Beta-propeller protein-associated neurodegeneration (BPAN), formerly known as static encephalopathy of childhood with neurodegeneration in adulthood (SENDA), is a subtype of neurodegeneration with brain iron accumulation (NBIA) caused by mutations in the WDR45 gene on the X chromosome. BPAN is the most recently identified and one of the more common NBIA subtypes, accounting for approximately 35–40 of molecularly confirmed NBIA cases (Hayflick et al., 2013). The disorder is characterized by a unique biphasic clinical course: childhood-onset global developmental delay that remains relatively static, followed by progressive dystonia, parkinsonism, and dementia in adolescence or early adulthood (Haack et al., 2012). [1]
BPAN occurs predominantly in females due to its X-linked dominant inheritance pattern, with most cases arising de novo. Affected males are rare and typically present with more severe phenotypes, consistent with hemizygous loss of WDR45 protein function (Saitsu et al., 2013). [2]
The WDR45 gene (also known as WIPI4) is located on chromosome Xp11.23 and encodes a 360-amino-acid protein belonging to the WIPI (WD-repeat protein interacting with phosphoinositides) family. The WDR45 protein contains a seven-bladed beta-propeller structure — from which the disease derives its name — and functions as a critical component of the autophagy machinery (Saitsu et al., 2013). [3]
More than 100 different pathogenic variants have been identified in WDR45, including missense, nonsense, frameshift, and splice-site mutations, as well as partial or complete gene deletions. Most mutations result in loss of function of the WDR45 protein. No clear genotype-phenotype correlations have been established, likely because the variable X-inactivation patterns in females contribute significantly to clinical heterogeneity (Stige et al., 2021). [4]
WDR45/WIPI4 plays an essential role in autophagy, specifically in the formation of autophagosomes. It binds phosphatidylinositol 3-phosphate (PI3P) on nascent autophagosomal membranes and recruits ATG2, facilitating lipid transfer to expand the phagophore membrane (Bakula et al., 2017). Loss of WDR45 function leads to: [5]
Studies in WDR45 knockout mice demonstrate accumulation of ubiquitin-positive protein aggregates, swollen axons, neuronal loss particularly in the substantia nigra and cerebellum, and progressive motor and cognitive deficits that recapitulate the human disease (Zhao et al., 2015). [^6]
The precise mechanism linking autophagy dysfunction to brain iron accumulation in BPAN remains under investigation. The leading hypothesis involves disrupted ferritinophagy: under normal conditions, selective autophagy of ferritin (NCOA4-mediated ferritinophagy) releases iron from ferritin cages for cellular use. When autophagy is impaired, iron-laden ferritin accumulates within cells, and paradoxically, bioavailable iron may be reduced, triggering compensatory upregulation of iron import pathways. Over time, this leads to total iron overload in vulnerable brain regions (Mancias et al., 2014). The preferential involvement of the globus pallidus and substantia nigra reflects the high baseline iron content and metabolic demands of these regions. [^7]
The first phase of BPAN typically manifests in infancy or early childhood with: [^8]
During this phase, cognitive and motor abilities remain relatively stable or show slow improvement with developmental interventions. Brain MRI may be normal or show subtle signal changes in the substantia nigra (Hayflick et al., 2013). [^9]
The second phase typically begins in the late teens to early 30s and is marked by abrupt neurological deterioration: [^10]
The progression during Phase 2 is relentless, typically leading to severe disability within 5–10 years of onset. Life expectancy is reduced, though data on long-term survival are limited given the recent recognition of the disorder (Gregory & Hayflick, 2017). [^11]
Postmortem studies of BPAN brains reveal a distinctive neuropathological profile: [^12]
The co-occurrence of tau, alpha-synuclein, and iron pathology in BPAN is remarkable and positions the disease at the intersection of multiple neurodegenerative proteinopathies (Hayflick et al., 2013). [^13]
Brain MRI findings in BPAN evolve with disease progression:
Phase 1 (Childhood):
Phase 2 (Adult):
The T1 hyperintense "halo" surrounding the substantia nigra on T1-weighted images is considered a diagnostic hallmark of BPAN and may be present before clinical neurodegeneration becomes apparent (Kruer et al., 2012).
Diagnosis of BPAN requires integration of clinical, neuroimaging, and genetic findings:
Differential diagnosis includes other NBIA subtypes (particularly PKAN and PLAN), Rett syndrome, juvenile Parkinson's disease, and other causes of childhood-onset epileptic encephalopathy (Gregory & Hayflick, 2017).
There is currently no disease-modifying therapy for BPAN. Management is supportive and symptomatic:
BPAN occupies a unique position in the neurodegenerative disease landscape due to its mixed proteinopathy:
This section highlights recent publications relevant to this disease.
A Comprehensive Overview of the Clinical, Electrophysiological, and Neuroimaging Features of BPAN: Insights From a New Case Series. ↩︎
A dwdr45 knock-out drosophila model to decipher the role of autophagy in BPAN. ↩︎
New biphenylvinylanthracene-based polymers for organic electronics applications: effect of the acceptor group on optoelectronic properties. ↩︎
Generation of two human iPSC lines from fibroblasts of BPAN patients carrying pathogenic variants in the WDR45 gene. ↩︎
Autophagy and mitophagy at the synapse and beyond: implications for learning, memory and neurological disorders. ↩︎