Oxysterol 7a-hydroxylase

This isoform was found as the result of a mouse deficient in the oxysterol 7a-hydroxylase gene (CYP7B1 in humans), which nonetheless did not accumulate 24-hydroxy cholesterol from the CYP46pathway, suggesting the existence of another 7ot-hydroxylase (Li-Hawkins et al., 2000). However, very httle evidence exists for its expression in human brain. Nishimura et al. quantified CYP39A1 mRNA by RT-RT-PCR and reported some 100-fold less than they reported for CYP51 mRNA (Nishimura et al., 2003). There is no other evidence for the expression of this isoform in the brain as of this writing and this is beheved to be the first report of its kind. The isoform has been shown to be sexually dimorphic in the liver (Li-Hawkins et al., 2000). 

The initial steps in BA synthesis are characterised by the introduction of a hy-droxylic group in the la position, or in position 27, followed by another in the la position into the cholesterol nucleus. Both synthetic pathways (the neutral and the acidic pathways) possess a distinct microsomal 7-oxysterol hydroxylase, which is regulated by different genes. The most recently described disorder of BA synthesis is cholesterol 7a-hydroxylase deficiency, in which their decreased production through the classical pathway is partially balanced by activation of the alternative pathway. Cholesterol levels increase in the liver, with a consequent low-density lipoprotein hypercholesterolemia, and cholesterol gallstones may result, although there is no liver disease. In contrast, a defect in the conversion of 27-hydroxy-cholesterol to la,27-dihydroxy-cholesterol due to deficiency of the oxysterol 7a-hydroxylase specific for the alternate pathway, causes severe neonatal liver disease [8]. 

Fig. 4. The bile acid biosynthetic pathways. The classical pathway operates entirely in the liver and cholesterol 7a-hydroxylase (CYP7A1) initiates the pathway. In other tissues, the entry of cholesterol into the alternate pathways is facilitated by sterol 27-hydroxylase (CYP27A1), cholesterol 24-hydroxylase (CYP46A1), and cholesterol 25-hydroxylase (CH25H). The oxysterols generated by these enzynies are 7a-hydroxylated by oxysterol 7a-hydroxylases CYP7B1 and CYP39A1, and the products enter the latter steps of the classical pathway.

Schwarz, M., Lund, E.G., Lathe, R., Bjorkhem, I., Russell, D.W. 1997. Identification and characterization of a mouse oxysterol 7a-hydroxylase cDNA. J. Biol. Chem. 272 23995-24001. 

Disruption of cholesterol 7a-hydroxylase gene in mice. 11. Bile acid deficiency is overcome by induction of oxysterol 7a-hydroxylase. J. Biol. Chem. 271,18024-18031. 

K. Wikvall (2000). Oxysterol 7a-hydroxylase activity by cholesterol 7a-hydroxylase (CYP7A). J. Biol. Chem. 275, 34046-34053. 

CH25H5 followed by oxysterol 7a-hydroxylase (CYP7Bl)/and hydroxysteroid dehydrogenase 3B7 (HSD3B7)... 

Norlin M, Chiang JY (2004) Transcriptional regulation of human oxysterol 7a-hydroxylase by sterol response element binding protein. Biochem Biophys Res Commun 316 158 164... 

Setchell KD, Sehwaiz M, O Connell NC, Lund EG, Davis DL, Lathe R, Thompson HR, Weslie Tyson R, Sokol RJ, Russell DW (1998) Identifiea-tion of a new inborn error in bile aeid synthesis mutation of the oxysterol 7a-hydroxylase gene causes severe neonatal liver disease. J Clin Invest 102 1690-1703... 

Wu Z, Martin KO, Javitt NB, Chiang JY (1999) Structure and functions of human oxysterol 7a-hydroxylase cDNAs and gene CYP7B1. J Lipid Res 40 2195 2203... 

Steckelbroeck S, Watzka M, Lutjohann D, Makio-la P, Nassen A, Hans VH, Clusmann H, Reissinger A, Ludwig M, Siekmann L, Klingmuller D (2002) Characterization of the dehydroepiandrosterone (DHEA) metabolism via oxysterol 7a-hydroxylase and 17-ketosteroid reductase activity in the human brain. JNeurochem 83 713-726... 

Oxysterols are formed in the first steps of all cholesterol metabolism. The most abundant oxysterol in mammalian brain is 24S-hydroxycholesterol, also known as cerebrosterol. Cerebrosterol is present at a level of 5-50 ng/mg in wet brain and is formed from cholesterol in a cytochrome P450 46A1 (CYP46A1)-catalyzed reaction [56]. CYP46A1 is only expressed in nervous tissue. CYP46A1 also has 25- and 26-hydroxylase activity [57] and low levels of both 25- and 26-hydroxycholesterols are found in brain (1-5 ng/mg) [48,58]. Oxysterol 7a-hydroxylase (CYP7B1) is also expressed in nervous tissue [59] and its substrates 25- and 26-hydroxycholesterols are 7a-hydroxylated by cells of the nervous system and their 7a-hydroxylated products found to be present in brain itself [49,60]. 

Fig. 32.2. The alternative ( acidic ) pathway for the synthesis of bile acids, which starts with conversion of cholesterol to 27-hydroxycholesterol and results predominantly in the production of chenodeoxycholic acid. Also shown is the route of the classical pathway via 7a-hydroxycholesterol. The alternative pathway illustrates that side-chain modification can proceed modification of the steroid nucleus, although the exact sequence and relative contributions of these different pathways is unclear, with most of the synthetic enzymes being shared by both pathways. A deficiency of oxysterol 7a-hydroxylase (32.4), which is unique to the alternative pathway, is shown, and the abnormal metabolites that are readily detected in urine by LSl-MS are highlighted...

GC-MS analysis is used to confirm the identities of ions in the LSI-MS urine spectrum and show that the excretion of abnormal cholanoids is >20 times normal. In the case of 5 ff-reductase deficiency GC-MS analysis should show that S-oxo-A" bile acids account for >70% of the total urinary bile acid excretion. In the case of sterol 27-hydroxylase deficiency (CTX), GC-MS analysis should indicate that the major cholestane pentols in the urine are 3,7,12,22,25 and 3,7,12,23,25-pentols. (One patient has been described who had familial cholestatic liver disease associated with greatly increased urinary excretion of 5jff-cholestane-3a,7a,12a,24 S,25-pentol [see previous table]). Liquid secondary ion-tandem mass spectrometry (LSI-MS/ MS) is an alternative method to GC-MS and can rapidly confirm the identity of a number of diagnostic ions that are found in the LSI-MS spectrum of urine. These include sulphated and taurine-conjugated abnormal metabolites such as those observed in 3 ff-HSDH deficiency (32.1), 5)9-reductase deficiency (32.2), oxysterol 7a-hydroxylase deficiency (32.4) and peroxisomal disorders [13]. 

Oxysterol 7a-hydroxylase Gene analysis has been conducted on the sole patient described with this disorder. 

Diagnosis of the 4 inborn errors of bile acid synthesis discussed in this chapter is important because three are treatable by oral bile acid supplementation (such treatment was not successful in the single reported case of oxysterol 7a-hydroxylase deficiency). Liquid secondary ion mass spectrometry (LSI-MS) is a simple and rapid method which can be used to screen urine samples for abnormal cholanoids (bile acids and bile alchohols). It should be applied to neonates with unexplained cholestatic liver disease, particularly if familial and associated with steatorrhoea and fat-soluble vitamin malabsorption, to infants and children with developmental delay whether or not this is associated with specific features suggestive of a peroxisomal disorder (e.g. hypotonia, seizures, dysmorphic features, ocular and auditory abnormalities and hepatic dysfunction) or CTX (e.g. juvenile cataracts). 

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