Background: The mechanisms responsible for inappropriate insulin release from β-cells in Congenital Hyperinsulinism in Infancy (CHI) have largely focused upon defects in KATP channels. Little is known about insulin biogenesis, the profiles of insulin in insulin-containing secretory granules or whether the impact of KATP 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 KATP 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.
23 - 25 Nov 2016
British Society for Paediatric Endocrinology and Diabetes