血漿升糖素濃度之全基因組相關性分析

Abstract

胰島素與升糖素為血糖調節中的關鍵賀爾蒙，兩者的生理作用相反且相互調節，分別由β細胞與α細胞分泌，升糖素上升會增加肝醣分解及糖質新生作用，而導致血清中的血糖濃度上升。第二型糖尿病患者的胰島質量比正常人輕且較小，同時β細胞數量佔整體胰島細胞比例會下降、α細胞的佔比則上升；第二型糖尿病致病機轉所謂的「厄運八重奏」（Ominous octet）將升糖素濃度上升被視為糖尿病指標之一。 本研究以臺灣人體資料庫檢體進行全基因體關聯研究（GWAS），試圖針對本國人種族，找尋影響升糖素分泌的遺傳因子與探討可能的調控機轉，為後續血糖調控研究提供新的方向。本實驗透過AlphaLISA®免疫分析定量升糖素，以臺灣人體資料庫基因型資料，分析了一共1,746位受試者，共找到6個具高關聯性之位點，分別為rs138661318位於第二條染色體基因座EPHA4 (EPH receptor a4)上、rs116063643與rs181373404位於第三條染色體上、rs144466533位於第五條染色體基因座SIL1 (SIL1 nucleotide exchange factor)上、rs80065348與rs201869328分別位於第六條染色體基因座FAM135A (family with sequence similarity 135 member A)與BACH2 (BTB domain and CNC homolog 2)上。 進一部探討各位點調控升糖素分泌的可能機制，EPHA4基因參與細胞間的訊息傳遞，轉譯出的蛋白Eph藉調控Eph/ephrin訊號傳遞調控升糖素分泌；NFKBIZ轉譯之蛋白為NF-kappa-B inhibitor zeta，NFkB路徑被視為會影響胰島素阻抗，NFKBIZ為NFkB路徑調節的因子之一，大鼠接受90%胰臟切除後，NFkB路徑基因（包含NFKBIZ）表現量大幅上升，顯示NFkB路徑基因與β細胞再生有關。由此推測，NFKBIZ被視為藉由抑制發炎反應而影響胰島素阻抗性，進而影響胰島素分泌；SIL1轉譯之蛋白為SIL1 Nucleotide Exchange Factor，根據過去動物實驗， SIL1基因剔除小鼠，胰島細胞數量較少且胰島體積較小、胰島素濃度降低，由於胰島素會抑制升糖素分泌，由此推測，SIL1調控胰島胰島素分泌，進而影響升糖素的濃度。在過去針對糖尿病致病基因研究中，FAM135A被發現與糖尿病有顯著相關的基因，但相關機轉仍須持續研究。最後，BACH2轉譯出的BTB domain and CNC homolog 2蛋白負責協助B細胞與T細胞的分化與形成，同時也參與胰島β細胞的細胞凋亡與免疫反應。據過去研究發現，剔除BACH2基因能恢復失效之β細胞、改善胰島素分泌不足的狀況，由此可推知， BACH2基因是藉由調控β細胞與受體結合之能力而影響胰島素分泌，進而影響升糖素的濃度。

Insulin and glucagon are key hormones in blood sugar regulation. Their physiological effects are opposite and regulate each other. Insulin and glucagon are secreted by β cells and α cells, respectively. The increase of glucagon will increase the decomposition of glycogen, and increase the effect of gluconeogenesis, which will lead to the increase of blood sugar concentration. The mass of islets in patients with type 2 diabetes is lighter and smaller than that of normal people. At the same time, the proportion of β cells in the total islet cells will decrease, and the proportion of α cells will increase. The mechanism of type 2 diabetes (so-called "Ominous octet”) views the increase the concentration of glucagon as one of the indicators of diabetes. In this study, a genome-wide association study (GWAS) was carried out with samples from the Taiwan Biobank, trying to find the genetic factors that affect the secretion of glucagon and explore the possible regulatory mechanism for Taiwanese population, providing a new direction for the further research on blood sugar regulation. In this experiment, glucagon was quantified by AlphaLISA®. 1,746 subjects were analyzed based on the genotype data from the Taiwan Biobank database, and a total of 6 significantly correlated loci were found, rs138661318 is located on chromosome 2, rs116063643 and rs181373404 are located on chromosome 3, rs144466533 is located on chromosome 5, rs80065348 and rs201869328 are located on chromosome 6. For further discussion, the possible mechanism of regulation of glucagon secretion by each locus was explored. EPHA4, is involved in the communication between cells, which encodes Eph receptors (Eph). Eph regulates the secretion of glucagon by regulating Eph/ephrin signal pathway. The protein encoded by NFKBIZ is the NF-kappa-B inhibitor zeta. The NFkB pathway is considered to affect insulin resistance. NFKBIZ is one of the factors regulating the NFkB pathway. After 90% pancreatectomy, NFkB pathway genes (including NFKBIZ) expression increased sharply, showing that NFkB pathway genes are related to β-cell regeneration. It is speculated that NFKBIZ is considered to affect insulin resistance by inhibiting the inflammatory response, mainly affecting the insulin pathway. The protein encoded by SIL1 is SIL1 Nucleotide Exchange Factor. According to previous animal studies, SIL1 knockout mice have fewer islets, smaller islet volume, and lower insulin concentration. Therefore, it is speculated that SIL1 regulates the type of islet β cells, thereby affecting the ability of insulin secretion. Because insulin and glucagon regulate each other, This in turn affects the concentration of glucagon. In previous studies, FAM135A was considered as a gene significantly related to diabetes, but the related regulatory mechanism is remained unclear. Last, BACH2 encodes BTB domain and CNC homolog 2 protein, which is responsible for assisting the differentiation and formation of B cells and T cells and is also involved in the apoptosis and immune response of pancreatic β cells. According to the previous publication, inhibiting the islet BACH2 can restore the failed β cells and improve the condition of insufficient insulin secretion. It can be deduced that the BACH2 is activated by regulating the ability of β cells to bind to receptors. Affect insulin secretion, thereby affecting the concentration of glucagon.