Loss-of-Function Alanyl-tRNA Synthetase Mutations Cause an Autosomal-Recessive Early-Onset Epileptic Encephalopathy with Persistent Myelination Defect

Abstract

Mutations in genes encoding aminoacyl-tRNA synthetases are known to cause leukodystrophies and genetic leukoencephalopathies-heritable disorders that result in white matter abnormalities in the central nervous system. Here we report three individuals (two siblings and an unrelated individual) with severe infantile epileptic encephalopathy, clubfoot, absent deep tendon reflexes, extrapyramidal symptoms, and persistently deficient myelination on MRI. Analysis by whole exome sequencing identified mutations in the nuclear-encoded alanyl-tRNA synthetase (AARS) in these two unrelated families: the two affected siblings are compound heterozygous for p.Lys81Thr and p.Arg751Gly AARS, and the single affected child is homozygous for p.Arg751Gly AARS. The two identified mutations were found to result in a significant reduction in function. Mutations in AARS were previously associated with an autosomal-dominant inherited form of axonal neuropathy, Charcot-Marie-Tooth disease type 2N (CMT2N). The autosomal-recessive AARS mutations identified in the individuals described here, however, cause a severe infantile epileptic encephalopathy with a central myelin defect and peripheral neuropathy, demonstrating that defects of alanyl-tRNA charging can result in a wide spectrum of disease manifestations.

Whole exome sequencing in patients with white matter abnormalities

“…Nine of the twenty-five cases had mutations in genes associated with disorders classically defined as leukodystrophies 3 (Table 1 & Supplemental Table S1): two patients were identified with TUBB4A (MIM:602662) related hypomyelination (Hypomyelination with atrophy of the basal ganglia and cerebellum [MIM:612438]) 24 ; two patients were identified with early onset Vanishing White Matter Disease (MIM:603896) (genotype EIF2B2 [MIM:606454] and EIF2B5 [MIM:603945]) 25, 26 ; three families were identified with t-RNA synthetase disorders ( AARS [MIM:601065] and DARS [MIM:603084]) 27–29 ; and two families identified with a POLR3-related disorder ( POLR3B [MIM:614366] and POLR1C [MIM:610060]) (Table 1, Supplemental Table S2 and Supplemental Case Reports). 30 The remaining individuals had mutations in genes associated with genetic leukoencephalopathies.…”

Regulation of mRNA Translation in Neurons—A Matter of Life and Death

“…In contrast, other CMT-linked mutations have been reported to cause a loss of charging activity, suggesting that haploinsufficiency of the enzyme may be a possible contributing factor (Griffin et al, 2014; McLaughlin et al, 2012). Supporting this hypothesis, recessive mutations in synthetases have been linked to neurodegenerative disease, as well as many multisystem disorders involving a wider range of tissues, and many of these mutations have been found to reduce aminoacylation activity in vitro (Kodera et al, 2015; McLaughlin et al, 2010; van Meel et al, 2013; Meyer-Schuman and Antonellis, 2017; Nakayama et al, 2017; Puffenberger et al, 2012; Simons et al, 2015; Taft et al, 2013; Wolf et al, 2014; Zhang et al, 2014a). Impaired synthetase function may reduce the amount of charged tRNA available for translation elongation, with a possible increase in the levels of uncharged tRNA.…”

Predicting the pathogenicity of aminoacyl-tRNA synthetase mutations

“…Steady-state kinetics are calculated by fitting the initial rate of aminoacylation as a function of tRNA concentration (or the concentration of another substrate) to the Michaelis–Menten equation [43]. Aminoacylation assays have been used to demonstrate a loss-of-function effect on tRNA charging for mutations in 15 genes implicated in recessive disease ( AARS , CARS2 , DARS2 , FARS2 , IARS , KARS , LARS2 , MARS , MARS2 , QARS , RARS2 , SARS2 , TARS2 , VARS2 , and YARS2 ) [4,44–59]. Interestingly, aminoacylation assays have also revealed a loss-of-function effect of GARS , YARS , and AARS mutations that cause dominant peripheral neuropathy (Table 2); HARS mutations associated with dominant CMT disease have not been tested.…”

“…Yeast complementation assays have been used to demonstrate a loss-of-function effect on ARS function for mutations in eight genes implicated in recessive diseases ( AARS , AARS2 , HARS2 , IARS , KARS , LARS2 , MARS , VARS2 ) [6,44,47,48,52,57,63,64]. Similar to aminoacylation assays, in vivo studies in yeast have revealed a loss-of-function effect for GARS , YARS , AARS , and HARS mutations that cause dominant CMT disease (Table 2).…”