Predicting Depression: How Scientists Identify Patients at Risk

Research on Biochemical Markers for Depression

Understanding Major Depressive Disorder

Scientists are exploring the potential of identifying molecules that could serve as biochemical markers for depression, aiming to predict individuals at risk of developing this condition. Major depressive disorder (MDD), often referred to as clinical depression, is a complex mood disorder that poses challenges in both diagnosis and treatment. The disorder is characterized by persistent feelings of sadness, anhedonia (the inability to experience pleasure), and cognitive impairments. Symptoms vary widely among individuals, complicating the diagnostic process, which predominantly relies on subjective assessments from tools like the Diagnostic and Statistical Manual of Mental Disorders. While our grasp of the factors contributing to depression is expanding, it remains limited in scope.

Causes of Depression

Historically, depression was primarily attributed to deficiencies in monoamine neurotransmitters, such as dopamine and serotonin. However, contemporary research indicates that numerous dysregulated pathways contribute to various dimensions of depression. These include factors like growth hormones, metabolic irregularities, and inflammation. Many symptoms of depression overlap with those of physical illnesses, such as fatigue and impaired concentration, prompting scientists to investigate the role of inflammation in depressive disorders. The kynurenine pathway, which connects peripheral inflammation to the central nervous system, plays a crucial role in this context. When activated, this pathway converts tryptophan into kynurenine, subsequently depleting tryptophan levels essential for serotonin production. The activation of the kynurenine pathway also leads to the generation of free radicals and neurotoxins, both of which may exacerbate depressive symptoms. This metabolic pathway produces several molecules that might serve as biomarkers for depression—molecules whose presence or absence could aid in diagnosing the disease or assessing treatment efficacy.

Potential of Biomarkers in Predicting Depression

Study on Kynurenine Pathway Molecules

In a recent study published in *Scientific Reports*, researchers from Japan aimed to determine if molecules derived from the kynurenine pathway could predict susceptibility to depression. The study analyzed 120 samples from the Resource Center for Health Science database in Kyoto, comparing 61 individuals identified as high-risk for major depressive disorder (HRMDD) with 59 age-matched healthy controls. The concentration levels of various kynurenine pathway molecules were compared across both groups. The findings indicated that anthranilic acid levels were significantly elevated in the HRMDD group, while tryptophan levels were found to be lower in women from this group compared to their healthy counterparts.

Anthranilic Acid as a Biomarker

Subsequently, researchers focused on patients who later developed depression, analyzing samples from their healthy state and comparing them to those diagnosed with depression. This analysis included 33 HRMDD subjects and 66 age-matched healthy controls, taking into account two patients on antidepressants and three on tranquilizers. The researchers calculated ratios of metabolite concentrations when patients were healthy versus when they were depressed. Notably, anthranilic acid emerged as the only molecule whose ratio correlated with changes in depression status, as measured by the Centre for Epidemiological Studies Depression scale. This suggests that anthranilic acid may serve as a promising biomarker for individuals at high risk of major depressive disorder.

Link Between Chronic Pain and Depression

The research team further explored the implications of these molecules in chronic pain disorders, recognizing the strong association between chronic pain and depression, both of which are linked to inflammation. They compared individuals with chronic pain who experienced depression to those without. However, no significant differences were found in kynurenine pathway molecules between the two groups.

Changes in Anthranilic Acid in Animal Models

The researchers also investigated whether changes in anthranilic acid concentrations could be observed in mice subjected to depression-like symptoms induced by chronic unpredictable mild stress. The results indicated an increase in anthranilic acid levels in the mice exhibiting these symptoms compared to control subjects.

Limitations and Future Directions

Challenges in Diagnostic Tools

Despite the promising findings regarding anthranilic acid, it is unlikely that a single biomarker will suffice for predictive diagnostics. A single marker lacks the necessary specificity and sensitivity for effective diagnosis. Moreover, similar shifts in anthranilic acid and other kynurenine pathway metabolites have been observed in conditions such as bipolar disorder and schizophrenia. The prevailing view in the field suggests that comprehensive panels of multiple biomarkers will be necessary to provide a thorough evaluation of a patient’s condition. Further research is essential to ascertain the viability of anthranilic acid as a biomarker for depression.

Conclusion

The exploration of biomarkers like anthranilic acid presents a significant step forward in understanding and potentially diagnosing depression. However, extensive studies are needed to establish their role in clinical settings and to develop effective diagnostic tools.

References

Réus, G. Z. et al. Kynurenine pathway dysfunction in the pathophysiology and treatment of depression: Evidences from animal and human studies. Journal of Psychiatric Research 68, 316-328 (2015).
Strawbridge, R., Young, A. H. & Cleare, A. J. Biomarkers for depression: recent insights, current challenges and future prospects. Neuropsychiatric Disease and Treatment (2017).
Müller, N., Myint, A.-M. & Schwarz, M. J. Inflammatory biomarkers and depression. Neurotoxicity Research 19, 308-318 (2011).
Schmidt, H. D., Shelton, R. C. & Duman, R. S. Functional biomarkers of depression: diagnosis, treatment, and pathophysiology. Neuropsychopharmacology 36, 2375-2394 (2011).
Sakurai, M. et al. Serum Metabolic Profiles of the Tryptophan-Kynurenine Pathway in the high-risk subjects of major depressive disorder. Scientific Reports 10, 1961, doi:10.1038/s41598-020-58806-w (2020).