Biallelic KARS pathogenic variants cause an early-onset progressive leukodystrophy.

The leukodystrophies cause severe neurodevelopmental defects from birth and follow an incurable and progressive course that often leads to premature death. It has recently been reported that abnormalities in aminoacyl t-RNA synthetase (ARS) genes are linked to various unique leukodystrophies and leukoencephalopathies. Aminoacyl t-RNA synthetase proteins are fundamentally known as the first enzymes of translation, catalysing the conjugation of amino acids to cognate tRNAs for protein synthesis. It is known that certain aminoacyl t-RNA synthetase have multiple non-canonical roles in both transcription and translation, and their disruption results in varied and complicated phenotypes. We clinically and genetically studied seven patients (six male and one female; aged 2 to 12 years) from five unrelated families who all showed the same phenotypes of severe developmental delay or arrest (7/7), hypotonia (6/7), deafness (7/7) and inability to speak (6/7). The subjects further developed intractable epilepsy (7/7) and nystagmus (6/6) with increasing age. They demonstrated characteristic laboratory data, including increased lactate and/or pyruvate levels (7/7), and imaging findings (7/7), including calcification and abnormal signals in the white matter and pathological involvement (2/2) of the corticospinal tracts. Through whole-exome sequencing, we discovered genetic abnormalities in lysyl-tRNA synthetase (KARS). All patients harboured the variant [c.1786C>T, p.Leu596Phe] KARS isoform 1 ([c.1702C>T, p.Leu568Phe] of KARS isoform 2) either in the homozygous state or compound heterozygous state with the following KARS variants, [c.879+1G>A; c.1786C>T, p.Glu252_Glu293del; p.Leu596Phe] ([c.795+1G>A; c.1702C>T, p.Glu224_Glu255del; p.Leu568Phe]) and [c.650G>A; c.1786C>T, p.Gly217Asp; p.Leu596Phe] ([c.566G>A; c.1702C>T, p.Gly189Asp; p.Leu568Phe]). Moreover, similarly disrupted lysyl-tRNA synthetase (LysRS) proteins showed reduced enzymatic activities and abnormal CNSs in Xenopus embryos. Additionally, LysRS acts as a non-canonical inducer of the immune response and has transcriptional activity. We speculated that the complex functions of the abnormal LysRS proteins led to the severe phenotypes in our patients. These KARS pathological variants are novel, including the variant [c.1786C>T; p.Leu596Phe] (c.1702C>T; p.Leu568Phe) shared by all patients in the homozygous or compound-heterozygous state. This common position may play an important role in the development of severe progressive leukodystrophy. Further research is warranted to further elucidate this relationship and to investigate how specific mutated LysRS proteins function to understand the broad spectrum of KARS-related diseases.

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