163D.H. Best and J.J. Swensen (eds.), Molecular Genetics and Personalized Medicine, Molecular and Translational Medicine, DOI 10.1007/978-1-61779-530-5_8, Springer Science+Business Media, LLC 2012
Metabolic disorders affect the transformation of nutrients in energy or in other compounds necessary for growth. They are due to defi ciencies in enzymes or trans-porters involved in the metabolism of sugars, amino acids, fatty acids, and macro-molecules. There is a wide spectrum of clinical presentation with onset of symptoms ranging from the newborn period to adult life. The consequences of untreated met-abolic disorders can be extremely severe and devastating, independently from the age of onset. Early identifi cation and treatment of these conditions before symp-toms appear and irreversible damage has occurred can improve overall outcome and quality of life, with reduction of morbidity, mortality, and disabilities. In recent years, advances in technology, such as the introduction of tandem mass spectrom-etry, has enabled neonatal screening for many diseases caused by impaired metab-olism of amino acids and fatty acids through multiplex analysis of several metabolites. This chapter will discuss some of the most common disorders of metabolism identifi ed through universal newborn screening and how molecular genetics can integrate the screening and biochemical tests to reach a defi nite diag-nosis of metabolic disorders in the asymptomatic patient.
M. Pasquali , Ph.D., FACMG (*) Department of Pathology , University of Utah and ARUP Laboratories , 500 Chipeta Way , Salt Lake City , UT 84108 , USA e-mail: firstname.lastname@example.org
N. Longo , M.D., Ph.D. Division of Medical Genetics, Department of Pediatrics , University of Utah, 2C412 SOM, 50 N Mario Capecchi Drive, School of Medicine , Salt Lake City , UT 84132, USA e-mail: Nicola.Longo@hsc.utah.edu
Chapter 8 Newborn Screening for Metabolic Disorders
Marzia Pasquali and Nicola Longo
164 M. Pasquali and N. Longo
Newborn screening is a public health activity that started in the early 1960s, thanks to Dr. Robert Guthrie, who developed a bacterial inhibition assay to screen for phenylketonuria using newborns blood spotted and dried on fi lter paper [ 1 ] .
Since then, millions of infants in the United States have been screened for a variety of genetic and congenital disorders.
Each state in the United States decides the panel of disorders included in their newborn screening programs. The use of tandem mass spectrometry (MS/MS), detecting simultaneously amino acids and acylcarnitines in the same blood spot, has greatly increased the number of disorders amenable to newborn screening [ 2, 3 ] . In 2005, the American College of Medical Genetics (ACMG) released a report, commissioned by the Maternal and Child Health Bureau (MCHB) of HRSA (Health Resources and Services Administration), with recommendations for a uniform panel for newborn screening [ 4 ] . This report recommended screening all newborns in the United States for 29 conditions, including fi ve fatty acid oxidation disorders, nine organic acidemias, six aminoacidopathies, three hemoglobinopathies, and six other disorders (Table 8.1 ). Of these conditions, 20 are screened for using tandem mass spectrometry, while the others use more traditional methods (isoelectrofocusing, immunoassay, HPLC, etc.). At the beginning of 2011, the addition of SCID (severe combined immunodefi ciency) to the uniform panel was recommended and intro-duced the fi rst DNA-based screening test in newborn screening [ 5 ] .
Two main classes of metabolites are detected by tandem mass spectrometry: amino acids and acylcarnitines. Amino acids become elevated in certain aminoaci-dopathies and urea cycle defects (phenylketonuria, tyrosinemia, maple syrup urine disease, etc.), while the study of the acylcarnitine profi le can identify defects of fatty acid oxidation (medium-chain acyl-CoA dehydrogenase defi ciency, very-long-chain acyl-CoA dehydrogenase defi ciency, and others) and organic acidemias (propionic acidemia, methylmalonic acidemia, glutaric acidemia type I, etc.). Disorders of car-bohydrate metabolism (such as galactosemia) cannot yet be detected by MS/MS; however, methods are in development [ 6 8 ] .
Blood is collected from a heel stick from each newborn and spotted on a fi lter paper card, and then the card is sent to a centralized laboratory where testing occurs. Typically, the sample is collected prior to discharge from the hospital of birth, between 24 and 48 h of life. This allows enough time to have a build-up of abnormal metabolites as a result of a full feeding, necessary to identify amino acidopathies, yet maintaining the ability to identify fatty acid oxidation disorders, which require a more stressed sample. Abnormal results, i.e., results outside the range observed for the normal population, are followed up with diagnostic tests to confi rm or exclude a diagnosis of a metabolic disorder.
Typically, biochemical genetics tests (amino acids, organic acids, acylcarnitines analyses in blood, and/or urine) represent the fi rst line of testing and are suffi cient to diagnose or exclude a metabolic disorder. This is especially true when patients are symptomatic and their biochemical phenotype is grossly abnormal. With the expansion of newborn screening, metabolic disorders are now identifi ed before the
1658 Newborn Screening for Metabolic Disorders
Table 8.1 Recommended uniform screening panel (core conditions) of the Secretarys Advisory Committee on Heritable Disorders in Newborns and Children ACMG code Core condition
Organic acidemia PROP Propionic acidemia MUT Methylmalonic acidemia (methylmalonyl-CoA mutase) Cbl A,B Methylmalonic acidemia (cobalamin disorders) IVA Isovaleric acidemia 3-MCC 3-Methylcrotonyl-CoA carboxylase defi ciency HMG 3-Hydroxy-3-methylglutaric aciduria MCD Holocarboxylase synthase defi ciency KT -Ketothiolase defi ciency GA1 Glutaric acidemia type I
Fatty acid oxidation CUD Carnitine uptake defect/carnitine transport defect MCAD Medium-chain acyl-CoA dehydrogenase defi ciency VLCAD Very-long-chain acyl-CoA dehydrogenase defi ciency LCHAD Long-chain L-3 hydroxyacyl-CoA dehydrogenase defi ciency TFP Trifunctional protein defi ciency
Amino acidopathy ASA Argininosuccinic aciduria CIT Citrullinemia, type I MSUD Maple syrup urine disease HCY Homocystinuria PKU Classic phenylketonuria TYR I Tyrosinemia, type I
Endocrinopathy CH Primary congenital hypothyroidism CAH Congenital adrenal hyperplasia
Hemoglobinopathy Hb SS S,S disease (sickle cell anemia) Hb S/Th S, b -thalassemia Hb S/C S,C disease
Others BIOT Biotinidase defi ciency CCCHD Critical cyanotic congenital heart disease (pending secretary approval) CF Cystic fi brosis GALT Classic galactosemia HEAR Hearing loss SCID Severe combined immunodefi ciencies The nomenclature for conditions is based on the report Naming and counting disorders (conditions) included in newborn screening panels Pediatrics 2006;117(5) Suppl:S308-14
patients become symptomatic, at least in the majority of the cases. This brings an additional level of complexity, as the biochemical phenotype is now less obvious, and the fi nal diagnosis may require additional tests such as enzyme/receptor/trans-porter assays and/or molecular analysis. In addition, for some metabolic condi-tions, heterozygotes may show the same abnormalities as the affected individuals;
166 M. Pasquali and N. Longo
therefore, the inclusion of molecular analysis in confi rmatory algorithms (see, e.g., those recommended by the American College of Medical Genetics: http://www.acmg.net/AM/Template.cfm?Section=NBS_ACT_Sheets_and_Algorithms_Table&Template=/CM/HTMLDisplay.cfm&ContentID=5072 ) is increasingly used.
This chapter will review the groups of metabolic disorders identifi ed by newborn screening, with special emphasis on the molecular diagnosis of these conditions.
Disorders of Amino Acid Metabolism
The concentrations of free amino acids in physiological fl uids refl ects the balance between exogenous intake, endogenous release from the catabolism of proteins, the kidney function responsible for fi ltration and reabsorption, and the body utilization to synthesize proteins or produce energy. Changes in any of these steps can result in accumulation or defi ciency of one or more amino acids.
The aminoacidopathies phenylketonuria, maple syrup urine disease, homocysti-nuria, tyrosinemia type I, and the two defects of the urea cycle, citrullinemia and argininosuccinic aciduria, are the amino acid disorders included in the core panel of conditions recommended by the American College of Medical Genetics to be screened for in each infant (Table 8.1 ). The characteristic amino acids elevated in these conditions are identifi ed by tandem mass spectrometry. Other disorders of the urea cycle (such as ornithine transcarbamylase defi ciency and carbamyl phosphate synthase defi ciency), characterized by low concentrations of citrulline and arginine, are not included in this panel because the reliability of newborn screening in identi-fying analytes present at low concentrations is not optimal. To overcome this prob-lem, the use of ratios of amino acids seems promising. Table 8.2 lists selected disorders of amino acid metabolism identifi able by newborn screening and the strat-egy for confi rming or excluding the diagnosis.
Phenylketonuria (PKU) results from the impaired conversion of p