Most fatty-acid-oxidation disorders are amenable to therapy by diet modification, and early recognition may prevent catastrophic illness

photoThe sudden, unexpected death of an infant, with no apparent cause, is tragic. Many of these cases are certified as sudden infant death syndrome (SIDS). Of course, not all sudden, unexpected deaths belong in the SIDS category. In many cases, the cause can be identified after a careful review of the circumstances of death and a complete autopsy by an experienced pathologist. The incidence of deaths certified as SIDS has gradually declined from 3 per 1,000 live births 3 decades ago to approximately 1 per 1,000 births,1 partly because of better death investigations. There is accumulating evidence that implicates pediatric fatty-acid–oxidation disorders as a significant cause of sudden, unexplained death. Fatty-acid–oxidation disorders are an emerging group of inherited, inborn errors of metabolism. The evidence that links these disorders to SIDS emphasizes disorders of mitochondrial trifunctional protein (MTP).

Fatty-acid–oxidation defects
b-Oxidation of fatty acids is the major source of energy for skeletal muscle and the heart, and it plays an essential role in the intermediary metabolism in the liver. There are four reactions in the b-oxidation spiral mediated, successively, by acyl-coenzyme A (CoA) dehydrogenase, 2-enoyl-CoA hydratase, 3-hydroxyacyl-CoA dehydrogenase, and 3-ketoacyl-CoA thiolase (Figure 1). As the acyl-CoA becomes shorter, it requires enzymes with different substrate specificity. For instance, very-long–chain acyl CoA dehydrogenase (VLCAD), medium-chain acyl CoA dehydrogenase (MCAD), and short-chain acyl CoA dehydrogenase catalyze the first step in the spiral. To date, two 3-hydroxyacyl-CoA dehydrogenases have been identified as catalyzing the third step: long-chain acyl CoA dehydrogenase (LCHAD) and short-chain 3-hydroxyacyl-CoA dehydrogenase.

figure 1

MTP is a hetero-octamer of four a and four b subunits.2,3 The a-subunit N-terminal domain contains the long-chain 3-enoyl-CoA hydratase activity, while LCHAD activity resides in the C-terminal domain. The b-subunit has the long-chain 3-ketoacyl-CoA thiolase activity. The human complementary DNAs encoding both subunits have been recently isolated and characterized.4,5 Both the a-subunit and b-subunit genes are localized to 2q24.1 through 23.3.6,7

Fatty-acid–oxidation defects are recessively inherited. Patients often present at a few months of age with an acute metabolic crisis (a Reye-like syndrome of hypoglycemia and hepatic encephalopathy) that may progress to coma and death, if not treated. Others may present with cardiomyopathy, neuromuscular manifestations, or sudden unexpected death.8 A more chronic phenotype with a slowly progressive neuromuscular dysfunction was also reported in a few patients.9 MCAD deficiency was first reported in 198310 and is considered the most common b-oxidation defect. LCHAD deficiency was first described in 1989,11 and, since that time, many LCHAD-deficient cases have been reported.4,5,12-14 The rapid recognition of this disorder suggests that it is common. Biochemical studies identified two groups of LCHAD-deficient patients.11-13,15-18 The first group had LCHAD deficiency with marked reduction in LCHAD activity, but preservation of the activity of the other two enzymes in the MTP. The second group has MTP deficiency such that all the three enzyme activities in the MTP are deficient. The majority of patients reported in the literature have only LCHAD deficiency.

In a recent report,19 we (at Wake Forest University, Winston-Salem, NC) documented the genotypes and phenotypes in 24 patients with LCHAD deficiency and MTP deficiency. Patients with the more common LCHAD deficiency carried a prevalent mutation (G1528C, E474Q) on one or both alleles and presented predominantly with a hepatic phenotype, whereas patients with MTP deficiency presented predominantly with dilated cardiomyopathy or neuromyopathy and carried mutations other than the prevalent E474Q mutation.19,20 We found that approximately 1% of the US population is heterozygous for an MTP a-subunit mutation.

An unexpected feature of this disorder is the development of acute fatty liver of pregnancy (AFLP) and/or hemolysis, elevated liver enzymes, and low platelets syndrome (HELLP) in some heterozygous women who carried LCHAD-

deficient fetuses. AFLP is a devastating disorder of the third trimester that carries a significant risk of neonatal and maternal morbidity and mortality.21,22 HELLP syndrome is also a serious third-trimester disorder, but prognosis is better than with AFLP. In a recent report,19 we studied the maternal phenotypes in 24 families with MTP defects and documented a fetal-maternal interaction; 79% of women who carried fetuses with LCHAD deficiency developed AFLP or HELLP syndrome. All fetuses in these affected pregnancies were homozygous for the G1528C mutation or compound heterozygous with one mutation at G1528C.

Affected children with defects in fatty-acid oxidation are treated with diet modifications. Diets in which more than 75% of energy comes from carbohydrate and less than 10% of energy comes from fat, with frequent feedings, are effective in treating these genetic disorders.23 Substitution of medium-chain fatty acids for long-chain fatty acids is also effective in the treatment of long-chain fatty-acid–oxidation defects.24

Fatty-acid–oxidation defects as a cause of SIDS
Fatty-acid–oxidation defects have been implicated as a significant cause of SIDS based on the deaths, certified as due to SIDS, of infants who were later found to be affected by a fatty-acid–oxidation disorder (either postmortem or retrospectively following the diagnosis of an affected sibling).25-30 Some patients with fatty-acid–oxidation defects die in their first episode of fasting intolerance; if appropriate investigations are not undertaken, these cases often meet the criteria for SIDS. There is still, however, great controversy concerning the proportion of SIDS attributable to fatty-acid–oxidation defects. For instance, Harpey et al31 studied 184 siblings of SIDS victims and found defects in b-oxidation in 15%, whereas Holton et al32 were unable to identify any b-oxidation defects in a study of 88 cases of SIDS. Others postulated that fatty-acid–oxidation defects might be responsible for as many as 5% of SIDS cases.28,29,33 These differing results probably reflect differences in the selection of cases, along with methodological variety. Most of these studies used biochemical assays, which have some limitations or associated problems, in screening for fatty-acid–oxidation defects.34 Molecular analysis in a large number of SIDS cases revealed that MCAD deficiency secondary to homozygosity for the common A985G mutation is not a significant cause of SIDS.35-38 One possible explanation for this difference between the biochemical and molecular studies is that mutations other than the common A985G may cause sudden death. In one series, 20% of all MCAD-deficient cases were not homozygous for the A985G mutation.38 More recent evidence based on the prospective screening of a large number of newborns suggests a lower frequency of homozygosity for A985G.39

A serious repercussion of the studies that focused on the prevalent mutation in MCAD is the tendency to ignore all other fatty-acid–oxidation disorders as possible causes of sudden death. In a study by Boles et al,40 the authors retrospectively studied liver tissue from 418 cases of sudden death in the first year of life (including 313 cases certified as SIDS) using biochemical assays and found 25 cases with profiles suggestive of fatty-acid–oxidation defects. Only two of these cases were consistent with the biochemical profile seen in MCAD deficiency; the majority were suggestive of long-chain fatty-acid–oxidation defects.40

MTP deficiency and LCHAD deficiency have been linked to multiple cases of sudden infant death. Similarly, there are several case reports of VLCAD deficiency presenting, initially, as sudden unexplained death.41,42

SIDS in Families with MTP Defects
Over the past few years, we have studied 35 families with documented MTP defects in their offspring. Thirteen of the 35 affected children (37%) died at presentation, and one died at 18 months of age, despite treatment. Eleven died within the first year of life. Five of these deaths were considered sudden and unexplained, and the cases were referred to the medical examiner. One was diagnosed with MTP deficiency early in life; he improved dramatically with appropriate dietary modification and transient treatment for congestive heart failure. He had normal cardiac function prior to his sudden, unexplained death at 18 months of age. Molecular analysis, in this patient, revealed two mutations leading to undetectable MTP. The second patient had evidence of mild cholestasis and vitamin-D deficiency with hypocalcemia at 2 months of age, for which he was treated with a vitamin D supplement. At 8 months of age, he died suddenly. His autopsy revealed microvesicular and macrovesicular fatty infiltration of the liver. Molecular testing for the MCAD common mutation (A985G) was negative. Only after later molecular testing for MTP mutations based on the patient’s history of sudden unexplained death, combined with a maternal history of AFLP, was a correct diagnosis made. This patient was homozygous for the common G1528C mutation.43 The third and fourth patients also had maternal histories of AFLP and died suddenly at a few months of age. Their autopsies revealed microvesicular and macrovesicular steatosis. Molecular testing revealed homozygosity for the common G1528C mutation. The fifth patient was the product of an uncomplicated pregnancy. He died suddenly at 8 months of age, and his death was certified as SIDS. A year later, during genetic counseling for a subsequent pregnancy, our testing of the DNA isolated from a paraffin block revealed homozygosity for the G1528C mutation. Six of the 35 families studied had undergone the sudden, unexpected death, certified as SIDS, of a sibling of unknown genotype. Molecular analysis, in these families, revealed the G1528C mutation on at least one allele. This suggests that molecular screening for this mutation is probably justifiable in SIDS cases with diffuse microvesicular steatosis in the liver.

A Mouse Model
Recently, we generated a mouse model for MTP deficiency.44 The induced mutation in the mouse genome causes complete absence of the MTP enzyme complex and deficiency of all three enzymes. When heterozygous mice were mated, all homozygous pups suffered sudden neonatal death between 6 and 36 hours after birth, with significant neonatal hypoglycemia and elevated liver enzymes prior to death. These observations were confirmed in more than 200 litters. Analysis of fatty-acid metabolites in the serum, urine, and liver revealed changes identical to those observed in the human deficiency. Histological analysis in a large number of mice revealed significant, acute degenerative changes and necrosis in the cardiac and diaphragmatic myocytes in the homozygotes at the time of death, as well as hepatic steatosis. Electron microscopy on liver sections showed mitochondrial swelling and distortion with significant cytoplasmic lipid accumulation.

This mouse model documents that intact long-chain fatty-acid oxidation is essential for survival after birth and confirms the observed association in humans between MTP defects and sudden, unexpected death. Furthermore, our biochemical and histopathological analyses in the MTP-deficient mice offer insight into the probable mechanism of that sudden death. The histological analyses suggest that the underlying etiology for sudden death of this kind consists of the cardiac and diaphragmatic lesions. We suggest that the cardiac lesions may have caused cardiac arrhythmias, as found in newborns with documented long-chain fatty-acid–oxidation disorders.45 We also suggest that the diaphragmatic lesions may have caused dysfunction of the diaphragm and subsequent respiratory insufficiency.

Conclusion
Fatty-acid–oxidation disorders cause sudden, unexpected death and probably account for a small number of SIDS cases. Awareness of this association is crucial. Identification of these patients, even if they make up a small proportion of SIDS cases, is important because of the impact on affected families. Most fatty-acid–oxidation disorders are amenable to therapy by diet modification, and early recognition may prevent catastrophic illness. The recent characterization of these disorders at the gene level allows molecular testing and diagnosis with certainty. In addition, identification of the molecular defect allows for genetic counseling for future pregnancies, including molecular prenatal diagnosis.46

Jamal A. Ibdah, MD, PhD, is a physician, Division of Gastroenterology, Department of Internal Medicine, Wake Forest University School of Medicine, Winston-Salem, NC.

References
1. US Bureau of the Census. Statistical Abstract of the United States. 113th ed. Washington, DC: US GPO; 1993:89.
2. Uchida Y, Izai K, Orii T, Hashimoto T. Novel fatty acid b-oxidation enzymes in rat liver mitochondria. J Biol Chem. 1992;267:1034-1041.
3. Jackson S, Kler RS, Barlett K, et al. Combined enzyme defect of mitochondrial fatty acid oxidation. J Clin Invest. 1992;90:1219-1225.
4. Sims HF, Brackett JC, Powell CK, et al. The molecular basis of pediatric long chain 3-hydroxyacyl-CoA dehydrogenase deficiency associated with maternal acute fatty liver of pregnancy. Proc Natl Acad Sci. 1995;92:841-845.
5. Kamijo T, Aoyama A, Komiyama A, Hashimoto T. Structural analysis of cDNAs for subunits of human mitochondrial fatty acid b-oxidation trifunctional protein. Biochem Biophys Res Commun. 1994;199:818-825.
6. Ushikubo S, Aoyama T, Kamijo T, et al. Molecular characterization of mitochondrial trifunctional protein deficiency: formation of the enzyme complex is important for stabilization of both a- and b-subunits. Am J Hum Genet. 1996;58:979-988.
7. Ijlst L, Ruiter JP, Hoovers JM, Jakobs ME, Wanders RJ. Common missense mutation G1528C mutation in long-chain 3-hydroxyacyl-CoA dehydrogenase deficiency. J Clin Invest. 1996;98:1028-1033.
8. Pollitt RJ. Disorders of mitochondrial long-chain fatty acid oxidation. J Inherit Metab Dis. 1995;18:473-490.
9. Tein I, Donner EJ, Hale DE, Murphy EG. Clinical and neurophysiologic response of myopathy and neuropathy in long-chain L-3-hydroxyacyl-CoA dehydrogenase deficiency to oral prednisone. Ped Neurol. 1995;12:68-76.
10. Tanaka K, Yokota I, Coates PM, et al. Mutations in the MCAD gene. Human Mutation. 1992;1:271-279.
11. Wanders RJA, Duran M, Ijlst L, et al. Sudden infant death and long-chain 3-hydroxyacyl-CoA dehydrogenase. Lancet. 1989;II:52-53.
12. Ijlst L, Ushikubo S, Kamijo T, et al. Long-chain 3-hydroxyacyl-CoA dehydrogenase deficiency: high frequency of the G1528C mutation with no apparent correlation with the clinical phenotype. J Inherit Metabol Dis. 1995;18:241-244.
13. Hagenfeldt N, Venizelos N, Dobeln U. Clinical and biochemical presentation of long chain 3-hydroxyacyl-CoA dehydrogenase deficiency. J Inherit Metabol Dis. 1995;18:245-248.
14. Tyni T, Palotie A, Viinikka L, et al. Long-chain 3-hydroxyacyl-coenzyme A dehydrogenase deficiency with the G1528C mutation: clinical presentation in 13 patients. J Pediatr. 1997;130:67-76.
15. Kamijo T, Wanders RJA, Saudubray JM, et al. Mitochondrial trifunctional protein deficiency. J Clin Invest. 1994;93:1740-1747.
16. Brackett JC, Sims HF, Rinaldo P, et al. Two a-subunit donor splice site mutations cause human trifunctional protein deficiency. J Clin Invest. 1995;95:2076-2082.
17. Isaacs JD, Sims HF, Powell CK, et al. Maternal acute fatty liver of pregnancy associated with fetal MTP deficiency. Pediatr Res. 1996;40:393-398.
18. Ijlst L, Wanders RJA, Ushikubo S, Kamijo T, Hashimoto T. Molecular basis of long-chain 3-hydroxyacyl-CoA dehydrogenase deficiency: identification of the major disease-causing mutation in the a-subunit of the mitochondrial trifunctional protein. Biochem Biophys Acta. 1994;1215:347-350.
19. Ibdah JA, Bennett MJ, Rinaldo P, et al. A fetal fatty acid oxidation disorder as a cause of liver disease in pregnant women. N Engl J Med. 1999;340:1723-1731.
20. Ibdah JA, Tein I, Dionisi-Vici C, et al. Mild trifunctional protein deficiency is associated with progressive neuropathy and myopathy and suggests a novel genotype-phenotype correlation. J Clin Invest. 1998;102:1193-1199.
21. Riely CA. Acute fatty liver of pregnancy. Semin Liver Dis. 1987;7:47-54.
22. Knox TA, Olans LB. Liver disease in pregnancy. N Engl J Med. 1996;335:569-576.
23. Przyrembel H, Jacobs C, Wanders RJA. Long chain 3-hydroxyacyl-CoA dehydrogenase deficiency. J Inherit Metabol Dis. 1991;14:674-680.
24. Brown-Harrison MC, Nada MA, Roe CR. Very long chain acyl-CoA dehydrogenase deficiency: successful treatment of acute cardiomyopathy. Biochem Mol Med. 1996;58:59-65.
25. Bennett MJ, Allison F, Pollitt RJ, Variend S. Fatty acid oxidation defects as causes of unexpected death in infancy. Prog Clin Biol Res. 1990;321:349-364.
26. Sudden infant death and inherited disorders of fat oxidation. Lancet. 1986;II:1073-1075.
27. Bennett MJ, Hale DE, Coates PM, Stanley CA. Postmortem recognition of fatty acid oxidation disorders. Pediatr Pathol. 1991;11:365-370.
28. Boles RG, Martin SK, Blitzer MG, Rinaldo P. Biochemical diagnosis of fatty acid oxidation disorders by metabolite analysis of postmortem liver. Hum Pathol. 1994;25:735-741.
29. Bennett MJ, Powell S. Metabolic diseases and sudden, unexpected death in infancy. Hum Pathol. 1994;25:742-746.
30. Howat AJ, Bennett MJ, Shaw L, Variend S. Medium chain acylcoenzyme A deficiency presenting as sudden infant death syndrome. BMJ. 1984;288:397.
31. Harpey JP, Charpentier C, Paturneau-Jouas M. Sudden infant death syndrome and inherited disorders of fatty acid b-oxidation. Biol Neonate. 1990;58:S70-S80.
32. Holton JB, Allen JT, Green CA. Inherited metabolic diseases in the sudden infant death syndrome. Arch Dis Child. 1991;66:1315-1317.
33. Pollitt RJ. Defects in mitochondrial fatty acid oxidation: clinical presentation and their role in sudden infant death. Pädiatrie und Padologie. 1993;28:13-17.
34. Bonham JR, Downing M. Metabolic deficiencies and SIDS. J Clin Pathol. 1992;45:S33-S38.
35. Arens R, Gozal D, Jain K, et al. Prevalence of medium-chain acyl-coenzyme A dehydrogenase deficiency in the sudden infant death syndrome. J Pediatr. 1993;122:715-718.
36. Dundra M, Lanyon WG, Connor JM. Scottish frequency of the common G985 mutation in the medium-chain acyl-CoA dehydrogenase (MCAD) gene and the role of MCAD deficiency in sudden infant death syndrome (SIDS). J Inherit Metabol Dis. 1993;16:991-993.
37. Miller ME, Brooks JG, Forbes N, Insel R. Frequency of medium-chain acyl-CoA dehydrogenase deficiency G-985 mutation in sudden infant death syndrome. Pediatr Res. 1992;31:305-307.
38. Anderson BS, Bross P, Jensen TG, et al. Molecular diagnosis and characterization of medium-chain acyl-CoA dehydrogenase deficiency. Scand J Clin Lab Invest. 1995;220:9-25.
39. Andresen BS, Dobrowolski SF, O’Reilly L, et al. Medium-chain acyl-CoA dehydrogenase (MCAD) mutations identified by MS/MS-based prospective screening of newborns differ from those observed in patients with clinical symptoms: identification and characterization of a new, prevalent mutation that results in mild MCAD deficiency. Am J Hum Genet. 2001;68:1408-1418.
40. Boles RG, Buck EA, Blitzer MG, et al Retrospective biochemical screening of fatty acid oxidation disorders in postmortem livers of 418 cases of sudden death in the first year of life. J Pediatr. 1998;132:924-933.
41. Strauss AW, Powell CK, Hale DE, et al. Molecular basis of human mitochondrial very-long-chain acyl-CoA dehydrogenase deficiency causing cardiomyopathy and sudden death in childhood. Proc Natl Acad Sci. 1995;92:10496-10500.
42. Mathur A, Sims HF, Gopalakrishnan D, et al. Molecular heterogeneity in very-long-chain acyl-CoA dehydrogenase deficiency causing pediatric cardiomyopathy and sudden death. Circulation. 1999;99:1337-1341.
43. Ibdah JA, Dasouki MJ, Strauss AW. Long chain 3-hydroxyacyl-CoA deficiency: variable expressivity of maternal illness and unusual presentation with infantile hypocalcemia and cholestasis. J Inherit Metabol Dis. 1999;22:811-814.
44. Ibdah JA, Paul H, Zhao Y, et al. Lack of mitochondrial trifunctional protein in mice causes neonatal hypoglycemia and sudden death. J Clin Invest. 2001;107:1403-1409.
45. Bonnet D, Martin D, De Lonlay P, et al. Arrhythmias and conduction defects as presenting symptoms of fatty acid oxidation disorders in children. Circulation. 1999;100:2248-2253.
46. Ibdah JA, Zhao Y, Viola J, Gibson B, Bennett MJ, Strauss AW. Molecular prenatal diagnosis in families with mitochondrial trifunctional protein mutations. J Pediatr. 2001;138:396-399.