Recently, the relationship between GLP1R polymorphisms and T2DM was reported to vary with ethnicity . However, current studies mainly focus on whether GLP1R gene variants increase the risk of T2DM without age stratification . The prevalence of EOD has increased rapidly worldwide, especially in Asian countries . In this study, we investigated the relationship between genetic variations of the GLP1R gene and susceptibility to EOD.
We defined adult T2DM patients diagnosed at age ≤ 45 years as having EOD. Consistent with the results of the 2010 General Survey of Diabetes in China , this study found that blood glucose, blood lipid, BMI, insulin resistance, and other indexes in EOD patients were significantly higher than those in non-diabetic subjects. In sex analysis, the proportion of male patients in the EOD group was significantly higher than that of female patients (P<0.01), which may be related to the complex interaction between genes and the environment. Recently, gene variants, such as TRIB3 (rs2295490), ADIPOQ (rs10937273), LEPR (rs1892534), and TCF7L2 (rs7903146), were identified as risk factors for EOD [20–23]. GLP1R is a G-protein-coupled receptor with a typical seven-transmembrane α-helical core domain (TMD) and an extracellular domain (ECD), playing a critical role in hormone signal transduction and receptor activation. Once the ligand contacts the receptor, the ECD of GLP1R firstly binds to the C-terminal of the ligand, followed by binding of the N-terminal of the ligand to the TMD of the receptor, thus activating the receptor. Amino acid residues in the extracellular domain of GLP1R play a critical role in its self-folding and structural stability. Previous studies have reported that GLP1R gene polymorphisms are closely related to insulin secretion and response to GLP1R analogs in diabetes patients. In hyperglycemic clamp experiments in healthy individuals, Sathananthan showed that rs3765467 was associated with altered β-cell response to GLP1 infusion . Moreover, the rs6923761 polymorphism of the GLP1R gene is closely related to fasting serum levels of GLP1 and adipocytokine in newly diagnosed T2DM patients . In addition, carriers of genotype GG at rs4714210 in GLP1R showed a decreased risk of coronary heart disease . However, the correlation between GLP1R gene polymorphism and the onset age of T2DM, especially EOD, has not been reported. This study is the first to investigate GLP1R gene polymorphisms and their effect on clinical characteristics in a Chinese EOD population. We found that among the 16 SNP sites with MAF > 0.05, only the polymorphism at position rs3765467 in GLP1R was statistically associated with EOD. Previous studies have indicated that the most frequent substitution in GLP1R is Gly-168 to Ser (G→A, rs6923761) . However, the MAF of rs6923761 was < 0.01 in this study, which might be attributed to the limited samples used and ethnic differences; thus, statistical analysis of the data could not be carried out.
The rs3765467 polymorphism involves an Arg-131 to Gln (G→A) substitution, which affects the binding efficiency of the ligand and GLP1R, thereby interfering with downstream signal transduction. Dipeptidyl peptidase 4 (DPP-4) inhibitors are common hypoglycemic drugs that can prevent the degradation of GLP1. Polymorphism at position rs3765467 in GLP1R may influence response to DPP-4 inhibitors. Patients with the rs3765467 GA+AA genotype had a relatively better response to DPP-4 inhibitors and a greater reduction in HbA1c . Another study found that the GLP1R rs3765467 G > A variant could significantly reduce insulin secretion and cyclic AMP (cAMP) concentration in response to high glucose exposure, which might promote β cell apoptosis . In this study, carriers of allele A at rs3765467 were independently associated with a risk of EOD after adjusting for sex and BMI. Furthermore, compared with that of the GG genotype, the GA+AA genotype decreased the risk of EOD, further confirming that the rs3765467 polymorphism is closely related to susceptibility to EOD. However, neither the distribution of the genotype nor the allelic frequency of rs3765467 was statistically different between the LOD and non-diabetic population. We speculated that the presence of the A allele at position rs3765467 may be specifically associated with a reduced susceptibility to EOD.
Previous studies have shown that patients with EOD suffer from a more severe impairment of islet β-cell function and require insulin treatment at an earlier time . In this study, the effect of genotypes on clinical characteristics among EOD patients was also investigated. No significant differences were found in HOMA-IR and HOMA-β between rs3765467 GG and GA+AA genotype carriers. However, the association between these genotypes and insulin levels could not be assessed accurately by HOMA-IR and HOMA-β, as most patients had been treated with hypoglycemic agents and/or insulin. Herein, we also assessed the levels of CP, a marker of insulin secretion, between these genotypes. Compared with those of the rs3765467 GG genotype carriers, patients with the GA+AA genotypes showed increased FCP levels in the EOD group. We speculated that the GLP1R rs3765467 (G→A) mutation may ameliorate the impaired islet function of β-cells in EOD patients, thereby decreasing the risk of EOD. Thus, the mutated A allele at position rs3765467 may be a protective factor to improve the secretory function of islet cells.
Although this study showed a correlation between GLP1R rs3765467 and susceptibility to EOD, it has a few limitations. First, this is a single-center study, and there may be sample selection bias. Second, the rs3765467 polymorphism had no significant effect on other clinical parameters in EOD patients, which may be attributed to the small sample size and our inability to detect weak associations. Third, no association was detected for all the SNPs in GLP1R between the LOD and control groups. As SNPs with a MAF < 0.01 in the GLP1R gene could not be detected, the slight effects of these GLP1R variants were also possibly undetected. Therefore, further replications in other cohorts are needed to confirm the association of genetic variants of the GLP1R gene with susceptibility to EOD.