The Arg16Gly variant of the beta2-adrenergic receptor predisposes to hypoglycemia unawareness in type 1 diabetes mellitus

Experimental pharmacological approaches to reverse impaired awareness of hypoglycemia-a review

The colossal global burden of diabetes management is compounded by the serious complication of hypoglycemia. Protective physiologic hormonal and neurogenic counterregulatory responses to hypoglycemia are essential to preserve glucose homeostasis and avert serious morbidity. With recurrent exposure to hypoglycemic episodes over time, these counterregulatory responses to hypoglycemia can diminish, resulting in an impaired awareness of hypoglycemia (IAH). IAH is characterized by sudden neuroglycopenia rather than preceding cautionary autonomic symptoms. IAH increases the risk of subsequent sudden and severe hypoglycemic episodes in patients with diabetes. The postulated causative mechanisms behind IAH are complex and varied. It is therefore challenging to identify a single effective therapeutic strategy. In this review, we closely examine the efficacy and feasibility of a myriad of pharmaceutical interventions in preventing and treating IAH as described in clinical and preclinical studies. Pharmaceutical agents outlined include N-acetyl cysteine, GABA A receptor blockers, opioid receptor antagonists, AMP activated protein kinase agonists, potassium channel openers, dehydroepiandrosterone, metoclopramide, antiadrenergic agents, antidiabetic agents and glucagon.

Hypoglycemia, Vascular Disease and Cognitive Dysfunction in Diabetes: Insights from Text Mining-Based Reconstruction and Bioinformatics Analysis of the Gene Networks

Hypoglycemia has been recognized as a risk factor for diabetic vascular complications and cognitive decline, but the molecular mechanisms of the effect of hypoglycemia on target organs are not fully understood. In this work, gene networks of hypoglycemia and cardiovascular disease, diabetic retinopathy, diabetic nephropathy, diabetic neuropathy, cognitive decline, and Alzheimer's disease were reconstructed using ANDSystem, a text-mining-based tool. The gene network of hypoglycemia included 141 genes and 2467 interactions. Enrichment analysis of Gene Ontology (GO) biological processes showed that the regulation of insulin secretion, glucose homeostasis, apoptosis, nitric oxide biosynthesis, and cell signaling are significantly enriched for hypoglycemia. Among the network hubs, INS, IL6, LEP, TNF, IL1B, EGFR, and FOS had the highest betweenness centrality, while GPR142, MBOAT4, SLC5A4, IGFBP6, PPY, G6PC1, SLC2A2, GYS2, GCGR, and AQP7 demonstrated the highest cross-talk specificity. Hypoglycemia-related genes were overrepresented in the gene networks of diabetic complications and comorbidity; moreover, 14 genes were mutual for all studied disorders. Eleven GO biological processes (glucose homeostasis, nitric oxide biosynthesis, smooth muscle cell proliferation, ERK1 and ERK2 cascade, etc.) were overrepresented in all reconstructed networks. The obtained results expand our understanding of the molecular mechanisms underlying the deteriorating effects of hypoglycemia in diabetes-associated vascular disease and cognitive dysfunction.

Impact of Genetic Polymorphism in the β2-Receptor Gene on Risk of Severe Hypoglycemia in Patients With Type 1 Diabetes

CONTEXT

Severe hypoglycemic events are unevenly distributed in people with type 1 diabetes, making a genetic influence probable. Of the common adrenoceptor β-2 receptor gene (ADRB2) polymorphisms, the Arg16 allele is associated with receptor downregulation and reduced agonist-mediated endogenous glucose production.

OBJECTIVE

We tested the hypothesis that the Arg16 variant is associated with severe hypoglycemia.

METHOD

A cohort of 311 patients with type 1 diabetes reported severe hypoglycemic events retrospectively in a validated questionnaire. The patients were characterized by diabetes history, state of hypoglycemia awareness, C-peptide status, HbA1c, and ADRB2 genotype.

RESULTS

The ADRB2 Gly16Arg genotype distribution was in Hardy-Weinberg equilibrium. The rate of severe hypoglycemia differed among all genotypes (P = 0.01). Patients homozygous for the Arg16 genotype (AA; n = 60) had a relative rate (RR) of severe hypoglycemia of 2.2 (95% CI, 1.3 to 3.6) compared with patients homozygous for the Gly16 genotype (GG; n = 116; P = 0.002). Among patients with impaired awareness or unawareness (n = 175), those with the AA genotype (n = 33) had an RR of severe hypoglycemia of 3.2 (95% CI, 1.7 to 6.0) compared with patients with the GG genotype (n = 58; P < 0.000). Genotype was not associated with state of hypoglycemia awareness per se, as assessed by any of three classification methods. The difference was not explained by other risk factors.

CONCLUSION

Genetic polymorphism in ADRB2 is associated with risk of severe hypoglycemia in individuals with type 1 diabetes, especially in those with impaired hypoglycemia awareness.

Impact of the TCF7L2 genotype on risk of hypoglycaemia and glucagon secretion during hypoglycaemia

INTRODUCTION

In healthy carriers of the T allele of the transcription factor 7-like 2 (TCF7L2), fasting plasma glucagon concentrations are lower compared with those with the C allele. We hypothesised that presence of the T allele is associated with a diminished glucagon response during hypoglycaemia and a higher frequency of severe hypoglycaemia (SH) in type 1 diabetes (T1DM).

MATERIAL AND METHODS

This is a post hoc study of an earlier prospective observational study of SH and four mechanistic studies of physiological responses to hypoglycaemia. 269 patients with T1DM were followed in a one-year observational study. A log-linear negative binomial model was applied with events of SH as dependent variable and TCF7L2 alleles as explanatory variable. In four experimental studies including 65 people, TCF7L2 genotyping was done and plasma glucagon concentration during experimental hypoglycaemia was determined.

RESULTS

Incidences of SH were TT 0.54, TC 0.98 and CC 1.01 episodes per patient-year with no significant difference between groups. During experimental hypoglycaemia, the TCF7L2 polymorphism did not influence glucagon secretion.

DISCUSSION

Patients with T1DM carrying the T allele of the TCF7L2 polymorphism do not exhibit diminished glucagon response during hypoglycaemia and are not at increased risk of severe hypoglycaemia compared with carriers of the C allele.

The effect of antecedent hypoglycaemia on β₂-adrenergic sensitivity in healthy participants with the Arg16Gly polymorphism of the β₂-adrenergic receptor

AIMS/HYPOTHESIS

Homozygosity for glycine at codon 16 (GlyGly) of the β(2)-adrenergic receptor may alter receptor sensitivity upon chronic stimulation and has been implicated in the pathogenesis of hypoglycaemia unawareness. We compared the effect of antecedent hypoglycaemia on β(2)-adrenergic receptor sensitivity between GlyGly participants and those with arginine 16 homozygosity (ArgArg) for the β(2)-adrenergic receptor.

METHODS

We enrolled 16 healthy participants, who were either GlyGly (n = 8) or ArgArg (n = 8). They participated randomly in two 2 day experiments. Day 1 consisted of two 2-h hyperinsulinaemic hypoglycaemic (2.8 mmol/l) or euglycaemic (4.8 mmol/l) glucose clamps. On day 2, we measured the forearm vasodilator response to the β(2)-adrenergic receptor agonist salbutamol and the dose of isoprenaline required to increase the heart rate by 25 bpm (IC(25)).

RESULTS

The vasodilator response to salbutamol tended to be greater after antecedent hypoglycaemia than after euglycaemia (p = 0.078), consistent with increased β(2)-adrenergic receptor sensitivity. This effect was driven by a significant increase in β(2)-adrenergic receptor sensitivity following hypoglycaemia compared with euglycaemia in ArgArg participants (p = 0.019), whereas no such effect was observed in the GlyGly participants. Antecedent hypoglycaemia tended to decrease the IC(25) in ArgArg participants, whereas the reverse occurred in the GlyGly participants (GlyGly vs ArgArg group p = 0.047).

CONCLUSION/INTERPRETATION

Antecedent hypoglycaemia did not affect β(2)-adrenergic receptor sensitivity in healthy GlyGly participants, but increased it in ArgArg participants. If these results also hold for participants with type 1 diabetes, such an increase in β(2)-adrenergic receptor sensitivity may potentially reduce the risk of repeated hypoglycaemia and the subsequent development of hypoglycaemia unawareness in ArgArg diabetic participants.

TRIAL REGISTRATION

ClinicalTrials.gov NCT00160056.

Angiotensin-converting enzyme and angiotensin II receptor subtype 2 genotypes in type 1 diabetes and severe hypoglycaemia requiring emergency treatment: a case cohort study

AIMS

In type 1 diabetes, individual susceptibility to severe hypoglycaemia is likely to be influenced by genetic factors. We have previously reported an association of the deletion (D-) allele of the angiotensin-converting enzyme (ACE) insertion/deletion (I/D) polymorphism and the A-allele of the angiotensin II receptor subtype 2 (AT2R) 1675 G>A polymorphism with risk of severe hypoglycaemia in such patients. The aim of this study was to test the hypothesis that these alleles are more frequent in patients suffering from the most severe episodes of hypoglycaemia requiring medical emergency treatment.

METHODS

The case cohort study consisted of 108 cases of type 1 diabetic patients with severe hypoglycaemia requiring medical emergency treatment during a 1-year period and 262 consecutive controls without such events. ACE I/D and AT2R 1675G>A genotype distributions were compared between cases and controls.

RESULTS

The proportion of D-allele carriers was higher amongst cases than controls (83 vs. 73%; P=0.032). In contrast, AT2R genotype distribution was similar in cases and controls. In a multiple regression analysis, D-allele carriage remained a significant risk factor for being a case [odds ratio: 1.9 (1.0-3.6)] together with male sex, impaired symptomatic awareness of hypoglycaemia and presence of nephropathy.

CONCLUSION

The D-allele of the ACE gene is associated with severe hypoglycaemia requiring emergency treatment in type 1 diabetic patients with preserved spontaneous ACE activity. This supports the association between high ACE activity and occurrence of severe hypoglycaemia.