Influence of OCTN2 Variants on Carnitine Status and Plasma Triglycerides

Overview

The current study is part of a large multi-investigator grant to look at the pharmacogenetics of a number of membrane transporters. Previously, the investigators have recruited a cohort of healthy volunteers (Studies of Pharmacogenetics in Ethnically-Diverse Populations, or SOPHIE) and have resequenced the coding region of a number of membrane transporter genes to identify genetic polymorphisms in these genes. Subjects in this cohort have agreed to be called back for recruitment in further studies based on their own genetic sequence, allowing the investigators the possibility to prospectively study the influence of genetic polymorphisms on particular phenotypes (i.e., genotype-to-phenotype studies). The investigators plan to take a genotype-to-phenotype approach to study the influence of specific polymorphisms in the novel organic cation transporter 2 (OCTN2) gene on carnitine and lipid metabolism in healthy subjects.

Although OCTN2 is fairly well studied in its relationship with SCD, little is known about the carrier frequency of disease-causing alleles of OCTN2, or of more common functional polymorphisms in this gene. To address these issues, we screened for genetic variants in the OCTN2 coding region by direct sequencing of the exons and flanking intronic region of OCTN2 in a large sample (n = 276) of ethnically diverse subjects. In addition, we established lymphoblastoid cell lines from subjects homozygous for either allele of the previously identified promoter region variant, -207G\>C. We found eight amino acid sequence variants of OCTN2, of which three (Phe17Leu, Leu144Phe, and Pro549Ser) were polymorphic in at least one ethnic group. When assayed for functional activity by expression in human embryonic kidney 293 cells, using as probes both the endogenous substrate (l-carnitine) and the organic cation tetraethylammonium, three variants showed functional differences from the reference OCTN2 (Phe17Leu, Tyr449Asp, Val481Phe; p \< 0.05). Further studies of the Phe17Leu polymorphism showed a reduced V(max) for l-carnitine transport to approximately 50% of the reference OCTN2. Confocal microscopy studies using an OCTN2-GFP fusion protein showed that Phe17Leu had distinct subcellular localization from the reference OCTN2, with diffuse cytoplasmic retention of Phe17Leu, in contrast to reference OCTN2, which localized specifically to the plasma membrane. Lymphoblasts from subjects homozygous for the -207G allele showed increased l-carnitine transport compared with the -207C/C homozygotes (p \< 0.05). This study suggests that although loss-of-function mutations in OCTN2 are likely to be rare, common variants of OCTN2 found in healthy populations may contribute to variation in the disposition of carnitine and some clinically used drugs.