Nutrient sulphate is important for numerous cellular and metabolic processes, particularly in fetal growth and development. Sulphate is supplied from mother to fetus via the placenta. Previously, we localised expression of the SLC13A4 sulphate transporter to the syncytiotrophoblast layer of human (and mouse) placentae, where it is proposed to mediate transport of sulphate between mother and fetus. The consequences of perturbed SLC13A4 function on human fetal development is unknown but warrants investigation based on fetal demise in Slc13a4 null mice.
In this study, we curated 52 known genetic variants in the human SLC13A4 gene from the NCBI and NHLBI GO ESP databases and further characterized the functional consequences of six variants (L72S, F309C, V512M, I569V, N299S and E359Q) which are conserved across multiple species and predict perturbed structural stability.
EGFP-SLC13A4 fusion proteins expressed in MDCK cells showed sorting of control and missense variants (N299S, E359Q, V512M and I569V) primarily to the apical membrane, whereas SLC13A4 harbouring the F309C variant was sorted to both apical and basolateral membranes. The L72S frameshift variant was retained intracellularly with no EGFP signal detected on the plasma membrane. Functional analysis of the variants using a radiotracer 35S-sulfate uptake assay, showed similar sulphate uptake between control SLC13A4 and the F309C, E359Q, V512M and I569V variants, whereas L72S completely abolished SLC13A4-mediated sulphate uptake.
This is the first study to functionally characterise known variants in the human SLC13A4 gene. Our findings show complete loss of function for L72S, and suggest that F309C leads to missorting of SLC13A4. Further studies are warranted to assess the consequences of genetic variants in SLC13A4 on sulphate transport function in vivo and on fetal outcomes. These studies have the potential for development of prenatal screening for SLC13A4 mutations and genetic counselling.