Single nucleotide polymorphisms in the human Na+-dicarboxylate cotransporter affect transport activity and protein expression

The sodium-coupled transport of citric acid cycleintermediates in the intestine and kidney is mediated by the Na+-dicarboxylate cotransporter, NaDC1. In the kidney, NaDC1 plays an important role in regulating succinateand citrate concentrations in the urine, which may have physiological consequences including the development of kidney stones. In the present study, the impact of nonsynonymous single nucleotide polymorphisms (SNPs) on NaDC1 expression and function was characterized using the COS-7 cell heterologous expressionsystem. The I550V variant had an increased sensitivity to lithium inhibition although there were no significant effects on protein abundance. The L44F variant had no significant effects on expression or function. The membrane protein abundance of the M45L, V117I, and F254L variants was decreased, with corresponding decreases in transport activity. The A310P variant had decreased protein abundance as well as a change in substrate selectivity. The P385S variant had a large decrease in succinatetransport Vmax, as well as altered substrate selectivity, and a change in the protein glycosylation pattern. The most damaging variant was V477M, which had decreased affinity for both succinateand sodium. The V477M variant also exhibited stimulation by lithium, indicating a change in the high-affinity cation binding site. We conclude that most of the naturally occurring nonsynonymous SNPs affect protein processing of NaDC1, and several also affect functional properties. All of these mutations are predicted to decrease transport activity in vivo, which would result in decreased intestinal and renal absorption of citric acid cycleintermediates.