Defects in Metabolism of Lipids

Clinical Manifestations

Previously undiagnosed affected patients usually present in the first 3 mo-5 yr of life with episodes of acute illness triggered by prolonged fasting (longer than 12-16 hr). Signs and symptoms include vomiting and lethargy, which rapidly progress to coma or seizures and cardiorespiratory collapse. Sudden unexpected infant death may occur. The liver may be slightly enlarged with fat deposition. Attacks are rare until the infant is beyond the first few months of life, presumably because of more frequent feedings at a younger age. Affected older infants are at higher risk of illness as they begin to fast through the night or are exposed to fasting stress during an intercurrent childhood illness. Presentation in the first days of life with neonatal hypoglycemia has been reported in newborns that were fasted inadvertently. Diagnosis of MCAD has occasionally been documented in previously healthy teenage and adult individuals, indicating that even patients who have been asymptomatic in infancy are still at risk for metabolic decompensation if exposed to sufficient periods of fasting. An unknown number may remain asymptomatic. Prior to routine newborn screening testing, as many as 25% of MCAD deficient cases died or suffered severe brain damage from their first episode. Most patients are now diagnosed in the newborn period by blood spot acylcarnitine screening, allowing the initiation of early treatment and prevention of many of the severe signs and symptoms.

Laboratory Findings

During acute episodes, hypoglycemia is usually present. Plasma and urinary ketone concentrations are inappropriately low (hypoketotic hypoglycemia). Because of the hypoketonemia, there is little or no metabolic acidosis, which is expected to be present in many children with hypoglycemia. Tests of liver function are abnormal, with elevations of liver enzymes (alanine aminotransferase, aspartate aminotransferase), elevated blood ammonia, and prolonged prothrombin and partial thromboplastin times. Liver biopsy at times of acute illness shows microvesicular or macrovesicular steatosis from triglyceride accumulation. During fasting stress or at times of acute illness, urinary organic acid profiles by gas chromatography/mass spectrometry show inappropriately low concentrations of ketones and elevated levels of medium-chain dicarboxylic acids (adipic, suberic, and sebacic acids) that derive from microsomal and peroxisomal omega oxidation of accumulated medium-chain fatty acids. Plasma and tissue concentrations of total carnitine are reduced to 25-50% of normal, and the fraction of total esterified carnitine is increased. This pattern of secondary carnitine deficiency is seen in most fatty acid oxidation defects and reflects competition between increased acylcarnitine levels and free carnitine for transport at the plasma membrane. Significant exceptions to this rule are the plasma membrane carnitine transporter, CPT-IA and β-hydroxy-β-methylglutaryl-CoA (HMG-CoA) synthase deficiencies that do not manifest secondary carnitine deficiency.

Diagnostic metabolite patterns include increased plasma C_8:0, C_10:0, and C_10:1 acylcarnitine species and increased urinary acylglycines including hexanoyl-propionyl, suberyl-propionyl, and 3-phenylpropionyl glycines. Newborn screening programs using tandem mass spectrometry, which almost all babies born in the United States receive, can diagnose presymptomatic MCAD deficiency based on the detection of the abnormal acylcarnitines in filter paper blood spots. In many cases, the diagnosis can be confirmed by finding the common A985G mutation. A second common variant, T199C, has been detected in infants with characteristic acylcarnitines in newborn screening tests. Interestingly, this allele has not been seen to date in symptomatic MCAD patients; it may represent a milder mutation.

Treatment

Acute illnesses should be promptly treated with intravenous fluids containing 10% dextrose to treat or prevent hypoglycemia and to suppress lipolysis as rapidly as possible (see Chapter 92). Chronic therapy consists of avoiding fasting. This usually requires simply adjusting the diet to ensure that overnight fasting periods are limited to <10-12 hr. Restricting dietary fat or treatment with carnitine is controversial. The necessity for active therapeutic intervention for individuals with the T199C variant has not yet been established.

Prognosis

Up to 25% of unrecognized patients may die during their first attack of illness. There is frequently a history of a previous sibling death that is presumed to be from an unrecognized MCAD deficiency. Some patients may suffer permanent brain injury during an attack of profound hypoglycemia. The prognosis for survivors without brain damage is excellent because progressive cognitive impairment or cardiomyopathy does not occur in MCAD deficiency. Muscle pain and reduced exercise tolerance may become evident with increasing age. Fasting tolerance improves with age and the risk of illness decreases. Because as many as 35% of affected patients have never had an episode, testing of siblings of affected patients is important to detect asymptomatic family members.

Very-Long-Chain Acyl-Coenzyme A Dehydrogenase Deficiency

Very-long-chain acyl-CoA dehydrogenase (VLCAD) deficiency is the second most commonly diagnosed disorder of fatty acid oxidation. It was originally termed long-chain acyl-CoA dehydrogenase deficiency before the existence of the inner mitochondrial membrane-bound VLCAD was known. All patients previously diagnosed as having long-chain acyl-CoA dehydrogenase deficiency have VLCAD gene defects. Patients with VLCAD deficiency have no ability to oxidize physiologic long-chain fatty acids and are usually more severely affected than those with MCAD deficiency who have a milder oxidative defect. VLCAD deficiency presents earlier in infancy and has more chronic problems with muscle weakness or episodes of muscle pain and rhabdomyolysis. Cardiomyopathy may be present during acute attacks provoked by fasting. The left ventricle may be hypertrophic or dilated and show poor contractility on echocardiography. Sudden unexpected death has occurred in several patients, but most who survived the initial episode showed improvement, including normalization of cardiac function. Other physical and routine laboratory features are similar to those of MCAD deficiency, including secondary carnitine deficiency. The urinary organic acid profile shows a nonketotic dicarboxylic aciduria with increased levels of C_6-12 dicarboxylic acids. Diagnosis may be suggested by an abnormal acylcarnitine profile with plasma or blood spot C_{14:0, 14:1, 14:2} acylcarnitine species; however, the specific diagnosis requires mutational analysis of the VLCAD gene. Treatment is based primarily on avoidance of fasts for longer than 10-12 hr. Continuous intragastric feeding is useful in some patients.

Short-Chain Acyl-Coenzyme A Dehydrogenase Deficiency

A small number of patients with 2 clear null mutations in the short-chain acyl-CoA dehydrogenase (SCAD) gene have been described with variable phenotype. Most individuals classified as being SCAD deficient have polymorphic DNA changes in the SCAD gene; for example, 2 common polymorphisms are G185S and R147W, which are homozygously present in 7% of the population. Some investigators argue that these may be susceptibility changes, which require a second, as yet unknown, genetic mutation to express a clinical phenotype; while others believe that SCAD deficiency is a harmless biochemical condition. This autosomal recessive disorder presents with neonatal hypoglycemia and may have normal levels of ketone bodies. The diagnosis is indicated by elevated levels of butyrylcarnitine (C4-carnitine) on newborn blood spots or plasma and increased excretion of urinary ethylmalonic acid and butyrylglycine. These metabolic abnormalities are most pronounced in patients with null mutations and variably present in patients who are homozygous for the common polymorphisms.

The necessity for treatment in SCAD deficiency has not yet been established. It has been proposed that long-term evaluation of asymptomatic individuals is necessary to determine whether this is or is not a real disease. Although most individuals with SCAD deficiency remain asymptomatic throughout life, it has been proposed that there is a subset of individuals with SCAD deficiency with severe manifestations, including dysmorphic facial features, feeding difficulties/failure to thrive, metabolic acidosis, ketotic hypoglycemia, lethargy, developmental delay, seizures, hypotonia, dystonia, and myopathy.

Long-Chain 3-Hydroxyacyl-CoA Dehydrogenase/Mitochondrial Trifunctional Protein Deficiency

The LCHAD enzyme is part of a MTP, which also contains 2 other steps in β-oxidation: long-chain enoyl CoA hydratase and long-chain β-ketothiolase. It is a heterooctameric protein composed of 4 α and 4 β chains that derive from distinct contiguous genes with a common promoter region. In some patients, only the LCHAD activity of the MTP is affected (LCHAD deficiency), whereas others have deficiencies of all 3 activities (MTP deficiency).

Clinical manifestations include attacks of acute hypoketotic hypoglycemia similar to MCAD deficiency; patients often show evidence of more severe disease, including cardiomyopathy, muscle cramps and weakness, and abnormal liver function (cholestasis). Toxic effects of fatty acid metabolites may produce pigmented retinopathy leading to blindness, progressive liver failure, peripheral neuropathy, and rhabdomyolysis. Life-threatening obstetric complications, acute fatty liver of pregnancy, and HELLP syndrome are observed in heterozygous mothers carrying homozygotic fetuses affected with LCHAD/MTP deficiency. Sudden unexpected infant death may occur. The diagnosis is indicated by elevated levels of blood spot or plasma 3-hydroxy acylcarnitines of chain lengths C₁₆-C₁₈. Urinary organic acid profile in patients may show increases in levels of 3-hydroxydicarboxylic acids of chain lengths C₆-C₁₄. Secondary carnitine deficiency is common. A common mutation in the α subunit, E474Q, is seen in more than 60% of LCHAD-deficient patients. This mutation in the fetus is especially associated with the obstetric complications, but other mutations in either subunit may also be linked to maternal illness.

Treatment is similar to that for MCAD or VLCAD deficiency; that is, avoiding fasting stress. Some investigators have suggested that dietary supplements with medium-chain triglyceride oil to bypass the defect in long-chain fatty acid oxidation and docosahexaenoic acid (for protection against the retinal changes) may be useful. Liver transplantation has been attempted in cases with severe liver failure, but does not ameliorate the metabolic abnormalities or prevent the myopathic or retinal complications.

Short-Chain 3-Hydroxyacyl-Coenzyme A Dehydrogenase Deficiency

Only 12 patients with proven mutations of short-chain 3-hydroxyacyl-CoA dehydrogenase (SCHAD) have been reported, although a few additional unpublished cases are known to the authors. Most cases with recessive mutations of the SCHAD gene have presented with episodes of hypoketotic hypoglycemia that was caused by hyperinsulinism. In contrast to patients with other forms of fatty acid oxidation disorders, these cases required specific therapy with diazoxide for hyperinsulinism to avoid recurrent hypoglycemia. A single case with compound heterozygous mutations presented with fulminant hepatic failure at age 10 mo. The SCHAD protein has a nonenzymatic function (moonlighting) in which it directly interacts with glutamate dehydrogenase (GDH) to inhibit its activity. In the absence of an SCHAD protein, this inhibition is removed leading to upregulation of GDH enzyme activity, a recognized cause of hyperinsulinism usually caused by activating mutations of the GDH gene. This severe deficiency of SCHAD protein often presents predominantly as protein sensitive hypoglycemia rather than as fasting hypoglycemia. It appears that if a SCHAD protein is present the inhibition of GDH is maintained even when there is no SCHAD enzyme activity; these patients may present with a more traditional fatty acid oxidation defect. Specific metabolic markers for SCHAD deficiency include elevated plasma C4-hydroxy acylcarnitine and urine 3-hydroxyglutaric acid.

Treatment of SCHAD deficient patients with hyperinsulinism is with diazoxide. There is insufficient experience with the non-hyperinsulinemic form of SCHAD deficiency at present to recommend treatment modalities, but prevention of fasting seems advisable, which is similar to other fatty acid oxidation disorders.

Carnitine Palmitoyltransferase-IA Deficiency

Several dozen infants and children have been described with a deficiency of the liver and kidney carnitine palmitoyltransferase-I (CPT-I) isozyme (CPT-IA). Clinical manifestations include fasting hypoketotic hypoglycemia, occasionally with markedly abnormal liver function tests and, rarely, with renal tubular acidosis. The heart and skeletal muscle are not involved because the muscle isozyme is unaffected. Fasting urinary organic acid profile sometimes shows a hypoketotic C₆-C₁₂ dicarboxylic aciduria but may be normal. Plasma acylcarnitine analysis demonstrates mostly free carnitine with very little acylated carnitine. This observation has been used to establish CPT-IA diagnosis on newborn screening by tandem mass spectrometry. CPT-IA deficiency is the only fatty acid oxidation disorder in which plasma total carnitine levels are elevated often to 150-200% of normal. This may be explained by the fact that the inhibitory effects of long-chain acylcarnitines on the renal tubular carnitine transporter are absent in CPT-IA deficiency. The enzyme defect can be demonstrated in cultured fibroblasts or lymphoblasts. CPT-IA deficiency in the fetus has been associated with acute fatty liver of pregnancy in the mother in a single case report. A common variant in the CPT-IA gene has been identified in individuals of Inuit background in the United States and First Nations tribes in Canada and Greenland. The variant is detected by a positive newborn acylcarnitine screen; enzyme activity is reduced by 80% and regulation by malonyl-CoA is lost. It has not been established if this is a pathologic DNA variant or an adaptation to ancient Inuit and First Nations high-fat diets. This variant is associated with an increased risk for sudden infant death syndrome. Treatment for the severe form of CPT-IA deficiency is similar to that for MCAD deficiency with avoidance of situations where fasting ketogenesis is necessary.

Carnitine:Acylcarnitine Translocase Deficiency

This defect of the inner mitochondrial membrane carrier protein for fatty acylcarnitines blocks the entry of long-chain fatty acids into the mitochondria for oxidation. The clinical phenotype of this disorder is characterized by a severe and generalized impairment of fatty acid oxidation. Most newborn patients present with attacks of fasting-induced hypoglycemia, hyperammonemia, and cardiorespiratory collapse. All symptomatic newborns have had evidence of cardiomyopathy and muscle weakness. Several patients with a partial translocase deficiency and milder disease without cardiac involvement have also been identified. No distinctive urinary or plasma organic acids are noted, although increased levels of plasma long-chain acylcarnitines of chain lengths C₁₆-C₁₈ are reported. Diagnosis can be confirmed using genetic analysis. Functional carnitine:acylcarnitine translocase activity can be measured in cultured fibroblasts or lymphoblasts. Treatment is similar to that of other long-chain fatty acid oxidation disorders.

Carnitine Palmitoyltransferase-II Deficiency

Three forms of CPT-II deficiency have been described. The severe neonatal lethal presentation of this disorder is associated with a profound enzyme deficiency, and early death has been reported in several newborns with dysplastic kidneys, cerebral malformations, and mild facial anomalies. A milder, second defect, is associated with an adult presentation of episodic rhabdomyolysis. The first episode usually does not occur until late childhood or early adulthood. Attacks may be precipitated by prolonged exercise. There is aching muscle pain and myoglobinuria that may be severe enough to cause renal failure. Serum levels of creatine kinase are elevated to 5,000-100,000 units/L. Fasting hypoglycemia has not been described, but fasting may contribute to attacks of myoglobinuria. Muscle biopsy shows increased deposition of neutral fat. The myopathic presentation of CPT-II deficiency is associated with a common mutation S113L. This mutation produces a heat-labile protein that is unstable to increased muscle temperature during exercise resulting in the myopathic presentation. The third intermediate form of CPT-II deficiency presents in infancy/early childhood with fasting-induced hepatic failure, cardiomyopathy, and skeletal myopathy with hypoketotic hypoglycemia, but does not have the severe developmental changes seen in the neonatal lethal presentation. This pattern is similar to that seen in VLCAD deficiency and management is identical.

Diagnosis of all forms of CPT-II deficiency can be made by a combination of molecular analysis and demonstrating deficient enzyme activity in muscle or other tissues and in cultured fibroblasts.

β-Hydroxy-β-Methylglutaryl-Coenzyme A Lyase Deficiency

See Chapter 85.6.

Succinyl-Coenzyme A:3-Ketoacid-Coenzyme A Transferase Deficiency

See Chapter 85.6.

Several patients with succinyl-CoA:3-ketoacid-CoA transferase (SCOT) deficiency have been reported. The characteristic presentation is an infant with recurrent episodes of severe ketoacidosis induced by fasting. Plasma acylcarnitine and urine organic acid abnormalities do not distinguish SCOT deficiency from other causes of ketoacidosis. Treatment of episodes requires infusion of glucose and large amounts of bicarbonate until metabolically stable. Patients usually exhibit inappropriate hyperketonemia even between episodes of illness. SCOT is responsible for activating acetoacetate in peripheral tissues using succinyl CoA as a donor to form acetoacetyl-CoA. Deficient enzyme activity can be demonstrated in brain, muscle, and fibroblasts from affected patients. The gene has been cloned, and numerous mutations have been characterized.

β-Ketothiolase Deficiency

See Chapter 85.6.

Etiology

Peroxisomal disorders are subdivided into 2 major categories (Table 86-2).

In category A, the peroxisomal biogenesis disorders (PBDs), the basic defect is the failure to import 1 or more proteins into the organelle. In category B, defects affect a single peroxisomal protein. The peroxisome is present in all cells except mature erythrocytes and is a subcellular organelle surrounded by a single membrane; more than 50 peroxisomal enzymes are identified. Some enzymes are involved in the production and decomposition of hydrogen peroxide; others are concerned with lipid and amino acid metabolism. Most peroxisomal enzymes are first synthesized in their mature form on free polyribosomes and enter the cytoplasm. Proteins that are destined for the peroxisome contain specific peroxisome targeting sequences (PTSs). Most peroxisomal matrix proteins contain PTS1, a 3-amino acid sequence at the carboxyl terminus. PTS2 is an aminoterminal sequence that is critical for the import of enzymes involved in plasmalogen and branched-chain fatty acid metabolism. Import of proteins involves a complex series of reactions that involves at least 23 distinct proteins. These proteins, referred to as peroxins, are encoded by PEX genes.

Epidemiology

Except for X-linked adrenoleukodystrophy (ALD), all the peroxisomal disorders in Table 86-2 are autosomal recessive traits. ALD is the most common peroxisomal disorder, with an estimated incidence of 1 in 17,000 live births. The combined incidence of the other peroxisomal disorders is estimated to be 1 in 50,000 live births.

Pathology

Absence or reduction in the number of peroxisomes is pathognomonic for disorders of peroxisome biogenesis. In most disorders, there are membranous sacs that contain peroxisomal integral membrane proteins, which lack the normal complement of matrix proteins; these are peroxisome “ghosts.” Pathologic changes are observed in most organs and include profound and characteristic defects in neuronal migration; micronodular cirrhosis of the liver; renal cysts; chondrodysplasia punctata; sensorineural hearing loss; retinopathy; congenital heart disease; and dysmorphic features.

Pathogenesis

It is likely that all pathologic changes are secondary to the peroxisome defect. Multiple peroxisomal enzymes fail to function in the PBDs (Table 86-3). The enzymes that are diminished or absent are synthesized but are degraded abnormally fast because they may be unprotected outside of the peroxisome. It is not clear how defective peroxisome functions lead to the widespread pathologic manifestations.

Mutations in 12 different PEX genes have been identified in PBDs. The pattern and severity of pathologic features vary with the nature of the import defects and the degree to which import is impaired. These gene defects lead to disorders that were named before their relationship to the peroxisome was recognized, namely, Zellweger syndrome, neonatal ALD, infantile Refsum disease, and rhizomelic chondrodysplasia punctata (RCDP). The first 3 disorders are considered to form a clinical continuum, with Zellweger syndrome the most severe, infantile Refsum disease the least severe, and neonatal ALD intermediate. They can be caused by mutations in any of the 11 genes involved in peroxisome assembly. The specific gene defects cannot be distinguished on the basis of clinical features. The clinical severity varies with the degree to which protein import is impaired. Mutations that abolish import completely are often associated with the Zellweger syndrome phenotype, whereas a missense mutation, in which some degree of import function is retained, leads to the somewhat milder phenotypes. A defect in PEX7, which involves the import of proteins that utilize PTS2, is associated with RCDP. PEX7 defects that leave import partially intact are associated with milder phenotypes, some of which resemble classic Refsum disease.

The genetic disorders that involve single peroxisomal enzymes usually have clinical manifestations that are more restricted and relate to the single biochemical defect. The primary adrenal insufficiency of ALD is caused by accumulation of very-long-chain fatty acids (VLCFAs) in the adrenal cortex, and the peripheral neuropathy in Refsum disease is caused by the accumulation of phytanic acid in Schwann cells and myelin.

Peroxisomal Biogenesis Disorders with Milder or Atypical Phenotypes

Newborn infants with Zellweger syndrome show striking and consistent recognizable abnormalities. Of central diagnostic importance are the typical facial appearance (high forehead, unslanting palpebral fissures, hypoplastic supraorbital ridges, and epicanthal folds; Fig. 86-3), severe weakness and hypotonia, neonatal seizures, and eye abnormalities. Because of the hypotonia and craniofacial appearance, Down syndrome may be suspected. Infants with Zellweger syndrome rarely live more than a few months. More than 90% show postnatal growth failure. Table 86-4 lists the main clinical abnormalities.

Table 86-4

Main Clinical Abnormalities in Zellweger Syndrome

ABNORMAL FEATURE	Cases in Which the Feature Was Present
	NO.	%
High forehead	58	97
Flat occiput	13	81
Large fontanelle(s), wide sutures	55	96
Shallow orbital ridges	33	100
Low/broad nasal bridge	23	100
Epicanthus	33	92
High arched palate	35	95
External ear deformity	39	97
Micrognathia	18	100
Redundant skin fold of neck	13	100
Brushfield spots	5	83
Cataract/cloudy cornea	30	86
Glaucoma	7	58
Abnormal retinal pigmentation	6	40
Optic disc pallor	17	74
Severe hypotonia	94	99
Abnormal Moro response	26	100
Hyporeflexia or areflexia	56	98
Poor sucking	74	96
Gavage feeding	26	100
Epileptic seizures	56	92
Psychomotor retardation	45	100
Impaired hearing	9	40
Nystagmus	30	81

Patients with neonatal ALD show fewer, less-prominent craniofacial features. Neonatal seizures occur frequently. Some degree of psychomotor developmental delay is present; function remains in the severely or profoundly retarded range, and development may regress after 3-5 yr of age, probably from a progressive leukodystrophy. Hepatomegaly, impaired liver function, pigmentary degeneration of the retina, and severely impaired hearing are invariably present. Adrenocortical function is usually impaired and may require adrenal hormone replacement. Chondrodysplasia punctata and renal cysts are absent.

Patients with infantile Refsum disease have survived to adulthood. They are able to walk, although gait may be ataxic and broad based. Cognitive function is in the severely impaired range. All have sensorineural hearing loss and pigmentary degeneration of the retina. They have moderately dysmorphic features that may include epicanthal folds, a flat bridge of the nose, and low-set ears. Early hypotonia and hepatomegaly with impaired function are common. Levels of plasma cholesterol and high- and low-density lipoprotein are often moderately reduced. Chondrodysplasia punctata and renal cortical cysts are absent. Postmortem study in infantile Refsum disease reveals micronodular liver cirrhosis and small hypoplastic adrenals. The brain shows no malformations, except for severe hypoplasia of the cerebellar granule layer and ectopic locations of the Purkinje cells in the molecular layer. The mode of inheritance is autosomal recessive.

Some patients with PBDs have milder and atypical phenotypes. They may present with peripheral neuropathy or with retinopathy, impaired vision, or cataracts in childhood, adolescence, or adulthood and have been diagnosed to have Charcot-Marie-Tooth disease or Usher syndrome. Some patients have survived to the 5th decade. Defects in PEX7, which most commonly lead to the RCDP phenotype, may also lead to a milder phenotype with clinical manifestations similar to those of classical Refsum disease (phytanoyl-CoA hydroxylase deficiency).

Rhizomelic Chondrodysplasia Punctata

RCDP is characterized by the presence of stippled foci of calcification within the hyaline cartilage and is associated with dwarfing, cataracts (72%), and multiple malformations caused by contractures. Vertebral bodies have a coronal cleft filled by cartilage that is a result of an embryonic arrest. Disproportionate short stature affects the proximal parts of the extremities (Fig. 86-4A). Radiologic abnormalities consist of shortening of the proximal limb bones, metaphyseal cupping, and disturbed ossification (Fig. 86-4B). Height, weight, and head circumference are less than the 3rd percentile, and these children have a severe intellectual disability. Skin changes such as those observed in ichthyosiform erythroderma are present in approximately 25% of patients.

Laboratory Findings

Laboratory tests for peroxisomal disorders can be viewed at 3 levels of complexity.

Diagnosis

There are several noninvasive laboratory tests that permit precise and early diagnosis of peroxisomal disorders (see Table 86-4). The challenge in PBDs is to differentiate them from the large variety of other conditions that can cause hypotonia, seizures, failure to thrive, or dysmorphic features. Experienced clinicians can readily recognize classic Zellweger syndrome by its clinical manifestations. However, more mildly affected PBD patients often do not show the full clinical spectrum of disease and may be identifiable only by laboratory assays. Clinical features that serve as indications for these diagnostic assays include severe intellectual disability; weakness and hypotonia; dysmorphic features; neonatal seizures; retinopathy, glaucoma, or cataracts; hearing deficits; enlarged liver and impaired liver function; and chondrodysplasia punctata. The presence of 1 or more of these abnormalities increases the likelihood of this diagnosis. Atypical milder forms presenting as peripheral neuropathy have also been described.

Some patients with the isolated defects of peroxisomal fatty acid oxidation (group B) resemble those with group A disorders and can be detected by the demonstration of abnormally high levels of VLCFAs.

Patients with RCDP must be distinguished from patients with other causes of chondrodysplasia punctata. In addition to warfarin embryopathy and Zellweger syndrome, these disorders include the milder autosomal dominant form of chondrodysplasia punctata (Conradi-Hünermann syndrome), which is characterized by longer survival, absence of severe limb shortening, and usually intact intellect; an X-linked dominant form; and an X-linked recessive form associated with a deletion of the terminal portion of the short arm of the X chromosome. RCDP is suspected clinically because of the shortness of limbs, psychomotor retardation, and ichthyosis. The most decisive laboratory test is the demonstration of abnormally low plasmalogen levels in red blood cells and an impaired capacity to synthesize plasmalogens in cultured skin fibroblasts. These biochemical defects are not present in other types of chondrodysplasia punctata. Chondrodysplasia punctata may also be associated with a defect of 3β-hydroxysteroid-Δ⁸,Δ⁷-isomerase, an enzyme involved in biosynthesis of cholesterol.

Complications

Patients with Zellweger cerebrohepatorenal syndrome have multiple disabilities involving muscle tone, swallowing, cardiac abnormalities, liver disease, and seizures. These conditions are treated symptomatically, but the prognosis is poor, and most patients succumb in the first year of life. Patients with RCDP may develop quadriparesis owing to compression at the base of the brain.

Treatment

The most effective therapy is the dietary treatment of classic Refsum disease with a phytanic acid-restricted diet.

For patients with the somewhat milder variants of the peroxisome import disorders, success has been achieved with multidisciplinary early intervention, including physical and occupational therapy, hearing aids or cochlear implants, alternative communication, nutrition, and support for the parents. Although most patients continue to function in the severely delayed range, some make significant gains in self-help skills, and several are in stable condition in their teens or even early 20s.

Attempts to mitigate some of the secondary biochemical abnormalities include the oral administration of docosahexaenoic acid. The levels of this substance are greatly reduced in patients with disorders of peroxisome biogenesis and this therapy normalizes the plasma levels of this substance. Although there were anecdotal reports of clinical improvement, a randomized placebo-controlled study failed to find benefit.

Genetic Counseling

All the peroxisomal disorders, except hyperoxaluria type 1, can be diagnosed prenatally in the 1st or 2nd trimester. The tests are similar to those described for postnatal diagnosis (see Table 86-5) and use chorionic villus sampling or amniocytes. Because of the 25% recurrence risk, couples with an affected child must be advised about the availability of prenatal diagnosis. Heterozygotes can be identified in ALD and in those disorders in which the molecular defect has been identified.

Etiology

The key biochemical abnormality is the tissue accumulation of saturated VLCFAs, with a carbon chain length of 24 or more. Excess hexacosanoic acid (C_26:0) is the most striking and characteristic feature. This accumulation of fatty acids is caused by genetically deficient peroxisomal degradation of fatty acid. The gene that is defective (ABCD1) codes for a peroxisomal membrane protein (ALDP, the ALD protein). Most families have a mutation that is “private” (unique to that kindred) and these are updated on the website, http://x-ald.nl. The mechanism by which the ALDP defect leads to VLCFA accumulation appears to be a disruption of transport of saturated fatty acids into the peroxisome with resultant continued elongation of progressively longer fatty acids.

Epidemiology

The minimum incidence of ALD in males is 1 in 21,000, and the combined incidence of ALD males and heterozygous females in the general population is estimated to be 1 in 17,000. All races are affected. The various phenotypes often occur in members of the same kindred.

Pathology

Characteristic lamellar cytoplasmic inclusions can be demonstrated with the electron microscope in adrenocortical cells, testicular Leydig cells, and nervous system macrophages. These inclusions probably consist of cholesterol esterified with VLCFA. They are most prominent in cells of the zona fasciculata of the adrenal cortex, which at first are distended with lipid and later atrophy.

The nervous system can display 2 types of lesions. In the severe childhood cerebral form and in the rapidly progressive adult forms, demyelination is associated with an inflammatory response manifested by the accumulation of perivascular lymphocytes that is most intense in the parietooccipital region. In the slowly progressive adult form, adrenomyeloneuropathy, the main finding is a distal axonopathy that affects the long tracts in the spinal cord. The inflammatory response is mild or absent.

Pathogenesis

The adrenal dysfunction is probably a direct consequence of the accumulation of VLCFAs. The cells in the zona fasciculata are distended with abnormal lipids. Cholesterol esterified with VLCFA is relatively resistant to adrenocorticotropic hormone (ACTH)-stimulated cholesterol ester hydrolases, and this limits the capacity to convert cholesterol to active steroids. In addition, C_26:0 excess increases the viscosity of the plasma membrane and this may interfere with receptor and other cellular functions.

There is no correlation between the neurologic phenotype and the nature of the mutation or the severity of the biochemical defect as assessed by plasma levels of VLCFAs or between the degree of adrenal involvement and nervous system involvement. The severity of the illness and rate of progression correlate with the intensity of the inflammatory response. The inflammatory response may be cytokine mediated and may involve an autoimmune response triggered in an unknown way by the excess of VLCFAs. Mitochondrial damage and oxidative stress also contribute. Approximately half of the patients do not experience the inflammatory response; this difference is not understood.

Clinical Manifestations

There are 5 relatively distinct phenotypes, 3 of which are present in childhood with symptoms and signs. In all the phenotypes, development is usually normal in the first 3-4 yr of life.

In the childhood cerebral form of ALD, symptoms are first noted most commonly between the ages of 4 and 8 yr. The most common initial manifestations are hyperactivity, which is often mistaken for an attention deficit disorder, and worsening school performance in a child who had previously been a good student. Auditory discrimination is often impaired, although tone perception is preserved. This may be evidenced by difficulty in using the telephone and greatly impaired performance on intelligence tests in items that are presented verbally. Spatial orientation is often impaired. Other initial symptoms are disturbances of vision, ataxia, poor handwriting, seizures, and strabismus. Visual disturbances are often caused by involvement of the cerebral cortex, which leads to variable and seemingly inconsistent visual capacity. Seizures occur in nearly all patients and may represent the first manifestation of the disease. Some patients present with increased intracranial pressure or with unilateral mass lesions. Impaired cortisol response to ACTH stimulation is present in 85% of patients, and mild hyperpigmentation is noted. In most patients with this phenotype, adrenal dysfunction is recognized only after the condition is diagnosed because of the cerebral symptoms. Cerebral childhood ALD tends to progress rapidly with increasing spasticity and paralysis, visual and hearing loss, and loss of ability to speak or swallow. The mean interval between the first neurologic symptom and an apparently vegetative state is 1.9 yr. Patients may continue in this apparently vegetative state for 10 yr or more.

Adolescent ALD designates patients who experience neurologic symptoms between the ages of 10 and 21 yr. The manifestations resemble those of childhood cerebral ALD except that progression is slower. Approximately 10% of patients present acutely with status epilepticus, adrenal crisis, acute encephalopathy, or coma.

Adrenomyeloneuropathy first manifests in late adolescence or adulthood as a progressive paraparesis caused by long tract degeneration in the spinal cord. Approximately half of the patients also have involvement of the cerebral white matter.

The “Addison only” phenotype is an important condition. Of male patients with Addison disease, 25% may have the biochemical defect of ALD. Many of these patients have intact neurologic systems, whereas others have subtle neurologic signs. Many acquire adrenomyeloneuropathy in adulthood.

The term “asymptomatic ALD” is applied to persons who have the biochemical defect of ALD but are free of neurologic or endocrinal disturbances. Nearly all persons with the gene defect eventually become neurologically symptomatic.

Approximately 50% of female heterozygotes acquire a syndrome that resembles adrenomyeloneuropathy but is milder and of later onset. Adrenal insufficiency and cerebral disease are rare.

Cases of typical ALD have occurred in relatives of those with adrenomyeloneuropathy. One of the most difficult problems in the management of X-linked ALD is the common observation that affected individuals in the same family may have quite different clinical courses. For example, in 1 family, 1 affected boy had severe classic ALD culminating in death by age 10 yr; another affected male (a brother) had late-onset adrenomyeloneuropathy, and a third had no symptoms at all.

Laboratory and Radiographic Findings

The most specific and important laboratory finding is the demonstration of abnormally high levels of VLCFA in plasma, red blood cells, or cultured skin fibroblasts. The test should be performed in a laboratory that has experience with this specialized procedure. Positive results are obtained in all male patients with ALD and in approximately 85% of female carriers of ALD. Mutation analysis is the most reliable method for the identification of carriers.

Neuroimaging

Patients with childhood cerebral or adolescent ALD show cerebral white matter lesions that are characteristic with respect to location and attenuation patterns on MRI. In 80% of patients, the lesions are symmetric and involve the periventricular white matter in the posterior parietal and occipital lobes. Approximately 50% show a garland of accumulated contrast material adjacent and anterior to the posterior hypodense lesions (Fig. 86-5A). This zone corresponds to the zones of intense perivascular lymphocytic infiltration where the blood–brain barrier breaks down. In 12% of patients, the initial lesions are frontal. Unilateral lesions that produce a mass effect suggestive of a brain tumor may occur. MRI provides a clearer delineation of normal and abnormal white matter than does CT (Fig. 86-5B).

Diagnosis and Differential Diagnosis

The earliest manifestations of childhood cerebral ALD are difficult to distinguish from the more common attention-deficit disorders or learning disabilities. Rapid progression, signs of dementia, or difficulty in auditory discrimination suggest ALD. Even in early stages, CT or MRI may show strikingly abnormal changes. Other leukodystrophies or multiple sclerosis may mimic these radiographic findings, although early ALD has more of a predilection for the posterior brain than its mimics. Definitive diagnosis depends on demonstration of VLCFA excess, which occurs only in ALD and the other peroxisomal disorders.

Cerebral forms of ALD may present as increased intracranial pressure and unilateral mass lesions. These have been misdiagnosed as gliomas, even after brain biopsy, and several patients have received radiotherapy before the correct diagnosis was made. Measurement of VLCFA in plasma or brain biopsy specimens is the most reliable differentiating test.

Adolescent or adult cerebral ALD can be confused with psychiatric disorders, dementing disorders, or epilepsy. The first clue to the diagnosis of ALD may be the demonstration of white matter lesions by neuroimaging; assays of VLCFA are confirmatory.

ALD cannot be distinguished clinically from other forms of Addison disease; it is recommended that assays of VLCFA levels be performed in all male patients with Addison disease. ALD patients do not usually have antibodies to adrenal tissue in their plasma.

Complications

An avoidable complication is the occurrence of adrenal insufficiency. The most difficult neurologic problems are those related to bed rest, contracture, coma, and swallowing disturbances. Other complications involve behavioral disturbances and injuries associated with defects of spatial orientation, impaired vision and hearing, and seizures.

Treatment

Corticosteroid replacement for adrenal insufficiency or adrenocortical hypofunction is effective. It may be lifesaving and increase general strength and well-being, but it does not alter the course of the neurologic disability.

Genetic Counseling and Prevention

Genetic counseling and primary and secondary prevention of ALD are of crucial importance. Extended family screening should be offered to all at-risk relatives of symptomatic patients; one program led to the identification of more than 250 asymptomatic affected males and 1,200 women heterozygous for ALD. The plasma assay permits reliable identification of affected males in whom plasma VLCFA levels are increased already on the day of birth. Identification of asymptomatic males permits institution of steroid replacement therapy when appropriate and prevents the occurrence of adrenal crisis, which may be fatal. Monitoring of brain MRI also permits identification of patients who are candidates for BMT at a stage when this procedure has the greatest chance of success. Plasma VLCFA assay is recommended in all male patients with Addison disease. ALD has been shown to be the cause of adrenal insufficiency in more than 25% of boys with Addison disease of unknown cause. Identification of women heterozygous for ALD is more difficult than that of affected males. Plasma VLCFA levels are normal in 15-20% of heterozygous women, and failure to note this has led to serious errors in genetic counseling. DNA analysis permits accurate identification of carriers, provided that the mutation has been defined in a family member, and is the procedure recommended for the identification of heterozygous women.

Prenatal diagnosis of affected male fetuses can be achieved by measurement of VLCFA levels in cultured amniocytes or chorionic villus cells and by mutation analysis. Whenever a new patient with ALD is identified, a detailed pedigree should be constructed and efforts should be made to identify all at-risk female carriers and affected males. These investigations should be accompanied by careful and sympathetic attention to social, emotional, and ethical issues during counseling.

Transport of Exogenous (Dietary) Lipids

All dietary fat with the exception of medium-chain triglycerides is efficiently carried into the circulation by way of lymphatic drainage from the intestinal mucosa. Triglyceride and cholesteryl esters combine with apoA and apoB-48 in the intestinal mucosa to form chylomicrons, which are carried into the peripheral circulation via the lymphatic system. HDL particles contribute apoC-II to the chylomicrons, required for the activation of lipoprotein lipase (LPL) within the capillary endothelium of adipose, heart, and skeletal muscle tissue. Free fatty acids are oxidized, esterified for storage as triglycerides, or released into the circulation bound to albumin for transport to the liver. After hydrolysis of the triglyceride core from the chylomicron, apoC particles are recirculated back to HDL. The subsequent contribution of apoE from HDL to the remnant chylomicron facilitates binding of the particle to hepatic LDL receptors (LDL-R). Within the hepatocyte, the chylomicron remnant may be incorporated into membranes, resecreted as lipoprotein back into the circulation, or secreted as bile acids. Normally, all dietary fat is disposed of within 8 hr after the last meal, an exception being individuals with a disorder of chylomicron metabolism. Postprandial hyperlipidemia is a risk factor for atherosclerosis. Abnormal transport of chylomicrons and their remnants may result in their absorption into the blood vessel wall as foam cells, caused by the ingestion of cholesteryl esters by macrophages, the earliest stage in the development of fatty streaks.

Transport of Endogenous Lipids from the Liver

The formation and secretion of VLDL from the liver and its catabolism to IDL and LDL particles describe the endogenous lipoprotein pathway. Fatty acids used in the hepatic formation of VLDL are derived primarily by uptake from the circulation. VLDL appears to be transported from the liver as rapidly as it is synthesized, and it consists of triglycerides, cholesteryl esters, phospholipids, and apoB-100. Nascent particles of VLDL secreted into the circulation combine with apoC and apoE. The size of the VLDL particle is determined by the amount of triglyceride present, progressively shrinking in size as triglyceride is hydrolyzed by the action of LPL, yielding free fatty acids for utilization or storage in muscle and adipose tissue. Hydrolysis of approximately 80% of the triglyceride present in VLDL particles produces IDL particles containing an equal amount of cholesterol and triglyceride. The remaining remnant IDL is converted to LDL for delivery to peripheral tissues or to the liver. ApoE is attached to the remnant IDL particle to allow binding to the cell and subsequent incorporation into the lysosome. Individuals with deficiency of either apoE2 or hepatic triglyceride lipase accumulate IDL in the plasma.

LDL particles account for approximately 70% of the plasma cholesterol in normal individuals. LDL receptors are present on the surfaces of nearly all cells. Most LDL is taken up by the liver, and the rest is transported to peripheral tissues such as the adrenal glands and gonads for steroid synthesis. Dyslipidemia is greatly influenced by LDL-R activity. The efficiency with which VLDL is converted into LDL is also important in lipid homeostasis.