: his white blood cells (the city's security guards) had giant, messy clumps inside them. This usually points to a rare immune disease, but this boy's symptoms were too severe for that. It was a clue that his "trash and delivery" system was fundamentally broken.

The Genetic Culprit: Two Bad Instructions

The scientists looked at the boy's DNA and found the problem in the WDR91 gene. Think of this gene as the instruction manual for building the WDR91 delivery manager.

The boy inherited two broken copies of this manual (one from his mom, one from his dad):

1. The "Nonsense" Copy: One copy had a typo that told the factory to stop building the protein halfway through. The result? Zero working managers.
2. The "Glitchy" Copy: The other copy had a tiny typo that changed one single letter in the instructions. The factory built the protein, but it was unstable and fell apart quickly.

It's like trying to run a delivery company with one manager who never shows up and another who shows up but quits after five minutes.

What Went Wrong? (The Science Simplified)

The researchers tested this in the lab using cells to see exactly what happens when WDR91 is missing. They found two major failures:

1. The Sorting Station Jam (Endosomal Maturation)

Inside the cell, packages arrive at a "sorting station" (early endosomes). They need to be upgraded to "late endosomes" to be sent to the recycling plant or the trash.

• The Normal Job: WDR91 acts like a traffic cop that guides these packages from the early station to the late station.
• The Breakdown: Without WDR91, the packages get stuck at the early station. They pile up, the station gets clogged, and the traffic stops. The cell can't process new materials or clear out old ones.

2. The Recycling Plant Overload (Autophagy)

Cells also have a recycling process called autophagy (literally "self-eating") where they eat their own damaged parts to stay healthy.

• The Normal Job: WDR91 helps the recycling trucks (autophagosomes) merge with the trash compactor (lysosomes) to get rid of the waste.
• The Breakdown: Without WDR91, the recycling trucks arrive but can't merge with the compactor. They just sit there, piling up with garbage. The cell becomes toxic with its own waste, which is especially deadly for brain cells that can't easily replace themselves.

The "Giant Granules" Clue

Remember those giant clumps in the boy's blood cells? The researchers realized that because the delivery system is broken, the cell's "trash bags" (lysosomes) can't fuse properly. They just keep merging into one giant, useless blob. This happens in both the brain cells and the blood cells, explaining why the boy had both brain issues and weird blood cells.

Why This Matters

This paper is a big deal for three reasons:

1. It Solves a Mystery: It confirms that broken WDR91 genes cause this specific, severe brain disorder. Before this, doctors didn't know exactly what was wrong with patients who had these symptoms.
2. It Explains the "Why": It shows us how the disease happens. It's not just a random glitch; it's a specific failure in how cells sort trash and recycle parts.
3. It Offers a Diagnostic Clue: The researchers found that looking for those "giant clumps" in a blood smear could be a quick, cheap way to spot this disease early, even before genetic testing is done.

The Takeaway

Think of the brain as a highly organized city. The WDR91 protein is the essential manager keeping the streets clear and the recycling plants running. When this manager is missing or unstable, the city gets clogged with trash, the streets jam, and the city (the brain) begins to shrink and fail. This research helps doctors understand the mechanics of this failure, paving the way for better diagnosis and, hopefully, future treatments.

Technical Summary

1. Problem Statement

Biallelic variants in genes regulating endosomal, lysosomal, and autophagy pathways are increasingly linked to severe neurodevelopmental disorders (NDDs). While WDR91 (encoding a Rab7 effector essential for endosomal maturation and lysosomal homeostasis) has been implicated in mouse models of neurodegeneration, its role in human disease was previously defined only by descriptive case reports lacking functional validation. The specific pathogenic mechanisms of WDR91 variants in humans, particularly regarding protein stability, endosomal trafficking, and autophagy, remained unresolved.

2. Methodology

The study employed a multidisciplinary approach combining clinical genetics, in silico structural analysis, and cellular functional assays:

• Clinical & Genetic Analysis:
  • Patient: A male proband with severe NDD, progressive microcephaly, microlissencephaly, and epilepsy.
  • Sequencing: Trio-based whole-exome sequencing (WES) identified compound heterozygous variants in WDR91: a paternal nonsense variant (p.Gln215*) and a maternal missense variant (p.Tyr15Asn).
  • Validation: Sanger sequencing confirmed segregation; variants were absent from population databases (gnomAD, 1000 Genomes).
• In Silico Analysis:
  • Conservation: Multiple sequence alignment and PhyloP/PhastCons scores confirmed strict evolutionary conservation of Tyr15.
  • Structural Modeling: AlphaFold structures were analyzed using PyMOL and FoldX to assess stability changes.
  • Motif Prediction: Tools (NetPhos, GPS-UbER, DEGRONOPEDIA) were used to predict post-translational modifications and degron motifs (specifically APC/C D-box motifs) in the N-terminus.
• Cellular Models:
  • CRISPR/Cas9: Generated WDR91 knockout (KO) HEK293T cell lines.
  • Reconstitution: KO cells were transduced with lentiviral vectors expressing Wild-Type (WT), p.Gln215*, or p.Tyr15Asn WDR91.
  • Patient-Derived Cells: Primary CD3+ T lymphoblasts were cultured from the patient and healthy controls.
• Functional Assays:
  • Protein Stability: Western blotting with cycloheximide (CHX) chase and MG132 (proteasome inhibitor) treatments to measure synthesis vs. degradation rates.
  • Endosomal Maturation: Live-cell confocal microscopy tracking the conversion of early endosomes (2×FYVE-mCherry) to late endosomes (GFP-Rab7).
  • Colocalization: Immunofluorescence to assess WDR91 localization relative to EEA1 (early) and Rab7 (late) markers.
  • Autophagy: RNA-seq for transcriptomic profiling; flow cytometry using autophagy detection probes (with/without bafilomycin A1) to measure autophagic flux.

3. Key Results

A. Genetic and Protein Expression Findings

• p.Gln215:* This truncating variant leads to a complete absence of WDR91 protein, likely due to nonsense-mediated decay (NMD) or degradation of the truncated protein lacking the essential WD40 domain.
• p.Tyr15Asn: This missense variant results in reduced protein abundance (steady-state levels) compared to WT.
  • Mechanism: CHX chase assays revealed accelerated degradation of the mutant protein, while synthesis rates remained normal.
  • Structural Insight: The variant does not cause global protein destabilization but induces a localized change in the N-terminal region (residues 11–18), which contains a predicted APC/C-associated D-box degron. The mutation likely increases degron accessibility or recognition, leading to enhanced ubiquitination and proteasomal degradation.
• Patient Cells: Primary T cells from the patient showed significantly reduced endogenous WDR91 levels, confirming the functional impact of the compound heterozygous state.

B. Endosomal Maturation Defects

• Delayed Conversion: Both the p.Gln215* (null) and p.Tyr15Asn (hypomorphic) variants caused a significant delay in the conversion of early (PtdIns3P+) to late (Rab7+) endosomes in live-cell imaging.
• Colocalization: In cells expressing p.Tyr15Asn, WDR91 showed reduced colocalization with Rab7-positive compartments, suggesting the mutation impairs the efficient recruitment of WDR91 to active Rab7 endosomes.
• Phenotype: Enlarged endosomal structures were observed in mutant cells, consistent with a block in maturation.

C. Autophagy Dysregulation

• Transcriptional Upregulation: RNA-seq of WDR91 KO cells showed significant enrichment of autophagy-related genes (initiation, elongation, and maturation factors), suggesting a compensatory response to trafficking defects.
• Impaired Flux:
  • WDR91 KO cells and patient-derived T cells exhibited a significant accumulation of autophagosomes and autolysosomes under lysosomal inhibition, indicating impaired autophagic turnover.
  • Context-Dependent Rescue: In overexpression systems (HEK293T), p.Tyr15Asn could restore autophagic flux to near-WT levels, likely due to supraphysiological protein levels overcoming the degradation defect. However, in patient-derived cells (where protein levels are naturally low), autophagic turnover remained defective.

D. Clinical Correlates

• The patient presented with giant cytoplasmic granules in neutrophils, a cytological feature previously associated with lysosome-related organelle disorders (e.g., Chediak-Higashi). This suggests WDR91 dysfunction extends beyond the nervous system to affect myeloid cell granule homeostasis.

4. Significance and Contributions

• Establishment of Pathogenicity: This study provides the first comprehensive functional validation linking biallelic WDR91 variants to a severe human neurodevelopmental disorder, moving beyond descriptive case reports.
• Mechanistic Insight: It elucidates a dual-pathway mechanism of disease:
  1. Endosomal Maturation Failure: Impaired early-to-late endosome conversion disrupts neuronal maintenance and cortical development.
  2. Autophagy Dysregulation: Defective WDR91 leads to autophagic flux blockage, contributing to progressive neurodegeneration.
• Variant Characterization: The study defines p.Tyr15Asn as a partial loss-of-function allele whose severity is context-dependent (protein abundance). It identifies a novel mechanism where an N-terminal degron mutation accelerates protein turnover.
• Diagnostic Clue: The presence of giant neutrophil granules in a patient with severe NDD and WDR91 variants (and previously in WDR81 variants) suggests a shared pathophysiology of endolysosomal trafficking defects that can serve as a diagnostic biomarker.
• Clinical Expansion: The findings expand the spectrum of autophagy-related NDDs, linking Rab7-dependent endosomal maturation directly to autophagic regulation in human brain development.

Conclusion

The authors demonstrate that WDR91 deficiency causes a severe neurodevelopmental disorder characterized by microcephaly, lissencephaly, and epilepsy. The disease mechanism involves a compound heterozygous state where one allele abolishes protein expression and the other reduces protein stability via accelerated degradation. This deficiency disrupts the critical interface between endosomal maturation and autophagy, leading to neuronal dysfunction and progressive brain atrophy.