December 2021 — Biochemical Genetics

Carnitine uptake defect

Several metabolic disorders can present with sudden unexpected childhood death. These include mitochondrial β-oxidation disorders, mitochondrial respiratory-chain disorders, and several organic acidemias. Many of these metabolic disorders can be detected postmortem by examining acylcarnitines in spotted blood and bile samples.

Carnitine uptake defect (CUD)

Carnitine is essential for the transport of long-chain fatty acids over the mitochondrial inner membrane. CUD is an autosomal recessive disease caused by defects in the organic cation/carnitine transporter (OCNT2) — a high-affinity carnitine transporter encoded by the gene SLC22A5. OCNT2 is responsible for carnitine uptake in the kidney, heart, muscle, and fibroblast but not in the liver. Defects of the carnitine transporter impair renal reabsorption and muscle uptake of carnitine and, consequently, cause primary carnitine deficiency, characterized by extremely low plasma carnitine concentration and impairment of mitochondrial fatty acid oxidation. 

Symptoms may present in infancy (usually before 2 years of age) with hypoglycemia, liver dysfunction, and hyperammonemia. Sudden and unexpected deaths have been reported. Alternatively, symptoms may present later in childhood with progressive cardiomyopathy, often accompanied by skeletal muscle weakness, or in adulthood with fatigue and arrhythmias. 

Acylcarnitine analysis of dried blood spots from untreated individuals with CUD shows abnormally low levels of free carnitine and acylcarnitines (low total carnitine). 

Glutaric aciduria type II (GA-II)

GA-II is caused by the inability to transfer electrons from multiple flavoprotein acyl-CoA dehydrogenases to the respiratory chain. These enzymes are involved in various metabolic pathways, including fatty acid β-oxidation and amino acid catabolism. The condition is consequently also known as multiple acyl-CoA dehydrogenase deficiency. 

The metabolic defects are, most commonly, in the electron transport flavoprotein (ETF) and the ETF-ubiquinone oxidoreductase, which together transfer electrons from mitochondrial FAD-linked dehydrogenases to the ubiquinone pool of the respiratory chain. GA-II also can result from defects of riboflavin transport and processing. 

Symptoms may present at any age and vary widely in severity. The severe, neonatal-onset form presents with hypoketotic hypoglycemia, metabolic acidosis, and hyperammonemia in the first days of life. Congenital anomalies may be present, including cystic kidneys, facial dysmorphism, and neuronal migration defects. Many affected individuals die within a week of birth. Those that survive the neonatal period may develop cardiomyopathy and die within months. The milder form can present at any age and is most often associated with muscle weakness, exercise intolerance, and muscle pain. 

Acylcarnitine analysis of blood spots from individuals with GA-II typically shows increased levels of multiple acylcarnitines, from C₄ to C₁₆, reflecting the various metabolic pathways affected. 

Medium-chain acyl-CoA dehydrogenase (MCAD) deficiency

MCAD is a FAD-linked dehydrogenase that catalyzes the first step of the mitochondrial β-oxidation spiral, i.e., the oxidation of the α-β carbon bond of acyl-CoAs, with a high affinity towards medium-chain length fatty acids (C₆-C₁₀).

MCAD deficiency is an autosomal recessive disorder caused by defects of the gene ACADM, which leads to impaired mitochondrial oxidation of medium chain-length fatty acids. Affected individuals appear normal until an episode of acute metabolic decompensation is provoked by a period of fasting, often concurrent with an infection. The first episode of acute metabolic decompensation may occur at any age — from the neonate period to adulthood. A small percentage develop severe, life-threatening symptoms during the first week of life, at times, before the result from newborn screening are available. Before widespread newborn screening for MCAD deficiency, affected individuals typically presented between 3 months and 24 months with hypoketotic hypoglycemia, encephalopathy, and liver dysfunction. Unexpected death during the first metabolic decompensation was common. 

Acylcarnitine analysis of dried blood spots and bile samples from MCAD-deficient individuals show elevated levels of medium chain-length acylcarnitines (C₆-C₁₀), most often with the characteristic pattern of C₆<C₈>C₁₀.

Very-long-chain Acyl-CoA dehydrogenase (VLCAD) deficiency 

VLCAD is a FAD-linked dehydrogenase that catalyzes the first step of mitochondrial β-oxidation with an affinity toward long-chain fatty acids (C₁₄-C₂₀).

VLCAD deficiency is an autosomal recessive disorder caused by a defect in the gene ACADVL, which leads impaired mitochondrial oxidation of long chain-length fatty acids. Three distinct phenotypes have been observed: a severe neonatal form that presents with cardiomyopathy, hypotonia, and hepatomegaly; a childhood-onset form that presents with hypoketotic hypoglycemia and hepatomegaly, but without cardiomyopathy; and a late-onset form that presents with exercise- or illness-induced rhabdomyolysis, muscle weakness, and exercise intolerance. VLCAD deficiency may result in unexpected and sudden death if not detected and treated. 

Acylcarnitine analysis of dried blood spots and bile samples from VLCAD deficient individuals show elevated levels of long chain-length acylcarnitines (C₁₄-C₁₈), particularly C_14:1.

References

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