Novel mutations in CPT 1A define molecular heterogeneity of hepatic carnitine palmitoyltransferase I deficiency

Abstract

Liver carnitine palmitoyltransferase I (CPT I) deficiency is a rare disorder of hepatic mitochondrial long-chain fatty acid oxidation. It characteristically presents with symptoms associated with failure of ketogenesis (hypoketotic hypoglycemia). The disorder is due to mutations in the CPT 1A gene for which few patients have been characterized. We present here four novel mutations in five patients from four families with severe enzyme deficiency. Three of these are missense mutations (G465W, R316G, and F343V) and the fourth a nonsense mutation (R160X). Other than small Inuit and Hutterite populations in Canada and the Northern plains, there is complete heterogeneity of disease-causing mutations within CPT I deficient families with each demonstrating unique mutations. Because there are no easily recognizable disease-specific metabolite markers, diagnostic confirmation of this disorder requires a combination of enzymatic analysis and whole gene sequencing.

Introduction

Carnitine palmitoyltransferase I (CPT 1, EC 2.3.1.21) activity is located on the inner aspect of the outer mitochondrial membrane and catalyzes the conversion of long-chain acyl-CoAs into their respective acylcarnitine species. This facilitates the transport of long-chain fatty acids into the mitochondrion via the action of carnitine acylcarnitine translocase. Carnitine palmitoyltransferase II (CPT 2) which is located on the inner mitochondrial membrane carries out the reverse reaction to CPT I and serves to regenerate long-chain acyl-CoAs for the process of fatty acid β-oxidation [1].

CPT I enzyme activity is encoded by three distinct genes with differential tissue expression of their products. CPT 1A is located on chromosome 11q13.1–q13.2 and encodes a 773 amino acid polypeptide which is predominantly expressed in liver, kidney, leucocytes, and fibroblasts and is known as the liver or L-CPT I. CPT 1B is on chromosome 22q13.3-ter and encodes a 772 amino acid polypeptide which is 63% identical to L-CPT I but is predominantly expressed in muscle (M-CPT I). CPT II is a ubiquitously expressed protein of 658 amino acid residues which is encoded by a gene on chromosome 1p32 [2]. CPT 1C was recently located to chromosome 19q13.33 which encodes a highly related polypeptide of 810 amino acids which is expressed predominantly in brain and testis [3].

Genetic defects of CPT II (MIM#600650) represent relatively common inborn errors of mitochondrial fatty acid oxidation. Hundreds of cases have been characterized at the clinical, biochemical, and molecular levels [4], [5], [6], [7]. In contrast, despite being first identified in 1981, L-CPT I deficiency (MIM#600528) remains a rare disorder of fatty acid oxidation with only approximately 30 cases in the published literature or known to us. Even fewer cases have been fully described at the molecular genetic level [8], [9], [10], [11], [12], [13], [14].

We present here five new cases in four families of L-CPT I deficiency for which four novel mutations have been identified in the CPT 1A gene. These cases emphasize the genetic heterogeneity of this disorder.