Genomic organization, expression, and alternate splicing of the mouse fatty aldehyde dehydrogenase gene

Genetics and prospective therapeutic targets for Sjögren-Larsson Syndrome

INTRODUCTION

Sjögren-Larsson syndrome (SLS) is a rare neurocutaneous disease characterized by ichthyosis, spasticity, intellectual disability and a distinctive retinopathy. It is caused by inactivating mutations in ALDH3A2, which codes for fatty aldehyde dehydrogenase (FALDH) and results in abnormal metabolism of long-chain aliphatic aldehydes and alcohols. The potential disease mechanisms leading to symptoms include 1) accumulation of toxic fatty aldehydes that form covalent adducts with lipids and membrane proteins; 2) physical disruption of multi-lamellar membranes in skin and brain; 3) abnormal activation of the JNK cell signaling pathway; and 4) defective farnesol metabolism resulting in abnormal PPAR-α dependent gene expression. Currently, no effective pathogenesis-based therapy is available.

AREAS COVERED

The clinical, pathologic and genetic features of SLS are summarized. The biochemical abnormalities caused by deficient activity of FALDH are reviewed in the context of proposed pathogenic mechanisms and potential therapeutic interventions.

EXPERT OPINION

The most promising pharmacologic approach to SLS involves blocking the formation of potentially harmful fatty aldehyde adducts using aldehyde scavenging drugs, currently in phase 2 clinical trials. Other approaches needing further investigation include: 1) ALDH-specific activator drugs and PPAR-α agonists to increase mutant FALDH activity; 2) inhibitors of the JNK phosphorylation cascade; 3) antioxidants to decrease aldehyde load; 4) dietary lipid modification; and 5) gene therapy.

Plasmalogens and fatty alcohols in rhizomelic chondrodysplasia punctata and Sjögren-Larsson syndrome

Plasmalogens are a special class of ether-phospholipids, best recognized by their vinyl-ether bond at the sn-1 position of the glycerobackbone and by the observation that their deficiency causes rhizomelic chondrodysplasia punctata (RCDP). The complex plasmalogen biosynthetic pathway involves multiple enzymatic steps carried-out in peroxisomes and in the endoplasmic reticulum. The rate limiting step in the biosynthesis of plasmalogens resides in the formation of the fatty alcohol responsible for the formation of an intermediate with an alkyl-linked moiety. The regulation in the biosynthesis of plasmalogens also takes place at this step using a feedback mechanism to stimulate or inhibit the biosynthesis. As such, fatty alcohols play a relevant role in the formation of ether-phospholipids. These advances in our understanding of complex lipid biosynthesis brought two seemingly distinct disorders into the spotlight. Sjögren-Larsson syndrome (SLS) is caused by defects in the microsomal fatty aldehyde dehydrogenase (FALDH) leading to the accumulation of fatty alcohols and fatty aldehydes. In RCDP cells, the defect in plasmalogens is thought to generate a feedback signal to increase their biosynthesis, through the activity of fatty acid reductases to produce fatty alcohols. However, the enzymatic defects in either glyceronephosphate O-acyltransferase (GNPAT) or alkylglycerone phosphate synthase (AGPS) disrupt the biosynthesis and result in the accumulation of the fatty alcohols. A detailed characterization on the processes and enzymes that govern these intricate biosynthetic pathways, as well as, the metabolic characterization of defects along the pathway should increase our understanding of the causes and mechanisms behind these disorders.

Fatty aldehyde dehydrogenase multigene family involved in the assimilation of n-alkanes in Yarrowia lipolytica

In the n-alkane assimilating yeast Yarrowia lipolytica, n-alkanes are oxidized to fatty acids via fatty alcohols and fatty aldehydes, after which they are utilized as carbon sources. Here, we show that four genes (HFD1-HFD4) encoding fatty aldehyde dehydrogenases (FALDHs) are involved in the metabolism of n-alkanes in Y. lipolytica. A mutant, in which all of four HFD genes are deleted (Δhfd1-4 strain), could not grow on n-alkanes of 12-18 carbons; however, the expression of one of those HFD genes restored its growth on n-alkanes. Production of Hfd2Ap or Hfd2Bp, translation products of transcript variants generated from HFD2 by the absence or presence of splicing, also supported the growth of the Δhfd1-4 strain on n-alkanes. The FALDH activity in the extract of the wild-type strain was increased when cells were incubated in the presence of n-decane, whereas this elevation in FALDH activity by n-decane was not observed in Δhfd1-4 strain extract. Substantial FALDH activities were detected in the extracts of Escherichia coli cells expressing the HFD genes. Fluorescent microscopic observation suggests that Hfd3p and Hfd2Bp are localized predominantly in the peroxisome, whereas Hfd1p and Hfd2Ap are localized in both the endoplasmic reticulum and the peroxisome. These results suggest that the HFD multigene family is responsible for the oxidation of fatty aldehydes to fatty acids in the metabolism of n-alkanes, and raise the possibility that Hfd proteins have diversified by gene multiplication and RNA splicing to efficiently assimilate or detoxify fatty aldehydes in Y. lipolytica.

Fatty aldehyde and fatty alcohol metabolism: review and importance for epidermal structure and function

Normal fatty aldehyde and alcohol metabolism is essential for epidermal differentiation and function. Long-chain aldehydes are produced by catabolism of several lipids including fatty alcohols, sphingolipids, ether glycerolipids, isoprenoid alcohols and certain aliphatic lipids that undergo α- or ω-oxidation. The fatty aldehyde generated by these pathways is chiefly metabolized to fatty acid by fatty aldehyde dehydrogenase (FALDH, alternately known as ALDH3A2), which also functions to oxidize fatty alcohols as a component of the fatty alcohol:NAD oxidoreductase (FAO) enzyme complex. Genetic deficiency of FALDH/FAO in patients with Sjögren-Larsson syndrome (SLS) results in accumulation of fatty aldehydes, fatty alcohols and related lipids (ether glycerolipids, wax esters) in cultured keratinocytes. These biochemical changes are associated with abnormalities in formation of lamellar bodies in the stratum granulosum and impaired delivery of their precursor membranes to the stratum corneum (SC). The defective extracellular SC membranes are responsible for a leaky epidermal water barrier and ichthyosis. Although lamellar bodies appear to be the pathogenic target for abnormal fatty aldehyde/alcohol metabolism in SLS, the precise biochemical mechanisms are yet to be elucidated. Nevertheless, studies in SLS highlight the critical importance of FALDH and normal fatty aldehyde/alcohol metabolism for epidermal function. This article is part of a Special Issue entitled The Important Role of Lipids in the Epidermis and their Role in the Formation and Maintenance of the Cutaneous Barrier. Guest Editors: Kenneth R. Feingold and Peter Elias.

Monitoring of fatty aldehyde dehydrogenase by formation of pyrenedecanoic acid from pyrenedecanal

Fatty aldehyde dehydrogenase (EC 1.2.1.48) converts long-chain fatty aldehydes to the corresponding acids. Deficiency in this enzyme causes the Sjogren Larsson Syndrome, a rare inherited disorder characterized by ichthyosis, spasticity, and mental retardation. Using a fluorescent aldehyde, pyrenedecanal, and HPLC with fluorescence detection, we developed a novel method to monitor fatty aldehyde dehydrogenase activity by quantification of the product pyrenedecanoic acid together with the substrate pyrenedecanal and possible side products, such as aldehyde adducts. As shown with recombinant enzymes, pyrenedecanal showed a high preference for fatty aldehyde dehydrogenase compared with other aldehyde dehydrogenases. The method allowed detection of fatty aldehyde dehydrogenase activity in nanogram amounts of microsomal or tissue protein and microgram amounts of Sjogren Larsson syndrome patients' skin fibroblast protein. It could successfully be adapted for the analysis of fatty aldehyde dehydrogenase activity in gel slices derived from low-temperature SDS-PAGE, showing that fatty aldehyde dehydrogenase activity from solubilized rat liver microsomes migrates as a dimer. Thus, monitoring of pyrenedecanoic acid formation from pyrenedecanal by HPLC with fluorescence detection provides a robust and sensitive method for determination of fatty aldehyde dehydrogenase activity.

Characterisation of recombinant human fatty aldehyde dehydrogenase: implications for Sjögren-Larsson syndrome

Fatty aldehyde dehydrogenase (FALDH) is an NAD+-dependent oxidoreductase involved in the metabolism of fatty alcohols. Enzyme activity has been implicated in the pathology of diabetes and cancer. Mutations in the human gene inactivate the enzyme and cause accumulation of fatty alcohols in Sjögren-Larsson syndrome, a neurological disorder resulting in physical and mental handicaps. Microsomal FALDH was expressed in E. coli and purified. Using an in vitro activity assay an optimum pH of approximately 9.5 and temperature of approximately 35 degrees C were determined. Medium- and long-chain fatty aldehydes were converted to the corresponding acids and kinetic parameters determined. The enzyme showed high activity with heptanal, tetradecanal, hexadecanal and octadecanal with lower activities for the other tested substrates. The enzyme was also able to convert some fatty alcohol substrates to their corresponding aldehydes and acids, at 25-30% the rate of aldehyde oxidation. A structural model of FALDH has been constructed, and catalytically important residues have been proposed to be involved in alcohol and aldehyde oxidation: Gln-120, Glu-207, Cys-241, Phe-333, Tyr-410 and His-411. These results place FALDH in a central role in the fatty alcohol/acid interconversion cycle, and provide a direct link between enzyme inactivation and disease pathology caused by accumulation of alcohols.

Alpha-methylacyl-CoA racemase--an 'obscure' metabolic enzyme takes centre stage

Branched-chain lipids are important components of the human diet and are used as drug molecules, e.g. ibuprofen. Owing to the presence of methyl groups on their carbon chains, they cannot be metabolized in mitochondria, and instead are processed and degraded in peroxisomes. Several different oxidative degradation pathways for these lipids are known, including alpha-oxidation, beta-oxidation, and omega-oxidation. Dietary branched-chain lipids (especially phytanic acid) have attracted much attention in recent years, due to their link with prostate, breast, colon and other cancers as well as their role in neurological disease. A central role in all the metabolic pathways is played by alpha-methylacyl-CoA racemase (AMACR), which regulates metabolism of these lipids and drugs. AMACR catalyses the chiral inversion of a diverse number of 2-methyl acids (as their CoA esters), and regulates the entry of branched-chain lipids into the peroxisomal and mitochondrial beta-oxidation pathways. This review brings together advances in the different disciplines, and considers new research in both the metabolism of branched-chain lipids and their role in cancer, with particular emphasis on the crucial role played by AMACR. These recent advances enable new preventative and treatment strategies for cancer.

Tissue distribution, ontogeny, and regulation of aldehyde dehydrogenase (Aldh) enzymes mRNA by prototypical microsomal enzyme inducers in mice

Aldehyde dehydrogenases (Aldhs) are a group of nicotinamide adenine dinucleotide phosphate-dependent enzymes that catalyze the oxidation of a wide spectrum of aldehydes to carboxylic acids. Tissue distribution and developmental changes in the expression of the messenger RNA (mRNA) of 15 Aldh enzymes were quantified in male and female mice tissues using the branched DNA signal amplification assay. Furthermore, the regulation of the mRNA expression of Aldhs by 15 typical microsomal enzyme inducers (MEIs) was studied. Aldh1a1 mRNA expression was highest in ovary; 1a2 in testis; 1a3 in placenta; 1a7 in lung; 1b1 in small intestine; 2 in liver; 3a1 in stomach; 3a2 and 3b1 expression was ubiquitous; 4a1, 6a1, 7a1, and 8a1 in liver and kidney; 9a1 in liver, kidney, and small intestine; and 18a1 in ovary and small intestine. mRNAs of different Aldh enzymes were detected at lower levels in fetuses than adult mice and gradually increased after birth to reach adult levels between 15 and 45 days of age, when the gender difference began to appear. Aromatic hydrocarbon receptor (AhR) ligands induced the liver mRNA expression of Aldh1a7, 1b1, and 3a1, constitutive androstane receptor (CAR) activators induced Aldh1a1 and 1a7, whereas pregnane X receptor (PXR) ligands and NF-E2 related factor 2 (Nrf2) activators induced Aldh1a1, 1a7, and 1b1. Peroxisome proliferator activator receptor alpha (PPAR alpha) ligands induced the mRNA expression in liver of almost all Aldhs. The Aldh organ-specific distribution may be important in elucidating their role in metabolism, elimination, and organ-specific toxicity of xenobiotics. Finally, in contrast to other phase-I metabolic enzymes such as CYP450 enzymes, Aldh mRNA expression seems to be generally insensitive to typical microsomal inducers except PPAR alpha ligands.

Dual subcellular localization in the endoplasmic reticulum and peroxisomes and a vital role in protecting against oxidative stress of fatty aldehyde dehydrogenase are achieved by alternative splicing

Fatty aldehyde dehydrogenase (FALDH, ALDH3A2) is thought to be involved in the degradation of phytanic acid, a saturated branched chain fatty acid derived from chlorophyll. However, the identity, subcellular distribution, and physiological roles of FALDH are unclear because several variants produced by alternative splicing are present in varying amounts at different subcellular locations. Subcellular fractionation experiments do not provide a clear-cut conclusion because of the incomplete separation of organelles. We established human cell lines heterologously expressing mouse FALDH from each cDNA without tagging under the control of an inducible promoter and detected the variant FALDH proteins using a mouse FALDH-specific antibody. One variant, FALDH-V, was exclusively detected in peroxisomal membranes. Human FALDH-V with an amino-terminal Myc sequence also localized to peroxisomes. The most dominant form, FALDH-N, and other variants examined, however, were distributed in the endoplasmic reticulum. A gas chromatography-mass spectrometry-based analysis of metabolites in FALDH-expressing cells incubated with phytol or phytanic acid showed that FALDH-V, not FALDH-N, is the key aldehyde dehydrogenase in the degradation pathway and that it protects peroxisomes from oxidative stress. In contrast, both FALDHs had a protective effect against oxidative stress induced by a model aldehyde for lipid peroxidation, dodecanal. These results suggest that FALDH variants are produced by alternative splicing and share an important role in protecting against oxidative stress in an organelle-specific manner.

Sjögren-Larsson syndrome: molecular genetics and biochemical pathogenesis of fatty aldehyde dehydrogenase deficiency

Sjögren-Larsson syndrome (SLS) is an inherited neurocutaneous disorder caused by mutations in the ALDH3A2 gene that encodes fatty aldehyde dehydrogenase (FALDH), an enzyme that catalyzes the oxidation of fatty aldehyde to fatty acid. Affected patients display ichthyosis, mental retardation and spastic diplegia. More than 70 mutations in ALDH3A2 have been discovered in SLS patients including amino acid substitutions, deletions, insertions and splicing errors. Most mutations are private, but several common mutations reflect founder effects, consanguinity or recurrent mutational events. FALDH oxidizes fatty aldehyde substrates arising from metabolism of fatty alcohols, leukotriene B4, ether glycerolipids and other potential sources such as sphingolipids. The pathogenesis of the cutaneous and neurologic symptoms is thought to result from abnormal lipid accumulation in the membranes of skin and brain; the formation of aldehyde Schiff base adducts with amine-containing lipids or proteins; or defective eicosanoid metabolism. Therapeutic approaches are being developed to target specific metabolic defects associated with FALDH deficiency or to correct the genetic defect by gene transfer.

Sjögren-Larsson syndrome: diversity of mutations and polymorphisms in the fatty aldehyde dehydrogenase gene (ALDH3A2)

Sjögren-Larsson syndrome (SLS) is an autosomal recessive disorder characterized by ichthyosis, mental retardation, and spastic diplegia or tetraplegia. The disease is caused by mutations in the ALDH3A2 gene (also known as FALDH and ALDH10) on chromosome 17p11.2 that encodes fatty aldehyde dehydrogenase (FALDH), an enzyme that catalyzes the oxidation of long-chain aldehydes derived from lipid metabolism. In SLS patients, 72 mutations have been identified, with a distribution that is scattered throughout the ALDH3A2 gene. Most mutations are private but several common mutations have been detected, which probably reflect founder effects or recurrent mutational events. Missense mutations comprise the most abundant class (38%) and expression studies indicate that most of these result in a profound reduction in enzyme activity. Deletions account for about 25% of the mutations and range from single nucleotides to entire exons. Twelve splice-site mutations have been demonstrated to cause aberrant splicing in cultured fibroblasts. To date, more than a dozen intragenic ALDH3A2 polymorphisms consisting of SNPs and one microsatellite marker have been characterized, although none of them alter the FALDH protein sequence. The striking mutational diversity in SLS offers a challenge for DNA-based diagnosis, but promises to provide a wealth of information about enzyme structure-function correlations.

Genome-wide detection of tissue-specific alternative splicing in the human transcriptome

We have developed an automated method for discovering tissue-specific regulation of alternative splicing through a genome-wide analysis of expressed sequence tags (ESTs). Using this approach, we have identified 667 tissue-specific alternative splice forms of human genes. We validated our muscle-specific and brain-specific splice forms for known genes. A high fraction (8/10) were reported to have a matching tissue specificity by independent studies in the published literature. The number of tissue-specific alternative splice forms is highest in brain, while eye-retina, muscle, skin, testis and lymph have the greatest enrichment of tissue-specific splicing. Overall, 10-30% of human alternatively spliced genes in our data show evidence of tissue-specific splice forms. Seventy-eight percent of our tissue-specific alternative splices appear to be novel discoveries. We present bioinformatics analysis of several tissue-specific splice forms, including automated protein isoform sequence and domain prediction, showing how our data can provide valuable insights into gene function in different tissues. For example, we have discovered a novel kidney-specific alternative splice form of the WNK1 gene, which appears to specifically disrupt its N-terminal kinase domain and may play a role in PHAII hypertension. Our database greatly expands knowledge of tissue-specific alternative splicing and provides a comprehensive dataset for investigating its functional roles and regulation in different human tissues.

Fatty aldehyde dehydrogenase: genomic structure, expression and mutation analysis in Sjögren-Larsson syndrome

Fatty aldehyde dehydrogenase (FALDH) is a microsomal enzyme that catalyzes the oxidation of medium- and long-chain aliphatic aldehydes derived from metabolism of fatty alcohol, phytanic acid, ether glycerolipids and leukotriene B4. The FALDH gene (ALDH3A2) in man and mouse consists of 11 exons and is closely linked to the gene for ALDH3. In both species, alternative splicing results in formation of a second minor protein, FALDHv, that has a unique carboxy-terminal end. The functional significance of this alternate protein is not known. In humans, mutations in the FALDH gene cause Sjögren-Larsson syndrome (SLS), which is characterized by ichthyosis, mental retardation and spasticity. Missense mutations involving 24 amino acid positions in FALDH have been identified. These amino acids are more highly conserved among related class 3 aldehyde dehydrogenase enzymes than expected, suggesting that they are critically important for protein folding, catalysis or stability. Studies of mutations in SLS should prove useful for understanding structure-function correlations in FALDH and other aldehyde dehydrogenase proteins.