Inherited Disorders of Intermediary Metabolism

medizinische genetik 2021; 33(1): 21–27

Johannes Zschocke* Inherited disorders of intermediary metabolism – a group-based approach https://doi.org/10.1515/medgen-2021-2053 amino acids, peptides/amines, carbohydrates, and fatty Received February 5, 2021; accepted March 18, 2021 acids/carnitine/ketones. The disorders of intermediary en- Abstract: In the recently developed International Classi- ergy metabolism include a large number of diverse con- fcation of Inherited Metabolic Disorders (ICIMD), more ditions that interfere mostly with mitochondrial oxidative than one third of the 1450 listed conditions involve gene phosphorylation. Two categories are classifed as “other” products required for intermediary metabolism. 225 of disorders of intermediary metabolism; one of them covers these diseases represent defciencies of enzymes or trans- the small but growing number of disorders of metabolite port proteins in the breakdown of nutrients, many of repair. This category comprises defciencies of enzymes which cause acute “metabolic” presentations with typical that do not have a function in a particular biochemical biochemical features that are amenable to specifc treat- pathway but remove metabolites generated through non- ments. A group-based approach to these conditions not specifc side reactions of other enzymes. The ICIMD has only assists in understanding and remembering them but been endorsed by all international societies for inherited facilitates the best choice of diagnostic tests and acute metabolic disorders and is due to be used for the Or- treatment. This review describes the basic characteristics phanet classifcation and diferent electronic database re- of the 25 disease groups in the four categories of nutrient sources. Its hierarchical structure can also serve as a ba- breakdown in intermediary metabolism, outlines the often sis for didactic purposes and textbooks [2]. Here we review relatively straight-forward diagnostic approach, and sum- the prototypical clinical and biochemical presentations as marizes important therapeutic principles. It should also well as main diagnostic and therapeutic approaches to assist in the retrospective identifcation of likely metabolic the diferent disorders of nutrient breakdown in intermedi- disorders in the family history for genetic counselling. ary metabolism. This category contains many of the well- known inherited metabolic diseases that share elements Keywords: inherited metabolic diseases, intermediary of the clinical presentation and are recognized by stan- metabolism, classifcation, ICIMD dard “metabolic analyses.” The discussion of the clinical approach to other metabolic disease groups is beyond the scope of this manuscript. Introduction

The recently developed International Classifcation of In- herited Metabolic Disorders (ICIMD) [1] contains a total Simplifed overview of intermediary number of 1450 individual disorders. They are defned metabolism as primary disturbances of a biochemical pathway with a known (or assumed) primary genetic cause, irrespec- Breakdown of the three major nutrient components – car- tive of the clinical consequences. Most conditions are bohydrates, proteins/peptides, and lipids – ultimately re- caused by reduced-function mutations in single genes al- sults in oxidative phosphorylation in the mitochondria for though some are due to activating mutations or other the generation of adenosine triphosphate (ATP). The main genetic or genomic alterations, e. g. in the mitochon- elements relevant for clinical needs and the relevant bio- drial genome. Within the ICIMD the disorders are hier- chemical tests are depicted – very simplifed – in Figure 1. archically structured into 24 categories comprising 124 – Cytosolic breakdown of glucose leads to pyruvate, disease groups (Table 1). The frst four categories refer which can be reversibly converted to lactate, the end to the disorders of nutrient breakdown in intermediary product of anaerobic glycolysis. After transport into metabolism and comprise disorders of the metabolism of the mitochondria, pyruvate is either fed into the Krebs cycle via the pyruvate dehydrogenase complex, or *Corresponding author: Johannes Zschocke, Institute of Human Genetics, Medical University Innsbruck, Peter-Mayr-Str. 1, 6020 serves as precursor of various metabolites via pyru- Innsbruck, Austria, e-mail: [email protected] vate carboxylase. Glycogen, the main storage form of

Open Access. © 2021 Zschocke, published by De Gruyter. This work is licensed under the Creative Commons Attribution 4.0 International License. 22 | J. Zschocke, Inherited disorders of intermediary metabolism – a group-based approach

Table 1: Disease categories in the International Classifcation of Inherited Metabolic Disorders.

Disorders of amino acid metabolism Disorders of peptide and amine metabolism Disorders of intermediary metabolism: nutrients Disorders of carbohydrate metabolism Disorders of fatty acid and ketone body metabolism Disorders of energy substrate metabolism Mitochondrial DNA-related disorders Nuclear-encoded disorders of oxidative phosphorylation Disorder of intermediary metabolism: energy Disorders of mitochondrial cofactor biosynthesis Disorders of mitochondrial DNA maintenance and replication Disorders of mitochondrial gene expression Other disorders of mitochondrial function Disorders of metabolite repair/proofreading Disorders of intermediary metabolism: others Miscellaneous disorders of intermediary metabolism Disorders of lipid metabolism Disorders of lipid metabolism and transport Disorders of lipoprotein metabolism Disorders of nucleobase, nucleotide, and nucleic acid metabolism Disorders of heterocyclic compounds Disorders of tetrapyrrole metabolism Congenital disorders of glycosylation Disorders of complex molecule and organelle metabolism Disorders of organelle biogenesis, dynamics, and interactions Disorders of complex molecule degradation Disorders of vitamin and cofactor metabolism Disorders of cofactor and mineral metabolism Disorders of trace elements and metals Neurotransmitter disorders Disorders of metabolic cell signalling Endocrine metabolic disorders

– Amino acids released from breakdown of proteins and peptides are often modifed in the cytosol before re- moval of the amino group. Deamination results in the generation of organic acids that are further catabo- lized and fnally enter the mitochondrial Krebs cy- + cle. Free ammonium ions (NH4 ) or amino groups are detoxifed mostly in the urea cycle. – Lipids in the form of triglycerides are the main long- term storage compounds for nutritive energy. Their re- lease is started after a few hours of fasting. Coenzyme A (CoA)-activated long-chain fatty acids are trans- Figure 1: Intermediary metabolism: overview and diagnostic tests. ported via the carnitine shuttle into the mitochon- dria, where they are broken down through successive Diferent biochemical categories and laboratory analyses are high- beta-oxidation cycles. This process generates large lighted in diferent colours. Yellow felds indicate basic laboratory tests that should be performed in every patient with an acute ill- amounts of acetyl-CoA, which enters the Krebs cycle ness potentially due to in inherited metabolic disease. Orange felds or – in the liver – is converted into ketone bodies that special metabolic investigations that may identify specifc metabolic function as energy source, e. g. for the brain. changes. In the subsequent Figures 2–7, felds that are shaded dark grey are not of primary relevance for the diagnosis of diseases in the The diferent nutrient breakdown pathways can be as- respective group. sessed with some basic laboratory tests which should be available for the investigation of patients with acute pre- glucose, is required for rapid provision of chemical en- sentations in every hospital: blood glucose, ketones (urine ergy, e. g. in the muscle or in the frst hours of fasting. stix test; quantifcation in blood is more reliable but avail- The other two main nutrient hexoses – galactose and able only in some centres), acid base status, lactate, and fructose – are fed into the glucose pathways through ammonium (in blood). Serum triglyceride levels may pro- specifc enzymes. vide information on lipolysis although the analysis of free J. Zschocke, Inherited disorders of intermediary metabolism – a group-based approach | 23 fatty acids is preferable for exact quantifcation. Selective metabolic screening tests that may show specifc abnor- malities diagnostic for particular disorders of intermediary metabolism include amino acid analysis in plasma/serum, urinary organic acid analysis, and the quantifcation of acylcarnitines (carnitine esters of mitochondrial CoA com- pounds) in dried blood spots. These laboratory tests in combination with the individual clinical presentation may allow a rapid diagnosis and start of specifc treatment in patients with an acute “metabolic” presentation.

Figure 2: Urea cycle disorders. Disorders of amino acid metabolism later in life include acute/episodic or chronic neurologic Defciencies of enzymes involved in amino acid symptoms. Essential for diagnosis is the determination of metabolism frequently result in the accumulation of blood ammonium in the acute situation (Figure 2). Plasma toxic substances leading to acute or chronic organ dam- amino acid analysis usually shows elevated glutamine age. The brain, liver, and kidneys are most frequently concentrations and may reveal specifc alterations of other afected. Development of acute symptoms in many dis- amino acids. Treatment includes controlled protein intake, eases is often linked to increased breakdown of protein maintenance of an anabolic state, removal of ammonium derived from endogenous catabolism, e. g. during an ill- if necessary by haemodiafltration, and amino group re- ness. Rapid diagnosis in these cases may be essential for moval with specifc drugs [3]. The most frequent and im- efective emergency treatment. Other disorders of amino portant urea cycle disorder is ornithine transcarbamylase acid metabolism cause chronic, often progressive organ (OTC) defciency, which is inherited as an X-linked trait dysfunction. Treatment strategies include (a) reversion and not only causes mostly severe neonatal hyperammon- and avoidance of protein catabolic states in diseases with aemia in boys but may trigger acute lethal metabolic de- acute presentations; (b) specifc detoxifcation measures compensation in previously healthy heterozygous females as required; (c) controlled protein intake with specifc at any age. reduction of the precursor amino acid in the afected path- way; and (d) supplementation of necessary other amino acids, vitamins, minerals, and trace elements. Most condi- Organic acidurias tions are asymptomatic at birth, and many are recognized in expanded newborn screening programs. The group of organic acidurias comprises inherited disor- ders in the metabolism of CoA-activated carboxylic acids mostly derived from deamination of amino acids [4, 5]. Urea cycle disorders and inherited The enzymes involved are usually localized in the mito- hyperammonaemias chondria, and the biochemical disturbance often inter- feres with energy metabolism. As stated in the name, Free ammonium derived mostly from amino acid break- specifc organic acidurias are recognized by urinary or- down is a highly neurotoxic metabolite and requires rapid ganic acid analysis but they also generate specifc acyl- and efective removal. This is achieved by producing carnitine patterns (Figure 3). Most of the classical or- water-soluble urea from carbamoyl phosphate (from am- ganic acidurias – such as propionic, methylmalonic, and monium and bicarbonate) and the amino group of aspar- isovaleric aciduria – are caused by enzyme defcien- tate in the four-enzyme urea cycle. Depending on the func- cies in branched-chain amino acid (valine, leucine, and tional severity, disorders that primarily afect ammonium isoleucine) breakdown, and typically manifest as acute or detoxifcation present with acute or chronic-fuctuating intermittent encephalopathy sometimes associated with brain dysfunction. In the newborn period, severely af- other organ damage (e. g. heart, kidney). Basic metabolic fected children after a brief symptom-free interval develop tests often show metabolic acidosis with elevated con- a rapidly progressive encephalopathy with lethargy, hy- centrations of ketones, lactate, and ammonium; hypogly- perventilation, seizures, and coma. Manifestation patterns caemia may occur. No systemic metabolic derangement 24 | J. Zschocke, Inherited disorders of intermediary metabolism – a group-based approach

progressive intellectual disability), alkaptonuria (gen- eration of dark pigments, osteoarthritis), and three types of tyrosinaemia. Accumulating metabolites in tyrosinaemia type 1 are highly toxic; their produc- tion is prevented by pharmaceutical blockage of an upstream enzyme with the drug nitisinone (NTBC). – Disorders of the metabolism of sulphur-containing amino acids (methionine, homocysteine, cysteine) and hydrogen sulphide cause predominately neurological symptoms. S-adenosylmethionin is the most impor- tant methyl group donor in cellular metabolism, and Figure 3: Classical organic acidurias. its replenishment requires remethylation of homocys- teine to methionine linked to the folate cycle. Classical homocystinuria causes a Marfan-like disease. is usually found in the cerebral organic acidurias (e. g. – Disorders of serine metabolism are multi-system dis- glutaric aciduria) which may show specifc neurological orders, in severe cases with prenatal onset (Neu–Lax- symptoms and neuroradiological alterations. Some genet- ova syndrome). Non-ketotic hyperglycinaemia caused ically and biochemically defned organic acidurias can re- by the mitochondrial glycine cleavage system is an im- main asymptomatic throughout life. portant epileptic encephalopathy. – Disorders afecting the other amino acids (ornithine, proline, and hydroxyproline; lysine, hydroxyly- Aminoacidopathies sine, and tryptophan; glutamate/glutamine and as- Breakdown of the 20 proteinogenic amino acids requires partate/asparagine; and histidine) are mostly rare a large number of specifc enzymes. The initial steps are and cause a wide range of neurological and non- mostly localized in the cytosol. Defciencies of these en- neurological manifestations. Some conditions (such zymes cause clinical manifestations usually depending on as histidinaemia) remain asymptomatic. Alanine – the specifc toxicity of the accumulating metabolites. Diag- the amino acid corresponding to pyruvate – is often nosis is based on the analysis of plasma amino acids (Fig- elevated in mitochondrial disorders. ure 4); urinary amino acid analysis is not relevant for the – The disorders of amino acid transport include cystin- majority of conditions. uria, a renal reabsorption defciency of lysine, argi- – Most disorders of branched-chain amino acid nine, ornithine, and cysteine, responsible for 6–8 % of metabolism are classifed as organic acidurias; the renal stones in childhood. main exception is maple syrup urine disease, which causes neonatal or late-onset encephalopathy. – The disorders of phenylalanine and tyrosine metabolism include phenylketonuria (PKU, untreated Disorders of peptide and amine metabolism

Four disease groups – disorders of the metabolism of glu- tathione, other peptides, methylamines, and polyamines – are classifed as disorders of peptide and amine metabolism. Diagnosis usually requires special biochem- ical tests or molecular genetic analyses. Most of the con- ditions are rare; a notable exception is trimethylaminuria or fsh odour syndrome, which is characterized by un- pleasant fsh-like body odour exacerbated by the intake of choline-containing foods or carnitine [6]. An attenuated form linked to homozygosity for a common variant allele of the FMO3 gene is predicted to afect up to 4 % of Euro- Figure 4: Aminoacidopathies. peans [7]. J. Zschocke, Inherited disorders of intermediary metabolism – a group-based approach | 25

Disorders of carbohydrate tous or may have two (liver- or muscle-specifc) isoforms. Correspondingly, diseases in this group have three proto- metabolism typic clinical presentations: – Liver glycogenoses typically present with hypogly- Disorders of galactose and fructose caemia at the onset of fasting (e. g. 3–4 hours af- metabolism ter meals), hepatomegaly and other storage features, growth retardation, and variable other features. The There are two main nutritive disaccharides: lactose (milk most frequent disease is GSD type 1 (von Gierke). Lab- sugar, galactose-glucose disaccharide) and sucrose (table oratory analyses in the acute phase show marked li- sugar, glucose-fructose disaccharide). Utilization of galac- paemia and elevated triglycerides caused by massive tose (Gal) and fructose (Fru) involves phosphorylation at lipolysis, elevated lactate and uric acid, and ketonuria the carbon-1 position; the respective metabolites Gal-1-P (Figure 6). Treatment is based on frequent meals, and Fru-1-P are toxic particularly for the liver and kidneys. slowly resorbed carbohydrates, and continuous naso- Correspondingly, defciencies of the enzymes required for gastric tube feeding overnight. Gal-1-P and Fru-1-P processing cause progressive liver and – Muscle glycogenoses frequently cause exercise intol- renal dysfunction starting with the intake of the respective erance, muscle cramps, and myoglobinuria; treatment disaccharide [8]. Classical galactosaemia becomes mani- is mostly symptomatic. GSD type 2 (Pompe) is charac- fest with milk feeds after birth and is also an important terized by severe muscle weakness and cardiomyopa- cause of cataracts; it is detected by analyses of galac- thy, which can be treated with enzyme replacement tose metabolites (Figure 5) and enzyme studies in new- therapy. born screening programs. Hereditary fructose intolerance – Mixed/generalized glycogenoses show both liver and becomes manifest after weaning or upon addition of su- muscle manifestations including cardiomyopathy. crose/fructose to the diet; there is no specifc biochemical analysis, and the diagnosis is made by mutation analysis. Diagnosis in all glycogen storage disorders is based on Both conditions are treated by removal of the respective molecular genetic analyses in conjunction with enzyme carbohydrate from the diet. studies.

Figure 5: Disorders of galactose and fructose metabolism. Figure 6: Glycogen storage disease type 1.

Disorders of glycogen metabolism (glycogen Other disorders of carbohydrate metabolism storage diseases [GSDs], glycogenoses) Group-specifc clinical presentations based on the func- Glycogen, the main storage form of glucose in humans, is tion in intermediary metabolism can also be defned for particularly abundant in the liver – where it is used for glu- the other disorders of carbohydrate metabolism: cose homeostasis – and the muscle – where it is used for – Disorders of gluconeogenesis show recurrent hypo- the provision of energy during exercise [9, 10]. Enzymes glycaemia, elevated lactate, and ketosis. Fructose- required for glycogen synthesis or release may be ubiqui- 1,6-bisphosphatase defciency, which is close to 26 | J. Zschocke, Inherited disorders of intermediary metabolism – a group-based approach

Glc-6-P in gluconeogenesis, also has marked hep- acid oxidation disorder which is now fortunately in- atomegaly and may resemble GSD type 1. Progressive cluded in most newborn screening programs. neurodegeneration and severe lactic acidosis are fea- – Severe defciency of the carnitine shuttle and long- tures of enzyme defciencies closer to the Krebs cycle, chain fatty acid oxidation enzymes can present like a such as in pyruvate carboxylase defciency. respiratory chain defect in the neonatal period, with – Disorders of glycolysis can cause haemolytic anaemia progressive coma, cardiomyopathy, liver failure, se- combined with variable neurologic, muscle, and reti- vere lactic acidosis, and multi-system organ failure. nal manifestations. – Attenuated variants of long-chain fatty acid oxidation – The disorders of pentose metabolism include and the carnitine shuttle often show adolescent or glucose-6-phosphate dehydrogenase defciency, an adult onset chronic muscle weakness, pain, recurrent X-chromosomal disease with recurrent haemolysis rhabdomyolysis, and/or cardiomyopathy. triggered by specifc foods (favism), drugs, and other factors, prevalent in malaria regions. – Disorders of carbohydrate transmembrane transport Specifc treatment involves the avoidance of fasting, rever- and absorption cause a variety of renal, gastrointesti- sion of catabolic states, and supplementation of medium- nal, neurological, or multi-system manifestations, chain fatty acids and/or carnitine in some conditions. based on the defcient transport functions.

Conclusion Disorders of fatty acid and ketone body metabolism Many disorders of nutrient breakdown in intermediary metabolism are relatively easy to diagnose based on the Enzyme defciencies in the mitochondrial import and oxi- clinical presentation in relation to metabolic states, and on dation of CoA-activated fatty acids, or in the production of the results of a few basic laboratory tests combined with ketone bodies, have variable but overlapping clinical pre- selective metabolic screening analyses. This is essential sentations that are typically related to catabolic states, e. g. for rapid implementation of efective specifc treatments after 8–12 hours of fasting [11]. Urinary organic acid anal- which are available for many conditions. The exact diag- ysis and acylcarnitine analysis in dried blood spots may nosis is usually confrmed by molecular genetic analyses. show specifc abnormalities (Figure 7). – Insufcient ketogenesis in conjunction with inhibition of gluconeogenesis causes potentially life-threatening acute hypoketotic hypoglycaemic coma, often in the References second half of the frst year of life. This is the typi- cal presentation of medium-chain acyl-CoA dehydro- [1] Ferreira CR, Rahman S, Keller M, Zschocke J. (IA Group). An international classifcation of inherited metabolic disorders genase defciency, the most frequent inherited fatty (ICIMD). J Inherit Metab Dis. 2021;44:164–77. [2] Zschocke J, Hofmann GF. Vademecum Metabolicum – Diagnosis and Treatment of Inherited Metabolic Disorders. Stuttgart, Germany: Thieme; 2021. [3] Haberle J, Burlina A, Chakrapani A et al. Suggested guidelines for the diagnosis and management of urea cycle disorders: First revision. J Inherit Metab Dis. 2019;42:1192–230. [4] Wajner M. Neurological manifestations of organic acidurias. Nat Rev Neurol. 2019;15:253–71. [5] Dimitrov B, Molema F, Williams M et al. Organic acidurias: Major gaps, new challenges, and a yet unfulflled promise. J Inherit Metab Dis. 2021;44:9–21. [6] Chalmers RA, Bain MD, Michelakakis H, Zschocke J, Iles RA. Diagnosis and management of trimethylaminuria (FMO3 defciency) in children. J Inherit Metab Dis. 2006;29:162–72. [7] Zschocke J, Kohlmueller D, Quak E, Meissner T, Hofmann GF, Mayatepek E. Mild trimethylaminuria caused by common Figure 7: Disorders of fatty acid and ketone body metabolism. variants in FMO3 gene. Lancet. 1999;354:834–5. J. Zschocke, Inherited disorders of intermediary metabolism – a group-based approach | 27

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