Quantitative Fibroblast Acylcarnitine Profiling In The Diagnostic and Prognostic Assessment of Mitochondrial fatty acid [beta]-oxidation disorders

Abstract

Mitochondrial fatty acid ß-oxidation disorders are a group of clinically and biochemically heterogeneous defects mainly associated with intolerance to catabolic stress. The diseases are potentially fatal, but treatable and the prognosis for most diagnosed disorders is generally favourable. Early diagnosis is thus important to prevent morbidity and mortality. This project describes an improved and validated quantitative fibroblast acylcarnitine profile assay for the investigation of suspected fatty acid ß-oxidation disorders. Intact cells were incubated with deuterium-labelled hexadecanoate and L-carnitine, and the accumulated acylcarnitines in the medium analysed using electrospray tandem mass spectrometry. This modified procedure is less demanding technically, requires fewer cells and better reflects the in vivo acylcarnitine status than previously published methods. Mitochondrial fatty acid ß-oxidation is coupled to the respiratory chain. Functional defects of one pathway may lead to secondary alterations in flux through the other. The diagnostic specificity and the prognostic potential of the in vitro acylcarnitine profile assay were investigated in fibroblasts from 14 normal controls, 38 patients with eight enzyme deficiencies of fatty acid ß-oxidation presenting with various phenotypes, and 16 patients with primary respiratory chain defects including both isolated and multiple enzyme complex defects. All fatty acid ß-oxidation deficient cell lines revealed disease-specific acylcarnitine profiles related to the sites of defects irrespective of the severity of symptoms or of different mutation. Preliminary studies suggested a correlation between severity of symptoms and higher concentrations of long-chain acylcarnitine species. However, the fibroblast acylcarnitine profiles from some patients with respiratory chain defects were similar to those of controls, whereas others had abnormal profiles resembling those found in fatty acid ß-oxidation disorders. In vitro acylcarnitine profiling is useful for the detection of fatty acid ß-oxidation deficiencies, and perhaps the prediction of disease severity and prognostic evaluation facilitating decisions of therapeutic intervention and genetic counselling. However, abnormal profiles do not exclusively indicate these disorders, and primary defects of the respiratory chain remain a possibility. Awareness of this diagnostic pitfall will aid in the selection of subsequent confirmatory tests and therapeutic options.