Defective transport of glucose across the blood—brain barrier is caused by deficiency in the glucose transporter protein

Glucose crosses the blood—brain barrier by a mechanism of facilitated diffusion (Chaps. 5 and 32). This stereospecific system has a relatively high K_m for glucose, approximately 6 mM. Normally, transport of glucose across the blood—brain barrier is not rate-limiting for cerebral metabolism. Two patients were reported with a defect involving the GLUT-1 carrier protein [2]. The clinical presentation was infantile-onset seizures and developmental delay. One patient had deceleration of head growth with resulting microcephaly. The metabolic signature of this condition is a persistent hypoglycorrhachia with low-normal or low CSF lactate values. The patients responded to a ketogenic diet that was implemented to provide ketone bodies as an alternative fuel source for cerebral metabolism [2,3]. The GLUT-1 protein also is present in erythrocyte membranes. Decreased binding of cytochalasin B, a ligand that selectively binds to glucose transporters, was documented in both cases, and decreased uptake of 3-O-methylglucose by freshly isolated erythrocytes was documented in one case. The molecular and genetic basis for this condition involves mutation in the GLUT-1 gene on chromosome 1 [9a]. These patients may be misdiagnosed as examples of cerebral palsy, suspected hypoglycemia or sudden infant death syndrome.

One class of carbohydrate and fatty acid metabolism disorders is caused by defects in enzymes that function in the brain

Acid maltase deficiency is characterized by large amounts of glycogen in the perikaryon of glial cells in both gray and white matter, whereas cortical neurons contain much smaller quantities of glycogen. In the spinal cord, the neurons of the anterior horn appear ballooned and contain glycogen, as shown by the abundant PAS-positive material that is digested by diastase. Schwann cells of both anterior and posterior spinal roots and of peripheral nerves also contain excessive glycogen. By electron microscopy, the most striking feature is the presence of glycogen granules within membrane-bound vacuoles. These glycogen-laden vacuoles are particularly abundant in anterior horn cells, in neurons of brainstem motor nuclei and in Schwann cells, whereas they are scarce in cortical neurons. Glycogen is increased in postmortem brain, and acid maltase activity is undetectable. The severe involvement of spinal and brainstem motor neurons and the massive accumulation of glycogen in muscle contribute to the profound hypotonia, weakness and hyporeflexia seen in Pompe's disease [1].

Debrancher enzyme deficiency appears to be generalized. Accordingly, although neither pathology nor debrancher enzyme activity has been reported, increased glycogen concentration has been observed in the brain of a patient. Thus, in debrancher enzyme deficiency, the nervous system seems to be involved biochemically, although clinical signs of brain dysfunction are limited to hypoglycemic seizures in childhood [1].

Branching enzyme deficiency has been described in multiple tissues, reflecting the fact that the enzyme is expressed as a single molecular form. Although signs and symptoms of brain dysfunction are not prominent in brancher enzyme deficiency, deposits of abnormal polysaccharide, in the form of PAS-positive spheroids, were seen in subpial and perivascular zones of the brainstem and spinal cord but never within neurons. Electron microscopy showed that the spheroids were composed of branched osmiophilic filaments, 600 nm in diameter, and located within distended astrocytic processes [1].

Phosphoglycerate kinase deficiency, as it is most commonly manifest, includes nonspherocytic hemolytic anemia and CNS dysfunction. Neurological problems vary in severity. All patients show some degree of mental retardation, with delayed language acquisition and behavioral abnormalities, and some have hemiplegia or seizures. The enzyme defect has been directly proven in the brain, and the severe brain involvement can be explained by impairment of the glycolytic pathway. The lack of symptoms of brain dysfunction in some patients with PGK deficiency, such as the two patients with recurrent myoglobinuria described above, are probably attributable to the presence of sufficient residual enzyme activity to prevent severe energy shortage.

Lafora and other polyglucosan-storage diseases manifest an accumulation of an abnormal glucose polymer resembling amylopectin, termed polyglucosan, in the CNS and PNS as well as in other tissues, but the biochemical defect(s) remains unknown [1].

Lafora's disease is transmitted as an autosomal recessive trait and is characterized by epilepsy, myoclonus and dementia. Other neurological manifestations include ataxia, dysarthria, spasticity and rigidity. Onset is in adolescence, and death occurs in most patients before 25 years of age.

The pathological hallmark of the disease is the presence in the brain of Lafora bodies: round, basophilic, PAS-positive intracellular inclusions varying in size from small, “dust-like” bodies less than 3 nm in diameter to large bodies up to 30 nm in diameter. Lafora bodies are typically seen in neuronal perikarya and processes, not in glial cells, and are more abundant in cerebral cortex, substantia nigra, thalamus, globus pallidus and dentate nucleus. Ultrastructural studies have shown that Lafora bodies consist of two components: amorphous electron-dense granules and irregular branched filaments.

Although the storage material is histochemically and biochemically similar to the polysaccharide that accumulates in branching enzyme deficiency, brancher enzyme activity was normal in brain and muscle from one patient. A different form of polyglucosan body disease was described in patients with a characteristic neurological syndrome consisting of progressive upper and lower motoneuron involvement, sensory loss, neurogenic bladder, and, in one-half of the patients, dementia without myoclonus or epilepsy. Onset is in the fifth or sixth decade, and the course varies between 3 and 30 years. Polyglucosan bodies are disseminated throughout the CNS and PNS in processes of neurons and astrocytes but not in perikarya. Other tissues are also affected, including liver, heart and skeletal and smooth muscle. In Ashkenazi Jewish patients with this disorder, but not in patients of different ethnic origins, branching enzyme activity is decreased in leukocytes, peripheral nerve and, presumably, brain but is normal in muscle [1]. The molecular basis for the differences in organs affected and clinical course between “typical” branching enzyme deficiency (see above) and polyglucosan body disease remains to be explained. The observation that branching deficiency in polyglucosan body disease is confined to Ashkenazi Jewish patients suggests that this disorder is biochemically heterogeneous.

Another class of carbohydrate and fatty acid metabolism disorders is caused by systemic metabolic defects that affect the brain

Glucose-6-phosphatase deficiency (glycogenosis type I, Von Gierke's disease) results in hypoglycemia and excessive intracellular accumulation of glucose-6-phosphate (Fig. 42-1). Hypoglycemia may produce lethargy, coma, seizures and brain damage in gluconeogenic and glycogen synthetase deficiencies [6]. As a result, there is formation of lactic acid, uric acid and lipids. A second form of the disease (type Ib) has been described. The defect in this form involves the glucose-6-phosphate translocation system that is important in facilitating the movement of the substrate into the microsomal compartment for enzymatic conversion to glucose by glucose-6-phosphatase. The clinical features of types Ia and Ib are similar, but normal enzyme activity is present in type Ib. Hepatomegaly, bleeding diathesis and neutropenia are present. The neurological signs result from the chronic hypoglycemia. Recent studies indicate that lactate may be used by the brain as an alternative cerebral metabolic fuel when hypoglycemia is associated with lactic acidosis. Nocturnal intragastric feeding and frequent daytime meals ameliorate most of the clinical and metabolic abnormalities of this condition.

Fructose-1,6-bisphosphatase deficiency, first described by Baker and Winegrad in 1970, has now been reported in approximately 30 cases. It is more common in women and is inherited as an autosomal recessive disorder. Initial manifestations are not strikingly dissimilar from those of glucose-6-phosphatase deficiency. Neonatal hypoglycemia is a common presenting feature, associated with profound metabolic acidosis, irritability or coma, apneic spells, dyspnea, tachycardia, hypotonia and moderate hepatomegaly. Lactate, alanine, uric acid and ketone bodies are elevated in the blood and urine [10]. The enzyme is deficient in liver, kidney, jejunum and leukocytes. Muscle fructose-1,6-bisphosphatase activity is normal.

Fructose-1,6-bisphosphatase is an important rate-limiting step in gluconeogenesis. This gluconeogenic step antagonizes the opposite reaction that forms fructose-1,6-bisphosphate from fructose-6-phosphate and ATP (see Chap. 31). A futile cycle exists between these two enzymes, one forming fructose-1,6-bisphosphate and the other disposing of this substrate. Small amounts of fructose-2,6-bisphosphate also are formed by the PFK reaction. This metabolite stimulates the PFK reaction and inhibits the fructose-1,6-bisphosphatase reaction. This finding nicely explains the subtle interplay between the key rate-limiting step in glycolysis, which is PFK-dependent, and the rate-limiting step in gluconeogenesis catalyzed by fructose-1,6-bisphosphatase.

Phosphoenolpyruvate carboxykinase (PEPCK) deficiency is a distinctly rare and even more devastating clinically than deficiencies of glucose-6-phosphatase or fructose-1,6-bisphosphatase. PEPCK activity is almost equally distributed between a cytosolic form and a mitochondrial form. These two forms have similar molecular weights but differ by their kinetic and immunochemical properties. The cytosolic activity is responsive to fasting and various hormonal stimuli. Hypoglycemia is severe and intractable in the absence of PEPCK [10]. A young child with cytosolic PEPCK deficiency had severe cerebral atrophy, optic atrophy and fatty infiltration of liver and kidney.

Pyruvate carboxylase deficiency has been documented in 36 cases [9,11]. This enzyme is mitochondrial in location and catalyzes the conversion of pyruvate to oxaloacetate in a biotin-dependent manner (Chaps. 35 and 39). The first report of pyruvate carboxylase deficiency involved an infant with subacute necrotizing encephalomyelopathy, or Leigh's syndrome. Subsequent reports have failed to confirm this causal relationship between pyruvate carboxylase deficiency and the neuropathological features of Leigh's syndrome. Leigh's syndrome has now been assigned to several other biochemical defects, including pyruvate dehydrogenase deficiency, cytochrome-oxidase deficiency, biotinidase deficiency and defects involving complex I and complex V of the respiratory chain.

Most patients with pyruvate-carboxylase deficiency present with failure to thrive, developmental delay, recurrent seizures and metabolic acidosis. Lactate, pyruvate, alanine, β-hydroxybutyrate and acetoacetate concentrations are elevated in blood and urine. Hypoglycemia is not a consistent finding despite the fact that pyruvate carboxylase is the first rate-limiting step in gluconeogenesis.

Sixteen patients had an associated hyperammonemia, citrullinemia and hyperlysinemia. This presentation is the most malignant, with death in early infancy. This French phenotype is commonly associated with the absence of any immunological cross-reacting material (CRM) corresponding to the pyruvate carboxylase apoenzyme protein.

The North American phenotype is associated with the presence of CRM. Possibly as a result, the clinical presentation is less devastating in early infancy, although the outcome is almost invariably fatal in later infancy or early childhood. These patients do not have the associated abnormalities of ammonia metabolism, and the serum aspartic acid concentrations are not as severely depleted. Only one patient has been described with the North American phenotype and a benign clinical syndrome. She has had recurrent episodes of metabolic acidosis requiring hospitalization. Otherwise, her growth and neurological development have been normal.

Prenatal and postnatal diagnoses can be made by enzyme assay of cultured amniocytes, fibroblasts or white blood cells. Treatment remains symptomatic. Sodium bicarbonate is necessary to correct the acidosis. Aspartic acid supplementation will improve the systemic condition but has no effect on the neurological disturbances. Biotin supplementation is of no value.

Biotin-dependent syndromes are manifest in infants, who may present with developmental delay and may demonstrate laboratory abnormalities resulting from deficiencies of the four biotin-dependent carboxylases (see Chap. 39). Three of the carboxylases, located in the mitochondria, are involved in organic acid metabolism. Multiple carboxylase deficiency, when present in the newborn period, is the result of a deficiency of holocarboxylase synthetase, the enzyme that catalyzes the binding of biotin to the apocarboxylase. These infants often die shortly after birth. Older infants gradually develop neurological signs, with developmental delay and seizures associated with alopecia, rash and immunodeficiency. There is a deficiency of biotinidase, the enzyme responsible for the breakdown of biocytin, the lysyl derivative of biotin, to free biotin. Biotinidase deficiency can be recognized at birth by measuring the serum activity. Biotinidase deficiency occurs in 1 in 41,000 live births, and it is eminently treatable by the oral administration of biotin.

Glycogen synthetase deficiency has been described in three families. It caused stunted growth and severe fasting hypoglycemia with ketonuria. Mental retardation was reported in the three children who survived past infancy. The liver was virtually devoid of glycogen and showed fatty degeneration in all cases. In two patients, the brain showed diffuse, nonspecific changes in the white matter, seen as the presence of reactive astrocytes and increased microglia, which were considered secondary to prolonged hypoglycemia or anoxia. Biochemical studies showed that glycogen-synthetase activity was markedly decreased in liver but normal in muscle, erythrocytes and leukocytes, suggesting the existence of multiple tissue-specific isoenzymes under separate genetic control. It is not known whether brain glycogen synthetase is different from that in liver.

In liver phosphorylase deficiency (glycogenosis type VI, Hers' disease; Fig. 42-1) and in two genetic forms of phosphorylase kinase deficiency, one of which is X-linked recessive, the other of which is autosomal recessive, hypoglycemia is either absent or mild. Symptoms of brain dysfunction do not usually occur (type VIII, Fig. 42-1) [1].

Fatty acid oxidation defects often produce recurrent disturbances of brain function [4,6,9]. Drowsiness, stupor and coma occur during acute metabolic crises and mimic the Reye's syndrome phenotype. The neurological symptoms have been attributed to hypoglycemia, hypoketonemia and the deleterious effects of potentially toxic organic acids. Hypoglycemia is caused by a continuing demand for glucose by brain and other organs, resulting from the primary biochemical defect of fatty-acid oxidation (Fig. 42-2). Avoidance of catabolic circumstances that require the utilization of fatty acids is the basic principle of treatment. l-Carnitine supplementation is recommended for all conditions associated with generalized carnitine deficiency. Some patients may benefit from medium-chain triglyceride supplementation, as discussed previously. Certain forms of ETF-oxidoreductase deficiency respond to riboflavin supplementation. The riboflavin-responsive multiple acyl CoA dehydrogenase deficiency represents the milder form of glutaric aciduria type II.