Dual Role of Dysfunctional Asc-1 Transporter in Distinct Human Pathologies, Human Startle Disease, and Developmental Delay

Abstract

Human startle disease is associated with mutations in distinct genes encoding glycine receptors, transporters or interacting proteins at glycinergic synapses in spinal cord and brainstem. However, a significant number of diagnosed patients does not carry a mutation in the common genesGLRA1,GLRB, andSLC6A5. Recently, studies onSLC7A10(Asc-1 alanine-serine-cysteine transporter) knockout mice displaying a startle disease-like phenotype hypothesized that this transporter might represent a novel candidate for human startle disease. Here, we screened 51 patients from our patient cohort negative for the common genes and found three exonic (one missense, two synonymous), seven intronic, and single nucleotide changes in the 5’ and 3’ untranslated regions. The identified missense mutation Asc-1G307Rfrom a patient with startle disease and developmental delay was investigated in functional studies. At the molecular level, the mutation Asc-1G307Rdid not interfere with cell-surface expression, but disrupted glycine uptake. Substitution of glycine at position 307 to other amino acids e.g. to alanine or tryptophan did not affect trafficking or glycine transport. By contrast, G307K disrupted glycine transport similar to the G307R mutation found in the patient. Structurally, the disrupted function in variants carrying positively charged residues can be explained by local structural rearrangements due to the large positively charged side chain. Thus, our data suggest thatSLC7A10may represent a rare but novel gene associated with human startle disease and developmental delay.Statement of SignificancePatients diagnosed with startle disease/hyperekplexia do not always carry mutations in the known associated disease genes (GLRA1, GLRB, SLC6A5). Recently, murine studies suggestedSLC7A10as candidate gene for human startle disease. Within our patient cohort, we found one missense mutation, three exonic silent mutations but also intronic single nucleotide variations as well as variations in the 5’ and 3’ untranslated regions inSLC7A10. We further concentrated on the structural and functional alterations due to identified missense mutations in Asc-1. The exchange of Gly307 with positively charged residues leads to structural rearrangements accompanied by loss of function. Our data provide evidence that in humansSLC7A10is a rare gene associated with human startle disease.

Over-Production of the Human SLC7A10 in E. coli and Functional Assay in Proteoliposomes

“…For all these reasons, hASC-1 transporter is nowadays considered a key player in neurological disorders, metabolic diseases related to obesity, and type 2 diabetes [ 7 ]. Very recently, the SLC7A10 gene has been proposed as a candidate for human startle disease: the G307R mutation found in patients led to structural rearrangements causing the loss of function of the transporter [ 10 ]. Moreover, the growing body of evidence linking metabolic changes to cell senescence and aging processes makes membrane transporters of nutrients, such as hASC-1, novel and under-evaluated markers for age-related diseases [ 9 ].…”