A deficiency of BH4, an essential cofactor o f NO biosynthesis, has also been implicated in ED (a feature o f atherosclerosis) and this hypothesis is supported by several lines o f evidence {See Section 5.2.1, Chapter 5) (Wemer-Felmayer et a l, 1993; Cosentino and Katusic, 1995; Busse and Fleming, 1996).
BH4 is synthesised by GTP cyclohydrolase I [GTFCH; E.C. 3.5.4.16; Mw = 125 000],
which is the first and rate-limiting enzyme o f the pterin pathway {Figure 5.1, Chapter 5) (Tahey and Marietta, 1989; Kwon et a l, 1989, Gütlich et a l, 1994). GTFCH cleaves GTF to 7,8-dihydroneopterin triphosphate (7,8-NH2-TF) which is converted to
BH4 by two consecutive enzymes: 6-pyruvoyl tetrahydropterin synthase (FTPS) and
sepiapterin reductase (SR) (Nichol et a l, 1985; Kaufman, 1978; Blau and Niederweiser, 1985; Curtius et a l, 1986). Alternatively 7,8-NH2-TF is cleaved to 7,8-
dihydroneopterin (7,8-NH2) by phosphorylases, which is further degraded to D-
erythro-ntopXQxm (Huber et a l, 1984; Werner et a l, 1990). In most human cells, the majority of 7,8-NH2-TF formed is metabolised to BH4. As FTPS and SR are
constitutive enzymes, that is their activities are unaffected by cytokines, they are present in low amounts in monocytes and macrophages (Werner et a l, 1990; Werner et a l, 1993). These cells accumulate 7,8-NH2-TF, which after hydrolysis by
ubiquitous phosphatases and partial oxidation is excreted as 7,8-NH2 or neopterin
(Werner et al., 1989; Prior et a l, 1986). To regulate its own synthesis, BH4 acts as a feedback inhibitor by forming a ternary 'inactive' complex with GTFCH and a GTFCH Feedback Regulatory Protein (GFRF) in the liver and the kidney as well as in other tissues, such as lung, heart and spleen (Xie et a l, 1998). At low GTF concentrations, phenylalanine converts this inactive complex to an active complex (Harada et al.,
1993).
GTFCH is encoded by the gene GCHl (Blau and Niederweiser, 1985; Duch and Smith, 1991). Polymorphisms within GCHl may have the potential to influence BH4
production, indirectly limiting endothelial NO production, by modulating GTFCH gene expression {Figure 1.8). However, surprisingly to date, there have been no studies on common sequence variation within the GCHl gene promoter and its coding sequences. Also, there is no information on the effects of any polymorphism on BH4
Figure 1.8: Hypothetical scheme demonstrating putative interconnections between L-arginine-NO pathway and pterins biosynthesis in eukaryotic cells.
GTP plays a dual in the biology of NO as it is a precursor to both BH4 (cofactor of eNOS) and
to cGMP (mediator of vasodilatation). NO activates sGC which catalyses the conversion of GTP to cGMP. BH4 oxidation to its quinoid isoform (Q -BH 2) is believed to provide two of the five electrons necessary for the oxidation of L-arginine to L-citrulline and NO. Neopterin is formed as a by-product of the pterin pathway while NO^ (i.e. NOT and NO2 anions) are
formed as oxidation products of NO.
G C H l G^^ 1 GTPCH Y \ 1 cGMP vasodilatation
neopterin 7,8-dihydroneopterin <- 7,8-dihydroneopterin t Phosphatases sGC Ik triphosphate 6-pyrnvoyl-letra- hydropterin synthase NO + L-citrulline m s 3 r s Sepiapterin eNOS L-arg reductase 23
Ichinose et al. cloned human GCHl in 1995, which spans -3 0 kb and is composed of six exons (Ichinose et a l, 1995a) {Figure 5.2, Chapter 5). The gene was mapped to region q21-22 o f chromosome 14 (Thony et a l, 1995). Each exon/intron boundary conforms to the GT/AG rule (Breatnach and Chambon, 1981). Canonical features of an eucaryotic promoter such as a CCAAT- and a TATA-box are located 64- and 21-bp upstream o f the tsp and several consensus sequences for the binding o f Spl are present (Witter et a l, 1996). Upstream of the tsp, there is a 452-bp GC-rich region acting as the promoter for the transcription of various housekeeping genes (Witter et a l, 1996). Analyses o f the promoter shows the presence o f potential binding sites for the following TFs: CCAAT-box/enhancer binding protein (C/EBP), hepatic nuclear factor- 1 (HNFl), upstream stimulatory factor (USF), AP-1, serum response factor (SRF), and nuclear factor NF-kB {Figure 5.3, Chapter 5). Cytokines elieit increases in NO and BH4 levels via NF-kB and AP-1 activation in murine cells (Togari et a l, 1998).
In humans, GTPCH molecules are encoded by at least three mRNAs, distinct in their 3 ’ ends (Togari et a l, 1992). The different isoforms of GCHl mRNAs result from alternative use o f the splice acceptor site at exon 6 . Splicing between the 5'- and 3'-
splices site o f intron 5 produced type 1 mRNA, splicing between the 5'-splice site o f intron 5 and the middle of exon 6 generates type 2 mRNA, whereas intron 5 is not
spliced out in the transcript corresponding to type 3 mRNA (Ichinose et a l, 1995a). The latter derives most probably from an immature mRNA. However, only type 1 mRNA encodes an active GTPCH while the other two are defective in GTPCH activity (Gütlich et a l, 1994a; Ichinose et a l, 1995a). The total length o f the GCHl mRNA was shown to be 3.6 kb with the 5'-untranslated region (5'-UTR) plus the open reading
et a l, 1992). At present, there is no information concerning the functional relevance o f 5' UTR of GCHl. However, studies on 5'-UTR in other genes suggest that this region is important in the regulation of gene expression, and that sequence changes can lead to a decreased enzyme activity of the gene product (Bandmann et a l, 1998).
A mouse mutant deficient in GTPCH activity, generated by chemical mutagenesis, showed that the hph-1 mutation affects the steady state level of GCHl mRNA instead o f altering the sequences within the reading frame o f the gene (McDonald et a l, 1988; Hyland et al., 1996). This occurs due to either a mutation occurring in the regulatory region o f GCHl affecting either the binding o f a TF to the promoter, or due to the release o f an inhibitory factor, or a mutation in the gene o f a regulatory factor thus causing a decreased hepatic GTPCH activity (Gutlich et a l, 1994b).
About 52 mutations have been found along human GCHl, mostly in exons 1 and 6,
with considerable heterogeneity of phenotypic expression (Bandmann et a l, 1996; Brique et a l, 1999). A possible explanation for the varied clinical manifestations is due to alternative splicing of GCHl mRNA resulting in different levels of mutant/normal mRNA ratios (Hirano et a l, 1996; 1999). Deficiency in GTPCH activity in humans results in a lack o f BH4 thereby impairing both the biosynthesis of
catecholamines and serotonin leading to hyperphenylalaninemia, dopamine responsive dystonia (DRD) and hereditary progressive dystonia (HPD) (Bandmann et a l, 1996; Brique et a l, 1999). DRD is a hereditary lower limb dystonia disorder (Nygaard et a l, 1993) while HPD is characterised by childhood-onset dystonia and marked diurnal fluctuation (Ichinose et a l, 1994; 1995b). However the effect o f BH4 deficiency on
vascular tone, BP and ED in these patients has not been documented.