Genotype-phenotype spectrum and correlation of PHARC Syndrome due to pathogenic ABHD12 variants.

BACKGROUND: A comprehensive understanding of the genetic basis of rare diseases and their regulatory mechanisms is essential for human molecular genetics. However, the genetic mutant spectrum of pathogenic genes within the Chinese population remains underrepresented. Here, we reported previously unreported functional ABHD12 variants in two Chinese families and explored the correlation between genetic polymorphisms and phenotypes linked to PHARC syndrome. METHODS: Participants with biallelic pathogenic ABHD12 variants were recruited from the Chinese Deafness Genetics Cohort. These participants underwent whole-genome sequencing. Subsequently, a comprehensive literature review was conducted. RESULTS: Two Han Chinese families were identified, one with a compound heterozygous variant and the other with a novel homozygous variant in ABHD12. Among 65 PHARC patients, including 62 from the literature and 3 from this study, approximately 90% (57 out of 63) exhibited hearing loss, 82% (50 out of 61) had cataracts, 82% (46 out of 56) presented with retinitis pigmentosa, 79% (42 out of 53) experienced polyneuropathy, and 63% (36 out of 57) displayed ataxia. Seventeen different patterns were observed in the five main phenotypes of PHARC syndrome. A total of 33 pathogenic variants were identified in the ABHD12. Compared with other genotypes, individuals with biallelic truncating variants showed a higher incidence of polyneuropathy (p = 0.006), but no statistically significant differences were observed in the incidence of hearing loss, ataxia, retinitis pigmentosa and cataracts. CONCLUSIONS: The diagnosis of PHARC syndrome is challenging because of its genetic heterogeneity. Therefore, exploring novel variants and establishing genotype-phenotype correlations can significantly enhance gene diagnosis and genetic counseling for this complex disease.

Coexistence of Retinitis Pigmentosa and Ataxia in Patients with PHARC, PCARP, and Oliver-McFarlane Syndromes.

Retinitis pigmentosa (RP) is an inherited retinal dystrophy caused by the loss of photoreceptors and retinal pigment epithelial atrophy, leading to severe visual impairment or blindness. RP can be classified as nonsyndromic or syndromic with complex clinical phenotypes. Three unrelated Polish probands affected with retinitis pigmentosa coexisting with cerebellar ataxia were recruited for this study. Clinical heterogeneity and delayed appearance of typical disease symptoms significantly prolonged the patients' diagnostic process. Therefore, many clinical and genetic tests have been performed in the past. Here, we provide detailed clinical and genetic analysis results of the patients. Whole-exome sequencing (WES) and targeted NGS analysis allow the identification of four novel and two previously reported variants in the following genes: ABHD12, FLVCR1, and PNPLA6. The use of next-generation sequencing (NGS) methods finally allowed for confirmation of the clinical diagnosis. Ultra-rare diseases such as PHARC, PCARP, and Oliver-McFarlane syndromes were diagnosed in patients, respectively. Our findings confirmed the importance of the application of next-generation sequencing methods, especially in ultra-rare genetic disorders with overlapping features.

Unilateral Cataract and Retinitis Pigmentosa in a Patient With Polyneuropathy, Hearing Loss, Ataxia, Retinitis Pigmentosa, and Cataract (PHARC) Syndrome: A Case Report.

Patients with mutations in the α/ß-hydrolase (ABHD) 12 gene develop ocular complications including cataracts and retinitis pigmentosa (RP), as part of the polyneuropathy, hearing loss, ataxia, RP, and cataract (PHARC) syndrome. A chart review on a patient with a heterozygous mutation on the ABHD12 gene underwent a comprehensive ophthalmic evaluation. Visual acuity was 0 and 1.3 (logMAR) on the right eye (OD) and left eye (OS), respectively. There was pseudophakia in the OS. Fundus examination in OD was normal and pale optic nerve, attenuated vessels, cystoid macular edema, and mid-peripheral bony spicules were found in OS. Visual field test showed a ring scotoma in the OS. Macular optical coherence tomography (OCT) and fundus autofluorescence were compatible with cystoid macular edema of the OS. The electroretinogram (ERG) of left eye was flat. Patient's systemic findings included: polyneuropathy and hearing loss. Unilateral presentation of cataract and RP in a patient with a heterozygous pathogenic mutation on the ABHD12 gene is rare. This could be due to mosaicism. Retinal follow-up is warranted in this patient since manifestations may occur later in the contralateral eye. A heterozygous pathogenic mutation on the ABHD12 gene may lead to partial ocular and systemic manifestations of the PHARC syndrome.

PHARC syndrome which an ultra-rare syndrome with retinitis pigmentosa and cataracts: case report and review of the literature.

BACKGROUND: PHARC syndrome (MIM:612674) is a rare neurodegenerative disorder characterized by demyelinating polyneuropathy, hearing loss, ataxia, retinitis pigmentosa, and cataracts (PHARC). The syndrome is caused by mutations in the ABHD12 gene, which encodes αß-hydrolase domain-containing protein 12 related to endocannabinoid metabolism. PHARC syndrome is one of the rare diseases; so far, only 51 patients have been reported in the literature. METHODS: We evaluated the 25-year-old male patient referred to us due to vision loss, cataracts, and hearing loss. Ophthalmological examinations and genetic analyses were performed using targeted next-generation sequencing. RESULTS: In the genetic analysis, the patient was diagnosed with PHARC syndrome by detecting homozygous (NM_001042472.3): c.871del (p.Tyr291IlefsTer28) novel pathogenic variation in the ABHD12 gene. Following the molecular diagnosis, he was referred to the neurology department for reverse phenotyping and sensorimotor demyelinating polyneuropathy was detected in the neurological evaluation. CONCLUSIONS: In this study, we report a novel variation in ABHD12 gene in the first Turkish-origin PHARC patient. We present this study to contribute genotype-phenotype correlation of PHARC syndrome and emphasize the importance of molecular genetic diagnosis in order to determine the appropriate clinical approach. This report is essential for expanding the phenotypic spectrum in different populations and understanding the genotype-phenotype correlation of PHARC syndrome via novel pathogenic variation in the ABHD12 gene.

Genetic insights into PHARC syndrome: identification of a novel frameshift mutation in ABHD12.

BACKGROUND: Mutations in ABHD12 (OMIM: 613,599) are associated with polyneuropathy, hearing loss, ataxia, retinitis pigmentosa, and cataract (PHARC) syndrome (OMIM: 612674), which is a rare autosomal recessive neurodegenerative disease. PHARC syndrome is easily misdiagnosed as other neurologic disorders, such as retinitis pigmentosa, Charcot-Marie-Tooth disease, and Refsum disease, due to phenotype variability and slow progression. This paper presents a novel mutation in ABHD12 in two affected siblings with PHARC syndrome phenotypes. In addition, we summarize genotype-phenotype information of the previously reported patients with ABHD12 mutation. METHODS: Following a thorough medical evaluation, whole-exome sequencing was done on the proband to look for potential genetic causes. This was followed by confirmation of identified variant in the proband and segregation analysis in the family by Sanger sequencing. The variants were interpreted based on the American College of Medical Genetics and Genomics (ACMG) guidelines. RESULTS: A novel pathogenic homozygous frameshift variant, NM_001042472.3:c.601dup, p.(Val201GlyfsTer4), was identified in exon 6 of ABHD12 (ACMG criteria: PVS1 and PM2, PM1, PM4, PP3, and PP4). Through Sanger sequencing, we showed that this variant is co-segregated with the disease in the family. Further medical evaluations confirmed the compatibility of the patients' phenotype with PHARC syndrome. CONCLUSIONS: Our findings expand the spectrum of mutations in the ABHD12 and emphasize the significance of multidisciplinary diagnostic collaboration among clinicians and geneticists to solve the differential diagnosis of related disorders. Moreover, a summary based on mutations found so far in the ABHD12 gene did not suggest a clear genotype-phenotype correlation for PHARC syndrome.

Increase in Cellular Lysophosphatidylserine Content Exacerbates Inflammatory Responses in LPS-Activated Microglia.

Mutations in alpha/beta-hydrolase domain containing (ABHD) 12 gene, which encodes lysophosphatidylserine (LysoPS) lipase, cause the neurodegenerative disease PHARC (Polyneuropathy, Hearing loss, Ataxia, Retinitis pigmentosa, Cataract). Since ABHD12 is expressed by microglia in the central nervous system and is localized to the endoplasmic reticulum, accumulation of intracellular LysoPS by ABHD12 mutations is assumed to be one of the pathological mechanisms associated with microglial activation in PHARC. However, the role of microglia in the PHARC brain and the relationship between microglial function and cellular LysoPS content remains unclear. Therefore, we explored the influence of cellular LysoPS content in microglial inflammatory responses. We evaluated the effects of inhibitors of cellular LysoPS metabolism, KC01 and DO-264, on inflammatory responses using a lipopolysaccharide (LPS)-stimulated mouse microglial cell line, BV-2 and primary microglia. Treatment of DO-264, an inhibitor of cellular LysoPS degradation, enhanced LPS-induced phagocytosis concomitant with the increase in cellular LysoPS content in BV-2 cells. On the other hand, treatment with KC01, an agent had been developed as an inhibitor of LysoPS synthase, reduced phagocytosis without affecting cellular LysoPS content. Such effects of both inhibitors on phagocytosis were also confirmed using primary microglia. KC01 treatment decreased nitric oxide (NO) production, accompanied by a reduction in inducible NO synthase expression in BV-2 microglia. KC01 also suppressed LPS-induced generation of intracellular reactive oxygen species and cytokines such as interleukin-6. Our results suggest that increase in cellular LysoPS levels can exacerbate microglial inflammatory responses. Treatment to prevent the increase in cellular LysoPS in microglia may have therapeutic potential for PHARC.

The loss of enzymatic activity of the PHARC-associated lipase ABHD12 results in increased phagocytosis that causes neuroinflammation.

Phagocytosis is an important evolutionary conserved process, essential for clearing pathogens and cellular debris in higher organisms, including humans. This well-orchestrated innate immunological response is intricately regulated by numerous cellular factors, important amongst which are the immunomodulatory lysophosphatidylserines (lyso-PSs) and the pro-apoptotic oxidized phosphatidylserines (PSs) signalling lipids. Interestingly, in mammals, both these signalling lipids are physiologically regulated by the lipase ABHD12, mutations of which cause the human neurological disorder PHARC. Despite the biomedical significance of this lipase, detailed mechanistic studies and the specific contribution of ABHD12 to innate processes like phagocytosis remain poorly understood. Here, by immunohistochemical and immunofluorescence approaches, using the murine model of PHARC, we show, that upon an inflammatory stimulus, activated microglial cells in the cerebellum of mice deficient in ABHD12 have an amoeboid morphology, increased soma size and display heightened phagocytosis activity. We also report that upon an inflammatory stimulus, cerebellar levels of ABHD12 increase to possibly metabolize the heightened oxidized PS levels, temper phagocytosis and, in turn, control neuroinflammation during oxidative stress. Next, to complement these findings, with the use of biochemical approaches in cultured microglial cells, we show that the pharmacological inhibition and/or genetic deletion of ABHD12 results in increased phagocytic uptake in a fluorescent bead uptake assay. Together, our studies provide compelling evidence that ABHD12 plays an important role in regulating phagocytosis in cerebellar microglial cells and provides a possible explanation, as to why human PHARC subjects display neuroinflammation and atrophy in the cerebellum.

The Phenotypic Spectrum of Patients with PHARC Syndrome Due to Variants in ABHD12: An Ophthalmic Perspective.

This study investigated the phenotypic spectrum of PHARC (polyneuropathy, hearing loss, ataxia, retinitis pigmentosa and early-onset cataract) syndrome caused by biallelic variants in the ABHD12 gene. A total of 15 patients from 12 different families were included, with a mean age of 36.7 years (standard deviation [SD] ± 11.0; range from 17.5 to 53.9) at the most recent examination. The presence and onset of neurological, audiological and ophthalmic symptoms were variable, with no evident order of symptom appearance. The mean best-corrected visual acuity was 1.1 logMAR (SD ± 0.9; range from 0.1 to 2.8; equivalent to 20/250 Snellen) and showed a trend of progressive decline. Different types of cataract were observed in 13 out of 15 patients (87%), which also included congenital forms of cataract. Fundus examination revealed macular involvement in all patients, ranging from alterations of the retinal pigment epithelium to macular atrophy. Intraretinal spicular hyperpigmentation was observed in 7 out of 15 patients (47%). From an ophthalmic perspective, clinical manifestations in patients with PHARC demonstrate variability with regard to their onset and severity. Given the variable nature of PHARC, an early multidisciplinary assessment is recommended to assess disease severity.

Fatty acid chain length drives lysophosphatidylserine-dependent immunological outputs.

In humans, lysophosphatidylserines (lyso-PSs) are potent lipid regulators of important immunological processes. Given their structural diversity and commercial paucity, here we report the synthesis of methyl esters of lyso-PS (Me-lyso-PSs) containing medium- to very-long-chain (VLC) lipid tails. We show that Me-lyso-PSs are excellent substrates for the lyso-PS lipase ABHD12, and that these synthetic lipids are acted upon by cellular carboxylesterases to produce lyso-PSs. Next, in macrophages we demonstrate that VLC lyso-PSs orchestrate pro-inflammatory responses and in turn neuroinflammation via a Toll-like receptor 2 (TLR2)-dependent pathway. We also show that long-chain (LC) lyso-PSs robustly induce intracellular cyclic AMP production, cytosolic calcium influx, and phosphorylation of the nodal extracellular signal-regulated kinase to regulate macrophage activation via a TLR2-independent pathway. Finally, we report that LC lyso-PSs potently elicit histamine release during the mast cell degranulation process, and that ABHD12 is the major lyso-PS lipase in these immune cells.

Elevated Levels of Arachidonic Acid-Derived Lipids Including Prostaglandins and Endocannabinoids Are Present Throughout ABHD12 Knockout Brains: Novel Insights Into the Neurodegenerative Phenotype.

Derived from arachidonic acid (AA), the endogenous cannabinoid (eCB) 2-arachidonoyl glycerol (2-AG) is a substrate for α/ß hydrolase domain-12 (ABHD12). Loss-of-function mutations of ABHD12 are associated with the neurodegenerative disorder polyneuropathy, hearing loss, ataxia, retinitis pigmentosa, and cataract (PHARC). ABHD12 knockout (KO) mice show PHARC-like behaviors in older adulthood. Here, we test the hypothesis that ABHD12 deletion age-dependently regulates bioactive lipids in the CNS. Lipidomics analysis of the brainstem, cerebellum, cortex, hippocampus, hypothalamus, midbrain, striatum and thalamus from male young (3-4 months) and older (7 months) adult ABHD12 KO and age-matched wild-type (WT) mice was performed on over 80 lipids via HPLC/MS/MS, including eCBs, lipoamines, 2-acyl glycerols, free fatty acids, and prostaglandins (PGs). Aging and ABHD12 deletion drove widespread changes in the CNS lipidome; however, the effects of ABHD12 deletion were similar between old and young mice, meaning that many alterations in the lipidome precede PHARC-like symptoms. AA-derived lipids were particularly sensitive to ABHD12 deletion. 2-AG increased in the striatum, hippocampus, cerebellum, thalamus, midbrain, and brainstem, whereas the eCB N-arachidonoyl ethanolamine (AEA) increased in all 8 brain regions, along with at least 2-PGs. Aging also had a widespread effect on the lipidome and more age-related changes in bioactive lipids were found in ABHD12 KO mice than WT suggesting that ABHD12 deletion exacerbates the effects of age. The most robust effects of aging (independent of genotype) across the CNS were decreases in N-acyl GABAs and N-acyl glycines. In conclusion, levels of bioactive lipids are dynamic throughout adulthood and deleting ABHD12 disrupts the wider lipidome, modulating multiple AA-derived lipids with potential consequences for neuropathology.

A novel ABHD12 nonsense variant in Usher syndrome type 3 family with genotype-phenotype spectrum review.

Usher syndrome (USH) is a clinically common autosomal recessive disorder characterized by retinitis pigmentosa (RP) and sensorineural hearing loss with or without vestibular dysfunction. In this study, we identified a Hunan family of Chinese descent with two affected members clinically diagnosed with Usher syndrome type 3 (USH3) displaying hearing, visual acuity, and olfactory decline. Whole-exome sequencing (WES) identified a nonsense variant in ABHD12 gene that was confirmed to be segregated in this family by Sanger sequencing and exhibited a recessive inheritance pattern. In this family, two patients carried homozygous variant in the ABHD12 (NM_015600: c.249C>G). Mutation of ABHD12, an enzyme that hydrolyzes an endocannabinoid lipid transmitter, caused incomplete PHARC syndrome, as demonstrated in previous reports. Therefore, we also conducted a summary based on variants in ABHD12 in PHARC patients, and in PHARC patients showing that there was no obvious correlation between the genotype and phenotype. We believe that this should be considered during the differential diagnosis of USH. Our findings predicted the potential function of this gene in the development of hearing and vision loss, particularly with regard to impaired signal transmission, and identified a novel nonsense variant to expand the variant spectrum in ABHD12.

Structural properties and role of the endocannabinoid lipases ABHD6 and ABHD12 in lipid signalling and disease.

The endocannabinoid (eCB) system is an important part of both the human central nervous system (CNS) and peripheral tissues. It is involved in the regulation of various physiological and neuronal processes and has been associated with various diseases. The eCB system is a complex network composed of receptor molecules, their cannabinoid ligands, and enzymes regulating the synthesis, release, uptake, and degradation of the signalling molecules. Although the eCB system and the molecular processes of eCB signalling have been studied extensively over the past decades, the involved molecules and underlying signalling mechanisms have not been described in full detail. An example pose the two poorly characterised eCB-degrading enzymes α/ß-hydrolase domain protein six (ABHD6) and ABHD12, which have been shown to hydrolyse 2-arachidonoyl glycerol-the main eCB in the CNS. We review the current knowledge about the eCB system and the role of ABHD6 and ABHD12 within this important signalling system and associated diseases. Homology modelling and multiple sequence alignments highlight the structural features of the studied enzymes and their similarities, as well as the structural basis of disease-related ABHD12 mutations. However, homologies within the ABHD family are very low, and even the closest homologues have widely varying substrate preferences. Detailed experimental analyses at the molecular level will be necessary to understand these important enzymes in full detail.

Biochemical characterization of the PHARC-associated serine hydrolase ABHD12 reveals its preference for very-long-chain lipids.

Polyneuropathy, hearing loss, ataxia, retinitis pigmentosa, and cataract (PHARC) is a rare genetic human neurological disorder caused by null mutations to the Abhd12 gene, which encodes the integral membrane serine hydrolase enzyme ABHD12. Although the role that ABHD12 plays in PHARC is understood, the thorough biochemical characterization of ABHD12 is lacking. Here, we report the facile synthesis of mono-1-(fatty)acyl-glycerol lipids of varying chain lengths and unsaturation and use this lipid substrate library to biochemically characterize recombinant mammalian ABHD12. The substrate profiling study for ABHD12 suggested that this enzyme requires glycosylation for optimal activity and that it has a strong preference for very-long-chain lipid substrates. We further validated this substrate profile against brain membrane lysates generated from WT and ABHD12 knockout mice. Finally, using cellular organelle fractionation and immunofluorescence assays, we show that mammalian ABHD12 is enriched on the endoplasmic reticulum membrane, where most of the very-long-chain fatty acids are biosynthesized in cells. Taken together, our findings provide a biochemical explanation for why very-long-chain lipids (such as lysophosphatidylserine lipids) accumulate in the brains of ABHD12 knockout mice, which is a murine model of PHARC.

Phenotypical features of two patients diagnosed with PHARC syndrome and carriers of a new homozygous mutation in the ABHD12 gene.

PHARC (Polyneuropathy, Hearing loss, Ataxia, Retinitis pigmentosa and Cataracts) (MIM# 612674) is an autosomal recessive neurodegenerative disease caused by mutations in the ABHD12 gene. We evaluated two Spanish siblings affected with pes cavus, sensorimotor neuropathy, hearing loss, retinitis pigmentosa and juvenile cataracts in whom the genetic test of ABHD12 revealed a novel homozygous frameshift mutation, c.211_223del (p.Arg71Tyrfs*26). The earliest clinical manifestation in these patients was a demyelinating neuropathy manifested with a Charcot-Marie-Tooth phenotype over three decades. Progressive hearing loss, cataracts and retinitis pigmentosa appeared after the age of 30. We herein describe the complete clinical picture of these two patients, and focus particularly on neuropathy characteristics. This study supports the fact that although PHARC is rare, its phenotype is very characteristic and we should include its study in patients affected with demyelinating polyneuropathy, hearing loss and retinopathy.

A complex homozygous mutation in ABHD12 responsible for PHARC syndrome discovered with NGS and review of the literature.

PHARC syndrome (MIM612674) is an autosomal recessive neurodegenerative pathology that leads to demyelinating Polyneuropathy, Hearing loss, cerebellar Ataxia, Retinitis pigmentosa, and early-onset Cataracts (PHARC). These various symptoms can appear at different ages. PHARC syndrome is caused by mutations in ABHD12 (α-ß hydrolase domain 12), of which several have been described. We report here a new complex homozygous mutation c.379_385delAACTACTinsGATTCCTTATATACCATTGTAGTCTTACTGCTTTTGGTGAACACA (p.Asn127Aspfs*23). This mutation was detected in a 36-year-old man, who presented neuropathic symptoms from the age of 15, using a next-generation sequencing panel. This result suggests that the involvement of ABHD12 in polyneuropathies is possibly underestimated. We then performed a comparative study of other patients presenting ABHD12 mutations and searched for genotype-phenotype correlations and functional explanations in this heterogeneous population.

Functional validation of ABHD12 mutations in the neurodegenerative disease PHARC.

ABHD12 mutations have been linked to neurodegenerative PHARC (polyneuropathy, hearing loss, ataxia, retinitis pigmentosa, and early-onset cataract), a rare, progressive, autosomal, recessive disease. Although ABHD12 is suspected to play a role in the lysophosphatidylserine and/or endocannabinoid pathways, its precise functional role(s) leading to PHARC disease had not previously been characterized. Cell and zebrafish models were designed to demonstrate the causal link between an identified new missense mutation p.T253R, characterized in ABHD12 from a young patient, the previously characterized p.T202I and p.R352* mutations, and the associated PHARC. Measuring ABHD12 monoacylglycerol lipase activity in transfected HEK293 cells demonstrated inhibition with mutated isoforms. Both the expression pattern of zebrafish abhd12 and the phenotype of specific antisense morpholino oligonucleotide gene knockdown morphants were consistent with human PHARC hallmarks. High abhd12 transcript levels were found in the optic tectum and tract, colocalized with myelin basic protein, and in the spinal cord. Morphants have myelination defects and concomitant functional deficits, characterized by progressive ataxia and motor skill impairment. A disruption of retina architecture and retinotectal projections was observed, together with an inhibition of lens clarification and a low number of mechanosensory hair cells in the inner ear and lateral line system. The severe phenotypes in abhd12 knockdown morphants were rescued by introducing wild-type human ABHD12 mRNA, but not by mutation-harboring mRNAs. Zebrafish may provide a suitable vertebrate model for ABHD12 insufficiency and the study of functional impairment and potential therapeutic rescue of this rare, neurodegenerative disease.

Novel ABHD12 mutations in PHARC patients: the differential diagnosis of deaf-blindness.

OBJECTIVE: This study examines ABHD12 mutation analysis in 2 PHARC patients, originally thought to be Usher syndrome. METHODS: The ABHD12 gene of 2 patients, who suffered from deaf-blindness and dysfunctional central and peripheral nervous systems, were sequenced. RESULTS: We identified that both cases carried the same novel splice site mutation in the ABHD12 gene. However, 1 had epilepsy and the other had peripheral neuropathy. Based on haplotype analysis, the mutation is likely not a hot spot, but rather could be attributable to a common ancestor. CONCLUSION: This study shows that PHARC has phenotypic variability, even within a family, which is consistent with previous reports. Differential diagnosis of "deaf-blindness" diseases is crucial. Confirming the presence of associated symptoms is necessary for differentiating some deaf-blindness syndromes. In addition, mutation analysis is a useful tool for confirming the diagnosis.

Two novel mutations in ABHD12: expansion of the mutation spectrum in PHARC and assessment of their functional effects.

PHARC (polyneuropathy, hearing loss, ataxia, retinitis pigmentosa, and cataracts) is a recently described autosomal-recessive neurodegenerative disease caused by mutations in the α-ß-hydrolase domain-containing 12 gene (ABHD12). Only five homozygous ABHD12 mutations have been reported and the pathogenesis of PHARC remains unclear. We evaluated a woman who manifested short stature as well as the typical features of PHARC. Sequence analysis of ABHD12 revealed a novel heterozygous c.1129A>T (p.Lys377*) mutation. Targeted comparative genomic hybridization detected a 59-kb deletion that encompasses exon 1 of ABHD12 and exons 1-4 of an adjacent gene, GINS1, and includes the promoters of both genes. The heterozygous deletion was also carried by the patient's asymptomatic mother. Quantitative reverse transcription-PCR demonstrated ∼50% decreased expression of ABHD12 RNA in lymphoblastoid cell lines from both individuals. Activity-based protein profiling of serine hydrolases revealed absence of ABHD12 hydrolase activity in the patient and 50% reduction in her mother. This is the first report of compound heterozygosity in PHARC and the first study to describe how a mutation might affect ABHD12 expression and function. The possible involvement of haploinsufficiency for GINS1, a DNA replication complex protein, in the short stature of the patient and her mother requires further studies.