Mutations Disrupting Selenocysteine Formation Cause Progressive Cerebello-Cerebral Atrophy

Abstract

The essential micronutrient selenium is found in proteins as selenocysteine (Sec), the only genetically encoded amino acid whose biosynthesis occurs on its cognate tRNA in humans. In the final step of selenocysteine formation, the essential enzyme SepSecS catalyzes the conversion of Sep-tRNA to Sec-tRNA. We demonstrate that SepSecS mutations cause autosomal-recessive progressive cerebellocerebral atrophy (PCCA) in Jews of Iraqi and Moroccan ancestry. Both founder mutations, common in these two populations, disrupt the sole route to the biosynthesis of the 21st amino acid, Sec, and thus to the generation of selenoproteins in humans.

Selenoprotein biosynthesis defect causes progressive encephalopathy with elevated lactate

“…SEPSECS mutations were previously described in a single report, underlying progressive cerebellocerebral atrophy with profound mental retardation, progressive microcephaly, severe spasticity, and myoclonic or generalized tonic-clonic seizures, later classified as pontocerebellar hypoplasia type 2D (PCH2D) (MIM 613811). 12,14 The MRI findings of progressive cerebellar atrophy, followed by cerebral atrophy involving both white and gray matter, mimicked the findings in our patients. In contrast to a normal metabolic profile of patients with PCH2D, 12 our patients had lactacidemia, and they also presented with axonal neuropathy.…”

“…12,14 The MRI findings of progressive cerebellar atrophy, followed by cerebral atrophy involving both white and gray matter, mimicked the findings in our patients. In contrast to a normal metabolic profile of patients with PCH2D, 12 our patients had lactacidemia, and they also presented with axonal neuropathy. Similarities between the patients with PCH2D and the patients described here support the disease-causing role of the identified mutations, thus extending the phenotypes caused by defective selenocysteine biosynthesis.…”

“…The previously reported SEPSECS mutations of patients with PCH2D displayed no detectable activity in an anaerobic in vivo assay for SEPSECS activity. 12 Thus, the differences in the phenotypes of our patients and those reported previously may be caused by differences in the residual SEPSECS activity. The fact that the fibroblasts, myoblasts, and myotubes of our patients did not display decreased selenoprotein levels (not shown) suggests that the residual SEPSECS activity was sufficient to maintain selenoprotein synthesis in these cell types but not in the brain.…”

“…To functionally confirm the mutation effects, we showed that both mutations severely affected SEPSECS activity in vivo in an anaerobic Escherichia coli assay. 12 For the assay, we utilized the DselA strain and inspected the ability of human SEPSECS to restore the benzyl viologen-reducing activity of an E coli selenoprotein, the formate dehydrogenase H. As predicted, p.Tyr429* mutant was completely inactive in this assay, while the p.Thr325Ser mutant was active albeit at the decreased level ( figure 3F). …”

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).…”