dentification and characterization of novel PAX4 variants in patients with suspected MODY9.

INTRODUCTION: Maturity-onset diabetes of the young (MODY) is a monogenic form of diabetes. MODY type 9 (MODY9) is a rare subtype caused by variants in the PAX4 gene. However, the pathogenicity and mechanisms of many PAX4 variants remain unclear. This study aimed to evaluate the clinical relevance and pathogenic mechanisms of three novel PAX4 variants identified in patients with suspected MODY. RESEARCH DESIGN AND METHODS: Three unrelated patients with early-onset diabetes and a family history of the disease were screened for PAX4 variants using whole-exome and Sanger sequencing. In silico predictions, evolutionary conservation analysis, and structural modeling were performed. Functional studies were conducted in MIN6 cells to assess protein expression, subcellular localization, and degradation pathways. RESULTS: Three novel PAX4 variants (c.83delA; p.Gln28ArgfsTer6, c.35T>C; p.Leu12Pro, and c.488G>C; p.Arg163Pro) were identified. Expression of p.Gln28ArgfsTer6 was undetectable, likely due to nonsense-mediated decay. In contrast, p.Leu12Pro and p.Arg163Pro retained nuclear localization but resulted in markedly reduced protein levels. Treatment with the proteasome inhibitor MG132 restored protein levels of the missense mutants, indicating enhanced proteasomal degradation as the likely mechanism. These findings suggest that certain PAX4 variants impair β-cell function by destabilizing the protein post-translationally. CONCLUSIONS: This study expands the spectrum of PAX4 variants and provides novel mechanistic insights into the pathogenesis of MODY9. Our results highlight the importance of assessing protein-level consequences for variant interpretation and support the integration of functional assays into MODY genetic diagnostic workflows.