Identification of a familial hyperinsulinism-causing mutation in the sulfonylurea receptor 1 that prevents normal...

Area of Science:

• Molecular biology
• Cell biology
• Genetics

Background:

• Persistent hyperinsulinemic hypoglycemia of infancy (PHI) is a rare genetic disorder.
• Mutations in pancreatic ATP-sensitive potassium (K(ATP)) channel subunits, sulfonylurea receptor 1 (SUR1) and Kir6.2, are known causes of PHI.
• Understanding the molecular mechanisms underlying these mutations is crucial for developing effective treatments.

Purpose of the Study:

• To investigate the functional and trafficking consequences of a novel SUR1 mutation, L1544P, identified in a patient with PHI.
• To elucidate the role of endoplasmic reticulum (ER) retention signals in K(ATP) channel assembly and surface expression.

Main Methods:

• Co-expression of wild-type and mutant SUR1 with Kir6.2 in COS cells.
• Functional assessment of K(ATP) channel activity using MgADP response.
• Quantification of channel surface expression via cell surface biotinylation.
• Analysis of protein degradation pathways (lysosomal and proteasomal).
• Site-directed mutagenesis to remove ER retention motifs (RKR).

Main Results:

• The L1544P SUR1 mutation significantly reduced K(ATP) channel response to MgADP (approx. 10% of wild-type).
• Surface expression of K(ATP) channels with L1544P SUR1 was markedly decreased (approx. 19% of wild-type).
• The mutation did not affect the steady-state level of SUR1 protein, and degradation pathways were not implicated.
• Removal of the RKR ER retention motif in SUR1 or Kir6.2 individually increased mutant channel surface expression.
• Simultaneous removal of the RKR motif in both subunits restored surface expression to wild-type levels.

Conclusions:

• The L1544P mutation impairs K(ATP) channel function and surface expression in PHI.
• The mutation likely interferes with proper K(ATP) channel trafficking by disrupting the shielding of ER retention signals.
• Targeting ER trafficking mechanisms may offer therapeutic strategies for PHI caused by such mutations.