Hypermorphic and hypomorphic AARS alleles in patients with CMT2N expand clinical and molecular heterogeneities

Abstract

Aminoacyl-tRNA synthetases (ARSs) are ubiquitously expressed enzymes implicated in several dominant and recessive disease phenotypes. The canonical function of ARSs is to couple an amino acid to a cognate transfer RNA (tRNA). We identified three novel disease-associated missense mutations in the alanyl-tRNA synthetase (AARS) gene in three families with dominant axonal Charcot-Marie-Tooth (CMT) disease. Two mutations (p.Arg326Trp and p.Glu337Lys) are located near a recurrent pathologic change in AARS, p.Arg329His. The third (p.Ser627Leu) is in the editing domain of the protein in which hitherto only mutations associated with recessive encephalopathies have been described. Yeast complementation assays demonstrated that two mutations (p.Ser627Leu and p.Arg326Trp) represent loss-of-function alleles, while the third (p.Glu337Lys) represents a hypermorphic allele. Further, aminoacylation assays confirmed that the third mutation (p.Glu337Lys) increases tRNA charging velocity. To test the effect of each mutation in the context of a vertebrate nervous system, we developed a zebrafish assay. Remarkably, all three mutations caused a pathological phenotype of neural abnormalities when expressed in zebrafish, while expression of the human wild-type messenger RNA (mRNA) did not. Our data indicate that not only functional null or hypomorphic alleles, but also hypermorphic AARS alleles can cause dominantly inherited axonal CMT disease.

In-silico Analysis of NF1 Missense Variants in ClinVar: Translating Variant Predictions into Variant Interpretation and Classification

“…Twelve variants in this interval were listed at ClinVar, and classified as pathogenic 7, with conflicting interpretation (1) or VUS (4). After reclassification, 10/12 variants were in the LEANING PATHOGENIC category for both VEST3 and ClinPred (8 for REVEL), and only 2 were reclassified as VUS (4 in REVEL).…”

“…While for several DCGs pathogenicity could be the consequence of either loss-of-function or gain-of-function mutations, some other genes can be functionally impaired by more subtle mechanisms in combination with additive effects. In fact, both hypermorphic and hypomorphic alleles were recently reported in patients with dominant axonal Charcot-Marie-Tooth disease [1], and FMF-associated MEFV mutations were experimentally demonstrated to be of hypermorphic nature [2].…”

Neurodegenerative Charcot–Marie–Tooth disease as a case study to decipher novel functions of aminoacyl-tRNA synthetases

“…Most recently, zebrafish have been used successfully to demonstrate the dominant toxicity of CMT mutants (Table 1). Injection of mRNAs of three different AlaRS CMT mutants (R326W, E337K, and S627L) produced neural developmental toxicity in the embryos, whereas the same amount of WT mRNA did not (27).…”

“…The various aaRS-linked CMT subtypes usually present as axonal peripheral neuropathies, and sometimes the nerve conduction velocity (NCV) can be in the range associated with demyelination (26,27). The patients have JBC REVIEWS: aaRSs in Charcot-Marie-Tooth disease predominantly motor deficits, with highly variable symptoms in terms of severity (27). Although the lower limbs are mainly affected by CMT, most patients with GlyRS mutations have upper limb predominance (22,28).…”

“…Although AlaRS-linked CMT is designated as type 2, some CMT2N patients (i.e. some JBC REVIEWS: aaRSs in Charcot-Marie-Tooth disease patients carrying the E337K mutation) also exhibit demyelinating features (27).…”