Previous studies investigating steroidogenic enzymes in the brain have often focused on only a few genes at a time, using animal models or pooled human RNA to investigate their expression. For this study, we obtained RNA from individual archived human brain tissue samples in order to analyse the transcription of twelve genes involved with steroid biosynthesis and metabolism within the cerebellum and hippocampus.
We developed realtime RT-PCR assays for twelve genes involved in steroid biosynthesis and metabolism. Assays specificities and efficiencies were verified using human adrenal RNA. Following approval by the Local Resesarch Ethics Committee, frozen paired samples of human hippocampus and cerebellum were selected from the collection of postmortem brains held at the University of Glasgow (n=9 cases). Total RNA was isolated, DNased, reverse transcribed and then subjected to realtime PCR on an Applied Biosystems PRISM 7900HT machine using fluorogenic probes. Data were corrected relative to GAPDH expression and analysed using the REST384 program.
PCR of hippocampus and cerebellum cDNA samples resulted in the consistent detection of transcripts from the genes CYP11A1 (side-chain cleavage enzyme), HSD3B2 (3β-hydroxysteroid dehydrogenase), CYP21B (21-hydroxylase), CYP19 (aromatase), SRD5A2 (5α-reductase), HSD17B3 (17β-HSD3), HSD17B5 (17β-HSD5), HSD17B7 (17β-HSD7) and 11BHSD1 (11β-hydroxysteroid dehydrogenase type 1).
In contrast to our previous studies of the rat brain, no evidence for CYP11B1 (11β-hydroxylase) or CYP11B2 (aldosterone synthase) transcription in the human hippocampus and cerebellum was observed. 17α-Hydroxylase (CYP17) mRNA was also not detected.
This study shows that important differences exist between the human and rat brain in terms of steroidogenic gene expression. Furthermore, while it suggests that synthesis of aldosterone and cortisol, as well as de novo production of estradiol and testosterone, is not possible in human cerebellum and hippocampus, it appears likely that a more restricted pattern of steroid biosynthesis may exist fuelled by steroid precursors from other sources.