Selenoprotein biosynthesis defect causes progressive encephalopathy with elevated lactate

Abstract

Objective: We aimed to decipher the molecular genetic basis of disease in a cohort of children with a uniform clinical presentation of neonatal irritability, spastic or dystonic quadriplegia, virtually absent psychomotor development, axonal neuropathy, and elevated blood/CSF lactate. Methods:We performed whole-exome sequencing of blood DNA from the index patients. Detected compound heterozygous mutations were confirmed by Sanger sequencing. Structural predictions and a bacterial activity assay were performed to evaluate the functional consequences of the mutations. Mass spectrometry, Western blotting, and protein oxidation detection were used to analyze the effects of selenoprotein deficiency.Results: Neuropathology indicated laminar necrosis and severe loss of myelin, with neuron loss and astrogliosis. In 3 families, we identified a missense (p.Thr325Ser) and a nonsense (p.Tyr429*) mutation in SEPSECS, encoding the O-phosphoseryl-tRNA:selenocysteinyl-tRNA synthase, which was previously associated with progressive cerebellocerebral atrophy. We show that the mutations do not completely abolish the activity of SEPSECS, but lead to decreased selenoprotein levels, with demonstrated increase in oxidative protein damage in the patient brain. Conclusions:These results extend the phenotypes caused by defective selenocysteine biosynthesis, and suggest SEPSECS as a candidate gene for progressive encephalopathies with lactate elevation. Neurology ® 2015;85:306-315 GLOSSARY PCH2D 5 pontocerebellar hypoplasia type 2D; PEHO 5 progressive encephalopathy with edema, hypsarrhythmia, and optic atrophy; RC 5 respiratory chain; SRM-MS 5 selected reaction monitoring-mass spectrometry; T 4 5 thyroxine; tRNA 5 transfer RNA; TSH 5 thyroid-stimulating hormone; T 3 5 triiodothyronine.Mitochondrial dysfunction is a frequent cause of childhood encephalopathy. Besides the typical multisystemic disorders, an increasing number of mitochondrial defects are shown to cause a CNS-specific phenotype.1-5 Lactate elevation raises suspicion of mitochondrial involvement and may be observed even in encephalopathies in which muscle biopsies show normal mitochondrial respiratory chain (RC) function. [1][2][3]6 Within our cohort of pediatric patients, we identified patients with an undefined cause of cerebellocerebral atrophy, seizures, severe spasticity, and axonal neuropathy with lactate elevation. We report that despite many of the clinical and neuropathologic signs pointing toward mitochondrial impairment, the patients had novel mutations in the SEPSECS gene, which functions in cytoplasmic transfer RNA (tRNA)-charging in the selenoprotein biosynthesis pathway. We describe the uniform clinical, neuroradiologic, and neuropathologic features of this entity and a detailed mutation *These authors contributed equally to this work.From the

Mutation in human selenocysteine transfer RNA selectively disrupts selenoprotein synthesis

“…This is achieved via a unique, highly conserved, Sec-insertion machinery comprising trans-acting factors (e.g., SECIS binding protein 2 ) interacting with cis-acting Sec-insertion sequence (SECIS) elements, located in the 3′ UTR of most eukaryotic selenoprotein mRNAs ( Figure 1). Mutations in SECISBP2 cause a multisystem disorder with myopathic features due to selenoprotein N (SEPN1) deficiency, increased ROS attributable to lack of antioxidant selenoenzymes -glutathione peroxidases (GPxs) and thioredoxin reductases (TrxRs) -and thyroid dysfunction secondary to loss of selenoprotein deiodinases (3,4); O-phosphoseryl-tRNA:Sec tRNA synthase (SEPSECS) defects cause progressive cerebellocerebral atrophy, likely reflecting global disruption of selenoprotein synthesis (5,6).…”

Structural basis for early-onset neurological disorders caused by mutations in human selenocysteine synthase

“…It is likely that the Y429* mRNA is targeted for the nonsense-mediated mRNA decay, while any truncated nascent polypeptide is targeted to the proteasome for degradation. A previous report using the E. coli activity assay found the Y429* variant to be completely inactive4.…”

“…This analysis is in good agreement with the results of an indirect activity assay in the E. coli system that is based on the substitution of the bacterial selenocysteine synthase with the mutant human SepSecS enzyme. This activity assay indicated the A239T and Y334C mutations completely ablated enzyme activity, while the T325S mutation modestly diminished SepSecS activity34. However, since tRNA Sec is essential for mammalian development, it is reasonable to suggest that Y334C most likely exhibits marginal activity in vivo to explain the viability of the homozygous Y334C patients13.…”

“…While Ala239 and Thr325 are conserved to a reasonable degree, both Tyr334 and Tyr429 are highly conserved in archaea and eukaryotes34. In addition, the crystal structure of human SepSecS15 provided a platform for studies on structural and functional effects of these point mutations.…”

Neurological disorders, genetic correlations, and the role of exome sequencing

“…Defective selenocysteine biosynthesis was the conclusion in a study whose design was to potentially identify a genetic molecular basis for progressive encephalopathies with CSF or serum lactate elevation [34]. These researchers were successful in identifying new phenotypes for both a missense (p.Thr325Ser) and a nonsense (p.Tyr429*) mutation in their study that of three families, each of whom had a neuropathological presentation of laminar necrosis, severe myelin loss, neuron loss, and astrogliosis.…”