Acute liver failure in children: The role of defects in fatty acid oxidation
Authors: Estella M. Alonso
Journal: Hepatology (2005)

Acute liver failure (ALF) in childhood is an uncommon, yet devastating clinical entity. There is a wide spectrum of well-described hepatic injuries that can lead to liver failure in children, including acute presentation of Wilson's disease, autoimmune hepatitis, and drug toxicity.1-3 In contrast to adults, hepatitis caused by identifiable viral infection is a rare cause of liver failure in children in North America, a point well illustrated by the early experience of the Adult and Pediatric Acute Liver Failure Study Groups.4 The growing understanding of metabolic disorders including defects that alter β-oxidation of fatty acids and other mitochondrial functions has allowed physicians to identify these disorders in a subset of children with ALF. However, ALF in childhood is, for the most part, a syndromic diagnosis. Despite our best efforts, over half the patients that present between the ages of 2 and 12 years are classified as indeterminate, having no definable etiology for their liver injury. There are recognizable clinical patterns within this group; the school-aged child that presents with acute severe hepatitis that evolves to liver failure and aplastic anemia, the preschool-aged child that presents with abrupt onset of liver dysfunction and centrilobular hepatocyte injury following a febrile illness,5 and infants that present within the first few months of life with hypoglycemia and steatosis. Although it is rare that children with ALF are identified as having fatty acid oxidation (FAO) defects, are many others undetected? In this issue of HEPATOLOGY Shneider et al. attempt to answer this question by examining specimens from patients registered in the Pediatric Acute Liver Failure Study Group (PALFSG).6 ALF, acute liver failure; FAO, fatty acid oxidation; PALFSG, Pediatric Acute Liver Failure Study Group; MCAD, medium-chain acyl-CoA dehydrogenase, SIDS, Sudden Infant Death Syndrome; AFLP, acute fatty liver of pregnancy; HELLP, hemolysis, elevated liver function tests and low platelets; LCHAD, long-chain 3 hyroxyacyl CoA dehydrogenase. Fatty acids liberated from adipose tissue are a necessary energy source during periods of fasting (see Fig. 1). In the fasted state, mitochondrial β-oxidation of fatty acids generates ketone bodies which are then exported as an alternative fuel for cardiac and skeletal muscle. Infants and children become dependent on FAO earlier during fasting than adults due to reduced glycogen stores, immature activity of key enzymes of gluconeogenesis and glycolysis and increased basal metabolic needs.7 Defects of FAO and fatty acid transport have been associated with an array of clinical phenotypes.8 Expression of these defects can occur throughout an age range that spans from early infancy to adulthood. The best recognized clinical scenario is the infant who presents with nonketotic hypoglycemia following a minor febrile illness or a period of fasting and dehydration. Variable degrees of liver, skeletal, and cardiac muscle injury have been described in case reports. Cellular injury results from accumulation of toxic intermediate metabolites including acylcarnitines, ammonia, lactate and uric acid. Limited availability of reaction products impairs essential cellular functions including hepatic ketogenesis. Fatty Acid Oxidation Pathway. Long chain fatty acids are actively transported across the plasma membrane by several long chain fatty acid transporters (LCFAT). Carnitine is actively transported against a concentration gradient by tissue specific plasma membrane carnitine transporters (CAT). Following activation to Coenzyme A (Co-A) esters by Acyl-CoA synthetase the long chain fatty acid CoA esters (LC-Acyl-CoA) are shuttled across the mitochondrial membrane; (1) LC Acyl-CoA is transesterified to acylcarnitine by carnitine palmitoyl transferase I (CPT I) (2) LC-Acylcarnitine is transferred across the inner mitochondrial membrane by carnitine acylcarnitine translocase (CACT); (3) LC-Acylcarnitine is reesterified to LCL-Acyl-CoA by carnitine palmitoyl transferase II (CPT II). The resulting acyl-CoA ester enters the four step β oxidation spiral which produces one molecule of acetyl-CoA for every two carbons cleaved from the ester and transfers electrons to the respiratory chain via flavin adenine dinucleotide (FAD) and nicotinamide adenine denucleotide (NAD). The first step of the process is catalyzed by one of four chain length specific acyl-CoA-dehydrogenases; very long chain acyl-CoA-dehydrogenase (VLCAD), long chain acyl-CoA-dehydrogenase (LCAD), medium chain acyl-CoA-dehydrogenase (MCAD), and short chain acyl-CoA-dehydrogenase (SCAD). Medium chain and short chain fatty acids enter the mitochondrial matrix by diffusion and enter the β oxidation spiral via MCAD or SCAD. The next three reactions are catalyzed by a trifunctional protein complex (TFP) bound to the inner mitochondrial membrane. These enzymes are also chain length specific and include; enoyl-CoA-hydratase, long, medium and short chain 3-hydroxyacyl-CoA-dehydrogenase (LCHAD, MCHAD, SCHAD), and long, medium and short chain 3-ketoacyl-CoA-thiolase (LCKAT, MCKAT, SCKAT). Definitive diagnosis of FAO defects can be accomplished by functional studies and enzyme analysis of cultured skin fibroblasts or muscle biopsy specimens and by molecular techniques to identify specific genetic mutations. However, these analyses are usually not sent unless screening studies are suggestive of this type of metabolic disease. Such screening studies include urinary organic acid measurements and plasma carnitine levels with acylcarnitine profiles. Dicarboxylic acids are a byproduct of microsomal ω oxidation of fatty acids and detection of these compounds in the urine signifies increased or impaired FAO. The ratio of dicarboxylic acids to ketone bodies can help distinguish exaggerated FAO due to fasting from a metabolic defect. However, these products disappear briskly after the patient is started on a constant glucose source. Furthermore, urinary dicarboxylic acids are not elevated in transport defects of long-chain fatty acids. Thus, urinary testing cannot be used alone to screen for FAO defects. Total plasma carnitine levels are characteristically low, 25%–50% of normal, in stable patients with FAO defects. These levels may increase to near normal during illness or fasting. However, the percentage of acylcarnitines will frequently be elevated in FAO defects with the predominant ester being the intermediate product that accumulates proximal to the metabolic defect. Although these screening tests when used in combination will identify the majority of patients with FAO defects, investigators are concerned that cases are missed when patients do not have the typical patterns on screening studies. Medium-chain acyl-CoA dehydrogenase (MCAD) deficiency is the most common defect in FAO with a frequency as high as 1 in 10,000 births.9 Since the original description of MCAD deficiency in 1983, a variety of clinical syndromes have been linked to mitochondrial oxidation disorders.8 Over twenty different defects have now been described and some patients who previously would have been given syndromic diagnoses have been recognized as having FAO defects. Important examples include patients with recurrent or familial Reye's syndrome and infants with Sudden Infant Death Syndrome (SIDS). Investigators now debate whether the sharp decline in the incidence of Reye's Syndrome was due to the reduced use of salicylates in febrile children or to improved diagnosis of metabolic disorders.10-15 One study of survivors of Reye's syndrome in Australia, a country where salicylates were rarely used in children, has revealed that 69% of these previously categorized patients actually had other disorders with 26% having inherited diseases of fatty acid oxidation or carnitine metabolism.13 However, since salicylates can block β-oxidation of long chain fatty acids in control cell lines, the two etiologies may not be mutually exclusive.16 It is now well accepted that approximately 5% of infants previously classified as SIDS actually died secondary to metabolic disease. Evaluation of autopsy material from infants with sudden unexplained death revealed that an important fraction of these children also had identifiable defects in mitochondrial metabolism.17, 18 Likewise, it has been demonstrated that a significant proportion of infants born to mothers that develop acute fatty liver of pregnancy (AFLP) and HELLP syndrome (hemolysis, elevated liver function tests and low platelets) are affected with long-chain 3 hyroxyacyl CoA dehydrogenase (LCHAD) deficiency or other FAO disorders8, 19. Although acute liver injury is a part of the clinical phenotype of many of the FAO defects, liver failure is infrequently described. The usual pattern of liver injury in mitochondrial oxidation disorders includes moderately elevated transaminases, preserved coagulation factor synthesis and diffuse hepatic steatosis. However, case reports of fulminant hepatic failure in association with these defects are increasing20-25 The most notable of these descriptions of ALF was reported in association with a defect in transport of long-chain fatty acids.22 In these cases blood and urine specimens obtained during the acute illness showed minimal accumulation of dicarboxylic acids, acylglycines or acylcarnitines, the intermediate metabolites which are the usual screening markers for these disorders. However, the authors were able to demonstrate altered fatty acid uptake at the plasma membrane in skin fibroblast culture. This case illustrated the complexity of diagnosis of this spectrum of disorders and raised suspicions that some cases of sporadic ALF could be metabolic disease in disguise. In the current study, patients were selected from the prospective PALFSG cohort based on bile sample availability. This method restricted the analysis to patients who either underwent liver transplantation or had biliary specimens available at autopsy. Electronspray/tandem mass spectrometry screening of bile samples has enhanced our ability to detect these disorders and has been instrumental in establishing the relationship of FAO defects to SIDS.18 Post-mortem bile samples are more readily available than blood or urine and may actually be more reflective of acylcarnitine patterns at the hepatic mitochondrial level. Shneider et al have now used this technique to identify free and acylcarnitine patterns suggesting that some cases of indeterminant hepatitis in this cohort may actually have been the clinical expression of previously unrecognized homozygous metabolic defects. Furthermore, the abnormal patterns seen in the bile samples of patients with known liver disorders suggests that their outcomes may have been negatively impacted by heterozygous conditions. Unfortunately, the study design did not allow follow-up assessment of surviving patients for genetic or functional confirmation of these defects. It is likely that there are several distinct etiologies which will ultimately be found within the indeterminate group. As suggested by the authors, future studies should include comparison of profiles in bile with those in other more readily obtainable body fluids, assessment of FAO in skin fibroblast culture and molecular analysis for genetic defects. It is likely that this type of approach will separate patients with defects which are the primary cause of the liver injury from those in whom defects exaggerate the injury from other sources. However, the role of heterozygous defects must be explored with caution. We have previously described a 3 year old that presented with ALF of indeterminate etiology who required liver transplantation.25 Urine analysis revealed mild C6-C10 dicarboxylic and C8–C14 3-hydroxydicarboxylic aciduria in the absence of ketonuria. Plasma concentrations of medium chain free fatty acids were markedly elevated and the acylcarnitine: free carnitine ratio was 7.1 (normal ratio, 0.1–0.8). Liver histology revealed extensive zonal necrosis surrounding central veins and microvesicular steatosis in preserved periportal hepatocytes (Fig. 2). Initial molecular studies suggested two mutations in the M/SCHAD gene, but subsequent analysis revealed only one mutation in the conserved NAD-binding domain. This case highlights the difficulty in genetic interpretation. The heterozygous condition may have potentiated the liver injury or it may have been a completely unrelated finding. Important information will be gained as investigators follow the group of children who are identified through neonatal screening to have genetic defects in β-oxidation. Obviously, these children will benefit from preemptive management during febrile illness and fasting. It would be expected though, that if FAO defects are a significant cause of ALF of indeterminate etiology, then as more cases are identified early in infancy, the incidence of indeterminate hepatitis will fall. Photomicrograph of the liver demonstrating prominent centrilobular hepatocyte dropout with secondary parenchymal hemorrhage (lower left). A portal tract is present on the upper right. Marked microvesicular steatosis was also present (inset, lower right) (H&E stain, original magnification of main image = 40×, inset = 400×). Continuous intravenous glucose infusion of at least 10mg/kg/min to maintain serum glucose above 100 mg/dL. Avoidance of drugs that inhibit FAO such as valproic acid, nonsteroidal antiinflammatory agents and salicylates. Avoidance of intravenous lipid emulsions. L-carnitine therapy given either enterally or intravenously at dose of 100 mg/kg/day. More controversial interventions might include exchange transfusion or continuous hemofiltration to remove toxic intermediate metabolites. While it is difficult to conclusively recommend changes in therapy based on this preliminary report; the standard treatment of a FAO crisis poses little risk for the patient in liver failure and should be considered on an individual basis. A final note. The PALFSG provides an unmatched opportunity to better define the etiology of ALF in children. The network can collect human samples including liver tissue and skin samples to establish cell lines and confirm metabolic diagnoses. As investigators begin to separate out patients with known disorders such as FAO defects, it will become easier to recognize diagnostic clues in the remaining population. With determination, and a little luck, this study group will transform the entity of indeterminate ALF from a wastebasket diagnosis to a clearly defined array of illnesses with well-described prognostic and treatment implications.