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Mutations in ABHD12 Cause the Neurodegenerative Disease PHARC: An Inborn Error of Endocannabinoid Metabolism

2010; Elsevier BV; Volume: 87; Issue: 3 Linguagem: Inglês

10.1016/j.ajhg.2010.08.002

1537-6605

Torunn Fiskerstrand, Dorra H’mida-Ben Brahim, Stefan Johansson, M’zahem Abderrahim, Bjørn Ivar Haukanes, Nathalie Drouot, Julian Zimmermann, Andrew J. Cole, Christian A. Vedeler, Cecilie Bredrup, Mirna Assoum, Mériem Tazir, Thomas Klockgether, Abdelmadjid Hamri, Vidar M. Steen, Helge Boman, Laurence A. Bindoff, M. Kœnig, Per M. Knappskog,

Alcohol Consumption and Health Effects

Polyneuropathy, hearing loss, ataxia, retinitis pigmentosa, and cataract (PHARC) is a neurodegenerative disease marked by early-onset cataract and hearing loss, retinitis pigmentosa, and involvement of both the central and peripheral nervous systems, including demyelinating sensorimotor polyneuropathy and cerebellar ataxia. Previously, we mapped this Refsum-like disorder to a 16 Mb region on chromosome 20. Here we report that mutations in the ABHD12 gene cause PHARC disease and we describe the clinical manifestations in a total of 19 patients from four different countries. The ABHD12 enzyme was recently shown to hydrolyze 2-arachidonoyl glycerol (2-AG), the main endocannabinoid lipid transmitter that acts on cannabinoid receptors CB1 and CB2. Our data therefore represent an example of an inherited disorder related to endocannabinoid metabolism. The endocannabinoid system is involved in a wide range of physiological processes including neurotransmission, mood, appetite, pain appreciation, addiction behavior, and inflammation, and several potential drugs targeting these pathways are in development for clinical applications. Our findings show that ABHD12 performs essential functions in both the central and peripheral nervous systems and the eye. Any future drug-mediated interference with this enzyme should consider the potential risk of long-term adverse effects. Polyneuropathy, hearing loss, ataxia, retinitis pigmentosa, and cataract (PHARC) is a neurodegenerative disease marked by early-onset cataract and hearing loss, retinitis pigmentosa, and involvement of both the central and peripheral nervous systems, including demyelinating sensorimotor polyneuropathy and cerebellar ataxia. Previously, we mapped this Refsum-like disorder to a 16 Mb region on chromosome 20. Here we report that mutations in the ABHD12 gene cause PHARC disease and we describe the clinical manifestations in a total of 19 patients from four different countries. The ABHD12 enzyme was recently shown to hydrolyze 2-arachidonoyl glycerol (2-AG), the main endocannabinoid lipid transmitter that acts on cannabinoid receptors CB1 and CB2. Our data therefore represent an example of an inherited disorder related to endocannabinoid metabolism. The endocannabinoid system is involved in a wide range of physiological processes including neurotransmission, mood, appetite, pain appreciation, addiction behavior, and inflammation, and several potential drugs targeting these pathways are in development for clinical applications. Our findings show that ABHD12 performs essential functions in both the central and peripheral nervous systems and the eye. Any future drug-mediated interference with this enzyme should consider the potential risk of long-term adverse effects. Inherited neurodegenerative diseases affecting both the peripheral and central nervous systems and the eye can be caused by a variety of metabolic disturbances. Mitochondrial dysfunction is a potent cause,1Tucker E.J. Compton A.G. Thorburn D.R. Recent advances in the genetics of mitochondrial encephalopathies.Curr. Neurol. Neurosci. Rep. 2010; 10: 277-285Crossref PubMed Scopus (38) Google Scholar, 2Finsterer J. Mitochondrial ataxias.Can. J. Neurol. Sci. 2009; 36: 543-553Crossref PubMed Scopus (36) Google Scholar arising either from mutation in the mitochondrial genome—e.g., neuropathy, ataxia, retinitis pigmentosa (NARP, MIM 551500) and Kearns-Sayre syndrome (ophthalmoplegia, retinal pigmentation, ataxia, and frequently peripheral neuropathy, MIM 530000)—or from a mutated nuclear gene. Friedreich ataxia (MIM 229300) and POLG-related diseases (MIM 174763) are examples of the latter. Defects involving peroxisomal metabolism, such as Refsum disease (MIM 266500) and alpha-methylacyl-CoA racemase (AMACR; MIM 604489) deficiency, also give rise to similar phenotypes.3Wanders R.J. Komen J.C. Peroxisomes, Refsum's disease and the alpha- and omega-oxidation of phytanic acid.Biochem. Soc. Trans. 2007; 35: 865-869Crossref PubMed Scopus (36) Google Scholar Recently, in a Norwegian family we described a progressive, autosomal-recessive, neurodegenerative disease that we ascertained initially as a phenocopy for Refsum disease (Figures 1A–1E ). We named the disorder polyneuropathy, hearing loss, ataxia, retinitis pigmentosa, and cataract, or PHARC4Fiskerstrand T. Knappskog P. Majewski J. Wanders R.J. Boman H. Bindoff L.A. A novel Refsum-like disorder that maps to chromosome 20.Neurology. 2009; 72: 20-27Crossref PubMed Scopus (27) Google Scholar (MIM 612674). The disease is slowly progressive, with recognition of the first symptoms typically in the late teens. Although the condition has similarities to Refsum disease, patients do not have anosmia and both phytanic acid levels and peroxisomal function are normal. We mapped the disease to a 16 Mb region on chromosome 20.4Fiskerstrand T. Knappskog P. Majewski J. Wanders R.J. Boman H. Bindoff L.A. A novel Refsum-like disorder that maps to chromosome 20.Neurology. 2009; 72: 20-27Crossref PubMed Scopus (27) Google Scholar Subsequently, additional affected individuals in four countries were identified, and we used homozygosity mapping to identify candidate regions for the mutated gene, followed by sequencing of candidate genes. For the present study, DNA was obtained from 19 persons affected with PHARC disease and from healthy siblings and parents. The patients (10 females and 9 males) had a mean age of 32.5 years (range 6–62 years) and originated from Norway (n = 8), Algeria (n = 7), the United Arab Emirates (n = 3), and the USA (n = 1) (Table 1). In the previously published Norwegian family, individuals 1.1 and 1.2 are siblings and 1.3 is their third cousin. There are two affected siblings in families 2, 8, 9, and 10, and three affected in family 6. The adults gave informed consent to the investigation and publication of the results. The healthy individuals were not subject to clinical investigation, whereas the affected individuals have all been examined by neurologists, ophthalmologists, and otologists (Table 1). The study was approved of by the Regional Ethics Committee of Western Norway and by the local ethics committees of the University Hospitals of Bonn, Constantine, and Algiers.Table 1Clinical Findings and Results of Investigations in the 19 Patients with PHARC DiseaseFamily/CaseAge (yr) and SexSensory and Motor NeuropathyNeurography and EMGSensorineural Hearing LossAtaxiaMR/CT of BrainPyramidal Tract SignsRetinitis PigmentosaERGCataractNorway mutation: c.337_338delGAinsTTT [p.Asp113PhefsX15]1.162 F38 years; pes cavus; sensory loss; absent ankle reflexesDemyelinatingpolyneuropathyTwentiesNoNormalNo38 yearsRod-cone dystrophy28 years1.256 M37 years; pes cavus from childhoodDemyelinatingpolyneuropathyThirties37 years; gait ataxiaNormalExtensor plantar response at lower limbs; spasticity; hyperreflexia37 yearsRod-cone dystrophy37 years1.346 M38 years; no pes cavus; sensory loss distallyDemyelinatingpolyneuropathyFrom childhood43 years; gait ataxia; upper limb intention tremorCerebellar atrophyExtensor plantar response at lower limbs; spasticity; hyperreflexia46 yearsRod-cone dystrophy25 years2.158 M51 years; pes cavus; sensory loss; reduced tendon reflexesDemyelinating/axonalpolyneuropathyTwentiesNoCerebellar atrophyExtensor plantar response at lower limbs35 yearsRod-cone dystrophy26 years2.254 F53 years; pes cavus; normal sensibility; reduced tendon reflexesNDTwentiesNoNDNo25 yearsFlat25 years3.136 FPes cavus; normal sensibility; reduced tendon reflexes in lower limbsDemyelinatingpolyneuropathyDeaf by the age of 10YesAtrophy of vermis and medulla oblongataExtensor plantar response at right side; spasticity36 yearsRod-cone dystrophy32 years4.124 MPec cavus; hammertoes; reduced tendon reflexes in upper and lower limbsDemyelinatingpolyneuropathyLate in teensNoSlight ventricular assymmetry.No cerebellar atrophyIndifferent plantar responseNoNormal15 years5.116 MPes cavus; reduced sensibility; reduced tendon reflexes in upper limbs, absent in lower limbsDemyelinatingpolyneuropathy13 yearsNoNormalNoNoNormal16 years(slight)The Emirates mutation: 14 Kb deletion removing exon 16.124 MPec cavus from childhood; absent tendon reflexesAbnormalDeaf by the age of 14MildNormalIndifferent plantar responseTwentiesND15 years6.220 MPes cavus from age 4; absent tendon reflexesDemyelinatingpolyneuropathy6 years2 years; gait, limb, and speech ataxia;wheelchair-bound from age 10Cerebellar atrophy (age 3)Extensor plantar responseYesNDYes6.36 FAbsent tendon reflexesNDYesSpeech and limbCerebellar atrophyIndifferent plantar responseNoNDYesUSA mutation: c.1054C>T [p.Arg352X]7.150 F34 years; pes cavus; hammertoes; sensibility slightly reducedAbnormal17 years18 years; dysarthria; gait ataxia; jerky eye movements; tremor in handsCerebellar atrophy Increased signal in periventricular white matter.Flexor plantar response; spasticity; preserved reflexesTwentiesND22 yearsAlgeria mutation: c.846_852dupTAAGAGC [p.His285fsX1]8.111 MAbsent tendon reflexes and moderate muscle weakness of lower limbs; normal sensibilityNDNo3-4 years; limb and gait ataxia; horizontal nystagmus; dysarthria; dysmetria upper and lower limbs; delayed walking at 15 month; action and intention tremorCerebellar atrophyExtensor plantar response at lower limbsNoNDNo8.210 FAbsent tendon reflexes of lower limbs; normal sensibilityNDNo4–5 years; gait ataxiaVermian atrophyExtensor plantar response at lower limbsNoNDNo9.144 MPes cavus; sensory loss; absent tendon reflexes at lower limbs; scoliosisDemyelinatingpolyneuropathyYes7–10 years; gait and limb ataxia; cerebellar dysarthria; dysmetria at upper limbs with adiadocokinesia; head titubationVermian atrophyExtensor plantar response at lower limbs; macroglossiaamblyopiaND9.226 FPes cavus; sensory loss; reduced tendon reflexes at upper limbs, and absent at lower limbsSevere demyelinatingpolyneuropathyDeaf4–9 years; gait and limb ataxia; horizontal nystagmus; moderate dysarthria; dysmetria at upper and lower limbsVermian atrophyExtensor plantar response at lower limbs; tongue fasciculationsYesNDYes10.126 FPes cavus; sensory loss; absent tendon reflexesSevere demyelinatingpolyneuropathy on nerve biopsy6 years6–12 years;gait and limb ataxiaNormalIndifferent plantar responseNoNDNo10.219 F12 years; pes cavus; sensory loss; absent tendon reflexes at upper and lower limbsNDNoNDND11.132 FPes cavus; sensory loss and absent tendon reflexes at lower limbsAxonal polyneuropathyYes16–20 years; gait ataxia; dysarthria; dysmetria at upper limbsCerebellar atrophyExtensor plantar response at lower limbsDecreased visual acuity and amblyopiaNDNoData on patients from four different countries (11 families) are shown. All individuals in one family are siblings, except for 1.3, who is the third cousin of 1.1 and 1.2. All adult patients have polyneuropathy of demyelinating type and sensorineural hearing loss (three patients are deaf), and nearly all adult patients have developed cataracts. Retinitis pigmentosa is typically recognized in the twenties or thirties. Ataxia is present in about half of the patients, with cerebellar atrophy and pyramidal tract signs like spasticity and extensor plantar response. The onset of ataxia is highly variable, starting particularly early in the families from the Emirates and Algeria. Open table in a new tab Data on patients from four different countries (11 families) are shown. All individuals in one family are siblings, except for 1.3, who is the third cousin of 1.1 and 1.2. All adult patients have polyneuropathy of demyelinating type and sensorineural hearing loss (three patients are deaf), and nearly all adult patients have developed cataracts. Retinitis pigmentosa is typically recognized in the twenties or thirties. Ataxia is present in about half of the patients, with cerebellar atrophy and pyramidal tract signs like spasticity and extensor plantar response. The onset of ataxia is highly variable, starting particularly early in the families from the Emirates and Algeria. From the same region as the original Norwegian family (family 1, Table 1),4Fiskerstrand T. Knappskog P. Majewski J. Wanders R.J. Boman H. Bindoff L.A. A novel Refsum-like disorder that maps to chromosome 20.Neurology. 2009; 72: 20-27Crossref PubMed Scopus (27) Google Scholar we ascertained a further five, apparently unrelated, patients (including a brother and sister, family 2) with suspected PHARC disease (family 2-5, Table 1). Homozygosity mapping was performed with GeneChip 250K NspI arrays (GEO accession number GSE23151). The data were exported and treated for further analysis by the programs GTYPE and Progeny Lab. Regions of homozygosity were identified with the PLINK program5Purcell S. Neale B. Todd-Brown K. Thomas L. Ferreira M.A. Bender D. Maller J. Sklar P. de Bakker P.I. Daly M.J. Sham P.C. PLINK: A tool set for whole-genome association and population-based linkage analyses.Am. J. Hum. Genet. 2007; 81: 559-575Abstract Full Text Full Text PDF PubMed Scopus (16836) Google Scholar. All eight Norwegian patients from five families were homozygous for overlapping parts of the previously published 16 Mb region on chromosome 20 (Figure S1, available online), indicating distant relationship. The inclusion of these five additional patients enabled us to refine the candidate region to approximately 6.4 Mb (23,553,833–29,936,849 bp from pter, NCBI build 36.3). Twenty-three of approximately 60 genes in this region were sequenced, and a homozygous indel mutation in exon 3 in the ABHD12 gene (c.337_338 delGAinsTTT; Figure 1F, Figure S2) was identified in all eight patients. The reference sequence for ABHD12 was NM_001042472.1. This frameshift mutation predicts the replacement of an asparagine at codon 113 with phenylalanine leading to a downstream premature stop codon (p.Asp113PhefsX15). The mutation segregated fully with the disease in these families. We screened 190 local healthy blood donors and found two heterozygous carriers of this mutation, corresponding to a disease incidence of approximately 1/36,000 in this population. This indicates that the frequency of PHARC in Western Norway is comparable to, or may be even higher than, relevant differential diagnoses like Friedreich ataxia and Refsum disease. Concurrent mapping studies in one family from the United Arab Emirates and four families from Algeria were performed with Genechip 10K XbaI arrays followed by analysis on selected individuals with the GeneChip 6.0 array (Affymetrix, Santa Clara, USA). Regions of homozygosity were identified with the HomoSNP software (Figure S3). These patients, initially diagnosed with recessive ataxia, defined a 5.5 Mb linkage interval in the 20p11.21-q12 region on chromosome 20 (24,393,550–29,940,293 bp from pter, NCBI build 36.3, Figure S1). Twelve of the 29 genes of this region were sequenced, and a 14 Kb deletion (g.25,312,257_25,326,263 del14007insGG, NCBI Ref.Seq: NC_000020.10) in ABHD12, encompassing the promoter region and exon 1 of the gene (Figure 1F, Figures S4A–S4C), was identified in the family from the Emirates. No copy-number variations in this region have been reported to the Database of Genomic Variants (hg 18). The seven patients in the four Algerian families were homozygous for a 7 bp duplication in exon 9 (c.846_852 dupTAAGAGC) in ABHD12 (Figure S2), which directly replaces the histidine residue at codon 285 with a stop codon (p.His285fsX1). Also in these families the mutation segregated fully with the disease. Finally, a patient from the USA of French-Canadian heritage with suspected PHARC disease was found to be homozygous for a nonsense mutation (c.1054C>T) in exon 12 in ABHD12 (Figure 1F, Figures S1 and S2), leading to a predicted stop codon in position 352 in the protein (p.Arg352X). The finding of four different deleterious ABHD12 mutations in a total of 19 patients with PHARC disease from four countries clearly supports a causal genotype-phenotype relationship. The addition of several new families requires refinement of our earlier clinical description.4Fiskerstrand T. Knappskog P. Majewski J. Wanders R.J. Boman H. Bindoff L.A. A novel Refsum-like disorder that maps to chromosome 20.Neurology. 2009; 72: 20-27Crossref PubMed Scopus (27) Google Scholar The essential clinical features are summarized in Figures 1A–1E and Table 1. PHARC in the Norwegian patients, and in the single American patient, appears to be a slowly progressive disease with recognition of the first symptoms typically in the teens. Cataracts, hearing loss, and a predominantly demyelinating peripheral neuropathy are present in all adult patients (Table 1), whereas the presence and extent of ataxia is variable. Retinitis pigmentosa typically presents in young adult life (twenties or thirties), and electroretinograms in most patients show a rod-cone dysfunction. The disorder in families from Algeria and the Emirates shows an earlier onset of ataxia that has both central and peripheral characteristics (Table 1). No evidence of behavioral disturbances or abnormalities related to appetite was detected in our adult patients. Cerebral cortical function appears to be spared, with only one patient having mental retardation (case 9.1) and another epilepsy (case 7.1, myoclonic seizures). Adult heterozygous carriers of ABHD12 mutations do not have an obvious phenotype, implying that their residual enzyme activity is sufficient to avoid clinical symptoms. Each of the four different ABHD12 mutations is interpreted as a null mutation that would either abolish or severely reduce the activity of the encoding enzyme, α/β-hydrolase 12 (ABHD12). PHARC may, therefore, be considered a human ABHD12 knockout model. The question also arises whether less detrimental mutations may cause various incomplete phenotypes. The serious and progressive disease seen in our patients suggests that ABHD12 performs an essential function in the peripheral and central nervous systems and in the eye. This is supported by the high expression of ABHD12 in the brain, with a striking enrichment in microglia (Figure 2), as shown by our replotting of data from GNF Mouse Gene Atlas V3. Expression is also high in macrophages. Currently, the only known substrate for ABHD12 is the main endocannabinoid 2-arachidonoyl glycerol (2-AG) (Figure 1G). This compound has important functions in synaptic plasticity6Makara J.K. Mor M. Fegley D. Szabó S.I. Kathuria S. Astarita G. Duranti A. Tontini A. Tarzia G. Rivara S. et al.Selective inhibition of 2-AG hydrolysis enhances endocannabinoid signaling in hippocampus.Nat. Neurosci. 2005; 8: 1139-1141Crossref PubMed Scopus (185) Google Scholar, 7Straiker A. Hu S.S. Long J.Z. Arnold A. Wager-Miller J. Cravatt B.F. Mackie K. Monoacylglycerol lipase limits the duration of endocannabinoid-mediated depolarization-induced suppression of excitation in autaptic hippocampal neurons.Mol. Pharmacol. 2009; 76: 1220-1227Crossref PubMed Scopus (76) Google Scholar and neuroinflammation.8Zhang J. Chen C. Endocannabinoid 2-arachidonoylglycerol protects neurons by limiting COX-2 elevation.J. Biol. Chem. 2008; 283: 22601-22611Crossref PubMed Scopus (86) Google Scholar, 9Kreutz S. Koch M. Böttger C. Ghadban C. Korf H.W. Dehghani F. 2-Arachidonoylglycerol elicits neuroprotective effects on excitotoxically lesioned dentate gyrus granule cells via abnormal-cannabidiol-sensitive receptors on microglial cells.Glia. 2009; 57: 286-294Crossref PubMed Scopus (66) Google Scholar In acute ischemia and/or excitotoxicity, 2-AG appears to have neuroprotective properties,9Kreutz S. Koch M. Böttger C. Ghadban C. Korf H.W. Dehghani F. 2-Arachidonoylglycerol elicits neuroprotective effects on excitotoxically lesioned dentate gyrus granule cells via abnormal-cannabidiol-sensitive receptors on microglial cells.Glia. 2009; 57: 286-294Crossref PubMed Scopus (66) Google Scholar, 10Panikashvili D. Simeonidou C. Ben-Shabat S. Hanus L. Breuer A. Mechoulam R. Shohami E. An endogenous cannabinoid (2-AG) is neuroprotective after brain injury.Nature. 2001; 413: 527-531Crossref PubMed Scopus (596) Google Scholar, 11Di Marzo V. Targeting the endocannabinoid system: To enhance or reduce?.Nat. Rev. Drug Discov. 2008; 7: 438-455Crossref PubMed Scopus (625) Google Scholar but the effects of long-term increased levels of this metabolite have not been investigated. The endocannabinoid signaling system is the focus of increasing scientific interest, in part because of the potential for developing novel therapeutic agents.11Di Marzo V. Targeting the endocannabinoid system: To enhance or reduce?.Nat. Rev. Drug Discov. 2008; 7: 438-455Crossref PubMed Scopus (625) Google Scholar, 12Marrs W. Stella N. 2-AG + 2 new players = forecast for therapeutic advances.Chem. Biol. 2007; 14: 1309-1311Abstract Full Text Full Text PDF PubMed Scopus (7) Google Scholar, 13Kinsey S.G. Long J.Z. O'Neal S.T. Abdullah R.A. Poklis J.L. Boger D.L. Cravatt B.F. Lichtman A.H. Blockade of endocannabinoid-degrading enzymes attenuates neuropathic pain.J. Pharmacol. Exp. Ther. 2009; 330: 902-910Crossref PubMed Scopus (245) Google Scholar The system is tightly regulated and appears to be important for many physiological processes including neurotransmission, pain appreciation, appetite, mood, addiction behavior, body temperature, and inflammation.11Di Marzo V. Targeting the endocannabinoid system: To enhance or reduce?.Nat. Rev. Drug Discov. 2008; 7: 438-455Crossref PubMed Scopus (625) Google Scholar Key players in these pathways are the G protein-coupled cannabinoid receptors CB1 and CB2 and their endogenous ligands, endocannabinoids, as well as enzymes that synthesize or hydrolyze these ligands.14Wang J. Ueda N. Biology of endocannabinoid synthesis system.Prostaglandins Other Lipid Mediat. 2009; 89: 112-119Crossref PubMed Scopus (114) Google Scholar The most abundant endocannabinoid, 2-AG, (Figure 1G) is formed on demand from the membrane lipid diacylglycerol (by diacylglycerol lipase α or β).14Wang J. Ueda N. Biology of endocannabinoid synthesis system.Prostaglandins Other Lipid Mediat. 2009; 89: 112-119Crossref PubMed Scopus (114) Google Scholar Endocannabinoids act locally as lipid transmitters and are rapidly cleared by hydrolysis. Interestingly, our patients did not show overt cannabinomimetic effects. Several enzymes are involved in 2-AG hydrolysis15Marrs W. Stella N. Measuring endocannabinoid hydrolysis: Refining our tools and understanding.AAPS J. 2009; 11: 307-311Crossref PubMed Scopus (9) Google Scholar, 16Blankman J.L. Simon G.M. Cravatt B.F. A comprehensive profile of brain enzymes that hydrolyze the endocannabinoid 2-arachidonoylglycerol.Chem. Biol. 2007; 14: 1347-1356Abstract Full Text Full Text PDF PubMed Scopus (772) Google Scholar (Figure 1G), and there is evidence that these enzymes are differentially expressed in various cell types17Muccioli G.G. Xu C. Odah E. Cudaback E. Cisneros J.A. Lambert D.M. López Rodríguez M.L. Bajjalieh S. Stella N. Identification of a novel endocannabinoid-hydrolyzing enzyme expressed by microglial cells.J. Neurosci. 2007; 27: 2883-2889Crossref PubMed Scopus (141) Google Scholar and cellular compartments.7Straiker A. Hu S.S. Long J.Z. Arnold A. Wager-Miller J. Cravatt B.F. Mackie K. Monoacylglycerol lipase limits the duration of endocannabinoid-mediated depolarization-induced suppression of excitation in autaptic hippocampal neurons.Mol. Pharmacol. 2009; 76: 1220-1227Crossref PubMed Scopus (76) Google Scholar, 16Blankman J.L. Simon G.M. Cravatt B.F. A comprehensive profile of brain enzymes that hydrolyze the endocannabinoid 2-arachidonoylglycerol.Chem. Biol. 2007; 14: 1347-1356Abstract Full Text Full Text PDF PubMed Scopus (772) Google Scholar, 17Muccioli G.G. Xu C. Odah E. Cudaback E. Cisneros J.A. Lambert D.M. López Rodríguez M.L. Bajjalieh S. Stella N. Identification of a novel endocannabinoid-hydrolyzing enzyme expressed by microglial cells.J. Neurosci. 2007; 27: 2883-2889Crossref PubMed Scopus (141) Google Scholar In the mouse brain, monoacylglycerol lipase (MAGL) accounts for 85% of the hydrolase activity,11Di Marzo V. Targeting the endocannabinoid system: To enhance or reduce?.Nat. Rev. Drug Discov. 2008; 7: 438-455Crossref PubMed Scopus (625) Google Scholar, 17Muccioli G.G. Xu C. Odah E. Cudaback E. Cisneros J.A. Lambert D.M. López Rodríguez M.L. Bajjalieh S. Stella N. Identification of a novel endocannabinoid-hydrolyzing enzyme expressed by microglial cells.J. Neurosci. 2007; 27: 2883-2889Crossref PubMed Scopus (141) Google Scholar with additional contributions from ABHD12 and α/β-hydrolase 6 (ABHD6).16Blankman J.L. Simon G.M. Cravatt B.F. A comprehensive profile of brain enzymes that hydrolyze the endocannabinoid 2-arachidonoylglycerol.Chem. Biol. 2007; 14: 1347-1356Abstract Full Text Full Text PDF PubMed Scopus (772) Google Scholar The apparent paradox of a purported minor role of ABHD12 in 2-AG hydrolysis versus the serious PHARC phenotype in the brain and eye suggests either that ABHD12 is of crucial importance only in certain cell types12Marrs W. Stella N. 2-AG + 2 new players = forecast for therapeutic advances.Chem. Biol. 2007; 14: 1309-1311Abstract Full Text Full Text PDF PubMed Scopus (7) Google Scholar or that it is also acting on a hitherto unknown substrate other than 2-AG. The finding that microglial cells have a particularly high expression of ABHD12, but very low levels of MGLL (encoding MAGL) and ABHD6 (Figure 2), indicates that the former alternative of differential cellular expression exists. Moreover, microglia dysfunction is known to be involved in neurodegenerative diseases18Landreth G.E. Microglia in central nervous system diseases.J. Neuroimmune Pharmacol. 2009; 4: 369-370Crossref PubMed Scopus (10) Google Scholar as well as in retinal dystrophies.19Ebert S. Weigelt K. Walczak Y. Drobnik W. Mauerer R. Hume D.A. Weber B.H. Langmann T. Docosahexaenoic acid attenuates microglial activation and delays early retinal degeneration.J. Neurochem. 2009; 110: 1863-1875Crossref PubMed Scopus (65) Google Scholar Whether ABHD12 acts on more than one substrate is currently unknown, but many hydrolases have overlapping functions, including MAGL, which is involved in lipolysis20Guzmán M. A new age for MAGL.Chem. Biol. 2010; 17: 4-6Abstract Full Text Full Text PDF PubMed Scopus (8) Google Scholar as well as in hydrolyzing 2-AG. Despite great interest in manipulating 2-AG hydrolysis in vivo,8Zhang J. Chen C. Endocannabinoid 2-arachidonoylglycerol protects neurons by limiting COX-2 elevation.J. Biol. Chem. 2008; 283: 22601-22611Crossref PubMed Scopus (86) Google Scholar, 21Stella N. Endocannabinoid signaling in microglial cells.Neuropharmacology. 2009; 56: 244-253Crossref PubMed Scopus (181) Google Scholar knockout animal models have not yet been developed, and only recently a blocker of MAGL with substantial effect in vivo was reported.22Long J.Z. Li W. Booker L. Burston J.J. Kinsey S.G. Schlosburg J.E. Pavón F.J. Serrano A.M. Selley D.E. Parsons L.H. et al.Selective blockade of 2-arachidonoylglycerol hydrolysis produces cannabinoid behavioral effects.Nat. Chem. Biol. 2009; 5: 37-44Crossref PubMed Scopus (689) Google Scholar Notwithstanding this, inhibition of endocannabinoid hydrolases, including ABHD12, has been suggested as a potential therapy for neurodegenerative diseases such as multiple sclerosis.21Stella N. Endocannabinoid signaling in microglial cells.Neuropharmacology. 2009; 56: 244-253Crossref PubMed Scopus (181) Google Scholar However, the consequences of irreversible loss of ABHD12 function, as seen in our patients with PHARC, may serve as a cautionary reminder that any potential drug inhibiting this enzyme be thoroughly evaluated with respect to the potential risk of severe long-term adverse effects. In conclusion, mutations in the ABHD12-gene causes PHARC, a disease with serious dysfunction of the central and peripheral nervous systems, as well as hearing loss and impaired vision. Our findings have implications for clinicians working with both children and adults and suggest disrupted endocannabinoid metabolism as a cause of neurodegenerative disease. This work was supported by grants from Helse Vest (Western Norway Regional Health Authority, 911308, to P.K., T.F., H.B., V.M.S, and B.I.H.) and from the Agence Nationale pour la Recherche-Maladies Rares (ANR-05-MRAR-013-01, France, to M.K.). D.H.-B.B. was supported by the French association Connaître les Syndromes Cérébelleux. M.A. was supported by a BDI fellowship from the Centre National de la Recherche Scientifique (CNRS). We thank John Walker and Andrew Su for the kind permission to replot (Figure 2) gene expression data from GNF Mouse Gene Atlas V3. The technical assistance of Jorunn Skeie Bringsli, Guri Matre, Hilde Rusaas, Sigrid Erdal, Paal Borge, Christine Stansberg, Bård Kjersem, Christelle Thibault, Serge Vicaire, Jone Vignes, and Ingrid Bauer was highly appreciated. We thank the patients and their families for participating in this study. Download .pdf (.49 MB) Help with pdf files Document S1. Four Figures The URLs for data presented herein are as follows:BioGPS database, http://biogps.gnf.orgDatabase of Genomic Variants, http://projects.tcag.ca/variation/?source=hg18NCBI Build 36.3, http://www.ncbi.nlm.nih.gov/mapviewNCBI Gene Expression Omnibus, http://www.ncbi.nlm.nih.gov/geo/Online Mendelian Inheritance in Man (OMIM), http://www.ncbi.nlm.nih.gov/Omim/ Microarray data have been deposited in NCBI's Gene Expression Omnibus (GEO) and are accessible through GEO Series accession number GSE23151.