GTP cyclohydrolase I gene expression in the brains of male and female hph-1 mice.

The hph-1 mouse is characterized by low levels of GTP cyclohydrolase I (GTPCH) and tetrahydrobiopterin. A quantitative double-label in situ hybridization technique was used to examine CNS GTPCH mRNA expression within serotonin, dopamine, and norepinephrine neurons of male and female wild-type and hph-1 mice. In wild-type male and female animals the highest levels of GTPCH mRNA expression were observed within serotonin neurons, followed by norepinephrine and then dopamine neurons. Wild-type female animals were found to express lower levels of GTPCH mRNA in each cell type when compared with levels seen in wild-type males. GTPCH mRNA abundance in all three cell types was lower in hph-1 male than in wild-type male mice, with the greatest reduction in serotonin neurons. GTPCH mRNA levels were also lower in hph-1 female than in wild-type female mice, again with the greatest reduction occurring in serotonin neurons. Comparison of hph-1 male and hph-1 female mice revealed that the sex-linked difference in GTPCH mRNA expression observed in wild-type neurons was only present within female dopamine neurons. Overall, these results indicate that not only are basal levels of GTPCH mRNA expression heterogeneous across wild-type murine monoamine cell types but that gene expression is also modified in a sex-linked and cell-specific fashion by the hph-1 gene locus. The hph-1 mutation does not lie within the GTPCH mRNA coding region. The 5' flanking region of the GTPCH gene was cloned and sequenced and shown to be identical for both wild-type and hph-1 genomic DNA. Transient transfection assays performed in PC12 cells demonstrated that this 5' flanking region was sufficient to initiate transcription of a luciferase reporter gene. Although the hph-1 mutation does not lie within the 5' flanking region of the GTPCH gene, this region of the gene can function as a core promoter and is thus crucial to the control of GTPCH gene expression.