臺灣博碩士論文加值系統

引言
憂鬱症是一種常見的精神疾患，影響著世界上約三百萬人口，帶來的影響甚巨。由於憂鬱症的成因非常複雜，包含遺傳因素、社會因素、心理因素以及生理機轉等等，因此要確定憂鬱症與其他複雜疾病的因果關係是非常困難的。而糖尿病是一種常見的慢性疾病，其特徵在於血糖水平的異常，且常是憂鬱症的共病症，過去的研究已經證實了憂鬱症與糖尿病之間的雙向因果關係，然而兩種疾病的成因都極為複雜，究竟是因為患者的血糖水平不穩定或是其他如心理壓力等因素導致憂鬱症的出現，仍沒有明確的證據。因此，若是能確定血糖水平與憂鬱症之間的因果關係，將會對於預防疾病的發生很有幫助。由於血糖水平的測量易受外在因素影響，因此若利用基因的方法排除干擾，並確定其與憂鬱症之間的因果後，就能針對具有血糖水平異常風險的人提前預防糖尿病與憂鬱症，進而阻止疾病發生且達到疾病預防的效果。綜上所述，糖尿病可能經由血糖水平的異常增加憂鬱症的風險，但其因果關係仍需要進一步探討。
方法
本研究使用了105,385位來自台灣人體生物資料庫的受試者資料，在排除其他如思覺失調、帕金森氏症、癡呆症、酒精與物質濫用以及產後憂鬱等精神疾患後，將自述過去是否罹患憂鬱症與躁鬱症的受試者分別定義為健康人(100,622位)、憂鬱(3,435位)與躁鬱症(649位)患者，同時本研究亦納入五間台灣醫院中經臨床醫師診斷而招募的憂鬱症(749位)與躁鬱症(944位)患者，並收集其基因資訊。血糖值資料來自受試者空腹超過八小時後所採集的血液樣本。在針對單核甘酸多態性與個體進行一連串品質管控後，隨機抽樣將樣本分為兩個不重疊的群體，以分別進行全基因組關聯性分析，並進行雙樣本、兩步驟以及多變量孟德爾隨機化等分析流程，同時評估血糖與情緒疾患之間的因果相關以及此關係是否透過特定中介因子。
本研究分別使用羅吉斯回歸與線性回歸對情緒疾患以及血糖水平進行遺傳關聯性的檢驗，同時針對孟德爾隨機化進行水平多效性的測試與敏感度的分析。為了釐清因果的方向性，本研究分析了憂鬱症對於血糖水平的影響，以及相反的關係。最後，使用兩步驟及多變量孟德爾隨機化分析，推測是否存在中介因子效應。
結果
研究結果顯示，在全基因組關聯性分析中，對於四個性狀分別找到了多個顯著位點，並分別納入多個位點作為工具變量(空腹血糖值22個；糖化血色素44個；憂鬱症6個；躁鬱症15個)。孟德爾隨機化分析的結果中發現較高的糖化血色素值是憂鬱症的風險因子，為單向因果關係，然而其與躁鬱症無因果相關；而空腹血糖則與兩種情緒疾患皆無因果相關。後續的中介分析則指出，糖化血色素會增加膽固醇、三酸甘油脂以及低密度脂蛋白水平，然而血脂並非糖化血色素與憂鬱症之間的中介因子。
結論
本研究透過分析台灣人之情緒疾患以及血糖水平的基因位點，並利用孟德爾隨機化的方法排除複雜的干擾後，評估血糖水平與情緒疾患之間的因果相關。研究結果發現了較高的血糖水平導致憂鬱症的風險，且因果關係是單向的，控制血糖水平在降低憂鬱症風險中扮演著一定的角色。此外糖化血色素是高血脂水平的風險因子，然而血脂並非糖化血色素與憂鬱症之間的中介因子。本研究提供因果方向的訊息，未來的研究可進一部探討背後的生物途徑與調控機制。
關鍵字：憂鬱症、躁鬱症、空腹葡萄糖、糖化血色素、全基因組關聯性分析、孟德爾隨機化

Background
Major depressive disorder (MDD) is a common mental disease and the causes of MDD are very complicated, such as genetic factors, social factors, psychological status and physiological mechanisms. Therefore, it is very difficult to clarify the causal relationship between MDD and other complex diseases. Diabetes is a common chronic diseases characterized by abnormal blood sugar levels such as fasting glucose (FG) and glycated hemoglobin (HbA1c), and often coexist with depression. Although previous studies have confirmed the bidirectional causality between depression and diabetes, there is no evidence to confirm whether the reason of this relationship is abnormal blood sugar levels or other factors such as psychological stress. If the causal relationship between abnormal blood glucose levels and depression can be determined, it will be very helpful to prevent the disease. Because the measurement of blood sugar level is confounded by external factors easily, we use genetic methods which is useful to exclude the confounders to investigate the causality between blood sugar levels and depression in this study. After determining the causal relationship, we can prevent both diabetes and depression in advance, especially to these people which have abnormal blood sugar levels. In summary, diabetes may increase the risk of depression through abnormal blood glucose levels, but the real causality of blood sugar levels and depression still need to be investigated.
Materials and Methods
In this study, we used 105,385 participants’ data from Taiwan Biobank (TWB). After excluding individuals with other mental diseases such as schizophrenia, Parkinson's disease, dementia, alcohol and substance abuse, and postpartum depression, we define MDD (n=3,435), bipolar disorder (BP) (n=649) patients and healthy controls (HC) (n=100,622) by self-report as MDD, BPD or have no any mental disease in past history. This study also recruited MDD (n=749) and BPD (n=944) patients from five Taiwan hospitals diagnosed by clinician. We collected genotype and blood samples of participants with fasting for more than eight hours. After quality control of single nucleotide polymorphism (SNP) and individuals, we classify samples into two non-overlapping subgroups by random sampling and performed genome-wide association study (GWAS), two sample Mendelian randomization analysis (MR), two step Mendelian randomization (2SMR), multivariable Mendelian randomization analysis (MVMR) to evaluated the causality between blood glucose level and mood disorders and whether this relationship is through specific mediators. We performed GWAS by liner regression and logistic regression for blood sugar levels and mood disorders, both pleiotropy tests and sensitivity tests were executed in Mendelian randomization analysis. For investigating the causality, we performed bidirectional MR in this study, and speculate whether there is mediation effect by 2SMR and MVMR.
Results
In this study, we find some significant SNPs of four traits and included them as instrumental variables (22 SNPs for FG ; 44 SNPs for HbA1c ; 6 SNPs for MDD and 15 SNPs for BPD). After MR analysis, we confirm that higher HbA1c level is a risk factor of MDD, but the causal relationship is unidirectional. However, HbA1c is not causally related to BPD and fasting glucose has no causal relationship with both MDD and BPD. Subsequent mediation analysis pointed out that HbA1c will increase the level of cholesterol, triglyceride and low-density lipoprotein, but blood lipids are not the mediators between blood sugar level and depression.
Conclusion
In this study, we find significant SNPs of four traits (FG, HbA1c, MDD and BPD) in Taiwanese population, and investigate the causal relationships between blood sugar level and mood disorders after excluding confounders by MR method. We find that higher blood sugar level will increase the risk of MDD, but it is a one-way causal relationship. Controlling of blood sugar level plays an important role in reducing the risk of depression. In addition, HbA1c is a risk factor of high blood lipid levels, but blood lipids are not mediators between HbA1c and depression. This research provides the information of the causal relationship between blood sugar level and depression, it will be helpful for the subsequent research on the biological pathways and regulatory mechanisms of diabetes and depression and prevent disease.
Keywords: Major depressive disorder, bipolar disorder, fasting glucose, glycated hemoglobin (HbA1c), genome-wide association analysis (GWAS), Mendelian randomization analysis (MR)

Contents
中文摘要 i
Abstract iii
Contents vi
List of Tables viii
List of Figures x
List of Supplementary Tables xiii
Chapter 1 Introduction 1
1.1 Epidemiology of diabetes 1
1.2 Epidemiology of depression 1
1.3 Causal relationship between diabetes and depression 2
1.4 Causality between blood sugar levels and depression 2
1.5 Potential mediators 3
1.6 Mendelian randomization study (MR) 4
1.7 Aim of this study 5
Chapter 2 Materials and Methods 6
2.1 Participants 6
2.2 Measurements of blood sugars and blood lipids 6
2.3 Genotyping and Imputation 7
2.4 Quality control (QC) 8
2.5 Genome-wide association study (GWAS) 8
2.6 Mendelian randomization analysis 9
2.7 Mediation analysis 10
Chapter 3 Results 11
3.1 Basic demographic characteristics 11
3.2 Genome-wide association study (GWAS) 11
3.3 Mendelian randomization analysis (MR) 12
3.4 Mediation analysis 13
Chapter 4 Discussion 14
4.1 Significant genetic variants in GWAS 14
4.2 MR analysis between blood sugar levels and mood disorders 15
4.3 Mediation effect between HbA1c and MDD 17
4.4 Strengths and Limitations 18
4.5 Conclusion 19
Reference 20
Tables and Figures 26
Supplementary Tables 88

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