Mutations in ABHD12 cause the neurodegenerative disease PHARC: An inborn error of endocannabinoid metabolism

Abstract

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.

Figure 1

Phenotype and Genotype of PHARC Patients with Genetic Disruption of the 2-AG-Hydrolyzing Enzyme ABHD12 (A–E) Main symptoms in PHARC patients are shown. A summary of the symptoms and findings in each case is given in Table 1. (A) MRI scan of an American female aged 50 (case 7.1) showing cerebellar atrophy. (B) Star-shaped cataract of the posterior pole of the lens in a Norwegian male aged 24 (case 4.1). (C) Fundus of a Norwegian male aged 56 (case 1.2) showing bone-spicule-shaped pigment deposits in the mid-periphery, pallor of the optic disc, attenuation of retinal vessels, and maculopathy. (D) Audiogram of a Norwegian male aged 16 (case 5.1) showing sensorineural hearing loss of both right (red curve) and left (blue curve) ear, around 60 dB in the higher frequencies. (E) Signs of peripheral neuropathy with pes cavus and hammertoes in a Norwegian male aged 56 (case 1.2). (F) The ABHD12 gene is located 25,223,379–25,319,477 bp from pter on the reverse strand of chromosome 20 (NCBI build 36.3). Two isoforms containing the α/β-hydrolase domain have been identified, differing only in the last exon. The positions of the homozygous mutations found in the families from the Emiraties (14 Kb deletion), Norway (c.337_338 delGAinsTTT), Algeria (c.846_852 dupTAAGAGC), and USA (c.1054C>T) are indicated. (G) 2-AG is formed nearly exclusively by the hydrolysis of diacylglycerol (DAG), catalyzed by DAG lipases (α or β). The main pathway for formation of DAG from phospholipids is catalyzed by phospholipase Cß (PLCß). Several enzymes are responsible for the breakdown of 2-AG to arachidonate and glycerol. Although MAGL is responsible for 85% of the 2-AG hydrolysis in the mouse brain, ABHD12 and ABHD6 may be important for hydrolysis in specific cell types and/or cellular compartments. 2-AG is also a substrate for the inducible enzyme cyclooxygenase-2 (COX2), which is involved in neuroinflammation. COX2 converts 2-AG to the corresponding hydroperoxy derivative, which is further metabolized to prostaglandin E2 glycerol ester by prostaglandin E2 glycerol ester synthase (PGE2S).

Figure 2

Gene Expression of ABHD12, ABHD6, and MGLL in Mouse Tissues This is a replot of a subset of the GNF Mouse GeneAtlas V3 data, provided by Lattin et al. The data are published online in the BioGPS database under the alias GeneAtlas MOE430, and the NCBI GEO accession number is GSE10246. There is high expression of ABHD12, ABHD6, and MGLL (encoding MAGL) in different brain tissues (dark green bars). The highest level of ABHD12 is found in microglia (red bar, lower panel), and the expression is also high in the related cell types macrophages (red bars) and osteoclasts (light green bar). There is scarce expression of both ABHD6 (mid panel) and MGLL (upper panel) in microglia, macrophages, and osteoclasts. Bars represent the mean of two biological replicates (RNA from two separate pools from independent mice), and error bars show standard error of the mean. Regarding eye tissue, however, bars are the mean of two technical replicates (RNA from the same pool was split for two amplifications).