Mutations in DARS cause hypomyelination with brain stem and spinal cord involvement and leg spasticity

Abstract

Inherited white-matter disorders are a broad class of diseases for which treatment and classification are both challenging. Indeed, nearly half of the children presenting with a leukoencephalopathy remain without a specific diagnosis. Here, we report on the application of high-throughput genome and exome sequencing to a cohort of ten individuals with a leukoencephalopathy of unknown etiology and clinically characterized by hypomyelination with brain stem and spinal cord involvement and leg spasticity (HBSL), as well as the identification of compound-heterozygous and homozygous mutations in cytoplasmic aspartyl-tRNA synthetase (DARS). These mutations cause nonsynonymous changes to seven highly conserved amino acids, five of which are unchanged between yeast and man, in the DARS C-terminal lobe adjacent to, or within, the active-site pocket. Intriguingly, HBSL bears a striking resemblance to leukoencephalopathy with brain stem and spinal cord involvement and elevated lactate (LBSL), which is caused by mutations in the mitochondria-specific DARS2, suggesting that these two diseases might share a common underlying molecular pathology. These findings add to the growing body of evidence that mutations in tRNA synthetases can cause a broad range of neurologic disorders.

Figure 1

MRI Patterns Characteristic of HBSL MRI images of subjects 1, 2 and 4: (A), (B), and (D)–(L) show axial T2-weighted images, and (C) shows a sagittal T2-weighted image. In (A), (G), and (J), note the diffuse T2 hyperintense signal of almost the entire supratentorial white matter. In subject 1, only the anterior section of the posterior limb of the internal capsule and a narrow band of the subcortical white matter in the occipital lobe are spared. In all subjects, the superior (D, H, and K; red arrows) and inferior (E, I, and L; red arrows) cerebellar peduncles and the anterior brainstem (D, H, and K; blue arrows) are affected. In the spinal cord, the dorsal columns (C and F; blue arrows) and the lateral corticospinal tracts (F, red arrow) are hyperintense. We note that in several individuals, the white-matter signal on the T2-weighted images was more elevated than usual for hypomyelinating leukoencephalopathies, similar to what has recently been described for another disease, hypomyelination with congenital cataracts (MIM 610532).

Figure 2

DARS Mutations in Individuals with HBSL (A) Genomic organization of DARS in humans (UCSC Genome Browser hg19). (B) Mutations and their positions within the DARS cDNA. Note that all identified mutations fall in the 3′ end of DARS, which encodes the active site of the AspRS. No mutations were detected within the hinge domain or the anticodon-binding domain. (C) Five DARS mutations affect amino acids that are conserved in S. cerevisiae (Table 1), and the orientation of these amino acids within the protein can therefore be visualized with the crystal structure of the yeast DARS homolog, DSP1 (Protein Data Bank ID 1ASZ), in complex with tRNA-Asp. In this image, p.Ala274 (yeast p.Ala327) is shown in red, p.Asp367 (yeast p.Asp421) is shown in green, p.Pro464 (yeast p.Pro521) is shown in cyan, p.Arg487 (yeast p.Arg543) is shown in orange, and p.Arg494 (p.Arg554) is shown in magenta. Note that four of the affected amino acids sit within regions that stabilize the tRNA (orange ribbon) in complex with DSP1. Two alterations, p.Ala274Val and p.Asp367Tyr, sit in the active-site pocket and resulted from a compound-heterozygous mutation in a single subject (Table 1). An animation of this structure can be found online (DARS animation in Web Resources). Visualization of the predicted human DARS structure and the HBSL mutations can be found in Figure S1.