ARSs can be subdivided into two classes depending on the cell compartment in which they catalyze the aminoacylation reaction: Cytosolic ARSs and mitochondrial ARSs (mt-ARSs). Each ARS is specific for the charging of one tRNA with its corresponding amino acid, e.g., AspRS specifically links tRNA Asp to aspartate, and there is no redundancy amongst these enzymes. This process is known as tRNA charging and is an essential prerequisite for successful protein biosynthesis. AspRS belongs to a group of enzymes termed aminoacyl-tRNA synthetases (ARSs) that catalyze an aminoacylation reaction in which transfer ribonucleic acids (tRNAs) are linked to their cognate amino acids. The underlying cause of HBSL are missense mutations of the aspartyl-tRNA synthetase (AspRS) gene DARS1. For a comprehensive clinical review of HBSL see Muthiah et al. The clinical symptoms typically include motor deficits, leg spasticity, regression, or delay of developmental milestones, hypertonia, hyperreflexia, positive Babinski sign, nystagmus, and gait abnormalities in patients who are able to mobilize ( Taft et al., 2013 Wolf et al., 2015 Ong et al., 2020). An early onset form of HBSL usually results in a more severe course of disease. HBSL can be seen as a spectrum disorder with a high variance in severity (mild to severe forms) and onset of the disease (4 months to 22 years) ( Wolf et al., 2015).
Following the initial discovery, two additional case studies, together with the original study, described a total of 16 HBSL patients ( Wolf et al., 2015 Ong et al., 2020).
Hypomyelination with brain stem and spinal cord involvement and leg spasticity (HBSL) belongs to this group of diseases and was first described in 2013 ( Taft et al., 2013). The population incidence of all leukodystrophies taken together is relatively high with one in 7,600 live births ( Bonkowsky et al., 2010) underpinning the high unmet medical need. Leukodystrophies are inherited white matter disorders often associated with an early onset, lack of treatment options and premature death. 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. Taken together, Dars1 D367 Y/− mice model aspects of the clinical picture of the corresponding missense mutation in HBSL. Expression levels of the major myelin proteins were reduced in Dars1 D367 Y/− mice compared to controls. Despite this early developmental deficit, once they made it through early adolescence Dars1 D367 Y/− 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. Throughout postnatal life, Dars1 D367 Y/− mice remained smaller and lighter than their Dars1 D367 Y/+ littermates. Of the few Dars1 D367 Y/− mice that were born at term, 25% displayed microphthalmia. Only a small fraction of Dars1 D367 Y/− mice were born, and half of these mice died with hydrocephalus during the first 3 weeks of life. The resulting Dars1 D367 Y/− offspring displayed a strong developmental delay compared to control Dars1 D367 Y/+ littermates, starting during embryogenesis. As hypomorphic mutations are more severe in trans to a deletion, we crossed Dars1 D367 Y/ D367 Y mice with Dars1-null carriers. Surprisingly, mice carrying this mutation homozygously were phenotypically normal. To address this, we introduced the HBSL-causing Dars1 D367 Y point mutation into the mouse genome. Dars1 knockout mice are embryonic lethal precluding examination of the etiology. 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.
The clinical picture includes the regression of acquired motor milestones, spasticity, ataxia, seizures, nystagmus, and intellectual disabilities. 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.
1Translational Neuroscience Facility & Department of Physiology, School of Medical Sciences, UNSW Sydney, Kensington, NSW, Australia.