Objective: Hypomyelination with brain stem and spinal cord involvement and leg spasticity (HBSL) is a rare form of leukodystrophy presenting with varying clinical and imaging features. We report a case of HBSL to investigate the clinical and radiological characteristics of HBSL resulting from cytoplasmic aspartyl-tRNA synthetase gene (DARS) mutations. Subjects: We report a patient of HBSL with compound heterozygous mutations in DARS1. To study the potential genetic variations of the patient, targeted next-generation sequencing, whole-exome sequencing, and Sanger sequencing were used. We reviewed the clinical and radiological features of the patient. The literature was thoroughly evaluated. Results: The patient suffered from developmental regression associated with lower limbs spasticity, developmental delay, and paralysis of the lower limbs since childhood. Decreased T1 and increased T2 signals were observed on the bilateral basal, centrum ovale, frontal lobe, parietal lobe, and ganglia in cervical cord magnetic resonance imaging (MRI). The patient had two compound heterozygous mutations (NM_001349:c.1363T > C and NM_001349:c.821C > G) in the DARS1 gene. Conclusion: Two mutations in DARS1 were found to be associated with HBSL, one of them being reported for the first time. These findings can be valuable for diagnosing and providing genetic counseling to HBSL patients in the future.

Hypomyelination with brain stem and spinal cord involvement and leg spasticity (HBSL) is a leukodystrophy caused by missense mutations of the aspartyl-tRNA synthetase-encoding gene DARS1. The clinical picture includes the regression of acquired motor milestones, spasticity, ataxia, seizures, nystagmus, and intellectual disabilities. Morphologically, HBSL is characterized by a distinct pattern of hypomyelination in the central nervous system including the anterior brainstem, the cerebellar peduncles and the supratentorial white matter as well as the dorsal columns and the lateral corticospinal tracts of the spinal cord. Adequate HBSL animal models are lacking. Dars1 knockout mice are embryonic lethal precluding examination of the etiology. To address this, we introduced the HBSL-causing Dars1 D367Y point mutation into the mouse genome. Surprisingly, mice carrying this mutation homozygously were phenotypically normal. As hypomorphic mutations are more severe in trans to a deletion, we crossed Dars1 D367Y/D367Y mice with Dars1-null carriers. The resulting Dars1 D367Y/- offspring displayed a strong developmental delay compared to control Dars1 D367Y/+ littermates, starting during embryogenesis. Only a small fraction of Dars1 D367Y/- mice were born, and half of these mice died with hydrocephalus during the first 3 weeks of life. Of the few Dars1 D367Y/- mice that were born at term, 25% displayed microphthalmia. Throughout postnatal life, Dars1 D367Y/- mice remained smaller and lighter than their Dars1 D367Y/+ littermates. Despite this early developmental deficit, once they made it through early adolescence Dars1 D367Y/- mice were phenotypically inconspicuous for most of their adult life, until they developed late onset motor deficits as well as vacuolization and demyelination of the spinal cord white matter. Expression levels of the major myelin proteins were reduced in Dars1 D367Y/- mice compared to controls. Taken together, Dars1 D367Y/- mice model aspects of the clinical picture of the corresponding missense mutation in HBSL. This model will enable studies of late onset deficits, which is precluded in Dars1 knockout mice, and can be leveraged to test potential HBSL therapeutics including DARS1 gene replacement therapy.

Introduction: Leukodystrophies constitute heterogenous group of rare heritable disorders primarily affecting the white matter of central nervous system. These conditions are often under-appreciated among physicians. The first clinical manifestations of leukodystrophies are often nonspecific and can occur in different ages from neonatal to late adulthood periods. The diagnosis is, therefore, challenging in most cases.Area covered: Herein, the authors discuss different aspects of leukodystrophies. The authors used MEDLINE, EMBASE, and GOOGLE SCHOLAR to provide an extensive update about epidemiology, classifications, pathology, clinical findings, diagnostic tools, and treatments of leukodystrophies. Comprehensive evaluation of clinical findings, brain magnetic resonance imaging, and genetic studies play the key roles in the early diagnosis of individuals with leukodystrophies. No cure is available for most heritable white matter disorders but symptomatic treatments can significantly decrease the burden of events. New genetic methods and stem cell transplantation are also under investigation to further increase the quality and duration of life in affected population.Expert opinion: The improvements in molecular diagnostic tools allow us to identify the meticulous underlying etiology of leukodystrophies and result in higher diagnostic rates, new classifications of leukodystrophies based on genetic information, and replacement of symptomatic managements with more specific targeted therapies.Abbreviations: 4H: Hypomyelination, hypogonadotropic hypogonadism and hypodontia; AAV: Adeno-associated virus; AD: autosomal dominant; AGS: Aicardi-Goutieres syndrome; ALSP: Axonal spheroids and pigmented glia; APGBD: Adult polyglucosan body disease; AR: autosomal recessive; ASO: Antisense oligonucleotide therapy; AxD: Alexander disease; BAEP: Brainstem auditory evoked potentials; CAA: Cerebral amyloid angiopathy; CADASIL: Cerebral autosomal dominant arteriopathy with subcortical infarcts and leukoencephalopathy; CARASAL: Cathepsin A-related arteriopathy with strokes and leukoencephalopathy; CARASIL: Cerebral autosomal recessive arteriopathy with subcortical infarcts and leukoencephalopathy; CGH: Comparative genomic hybridization; ClC2: Chloride Ion Channel 2; CMTX: Charcot-Marie-Tooth disease, X-linked; CMV: Cytomegalovirus; CNS: central nervous system; CRISP/Cas9: Clustered regularly interspaced short palindromic repeat/CRISPR-associated 9; gRNA: Guide RNA; CTX: Cerebrotendinous xanthomatosis; DNA: Deoxyribonucleic acid; DSB: Double strand breaks; DTI: Diffusion tensor imaging; FLAIR: Fluid attenuated inversion recovery; GAN: Giant axonal neuropathy; H-ABC: Hypomyelination with atrophy of basal ganglia and cerebellum; HBSL: Hypomyelination with brainstem and spinal cord involvement and leg spasticity; HCC: Hypomyelination with congenital cataracts; HEMS: Hypomyelination of early myelinated structures; HMG CoA: Hydroxy methylglutaryl CoA; HSCT: Hematopoietic stem cell transplant; iPSC: Induced pluripotent stem cells; KSS: Kearns-Sayre syndrome; L-2-HGA: L-2-hydroxy glutaric aciduria; LBSL: Leukoencephalopathy with brainstem and spinal cord involvement and elevated lactate; LCC: Leukoencephalopathy with calcifications and cysts; LTBL: Leukoencephalopathy with thalamus and brainstem involvement and high lactate; MELAS: Mitochondrial myopathy, encephalopathy, lactic acidosis, and stroke; MERRF: Myoclonic epilepsy with ragged red fibers; MLC: Megalencephalic leukoencephalopathy with subcortical cysts; MLD: metachromatic leukodystrophy; MRI: magnetic resonance imaging; NCL: Neuronal ceroid lipofuscinosis; NGS: Next generation sequencing; ODDD: Oculodentodigital dysplasia; PCWH: Peripheral demyelinating neuropathy-central-dysmyelinating leukodystrophy-Waardenburg syndrome-Hirschprung disease; PMD: Pelizaeus-Merzbacher disease; PMDL: Pelizaeus-Merzbacher-like disease; RNA: Ribonucleic acid; TW: T-weighted; VWM: Vanishing white matter; WES: whole exome sequencing; WGS: whole genome sequencing; X-ALD: X-linked adrenoleukodystrophy; XLD: X-linked dominant; XLR: X-linked recessive.

Translation of mRNA into protein is an evolutionarily conserved, fundamental process of life. A prerequisite for translation is the accurate charging of tRNAs with their cognate amino acids, a reaction catalyzed by specific aminoacyl-tRNA synthetases. One of these enzymes is the aspartyl-tRNA synthetase DARS, which pairs aspartate with its corresponding tRNA. Missense mutations of the gene encoding DARS result in the leukodystrophy hypomyelination with brainstem and spinal cord involvement and leg spasticity (HBSL) with a distinct pattern of hypomyelination, motor abnormalities, and cognitive impairment. A thorough understanding of the DARS expression domains in the central nervous system is essential for the development of targeted therapies to treat HBSL. Here, we analyzed endogenous DARS expression on the mRNA and protein level in different brain regions and cell types of human post mortem brain tissue as well as in human stem cell derived neurons, oligodendrocytes, and astrocytes. DARS expression is significantly enriched in the cerebellum, a region affected in HBSL patients and important for motor control. Although obligatorily expressed in all cells, DARS shows a distinct expression pattern with enrichment in neurons but only low abundance in oligodendrocytes, astrocytes, and microglia. Our results reveal little homogeneity across the different cell types, largely matching previously published data in the murine brain. This human gene expression study will significantly contribute to the understanding of DARS gene function and HBSL pathology and will be instrumental for future development of animal models and targeted therapies. In particular, we anticipate high benefit from a gene replacement approach in neurons of HBSL mouse models, given the abundant endogenous DARS expression in this lineage cell.

Objective: To analyze the clinical and imaging features of hypomyelination with brain stem and spinal cord involvement and leg spasticity (HBSL) due to mutations in DARS, and to identify DARS mutations responsible for HBSL. Methods: Data on 2 HBSL patients who were admitted to the pediatric department of Peking University First Hospital from January 2009 through December 2016 were reviewed and the 2 patients were followed up. Targeted next generation sequencing, whole exome sequencing and Sanger sequencing were employed to identify potential genetic variations of the children and their parents. The clinical manifestations, MRI features and genotypic characteristics of two patients were reviewed, and the literature was reviewed. HBSL reported cases were searched with"leukoencephalopathies, DARS"on databases of PubMed, Wanfang, China National Knowledge Infrastructure and VIP from 1975 to 2017. The clinical manifestations and molecular features were analyzed. Results: Both patients showed delayed motor development, but had normal cognitive development. At the age of 8 years, case 1 reached the most significant motor development milestone of only standing with help during the last follow-up. At the age of 9, case 2 could walk independently during the last follow-up. On physical examination, both showed leg spastcity, active tendon reflex, positive Babinski sign. Both patients had brain MRI findings of high T2WI signal in bilateral deep cerebral white matter, slightly lower T1WI, and no abnormal DWI signal. Lesions of case 1 were relatively extensive and involved subcortical white matter, corpus callosum and internal capsule. Spinal MRI scans for both patients showed no abnormal signals. Novel mutations in DARS gene-namely, c.1498_1499insTCA (p.500_501insIle) and c.1210A>G (p.Met404Val) , c.1432A>G (p.Met478Val) and c.1210A>G (p.Met404Val) were identified in case 1 and case 2 respectively. On the database, 2 reports involving 13 foreign patients were retrieved. The age of disease onset was from 4 months to 18 years, and their initial symptoms were development delay or regression. Most of them presented with progressive lower extremity spasm, and the brain magnetic resonance imaging was characterized by hypomyelination in white matter. Clinical phenotypes of different age groups were significantly different. Conclusion: We have reported two patients with HBSL in China, and 3 novel mutations in DARS, which is helpful for the diagnosis and genetic counseling of HBSL. 目的： 探讨DARS基因突变所致髓鞘化低下伴脑干、脊髓受累及下肢痉挛的白质脑病的临床和影像学特点。 方法： 随访2009—2016年北京大学第一医院儿科就诊2例HBSL患儿的临床资料。通过靶向捕获二代测序技术、全外显子组测序、Sanger测序对患者及其父母进行遗传学分析，以"Leukoencephalopathies, DARS""白质脑病、DARS"为检索词检索PubMed数据库、万方、中国期刊全文和中国维普数据库（1975—2017年）文献报道的病例，总结临床表型、影像学特点及基因型。 结果： 2例均为运动发育落后，认知发育正常，例1至末次随访年龄8岁时最高运动里程碑仅为扶站，例2至末次随访年龄9岁时最高运动里程碑可独走。2例患儿查体示下肢痉挛重于上肢，膝腱反射亢进，双侧巴宾斯基征阳性。头颅磁共振成像示双侧深部白质对称性T2WI高信号，TIWI稍低信号，DWI未见高信号，例1累及皮层下白质、内囊前后肢及胼胝体。脊髓MRI均未见异常。2例DARS基因均存在复合杂合突变：例1为c.1498_1499insTCA（p.500_501insIle）和c.1210A>G（p.Met404Val），例2为c.1432A>G（p.Met478Val）和c.1210A>G（p.Met404Val），均为未报道的新突变。符合检索条件2篇文献报道13例患者，年龄4月龄至18岁，以运动发育落后或运动倒退为首发症状，多数呈进行性下肢痉挛，头颅磁共振表现为白质髓鞘化低下，不同起病年龄的临床表型差异较大。 结论： 2例HBSL患儿发现了3个相关新突变，扩展了对HBSL临床表型及基因谱的认识。.