Familial hyperinsulinism, more commonly known as persistent hyperinsulinaemic hypoglycaemia of infancy (PHHI), is an autosomal recessive disorder characterised by excess secretion of insulin and hypoglycaemia. In most patients, the disease is caused by mutations in either of the two subunits, Kir6.2 and the sulphonylurea receptor 1 (SUR1), which constitute the functional ATP-sensitive potassium (K_ATP) channel of the pancreatic β-cell. When expressed in mammalian cell lines, the mutant channels are either non-functional, or exhibit a reduced response to a fall in the [ATP]/[ADP] ratio (Shyng et al. 1998). We show that one such non-functional mutant R1394H-SUR1 when co-expressed with Kir6.2 in Xenopus oocytes gives rise to functional K_ATP channels, as determined using the two-electrode voltage-clamp technique (n = 3). We then asked whether the reported loss of function of R1394H-SUR1 and Kir6.2 in mammalian cells could be due to impaired trafficking of mutant channels to the cell membrane. Using immunostaining on stably transfected HEK293 cells, we found that the R1394H mutation in SUR1 abolishes membrane expression of the channel subunits; both R1394H-SUR1 and Kir6.2 exhibit a vesicular-like staining pattern (n = 14 independent experiments). The trafficking defect, unlike the ▓Dgr│F508-cystic fibrosis transmembrane conductance regulator mutant (Denning et al. 1992), could not be reversed by decreasing the incubation temperature of the HEK cells. Pretreatment for 12 h with 100 µM diazoxide (a drug used in the treatment of PHHI) fully reversed the trafficking defect and restored surface expression of the mutant channels (n = 5 independent experiments). The rescuing effect conferred by diazoxide was fully prevented in the presence of 1 µM glibenclamide, a K_ATP channel blocker (n = 3 independent experiments). Thus we report a novel mechanism by which PHHI mutations can cause the disease, and demonstrate a novel mechanism by which diazoxide could contribute to its therapeutic action, that is, its ability to recruit channels to the membrane.

Research funds were from MRC.

Denning, G.M., Anderson, M.P., Amara, J.F., Marshall, J., Smith, A.E. & Welsh, M.J. (1992). Nature 358, 761-764.

Shyng, S.-L., Ferrigni, T., Shephard, J.B., Nestorwicz, A., Glaser, B., Permutt, A.M. & Nichols, C.G. (1998). Diabetes 47, 1145-1151.