Functional Homology of WIPI4 and Atg18 Enables WDR45 Variant Interpretation for BPAN Diagnosis

β-propeller protein-associated neurodegeneration (BPAN) is the most prevalent subtype of neurodegeneration with brain iron accumulation (NBIA) and is caused by mutations in the WDR45 gene encoding the autophagy-related protein WIPI4. However, many WDR45 missense variants remain classified as variants of uncertain significance (VUS), highlighting the need for reliable functional assays to support their clinical interpretation. In this study, we identified the yeast ortholog of WDR45 and established a Saccharomyces cerevisiae-based functional complementation assay to assess the pathogenicity of WDR45 variants. We first showed that deletion of the β-propellers that bind polyphosphoinositides (PROPPIN)-encoding genes ATG18 or HSV2 causes mitochondrial dysfunction and impaired respiratory growth. Human WDR45/WIPI4 specifically rescued the respiratory defect of the atg18Δ strain, whereas WDR45B/WIPI3 complemented the hsv2Δ phenotype, establishing yeast Atg18 as the closest functional counterpart of WIPI4, thus clarifying PROPPIN orthology. We then evaluated a panel of WDR45 variants and found that benign variants restored normal growth, whereas truncating and pathogenic missense variants failed to complement the atg18Δ phenotype, validating the assay for functional variant classification. Finally, we analyzed several VUS identified in patients with clinically compatible BPAN and obtained functional evidence supporting their pathogenicity. Overall, our study establishes a simple, robust, and scalable yeast model that enables functional interpretation of WDR45 variants and improves molecular diagnosis of BPAN.