RARS1-related hypomyelinating leukodystrophy: Expanding the spectrum

Abstract

Objective: Biallelic variants in RARS1, encoding the cytoplasmic tRNA synthetase for arginine (ArgRS), cause a hypomyelinating leukodystrophy. This study aimed to investigate clinical, neuroradiological and genetic features of patients with RARS1-related disease, and to identify possible genotype-phenotype relationships.

Methods: We performed a multinational cross-sectional survey among 20 patients with biallelic RARS1 variants identified by next-generation sequencing techniques. Clinical data, brain MRI findings and genetic results were analyzed. Additionally, ArgRS activity was measured in fibroblasts of four patients, and translation of long and short ArgRS isoforms was quantified by western blot.

Results: Clinical presentation ranged from severe (onset in the first 3 months, usually with refractory epilepsy and early brain atrophy), to intermediate (onset in the first year with nystagmus and spasticity), and mild (onset around or after 12 months with minimal cognitive impairment and preserved independent walking). The most frequent RARS1 variant, c.5A>G, led to mild or intermediate phenotypes, whereas truncating variants and variants affecting amino acids close to the ArgRS active centre led to severe phenotypes. ArgRS activity was significantly reduced in three patients with intermediate and severe phenotypes; in a fourth patient with intermediate to severe presentation, we measured normal ArgRS activity, but found translation mainly of the short instead of the long ArgRS isoform.

Interpretation: Variants in RARS1 impair ArgRS activity and do not only lead to a classic hypomyelination presentation with nystagmus and spasticity, but to a wide spectrum, ranging from severe, early-onset epileptic encephalopathy with brain atrophy to mild disease with relatively preserved myelination.

Figure 1

Demonstrating the spectrum of MRI findings in selected patients with RARS1. Axial T2‐weighted and sagittal T1‐weighted (P19, P5, P4, P1, P20) and T2‐weighted images (P17, P14), from the most to the least severely affected patient. The severely affected patients have early‐onset cerebral atrophy and, in patient 5, also cerebellar atrophy, in addition to abnormal T2 hyperintense signal of the cerebral white matter, indicating myelin deficit. P19, the most severely affected patient, also has a simplified gyral pattern and a round, small cerebellum.

Figure 2

Additional findings beyond hypomyelination in RARS1 variants. (A and B) T2‐weighted axial images of patient 1 in the neonatal period, with simplified gyral pattern and thick posterior cortex. (C and D) Patient 15, at age 6 months, shows hyperintense T2‐signal of the ventrolateral thalamus (arrow).

Figure 3

ArgRS activity, isoform expression and RARS1 mutations (A) Activities of ArgRS, ValRS and GlyRS in fibroblasts of patients and controls, indicating significantly reduced ArgRS activity in patients 5, 1, and 2 (patient order according to severity with P5 most severely affected, P1 and P2 intermediately affected). (B) depicts a Western blot of ArgRS, with mainly a short transcript present in P4. (C) Is a model of ArgRS, GlyRS and AIMP1, with variants leading to a mild presentation indicated in green, variants associated with a moderate presentation in blue and variants causing a severe presentation in red. (D) Distribution of the variants (with the same color code as in (C) throughout the RARS1 gene. ArgRS, arginyl‐tRNA synthetase; GlyRS, Glycyl‐tRNA synthetase; ValRS, Valyl‐tRNA synthetase; AIMP1, aaRS complex‐interacting multifunctional protein 1.