UPDATE ON GENE COMPLETION AND ANNOTATIONS Analysis and update of the human aldehyde dehydrogenase ( ALDH)gene family Vasilis Vasiliou1*and Daniel W. Nebert 2 1 Molecular Toxicology and Environmental Health Sciences Program, Department of Pharmaceutical Sciences, University of Colorado Health Sciences Center,Denver, CO,USA 2 Department of Environmental Health and Center for Environmental Genetics (CEG), University of Cincinnati Medical Center, Cincinnati, OH, USA * Correspondence to:Tel: +1 303 315 6153; Fax: +1 303 315 6281; E-mail: [email protected] Date received: 23rd February2005 Abstract The aldehyde dehydrogenase ( ALDH)gene superfamily encodes enzymes that are critical for certain life processes and detoxification via the NAD(P)þ -dependent oxidation of numerous endogenous and exogenous aldehyde substrates, including pharmaceuticals and environmental pollutants. Analysis of the ALDH gene superfamily in the latest databases showed that the human genome contains 19 putativelyfunctional genes and three pseudogenes. Anumber of ALDH genes are upregulated as apart of the oxidative stress response and inexplicably overexpressed in various tumours, leading to problems during cancer chemotherapy. Mutations in ALDH genes cause inborn errors of metabolism —such as the Sjo¨ gren–Larsson syndrome,type II hyperprolinaemia and g -hydroxybutyric aciduria —and arelikely to contribute to several complex diseases, including cancer and Alzheimer’sdisease. The ALDH gene products appear to be multifunctional proteins, possessing both catalytic and non-catalytic properties. Keywords: human genome,aldehyde dehydrogenase gene family,genetic polymorphism, evolution, crystallins Introduction There are instances, however, in which ALDHscatalyse reactions yielding chemically reactive or bioactivemetabolites Aldehyde dehydrogenases (ALDHs; EC1.2.1.3) represent that are essential to the organism. Several ALDH enzymes — agroup of enzymes that oxidise awide range of endogenous including ALDH1A1, ALDH1A2 and ALDH1A3 —catalyse and exogenous aldehydes to their corresponding carboxylic the irreversible oxidationofretinal to retinoic acid.3 Whereas 1 acids. Endogenous aldehydes areformed during the meta- the light-absorbing properties of retinal are anecessary bolism of aminoacids, carbohydrates, lipids, biogenic amines, element for vision, thecarboxylic acid isomers, all-trans- vitamins and steroids. Biotransformations of alarge number of retinoic acid and/or 9-cis-retinoic acid, serve as ligands for the drugsand environmental chemicals also generate aldehydes. retinoic receptor (RAR)and the retinoid Xreceptor (RXR) Aldehydes arehighly reactiveelectrophilic compounds which that mediate geneexpression for growth and development.4 interactwiththiol and aminogroups, the resulting effects vary The importance of ALDH enzymes in retinoic acid formation from physiological and therapeutic to cytotoxic,mutagenic became evident from the fact that homozygous disruptionof or carcinogenic.Inthis respect, ALDHsefficiently oxidise and, the mouse Aldh1a2 gene results in an embryoniclethal in most instances, detoxify asignificantnumber of chemically phenotype due to defects in early heart morphogenesis,5,6 diverse aldehydes which otherwise would be harmful to the whereas Aldh1a3 null mice die shortly after birth, due to organism.Strong evidence supporting this notioncomes from respiratorydistress caused by choanal atresia.7 the fact that mutations in ALDH genes cause inbornerrorsof Formation of retinoic acid and g -aminobutyric acid metabolism associated with clinical phenotypes —such as (GABA) areamong the most intriguing functions of ALDHs Sjo¨ gren–Larssonsyndrome (SLS), type II hyperprolinaemia regarding bioactivation.GABA is implicated in theregulation 2 and g -hydroxybutyric aciduria. In addition, mutations in of the GABAergic,dopaminergic and opioid systems. Even ALDH genes contribute to clinically relevant diseases such as though the main pathway for GABA synthesis is the decar- cancer and Alzheimer’sdisease. boxylationof L -glutamate,this neurotransmitter can also be 138 q HENRYSTEWART PUBLICATIONS 1479–7364. HUMANGENOMICS .VOL 2. NO 2. 138–143 JUNE 2005 Analysis and update of the ALDH gene family UPDATE ON GENE COMPLETION AND ANNOTReviewATIONS formed from putrescine by directoxidativedeamination to balance. 12 Forexample,ithas been proposedthat ALDH3A1 give g -aminobutyraldehyde,which is then converted into mayscavenge hydroxyl radicals via the–SH groupsofCys and GABA by an ALDH. 8 Allinall, the ALDH gene family Met residues, and that both ALDH3A1 and ALDH1A1 may represents atruly diverse group of proteinswhich arecritical contribute to theantioxidant capacity of the cell by generating to metabolism. NADPH and/or NADH. 13 The enzymatic activity of ALDH3A1 generatesNADPH, which is linked to the regeneration of reduced glutathione (GSH) from its oxidised Multiple function(s) of the ALDH form(GSSG) via the glutathione reductase/peroxidase system. enzymes NAD(P)Hmay also function as adirect antioxidantby reducing glutathiyl radicals (GSz )ortyrosyl radicals.14 The Although the major function of ALDH enzymes is the expression of ALDH3A1 and ALDH1A1 at very high con- NAD(P)þ -dependentaldehyde oxidation, it has become centrations in the mammalian cornea and lens (crystallins) has increasingly clear that some, if not most, ALDHs exhibit led to additional hypothesesregarding the multifunctional multiple functions (Figure 1). For example,ALDH1A1, properties of these proteins —including astructural function ALDH2, ALDH3A1 and ALDH4A1 areknown to catalyse contributing to transparency. 15,16 Finally,the ALDH7A1 ester hydrolysis, suggestingthat the ALDHs mayhavemore gene product is similar to thegreen garden pea ‘26g protein’ than one catalytic function.9 Indeed, it has recently been involved in the regulation of turgor pressure, suggesting suggested that ALDH2 also possesses nitrate reductase activity, that the ALDH7A1 protein mighthaveosmoregulatory which catalyses the formation of 1,2-glyceryl dinitrate and properties. nitrite from nitroglycerin within mitochondria, leading to the production of cGMP and vasorelaxation.10 Aside from their catalytic properties,ALDH proteinsare Evolution of the ALDH genes capable of non-catalytic interactions with chemically diverse endogenous compounds and chemotherapeutic agents. In this ALDHshaveawide distribution in nature,ranging from context,ALDH1A1 has been identifiedasanandrogen- bacteria and yeaststoplants and animals.17 Sequence binding protein prominently expressed in humangenital comparisons indicate extensivesimilarity between bacterial fibroblasts; as acholesterol-binding protein in bovine lens and human ALDHs and suggest that the superfamily has a epithelium; and as acytosolic thyroid hormone-binding common ancestral gene,datingback to , 3billion yearsago. 18 protein in Xenopus . 11 ALDH1A1 has also been identifiedasa Asystematic nomenclatureschemefor the ALDH gene flavopyridol-binding protein in non-small cell lung carcinomas superfamily (in animals, plants, bacteria and yeasts) has been and as adaunorubicinbinding protein in rat liver. 1 Similar to developed, based on evolutionary divergence, 18 which has ALDH1A1, ALDH2 also displaysbinding capabilities with been implemented with biannual updates 19,20 and is available exogenous compounds, which became evident from its via the internet (http://www.aldh.org). identification as an acetaminophen binding protein. 1 ALDH proteins are conveniently classified into families and In addition, it has been suggested that someALDHsmay subfamilies based on the percentage of amino acid identity. playacritical role in cellular homeostasis by maintaining redox Proteinssharing $ 40 per cent identity areassigned to a particular family designated by an Arabic numeral, whereas those sharing $ 60 per cent identityare classified in the same • Aldehyde oxidation subfamily designated by aletter.These cut-off values follow • Catalytic • Ester hydrolysis the originalrecommendations by Margaret Dayhoff and 21 • Nitrate reductase were first applied to the cytochrome P-450 superfamily. At present, more than 130 additional gene superfamilies and • Endobiotics • Binding • Xenobiotics large gene families followthis same format. Functions of • Direct (GSH-like) • Antioxidant ALDHs • Indirect [generating NAD(P)H] Endogenous functions of ALDHs Antioxidants and oxidativestress increase the expression of • Structural • Cornea and lens crystallins certain ALDH genes, leading to increased protection of the cell against insult by environmental chemicals and drugs.22 • Regulatory • Osmotic pressure Increased expression of certain ALDHs in tumour cells, Figure 1. Multiple functions of aldehyde dehydrogenase however, leads to decreased cellular sensitivity to cyclopho- (ALDH) enzymes. Endobiotics, endogenous compounds. sphamide and other oxazaphosphorines and,thus, to clinical 23 Xenobiotics, foreign chemicals. problems in the treatment of cancer patients. The reason for certain ALDHs —and other non-P450 membersofthe [ Ah ] q HENRYSTEWART PUBLICATIONS 1479–7364. HUMANGENOMICS .VOL 2. NO 2. 138–143 JUNE 2005 139 UPDATERevieONw GENE COMPLETION AND ANNOTATIONS Vasiliou and Nebert battery—to be upregulated in some tumours24 remains dehydrogenase the number EC 1.2.1.31 —meaning that it an enigma. is closely related functionally to the other ALDH activities Numerous polymorphisms exist in the human ALDH (EC 1.2.1.3). genes, some of which cause inbornerrorsofmetabolism and The twomost recently discovered ALDH genes are