An international classification of inherited metabolic disorders (ICIMD)

Several initiatives at establishing a classification of inherited metabolic disorders have been published previously, some focusing on pathomechanisms, others on clinical manifestations, while yet another attempted a simplified approach of a comprehensive nosology. Some of these classifications suffered from shortcomings, such as lack of a mechanism for continuous update in light of a rapidly evolving field, or lack of widespread input from the metabolic community at large. Our classification-the International Classification of Inherited Metabolic Disorders, or International Classification of Inborn Metabolic Disorders (ICIMD)-includes 1450 disorders, and differs from prior approaches in that it benefited from input by a large number of experts in the field, and was endorsed by major metabolic societies around the globe. Several criteria such as pathway involvement and pathomechanisms were considered. The main purpose of the hierarchical, group-based approach of the ICIMD is an improved understanding of the interconnections between many individual conditions that may share functional, clinical, and diagnostic features. The ICIMD aims to include any primary genetic condition in which alteration of a biochemical pathway is intrinsic to specific biochemical, clinical, and/or pathophysiological features. As new disorders are discovered, we will seek the opinion of experts in the advisory board prior to inclusion in the appropriate group of the ICIMD, thus guaranteeing the continuing relevance of this classification via regular curation and expert advice.

Clinical manifestation of myeloperoxidase deficiency

Myeloperoxidase (MPO), an iron-containing heme protein localized in the azurophilic granules of neutrophil granulocytes and in the lysosomes of monocytes, is involved in the killing of several micro-organisms and foreign cells, including bacteria, fungi, viruses, red cells, and malignant and nonmalignant nucleated cells. Despite the primary role of the oxygen-dependent MPO system in the destruction of certain phagocytosed microbes, subjects with total or partial MPO deficiency generally do not have an increased frequency of infections, probably because other MPO-independent mechanism(s) for microbicidal activity compensate for the lack of MPO. Infectious diseases, especially with species of Candida, have been observed predominantly in MPO-deficient patients who also have diabetes mellitus, but the frequency of such cases is very low, less than 5% of reported MPO-deficient subjects. Evidence from a number of investigators indicates that individuals with total MPO deficiency show a high incidence of malignant tumors. Since MPO-deficient PMNs exhibit in vitro a depressed lytic action against malignant human cells, it can be speculated that the neutrophil MPO system plays a central role in the tumor surveillance of the host. However, any definitive conclusion on the association between MPO deficiency and the occurrence of cancers needs to be confirmed in further clinical studies. Clinical manifestations of this disorder depend on the nature of the defect; an acquired abnormality associated with other hematological or nonhematological diseases has been occasionally described, but the primary deficiency is the form more commonly reported. Another area of interest pertinent to MPO expression is related to the use of anti-MPO monoclonal antibodies for the lineage assignment of acute leukemic cells, the definition of FAB MO acute myeloid leukemia, the identification of biphenotypic acute leukemias, and their distinction from acute leukemia with minimal phenotypic deviation. The advantage of MPO monoclonal antibodies over the MPO cytochemical assay relies in the ability of the former method to recognize the enzymatically inactive precursor forms of MPO.

Psychiatric manifestations revealing inborn errors of metabolism in adolescents and adults

Inborn errors of metabolism (IEMs) may present in adolescence or adulthood as a psychiatric disorder. In some instances, an IEM is suspected because of informative family history or because psychiatric symptoms form part of a more diffuse clinical picture with systemic, cognitive or motor neurological signs. However, in some cases, psychiatric signs may be apparently isolated. We propose a schematic classification of IEMs into three groups according to the type of psychiatric signs at onset. Group 1 represents emergencies, in which disorders can present with acute and recurrent attacks of confusion, sometimes misdiagnosed as acute psychosis. Diseases in this group include urea cycle defects, homocysteine remethylation defects and porphyrias. Group 2 includes diseases with chronic psychiatric symptoms arising in adolescence or adulthood. Catatonia, visual hallucinations, and aggravation with treatments are often observed. This group includes homocystinurias, Wilson disease, adrenoleukodystrophy and some lysosomal disorders. Group 3 is characterized by mild mental retardation and late-onset behavioural or personality changes. This includes homocystinurias, cerebrotendinous xanthomatosis, nonketotic hyperglycinaemia, monoamine oxidase A deficiency, succinic semialdehyde dehydrogenase deficiency, creatine transporter deficiency, and alpha and beta mannosidosis. Because specific treatments should be more effective at the 'psychiatric stage' before the occurrence of irreversible neurological lesions, clinicians should be aware of atypical psychiatric symptoms or subtle organic signs that are suggestive of an IEM. Here we present an overview of IEMs potentially revealed by psychiatric problems in adolescence or adulthood and provide a diagnostic strategy to guide metabolic investigations.

Newborn mass screening and selective screening using electrospray tandem mass spectrometry in Japan

Electrospray tandem mass spectrometry was applied to detect a series of inherited metabolic disorders during a newborn-screening pilot study and a selective screening in Japan. In our mass screening of 102,200 newborns, five patients with propionic acidemia, two with methylmalonic acidemia, two with medium-chain acyl-CoA dehydrogenase deficiency, three with citrullinemia type II, and one with phenylketonuria were identified. In a selective screening of 164 patients with symptoms mainly related to hypoglycemia and/or hyperammonemia, 12 with fatty acid oxidation disorders and six with other disorders were found. The results indicated the importance of newborn screening using this technology in Japan.

The clinical phenotype of succinic semialdehyde dehydrogenase deficiency (4-hydroxybutyric aciduria): case reports of 23 new patients

OBJECTIVES

To further define the clinical spectrum of the disease for pediatric and metabolic specialists, and to suggest that the general pediatrician and pediatric neurologist consider succinic semialdehyde dehydrogenase (SSADH) deficiency in the differential diagnosis of patients with (idiopathic) mental retardation and emphasize the need for accurate, quantitative organic acid analysis in such patients.

PATIENTS

The clinical features of 23 patients (20 families) with SSADH deficiency (4-hydroxybutyric acid-uria) are presented. The age at diagnosis ranged from 3 months to 25 years in the 11 male and 12 female patients; consanguinity was noted in 39% of families.

OUTCOME MEASUREMENTS

The following abnormalities were observed (frequency in 23 patients): motor delay, including fine-motor skills, 78%; language delay, 78%; hypotonia, 74%; mental delay, 74%; seizures, 48%; decreased or absent reflexes, 39%; ataxia, 30%; behavioral problems, 30%; hyperkinesis, 30%; neonatal problems, 26%; and electroencephalographic abnormalities, 26%. Associated findings included psychoses, cranial magnetic resonance or computed tomographic abnormalities, and ocular problems in 22% or less of patients. Therapy with vigabatrin proved beneficial to varying degrees in 35% of the patients. Normal early development was noted in 30% of patients.

CONCLUSIONS

Our data imply that two groups of patients with SSADH deficiency exist, differentiated by the course of early development. Our recommendation would be that accurate, quantitative organic acid analysis in an appropriate specialist laboratory be requested for any patients presenting with two or more features of mental, motor, or language delay and hypotonia of unknown cause. Such analyses are the only definitive way to diagnose SSADH deficiency; the diagnosis can be confirmed by determination of enzyme activity in white cells from whole blood. We think that increased use of organic acid determination will lead to increased diagnosis of SSADH deficiency and a more accurate representation of disease frequency. As additional patients are identified, we should have a better understanding of both the metabolic and clinical profiles of SSADH deficiency.

Clinical approach to treatable inborn metabolic diseases: an introduction

In view of the major improvements in treatment, it has become increasingly important that in order for first-line physicians not to miss a treatable disorder they should be able initiate a simple method of clinical screening, particularly in the emergency room. We present a simplified classification of treatable inborn errors of metabolism in three groups. Group 1 includes inborn errors of intermediary metabolism that give rise to an acute or chronic intoxication. It encompasses aminoacidopathies, organic acidurias, urea cycle disorders, sugar intolerances, metal disorders and porphyrias. Clinical expression can be acute or systemic or can involve a specific organ, and can strike in the neonatal period or later and intermittently from infancy to late adulthood. Most of these disorders are treatable and require the emergency removal of the toxin by special diets, extracorporeal procedures, cleansing drugs or vitamins. Group 2 includes inborn errors of intermediary metabolism that affect the cytoplasmic and mitochondrial energetic processes. Cytoplasmic defects encompass those affecting glycolysis, glycogenosis, gluconeogenesis, hyperinsulinisms, and creatine and pentose phosphate pathways; the latter are untreatable. Mitochondrial defects include respiratory chain disorders, and Krebs cycle and pyruvate oxidation defects, mostly untreatable, and disorders of fatty acid oxidation and ketone bodies that are treatable. Group 3 involves cellular organelles and includes lysosomal, peroxisomal, glycosylation, and cholesterol synthesis defects. Among these, some lysosomal disorders can be efficiently treated by enzyme replacement or substrate reduction therapies. Physicians can be faced with the possibility of a treatable inborn error in an emergency, either in the neonatal period or late in infancy to adulthood, or as chronic and progressive symptoms--general (failure to thrive), neurological, or specific for various organs or systems. These symptoms are summarized in four tables. In addition, an extensive list of medications used in the treatment of inborn errors is presented.

Hepatic cirrhosis, dystonia, polycythaemia and hypermanganesaemia--a new metabolic disorder

We report a new constellation of clinical features consisting of hypermanganesaemia, liver cirrhosis, an extrapyramidal motor disorder and polycythaemia in a 12 year-old girl born to consanguineous parents. Blood manganese levels were >3000 nmol/L (normal range <320 nmol/L) and MRI revealed signal abnormalities of the basal ganglia consistent with manganese deposition. An older brother with the same phenotype died at 18 years, suggesting a potentially lethal, autosomal recessive disease. This disorder is probably caused by a defect of manganese metabolism with the accumulation of manganese in the liver and the basal ganglia similar to the copper accumulation in Wilson disease. In order to assess the genetic basis of this syndrome we investigated two candidate genes: ATP2C2 and ATP2A3 encoding the manganese-transporting calcium-ATPases, SPCA2 and SERCA3, respectively. Genotyping of the patient and the family for microsatellite markers surrounding ATP2C2 and ATP2A3 excluded these genes. The patient was found to be heterozygous for both gene loci. Despite the unknown pathophysiology, we were able to develop a successful treatment regime. Chelation therapy with disodium calcium edetate combined with iron supplementation is the treatment of choice, lowering blood manganese levels significantly and improving clinical symptoms.

The inborn errors of peroxisomal beta-oxidation: a review

In recent years a growing number of inherited diseases in man have been recognized in which there is an impairment in peroxisomal beta-oxidation. In some diseases this is due to the (virtual) absence of peroxisomes leading to a generalized loss of peroxisomal functions including peroxisomal beta-oxidation. In most inborn errors of peroxisomal beta-oxidation, however, peroxisomes are normally present and the impairment in peroxisomal beta-oxidation is due to the single or multiple loss of peroxisomal beta-oxidation enzyme activities. In all these disorders there is accumulation of very-long-chain fatty acids in plasma, which allows biochemical diagnosis of patients affected by an inborn error of peroxisomal beta-oxidation to be done via gas-chromatographic analysis of plasma very-long-chain fatty acids. Subsequent enzymic and immunological investigations are required to identify the precise enzymic defects in these patients. In all inborn errors of peroxisomal beta-oxidation known today there are multiple abnormalities, especially neurological with death usually occurring in the first decade of life. Prenatal diagnosis of these disorders has recently become possible.

The spectrum of hypoxanthine-guanine phosphoribosyltransferase (HPRT) deficiency. Clinical experience based on 22 patients from 18 Spanish families

The enzyme hypoxanthine-guanine phosphoribosyltransferase (HPRT) catalyzes the reutilization of hypoxanthine and guanine to the purine nucleotides IMP and GMP, respectively. HPRT deficiency is an X-linked disorder characterized by uric acid overproduction and variable neurologic impairment. The complete deficiency of HPRT is diagnostic of Lesch-Nyhan syndrome manifested by choreoathetosis, spasticity, mental retardation, and self-injurious behavior. In some HPRT-deficient patients the enzyme defect appeared to be "partial" and the neurologic symptoms mild to severe (Kelley-Seegmiller syndrome). This has prompted the classification of HPRT deficiency in 2 distinct groups: Lesch-Nyhan syndrome and Kelley-Seegmiller syndrome, which has created much confusion. A spectrum of clinical consequences of HPRT deficiency has been recognized in small series of patients, but the complete spectrum of the neurologic disorder has not been described in a single series of patients examined by the same observers. We analyzed our experience with 22 patients belonging to 18 different families with HPRT deficiency diagnosed at "La Paz" University Hospital in Madrid over the past 16 years. The clinical spectrum of these HPRT-deficient Spanish patients was similar to the different phenotypes occasionally reported in the literature, in some cases diagnosed as Lesch-Nyhan "variants." The clinical, biochemical, enzymatic, and molecular genetic studies on these 22 patients allowed us to delineate a new classification of HPRT deficiency. Based on the neurologic symptoms, dependency for personal care, HPRT activity in hemolysate and in intact erythrocytes, and predicted protein size, patients were classified into 4 groups: Group 1 (2 patients), normal development with no neurologic symptoms, HPRT activity was detectable in hemolysates and in intact erythrocytes, and the mutation did not affect the predicted protein size. Group 2 (3 patients) mild neurologic symptoms that did not prevent independent lives, HPRT activity was detectable in intact erythrocytes, and the protein size was normal. Group 3 (2 patients), severe neurologic impairment that precluded an independent life, no residual HPRT activity, and normal protein size. Group 4 (15 patients), clinical characteristics of Lesch-Nyhan syndrome (some may not show self-injurious behavior), no residual HPRT activity, and in most (7 of 8 patients in whom the mutation could be detected) the mutation affected the predicted protein size. This classification of HPRT deficiency into 4 groups may be more useful in terms of accuracy, reproducibility, assessment for treatment trials and prognosis. The study of this Spanish series allows us to conclude that HPRT deficiency may be manifested by a wide spectrum of neurologic symptoms; the overall severity of the disease is associated with mutations permitting some degree of residual enzyme activity; and mutation analysis provides a valuable tool for prognosis, carrier identification, and prenatal diagnosis.

Inborn errors of metabolism with 3-methylglutaconic aciduria as discriminative feature: proper classification and nomenclature

Increased urinary 3-methylglutaconic acid excretion is a relatively common finding in metabolic disorders, especially in mitochondrial disorders. In most cases 3-methylglutaconic acid is only slightly elevated and accompanied by other (disease specific) metabolites. There is, however, a group of disorders with significantly and consistently increased 3-methylglutaconic acid excretion, where the 3-methylglutaconic aciduria is a hallmark of the phenotype and the key to diagnosis. Until now these disorders were labelled by roman numbers (I-V) in the order of discovery regardless of pathomechanism. Especially, the so called "unspecified" 3-methylglutaconic aciduria type IV has been ever growing, leading to biochemical and clinical diagnostic confusion. Therefore, we propose the following pathomechanism based classification and a simplified diagnostic flow chart for these "inborn errors of metabolism with 3-methylglutaconic aciduria as discriminative feature". One should distinguish between "primary 3-methylglutaconic aciduria" formerly known as type I (3-methylglutaconyl-CoA hydratase deficiency, AUH defect) due to defective leucine catabolism and the--currently known--three groups of "secondary 3-methylglutaconic aciduria". The latter should be further classified and named by their defective protein or the historical name as follows: i) defective phospholipid remodelling (TAZ defect or Barth syndrome, SERAC1 defect or MEGDEL syndrome) and ii) mitochondrial membrane associated disorders (OPA3 defect or Costeff syndrome, DNAJC19 defect or DCMA syndrome, TMEM70 defect). The remaining patients with significant and consistent 3-methylglutaconic aciduria in whom the above mentioned syndromes have been excluded, should be referred to as "not otherwise specified (NOS) 3-MGA-uria" until elucidation of the underlying pathomechanism enables proper (possibly extended) classification.

Clinical approach to inherited metabolic disorders in neonates: an overview

There are almost one hundred inborn errors of metabolism which can start in the neonatal period, but less than 20 are amenable to treatment. In general, an extremely evocative clinical setting is the course of a full-term baby born after normal pregnancy and delivery who, after an initial symptom-free period deteriorates relentlessly for no apparent reason and does not respond to symptomatic therapy. Investigations routinely performed in all sick neonates yield normal results. Emergency treatment must be undertaken in parallel with investigations. Five main presentations can be observed: a neurologic deterioration 'intoxication' type mostly suggests maple syrup urine disease, methylmalonic, propionic, isovaleric acidaemias and urea cycle disorders. Isolated seizures is the revealing symptom of pyridoxine-responsive and folinic acid responsive seizures. A jaundice or a liver failure suggest galactosaemia, fructosaemia, tyrosinaemia type I (after 3 weeks), phosphomannoisomerase deficiency or bile acid synthesis defects. Cardiac failure and heartbeat disorders should first suggest mitochondrial fatty acid oxidation (FAO) disorders. Persistent hypoglycaemia is the presenting sign of glyco/gluconeogeneis defects, hyperinsulinism and FAO disorders. The first line investigation relies upon the collection at the same time of a few samples including blood gases electrolytes, prothrombin time, transaminases, ammonia and lactic acid, and the search for ketonuria. The storage of plasma, urine and blood (on filter paper) is an important element in the diagnosis. The utilization of these samples should be carefully planned after taking advice from specialists in inborn errors.

Emergency management of inherited metabolic diseases

Inherited metabolic diseases with acute severe manifestations can be divided into five categories: (1) disorders of the intoxication type, (2) disorders with reduced fasting tolerance, (3) disorders with disturbed energy metabolism, (4) disorders of neurotransmission and (5) disorders in which no specific emergency treatment is available. Diagnostic emergency laboratory evaluation should cover all differential diagnoses that are therapeutically relevant and should always include ammonia, glucose, lactate and acid-base status as well as testing the urine for ketones. These are indispensable for planning and conducting the first steps of metabolic emergency treatment and should be available within 30 min. According to the clinical situation and biochemical derangement, special metabolic investigations must be initiated in parallel. These include acylcarnitine profiling with tandem mass spectrometry (in plasma or dried blood spots) and analysis of amino acids in plasma and of organic acids in urine. The results of all laboratory investigations relevant to the diagnosis of metabolic disorders for which specific emergency therapy exists should be available within 24 h. There is general agreement with regard to some therapeutic strategies that are clearly explained by pathophysiology: in disorders with endogenous intoxication, anabolism must be promoted and specific detoxification measures initiated. In disorders with reduced fasting tolerance, administration of glucose at the rate of hepatic glucose production forms the basis of treatment. Correction of acidosis is a major goal in disorders with disturbed mitochondrial energy metabolism, while glucose supply may have to be limited. Many current therapeutic strategies are based on case reports and personal experiences at different metabolic centres. The aim of devising the 'best' management is often hampered by the lack of objective evidence of efficacy.

Genomic structure and mutational spectrum of the bicistronic MOCS1 gene defective in molybdenum cofactor deficiency type A

Molybdenum cofactor (MoCo) deficiency is a rare and devastating disease resulting in neonatal seizures and other neurological symptoms identical to those of sulphite oxidase deficiency. It is an autosomal recessive disease and no therapy is known. Most patients harbour MOCS1 mutations, which are found in both open reading frames of this unusual gene encoding the first two enzymes required in the MoCo biosynthesis pathway, MOCS1 A and MOCS1 B, in a single transcript. We describe genomic details as a prerequisite for comprehensive mutation analysis. In an initial cohort of 24 MoCo deficiency patients, we identified 13 different mutations on 34 chromosomes, with a mutation detection rate of 70%. Five mutations were observed in more than one patient and together accounted for two thirds of detected mutations. These comprise the most frequent mutation, R319Q, which is restricted to England, two Danish/German mutations (one missense and one splice site mutation), a missense mutation found in England and Germany, and a "Mediterranean" frameshift mutation. All patients with identified mutations are either homozygous or compound heterozygous for mutations in either of the two open reading frames corresponding to MOCS1 A and MOCS1 B, respectively. This observation suggests the existence of more than the two previously described complementation groups in MoCo biosynthesis.

The cost-effectiveness of expanding newborn screening for up to 21 inherited metabolic disorders using tandem mass spectrometry: results from a decision-analytic model

OBJECTIVES

In 2005, in Ontario, Canada, newborns were only screened for phenylketonuria (PKU) and hypothyroidism. Tandem mass spectrometry (MS/MS) has since been implemented as a new screening technology because it can screen for PKU and many other diseases simultaneously. We estimated the cost-effectiveness of using this technology to expand the Ontario newborn screening program to screen for each disease independently and for hypothetical bundles of up to 21 metabolic diseases.

METHODS

We constructed a decision-analytic model to estimate the incremental costs and life-years of survival that can be gained by screening or changing screening technologies. Costs and health benefits were estimated for a cohort of babies born in Ontario in 1 year. Secondary sources and expert opinion were used to estimate the test characteristics, disease prevalence, treatment effectiveness, disease progression rates, and mortality. The London Health Sciences Centre Case Costing Initiative, the Ontario Health Insurance Plan Schedule, and the Ontario Drug Benefits plan formulary were used to estimate costs.

RESULTS

Changing screening technologies, from the Guthrie test to MS/MS, for PKU detection had an incremental cost of $5,500,000 per life-year (LY) gained. We identified no diseases for which the incremental cost of screening for just that disease was less than $100,000 per LY gained. The incremental costs of screening ranged from $222,000 (HMG-CoA lyase deficiency) to $142,500,000 (glutaric acidemia type II) per LY gained. Screening for a bundle of diseases including PKU and the 14 most cost-effective diseases to screen for cost less than $70,000 per LY gained, and the incremental cost-effectiveness of adding each of the 14 diseases to the bundle was less than $100,000 per LY gained. The incremental cost of adding the 15th most cost-effective disease was $309,400 per LY gained.

CONCLUSIONS

Early diagnosis and treatment of metabolic disease is important to reduce disease severity and delay or prevent the onset of the disease. Screening at birth reduces the morbidity, mortality, and social burden associated with the irreversible effects of disease on the population. Our analysis suggests that the cost-efficiencies gained by using MS/MS to screen for bundles of diseases rather than just one disease are sufficient to warrant consideration of an expanded screening program. It is, however, not cost-effective to screen for all diseases that can be screened for using this technology.

Targeted Next Generation Sequencing in Patients with Inborn Errors of Metabolism

Background

Next-generation sequencing (NGS) technology has allowed the promotion of genetic diagnosis and are becoming increasingly inexpensive and faster. To evaluate the utility of NGS in the clinical field, a targeted genetic panel approach was designed for the diagnosis of a set of inborn errors of metabolism (IEM). The final aim of the study was to compare the findings for the diagnostic yield of NGS in patients who presented with consistent clinical and biochemical suspicion of IEM with those obtained for patients who did not have specific biomarkers.

Methods

The subjects studied (n = 146) were classified into two categories: Group 1 (n = 81), which consisted of patients with clinical and biochemical suspicion of IEM, and Group 2 (n = 65), which consisted of IEM cases with clinical suspicion and unspecific biomarkers. A total of 171 genes were analyzed using a custom targeted panel of genes followed by Sanger validation.

Results

Genetic diagnosis was achieved in 50% of patients (73/146). In addition, the diagnostic yield obtained for Group 1 was 78% (63/81), and this rate decreased to 15.4% (10/65) in Group 2 (X² = 76.171; p < 0.0001).

Conclusions

A rapid and effective genetic diagnosis was achieved in our cohort, particularly the group that had both clinical and biochemical indications for the diagnosis.

Supervised machine learning techniques for the classification of metabolic disorders in newborns

MOTIVATION

During the Bavarian newborn screening programme all newborns have been tested for about 20 inherited metabolic disorders. Owing to the amount and complexity of the generated experimental data, machine learning techniques provide a promising approach to investigate novel patterns in high-dimensional metabolic data which form the source for constructing classification rules with high discriminatory power.

RESULTS

Six machine learning techniques have been investigated for their classification accuracy focusing on two metabolic disorders, phenylketo nuria (PKU) and medium-chain acyl-CoA dehydrogenase deficiency (MCADD). Logistic regression analysis led to superior classification rules (sensitivity >96.8%, specificity >99.98%) compared to all investigated algorithms. Including novel constellations of metabolites into the models, the positive predictive value could be strongly increased (PKU 71.9% versus 16.2%, MCADD 88.4% versus 54.6% compared to the established diagnostic markers). Our results clearly prove that the mined data confirm the known and indicate some novel metabolic patterns which may contribute to a better understanding of newborn metabolism.

Proposal for a simplified classification of IMD based on a pathophysiological approach: A practical guide for clinicians

In view of the rapidly expanding number of IMD discovered by next generation sequencing, we propose a simplified classification of IMD that mixes elements from a clinical diagnostic perspective and a pathophysiological approach based on three large categories. We highlight the increasing importance of complex molecule metabolism and its connection with cell biology processes. Small molecule disorders have biomarkers and are divided in two subcategories: accumulation and deficiency. Accumulation of small molecules leads to acute or progressive postnatal "intoxication", present after a symptom-free interval, aggravated by catabolism and food intake. These treatable disorders must not be missed! Deficiency of small molecules is due to impaired synthesis of compounds distal to a block or altered transport of essential molecules. This subgroup shares many clinical characteristics with complex molecule disorders. Complex molecules (like glycogen, sphingolipids, phospholipids, glycosaminoglycans, glycolipids) are poorly diffusible. Accumulation of complex molecules leads to postnatal progressive storage like in glycogen and lysosomal storage disorders. Many are treatable. Deficiency of complex molecules is related to the synthesis and recycling of these molecules, which take place in organelles. They may interfere with fœtal development. Most present as neurodevelopmental or neurodegenerative disorders unrelated to food intake. Peroxisomal disorders, CDG defects of intracellular trafficking and processing, recycling of synaptic vesicles, and tRNA synthetases also belong to this category. Only few have biomarkers and are treatable. Disorders involving primarily energy metabolism encompass defects of membrane carriers of energetic molecules as well as cytoplasmic and mitochondrial metabolic defects. This oversimplified classification is connected to the most recent available nosology of IMD.

Differential diagnosis and management of neonatal hypoglycemia

Persistent hypoglycemia in the neonate is most often caused by hyperinsulinemia. Recent discoveries in the molecular and biochemical regulation of insulin secretion have increased dramatically our understanding of disorders responsible for syndromes of hyperinsulinemic hypoglycemia. This article focuses on defects and disorders of the KATP channel, activating mutation of glucokinase and glutamate dehydrogenase, and other disorders that may be associated with specific phenotypes to permit appropriate targeted therapies. It is essential to evaluate these entities carefully because of the emerging evidence that at least half, if not more, have focal disease, which can be cured by local excision rather than diffuse disease, which may not be cured even after near total pancreatectomy with risk for future diabetes. Delay in diagnosis may be associated with developmental delay. The mechanisms of hypoglycemia remain incompletely understood.

Mutation and biochemical analysis of patients belonging to the cblB complementation class of vitamin B12-dependent methylmalonic aciduria

Methylmalonic aciduria, cblB type (OMIM 251110) is an inborn error of vitamin B(12) metabolism that occurs due to mutations in the MMAB gene. MMAB encodes the enzyme ATP:cobalamin adenosyltransferase, which catalyzes the synthesis of the coenzyme adenosylcobalamin required for the activity of the mitochondrial enzyme methylmalonyl CoA mutase (MCM). MCM catalyzes the isomerization of methylmalonyl CoA to succinyl CoA. Deficient MCM activity results in methylmalonic aciduria and a susceptibility to life-threatening acidotic crises. The MMAB gene was sequenced from genomic DNA from a panel of 35 cblB patients, including five patients previously investigated. Nineteen MMAB mutations were identified, including 13 previously unknown mutations. These included 11 missense mutations, two duplications, one deletion, four splice-site mutations, and one nonsense mutation. None of these mutations was identified in 100 control alleles. Most of the missense mutations (9/11) were clustered in exon 7; many of these affected amino acid residues that are part of the probable active site of the enzyme. One previously described mutation, c.556C >T (p.R186W), was particularly common, accounting for 33% of pathogenic alleles. It was seen almost exclusively in patients of European background and was typically associated with presentation in the first year of life.

Presence of cytoplasmic factors functional in peroxisomal protein import implicates organelle-associated defects in several human peroxisomal disorders

Cells from patients with peroxisome-deficient disorders contain membrane ghosts devoid of most matrix contents instead of normal peroxisomes indicating that the underlying molecular defects impair the import of matrix proteins into these peroxisome ghosts. Genetic heterogeneity for the molecular defects was inferred from the assignment of patients with peroxisome-deficient disorders into nine complementation groups. The aim of our studies was to analyze cell lines from six different complementation groups in a systematic manner for the presence of peroxisome ghosts, the ability to import Ser-Lys-Leu-containing proteins into peroxisome ghosts and for the presence of cytosolic factors required for peroxisomal protein import. We show that each of the cell lines analyzed contains peroxisome ghosts, but is unable to import matrix proteins as judged by a peroxisomal import assay using permeabilized cells. The addition of wild type cytosol did not restore the capacity to import matrix proteins but cytosol prepared from these cell lines was functional in stimulation of peroxisomal protein import in a heterologous system. These results implicate organelle-associated molecular defects in each of the six cell lines analyzed.

Controversy in clinical endocrinology: problems with reclassification of insulin-like growth factor I production and action disorders

CONTEXT

Recent developments in the IGF field have raised questions on whether this is the right time to redefine IGF deficiency.

OBJECTIVE

In this controversy, arguments are made against the need for redefining IGF deficiency at this moment, suggesting instead to wait for further clinical developments.

CASE

Although a number of rare case reports of IGF deficiency with precise molecular etiologies have been described, the vast majority of the cases remain clinically defined and without a genetic diagnosis.

INTERVENTIONS

Because IGF products are now available for clinical use in IGF-deficient patients, we are still using GH stimulation and static IGF levels as our only clinical diagnostic and classification tools. POSITIONS: We need to develop additional clinical tools, side by side with molecular tools, for the diagnosis and subclassification of IGF deficiency. Chief among these are the IGF-generation test for identification of GH-insensitive patients and genetic panels of polymorphic changes in relevant genes.

CONCLUSIONS

Until further progress is made in the clinical classification of IGF deficiency, we should not change the current classification, and, when we do, it should be the responsibility of the relevant societies in the field to conduct a consensus statement on the topic first.

Peripheral neuropathy and inborn errors of metabolism in adults

Although they are classically viewed as paediatric diseases, it is now recognized that inborn errors of metabolism (IEMs) can present at any age from childhood to adulthood. IEMs can involve the peripheral nervous system, mostly as part of a more diffuse neurological or systemic clinical picture. However, in some cases, the neuropathy can be the unique initial sign. Here, based on our personal experience and on a comprehensive literature analysis, we review IEMs causing neuropathies in adults. Diseases were classified according to the predominant type of neuropathies into (1) acute neuropathies, (2) mononeuropathy multiplex, (3) chronic axonal polyneuropathies, (4) chronic demyelinating polyneuropathies, (5) small-fibre neuropathies, and (6) lower motor neuron disease.

Advances in analytical mass spectrometry to improve screening for inherited metabolic diseases

Gas chromatography/mass spectrometry became available more than 30 years ago and has subsequently profoundly contributed not only in the identification of a wide range of inborn errors but also as a key tool for clinical diagnostic screening of genetic metabolic disease. Due to extraordinary advances in liquid chromatography and mass spectrometry (MS) developed in the last decade, the utilisation of MS and the potential number of applications for the purpose of metabolic screening is currently undergoing considerable expansion.

CONCLUSIONS

This overview aims to describe only current new developments in clinically most relevant applications, in particular with focus on low molecular weight compounds.