Abnormal Sodium Stimulation of Carnitine Transport in Primary Carnitine Deficiency

Yuhuan Wang,Telly A Meadows,Nicola Longo

doi:10.1074/jbc.m000194200

Abstract

Primary carnitine deficiency is an autosomal recessive disorder of fatty acid oxidation characterized by hypoketotic hypoglycemia and skeletal and cardiac myopathy. It is caused by mutations in the sodium-dependent carnitine cotransporter OCTN2. The majority of natural mutations identified in this and other Na(+)/solute symporters introduce premature termination codons or impair insertion of the mutant transporter on the plasma membrane. Here we report that a missense mutation (E452K) identified in one patient with primary carnitine deficiency did not affect membrane targeting, as assessed with confocal microscopy of transporters tagged with the green fluorescent protein, but reduced carnitine transport by impairing sodium stimulation of carnitine transport. The natural mutation increased the concentration of sodium required to half-maximally stimulate carnitine transport (K(Na)) from the physiological value of 11.6 to 187 mm. Substitution of Glu(452) with glutamine (E452Q), aspartate (E452D), or alanine (E452A) caused intermediate increases in the K(Na). Carnitine transport decreased exponentially with increased K(Na). The E452K mutation is the first natural mutation in a mammalian cotransporter affecting sodium-coupled solute transfer and identifies a novel domain of the OCTN2 cotransporter involved in transmembrane sodium/solute transfer.

Highlights

Primary carnitine deficiency (On-line Mendelian Inheritance in Man (OMIM) no. 212140) is a recessively inherited disorder of fatty acid oxidation due to defective carnitine transport [1, 2]
We report that a missense mutation (E452K) identified in one patient with primary carnitine deficiency did not affect membrane targeting, as assessed with confocal microscopy of transporters tagged with the green fluorescent protein, but reduced carnitine transport by impairing sodium stimulation of carnitine transport
We have recently identified a missense mutation (E452K) associated with residual carnitine transport activity in a patient who presented at 7 years of age with severe cardiomyopathy and carnitine deficiency [10]

Summary

Introduction

Primary carnitine deficiency (On-line Mendelian Inheritance in Man (OMIM) no. 212140) is a recessively inherited disorder of fatty acid oxidation due to defective carnitine transport [1, 2]. Conjugation with the green fluorescent protein did not affect carnitine transport activity, which remained similar in CHO cells expressing the native OCTN2 cDNA and the normal cDNA with a C-terminal addition of the green fluorescent protein (Fig. 1A).

Results

Conclusion