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Schaaf-Yang syndrome (SYS) is an ultra-rare neurodevelopmental disorder caused by truncating mutations in MAGEL2 Heterologous expression of wild-type (WT) or a truncated (p.Gln638*) C-terminal HA-tagged MAGEL2 revealed a shift from a primarily cytoplasmic to a more nuclear localisation for the truncated protein variant. We now extend this analysis to six additional SYS mutations on a N-terminal FLAG-tagged MAGEL2. Our results replicate and extend our previous findings, showing that all the truncated MAGEL2 proteins consistently display a predominant nuclear localisation, irrespective of the C-terminal or N-terminal position and the chemistry of the tag. The variants associated with arthrogryposis multiplex congenita display a more pronounced nuclear retention phenotype, suggesting a correlation between clinical severity and the degree of nuclear mislocalisation. These results point to a neomorphic effect of truncated MAGEL2, which might contribute to the pathogenesis of SYS., Competing Interests: Competing interests: None declared., (© Author(s) (or their employer(s)) 2024. No commercial re-use. See rights and permissions. Published by BMJ.)

Background: Schaaf-Yang syndrome (SYS) is caused by truncating mutations in MAGEL2 , mapping to the Prader-Willi region (15q11-q13), with an observed phenotype partially overlapping that of Prader-Willi syndrome. MAGEL2 plays a role in retrograde transport and protein recycling regulation. Our aim is to contribute to the characterisation of SYS pathophysiology at clinical, genetic and molecular levels., Methods: We performed an extensive phenotypic and mutational revision of previously reported patients with SYS. We analysed the secretion levels of amyloid-β 1-40 peptide (Aβ 1-40 ) and performed targeted metabolomic and transcriptomic profiles in fibroblasts of patients with SYS (n=7) compared with controls (n=11). We also transfected cell lines with vectors encoding wild-type (WT) or mutated MAGEL2 to assess stability and subcellular localisation of the truncated protein., Results: Functional studies show significantly decreased levels of secreted Aβ 1-40 and intracellular glutamine in SYS fibroblasts compared with WT. We also identified 132 differentially expressed genes, including non-coding RNAs (ncRNAs) such as HOTAIR , and many of them related to developmental processes and mitotic mechanisms. The truncated form of MAGEL2 displayed a stability similar to the WT but it was significantly switched to the nucleus, compared with a mainly cytoplasmic distribution of the WT MAGEL2. Based on the updated knowledge, we offer guidelines for the clinical management of patients with SYS., Conclusion: A truncated MAGEL2 protein is stable and localises mainly in the nucleus, where it might exert a pathogenic neomorphic effect. Aβ 1-40 secretion levels and HOTAIR mRNA levels might be promising biomarkers for SYS. Our findings may improve SYS understanding and clinical management., Competing Interests: Competing interests: None declared., (© Author(s) (or their employer(s)) 2023. Re-use permitted under CC BY-NC. No commercial re-use. See rights and permissions. Published by BMJ.)