This piece is inspired by Amanda’s research on the epigenetic relationship between early-life adversity and depression/suicide undertaking. As well, it puts focus on the importance of the translation step of the central dogma in determining which differentially transcribed genes are actually synthesized into proteins, and therefore contribute to the depression and suicide phenotypes.

According to the World Health Organization (WHO), depression is currently the leading cause of global disability, with over 300 million people affected worldwide (1). Major Depressive Disorder (MDD) is characterized by depressed mood and anhedonia, as well as secondary symptoms of appetite or weight changes, insomnia or hypersomnia, psychomotor agitation or retardation, fatigue, deficits in cognition and concentration, feelings of worthlessness, excessive guilt, and suicidality (2). The relationship between depression and suicidality is of particular interest to global health, considering that approximately 60% of suicides occur in individuals with a diagnosis of MDD (3).

One of the leading factors associated with both MDD and suicide is early-life adversity (ELA), which includes childhood sexual, physical, and emotional abuse, as well as neglect. ELA has also been studied in association with epigenetic alterations: heritable changes that do not affect the genome, yet still result in changes to gene expression- which can include DNA methylation, chromatin accessibility changes, and other alterations to gene expression. There is ample evidence that exposure to ELA can trigger sustained epigenetic alterations in certain genes related to stress and emotional regulation, that likely contribute to the MDD/suicide phenotype in adulthood(4).

Gene expression, which refers to the process by which DNA base-pair sequences are converted into functional gene products, begins with transcription. During transcription, particular sequences from the genome are copied into RNA (notably into mRNA for coding sequences that will produce protein products), the sum of which can be referred to as the transcriptome. When the transcriptome is altered via epigenetic regulation, we can describe the changes in genes present as “differentially expressed” (5).

Following transcription, coding mRNAs undergo the process of translation, whereby they are bound by ribosomes. In the ribosome, transfer RNAs or “tRNAs” with attached amino acids are recruited to the codons (or 3 nucleotide sequences) of the mRNA that match the codons at the bottom of the tRNA structure. Different codons correspond to different amino acids, and in this way, the translation machinery “reads” the mRNA to identify which amino acids to build the protein sequences with. The amino acids attached to the tRNAs link together to form amino acid chains as the tRNAs enter and then leave the ribosome after transferring their amino acid to the forming protein chain. This is how complex proteins that make up every tissue in the body are eventually formed(6).

This is all of the particular interest within the context of the ELA/MDD/suicide pathway as epigenetic changes- such as DNA methylation and differential gene expression- have already been demonstrated in the brain (particularly in the frontal cortex, Anterior Cingulate Cortex, and Amygdala, and Habenula) of individuals who experienced ELA and later were diagnosed with MDD and died by suicide, compared to non-ELA MDD/suicides and controls, but differential expression of tRNA genes and translational regulation have not (5). This represents an unexplored but crucial piece of the framework that I investigate in my research.

Specifically, I aim to use a methodology called 4-leaf clover qPCR (a specialized form of quantitative PCR) to validate tRNA differentially expressed genes (DEGs) in the anterior cingulate cortex (ACC) of individuals with MDD who died of suicide. In general, qPCR works by attaching fluorescent probes to specific DNA or RNA sequences (that have been converted to DNA via library construction), amplifying them via qPCR, and then quantifying the number of reads that correspond to each probe (7).

By doing so, I am hoping to identify any tRNA DEGs that reflect altered levels of translation in MDD/suicide, in ways that influence the MDD/suicide phenotype. One such hypothesis is that initiator methionine tRNAs- which always begin the translation process- will be dysregulated in MDD/suicides compared to controls in ways that indicate altered protein synthesis at the synapses in neurons (a phenomenon already observed in MDD in another piece of the translation machinery-MTORC1) (6).

In this way- it can be demonstrated that ELA, transcription, and translation all play crucial roles in the development of depression and in conferring risk for suicide.

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References

1. World Health Organization. (2017). Depression and Other Common Mental Disorders Global Health Estimates. World Health Organization.

2. American Psychiatric Association. (2013). Diagnostic and statistical manual of mental disorders (5th ed.).

3. Cavanagh, J. T., Carson, A. J., Sharpe, M., & Lawrie, S. M. (2003). Psychological autopsy studies of suicide: A systematic review. Psychological Medicine, 33(3), 395-405.

4. Turecki, G. (2014). The molecular bases of the suicidal brain. Nature reviews neuroscience, 15(12), 802-816.

5. Schwanhäusser, B., Busse, D., Li, N., Dittmar, G., Schuchhardt, J., Wolf, J., . . . Selbach, M. (n.d.). Global quantification of mammalian gene expression control. Nature, 473, 337-342.

6. Laguesse, S., & Ron, D. (2019). Protein Translation and Psychiatric Disorders. The Neuroscientist, 26(1), 21-42.

7. Honda, S., Shigematsu, M., Morichika, K., Telonis, A. G., & Kirino, Y. (2015). Four-leaf clover qRT-PCR: A convenient method for selective quantification of mature tRNA. RNA Biology, 12(5), 501-508.