Influence of OCTN2 Variants on Carnitine Status and Plasma Triglycerides
Condition: Carnitine Transporter Deficiency
Study type: Observational

Summary: 
      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.
    

Detailed description: 
      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.
    

Intervention name(s): Fasting blood and urine collection
Intervention type(s): Other
Intervention description(s): Not applicable no drugs dispensed
Observational model: Cohort
Time perspective: Prospective
Sampling method: Non-Probability Sample
Study population: 
        Healthy individuals per screening laboratory results and health questionnaire
      
Biospecimen description: 
      Blood Draw (10cc) to determine eligibility (CBC, blood chemistries). Blood draw (20cc) at t=0
      in fasting state for baseline measurement of biochemical markers (including carnitine,
      acylcarnitines, creatinine, and total lipid panel) Urine collection at t=0 Blood draw (20cc)
      at t=2 hours for measurement of biochemical markers (including carnitine, acylcarnitines,
      creatinine, and total lipid panel) Urine collection at t=2 hours Transformation of blood to
      establish lymphoblastoid cell lines (from blood samples collected at t=0 and t=2 hours)
    
Keywords: L-carnitine