Alternative titles; symbolsTRYPTOPHAN HYDROXYLASE, NEURONAL; NTPHHGNC Approved Gene Symbol: TPH2Cytogenetic location: 12q21.1 Genomic coordinates (GRCh38): 1...
Alternative titles; symbols
HGNC Approved Gene Symbol: TPH2
Cytogenetic location: 12q21.1 Genomic coordinates (GRCh38): 12:71,938,844-72,032,439 (from NCBI)
Tryptophan hydroxylase (TPH; EC 188.8.131.52) is the rate-limiting enzyme in the synthesis of serotonin (5-hydroxytryptamine, or 5HT). 5HT is causally involved in numerous central nervous activities, and it has several functions in peripheral tissues, including the maintenance of vascular tone and gut motility.
▼ Cloning and Expression
Using the TPH (191060) sequence as query, Walther et al. (2003) identified a human genomic clone containing neuronal TPH, which they designated TPH2. They subsequently cloned the full-length mouse Tph2 sequence, as well as the human and rat homologs. By RNase protection assays specific for the Tph1 and Tph2 isoforms, they detected Tph1 mRNA in mouse duodenum, but not in brain. Tph2, however, was detected exclusively in brain. Mouse brainstem total RNA had about 150 times more Tph2 than Tph1.
▼ Gene Function
Walther et al. (2003) demonstrated that COS-7 cells expressing TPH2 acquired tryptophan-hydroxylating activity.
Zhang et al. (2004) created stable cell lines expressing variants of Tph2 from mice of different genotypes and found that 5HT levels from cells expressing arginine at position 447 were reduced by approximately 55% compared to cells expressing proline at position 447. BALB/cJ mice showed approximately 50% and approximately 70% reductions in synthesis of the 5HT precursor 5-hydroxytryptophan (5HTP) in the frontal and cortex striatum, respectively, as compared with 129X1/SvJ mice. Zhang et al. (2004) also found other differences in Tph2 genotypes in mice of different strains that correlated with synthesis of 5HT. They concluded that their data provided evidence for the fundamental role of TPH2 in 5HT synthesis in the central nervous system.
Clark et al. (2005) used in situ hybridization histochemistry and real-time RT-PCR to assess the effects of dexamethasone or estradiol on TPH2 mRNA levels in the dorsal raphe nucleus of both ovariectomized female and intact male mice. They found that TPH2 mRNA was regulated by glucocorticoids but not by estradiol. Clark et al. (2005) suggested that glucocorticoid-mediated reduction of TPH2 message may have relevance to the etiology of major depression in cases where elevated glucocorticoids are one hallmark of the disease.
Using fMRI, Brown et al. (2005) found that a -844G-T SNP in the upstream regulatory region of the TPH2 gene biased the reactivity of the amygdala, suggesting that the T allele may be associated with greater promoter activity.
By genomic sequence analysis, Walther et al. (2003) mapped the TPH2 gene to chromosome 12.
▼ Molecular Genetics
Association with Major Depressive Disorder
In individuals with unipolar major depression (see 608516), Zhang et al. (2005) identified a 1463G-A transition (607478.0001) in the TPH2 gene. This functional SNP in TPH2 replaces the highly conserved arg441 with his (R441H), which resulted in approximately 80% loss of function in serotonin production when TPH2 was expressed in PC12 cells. SNP analysis in a cohort of 87 patients with unipolar major depression revealed that 9 patients carried the mutant allele, whereas among 219 controls, 3 subjects carried this mutation. In addition, this functional SNP was not found in a cohort of 60 bipolar disorder patients. Identification of a loss-of-function mutation in TPH2 suggested that a defect in brain serotonin synthesis may represent an important risk factor for unipolar major depression.
Garriock et al. (2005), however, found no evidence of the TPH2 R441H mutation by sequence analysis of 182 patients with unipolar depression (83 were treatment resistant), 186 nondepressed controls, and 8 bipolar patients. The ethnicity and gender distribution was similar to that studied by Zhang et al. (2005).
Association with Attention Deficit Hyperactivity Disorder
Walitza et al. (2005) investigated the effect of polymorphic variants in the TPH2 gene in 225 children with ADHD (143465) in 103 families. Three SNPs in and downstream of the transcriptional control region were assessed using the pedigree disequilibrium test. Preferential transmissions were detected for 2 of the SNPs (rs4570625, p = 0.049; rs11178997, p = 0.034). Haplotype analysis revealed a trend of association between these 2 SNPs and ADHD (p = 0.064).
McKinney et al. (2008) identified a heterozygous mutation in the TPH2 gene (R303W; 607478.0002) in a mother and daughter with ADHD (ADHD7; 613003). In vitro functional expression studies showed loss of enzyme activity.
Association with Bipolar Disorder
In 2 cohorts of patients with bipolar affective disorder (BPAD; 125480) from Germany and Russia totaling 883 patients and 1,300 controls, Cichon et al. (2008) observed an association between disease and the minor alleles of 3 SNPs in haplotype 1 of the TPH2 gene (rs11178997, rs11178998, and rs7954758; odds ratio of 1.6, p value of 0.00073). Haplotype 1 covers part of the 5-prime regulatory region and exons 1 and 2 of the TPH2 gene. Cichon et al. (2008) also observed an association between bipolar disorder and a nonsynonymous SNP in the TPH2 gene (P206S; 607478.0003).
▼ Animal Model
Beaulieu et al. (2008) generated knockin mice with a Tph2 variant similar to the human R441H variant. Expression of this mutant Tph2 resulted in markedly decreased brain 5HT production and behavioral abnormalities related to depression and anxiety. The reduction of 5HT was associated with activation of Gsk3b (605004) in the frontal cortex. Inactivation of Gsk3b in Tph2 knockin mice alleviated the aberrant behaviors induced by 5HT deficiency. Beaulieu et al. (2008) concluded that Tph2 plays a role in serotonin homeostasis and identified GSK3B-mediated signaling as an important pathway involved in serotonin-related behaviors.
Alenina et al. (2009) found that Tph2-null mice were viable, but showed growth retardation and 50% lethality within the first 4 weeks of life, although some survived into adulthood. Tph2-null mice had significantly decreased serotonin levels in all brain regions analyzed. They demonstrated more extended daytime sleep, suppressed respiration, altered body temperature control, and decreased heart rate and blood pressure during the night. Tph2-null females showed impaired maternal care and increased aggression. Alenina et al. (2009) concluded that most serotonin in the central nervous system is generated by Tph2, and that this serotonin is involved in behavior and autonomic regulation.
Liu et al. (2011) reported that the neurotransmitter 5HT is required for male sexual preference. Wildtype male mice preferred females over males, but males lacking central serotonergic neurons lost sexual preference although they were not generally defective in olfaction or in pheromone sensing. A role for 5HT was demonstrated by the phenotype of mice lacking Tph2, which is required for the first step of 5HT synthesis in the brain. Thirty-five minutes after the injection of the intermediate 5-hydroxytryptophan (5HTP), which circumvented Tph2 to restore 5HT to the wildtype level, adult Tph2 knockout mice also preferred females over males. Liu et al. (2011) concluded that 5HT and serotonergic neurons in the adult brain regulate mammalian sexual preference.
▼ ALLELIC VARIANTS ( 3 Selected Examples):
.0001 UNIPOLAR DEPRESSION, SUSCEPTIBILITY TO
Zhang et al. (2005) identified a G-to-A transition at nucleotide 1463 of the TPH2 gene that replaces the highly conserved arg at position 441 with his (R441H). This mutation resulted in an approximately 80% loss of function in serotonin production when TPH2 was expressed in PC12 cells. SNP analysis in a cohort of 87 patients with unipolar major depression (see 608516) revealed that 9 patients carried the mutant allele, while among 219 controls, 3 subjects carried this mutation. Interestingly, depressed patients carrying this mutation were poorly responsive or unresponsive to SSRIs. The 3 control subjects carrying this mutation were not diagnosed as having unipolar major depression, but they displayed clinical symptoms of comorbid conditions. One of the 3 control subjects, carrying homozygous mutant alleles, had generalized anxiety symptoms, while the other 2, carrying heterozygous alleles, had mild depression and a family history of mental illness or drug and alcohol abuse, suggesting a potentially higher susceptibility for certain neuropsychiatric disorders in the presence of the mutant allele. This mutation was not identified in 60 patients with bipolar disorder. Garriock et al. (2005), however, found no evidence of the TPH2 R441H mutation by sequence analysis of 182 patients with unipolar depression (83 were treatment resistant), 186 nondepressed controls, and 8 bipolar patients. The ethnicity and gender distribution was similar to that studied by Zhang et al. (2005).
By in vitro functional expression studies, McKinney et al. (2009) demonstrated that the R441H mutant protein had moderately decreased stability and solubility, as well as decreased activity (9% of wildtype). The substitution is within the catalytic domain of the protein.
.0002 ATTENTION DEFICIT-HYPERACTIVITY DISORDER, SUSCEPTIBILITY TO, 7
In a Norwegian woman with ADHD (ADHD7; 613003), McKinney et al. (2008) identified a heterozygous 907C-T transition in exon 7 of the TPH2 gene, resulting in an arg303-to-trp (R303W) substitution in a highly conserved residue located at the entry of the active site. The patient also had a history of major depression. Her daughter, who also carried the mutation, had ADHD. Both reported an excellent response to stimulant medication. The mutation was not found in 323 additional ADHD patients or 87 controls. In vitro functional expression studies showed that the mutant protein had less than 5% residual enzyme activity and decreased solubility, compatible with a folding defect or increased hydrophobicity of the mutant protein. McKinney et al. (2008) concluded that a loss-of-function TPH2 mutation leads to reduced serotonin synthesis, which may result in increased susceptibility to ADHD and possibly other psychiatric disorders.
.0003 BIPOLAR AFFECTIVE DISORDER, SUSCEPTIBILITY TO
Cichon et al. (2008) found an association between a 757C-T transition in exon 6 of the TPH2 gene, resulting in a pro206-to-ser (P206S) substitution (rs17110563) and bipolar affective disorder (125480) in 2 cohorts from Germany and Russia, totaling 883 patients and 1,300 controls. In the combined sample, the ser206 allele yielded an odds ratio of 4.8 for development of the disorder (p value of 0.0024). In vitro functional expression studies showed that the P206S protein had similar catalytic activity as wildtype, but decreased solubility and thermal stability. Further studies suggested a mild dominant-negative effect.
By in vitro functional expression studies, McKinney et al. (2009) demonstrated that the P206S protein had significantly decreased stability and solubility, as well as decreased activity compared to wildtype. P206S is located on surface-exposed side chains and may result in reduced stability.