A missense mutation in the OCTN2 gene associated with residual carnitine transport activity

Primary carnitine deficiency is an autosomal recessive disorder of fatty acid oxidation caused by defective carnitine transport. This disease can present early in life with hypoketotic hypoglycemia and acute metabolic decompensation, or later in life with skeletal or cardiac myopathy. Mutations abolishing the function of OCTN2, an organic cation/carnitine transporter with twelve putative transmembrane spanning domains, were recently demonstrated in patients with early- and late-onset (up to seven years of age) presentation of this syndrome. Most of the reported mutations are null alleles. Here we evaluate the OCTN2 gene in a male patient who presented at seven years of age with severe dilated cardiomyopathy. Plasma carnitine levels were undetectable and carnitine transport by his fibroblasts was reduced to about 3% of normal controls. This patient was homozygous for a single base pair change in exon 8 of the OCTN2 gene (1354G>A) converting the codon for Glu 452 to Lys (E452K) in the predicted intracellular loop between transmembrane domains 10 and 11. Stable expression of the mutant E452K-OCTN2 cDNA in Chinese hamster ovary (CHO) cells caused a partial increase in carnitine transport to 2-4% of the levels measured in the wild type transporter. This reduced transport activity was associated with normal Km toward carnitine (3.1 +/- 1.1 microM), but markedly reduced Vmax. These results indicate that primary carnitine deficiency can be caused by mutations encoding for carnitine transporters with residual activity, and that the E452K affects a domain not involved in carnitine recognition.