Gene Function
● Canonical Function: The SARS1 gene, located on chromosome 1p13.3, encodes the cytosolic seryl-tRNA synthetase. This enzyme is a member of the class II aminoacyl-tRNA synthetase family and plays a fundamental role in protein biosynthesis. It catalyzes the covalent attachment of the amino acid L-serine to its cognate transfer RNA (tRNA-Ser) in a two-step, ATP-dependent reaction: first, serine is activated by ATP to form a seryl-adenylate intermediate (Ser-AMP), which is then transferred to the acceptor end of tRNA-Ser. This process, known as tRNA charging or aminoacylation, is an essential prerequisite for the accurate incorporation of serine into nascent polypeptide chains during mRNA translation. The enzyme is also capable of serylating tRNA-Sec, which is the first step in the biosynthesis of the 21st proteinogenic amino acid, selenocysteine.
● Non-Canonical Function: Beyond its canonical role in translation, a fraction of the SARS1 protein possesses a nuclear localization signal and can translocate to the nucleus. Within the nucleus, it functions as a transcriptional repressor by binding directly to the core promoter of the vascular endothelial growth factor A (VEGFA) gene. This activity prevents the binding of transcriptional activators like MYC and recruits the deacetylase SIRT2, thereby inhibiting VEGFA production and angiogenesis. This non-canonical function is dependent on a unique C-terminal domain known as UNE-S and is critical for the proper regulation of vascular development.
Pathophysiologic Mechanism
The clinical phenotypes associated with SARS1 mutations arise from distinct molecular mechanisms depending on the nature of the variant and the mode of inheritance.
● Loss-of-Function (Recessive): The autosomal recessive NEDMAS phenotype is caused by biallelic (homozygous or compound heterozygous) missense or other loss-of-function variants. These mutations lead to significantly reduced or absent enzyme activity, either through impaired catalytic function or reduced protein stability and expression. The resulting deficit in charged tRNA-Ser impairs the cell's capacity for protein synthesis. This defect disproportionately affects tissues with high metabolic and translational demands, such as the developing central nervous system, leading to the observed neurodevelopmental phenotype.
● Dominant-Negative (Dominant): The autosomal dominant form of complex spastic paraplegia has been linked to a de novo splice-site variant that results in an in-frame insertion of five amino acids near the enzyme's active site. Functional assays have demonstrated that the mutant protein produced from this allele not only possesses reduced function but also actively interferes with the function of the wild-type protein produced from the normal allele. This interference likely occurs through the disruption of the functional homodimer structure of the enzyme, leading to a dominant-negative effect.
The dual canonical and non-canonical functions of SARS1 introduce complexity into understanding the complete pathophysiology. While impaired protein synthesis is the most direct explanation for the severe neurological symptoms, it is plausible that the dysregulation of VEGFA and angiogenesis during critical periods of brain development could also contribute to the pathology. Future research is needed to determine whether known pathogenic mutations differentially affect these two distinct functions.
Diagnostic Testing
The diagnosis of SARS1-related disorders is typically established through molecular genetic testing.
● Sequencing: Given the non-specific clinical presentation, the diagnosis is most often made via unbiased, next-generation sequencing approaches, such as whole-exome sequencing (WES) or whole-genome sequencing (WGS), performed on a proband. Panel-based testing for neurodevelopmental disorders or leukodystrophies may also include the SARS1 gene.
● Variant Analysis: Following identification of a candidate variant (or variants), confirmation and segregation analysis in the parents via Sanger sequencing is standard practice to determine inheritance patterns (biallelic vs. de novo).
● Functional Studies: For novel variants of uncertain significance (VUS), functional validation may be required to establish pathogenicity. These studies can include enzymatic (serylation) assays performed on patient-derived fibroblasts or complementation assays in heterologous systems, such as Saccharomyces cerevisiae (yeast) models, to assess the variant's impact on protein function.
SARS1