Most fatty-acid-oxidation disorders are amenable to therapy by diet modification, and early recognition may prevent catastrophic illness
The 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-acidoxidation disorders as a significant cause of sudden, unexplained death. Fatty-acidoxidation 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).
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-longchain 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.
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-acidoxidation 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-acidoxidation defects.24
Fatty-acidoxidation defects as a cause of SIDS
Fatty-acidoxidation 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-acidoxidation disorder (either postmortem or retrospectively following the diagnosis of an affected sibling).25-30 Some patients with fatty-acidoxidation 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-acidoxidation 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-acidoxidation 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-acidoxidation 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-acidoxidation 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-acidoxidation defects. Only two of these cases were consistent with the biochemical profile seen in MCAD deficiency; the majority were suggestive of long-chain fatty-acidoxidation 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 patients 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-acidoxidation disorders.45 We also suggest that the diaphragmatic lesions may have caused dysfunction of the diaphragm and subsequent respiratory insufficiency.
Fatty-acidoxidation 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-acidoxidation 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.
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