Defects in Metabolism of Lipids

Medium-Chain Acyl-CoA Dehydrogenase Deficiency

Medium-chain acyl-CoA dehydrogenase ( MCAD ) deficiency is the most common fatty acid oxidation disorder. The disorder shows a strong founder effect; most patients have a northwestern European ancestry, and the majority of these patients are homozygous for a single common MCAD missense mutation, an A-G transition at cDNA position 985 (c.985A>G) that changes a lysine to glutamic acid at residue 329 (p.K329E).

Very-Long-Chain Acyl-CoA 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 may show poor contractility on echocardiography. Sudden unexpected death has occurred in several patients, but most who survive the initial episode show 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 >10-12 hr. Continuous intragastric feeding is useful in some patients.

Short-Chain Acyl-CoA Dehydrogenase Deficiency

A small number of patients with 2 null mutations in the short-chain acyl-CoA dehydrogenase (SCAD) gene have been described with variable phenotype. Most individuals classified as being SCAD deficient actually 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 variants may be susceptibility factors, which require a 2nd, as yet unknown, genetic mutation to express a clinical phenotype; many other investigators 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 are variably present in patients who are homozygous for the common polymorphisms.

The need 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. Most individuals with SCAD deficiency remain asymptomatic throughout life, but there may be a subset of individuals 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 the MTP, which also contains 2 other steps in β-oxidation: long-chain 2,3-enoyl CoA hydratase and long-chain β-ketothiolase. MTP is a hetero-octameric protein composed of 4 α and 4 β chains derived from distinct contiguous genes sharing 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; however, 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 (AFLP, HELLP syndrome) have been observed in heterozygous mothers carrying homozygous 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 C ₁₆ -C ₁₈ . Urinary organic acid profile in patients may show increased levels of 3-hydroxydicarboxylic acids of chain lengths C ₆ -C ₁₄ . 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 patients with severe liver failure, but does not ameliorate the metabolic abnormalities or prevent the myopathic or retinal complications.

Short-Chain 3-Hydroxyacyl-CoA Dehydrogenase Deficiency

Only 14 patients with proven mutations of short-chain 3-hydroxyacyl-CoA dehydrogenase ( SCHAD ) have been reported. Most cases with recessive mutations of the SCHAD gene have presented with episodes of hypoketotic hypoglycemia that was caused by hyperinsulinism. In contrast to those with other forms of fatty acid oxidation disorders, these patients required specific therapy with diazoxide for hyperinsulinism to avoid recurrent hypoglycemia. A single patient with compound heterozygous mutations presented with fulminant hepatic failure at age 10 mo. The SCHAD protein has a nonenzymatic function in which it directly interacts with glutamate dehydrogenase (GDH) to inhibit its activity. In the absence of SCHAD protein, this inhibition is removed, leading to upregulation of GDH enzyme activity, a recognized cause of hyperinsulinism usually from activating mutations of the GDH gene. Severe deficiency of SCHAD protein often presents predominantly as protein-sensitive hypoglycemia rather than as fasting hypoglycemia. It appears that if SCHAD protein is present, 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. Successful newborn screening for SCHAD deficiency has been recorded, but the sensitivity of the process has not yet been established.

Treatment of SCHAD-deficient patients with hyperinsulinism is with diazoxide. There is insufficient experience with the nonhyperinsulinemic form of SCHAD deficiency at present to recommend treatment modalities, but prevention of fasting seems advisable.

Short-Chain 2,3-Enoyl-CoA Hydratase Deficiency

This disorder, resulting from mutations in the ECHS1 gene, has only recently been defined. Many patients were identified through exome sequencing, and currently there are approximately 20 cases in the literature. The disorder affects a shared pathway of short-chain fatty acid and valine metabolism. The clinical phenotypes are more characteristic of mitochondrial disorders of pyruvate metabolism with predominantly a Leigh-like disease (see Chapter 616.2 ) with profound and often-fatal lactic acidosis. Currently, no treatment modalities or specific biomarkers have been established. Several patients were found to excrete increased levels of methacrylylglycine, a highly reactive and potentially toxic intermediate; 2-methyl-2.3 dihydroxybutyrate; S -(2-carboxypropyl) cysteine; and S -(2-carboxpropyl) cysteamine.

Plasma Membrane Carnitine Transport Defect (Primary Carnitine Deficiency)

Primary carnitine deficiency is the only genetic defect in which carnitine deficiency is the cause, rather than the consequence, of impaired fatty acid oxidation. The most common presentation is progressive cardiomyopathy with or without skeletal muscle weakness beginning at age 1-4 yr. A smaller number of patients may present with fasting hypoketotic hypoglycemia in the 1st yr of life, before the cardiomyopathy becomes symptomatic. The underlying defect involves the plasma membrane sodium gradient–dependent carnitine transporter that is present in heart, muscle, and kidney. This transporter is responsible both for maintaining intracellular carnitine concentrations 20-50–fold higher than plasma concentrations and for renal conservation of carnitine.

Diagnosis of the carnitine transporter defect is aided by patients having extremely reduced carnitine levels in plasma and muscle (1–2% of normal). Heterozygote parents have plasma carnitine levels approximately 50% of normal. Fasting ketogenesis may be normal because liver carnitine transport is normal, but it may become impaired if dietary carnitine intake is interrupted. The fasting urinary organic acid profile may show a hypoketotic dicarboxylic aciduria pattern if hepatic fatty acid oxidation is impaired, but is otherwise unremarkable. The defect in carnitine transport can be demonstrated clinically by the severe reduction in renal carnitine threshold or by in vitro assay of carnitine uptake using cultured fibroblasts or lymphoblasts. Mutations in the organic cation/carnitine transporter (OCTN2) underlie this disorder. Treatment with pharmacologic doses of oral carnitine (100-200 mg/kg/day) is highly effective in correcting the cardiomyopathy and muscle weakness, as well as any impairment in fasting ketogenesis. Muscle total carnitine concentrations remain <5% of normal on treatment.

Carnitine Palmitoyltransferase-IA Deficiency

Several dozen infants and children have been described with a deficiency of the liver and kidney CPT-I isozyme (CPT-IA). Clinical manifestations include fasting-induced hypoketotic hypoglycemia, occasionally with extremely abnormal LFTs and rarely with renal tubular acidosis. The heart and skeletal muscle are not involved because the muscle isozyme is unaffected. Fasting urinary organic acid profiles sometimes show a hypoketotic C ₆ -C ₁₂ dicarboxylic aciduria but may be normal. Plasma acylcarnitine analysis demonstrates mostly free carnitine with very little acylated carnitine. This observation has been used to identify CPT-IA deficiency on newborn screening by tandem mass spectrometry. CPT-IA deficiency is the only fatty acid oxidation disorder in which plasma total carnitine levels may be elevated, often to 150–200% of normal. This phenomenon is explained by the absence of inhibitory effects of long-chain acylcarnitines on the renal tubular carnitine transporter in CPT-IA deficiency. The enzyme defect can be demonstrated in cultured fibroblasts or lymphoblasts. CPT-IA deficiency in the fetus has been associated in a single case report with AFLP in the mother. A common variant in the CPTIA gene (c.1436C>t, p.P479L) has been identified in individuals of Inuit background in the United States, Canada, and Greenland. This variant is associated with an increased risk for sudden infant death syndrome (SIDS) in the Inuit population. The variant can be detected by newborn screening; enzyme activity is reduced by 80%, and regulation by malonyl-CoA is lost. It has not been established whether CPT-IA (c.1436C>t, p.P479L) is a pathologic enzyme variant or an adaptation to ancient Inuit high-fat diets. Treatment for the severe form of CPT-IA deficiency that is found in non-Inuit populations is similar to that for MCAD deficiency, with avoidance of situations where fasting ketogenesis is necessary. The need for treatment of the Inuit variant has not yet been determined.

Carnitine:Acylcarnitine Translocase Deficiency

This defect of the inner mitochondrial membrane carrier protein for long-chain 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 C ₁₆ -C ₁₈ 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. A severe neonatal lethal presentation associated with a profound enzyme deficiency and early death has been reported in several newborns in association with dysplastic kidneys, cerebral malformations, and mild facial anomalies. A milder defect is associated with an adult presentation of episodic rhabdomyolysis. The first episode usually does not occur until late childhood or early adulthood. Attacks are frequently 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. Hypoglycemia has not been described, but fasting may contribute to attacks of myoglobinuria. Muscle biopsy shows increased deposition of neutral fat. This adult myopathic presentation of CPT-II deficiency is associated with a common mutation, c.338C>T, p.S113L. This mutation produces a heat-labile protein that is unstable to increased muscle temperature during exercise resulting in the myopathic presentation. The 3rd, intermediate form of CPT-II deficiency presents in infancy or early childhood with fasting-induced hepatic failure, cardiomyopathy, and skeletal myopathy with hypoketotic hypoglycemia, but is not associated with the severe developmental changes seen in the neonatal lethal presentation. This pattern of illness 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 genetic analysis and demonstrating deficient enzyme activity in muscle or other tissues and in cultured fibroblasts.

Electron Transfer Flavoprotein and Electron Transfer Flavoprotein Dehydrogenase Deficiencies (Glutaric Acidemia Type 2, Multiple Acyl-CoA Dehydrogenation Defects)

ETF and ETF-DH function to transfer electrons into the mitochondrial electron transport chain from dehydrogenation reactions catalyzed by VLCAD, MCAD, and SCAD, as well as by glutaryl-CoA dehydrogenase and 4 enzymes involved in branched-chain amino acid (BCAA) oxidation. Deficiencies of ETF or ETF-DH produce illness that combines the features of impaired fatty acid oxidation and impaired oxidation of several amino acids. Complete deficiencies of either protein are associated with severe illness in the newborn period, characterized by acidosis, hypoketotic hypoglycemia, coma, hypotonia, cardiomyopathy, and an unusual odor of sweaty feet caused by isovaleryl-CoA dehydrogenase inhibition. Some affected neonates have had congenital facial dysmorphism and polycystic kidneys similar to that seen in severe CPT-II deficiency, which suggests that toxic effects of accumulated metabolites may occur in utero.

Diagnosis can be made from the newborn blood spot acylcarnitine profile and urinary organic acids; both tests show abnormalities corresponding to blocks in the oxidation of fatty acids (ethylmalonate and C ₆ -C ₁₀ dicarboxylic acids), lysine (glutarate), and BCAAs (isovaleryl-, isobutyryl-, and α-methylbutyryl-glycine). The diagnosis can be confirmed by genetic testing for ETF (2 genes, A and B) and ETF dehydrogenase. Most severely affected infants do not survive the neonatal period.

Partial deficiencies of ETF and ETF-DH produce a disorder that may mimic MCAD deficiency or other milder fatty acid oxidation defects. These patients have attacks of fasting hypoketotic coma. The urinary organic acid profile reveals primarily elevations of dicarboxylic acids and ethylmalonate, derived from short-chain fatty acid intermediates. Secondary carnitine deficiency is present. Some patients with mild forms of ETF/ETF-DH deficiency may benefit from treatment with high doses of riboflavin, a precursor of the various flavoproteins involved in electron transfer.

β-Hydroxy-β-Methylglutaryl-CoA Synthase Deficiency

See Chapter 103.6 .

HMG-CoA synthase is the rate-limiting step in the conversion of acetyl-CoA derived from fatty acid β-oxidation in the liver to ketones. Several patients with this defect have been identified. The presentation is one of fasting hypoketotic hypoglycemia without evidence of impaired cardiac or skeletal muscle function. Urinary organic acid profile shows only a nonspecific hypoketotic dicarboxylic aciduria. Plasma and tissue carnitine levels are normal, in contrast to all the other disorders of fatty acid oxidation. A separate synthase enzyme, present in cytosol for cholesterol biosynthesis, is not affected. The HMG-CoA synthase defect is expressed only in the liver (and kidney) and cannot be demonstrated in cultured fibroblasts. The diagnosis can be made by genetic mutation analysis. Avoiding fasting is usually a successful treatment.

β-Hydroxy-β-Methylglutaryl-CoA Lyase Deficiency (3 Hydroxy-3-Methylgutaric Aciduria)

See Chapter 103.6 .

Monocarboxylate Transporter-1 Deficiency

About 10 patients have been described with recurrent episodes of potentially lethal ketoacidosis, with or without hypoglycemia, caused by deficiency of monocarboxylate transporter 1 (MCT1), a plasma membrane carrier encoded by SLC16A1 that is required to transport ketones into tissues from plasma. Although the first cases identified were homozygous for inactivating mutations of MCT1, heterozygous carriers can also be affected. Affected patients developed severe ketoacidosis provoked by fasting or infections in their 1st years of life; hypoglycemia was not always present. The differential includes ketotic hypoglycemia associated with milder forms of glycogen storage disease, such as phosphorylase or phosphorylase kinase deficiency (see Chapter 105 ). Treatment for acute episodes includes IV dextrose to suppress lipolysis and inhibit ongoing ketogenesis. Long-term treatment includes avoidance of prolonged fasting stress. The diagnosis can be suspected by unusually severe ketosis and delayed suppression of ketones after starting treatment with dextrose. There are no specific metabolic markers or newborn screening methods. The diagnosis can be established by genetic sequencing of SLC16A1 .

Succinyl-CoA:3-Ketoacid-CoA Transferase Deficiency

See Chapter 103.6 .

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 the brain, muscle, and fibroblasts from affected patients. The gene has been cloned, and numerous mutations have been characterized.

β-Ketothiolase Deficiency

See Chapter 103.6 .

Etiology

Peroxisomal disorders are subdivided into two major categories ( Table 104.2 ). In the peroxisomal biogenesis disorders (PBDs) the basic defect is the failure to import 1 or more proteins into the organelle. In the other group, defects affect a single peroxisomal protein ( single-enzyme defects ). The peroxisome is present in all cells except mature erythrocytes and is a subcellular organelle surrounded by a single membrane; >50 peroxisomal enzymes are identified. Some enzymes are involved in production and decomposition of hydrogen peroxide and others in 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 listed in Table 104.2 are autosomal recessive diseases . 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, although with broader newborn screening it is expected that the actual incidences of all of the disorders of very-long-chain fatty acids will be more accurately established.

Pathology

Absence or reduction in the number of peroxisomes is pathognomonic for disorders of peroxisome biogenesis. In most disorders, membranous sacs 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

All pathologic changes likely are secondary to the peroxisome defect. Multiple peroxisomal enzymes fail to function in the PBDs ( Table 104.3 ). The enzymes that are diminished or absent are synthesized but are degraded abnormally fast because they may be unprotected outside 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 of import impairment. 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 by 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 (adult) 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 VLCFAs in the adrenal cortex, and the peripheral neuropathy in adult Refsum disease is caused by the accumulation of phytanic acid in Schwann cells and myelin.

Zellweger Spectrum Disorder

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. 104.3 ), 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 104.4 lists the main clinical abnormalities.

Table 104.4

Main Clinical Abnormalities in Zellweger Syndrome

From Heymans HAS: Cerebro-hepato-renal (Zellweger) syndrome: clinical and biochemical consequences of peroxisomal dysfunctions, Thesis, University of Amsterdam, 1984.

ABNORMAL FEATURE	PATIENTS IN WHOM THE FEATURE WAS PRESENT
	Number	%
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 range of severe intellectual disability, 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 can walk, although gait may be ataxic and broad based. Cognitive function is generally impaired, but accurate assessment is limited, usually by the presence of both vision and hearing impairment. Almost all have some degree of sensorineural hearing loss and pigmentary degeneration of the retina. They have moderately dysmorphic features that may include epicanthal folds, a flat nose bridge, and low-set ears. Early hypotonia and hepatomegaly with impaired function are common. Levels of plasma cholesterol and high-density 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 fifth decade. Defects in PEX7, which most frequently lead to the RCDP phenotype, may also lead to a milder phenotype with clinical manifestations similar to those of adult Refsum disease.

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. 104.4 A ). Radiologic abnormalities consist of shortening of the proximal limb bones, metaphyseal cupping, and disturbed ossification ( Fig. 104.4 B ). 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

Diagnosis of a peroxisomal disorder often follows from a biochemical determination of an abnormality and then is confirmed through further genetic testing.

The biochemical characterization of peroxisomal disorders uses the generally available testing listed in Table 104.5 . Measurement of plasma VLCFA levels is the most common assay. It must be emphasized that although plasma VLCFA levels are elevated in many patients with peroxisomal disorders, this is not always the case. The most important exception is RCDP, in which VLCFA levels are normal, but plasma phytanic acid levels are increased and red blood cell (RBC) plasmalogen levels are reduced. In other peroxisomal disorders, the biochemical abnormalities are still more restricted. Therefore, a panel of tests is recommended and includes plasma levels of VLCFAs and phytanic, pristanic, and pipecolic acids and RBC levels of plasmalogens. Tandem mass spectrometry techniques also permit convenient quantitation of bile acids in plasma and urine. This panel of tests can be performed on very small amounts of venous blood and permits detection of most peroxisomal disorders. Furthermore, normal results make the presence of the typical peroxisomal disorder unlikely. Biochemical findings combined with the clinical presentation are often sufficient to arrive at a clinical diagnosis. Methods using dried blood spots of filter paper have been developed and are being incorporated into newborn screening assays.

The next step in diagnosis is generally to proceed to molecular DNA diagnosis, and many clinical laboratories provide a peroxisomal panel using next-generation technology. In some circumstances the diagnosis has been revealed through whole exome sequencing and the pathogenic nature of the alteration then confirmed through biochemical means.

Definition of the molecular defect in the proband is essential for carrier detection and speeds prenatal diagnosis . Characterization of the mutation may be of prognostic value in patients with PEX1 defects. This defect is present in approximately 60% of PBD patients, and about half the PEX1 defects have the G843D allele, which is associated with a significantly milder phenotype than found in other mutations.

Diagnosis

Several noninvasive laboratory tests permit precise and early diagnosis of peroxisomal disorders (see Table 104.5 ). 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 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 warrant diagnostic assay include 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 resemble those with Zellweger spectrum disorder 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. RCDP is suspected clinically because of the shortness of limbs, developmental delays, and ichthyosis. The most decisive laboratory test is the demonstration of abnormally low plasmalogen levels in RBCs and an alteration in PEX7 .

Complications

Patients with Zellweger 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 1st yr of life. Similarly, individuals with RCDP have multiple systemic and neurologic issues. In addition, they may develop quadriparesis from compression at the base of the brain.

Treatment

The most effective therapy is the dietary treatment of adult Refsum disease with a phytanic acid–restricted diet. However, this only applies to this specific condition.

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, augmentative and alternative communication, nutrition, and support for the families. Although most patients continue to function in the impaired 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 (DHA). DHA level is greatly reduced in patients with disorders of peroxisome biogenesis, and this therapy normalizes DHA plasma levels. Although there were anecdotal reports of clinical improvement with DHA therapy, a randomized placebo-controlled study failed to find benefit.

Genetic Counseling

All the discussed peroxisomal disorders can be diagnosed prenatally. Prenatal testing using chorionic villi sampling or amniocentesis will usually rely on genetic testing when the alteration is known, but biochemical measurements may be made using the same tests as described for postnatal diagnosis (see Table 104.5 ). Because of the 25% recurrence risk, couples with an affected child should be advised about the availability of prenatal diagnosis.

Etiology

The key biochemical abnormality in ALD 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 defective gene (ABCD1) codes for a peroxisomal membrane protein (ALDP, the ALD protein). Many alterations in ABCD1 have been determined to be pathogenic, with over half these being private or unique to the kindred. A curated database of mutations is maintained ( www.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. Increased implementation of newborn screening in the United States and other countries is expected to improve the accuracy of these incidence estimates.