25-OR: Investigating a Candidate Causal Allele in Type 2 Diabetes Susceptibility Gene PAM as a Cause of Neonatal Diabetes Mellitus

Abstract

Peptidylglycine α-amidating monooxygenase (PAM) catalyses the amidation of glycine-extended peptides, thereby maximising their biological potency. We recently showed that coding alleles in PAM mediate type 2 diabetes (T2D) risk through alterations to PAM protein activity and expression in the β cell. Identifying alleles along a continuum of functional severity informs on the relationship between protein perturbation and clinical effect. We hypothesised that rare, loss of function PAM alleles could cause severe β cell dysfunction presenting as neonatal diabetes. Sequencing of 15 probands presenting with diabetes at ≤ 6 months of age and their unaffected immediate family, identified a novel de novo nonsynonymous coding variant (p.R36S) in PAM in a single proband. We investigated the impact of the variant allele on amidating activity, protein stability and localisation as a measure of PAM function and the consequences on β cell viability. Recombinant R36S-PAM exhibited normal amidating activity in vitro which was consistent with PAM activity measured in patient serum (WT: 391pmol/μl/hr vs. R36S: 305pmol/μl/hr). In human β cells (EndoC-βh1) the secreted luminal R36S-PAM isoform displayed reduced protein expression (-78%, p=0.004) compared to WT-PAM, which could be rescued by inhibiting the proteosomal pathway (+120% of WT); whilst the non-secreted membrane-integral isoform was retained in the endoplasmic reticulum (ER). We observed that overexpression of R36S-PAM elevated levels of ER stress (p-IRE1: +196%, p=0.02; CHOP: +114%, p=0.051) and pro-apoptotic markers (ASK1: +145%, caspase-3: +154%) compared with WT-PAM. We have identified a novel, de novo coding variant, in a gene with a proven role in T2D risk, in a single patient with neonatal diabetes. Functional studies provide evidence for protein mislocalisation and instability and support a role for this allele in ER stress and β cell dysfunction consistent with neonatal diabetes pathogenesis. Disclosure S. Sengupta: None. L.L. Bonnycastle: None. B. Hastoy: None. A. Grotz: None. M.M. Umapathysivam: None. A. Raimondo: None. A. Swift: None. P.S. Chines: None. A. Clark: None. H. Huopio: None. F.S. Collins: None. M. Laakso: None. A.L. Gloyn: Consultant; Spouse/Partner; Eli Lilly and Company, Merck & Co., Inc. Consultant; Self; Merck & Co., Inc. Consultant; Spouse/Partner; Novo Nordisk A/S, Pfizer Inc. Research Support; Self; Novo Nordisk A/S. Speaker's Bureau; Self; Novo Nordisk A/S. Other Relationship; Self; Diabetes UK, European Foundation for the Study of Diabetes. Funding UK Wellcome Trust