Serotonin is everywhere, and it's a good thing, too.
"It is synthesized in the raphe nuclei, which project to virtually every part of the brain," Xiaodong Zhang, postdoctoral researcher at Duke University, told BioWorld Today. "That itself tells you it's a very important system."
And when that system goes awry, it leads to any number of disorders. Pharmaceutical market research firm IMS Health said selective serotonin reuptake inhibitors (SSRIs), and their cousins, selective norepinephrine reuptake inhibitors, are the third-largest therapeutic class, with 2004 U.S. sales of more than $10 billion through October. SSRIs increase the serotonin available to the brain and are used in the treatment of depression and anxiety disorders.
Earlier this year, the FDA ruled that all antidepressants, including SSRIs, needed to include "a boxed warning and expanded warning statements that alert health care providers to an increased risk of suicidality (suicidal thinking and behavior) in children and adolescents being treated with these agents," after studies had shown that to be true. (See BioWorld Today, Sept. 14, 2004, and Sept. 16, 2004.)
Despite those problems, SSRIs remain an important weapon in the treatment arsenal for major depression, as well as a definite improvement over earlier classes of antidepressants, due mainly to their reduced side effects. Of course, anything that helps predict how a given patient will react to SSRIs would be a boon to physicians. And in the Dec. 9, 2004, online issue of Neuron, scientists from Duke University report what might turn out to be such information. They have identified a gene variation in the key enzyme regulating serotonin synthesis that appears to affect susceptibility to both major depression and treatment with SSRIs.
The gene in question is tryptophan hydroxylase 2. It is the rate-limiting enzyme in the synthesis of serotonin, and involved in all manner of cheery emotions: Previous studies have linked SNPs in the TPH2 gene to depression, aggression, irritability and anger-related traits.
Tying SNP To Function By Focusing On Exons
However, those studies all identified SNPs located in noncoding introns, which make up the vast majority of the TPH2 gene - the whole gene is about 100,000 base pairs long, but coding exons make up only about 1,600 base pairs. "Most people look in the introns because they are much bigger," Zhang, who is lead author of the study, said. "But if you can find a SNP [in an exon], you can really address the functional consequences."
The Duke researchers previously had reported on a SNP in the mouse TPH2 gene that led to a decrease in serotonin production of 50 percent to 70 percent. In the new study, titled "Loss-of-Function Mutation in Tryptophan Hydroxylase-2 Identified in Unipolar Major Depression," they took their search to humans. They first analyzed the exons of TPH2 in about 300 individuals, and found a previously unidentified polymorphism that replaced a highly conserved amino acid near the protein's C terminal. Cells transfected with the mutant gene produced 70 percent to 80 percent less serotonin than those transfected with the wild-type allele.
The researchers then screened different populations for the presence of the mutant allele. About 10 percent of a group of people diagnosed with major depression had either one or two copies of the mutant gene, while only about 1.5 percent of control subjects did. Notably, while control subjects carrying the mutant allele did not meet the formal diagnostic criteria for major depression, all displayed clinical symptoms of conditions, such as generalized anxiety, mild depression and a family history of mental illness or drug and alcohol abuse.
Of the nine depressive patients that had the mutant gene in the Neuron study, all were either unresponsive to treatment with SSRIs altogether or only responded to the highest doses of the drugs. Zhang was cautious about making too much of the population studies at this point: "It's a relatively small-scale study, and we're not going to give any conclusion until a much larger study can be done. But they are very interesting findings."
The molecular mechanism by which the mutation affects serotonin levels remains a bit of a mystery. "So far, we still don't know what causes the loss of function," Zhang said.
The researchers already have excluded one possible mechanism. Tryptophan hydroxylase needs to oligomerize to do its job; that is, several identical subunits aggregate to form the functioning protein. A related mutation in the phenylalanine hydroxylase gene affects the proper folding of that enzyme and leads to a loss of protein expression and, consequently, to phenylketonuria.
Because the point mutation the group found is in the region of the gene at which the aggregation takes place, the scientists first hypothesized that the mutation they had identified might affect that aggregation, thereby rendering the protein ineffective. But when they transfected cells with wild-type, mutant or mixed alleles for the tryptophan hydroxylase gene, both protein expression and dimerization rates of all three combinations were similar.
The Duke group wants to do larger-scale association studies, and also is searching for further SNPs in the TPH2 exons. Zhang hopes that those SNPs will help improve treatment for serotonin-related disorders.
"If you have five treatment options and know a mutation that tells you that only two of them will likely be effective, it could give a virtual clue for clinicians to go straight to the right treatment," he said. "Maybe in the long run, that will be the way to go."