MEGDEL Syndrome in a Child From Palestine: Report of a Novel Mutation in SERAC1 Gene

LPGAT1 controls MEGDEL syndrome by coupling phosphatidylglycerol remodeling with mitochondrial transport

Phosphatidylglycerol (PG) is a mitochondrial phospholipid required for mitochondrial cristae structure and cardiolipin synthesis. PG must be remodeled to its mature form at the endoplasmic reticulum (ER) after mitochondrial biosynthesis to achieve its biological functions. Defective PG remodeling causes MEGDEL (non-alcohol fatty liver disease and 3-methylglutaconic aciduria with deafness, encephalopathy, and Leigh-like) syndrome through poorly defined mechanisms. Here, we identify LPGAT1, an acyltransferase that catalyzes PG remodeling, as a candidate gene for MEGDEL syndrome. We show that PG remodeling by LPGAT1 at the ER is closely coordinated with mitochondrial transport through interaction with the prohibitin/TIMM14 mitochondrial import motor. Accordingly, ablation of LPGAT1 or TIMM14 not only causes aberrant fatty acyl compositions but also ER retention of newly remodeled PG, leading to profound loss in mitochondrial crista structure and respiration. Consequently, genetic deletion of the LPGAT1 in mice leads to cardinal features of MEGDEL syndrome, including 3-methylglutaconic aciduria, deafness, dilated cardiomyopathy, and premature death, which are highly reminiscent of those caused by TIMM14 mutations in humans.

Two cases of MEGDHEL syndrome diagnosed with hyperammonemia

OBJECTIVES

MEGDHEL [3-methylglutaconic aciduria (MEG), deafness (D), hepatopathy (H), encephalopathy (E), and Leigh-like disease (L)] syndrome is an autosomal recessive disorder caused by mutations in the serine active site-containing protein 1 (SERAC1) gene. MEGDHEL syndrome is clinically characterized by sensorineural hearing loss, encephalopathy, hepatopathy, 3-methylglutaconic aciduria, and Leigh-like lesions on cranial magnetic resonance imaging. During the neonatal period, it has been reported to present with hypoglycemia, hyperammonemia, impaired liver functions, cholestasis, metabolic acidosis, and sepsis-like clinical findings. However, clinical findings in the neonatal period were reported as a result of the retrospective evaluation of patients diagnosed at an older age. Herein we reported two cases diagnosed as MEGDHEL syndrome during neonatal period in two different clinics with sepsis-like findings, impaired liver functions, and ammonia levels high enough to require dialysis.

CASE PRESENTATION

One of the cases was born 37 weeks of gestation with a birth weight of 2,060 g and initially presented with respiratory distress and feeding difficulties. The other case admitted to the neonatal intensive care unit had fed problems together with respiratory distress and circulatory failure within the first 24 h after initiation of parenteral nutrition.

CONCLUSIONS

MEGDHEL syndrome should be suspected in patients with sepsis-like clinical features and hyperammonemia.

A combined genome-wide association and molecular study of age-related hearing loss in H. sapiens

BACKGROUND

Sensorineural hearing loss is one of the most common sensory deficiencies. However, the molecular contribution to age-related hearing loss is not fully elucidated.

METHODS

We performed genome-wide association studies (GWAS) for hearing loss-related traits in the UK Biobank (N = 362,396) and selected a high confidence set of ten hearing-associated gene products for staining in human cochlear samples: EYA4, LMX1A, PTK2/FAK, UBE3B, MMP2, SYNJ2, GRM5, TRIOBP, LMO-7, and NOX4.

RESULTS

All proteins were found to be expressed in human cochlear structures. Our findings illustrate cochlear structures that mediate mechano-electric transduction of auditory stimuli, neuronal conductance, and neuronal plasticity to be involved in age-related hearing loss.

CONCLUSIONS

Our results suggest common genetic variation to influence structural resilience to damage as well as cochlear recovery after trauma, which protect against accumulated damage to cochlear structures and the development of hearing loss over time.

Severe neonatal MEGDHEL syndrome with a homozygous truncating mutation in SERAC1

In the diagnostic work-up of a newborn infant with a metabolic crisis, lethal multiorgan failure on day six of life, and increased excretion of 3-methylglutaconic acid, we found using whole genome sequencing a homozygous SERAC1 mutation indicating MEGDHEL syndrome (3-methylglutaconic aciduria with deafness-dystonia, hepatopathy, encephalopathy, and Leigh-like syndrome). The SERAC1 protein is located at the contact site between mitochondria and the endoplasmic reticulum (ER) and is crucial for cholesterol trafficking. Our aim was to investigate the effect of the homozygous truncating mutation on mitochondrial structure and function. In the patient fibroblasts, no SERAC1 protein was detected, the mitochondrial network was severely fragmented, and the cristae morphology was altered. Filipin staining showed uneven localization of unesterified cholesterol. The calcium buffer function between cytoplasm and mitochondria was deficient. In liver mitochondria, complexes I, III, and IV were clearly decreased. In transfected COS-1 cells the mutant protein with the a 45-amino acid C-terminal truncation was distributed throughout the cell, whereas wild-type SERAC1 partially colocalized with the mitochondrial marker MT-CO1. The structural and functional mitochondrial abnormalities, caused by the loss of SERAC1, suggest that the crucial disease mechanism is disrupted interplay between the ER and mitochondria leading to decreased influx of calcium to mitochondria and secondary respiratory chain deficiency.

Incidental Finding of MEGDEL Syndrome Based on Neuroimaging: Case Report

MEGDEL 3-methylglutaconic (MG) aciduria, deafness, encephalopathy, Leigh-like syndrome is an autosomal recessive disorder associated with infantile hypoglycemia, progressive psychomotor developmental delay, cerebellar atrophy with lesions in the basal ganglia, spasticity, dystonia, deafness, and transient liver problems, which typically occur in the first year of life. Other clinical presentations include failure to thrive, epilepsy, and optic nerve atrophy. The serine active site-containing 1 (SERAC1) mutation is localized at the mitochondria-associated membranes, which are responsible for encoding a phosphatidylglycerol remodeler essential for both mitochondrial function and intracellular cholesterol trafficking and is thus responsible for the disease. Diagnosis is confirmed by the elevation of and concentrations of 3-MG acid and 3-methylglutaric acid in the urine or by identification of bi-allelic SERAC1 pathogenic variants on molecular genetic testing. Different pathological variants of SERAC1 have been identified in MEGDEL syndrome to date. Here, we report a case of a child with MEGDEL syndrome due to SERAC1 mutation. The child presented with accidental finding by CT showing hypodensity on bilateral symmetric anterior putamen and caudate abnormal. Neurological examination was unremarkable. This report presents a new neuroimaging finding by CT of MEGDEL syndrome.

Complicated Hereditary Spastic Paraplegia Caused by SERAC1 Variants in a Chinese Family

BACKGROUND

The serine active site-containing protein 1 (SERAC1) biallelic variant usually causes MEGDEL syndrome, clinically characterized by increased excretion of 3-methylglutaconic in the urine, muscle hypotonia, sensorineural deafness, and Leigh-like lesions on brain MRI scans. In this study, we present a case from a Chinese family with disordered metabolism and dystonia owing to SERAC1 variants; the clinical phenotypes of the proband were different from those of MEGDEL syndrome but were similar to those juvenile-onset complicated hereditary spastic paraplegia. Thus, in this study, we aimed to confirm the relationship between SERAC1 variants and complicated hereditary spastic paraplegia.

METHODS

MRI and laboratory tests, including gas chromatography/mass spectrometry (GC/MS), were carried out for the proband. Whole-exome sequencing was used to detect the candidate SERAC1 variants. Variants were verified using Sanger sequencing. Various software programs (PolyPhen-2, MutationTaster, PROVEAN, and SIFT) were used to predict the pathogenicity of novel variants.

RESULTS

Brain MRI scans showed a symmetric flake abnormal signal shadow in the bilateral basal ganglia in T2-weighted image (T2WI) and fluid-attenuated inversion recovery (FLAIR) analyses. The excretion of 3-methylglutaconic acid was found to be increased in our GC/MS analysis. Whole-exome sequencing showed novel compound heterozygous variants, including a novel c.1495A>G (p.Met499Val) variant in exon 14 of SERAC1 inherited from the father and a novel c.721_722delAG (p.Leu242fs) variant in exon 8 inherited from the mother. The pathogenicity prediction results showed that these two variants were deleterious.

CONCLUSIONS

This study presented a patient with complicated hereditary spastic paraplegia caused by SERAC1 variants. These findings expand the number of known SERAC1 variants and the phenotypic spectrum associated with SERAC1 deficiency. This study may contribute to counseling and prevention of hereditary diseases through prenatal.

MEGDEL Syndrome

MEGDEL syndrome is an autosomal recessive disorder, clinically characterized by 3-methylglutaconic aciduria, psychomotor delay, muscle hypotonia, sensorineural deafness, and Leigh-like lesions on brain magnetic resonance imaging. MEGDEL syndrome is due to mutations in the serine active site-containing protein 1 (SERAC1) gene. The SERAC1 protein is localized at the interface between the mitochondria and the endoplasmic reticulum in the mitochondrion-associated membrane fraction, which is essential for phospholipid exchange. SERAC1 was identified as a key player in phosphatidylglycerol remodeling, which is essential for both mitochondrial function and intracellular cholesterol trafficking. Since the first description of MEGDEL syndrome in 2006, at least 102 patients have been reported. The phenotypic spectrum of MEGDEL syndrome is much broader than so far anticipated. In addition to the brain, ears, and gastrointestinal tract, the eyes, endocrine organs, heart, peripheral nerves, and the skeletal muscle may be affected. Diagnosing MEGDEL syndrome requires a multidisciplinary approach, including genetic confirmation of a SERAC1 mutation. Treatment is supportive, and the outcome is usually poor with early death, except for the juvenile-onset type.

Progressive deafness-dystonia due to SERAC1 mutations: A study of 67 cases

OBJECTIVE

3-Methylglutaconic aciduria, dystonia-deafness, hepatopathy, encephalopathy, Leigh-like syndrome (MEGDHEL) syndrome is caused by biallelic variants in SERAC1.

METHODS

This multicenter study addressed the course of disease for each organ system. Metabolic, neuroradiological, and genetic findings are reported.

RESULTS

Sixty-seven individuals (39 previously unreported) from 59 families were included (age range = 5 days-33.4 years, median age = 9 years). A total of 41 different SERAC1 variants were identified, including 20 that have not been reported before. With the exception of 2 families with a milder phenotype, all affected individuals showed a strikingly homogeneous phenotype and time course. Severe, reversible neonatal liver dysfunction and hypoglycemia were seen in >40% of all cases. Starting at a median age of 6 months, muscular hypotonia (91%) was seen, followed by progressive spasticity (82%, median onset = 15 months) and dystonia (82%, 18 months). The majority of affected individuals never learned to walk (68%). Seventy-nine percent suffered hearing loss, 58% never learned to speak, and nearly all had significant intellectual disability (88%). Magnetic resonance imaging features were accordingly homogenous, with bilateral basal ganglia involvement (98%); the characteristic "putaminal eye" was seen in 53%. The urinary marker 3-methylglutaconic aciduria was present in virtually all patients (98%). Supportive treatment focused on spasticity and drooling, and was effective in the individuals treated; hearing aids or cochlear implants did not improve communication skills.

INTERPRETATION

MEGDHEL syndrome is a progressive deafness-dystonia syndrome with frequent and reversible neonatal liver involvement and a strikingly homogenous course of disease. Ann Neurol 2017;82:1004-1015.

First missense mutation outside of SERAC1 lipase domain affecting intracellular cholesterol trafficking

We report the clinical and genetic findings in a Spanish boy who presented MEGDEL syndrome, a very rare inborn error of metabolism. Whole-exome sequencing uncovered a new homozygous mutation in the serine active site containing 1 (SERAC1) gene, which is essential for both mitochondrial function and intracellular cholesterol trafficking. Functional studies in patient fibroblasts showed that p.D224G mutation affects the intracellular cholesterol trafficking. Only three missense mutations in this gene have been described before, being p.D224G the first missense mutation outside of the SERAC1 serine-lipase domain. Therefore, we conclude that the defect in cholesterol trafficking is not limited to alterations in this specific part of the protein.

Disorders of phospholipid metabolism: an emerging class of mitochondrial disease due to defects in nuclear genes

The human nuclear and mitochondrial genomes co-exist within each cell. While the mitochondrial genome encodes for a limited number of proteins, transfer RNAs, and ribosomal RNAs, the vast majority of mitochondrial proteins are encoded in the nuclear genome. Of the multitude of mitochondrial disorders known to date, only a fifth are maternally inherited. The recent characterization of the mitochondrial proteome therefore serves as an important step toward delineating the nosology of a large spectrum of phenotypically heterogeneous diseases. Following the identification of the first nuclear gene defect to underlie a mitochondrial disorder, a plenitude of genetic variants that provoke mitochondrial pathophysiology have been molecularly elucidated and classified into six categories that impact: (1) oxidative phosphorylation (subunits and assembly factors); (2) mitochondrial DNA maintenance and expression; (3) mitochondrial protein import and assembly; (4) mitochondrial quality control (chaperones and proteases); (5) iron–sulfur cluster homeostasis; and (6) mitochondrial dynamics (fission and fusion). Here, we propose that an additional class of genetic variant be included in the classification schema to acknowledge the role of genetic defects in phospholipid biosynthesis, remodeling, and metabolism in mitochondrial pathophysiology. This seventh class includes a small but notable group of nuclear-encoded proteins whose dysfunction impacts normal mitochondrial phospholipid metabolism. The resulting human disorders present with a diverse array of pathologic consequences that reflect the variety of functions that phospholipids have in mitochondria and highlight the important role of proper membrane homeostasis in mitochondrial biology.