Novel RARS2 Variants: Updating the Diagnosis and Pathogenesis of Pontocerebellar Hypoplasia Type 6

• Published in: Pediatric neurology 2022-06, Vol.131, p.30-41
• Main Authors: Zhang, Yi, Yu, Yafen, Zhao, Xiangyue, Xu, Yufei, Chen, Lina, Li, Niu, Yao, Ruen, Wang, Jian, Yu, Tingting
• Format: Article
• Language: English
• Subjects: Apoptosis; Arginine-tRNA Ligase - genetics; Facial feature; HEK293 Cells; Humans; Mitochondria; Mutation - genetics; Olivopontocerebellar Atrophies - diagnosis; Olivopontocerebellar Atrophies - pathology; Pontocerebellar hypoplasia type 6; RARS2; Reactive Oxygen Species
• ISSN: 0887-8994; 1873-5150
• DOI: 10.1016/j.pediatrneurol.2022.04.002

Pontocerebellar hypoplasia type 6 (PCH6) is an early-onset encephalopathy with/without mitochondrial respiratory complex defects caused by recessive mutations in mitochondrial arginyl-tRNA synthetase (RARS2). Highly heterogeneous clinical phenotypes and numerous missense variations of uncertain significance make diagnosis difficult. Pathogenesis of PCH6 remains unclear. Facial characteristics of patients were assessed. Genetic tests were performed. Structure prediction was based on the template from AlphaFold Protein Structure Database. Expression of mutant RARS2 was tested in HEK293T cells. Patient-derived induced pluripotent stem cells (iPSCs) were detected for human mitochondrial tRNAArg (hmtRNAArg) steady-state level, mitochondrial respiratory complex (MRC) activity, oxygen consumption rate (OCR), extracellular acidification rate (ECAR), mitochondrial membrane potential (MMP), reactive oxygen species (ROS) abundance, and apoptosis level. The three pedigrees were diagnosed as PCH6 caused by compound heterozygous RARS2 variations. Five RARS2 variants were identified: c.3G>C(p.M1?), c.685C>T(p.R229∗), c.1060T>A(p.F354I), c.1210A>G(p.M404V), and c.1369G>A(p.G457R). RARS2 c.3G>C disrupted protein expression. RARS2 c.685C>T created a truncated protein lacking complete catalytic core and anticodon-binding domain. RARS2 c.1060T>A and c.1369G>A were predicted to cause structural abnormality. The hmtRNAArg steady-state abundance in a patient's iPSCs was unaffected. Mitochondrial energy metabolism was normal, including MRC activity, OCR, ECAR, and MMP, while mitochondria-related cellular characteristics, including ROS (P A are novel. Summarized facial features of PCH6 patients will facilitate diagnosis. Defective mitochondrial energy metabolism may not be key points, but mitochondria-related abnormal cellular physiology, including apoptosis, may be an underlying pathogenesis.