Type 2 Diabetes Risk Alleles Reveal a Role for Peptidylglycine Alpha-amidating Monooxygenase in Beta Cell Function

Abstract

149Word count: 4500Number of tables and figures: 5 . CC-BY-NC-ND 4.0 International license peer-reviewed) is the author/funder. It is made available under a The copyright holder for this preprint (which was not . http://dx.doi.org/10.1101/158642 doi: bioRxiv preprint first posted online Jul. 3, 2017; 2 ABSTRACT Molecular mechanisms underpinning the genetic risk for type 2 diabetes (T2D) remain poorly understood, hindering translation into new therapies. Recently, genome-wide studies identified two coding variants in Peptidylglycine Alpha-amidating Monooxygenase (PAM) associated with T2D risk and measures of beta cell dysfunction. Here, we demonstrate that both risk alleles impact negatively on overall PAM activity, but via distinct effects on expression and catalytic function. In a human beta cell model, PAM silencing caused decreased insulin content and altered dynamics of granule exocytosis. Analysis of primary human beta cells from cadaveric donors confirmed an effect on exocytosis in carriers of the p.D563G T2D-risk allele. Finally, we show that the granular packaging protein Chromogranin A is a PAM substrate and a strong candidate for mediating downstream effects on insulin secretion. Taken together, our results establish a role for PAM in beta cell function, and uncover a novel mechanism for T2D-associated PAM alleles.

Islet prohormone processing in health and disease

“…The D563G variant was identified in another European large‐scale whole‐genome sequencing study, and was shown again to be associated with increased T2D risk . PAM polymorphisms may directly contribute to islet secretory defects, as a recent report showed that human islets from D563G variants have a modest impairment in insulin granule exocytosis …”

“…For example, ChgA is processed into the vasostatins, pancreastatin, catestatin, parastatin and other products . Although little is known regarding the relative abundance of ChgA‐ and ChgB‐derived peptides in β‐cells, and their local effect on islet function, loss of Pam‐mediated amidation of ChgA has recently been suggested as a contributor to β‐cell dysfunction in carriers of Pam variants . ProSAAS, an inhibitor of Pc1/3, and 7B2 (secretogranin V), an inhibitor and chaperone of proPc2, are also synthesized as propeptides in β‐cells, and processed to mature products.…”

“…29 PAM polymorphisms may directly contribute to islet secretory defects, as a recent report showed that human islets from D563G variants have a modest impairment in insulin granule exocytosis. 59 Pam deletion is not well-tolerated in multiple model organisms,…”

Genetic discovery and translational decision support from exome sequencing of 20,791 type 2 diabetes cases and 24,440 controls from five ancestries

“…The gene-level signal in PAM also remained nominally significant ( p <0.05) even after removing the 35 strongest individually associated PAM variants, indicating a contribution from substantially more variants than p.Asp563Gly and p.Ser539Trp ( Supplementary Figure 14 ). Cellular characterization of p.Asp563Gly and p.Ser539Trp recently identified a novel mechanism for T2D risk through altered insulin storage and secretion 46 . Our results provide many more genetic variants – identifiable only through sequencing 17 – that could be characterized for further insights into the T2D risk mechanism mediated by PAM .…”

Beta-cell autophagy under the scope of hypoglycemic drugs; possible mechanism as a novel therapeutic target

“…Therefore, there should be different pathophysiological pathways for the development of the condition. Many of these mechanisms have been shown to have a genetic predisposition, including; Proopiomelanocortin gene mutation, Melanocortin receptor mutations, brain-derived neurotrophic factor and receptor mutations, glucose kinase mutations, hepatocyte nuclear factor mutations, mitochondrial DNA mutations, Insulin receptor mutations, viral oncogene homolog 2 (AKT2), v-akt murine thymoma and even lipodystrophy can alter the normal function of β-cell [29][30][31][32][33]. Apoptosis appears to be the leading cause of β-cell destruction in both types of diabetes mellitus through activation of interleukin (IL)-1β, nuclear factor (NF)-κB, and Fas pathways, through a process, known as insulitis [34].…”