Currently, more than 800 million people worldwide suffer from major depressive disorder (MDD). Due to the high incidence, early onset, chronic course, unnatural death and impaired social function and cognitive ability caused by this condition , depression will become the second leading contributor to disease burden by the year 2020 . MDD is a heterogeneous illness, and there are currently no effective methods to objectively assess the severity, endophenotypes, or prognosis of this disease. Additionally, its occurrence and development are influenced by social environmental factors. Thus, one effective measure to reduce the burden on society is to identify potential biomarkers to allow for earlier diagnosis and intervention.
So far, the search for diagnostic and prognostic biomarkers for MDD has made considerable progress. Previous research has shown that peripheral growth factors (BDNF, GDF-15) [3,4], neurotransmitter (Shank3) , endocrine factors (insulin, MMP09, melatonin) [6,7] and proinflammatory cytokines (thromboxane)  could be potential biomarkers for MDD. However, upon assessment of the predictive characteristics of these biomarkers for diagnostic purposes, they typically showed high false positive rates , and they cannot explain the interaction between gene and environment, that is to say the environmental and behavioral factors play an important role in the onset and development of MDD.
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