Defective urinary carnitine transport in heterozygotes for primary carnitine deficiency

PURPOSE

Primary carnitine deficiency is an autosomal recessive disorder caused by defective carnitine transport and manifests as nonketotic hypoglycemia or skeletal or heart myopathy.

METHODS

To define the mechanisms producing partially reduced plasma carnitine levels in the parents of affected patients, we examined carnitine transport in vivo and in the fibroblasts of a new patient and his heterozygous parents.

RESULTS

Kinetic analysis of carnitine transport in fibroblasts revealed an absence of saturable carnitine transport in the proband's cells and a partially impaired carnitine transport in fibroblasts from both parents, whose cells retained normal Km values toward carnitine (6-9 microM) but reduced Vmax. At steady state, normal fibroblasts accumulated carnitine to a concentration that was up to 80 times the extracellular value (0.5 microM). By contrast, cells from the proband had minimal carnitine accumulation, and cells from both parents had intermediate values of carnitine accumulation. Plasma carnitine levels were slightly below normal in both heterozygous, yet clinically normal, parents and in the paternal grandfather and the maternal grandmother. To define the mechanism producing partially decreased carnitine levels, we studied urinary carnitine losses in heterozygous parents compared with controls. Urinary losses increased linearly (P < 0.05) with plasma carnitine levels in normal controls. When urinary carnitine losses were normalized to plasma carnitine levels, a significant difference was observed between controls and heterozygous individuals (P < 0.01).

CONCLUSIONS

These results indicate that fibroblasts from heterozygotes for primary carnitine deficiency have a decreased capacity to accumulate carnitine and that heterozygotes have increased urinary losses, which may contribute to their reduced plasma carnitine levels.

Carnitine metabolism and its regulation in microorganisms and mammals

In procaryotes, L-carnitine may be used as both a carbon and nitrogen source for aerobic growth, or the carbon chain may be used selectively following cleavage trimethylamine. Under anaerobic conditions and in the absence of preferred substrates, some bacteria use carnitine, via crotonobetaine, as an electron acceptor. Formation of trimethylamine and lambda-butyrobetaine (from reduction of crotonobetaine) from L-carnitine by enteric bacteria has been demonstrated in rats and humans. Carnitine is not degraded by enzymes of eukaryotic origin. In higher organisms, carnitine has specific functions in intermediary metabolism. Concentrations of carnitine and its esters in cells of eukaryotes are rigorously maintained to provide optimal function. Carnitine homeostasis in mammals is preserved by a modest rate of endogenous synthesis, absorption from dietary sources, efficient reabsorption, and mechanisms present in most tissues that establish and maintain substantial concentration gradients between intracellular and extracellular carnitine pools.

Diagnosis of isovaleric acidaemia by tandem mass spectrometry: false positive result due to pivaloylcarnitine in a newborn screening programme

Tandem mass spectrometric analysis of acylcarnitines and amino acids has been applied in newborn screening programmes for the detection of several inborn errors of metabolism. We report a false positive result for isovaleric acidaemia in a newborn screening programme using this method. The newborn screening sample showed a very prominent signal corresponding to the mass of isovalerylcarnitine. Repeat samples (age 6 days) of blood and urine showed similar results. However, urine organic acids were normal. Acylcarnitine analysis in blood, breast milk and urine of the mother also showed a prominent signal of the same mass. Gas chromatography-mass spectrometry of the methyl esters demonstrated that the signal in the patient's urine was due to the presence of pivaloylcarnitine, which is isomeric with isovalerylcarnitine. The patient's mother was receiving an antibiotic containing a derivative of pivalic acid to treat a urinary tract infection. Follow-up samples in the patient and the mother confirmed a decrease in the levels of pivaloylcarnitine, concomitant with the discontinuation of the treatment. We conclude that pivaloylcarnitine can give a false positive result for isovaleric acidaemia in newborns whose mothers are on treatment with pivoxilsulbactam-containing antibiotics.

The effect of fasting, long-chain triglyceride load and carnitine load on plasma long-chain acylcarnitine levels in mitochondrial very long-chain acyl-CoA dehydrogenase deficiency

We studied a 10-year-old patient with very long-chain acyl-CoA dehydrogenase (VLCAD) deficiency who was originally (mis)diagnosed as having systemic carnitine deficiency. He was subjected to a fasting test, a long-chain triglyceride (LCT) loading test (1.5 g/kg) and an intravenous carnitine clearance test (0.25 mumol/kg per min). Plasma acylcarnitines were analysed using a quantitative GC-CI-MS method. During fasting, all long-chain acylcarnitines with a chain length of C14 and higher (especially C14:1) increased dramatically. Total plasma long-chain acylcarnitine reached a concentration of 28.6 mumol/L. LCT loading resulted in a moderate increase, mainly of the C18 esters. The carnitine infusion, which led to a supranormal plasma free carnitine concentration, gave only a slight but generalized rise of long-chain acylcarnitines. Although only one patient could be tested, the results suggest that the accumulation of potentially toxic long-chain acylcarnitines in VLCAD deficiency is provoked by fasting, LCT loading and carnitine supplementation. Therapy should be adjusted accordingly.

Retrospective biochemical screening of fatty acid oxidation disorders in postmortem livers of 418 cases of sudden death in the first year of life

OBJECTIVE

Fatty acid oxidation (FAO) disorders are frequently reported as the cause of sudden and unexpected death, but their postmortem recognition remains difficult. We have devised a biochemical protocol in which informative findings in liver tissue are microvesicular steatosis, elevated concentrations of C8-C16 fatty acids, glucose depletion, and low carnitine concentration.

STUDY DESIGN

We analyzed 27 cases representing five FAO disorders and compared the results with those obtained in a retrospective blinded analysis of 418 cases of sudden infant death (313 SIDS, 45 infections, and 34 accidents and abuse).

RESULTS

All cases of accidents and abuse correctly tested negative. Among the others, 25 (6%) showed at least two abnormal findings. Of these, 14 closely matched the biochemical profiles seen in specific FAO disorders. These included 2 cases with medium-chain acyl-CoA dehydrogenase deficiency, 4 cases consistent with glutaric acidemia type 2, 4 cases with either very long-chain acylcoenzyme A dehydrogenase deficiency or long-chain 3-hydroxy-acyl-coenzyme A dehydrogenase deficiency, and 4 cases predicted to be affected with carnitine uptake defect.

CONCLUSION

The results of this study support the view that approximately 5% of all cases of sudden infant death are likely caused by an FAO disorder.

Sudden neonatal death in carnitine transporter deficiency

A newborn infant died suddenly and unexpectedly on day 5 of life. Postmortem investigations led to a suspicion of carnitine transporter deficiency, a diagnosis supported by the finding that both parents are heterozygotes for this disorder. The fasting stress caused by poor breast-feeding with no formula supplements and, possibly, the vegetarian diet of the mother were likely the critical factors leading to neonatal death, an outcome previously not described in this disorder.

Screening blood spots for inborn errors of metabolism by electrospray tandem mass spectrometry with a microplate batch process and a computer algorithm for automated flagging of abnormal profiles

Metabolic profiling of amino acids and acylcarnitines from blood spots by automated electrospray tandem mass spectrometry (ESI-MS/MS) is a powerful diagnostic tool for inborn errors of metabolism. New approaches to sample preparation and data interpretation have helped establish the methodology as a robust, high-throughput neonatal screening method. We introduce an efficient 96-well-microplate batch process for blood-spot sample preparation, with which we can obtain high-quality profiles from 500-1000 samples per day per instrument. A computer-assisted metabolic profiling algorithm automatically flags abnormal profiles. We selected diagnostic parameters for the algorithm by comparing profiles from patients with known metabolic disorders and those from normal newborns. Reference range and cutoff values for the diagnostic parameters were established by measuring either metabolite concentrations or peak ratios of certain metabolite pairs. Rigorous testing of the algorithm demonstrates its outstanding clinical sensitivity in flagging abnormal profiles and its high cumulative specificity.

Stability of long-chain and short-chain 3-hydroxyacyl-CoA dehydrogenase activity in postmortem liver

Inherited enzyme defects in mitochondrial fatty acid oxidation (FAO) are associated with acute metabolic crisis and sudden death. Necropsy findings may be subtle, yielding no diagnosis and precluding genetic counseling. Preliminary identification of an FAO disorder requires the use of sophisticated tools (e.g., GC/MS) and specific body fluids, and the diagnosis rests on molecular analysis or enzyme assay. At present, confirmation of long-chain or short-chain 3-hydroxyacyl-CoA dehydrogenase deficiency relies on measurement of enzyme activity. Here, we report our examination of the effect of storage temperature (25, 4, −20, and −70 °C) and the postmortem interval on enzyme activities in rat and human liver. Enzyme activity decreases 50% in 30 h in samples stored at 25 °C, whereas 55 h at 4 °C is required to reach this value; freezing minimizes this loss. Regardless of rate of degradation, however, the short-chain to long-chain activity ratio remains constant—which should make it possible to differentiate postmortem degradation from enzyme deficiency.