Abstract Disclosure: H. Wu: None. W. Zhao: None. X. Chen: None. L. Gao: None. C. Xu: None. J. Zhao: None. X-linked hypophosphatemia (XLHR) is the most common genetic form of hypophosphatemic rickets (HR), which is caused by mutations in the phosphate regulating endopeptidase homolog X-linked (PHEX) gene. Till now, the genotype-phenotype relationship of XLHR and the pathogenic role of PHEX have not been fully determined. Here we summarized clinical features of 49 HR patients and detected 17 novel PHEX variants and 5 novel non-PHEX variants. Then we explored the pathogenesis of new-found variants from protein expression, glycosylation analysis, subcellular localization and endopeptidase activity. The results showed that missense variants (Q189H and X750R) slightly reduced protein expression without obviously altering the length and localization of the protein, while truncating variants significantly impaired the synthesis of PHEX and produced a shorter immature protein retained in cells. No evident correlation was observed between mutation types and clinical phenotypes. However, for the first time ever, we analyzed the relationship between PHEX activity and serum phosphorus level and found that patients with low PHEX activity tended to have severe hypophosphatemia and high rickets severity score (RSS). Then we established two new knock-in XLHR mouse models by introducing into mice two novel Phex variants (c.T1349C and c.C426G) found in patients using CRISPR/Cas9 technology. Both demonstrated the clinical manifestations of XLHR seen in the patients and PhexC426G mice showed more severe phenotype than PhexT[1]349C mice, which further confirmed the rationality of genotype-PHEX enzymatic activity correlation analysis. Therefore, our findings demonstrated that novel PHEX variants could disrupt protein function via affecting protein synthesis, post-translational modification, cellular trafficking and catalytic activity. Our study promoted the understanding of XLHR pathogenic mechanism and PHEX activity-phenotype correlation provided scientific support for precise diagnosis and treatment of XLHR. Presentation: 6/3/2024