Study of a Novel NUMB Loss-of-Function Mutation Leading to Hyperuricemia and Gout

2024-11-22 Fri
NUMB hyperuricemia gout uric acid excretion genetic variant
NUMB Gene Variant: A Novel Mechanism in Gout DevelopmentBackgroundGout is one of the most common forms of inflammatory arthritis worldwide, primarily triggered by the persistent elevation of serum uric acid levels, known as hyperuricemia. Hyperuricemia leads to the deposition of urate crystals, resulting in inflammation of joints and other tissues. While the development of gout is closely linked to both genetic and environmental factors, the precise molecular mechanisms remain largely unclear. The balance of uric acid metabolism is mainly achieved through its production in the liver and excretion by the kidneys and intestines, with around 70% of uric acid being excreted by the kidneys. However, many aspects of the molecular mechanism underlying impaired uric acid excretion are still unresolved.
Recently, a research team from the Affiliated Hospital of Qingdao University and the Karolinska Institute in Sweden discovered a rare variant in the NUMB gene that is associated with hereditary gout. This study, published in Cell Discovery, reveals a critical role for NUMB in regulating uric acid excretion and explains how its mutation can lead to gout.
Origin and Background of the StudyNUMB is a conserved clathrin adaptor protein that plays an important role in cell fate determination. It is involved in endocytosis and intracellular trafficking by interacting with membrane proteins. However, the role of NUMB in uric acid excretion had not been previously explored. The research team aimed to investigate whether NUMB gene mutations could be linked to impaired uric acid excretion and gout, and to uncover the underlying molecular mechanisms.
Research MethodsThe research team recruited a family with hereditary gout from Shandong, China, along with 300 patients with sporadic gout. By employing whole exome sequencing (WES), they identified a rare variant of the NUMB gene associated with gout. They further investigated the impact of this mutation on uric acid metabolism through a combination of cell experiments, mouse models, and proteomic analysis.
1. Family and Genetic Variant IdentificationIn this family, 11 out of 83 members across five generations were diagnosed with gout. WES analysis showed that all affected individuals carried the NUMB gene variant (R630H). Sanger sequencing further confirmed that this variant was not present in healthy family members or among 300 patients with sporadic gout.
2. Interaction between NUMB and Urate Transporter ABC-G2The study found that NUMB binds to the ABC-G2 protein through its YxNxxF motif, regulating the protein’s localization on the membrane of renal tubular epithelial cells (RTECs). Co-immunoprecipitation and colocalization experiments provided evidence for a direct interaction between NUMB and ABC-G2.
3. Impact of NUMB Mutation on Intracellular Distribution and FunctionThe NUMB R630H mutation led to abnormal aggregation of the NUMB protein, which was degraded through an autophagy-dependent pathway, significantly reducing NUMB levels in cells. CRISPR-Cas9 knockout of the NUMB gene caused ABC-G2 protein to relocate from the apical membrane to the basolateral membrane of polarized renal epithelial cells, impairing uric acid excretion.
4. Phenotypic Analysis of NUMB Mutation Mouse ModelThe research team used gene knock-in technology to establish a mouse model carrying the NUMB R630H mutation. Compared with normal mice, mutant mice exhibited significant hyperuricemia and reduced uric acid excretion. Furthermore, the localization of ABC-G2 at the apical membrane of renal tubules was reduced in mutant mice, resulting in chronic uric acid nephropathy.
Research Findings1. New Function of the NUMB ProteinThe study is the first to reveal that NUMB regulates uric acid excretion by guiding the apical localization of ABC-G2. This new mechanism highlights the previously unknown function of NUMB in maintaining cellular polarity.
2. Pathogenic Mechanism of NUMB R630H VariantThe NUMB R630H variant decreases functional NUMB protein expression, disrupting the apical localization of ABC-G2, leading to impaired uric acid excretion and hyperuricemia. Moreover, the mutation caused structural abnormalities in renal tubules and increased inflammation.
3. Validation through Mouse ModelThe NUMB variant mouse model successfully replicated hyperuricemia and associated kidney abnormalities observed in humans, providing an ideal animal model for further research into uric acid metabolism regulation and potential gout treatments.
Significance of the Study1. A New Perspective on Gout PathogenesisThis study reveals the critical role of NUMB in uric acid excretion and its involvement in gout development, offering a new perspective for understanding the molecular mechanisms of gout.
2. Potential for Clinical Diagnosis and TreatmentIdentification of the NUMB variant provides a new target for the molecular diagnosis of hereditary gout. Furthermore, restoring NUMB function or regulating the localization of ABC-G2 may become a novel strategy for the treatment of hyperuricemia and gout.
Future ProspectsFuture studies could further explore the role of NUMB in the polarized localization of other membrane proteins and its potential association with other diseases. Developing therapeutic agents targeting NUMB function could lead to significant advancements in the treatment.