Endocrine Abstracts

Background: The mechanisms responsible for inappropriate insulin release from β-cells in Congenital Hyperinsulinism in Infancy (CHI) have largely focused upon defects in K_ATP channels. Little is known about insulin biogenesis, the profiles of insulin in insulin-containing secretory granules or whether the impact of K_ATP channel defects (due to mutations in ABCC8 or KCNJ11) is the same in diffuse- and focal disease.

Objective and hypotheses: We aimed to define the ultrastructural properties of the insulin-containing granules in β-cells from different forms of CHI and to compare these with control β-cells.

Methods: Tissue was obtained from six patients with CHI who underwent surgery for the treatment of hypoglycaemia. All patients were positive for mutations in the K_ATP channel gene ABCC8. Morphometric analysis and immuno-gold labelling of insulin (I-Au) was applied to frozen tissue sections of control (n=4 cases) and CHI tissues (diffuse-CHI, n=3; focal-CHI, n=3). Data were acquired using Transmission Electron Microscopy from images stacks in each of the different groups; control n=60 cells, diffuse-CHI n=58 cells and focal-CHI n=61 cells.

Results: Three profiles of secretory granules were defined across all tissues; (i) mature granules with dense-core/crystalline insulin; (ii) immature secretory granules and (iii) secretory granules that were depleted of insulin (confirmed by I-Au labeling). We found that approximately 60% of secretory granules (n=3428) were depleted of insulin in focal-CHI compared to around 10% of granules in diffuse- (n=2258) and control β-cells (n=2577). The percentages of immature granules were significantly lower in focal-CHI (5.7±1.7%) compared with diffuse CHI (45.5±8.7%) and control samples (31.6±3.7%). In contrast, control β-cells had a higher proportion of crystalline granules (62.9±3.1%) compared with focal- (36.6±3.5%) and diffuse-CHI (42.7±1.9%). We also found a higher incidence of multi-vesicular secretory granule structures in focal- (74.7±3.3%) compared to diffuse-CHI and control β-cells (39.5±6.8% vs. 27.8±5.6%).

Conclusion: Our data imply that β-cells in focal-CHI have a greater secretory capacity (increased number multi-vesicular secretory granules and depleted granules) than in diffuse disease, despite the fact that both conditions associate with ABCC8 gene defects.