Multiple beta cell-independent mechanisms drive hypoglycemia in Timothy syndrome

Abstract

The canonical G406R gain of function mutation that reduces inactivation and increases Ca2+influx through the CACNA1C-encoded CaV1.2 voltage gated Ca2+channel underlies the multisystem disorder Timothy syndrome (TS), characterized by invariant Long QT syndrome and consequent life-threatening arrhythmias. Severe episodic hypoglycemia, which exacerbates arrhythmia risk, is among the myriad non-cardiac TS pathologies that are poorly characterized. While hypoglycemia is thought to result from increased Ca2+influx through CaV1.2 channels in pancreatic beta cells and consequent hyperinsulinism, this mechanism has never been demonstrated due to a lack of informative animal models, thus hampering development of preventive strategies. We generated a CaV1.2 G406R knockin mouse model that recapitulates key TS features including hypoglycemia. Unexpectedly, these mice did not show hyperactive beta cells or hyperinsulinism in the setting of normal intrinsic beta cell function, suggesting dysregulated glucose homeostasis. We discovered multiple alternative contributors to hypoglycemia, including perturbed counterregulatory hormone responses with defects in glucagon secretion and abnormal hypothalamic glucose sensing. Together, these data provide new insights into physiological contributions of the broadly expressed CaV1.2 channel and reveal integrated consequences of the mutant channel that underlie the life-threatening events in TS.