GTP Cyclohydrolase I Gene Expression in the Brains of Male and Female hph‐1 Mice

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Abstract
: The hph‐1 mouse is characterized by low levels of GTP cyclohydrolase I (GTPCH) and tetrahydrobiopterin. A quantitative double‐lable 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 GT‐PCH 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.