cDNA sequencing increases the molecular diagnostic yield in Chediak-Higashi syndrome.

Introduction: Chediak-Higashi syndrome (CHS) is rare autosomal recessive disorder caused by bi-allelic variants in the Lysosomal Trafficking Regulator (LYST) gene. Diagnosis is established by the detection of pathogenic variants in LYST in combination with clinical evidence of disease. Conventional molecular genetic testing of LYST by genomic DNA (gDNA) Sanger sequencing detects the majority of pathogenic variants, but some remain undetected for several individuals clinically diagnosed with CHS. In this study, cDNA Sanger sequencing was pursued as a complementary method to identify variant alleles that are undetected by gDNA Sanger sequencing and to increase molecular diagnostic yield. Methods: Six unrelated individuals with CHS were clinically evaluated and included in this study. gDNA Sanger sequencing and cDNA Sanger sequencing were performed to identify pathogenic LYST variants. Results: Ten novel LYST alleles were identified, including eight nonsense or frameshift variants and two in-frame deletions. Six of these were identified by conventional gDNA Sanger sequencing; cDNA Sanger sequencing was required to identify the remaining variant alleles. Conclusion: By utilizing cDNA sequencing as a complementary technique to identify LYST variants, a complete molecular diagnosis was obtained for all six CHS patients. In this small CHS cohort, the molecular diagnostic yield was increased, and canonical splice site variants identified from gDNA Sanger sequencing were validated by cDNA sequencing. The identification of novel LYST alleles will aid in diagnosing patients and these molecular diagnoses will also lead to genetic counseling, access to services and treatments and clinical trials in the future.

Progressive pulmonary fibrosis in a murine model of Hermansky-Pudlak syndrome.

BACKGROUND: HPS-1 is a genetic type of Hermansky-Pudlak syndrome (HPS) with highly penetrant pulmonary fibrosis (HPSPF), a restrictive lung disease that is similar to idiopathic pulmonary fibrosis (IPF). Hps1ep/ep (pale ear) is a naturally occurring HPS-1 mouse model that exhibits high sensitivity to bleomycin-induced pulmonary fibrosis (PF). Traditional methods of administering bleomycin as an intratracheal (IT) route to induce PF in this model often lead to severe acute lung injury and high mortality rates, complicating studies focusing on pathobiological mechanisms or exploration of therapeutic options for HPSPF. METHODS: To develop a murine model of HPSPF that closely mimics the progression of human pulmonary fibrosis, we investigated the pulmonary effects of systemic delivery of bleomycin in Hps1ep/ep mice using a subcutaneous minipump and compared results to oropharyngeal delivery of bleomycin. RESULTS: Our study revealed that systemic delivery of bleomycin induced limited, acute inflammation that resolved. The distinct inflammatory phase preceded a slow, gradually progressive fibrogenesis that was shown to be both time-dependent and dose-dependent. The fibrosis phase exhibited characteristics that better resembles human disease with focal regions of fibrosis that were predominantly found in peribronchovascular areas and in subpleural regions; central lung areas contained relatively less fibrosis. CONCLUSION: This model provides a preclinical tool that will allow researchers to study the mechanism of pulmonary fibrosis in HPS and provide a platform for the development of therapeutics to treat HPSPF. This method can be applied on studies of IPF or other monogenic disorders that lead to pulmonary fibrosis.

Diagnosis of Chediak Higashi disease in a 67-year old woman.

Chediak-Higashi disease is a rare disease caused by bi-allelic mutations in the lysosomal trafficking regulator gene, LYST. Individuals typically present in early childhood with partial oculocutaneous albinism, a bleeding diathesis, recurrent infections secondary to immune dysfunction, and risk of developing hemophagocytic lymphohistiocytosis (HLH). Without intervention, mortality is high in the first decade of life. However, some individuals with milder phenotypes have attenuated hematologic and immunologic presentations, and lower risk of HLH. Both classic and milder phenotypes develop progressive neurodegeneration in early adulthood. Here we present a remarkable patient diagnosed with Chediak-Higashi disease at age 67, many decades after the diagnosis is usually established. Diagnosis was suspected by observing the pathognomonic granules within leukocytes, and confirmed by identification of bi-allelic mutations in LYST, reduced LYST mRNA expression, enlarged lysosomes within fibroblasts, and decreased NK cell lytic activity. This case further expands the phenotype of Chediak-Higashi disease and highlights the need for increased awareness. Individuals with milder phenotypes may escape early diagnosis, but identification is important for close monitoring of potential complications, and to further our understanding of the function of LYST.

Lysosomal Storage and Albinism Due to Effects of a De Novo CLCN7 Variant on Lysosomal Acidification.

Optimal lysosome function requires maintenance of an acidic pH maintained by proton pumps in combination with a counterion transporter such as the Cl-/H+ exchanger, CLCN7 (ClC-7), encoded by CLCN7. The role of ClC-7 in maintaining lysosomal pH has been controversial. In this paper, we performed clinical and genetic evaluations of two children of different ethnicities. Both children had delayed myelination and development, organomegaly, and hypopigmentation, but neither had osteopetrosis. Whole-exome and -genome sequencing revealed a de novo c.2144A>G variant in CLCN7 in both affected children. This p.Tyr715Cys variant, located in the C-terminal domain of ClC-7, resulted in increased outward currents when it was heterologously expressed in Xenopus oocytes. Fibroblasts from probands displayed a lysosomal pH approximately 0.2 units lower than that of control cells, and treatment with chloroquine normalized the pH. Primary fibroblasts from both probands also exhibited markedly enlarged intracellular vacuoles; this finding was recapitulated by the overexpression of human p.Tyr715Cys CLCN7 in control fibroblasts, reflecting the dominant, gain-of-function nature of the variant. A mouse harboring the knock-in Clcn7 variant exhibited hypopigmentation, hepatomegaly resulting from abnormal storage, and enlarged vacuoles in cultured fibroblasts. Our results show that p.Tyr715Cys is a gain-of-function CLCN7 variant associated with developmental delay, organomegaly, and hypopigmentation resulting from lysosomal hyperacidity, abnormal storage, and enlarged intracellular vacuoles. Our data supports the hypothesis that the ClC-7 antiporter plays a critical role in maintaining lysosomal pH.

Neurologic involvement in patients with atypical Chediak-Higashi disease.

OBJECTIVE: To delineate the developmental and progressive neurodegenerative features in 9 young adults with the atypical form of Chediak-Higashi disease (CHD) enrolled in a natural history study. METHODS: Patients with atypical clinical features, but diagnostically confirmed CHD by standard evaluation of blood smears and molecular genotyping, underwent complete neurologic evaluation, MRI of the brain, electrophysiologic examination, and neuropsychological testing. Fibroblasts were collected to investigate the cellular phenotype and correlation with the clinical presentation. RESULTS: In 9 mildly affected patients with CHD, we documented learning and behavioral difficulties along with developmental structural abnormalities of the cerebellum and posterior fossa, which are apparent early in childhood. A range of progressive neurologic problems emerge in early adulthood, including cerebellar deficits, polyneuropathies, spasticity, cognitive decline, and parkinsonism. CONCLUSIONS: Patients with undiagnosed atypical CHD manifesting some of these wide-ranging yet nonspecific neurologic complaints may reside in general and specialty neurology clinics. The absence of the typical bleeding or infectious diathesis in mildly affected patients with CHD renders them difficult to diagnose. Identification of these individuals is important not only for close surveillance of potential CHD-related systemic complications but also for a full understanding of the natural history of CHD and the potential role of the disease-causing protein, LYST, to the pathophysiology of other neurodevelopmental and neurodegenerative disorders.

Neurologic involvement in patients with atypical Chediak-Higashi disease.

OBJECTIVE: To delineate the developmental and progressive neurodegenerative features in 9 young adults with the atypical form of Chediak-Higashi disease (CHD) enrolled in a natural history study. METHODS: Patients with atypical clinical features, but diagnostically confirmed CHD by standard evaluation of blood smears and molecular genotyping, underwent complete neurologic evaluation, MRI of the brain, electrophysiologic examination, and neuropsychological testing. Fibroblasts were collected to investigate the cellular phenotype and correlation with the clinical presentation. RESULTS: In 9 mildly affected patients with CHD, we documented learning and behavioral difficulties along with developmental structural abnormalities of the cerebellum and posterior fossa, which are apparent early in childhood. A range of progressive neurologic problems emerge in early adulthood, including cerebellar deficits, polyneuropathies, spasticity, cognitive decline, and parkinsonism. CONCLUSIONS: Patients with undiagnosed atypical CHD manifesting some of these wide-ranging yet nonspecific neurologic complaints may reside in general and specialty neurology clinics. The absence of the typical bleeding or infectious diathesis in mildly affected patients with CHD renders them difficult to diagnose. Identification of these individuals is important not only for close surveillance of potential CHD-related systemic complications but also for a full understanding of the natural history of CHD and the potential role of the disease-causing protein, LYST, to the pathophysiology of other neurodevelopmental and neurodegenerative disorders.

In vitro functional correction of Hermansky-Pudlak Syndrome type-1 by lentiviral-mediated gene transfer.

Hermansky-Pudlak syndrome (HPS) is a genetic disorder characterized by oculocutaneous albinism, bleeding tendency and susceptibility to pulmonary fibrosis. No curative therapy is available. Genetic correction directed to the lungs, bone marrow and/or gastro-intestinal tract might provide alternative forms of treatment for the diseases multi-systemic complications. We demonstrate that lentiviral-mediated gene transfer corrects the expression and function of the HPS1 gene in patient dermal melanocytes, which opens the way to development of gene therapy for HPS.

Two novel compound heterozygous mutations in OPA3 in two siblings with OPA3-related 3-methylglutaconic aciduria.

OPA3-related 3-methylglutaconic aciduria, or Costeff Optic Atrophy syndrome, is a neuro-ophthalmologic syndrome of early-onset bilateral optic atrophy and later-onset spasticity, and extrapyramidal dysfunction. Urinary excretion of 3-methylglutaconic acid and of 3-methylglutaric acid is markedly increased. OPA3-related 3-methylglutaconic aciduria is due to mutations in the OPA3 gene located at 19q13.2-13.3. Here we describe two siblings with novel compound heterozygous variants in OPA3: c.1A>G (p.1M>V) in the translation initiation codon in exon 1 and a second variant, c.142+5G>C in intron 1. On cDNA sequencing the c.1A>G appeared homozygous, indicating that the allele without the c.1A>G variant is degraded. This is likely due to an intronic variant; possibly the IVS1+5 splice site variant. The older female sibling initially presented with motor developmental delay and vertical nystagmus during her first year of life and was diagnosed subsequently with optic atrophy. Her brother presented with mildly increased hip muscle tone followed by vertical nystagmus within the first 6 months of life, and was found to have elevated urinary excretion of 3-methylglutaconic acid and 3-methylglutaric acid, and optic atrophy by 1.5 years of age. Currently, ages 16 and 7, both children exhibit ataxic gaits and dysarthric speech. Immunofluorescence studies on patient's cells showed fragmented mitochondrial morphology. Thus, though the exact function of OPA3 remains unknown, our experimental results and clinical summary provide evidence for the pathogenicity of the identified OPA3 variants and provide further evidence for a mitochondrial pathology in this disease.

Dysregulation of galectin-3. Implications for Hermansky-Pudlak syndrome pulmonary fibrosis.

The etiology of Hermansky-Pudlak syndrome (HPS) pulmonary fibrosis (HPSPF), a progressive interstitial lung disease with high mortality, is unknown. Galectin-3 is a ß-galactoside-binding lectin with profibrotic effects. The objective of this study was to investigate the involvement of galectin-3 in HPSPF. Galectin-3 was measured by ELISA, immunohistochemistry, and immunoblotting in human specimens from subjects with HPS and control subjects. Mechanisms of galectin-3 accumulation were studied by quantitative RT-PCR, Northern blot analysis, membrane biotinylation assays, and rescue of HPS1-deficient cells by transfection. Bronchoalveolar lavage galectin-3 concentrations were significantly higher in HPSPF compared with idiopathic pulmonary fibrosis or that from normal volunteers, and correlated with disease severity. Galectin-3 immunostaining was increased in HPSPF compared with idiopathic pulmonary fibrosis or normal lung tissue. Fibroblasts from subjects with HPS subtypes associated with pulmonary fibrosis had increased galectin-3 protein expression compared with cells from nonfibrotic HPS subtypes. Galectin-3 protein accumulation was associated with reduced Galectin-3 mRNA, normal Mucin 1 levels, and up-regulated microRNA-322 in HPSPF cells. Membrane biotinylation assays showed reduced galectin-3 and normal Mucin 1 expression at the plasma membrane in HPSPF cells compared with control cells, which suggests that galectin-3 is mistrafficked in these cells. Reconstitution of HPS1 cDNA into HPS1-deficient cells normalized galectin-3 protein and mRNA levels, as well as corrected galectin-3 trafficking to the membrane. Intracellular galectin-3 levels are regulated by HPS1 protein. Abnormal accumulation of galectin-3 may contribute to the pathogenesis of HPSPF.

Non-specific accumulation of glycosphingolipids in GNE myopathy.

BACKGROUND: UDP-GlcNAc 2-epimerase/ManNAc 6-kinase (GNE) is a bifunctional enzyme responsible for the first committed steps in the synthesis of sialic acid, a common terminal monosaccharide in both protein and lipid glycosylation. GNE mutations are responsible for a rare autosomal recessive neuromuscular disorder, GNE myopathy (also called hereditary inclusion body myopathy). The connection between the impairment of sialic acid synthesis and muscle pathology in GNE myopathy remains poorly understood. METHODS: Glycosphingolipid (GSL) analysis was performed by HPLC in multiple models of GNE myopathy, including patients' fibroblasts and plasma, control fibroblasts with inhibited GNE epimerase activity through a novel imino sugar, and tissues of Gne(M712T/M712T) knock-in mice. RESULTS: Not only neutral GSLs, but also sialylated GSLs, were significantly increased compared to controls in all tested models of GNE myopathy. Treatment of GNE myopathy fibroblasts with N-acetylmannosamine (ManNAc), a sialic acid precursor downstream of GNE epimerase activity, ameliorated the increased total GSL concentrations. CONCLUSION: GNE myopathy models have increased total GSL concentrations. ManNAc supplementation results in decrease of GSL levels, linking abnormal increase of total GSLs in GNE myopathy to defects in the sialic acid biosynthetic pathway. These data advocate for further exploring GSL concentrations as an informative biomarker, not only for GNE myopathy, but also for other disorders of sialic acid metabolism.

Disorders with similar clinical phenotypes reveal underlying genetic interaction: SATB2 acts as an activator of the UPF3B gene.

Two syndromic cognitive impairment disorders have very similar craniofacial dysmorphisms. One is caused by mutations of SATB2, a transcription regulator and the other by heterozygous mutations leading to premature stop codons in UPF3B, encoding a member of the nonsense-mediated mRNA decay complex. Here we demonstrate that the products of these two causative genes function in the same pathway. We show that the SATB2 nonsense mutation in our patient leads to a truncated protein that localizes to the nucleus, forms a dimer with wild-type SATB2 and interferes with its normal activity. This suggests that the SATB2 nonsense mutation has a dominant negative effect. The patient's leukocytes had significantly decreased UPF3B mRNA compared to controls. This effect was replicated both in vitro, where siRNA knockdown of SATB2 in HEK293 cells resulted in decreased UPF3B expression, and in vivo, where embryonic tissue of Satb2 knockout mice showed significantly decreased Upf3b expression. Furthermore, chromatin immunoprecipitation demonstrates that SATB2 binds to the UPF3B promoter, and a luciferase reporter assay confirmed that SATB2 expression significantly activates gene transcription using the UPF3B promoter. These findings indicate that SATB2 activates UPF3B expression through binding to its promoter. This study emphasizes the value of recognizing disorders with similar clinical phenotypes to explore underlying mechanisms of genetic interaction.

Oral monosaccharide therapies to reverse renal and muscle hyposialylation in a mouse model of GNE myopathy.

GNE myopathy, previously termed hereditary inclusion body myopathy (HIBM), is an adult-onset neuromuscular disorder characterized by progressive muscle weakness. The disorder results from biallelic mutations in GNE, encoding UDP-N-acetylglucosamine 2-epimerase/N-acetylmannosamine kinase, the key enzyme of sialic acid synthesis. GNE myopathy, associated with impaired glycan sialylation, has no approved therapy. Here we test potential sialylation-increasing monosaccharides for their effectiveness in prophylaxis (at the embryonic and neonatal stages) and therapy (after the onset of symptoms) by evaluating renal and muscle hyposialylation in a knock-in mouse model (Gne p.M712T) of GNE myopathy. We demonstrate that oral mannosamine (ManN), but not sialic acid (Neu5Ac), mannose (Man), galactose (Gal), or glucosamine (GlcN), administered to pregnant female mice has a similar prophylactic effect on renal hyposialylation, pathology and neonatal survival of mutant offspring, as previously shown for N-acetylmannosamine (ManNAc) therapy. ManN may be converted to ManNAc by a direct, yet unknown, pathway, or may act through another mode of action. The other sugars (Man, Gal, GlcN) may either not cross the placental barrier (Neu5Ac) and/or may not be able to directly increase sialylation. Because GNE myopathy patients will likely require treatment in adulthood after onset of symptoms, we also administered ManNAc (1 or 2g/kg/day for 12 weeks), Neu5Ac (2 g/kg/day for 12 weeks), or ManN (2 g/kg/day for 6 weeks) in drinking water to 6 month old mutant Gne p.M712T mice. All three therapies markedly improved the muscle and renal hyposialylation, as evidenced by lectin histochemistry for overall sialylation status and immunoblotting of specific sialoproteins. These preclinical data strongly support further evaluation of oral ManNAc, Neu5Ac and ManN as therapy for GNE myopathy and conceivably for certain glomerular diseases with hyposialylation.

Cellular and clinical report of new Griscelli syndrome type III cases.

The RAB27A/Melanophilin/Myosin-5a tripartite protein complex is required for capturing mature melanosomes in the peripheral actin network of melanocytes for subsequent transfer to keratinocytes. Mutations in any one member of this tripartite complex cause three forms of Griscelli syndrome (GS), each with distinct clinical features but with a similar cellular phenotype. To date, only one case of GS type III (GSIII), caused by mutations in the Melanophilin (MLPH) gene, has been reported. Here, we report seven new cases of GSIII in three distinct Arab pedigrees. All affected individuals carried a homozygous missense mutation (c.102C>T; p.R35W), located in the conserved Slp homology domain of MLPH, and had hypomelanosis of the skin and hair. We report the first cellular studies on GSIII melanocytes, which demonstrated that MLPH(R35W) causes perinuclear aggregation of melanosomes in melanocytes, typical for GS. Additionally, co-immunoprecipitation assays showed that MLPH(R35W) lost its interaction with RAB27A, indicating pathogenicity of the R35W mutation.

Exome sequencing identifies ACSF3 as a cause of combined malonic and methylmalonic aciduria.

We used exome sequencing to identify the genetic basis of combined malonic and methylmalonic aciduria (CMAMMA). We sequenced the exome of an individual with CMAMMA and followed up with sequencing of eight additional affected individuals (cases). This included one individual who was identified and diagnosed by searching an exome database. We identify mutations in ACSF3, encoding a putative methylmalonyl-CoA and malonyl-CoA synthetase as a cause of CMAMMA. We also examined a canine model of CMAMMA, which showed pathogenic mutations in a predicted ACSF3 ortholog. ACSF3 mutant alleles occur with a minor allele frequency of 0.0058 in ∼1,000 control individuals, predicting a CMAMMA population incidence of ∼1:30,000. ACSF3 deficiency is the first human disorder identified as caused by mutations in a gene encoding a member of the acyl-CoA synthetase family, a diverse group of evolutionarily conserved proteins, and may emerge as one of the more common human metabolic disorders.

NBEAL2 is mutated in gray platelet syndrome and is required for biogenesis of platelet α-granules.

Gray platelet syndrome (GPS) is an autosomal recessive bleeding disorder that is characterized by large platelets that lack α-granules. Here we show that mutations in NBEAL2 (neurobeachin-like 2), which encodes a BEACH/ARM/WD40 domain protein, cause GPS and that megakaryocytes and platelets from individuals with GPS express a unique combination of NBEAL2 transcripts. Proteomic analysis of sucrose-gradient subcellular fractions of platelets indicated that NBEAL2 localizes to the dense tubular system (endoplasmic reticulum) in platelets.

A BLOC-1 mutation screen reveals that PLDN is mutated in Hermansky-Pudlak Syndrome type 9.

Hermansky-Pudlak Syndrome (HPS) is an autosomal-recessive condition characterized by oculocutaneous albinism and a bleeding diathesis due to absent platelet delta granules. HPS is a genetically heterogeneous disorder of intracellular vesicle biogenesis. We first screened all our patients with HPS-like symptoms for mutations in the genes responsible for HPS-1 through HPS-6 and found no functional mutations in 38 individuals. We then examined all eight genes encoding the biogenesis of lysosome-related organelles complex-1, or BLOC-1, proteins in these individuals. This identified a homozygous nonsense mutation in PLDN in a boy with characteristic features of HPS. PLDN is mutated in the HPS mouse model pallid and encodes the protein pallidin, which interacts with the early endosomal t-SNARE syntaxin-13. We could not detect any full-length pallidin in our patient's cells despite normal mRNA expression of the mutant transcript. We could detect an alternative transcript that would skip the exon that harbored the mutation, but we demonstrate that if this transcript is translated into protein, although it correctly localizes to early endosomes, it does not interact with syntaxin-13. In our patient's melanocytes, the melanogenic protein TYRP1 showed aberrant localization, an increase in plasma-membrane trafficking, and a failure to reach melanosomes, explaining the boy's severe albinism and establishing his diagnosis as HPS-9.

Vascular pathology of medial arterial calcifications in NT5E deficiency: implications for the role of adenosine in pseudoxanthoma elasticum.

Arterial Calcification due to Deficiency of CD73 (ACDC) results from mutations in the NT5E gene encoding the 5' exonucleotidase, CD73. We now describe the third familial case of ACDC, including radiological and histopathological details of the arterial calcifications. The medial lesions involve the entire circumference of the elastic lamina, in contrast to the intimal plaque-like disease of atherosclerosis. The demonstration of broken and fragmented elastic fibers leading to generalized vascular calcification suggests an analogy to pseudoxanthoma elasticum (PXE), which exhibits similar histopathology. Classical PXE is caused by deficiency of ABCC6, a C type ABC transporter whose ligand is unknown. Other C type ABC proteins transport nucleotides, so the newly described role of adenosine in inhibiting vascular calcification, along with the similarity of ACDC and PXE with respect to vascular pathology, suggests that adenosine may be the ligand for ABCC6.

Novel 47.5-kb deletion in RAB27A results in severe Griscelli Syndrome Type 2.

Griscelli syndrome (GS), a rare autosomal recessive disorder characterized by partial albinism and immunological impairment and/or severe neurological impairment, results from mutations in the MYO5A (GS1), RAB27A (GS2), or MLPH (GS3) genes. We identified a Hispanic patient born of a consanguineous union who presented with immunodeficiency, partial albinism, hepatic dysfunction, hemophagocytosis, neurological impairment, nystagmus, and silvery hair indicative of Griscelli Syndrome Type 2 (GS2). We screened for point mutations, but only exons 2-6 of the patient's DNA could be PCR-amplified. Whole genome analysis using the Illumina 1M-Duo DNA Analysis BeadChip identified a homozygous deletion in the patient's DNA. The exact breakpoints of the 47.5-kb deletion were identified as chr15q15-q21.1: g.53332432_53379990del (NCBI Build 37.1); the patient lacks the promoter and 5'UTR regions of RAB27A, thus confirming the diagnosis of GS2.

A model of Costeff Syndrome reveals metabolic and protective functions of mitochondrial OPA3.

Costeff Syndrome, which is caused by mutations in the OPTIC ATROPHY 3 (OPA3) gene, is an early-onset syndrome characterized by urinary excretion of 3-methylglutaconic acid (MGC), optic atrophy and movement disorders, including ataxia and extrapyramidal dysfunction. The OPA3 protein is enriched in the inner mitochondrial membrane and has mitochondrial targeting signals, but a requirement for mitochondrial localization has not been demonstrated. We find zebrafish opa3 mRNA to be expressed in the optic nerve and retinal layers, the counterparts of which in humans have high mitochondrial activity. Transcripts of zebrafish opa3 are also expressed in the embryonic brain, inner ear, heart, liver, intestine and swim bladder. We isolated a zebrafish opa3 null allele for which homozygous mutants display increased MGC levels, optic nerve deficits, ataxia and an extrapyramidal movement disorder. This correspondence of metabolic, ophthalmologic and movement abnormalities between humans and zebrafish demonstrates a phylogenetic conservation of OPA3 function. We also find that delivery of exogenous Opa3 can reduce increased MGC levels in opa3 mutants, and this reduction requires the mitochondrial localization signals of Opa3. By manipulating MGC precursor availability, we infer that elevated MGC in opa3 mutants derives from extra-mitochondrial HMG-CoA through a non-canonical pathway. The opa3 mutants have normal mitochondrial oxidative phosphorylation profiles, but are nonetheless sensitive to inhibitors of the electron transport chain, which supports clinical recommendations that individuals with Costeff Syndrome avoid mitochondria-damaging agents. In summary, this paper introduces a faithful Costeff Syndrome model and demonstrates a requirement for mitochondrial OPA3 to limit HMG-CoA-derived MGC and protect the electron transport chain against inhibitory compounds.