Metabolic Disorders in the Center of Genetic Medicine
http://www.100md.com
《新英格兰医药杂志》
Genetics is the new frontier of medicine. Hardly an issue of any leading medical journal is published without one or more articles on a genetic disease or a topic closely related to genetics. A recent series of articles in the Journal was devoted to the importance of genetics in medical practice.1 The immediate impetus for such a series was, as is widely known, the sequencing of the human genome and the promise that that effort would lead to better diagnosis and treatment of disease.2 However, the knowledge that many metabolic disorders are genetic has long been seen as evidence, with clear implications for medical practice, that inherited genetic traits and propensities play a large part in disease.
At the center of knowledge about inherited metabolic diseases has been phenylketonuria, a disorder of phenylalanine metabolism, and newborn screening and dietary treatment have virtually eliminated mental retardation in patients with this disorder. From the understanding of the genetic basis of phenylketonuria has come the realization that there is a tangible benefit from diagnosing inherited disorders.3 Phenylketonuria also stimulated genetic research and federal funding of this research. One of the first-cloned genes involved in genetic disease was that for phenylalanine hydroxylase, the enzyme that is defective in phenylketonuria.4 The excitement of understanding phenylketonuria led to frequent metabolic testing of children with clinical disease that might be metabolic, especially those with developmental delay or other neurologic symptoms. Such strategies resulted in the discovery of many additional metabolic disorders. The need to understand the disorders motivated investigations that began to elucidate not only the metabolic defects but also areas of human biochemistry and pathology. Thus, as Garrod realized in the early 1900s as a result of his studies of alkaptonuria that led to the concept of the inborn error of metabolism, the importance of these disorders transcends their rarity; they can provide paths to greater knowledge about normal human biology and to deeper understanding of common problems.5
Homocystinuria provides a particularly striking example of this importance. This disorder is a well-delineated inborn error of methionine metabolism that causes ectopia lentis, skeletal abnormalities, mental retardation, and thromboembolism. Its cardinal biochemical feature is an increased level of homocysteine. A few years after homocystinuria was reported, Harvey Mudd and I and our colleagues studied an infant who not only had increased levels of homocysteine but also had other biochemical abnormalities that are not found in homocystinuria, including an increased level of methylmalonic acid. Eventually, we recognized that the infant had a new disorder in which vitamin B12 was not metabolized to the cobalamin coenzymes necessary to stimulate remethylation of homocysteine to methionine and to convert methylmalonic acid to succinic acid.6
On autopsy of this infant, McCully found vascular occlusions that resembled those seen in patients with homocystinuria; he suggested that the increase in the level of homocysteine was the common etiologic factor of the occlusions.7 Studies stimulated by these findings have now shown that almost 10 percent of the general population is homozygous for a thermolabile variant of methylenetetrahydrofolate reductase, an enzyme that is also responsible for remethylation. These people usually have hyperhomocysteinemia, which is considered a risk factor for cardiovascular disease.8 The reduced activity of methylenetetrahydrofolate reductase can be restored by administration of large amounts of folate, which lowers the level of homocysteine. These observations provide the basis for the current measurement of plasma homocysteine during health examinations and constitute the major reasons for the recommendation of an increase in folate intake and of a daily supplement of folic acid, especially among patients with hyperhomocysteinemia.
An article in this issue of the Journal illustrates, once again, the potential for wider implications of what is probably a rare metabolic disorder. H?berle and colleagues report on two unrelated infants who had dramatically reduced levels of glutamine due to a deficiency of glutamine synthetase, the enzyme that catalyzes the conversion of glutamate to glutamine.9 Both infants had profound cerebral disease that included malformations of the brain, and both died during the neonatal period. Molecular studies showed that each of the infants was homozygous for a different missense mutation in exon 6 of the glutamine synthetase gene; sequence alignment located the mutations in or near the glutamate-binding region of the enzyme. The immediate importance of this report is that it adds a genetic disorder to the list of those that should be considered in a neonate or, perhaps, an older infant with undiagnosed neurologic disease. The greater significance, however, is that these cases will lead to opportunities for better understanding the role that glutamine synthetase and glutamine play in cellular metabolism, especially with regard to brain functioning and development.
Glutamine is the most abundant free amino acid in the body. It plays a vital role in cell viability,10 cellular energy metabolism,11 and the availability of glutamate neurotransmitters in neurons.12 Glutamine is rendered available largely by glutamine synthetase–catalyzed amidation of glutamate.13 Thus, a glutamine deficiency resulting from defective glutamine synthetase is likely to be detrimental, as it appeared to be in the two infants described in this report.
The damage to these infants must have originated prenatally, as suggested by the brain malformations and somatic dysmorphologic features. Such prenatal damage is unusual, since a fetus with a metabolic disorder is usually protected in utero by the metabolically normal mother through placental exchange of metabolites, so that the fetus becomes affected only after birth. Why were these infants not protected as fetuses? Studies in animals indicate that the fetus requires a very large amount of glutamine that is normally provided from glutamate by placental glutamine synthetase.14 In fetal glutamine synthetase deficiency, the glutamine supplied by the mother may not adequately compensate for severe fetal glutamine deprivation. Perhaps in the future it would be beneficial to provide supplemental glutamate to a mother carrying an affected fetus.
The most profound effect in the infants reported on by H?berle and colleagues, however, was brain dysfunction. Patient 1 was neurologically devastated, with marked flaccidity; Patient 2 had convulsions. Glutamate is the principal excitatory neurotransmitter in the central nervous system, essential for glutamatergic neurons. Because it does not cross the blood–brain barrier, it must be incorporated into cells from extracellular glutamate. However, only astrocytes take up glutamate, whereas neurons take up glutamine. The mechanism for supplying neurons with glutamate is the glutamate–glutamine cycle, in which astrocytic glutamine synthetase converts glutamate to glutamine, which is released and taken up by the glutamatergic neurons. Within the neuron, glutamine is hydrolyzed back to glutamate (Figure 1). A deficiency of glutamine synthetase in the astrocytes could have resulted in neuronal deprivation of glutamate in these infants and produced the neurologic disease.
Figure 1. Glutamate–Glutamine Cycle.
Glutamate is taken up by an astrocyte, in which glutamine synthetase converts glutamate to glutamine. Glutamine is then released and taken up by glutamatergic neurons. Within the neuron, glutaminase hydrolyzes glutamine to reform glutamate. ADP denotes adenosine diphosphate, and Pi inorganic phosphate.
Finally, an interesting and important feature of the report by H?berle and colleagues is that the condition was identified in the infants because of reduced glutamine levels, rather than increased glutamate levels. Presumably, their normal levels of glutamate reflect the many other pathways available for glutamate degradation.13 It is very unusual to diagnose an amino acid disorder solely on the basis of a decrease in the level of a specific amino acid. Consequently, the interpretation of amino acid analysis is usually directed toward increased, rather than decreased, levels. Newborn screening for metabolic disorders is almost entirely based on high levels.15 Could amino acid disorders be missed because of this selectivity? The present report dramatically shows the value of closely observing low as well as high levels. A more comprehensive interpretation of amino acid findings will probably result in identifying other as yet unknown genetic causes of disease — and will continue our march toward a more basic understanding of human disease.
Source Information
From the Department of Medicine, Division of Genetics, Children's Hospital Boston, and Harvard Medical School — both in Boston.
References
Guttmacher AE, Collins FS. Welcome to the genomic era. N Engl J Med 2003;349:996-998.
Guttmacher AE, Collins FS. Genomic medicine -- a primer. N Engl J Med 2002;347:1512-1520.
Scriver CR, Clow CL. Phenylketonuria: epitome of human biochemical genetics. N Engl J Med 1980;303:1336-42, 1394.
Kwok SCM, Ledley FD, DiLella AG, Robson KJH, Woo SLC. Nucleotide sequence of a full-length complementary DNA clone and amino acid sequence of human phenylalanine hydroxylase. Biochemistry 1985;24:556-561.
Garrod AE. Inborn errors of metabolism: the Croonian Lectures delivered before the Royal College of Physicians of London, in June 1908. London: Oxford University Press, 1909.
Mudd SH, Levy HL, Abeles RH, Kennedy JP Jr. A derangement of B12 metabolism leading to homocystinemia, cystathioninemia, and methylmalonic aciduria. Biochem Biophys Res Commun 1969;35:121-126.
McCully KS. Vascular pathology of homocysteinemia: implications for the pathogenesis of arteriosclerosis. Am J Pathol 1969;56:111-128.
Kluijtmans LAJ, Kastelein JJP, Lindemans J, et al. Thermolabile methylenetetrahydrofolate reductase in coronary artery disease. Circulation 1997;96:2573-2577.
H?berle J, G?rg B, Rutsch F, et al. Congenital glutamine deficiency with glutamine synthetase mutations. N Engl J Med 2005;353:1926-1933.
Fumarola C, Zerbini A, Guidotti GG. Glutamine deprivation-mediated cell shrinkage induces ligand-independent CD95 receptor signaling and apoptosis. Cell Death Differ 2001;8:1004-1013.
Sumbilla CM, Zielke CL, Reed WD, Ozand PT, Zielke HR. Comparison of the oxidation of glutamine, glucose, ketone bodies and fatty acids by human diploid fibroblasts. Biochim Biophys Acta 1981;675:301-304.
Hertz L, Dringen R, Schousboe A, Robinson SR. Astrocytes: glutamate producers for neurons. J Neurosci Res 1999;57:417-428.
Meister A. Biochemistry of the amino acids. 2nd ed. New York: Academic Press, 1965.
Self JL, Spencer TE, Johnson GA, Hu J, Bazer FW, Wu G. Glutamine synthesis in the developing porcine placenta. Biol Reprod 2004;70:1444-1451.
Fearing MK, Levy HL. Expanded newborn screening using tandem mass spectrometry. Adv Pediatr 2003;50:81-111.(Harvey L. Levy, M.D.)
At the center of knowledge about inherited metabolic diseases has been phenylketonuria, a disorder of phenylalanine metabolism, and newborn screening and dietary treatment have virtually eliminated mental retardation in patients with this disorder. From the understanding of the genetic basis of phenylketonuria has come the realization that there is a tangible benefit from diagnosing inherited disorders.3 Phenylketonuria also stimulated genetic research and federal funding of this research. One of the first-cloned genes involved in genetic disease was that for phenylalanine hydroxylase, the enzyme that is defective in phenylketonuria.4 The excitement of understanding phenylketonuria led to frequent metabolic testing of children with clinical disease that might be metabolic, especially those with developmental delay or other neurologic symptoms. Such strategies resulted in the discovery of many additional metabolic disorders. The need to understand the disorders motivated investigations that began to elucidate not only the metabolic defects but also areas of human biochemistry and pathology. Thus, as Garrod realized in the early 1900s as a result of his studies of alkaptonuria that led to the concept of the inborn error of metabolism, the importance of these disorders transcends their rarity; they can provide paths to greater knowledge about normal human biology and to deeper understanding of common problems.5
Homocystinuria provides a particularly striking example of this importance. This disorder is a well-delineated inborn error of methionine metabolism that causes ectopia lentis, skeletal abnormalities, mental retardation, and thromboembolism. Its cardinal biochemical feature is an increased level of homocysteine. A few years after homocystinuria was reported, Harvey Mudd and I and our colleagues studied an infant who not only had increased levels of homocysteine but also had other biochemical abnormalities that are not found in homocystinuria, including an increased level of methylmalonic acid. Eventually, we recognized that the infant had a new disorder in which vitamin B12 was not metabolized to the cobalamin coenzymes necessary to stimulate remethylation of homocysteine to methionine and to convert methylmalonic acid to succinic acid.6
On autopsy of this infant, McCully found vascular occlusions that resembled those seen in patients with homocystinuria; he suggested that the increase in the level of homocysteine was the common etiologic factor of the occlusions.7 Studies stimulated by these findings have now shown that almost 10 percent of the general population is homozygous for a thermolabile variant of methylenetetrahydrofolate reductase, an enzyme that is also responsible for remethylation. These people usually have hyperhomocysteinemia, which is considered a risk factor for cardiovascular disease.8 The reduced activity of methylenetetrahydrofolate reductase can be restored by administration of large amounts of folate, which lowers the level of homocysteine. These observations provide the basis for the current measurement of plasma homocysteine during health examinations and constitute the major reasons for the recommendation of an increase in folate intake and of a daily supplement of folic acid, especially among patients with hyperhomocysteinemia.
An article in this issue of the Journal illustrates, once again, the potential for wider implications of what is probably a rare metabolic disorder. H?berle and colleagues report on two unrelated infants who had dramatically reduced levels of glutamine due to a deficiency of glutamine synthetase, the enzyme that catalyzes the conversion of glutamate to glutamine.9 Both infants had profound cerebral disease that included malformations of the brain, and both died during the neonatal period. Molecular studies showed that each of the infants was homozygous for a different missense mutation in exon 6 of the glutamine synthetase gene; sequence alignment located the mutations in or near the glutamate-binding region of the enzyme. The immediate importance of this report is that it adds a genetic disorder to the list of those that should be considered in a neonate or, perhaps, an older infant with undiagnosed neurologic disease. The greater significance, however, is that these cases will lead to opportunities for better understanding the role that glutamine synthetase and glutamine play in cellular metabolism, especially with regard to brain functioning and development.
Glutamine is the most abundant free amino acid in the body. It plays a vital role in cell viability,10 cellular energy metabolism,11 and the availability of glutamate neurotransmitters in neurons.12 Glutamine is rendered available largely by glutamine synthetase–catalyzed amidation of glutamate.13 Thus, a glutamine deficiency resulting from defective glutamine synthetase is likely to be detrimental, as it appeared to be in the two infants described in this report.
The damage to these infants must have originated prenatally, as suggested by the brain malformations and somatic dysmorphologic features. Such prenatal damage is unusual, since a fetus with a metabolic disorder is usually protected in utero by the metabolically normal mother through placental exchange of metabolites, so that the fetus becomes affected only after birth. Why were these infants not protected as fetuses? Studies in animals indicate that the fetus requires a very large amount of glutamine that is normally provided from glutamate by placental glutamine synthetase.14 In fetal glutamine synthetase deficiency, the glutamine supplied by the mother may not adequately compensate for severe fetal glutamine deprivation. Perhaps in the future it would be beneficial to provide supplemental glutamate to a mother carrying an affected fetus.
The most profound effect in the infants reported on by H?berle and colleagues, however, was brain dysfunction. Patient 1 was neurologically devastated, with marked flaccidity; Patient 2 had convulsions. Glutamate is the principal excitatory neurotransmitter in the central nervous system, essential for glutamatergic neurons. Because it does not cross the blood–brain barrier, it must be incorporated into cells from extracellular glutamate. However, only astrocytes take up glutamate, whereas neurons take up glutamine. The mechanism for supplying neurons with glutamate is the glutamate–glutamine cycle, in which astrocytic glutamine synthetase converts glutamate to glutamine, which is released and taken up by the glutamatergic neurons. Within the neuron, glutamine is hydrolyzed back to glutamate (Figure 1). A deficiency of glutamine synthetase in the astrocytes could have resulted in neuronal deprivation of glutamate in these infants and produced the neurologic disease.
Figure 1. Glutamate–Glutamine Cycle.
Glutamate is taken up by an astrocyte, in which glutamine synthetase converts glutamate to glutamine. Glutamine is then released and taken up by glutamatergic neurons. Within the neuron, glutaminase hydrolyzes glutamine to reform glutamate. ADP denotes adenosine diphosphate, and Pi inorganic phosphate.
Finally, an interesting and important feature of the report by H?berle and colleagues is that the condition was identified in the infants because of reduced glutamine levels, rather than increased glutamate levels. Presumably, their normal levels of glutamate reflect the many other pathways available for glutamate degradation.13 It is very unusual to diagnose an amino acid disorder solely on the basis of a decrease in the level of a specific amino acid. Consequently, the interpretation of amino acid analysis is usually directed toward increased, rather than decreased, levels. Newborn screening for metabolic disorders is almost entirely based on high levels.15 Could amino acid disorders be missed because of this selectivity? The present report dramatically shows the value of closely observing low as well as high levels. A more comprehensive interpretation of amino acid findings will probably result in identifying other as yet unknown genetic causes of disease — and will continue our march toward a more basic understanding of human disease.
Source Information
From the Department of Medicine, Division of Genetics, Children's Hospital Boston, and Harvard Medical School — both in Boston.
References
Guttmacher AE, Collins FS. Welcome to the genomic era. N Engl J Med 2003;349:996-998.
Guttmacher AE, Collins FS. Genomic medicine -- a primer. N Engl J Med 2002;347:1512-1520.
Scriver CR, Clow CL. Phenylketonuria: epitome of human biochemical genetics. N Engl J Med 1980;303:1336-42, 1394.
Kwok SCM, Ledley FD, DiLella AG, Robson KJH, Woo SLC. Nucleotide sequence of a full-length complementary DNA clone and amino acid sequence of human phenylalanine hydroxylase. Biochemistry 1985;24:556-561.
Garrod AE. Inborn errors of metabolism: the Croonian Lectures delivered before the Royal College of Physicians of London, in June 1908. London: Oxford University Press, 1909.
Mudd SH, Levy HL, Abeles RH, Kennedy JP Jr. A derangement of B12 metabolism leading to homocystinemia, cystathioninemia, and methylmalonic aciduria. Biochem Biophys Res Commun 1969;35:121-126.
McCully KS. Vascular pathology of homocysteinemia: implications for the pathogenesis of arteriosclerosis. Am J Pathol 1969;56:111-128.
Kluijtmans LAJ, Kastelein JJP, Lindemans J, et al. Thermolabile methylenetetrahydrofolate reductase in coronary artery disease. Circulation 1997;96:2573-2577.
H?berle J, G?rg B, Rutsch F, et al. Congenital glutamine deficiency with glutamine synthetase mutations. N Engl J Med 2005;353:1926-1933.
Fumarola C, Zerbini A, Guidotti GG. Glutamine deprivation-mediated cell shrinkage induces ligand-independent CD95 receptor signaling and apoptosis. Cell Death Differ 2001;8:1004-1013.
Sumbilla CM, Zielke CL, Reed WD, Ozand PT, Zielke HR. Comparison of the oxidation of glutamine, glucose, ketone bodies and fatty acids by human diploid fibroblasts. Biochim Biophys Acta 1981;675:301-304.
Hertz L, Dringen R, Schousboe A, Robinson SR. Astrocytes: glutamate producers for neurons. J Neurosci Res 1999;57:417-428.
Meister A. Biochemistry of the amino acids. 2nd ed. New York: Academic Press, 1965.
Self JL, Spencer TE, Johnson GA, Hu J, Bazer FW, Wu G. Glutamine synthesis in the developing porcine placenta. Biol Reprod 2004;70:1444-1451.
Fearing MK, Levy HL. Expanded newborn screening using tandem mass spectrometry. Adv Pediatr 2003;50:81-111.(Harvey L. Levy, M.D.)