Muscle Carnitine Palmitoyltransferase II (CPT II) Deficiency: A Conceptual Approach

Abstract

Carnitine palmitoyltransferase (CPT) catalyzes the transfer of long- and medium-chain fatty acids from cytoplasm into mitochondria, where oxidation of fatty acids takes place. Deficiency of CPT enzyme is associated with rare diseases of fatty acid metabolism. CPT is present in two subforms: CPT I at the outer mitochondrial membrane and carnitine palmitoyltransferase II (CPT II) inside the mitochondria. Deficiency of CPT II results in the most common inherited disorder of long-chain fatty acid oxidation affecting skeletal muscle. There is a lethal neonatal form, a severe infantile hepato-cardio-muscular form, and a rather mild myopathic form characterized by exercise-induced myalgia, weakness, and myoglobinuria. Total CPT activity (CPT I + CPT II) in muscles of CPT II-deficient patients is generally normal. Nevertheless, in some patients, not detectable to reduced total activities are also reported. CPT II protein is also shown in normal concentration in patients with normal CPT enzymatic activity. However, residual CPT II shows abnormal inhibition sensitivity towards malonyl-CoA, Triton X-100 and fatty acid metabolites in patients. Genetic studies have identified a common p.Ser113Leu mutation in the muscle form along with around 100 different rare mutations. The biochemical consequences of these mutations have been controversial. Hypotheses include lack of enzymatically active protein, partial enzyme deficiency and abnormally regulated enzyme. The recombinant enzyme experiments that we recently conducted have shown that CPT II enzyme is extremely thermoliable and is abnormally inhibited by different emulsifiers and detergents such as malonyl-CoA, palmitoyl-CoA, palmitoylcarnitine, Tween 20 and Triton X-100. Here, we present a conceptual overview on CPT II deficiency based on our own findings and on results from other studies addressing clinical, biochemical, histological, immunohistological and genetic aspects, as well as recent advancements in diagnosis and therapeutic strategies in this disorder.

Keywords: CPT II; Genotype-Phenotype; fatty acids; muscle; mutation; rhabdomyolysis.

Figure 1

Transport system for esterification of fatty acids through mitochondrial membranes (OMM: outer mitochondrial membrane; IMM: inner mitochondrial membrane, CPT I & II: carnitine palmitoyltransferase I and II; CACT: Carnitine- Acylcarnitine-Translocase; CoASH: free Coenzyme A).

Figure 2

Sudan staining of muscle biopsy sections (200× magnifications) of (a) CPT II deficient patient compound heterozygous for p.Ser113Leu/p.Arg151Gln mutations and (b) primary carnitine deficient patient. Red dots represent lipid accumulation.

Figure 3

Immunohistochemical staining of CPT II (200× magnification) (a) control muscle section and (b) muscle section of CPT II deficient patient (Figure adapted with kind permission of the authors from Lehmann & Zierz; 2014).

Figure 4

Residual CPT Activity (% of total CPT I and II) after pre-incubation with malonyl-CoA and Triton-X. Grey bars represent patients with mutations on both alleles (n = 40) and white bars represent healthy controls (n = 21). The error bars represent standard errors. Three manifesting heterozygotes patients are represented by closed circle, open circle and open triangle, respectively.