Abstracts

Rationale: Recessive variants in tRNA synthetases, like QARS, have been previously linked to epilepsy and developmental delay. In this study we focus on recessive variants in VARS, encoding a valine tRNA synthetase, which might play an important role in development and epilepsy. We explore in depth the genetics, the functional characteristics of VARS variants and the corresponding phenotype. Methods: Through international collaborations we identified 3 families with compound heterozygous or homozygous missense variants in VARS. All variants were identified with whole exome or whole genome sequencing and confirmed with conventional Sanger sequencing. A zebrafish knock out (ko) VARS model was developed using CRISPR-Cas9 to study the zebrafish phenotype. Complementation assays in yeast were performed to determine the effect of variants on valine tRNA synthetase activity. Results: We identified two compound heterozygous missense variants in VARS (p.Gly822Ser/p.Leu434Val) in two siblings of a non-consanguineous family with severe developmental delay (DD), early onset epilepsy and microcephaly. Two other compound heterozygous missense variants (p.Gln400Pro/p.Arg442Gln) were identified in an isolated patient with a similar phenotype. The fifth variant was a homozygous missense (p.Arg173Leu) identified in a consanguineous family in which five out of six pregnancies ended prematurely due to intra-uterine fetal death. Massive placental floor infarctions were noted. All compound heterozygous variants were located in the catalytic domain of the protein whereas the homozygous variant was located in the glutathione S-transferase C-terminal-like domain. Preliminary results of a VARS ko zebrafish model show premature lethality (death prior to 7 days post fertilization) and decreased locomotor activity in homozygous fish. Further analysis including local field potential recordings and rescue experiments of homozygous larvae with both wild type VARS constructs and mutated constructs are ongoing, as are the yeast complementation assays. Conclusions: We identified three families with novel recessive variants in VARS. All patients have a complex developmental disorder including severe DD, early onset epilepsy and microcephaly except for one family where intra-uterine lethality repeatedly occurred. All variants identified are homozygous or compound heterozygous missense variants. We hypothesize that total loss of function is not viable which is reinforced by the preliminary results of a zebrafish ko model. Ongoing functional experiments with zebrafish and yeast will give more insight into the pathogenicity of the identified variants and the corresponding phenotype. Funding: Funding: H.S. is PhD fellow of the Fund for Scientific Research Flanders (1125416N).