Background: Hyperinsulinaemic hypoglycaemia (HH) has two main histological subtypes: diffuse and focal. Diffuse HH are most commonly due to recessive or dominant ABCC8/KCNJ11 mutations. Focal HH results due to somatic loss of the maternal 11p allele involving the ABCC8 and KCNJ11 region in patients with paternally inherited ABCC8 or KCNJ11 mutation.
Aim: To molecular characterise a novel non-stop KCNJ11 mutations associated with a unique dual clinical phenotype of focal and diffuse HH.
Patients and methods: A female infant with paternally inherited heterozygous KCNJ11 mutation (c.1171T>C; p.*391Rext*94 Kir6.2), presented with diazoxide unresponsive HH. First 18F-DOPA-PET suggested focal lesion in tail, which was resected. Tissue histology and microsatellite analysis confirmed focal HH. Post-resection, HH persisted and repeat 18F-DOPA-PET suggested diffuse disease. A second limited pancreatectomy was undertaken. Tissue histology and microsatellite analysis confirmed diffuse HH. After second resection, the patient was diazoxide responsive. Point mutation of Kir6.2 was introduced in the human Kir6.2 cDNA in the pcDNA3.1 plasmid by site-directed mutagenesis. HEK293 cells were transfected with WT hamster SUR1 cDNA and mutant human Kir6.2 cDNA using FuGENE. Functional properties of channels were studied using whole-cell patch-clamp recordings. Both homogenous and heterozygous expressions of the mutants were studied.
Results: Currents equivalent to endogenous HEK293 cells currents were noticed when mutant p.*391Rext*94 Kir6.2 subunit was expressed with WT SUR1 subunit (53±10pA/pF vs. 80±33pA/pF). This suggested negligible KATP current, consistent with the diazoxide-unresponsive clinical picture before the resection of focal lesion. With heterozygous p.*391Rext*94 Kir6.2 expression (1:1 molar ratio of WT and mutant Kir6.2 units, with WT SUR1 subunits) to simulate the situation in the rest of the pancreas, significantly reduced but diazoxide responsive KATP currents were observed (325±54pA/pF vs. 147±43pA/pF; P=0.03).
Conclusion: This study describes the first reported dual focal and diffuse HH phenotype with KCNJ11 mutations. Molecular characterization supports the observed clinical phenotype.
Funding: This work was supported by MRC UK.
01 Oct 2015 - 03 Oct 2015