What we experienced mentally as a child may have unspoken impact on our characters as an adult. It is well-known that Hitler experienced a miserable adolescence when he was severely abused. This troubled upbringing was believed to have a dominant negative effect on Hitler’s moral psychological development. On the contrary, Beethoven grew into a great musician with the tough training from his abusive father. Both environmental stresses and our genetic factors shape us. But how does this happen?
Imagine an injury when we got cut by a knife. Suppose that a child got hurt on his finger when he was carelessly playing with a sharp knife. He was left with a scar that may be conspicuous even years later. Four things are important during this process: the finger, the knife, the injury represented by blood, and a hidden genetic profiling of the kid. This fourth hidden genetic factor can be easily overlooked, but it is very important. For example, when a kid with haemophilia is cut in the finger, he will experience excessive bleeding.
What resembles the finger, the knife, the injury and the genetics when we get mentally stressed? They are dopaminergic neurons, specific stress, glucocorticoids, and similar genetic factors. Dopaminergic neurons are located in the midbrain. Although fewer in numbers, they are involved in a lot of behavioral processes, including mood, reward, addiction, and in particular, stress. One specific example of mental stress is social isolation. Glucocorticoids are important hormones responsible for regulating a lot of physiological events. It is glucocorticoids that affect dopaminergic neurons. The hidden genetic factors can be complicated, and a great number of genes are related to mental stress response. One gene called DISC1 was found to predispose individuals to schizophrenia and clinical depression. When an adolescent boy gets socially isolated, the “knife” of stress leaves “cuts” in the dopaminergic neurons. He will experience changes in his dopaminergic neurons and in the secretion of glucocorticoids. These changes will reshape, to a degree, his personality.If the kid unfortunately has an altered DISC1, the changes will probably have more severe consequences.
Then how are the changes in dopaminergic neurons retained in the mind to allow a profound influence to present itself years later? It is unlikely that the genes in dopaminergic neurons have changed, because DNA sequences, except those in tumors, often remain the same during one's lifetime. However, DNA can undergo some modification, such as methylation. The modifications may last for a long time. In dopaminergic neurons, a gene named tyrosine hydroxylase can undergo DNA methylation, and such methylation can be burnt into memory that will affect behavior years later. Therefore, although the DNA sequence remains the same, the function has changed. The DNA modifications such as methylation, acetylation, and histone modification, constitute the so-called epigenetics, which is a hot research field in genetics.
What a surprise to know that our genetics is susceptible to mental trauma, and that DNA modifications can stay with us for so long! Please rest assured because the other side of the coin is that these discoveries bring forward new solutions. First, with the aid of personal genome sequencing, which is becoming affordable nowadays, a clinician can tell if a person is predisposed to sustain certain mental diseases. This can lead to early prevention and better diagnosis of mental illness. Secondly, scientists can develop new medicine to re-modify the DNA modifications and to help patients recover from mental trauma.
[1] Dopaminergic neurons. Int J Biochem Cell Biol. 2005 May;37(5):942-6.
[2] Adolescent Stress−Induced Epigenetic Control of Dopaminergic Neurons via Glucocorticoids. Science 339, 335 (2013)
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