3-Methylglutaconic Aciduria With Deafness, Encephalopathy, And Leigh-Like Syndrome

A number sign (#) is used with this entry because 3-methylglutaconic aciduria with deafness, encephalopathy, and Leigh-like syndrome (MEGDEL), also referred to as 3-methylglutaconic aciduria type VI (MGCA6), is caused by homozygous or compound heterozygous mutation in the SERAC1 gene (614725) on chromosome 6q25.

Description

MEGDEL is an autosomal recessive disorder characterized by childhood onset of delayed psychomotor development or psychomotor regression, sensorineural deafness, spasticity or dystonia, and increased excretion of 3-methylglutaconic acid. Brain imaging shows cerebral and cerebellar atrophy as well as lesions in the basal ganglia reminiscent of Leigh syndrome (256000). Laboratory studies show increased serum lactate and alanine, mitochondrial oxidative phosphorylation defects, abnormal mitochondria, abnormal phosphatidylglycerol and cardiolipin profiles in fibroblasts, and abnormal accumulation of unesterified cholesterol within cells (summary by Wortmann et al., 2012). About 50% of patients develop severe, but transient, liver dysfunction and/or signs of liver failure, in the neonatal period or during the first year of life, prompting some authors to suggest the name 'MEGDHEL' syndrome, with the 'H' referring to 'hepatopathy' (summary by Maas et al., 2017). Some patients may have a milder presentation with juvenile-onset spasticity and mild cognitive impairment, indicating a broader phenotypic spectrum (Roeben et al., 2018).

For a general phenotypic description and a discussion of genetic heterogeneity of 3-methylglutaconic aciduria, see MGCA type I (250950).

Clinical Features

Wortmann et al. (2006, 2009) reported 4 unrelated girls with an encephalomyopathy associated with mildly and intermittently increased urinary 3-methylglutaconic aciduria. Three children were born of 3 unrelated sets of consanguineous Turkish parents, and the fourth child was born of unrelated Dutch parents. All presented in the neonatal period with severe infections and had episodes of lactic acidosis and hypoglycemia. Other features included failure to thrive, loss of motor skills, mental retardation, sensorineural deafness, and hypotonia with progressive spasticity. One patient had delayed development, 1 did not develop at all, and the other 2 showed developmental regression during early childhood. Brain MRI showed cerebral and cerebellar atrophy as well as bilateral hyperintensities in the basal ganglia reminiscent of Leigh syndrome. Mitochondrial complex I deficiency was found in muscle and fibroblasts. Two patients died at age 3 and 16 years, respectively. Genetic analysis did not find mutations in several candidate genes.

Wortmann et al. (2012) reported 11 additional patients with MEGDEL, including 2 relatives of 1 of the Turkish patients reported by Wortmann et al. (2006). The phenotype was relatively homogeneous, with psychomotor retardation, spasticity or dystonia, sensorineural deafness, Leigh syndrome-like lesions on brain imaging, and brain atrophy. Laboratory studies showed 3-methylglutaconic aciduria, increased serum lactate, and defects in oxidative phosphorylation. Patient fibroblasts showed an increased phosphatidylglycerol 34:1 to phosphatidylglycerol 36:1 ratio as well as abnormal cardiolipin subspecies. There was abnormal intracellular accumulation of unesterified cholesterol, and some patients had low serum cholesterol.

Sarig et al. (2013) reported 4 males, 2 each from 2 unrelated, highly consanguineous families, with features consistent with MEGDEL syndrome, including 3-methylglutaconic aciduria, sensorineural deafness, encephalopathy, and brain magnetic resonance imaging with findings consistent with Leigh-like syndrome. All 4 patients presented at age 24 to 48 hours with hypotonia and evidence of liver dysfunction, including symptomatic hypoglycemia, lactic acidosis, elevated serum transaminase levels, coagulopathy, hyperammonemia, and markedly elevated serum alpha-fetoprotein. During periods of acute infection in the first year of life, the patients experienced a few episodes of liver dysfunction, but these episodes did not recur later in life. Electron microscopy of a liver biopsy from 1 patient showed hepatocyte steatosis with mitochondrial ultrastructural changes, consistent with a mitochondrial oxidative phosphorylation disorder and abnormal lipid metabolism. The patients subsequently developed features of MEGDEL syndrome. The families were found to have 2 homozygous mutations in the SERAC1 gene that led to decreased or absent gene expression. Sarig et al. (2013) suggested that neonatal liver disease is a component of the phenotypic spectrum of MEGDEL syndrome and proposed that the disease be renamed MEGDHEL syndrome.

Tort et al. (2013) reported a girl, born of unrelated parents, with MEGDEL. She presented on the fourth day of life with respiratory distress, refusal to feed, and jaundice, and was found to have metabolic acidosis with ketonuria, hyperammonemia, and elevated liver enzymes. During the first few years of life, she developed oral dyskinesia, hypotonia, truncal ataxia, episodic ketotic hypoglycemia, and psychomotor deterioration. Brain imaging showed features typical of Leigh syndrome. Urinary analysis showed 3-methylglutaconic and 3-methylglutaric aciduria. Sensorineural deafness and optic atrophy became apparent in her teenage years. At age 19 years, she was severely affected, with microcephaly, poor communication skills, inability to hold up her head, and multiple joint problems.

Maas et al. (2017) reviewed the clinical features of 67 patients with MEGDHEL, including 39 previously unreported individuals. Most (64%) of the families were consanguineous. Most of the individuals were of European ancestry, although several dozen patients were from Africa, Asia, the Middle East, or Australia, indicating that MEGDHEL is a panethnic disorder. With the exception of 2 families with a milder phenotype and onset later in childhood, all affected individuals showed a strikingly homogeneous phenotype and time course. Sixteen patients died at a median age of 9 years, mostly due to respiratory infections. In the neonatal period, 48% of patients showed liver dysfunction and hypoglycemia, with variable abnormal liver enzymes, increased bilirubin, and disturbed coagulation in the more severe cases. Signs of hepatic dysfunction during the first year of life were common in this group, but resolved afterwards. Other presenting symptoms of the disorder included neonatal sepsis, hypotonia, and delayed motor development. The majority of patients lost previously acquired skills in the first year of life. Patients had early-onset progressive spasticity, dystonia, and oropharyngeal dyskinesia, and most (78%) never learned to walk. All had intellectual disability that varied from mild (12%) to severe (73%), and nearly all (93%) were completely dependent for activities of daily living. Additional features included epilepsy (35%), sensorineural hearing impairment (79%), absent speech (58%), impaired vision (42%), retinal pigmentary changes (6%), optic atrophy (25%), recurrent respiratory infections (42%), scoliosis (39%), and poor feeding (79%), often necessitating tube feeding. Less common features included transient renal tubular dysfunction (12%) and cardiac abnormalities (7%). Laboratory studies showed increased urinary 3-MGA and increased serum lactate, and brain imaging showed lesions in the basal ganglia, consistent with Leigh syndrome. Maas et al. (2017) concluded that MEGDHEL is best classified as a disorder of the biosynthesis of complex lipids with secondary mitochondrial dysfunction, although MEGDHEL syndrome displays typical findings and the progressive course of a mitochondrial disorder. However, mitochondrial dysfunction in tissue varies greatly and may be unremarkable.

Clinical Variability

Roeben et al. (2018) reported a large consanguineous Iraqi kindred in which 6 individuals from 2 different family branches had a relatively mild form of MEGDEL, which was characterized by the authors as 'juvenile-onset complicated hereditary spastic paraplegia' (SPG). The patients ranged in age from 10 to 27 years. All patients were noted to have mild cognitive delay with learning disability and reduced verbal fluency between 2 and 7 years of age. In family branch I, 3 sibs had slowly progressive lower limb spasticity starting in adolescence, but retained the ability to walk. These 3 patients also had febrile seizures. The youngest sib, examined at age 10, did not show spasticity, seizures, or any neurologic signs besides mild cognitive impairment. In family branch II, both affected sibs presented with juvenile-onset progressive spasticity, which progressed more quickly to tetraspasticity. These individuals also had additional abnormalities, including dystonia, progressive speech reduction, and dysphagia. One 33-year-old sib had a sensorimotor mixed axonal and demyelinating peripheral neuropathy. Brain imaging of 5 individuals showed T2-hyperintensities in the basal ganglia, consistent with Leigh-like syndrome, and urine analysis of 2 patients showed a 10-fold increase of 3-MGA. None of the patients had optic atrophy, deafness, or a history of liver failure; history from the neonatal period was not available. Whole-exome sequencing identified a homozygous intronic variant in the SERAC1 gene (614725.0007). The mutation was confirmed by Sanger sequencing and segregated with the disorder in the family. Patient cells showed aberrant splicing and absence of the full-length SERAC1 protein, as well as impaired prostaglandin remodeling activity compared to wildtype. However, the prostaglandin imbalances were milder than those observed in patients with the classic infantile-onset disease, suggesting a basis for the milder phenotype in the Iraqi family. The findings expanded the phenotype resulting from SERAC1 mutations, showing a clinical spectrum of severity.

Molecular Genetics

In 15 individuals from 13 families with 3-methylglutaconic aciduria with deafness, encephalopathy, and Leigh-like syndrome, Wortmann et al. (2012) identified 14 different homozygous or compound heterozygous mutations in the SERAC1 gene (see, e.g., 614725.0001-614725.0005). The first 2 mutations were identified by exome sequencing and confirmed by Sanger sequencing. Four of the patients had previously been reported by Wortmann et al. (2006).

In a patient with MEGDEL, Tort et al. (2013) identified a homozygous truncating mutation in the SERAC1 gene (R68X; 614725.0006). The mutation was found by exome sequencing and segregated with the disorder in the family.

Among 67 patients with MEGDHEL, including 39 previously unreported individuals, Maas et al. (2017) identified 41 SERAC1 sequence variants, including 20 novel variants. The mutations were located throughout the gene, with no hotspots, although there were several recurrent mutations, suggesting founder effects in certain populations. Most of the mutations were predicted to result in a loss of function, suggesting that missense mutations may be better tolerated.

Population Genetics

Maas et al. (2017) estimated that approximately 27 children with MEGDHEL will be born each year worldwide.