BioWorld International Correspondent
LONDON - Researchers in Germany predict that vast tracts of psychiatric research will have to be revisited following their discovery that the enzyme that synthesizes the neurotransmitter serotonin is encoded by two genes rather than one.
Because only one of the two genes is expressed in brain and other nervous tissue, the group's finding will, in addition, pave the way for new drugs to manipulate brain levels of serotonin without causing side effects elsewhere in the body. Similarly, the ability to alter levels of serotonin in the gut and blood without affecting those in the brain will make it possible to develop novel compounds capable of inducing immunosuppression and dissolving unwanted blood clots.
Diego Walther and Michael Bader of the Max Delbruck Center for Molecular Medicine in Berlin-Buch, Germany, and their colleagues, report their work in a paper in the Jan. 3, 2002, issue of Science titled "Synthesis of Serotonin by A Second Tryptophan Hydroxylase Isoform."
Walther told BioWorld International, "Hundreds of linkage studies have been done over the past few decades, looking for mutations in the gene for the enzyme that synthesizes serotonin. Our study indicates that these investigations have focused on the wrong gene."
Serotonin, which is also known as 5-hydroxytryptamine (5-HT), has a key role in determining mood. It also regulates sleep, anxiety, alcoholism, drug abuse, food intake and sexual behavior. In tissues other than the central nervous system, it is essential for maintaining the tone of blood vessels, gut motility, control of bleeding and cell-mediated immune responses.
The rate-limiting enzyme in serotonin synthesis is tryptophan hydroxylase (TPH). Walther and his colleagues decided to investigate what would happen to mice if they knocked out the animals' gene for TPH. To their enormous surprise, they found that although these animals had virtually no serotonin in non-neural tissues, they had normal amounts of serotonin in their brains, and their behavior was unchanged on tests that normally detect abnormal brain levels of serotonin.
Over the past 20 years or so, there had been clues that there might be different versions of TPH. For example, TPH isolated from the brain appeared to have different biochemical parameters from TPH isolated from the gut. But, Walther said, no one had ever confirmed that this was the case.
"We decided to assume that a second isoform of TPH was responsible for what we observed, and that this was an independent genetic entity," he said. "So we started to search for a second gene encoding TPH."
They screened databases of the human genome, and identified sequences that were present on human chromosome 12, near a gene encoding an enzyme related to TPH, namely phenylalanine hydroxylase. Once researchers had obtained the full sequence of the gene, which they called tph2, they could see that it was very different from the original gene, now called tph1, although 70 percent of the deduced amino acid sequences were identical. Further experiments showed that messenger RNA for tph1 could be found in the duodenum of mice, but not in the brain, and that for tph2 could be found only in the brain. Highly sensitive tests showed that there was about 150 times more RNA for tph2 in the brain stem of wild-type mice than RNA for tph1.
Walther said, "The fact that our knockout mice have normal levels of serotonin in their brains indicates that this isoform is the more important one for serotonin synthesis in the brain, not the gene that was previously known. Consequently, studies that have been conducted to date on serotonin synthesis disorders were all conducted on the wrong gene, as this gene is hardly expressed at all in brain."
The impact on psychiatric research would be "profound," he added. "Moreover, our work will open new avenues for treatment and diagnosis because we can now manipulate one of the isoforms without affecting the other one, as they are slightly different."
Walther predicted that in the future it would be possible to treat psychiatric disorders by changing serotonin levels without causing side effects such as bleeding, which result from lowering serotonin levels in the blood. Equally, it would become possible to treat defects in hemostasis, including clot formation, caused by lack of serotonin without the risk of psychiatric side effects. Given serotonin's role in regulating cellular immune responses, new immunosuppressive drugs for transplant patients and to treat allergies would also follow, he predicted, again without the risk of affecting the central nervous system.
Next, the group is planning to make a knockout mouse lacking tph2. "We expect these animals to have major differences in brain development compared to wild-type mice," Walther said. "These studies will help us to find out how serotonin regulates brain development during embryogenesis."