Inactivating mutations of cytosolic phenylalanine hydroxylase (PAH) block the normal reaction of phenylalanine, molecular oxygen and tetrahydrobiopterin to form tyrosine, water, and 4 alpha-hydroxytetrahydrobiopterin. Excess phenylalanine accumulates as a result, driving the formation of abnormally high levels of phenylpyruvate, and phenyllactate (Guldberg et al. 1996; Mitchell et al. 2011) in reactions not annotated here.

Polycyclic aromatic hydrocarbons (PAHs) are pro-carcinogens which require further metabolic activation to ellicit their harmful effects. Aldo-keto reductases (AKRs) such as alcohol dehydrogenase [NADP+] (AKR1A1) can catalyse the oxidation of proximate carcinogenic PAH trans-dihydrodiols to reactive and redox active PAH o-quinones. Redox-cycling of PAH o-quinones generate reactive oxygen species and subsequent oxidative DNA damage. The proximate PAH carcinogen benzo[a]pyrene-7,8-trans-dihydrodiol (BaPtDHD) is oxidised by AKR1A1 to yield BaP-7,8-catechol which is unstable and auto-oxidises to yield BaP-7,8-dione (Zhang et al. 2012).

Inactivating mutations of cytosolic phenylalanine hydroxylase (PAH) block the normal reaction of phenylalanine, molecular oxygen and tetrahydrobiopterin to form tyrosine, water, and 4 alpha-hydroxytetrahydrobiopterin. Excess phenylalanine accumulates as a result, driving the formation of abnormally high levels of phenylpyruvate, and phenyllactate (Guldberg et al. 1996; Mitchell et al. 2011) in reactions not annotated here.

The human gene SLC22A6 encodes organic anion transporter1 (OAT1). It was originally characterized in mouse as Novel Kidney Transcript (NKT). OAT1 is located on the basolateral membrane of the proximal tubule in human kidney as well as in the brain (Reid G et al, 1998; Lu R et al, 1999; Hosoyamada M et al, 1999). The human gene SLC22A7 encodes organic anion transporter 2 (OAT2) and is highly expressed in the liver and kidney (Sun W et al, 2001; Kobayashi Y et al, 2005). The human gene SLC22A8 encodes organic anion transporter 3 (OAT3) which is expressed mainly in the brain and kidney (Race JE et al, 1999; Bakhiya A et al, 2003).
OAT1-3 transport organic anions such as p-aminohippurate and drugs such as cimetidine and acyclovir. This transport is is coupled with an efflux of one molecule of endogenous dicarboxylic acid such as alpha-ketoglutarate (2-oxoglutarate). OAT2 is classified as both a transporter of organic anions and sulphate conjugates.

The endogenous prostanoid prostacyclin is continuously produced by healthy vascular endothelial cells and inhibits platelet activation through interaction with the Gs-coupled prostacyclin receptor PTGIR. Prostacyclin prevents formation of the platelet plug involved in primary hemostasis (a part of blood clot formation) and is also an effective vasodilator.

Pulmonary arterial hypertension (PAH) is a chronic, progressive, multifactorial disease. The endothelin, nitric oxide (NO), and prostacyclin pathways are involved in PAH and combined therapies that target each of these pathways are currently recommended. The PTGIR agonists iloprost and treprostinil (Whittle et al. 2012) are indicated for PAH (Gąsecka et al. 2021). Selexipag is a novel oral PTGIR agonist drug with vasodilatory and antiproliferative effects. It is the prodrug, metabolised to the active form MRE-269 (Kuwano et al. 2007). Selexipag is well tolerated and safe when used to treat PAH in children (Hansmann et al. 2020, Bravo-Valenzuela et al. 2021).

The aryl hydrocarbon receptor (AHR) is a ligand-activated transcription factor that can control the expression of a diverse set of genes. Two major types of environmental compounds can activate AHR signaling: halogenated aromatic hydrocarbons such as 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and polycyclic aromatic hydrocarbons (PAH) such as benzo(a)pyrene. Unliganded AHR forms a complex in the cytosol with two copies of 90kD heat shock protein (HSP90AB1) (Forsythe et al. 2001), one X-associated protein (AIP) (Meyer et al. 1998), and one p23 molecular chaperone protein (PTGES3) (Nguyen et al. 2012, Beischlag et al. 2008). Here, the binding of TCDD is shown.

Endothelins (EDNs) are 21-amino acid vasoconstricting peptides produced primarily in the endothelium that play a key role in vascular homeostasis. They are potent and long-lasting vasoconstrictors. An imbalance and over-expression of EDNs can contribute to hypertension (high blood pressure). EDNs bind to two endothelin receptors, EDNRA and EDNRB. EDNRA is primarily located in the smooth muscle of blood vessels and upon EDN binding, leads to vasoconstriction and sodium retention, ultimately increasing blood pressure. EDNRB is primarily located on endothelial cells lining the internal walls of vasculature. EDN binding to EDNRB leads to the release of NO (nitric oxide), a powerful vasodilator.

EDNR antagonists (ERAs) are competitive antagonists of EDNs at EDNRs. Dual-acting ERAs such as macitentan (Ahn et al. 2014) and bosentan (Clozel et al. 1994, Weber et al. 1996) can bind to both EDNRA and EDNRB receptors but have greater sensitivity for EDNRA so these drugs can be used to treat PAH (Zheng et al. 2020).

Endothelins (EDNs) are 21-amino acid vasoconstricting peptides produced primarily in the endothelium that play a key role in vascular homeostasis. They are potent and long-lasting vasoconstrictors. An imbalance and over-expression of EDNs can contribute to hypertension (high blood pressure). EDNs bind to two endothelin receptors, EDNRA and EDNRB. EDNRA is primarily located in the smooth muscle of blood vessels and upon EDN binding, leads to vasoconstriction and sodium retention, ultimately increasing blood pressure. EDNRB is primarily located on endothelial cells lining the internal walls of vasculature. EDN binding to EDNRB leads to the release of NO (nitric oxide), a powerful vasodilator.

EDNR antagonists (ERAs) are competitive antagonists of EDNs at EDNRs. ERAs selective for EDNRA are primarily indicated for pulmonary arterial hypertension (PAH) (Zheng et al. 2020). EDNRA-selective approved drugs are sitaxentan (Wu et al. 1997) and ambrisentan (Bolli et al. 2004).

Ambrisentan has been shown to inhibit cancer cell migration, invasion and metastasis in vitro, suggesting a new therapeutic application for this drug (Kappes et al. 2020).

DAXX mutations identified in cancer are frequently nonsense and frameshift mutations that result in premature protein truncation. Mutant DAXX proteins are frequently lost or mis-localized and are usually undetectable in the nucleus. DAXX contains an ATRX-binding region that maps to PAH domains of DAXX, PAH1 and PAH2, which are located in the N-terminal portion of DAXX. At least the PAH1 domain is needed for binding to ATRX, and a recombinant construct consisting of amino acids 1-160 of DAXX, which includes the PAH1 domain, is able to bind to ATRX, but the binding is stronger if PAH2 domain is also included (if the recombinant DAXX construct consists of amino acids 1-260) (Tang et al. 2004). Minimally, amino acids 55-144 are required for DAXX binding to ATRX (Wang et al. 2017). All DAXX truncation mutants in which the stop codon occurs upstream of the amino acid 144 are annotated as candidate loss-of-function mutants. These include the following nonsense mutants:
DAXX K56*
DAXX E104*
DAXX C106*
DAXX S138*.
DAXX frameshift mutants in which the frameshift occurs upstream of the codon 144 are also annotated as candidates:
DAXX H26Tfs*118
DAXX A36Qfs*108
DAXX R48Vfs*93
DAXX E72Nfs*72
DAXX C74*
DAXX L98Vfs*13
DAXX A103Sfs*40.
Missense mutants of DAXX have not been functionally tested in the context of the full-length protein, just in the context of the DAXX fragment that consists of amino acids 55-144 (Wang et al. 2017) and are not shown here.

Soluble guanylate cyclase (sGC) modulators are small-molecule drugs that bind sGC and enhance nitric oxide (NO)-mediated cGMP signalling, resulting in vasodilation and inhibition of platelet aggregation. The suffix “ciguat” is the unique identifier for this drug class. There are two types of "ciguats"; NOsGC stimulators, which act through allosteric regulation and NOsGC activators, which occupy the heme binding site and work additively with NO (Kraehling & Sessa 2017).

NOsGC stimulators activate sGC independently of NO. This was demonstrated in human platelets by the first allosteric activator lificiguat (YC-1), a benzyl indazol derivative (Friebe et al. 1998). Lificiguat, synergistically with NO, stimulates sGC activity 200-800 fold to result in inhibition of platelet aggregation. A regulatory site on sGC was discovered to be the probable binding site (C239 and C244 regions) for sGC stimulators using the pyrazolopyridine BAY 41-2272, a lificiguat analog (Stasch et al. 2001). BAY 41-2272 induces vasodilation without developing nitrate tolerance, possesses antiplatelet activity and reduces mortality. Optimisation experiments on lificiguat led to the development of riociguat (BAY 63-252, Adempas). Riociguat, activates NOsGC 70-fold at therapeutic concentrations and is the only "ciguat" so far to be approved for the treatment of pulmonary arterial hypertension (PAH) and inoperable chronic thromboembolic pulmonary hypertension (CTEPH) (Schermuly et al. 2008).

Another NOsGC stimulator, vericiguat, shows an optimized pharmacokinetic profile and allows a once daily dosing regimen, unlike riociguat (3 times a day) (Breitenstein et al. 2017). Vericiguat is in clinical trials to determine its efficacy in heart failure, in patients with chronic failure and reduced ejection fraction (Gheorghiade et al. 2015). Praliciguat (IW-1973) is a novel clinical-stage NOsGC stimulator under clinical investigation for the treatment of heart failure with preserved ejection fraction and diabetic nephropathy (Tobin et al. 2018, Breitenstein et al. 2017).