Metabolic MedicinePart one

Hevi Pediatric Teaching Hospital

28-4-2013

Metabolic pathways

• The term metabolism refers to all biochemical processes and pathways in the body.

• Enzymes play a key role in many of these processes and changes in their function, as a result, of genetic mutation can lead to problems in these pathways.

• The major metabolic pathways for proteins, carbohydrates and lipids are closely integrated with key molecules, such as acetyl co-enzyme A via complex mechanisms.

• A genetic defect in any part of the major metabolic pathways is known as an inborn or congenital (if present from birth) error of metabolism.

• Inborn errors of metabolism can be divided into three pathophysiological diagnostic groups:

• Disorders that disrupt the synthesis or catabolism of complex molecules with symptoms that are permanent, progressive, independent of intercurrent events and not related to food intake.

• These include lysosomal disorders, peroxisomaldisorders and disorders of intracellular transport and processing.

• Disorders that lead to an acute or progressive accumulation of toxic compounds as a result of metabolic block.

• These include disorders of amino acid metabolism , organic acidurias, congenital urea cycle defects and sugar intolerances (galactosaemia).

• Disorders with symptoms due to a deficiency of energy production or utilisation within the liver, myocardium, muscle or brain.

• These include congenital lactic acidemias, fatty acid oxidation defects, gluconeogenesisdefects and mitochondrial respiratory chain disorders.

BASIC METABOLISM

• Carbohydrate metabolism

• Glucose has three metabolic pathway in the body:

• oxidation for energy,

• storage as glycogen,

• and conversion to amino acids and triglycerides.

Glycolysis

• Glycolysis takes place in the cytoplasm of all cells and describ'es the break down of one molecule of glucose to produce two molecules of pyruvate·

• It can occur under aerobic via the tricarboxylic acid (TCA) cycle and oxidative phosphorylation)

• or under anaerobic conditions via lactate.

• Glycolysis provides an emergency mechanism for energy production when oxygen is limited, i.e. in red cells (which have no mitochondria, thus glycolysis is their only means of energy production) or in skeletal muscle during exercise.

• Glycolysis also provides intermediates for other metabolic pathways, e.g. pentoses for DNA synthesis.

• Pyruvate metabolism:

• Pyruvate, produced by glycolysis and other metabolic pathways, can be converted to

• oxaloacetate (by pyruvate carboxylase) for entry into the TCA cycle,

• or acetyi-CoA (by pyruvate dehydrogenase)

• Tricarboxylic acid (TCA) cycle

• This cycle is present in all cells with mitochondria (not red cells) and provides the final common pathway for glucose, fatty acids and amino acid oxidation via acetyi-CoA or other TCA cycle intermediates.

• The cycle's main function is ATP production .

• The cycle also provides metabolic intermediates for other synthetic pathways, e.g. amino acid synthesis.

• Glycogen metabolism

• Glycogen is a branched glucose polymer stored in liver, kidney and muscle for the rapid release of glucose when needed.

• Liver glycogen is a store to release glucose to the rest of the body,

• whereas muscle glycogen supports muscleglycolysis only.

• Glycogen synthesis is promoted by insulin.

• Glycogenolysis is promoted by adrenalineand glucagon .

Protein metabolism

• Proteins are assembled from amino acids which are composed of amino group

• Proteins metabolized to urea via ammonia.

• Proteins also metabolized a carbon skeleton which has a number of potential metabolic fates:

• acetyi-CoA,

• pyruvate and

• ketone bodies.

• Amino acids may be used for protein synthesis,

• or may be converted to other non-essential amino" acids (transamination) or

• oxidized via the TCA cycle.

• Essential amino acids cannot be synthesized in the body.

• Protein cannot be stored and therefore any amino acids not used are catabolized,

• and hence to remain in neutral nitrogen balance, protein is an essential constituent of a healthy diet.

• Gluconeogenesis is the de novo synthesis of glucose from non-carbohydrate sources such as amino acids, lactate and glycerol.

• This usually occurs in the liver but also occurs in the kidney in prolonged starvation.

• Gluconeogenesis is promoted by glucagon, cortisol and adrenocorticotrophic hormone (ACTH).

Fat metabolism

• Fat has the highest caloric value and therefore is an essential energy source.

• Triglycerides comprise three fatty acid molecules and one glycerol molecule which are broken down by lipase (lipolysis).

• The released glycerol is converted to glyceraldehyde-3-phosphate in the liver, a key intermediate of both glycolysis and gluconeogenesis.

• The fatty acids undergo ~oxidation within mitochondria which shortens the fatty acid by two carbons per cycle releasing acetyi-CoA for entry to

• the TCA cycle or

• for the production of ketone bodies.

• Fatty acids can be synthesized from acetyi-CoA(lipogenesis).

• Essential fatty acids cannot be synthesized by the body

• The principal essential fatty acids are linoleicand a-linolenic acids.

APPROACH TO THE METABOLIC CASE

• Inheritance• Inborn errors of metabolism are individually rare but

collectively they have an incidence of about 1 per 3,000 to 4,000 births.

• Autosomal recessive inheritance is commonest.• Exceptions include:• • X-linked recessive- Lesch-Nyhan syndrome; Hunter

syndrome; ornithine transcarbamylase (OTC) deficiency; Fabry disease; adrenoleukodystrophy

• • Autosomal dominant - Porphyrias (some recessive)• • Matrilineal - Mitochondrial DNA mutations

• Presentation

• Presentation is non-specific, therefore clues should be sought in the history.

• The commonest misdiagnosis is sepsis.

Clues from history

• Consanguineous parents• Previous sudden infant death (especially late, i.e. > 6

months)• Ethnicity (for certain conditions only)• Previous multiple miscarriages (indicating non-viable

fetuses)• Maternal illness during pregnancy, e.g.:- Acute Fatty Liver of Pregnancy (AFLP) and Haemolysis,

Elevated Liver enzymes, Low Platelets (HELLP) syndrome association with carrying fetus with long-chain fat oxidation defect

- Increased fetal movements (in utero fits)

• Faddy eating (avoidance of foods that provoke feeling unwell)

• Previous encephalopathic or tachypnoeicepisodes (latter implies acidosis)

History

• Inborn errors of metabolism present at times of metabolic stress, e.g.:

• Neonatal period

• Weaning (increased oral intake, new challenges, e.g. fructose)

• End of first year (slowing in growth rate, therefore more protein catabolized as less used for growth. May exceed metabolic capacity of defective pathway)

• Intercurrent infections

• PubertyHistory

Examination

• Clinical examination may reveal few clues in many disorders of intermediary metabolism.

• Dysmorphic features may suggest certain diagnoses.

• Peroxisomal disorders and Zellweger’ssyndrome:

• Large fontanelle, high prominent forehead, hypoplastic supra-orbital ridges, epicanthic folds, flat nasal bridge

• Pyruvate dehydrogenase deficiency:

• Epicanthic folds, flat nasal bridge, small nose with anteverted flared alae nasi, long philtrum

Examination

• Cholesterol biosynthetic defects:

• Smith-Lemli-Opitz syndrome Epicanthic folds, flat nasal bridge, syndactyly, abnormal genitalia

Examination

• Lysosomal storage diseases (I cell disease):

• Hurler-like phenotype (coarse facial features, large tongue)

Examination

• Odours are usually unhelpful and rarely significant exceptions include:

Urine OdorInborn Error of Metabolism

Sweaty feetGultaric Academia

Maple syrupMaple Syrup urine disease

Boiled cabbageHypermethioninemia

Mousy or mustyPhenylketonuria

Rotten fishTrimethylaminuria

Cat urineMultiple crboxylase deficiency

Examination

• Eyes should be carefully examined for

• corneal clouding (mucopolysaccharidoses, cystinosis),

• cataracts (galactosaemia, peroxisomal, mitochondrial),

• pigmentary retinopathy (fat oxidation, mitochondrial) and

• cherry-red spot (lay-Sachs, Niemann-Pick, Sandhoff, GM1 ).

Examination

• Organomegaly is a key revealing sign.

• Hepatosplenomegaly is a feature of storage disorders.

• Massive hepatomegaly in the absence of splenomegaly suggests glycogen storage disease because glycogen is not stored in the spleen.

• More prominent splenomegaly is suggestive of Gaucher disease.

Examination

Investigation

• Perform investigations at the time of decompensation when diagnostic metabolites are most likely to be present and avoid the need for stress tests at a later date.

• Key initial metabolic investigations

• Blood gas (venous, capillary or arterial)

• Glucose

• Lactate

• Ammonia

• Amino acids (blood)

• Organic acids (urine)

• Acylcarnitines

• Ketones (urinary dipstick)Investigation

Acid-base status• Anion gap = Na + K - (CI + HC03 -)• A normal anion gap in the presence of metabolic

acidosis signifies bicarbonate loss rather than an excess of acid, e.g. renal or gut.

• Marked ketosis is unusual in the neonate and is therefore highly suggestive of an underlying metabolic disorder.

• Urea cycle defects may initially present with a mild respiratory alkalosis because ammonia acts directly on the brainstem as a respiratory stimulant.

Investigation

Hypoglycaemia

• Hypoglycaemia is defined as a blood glucose concentration of ~ 2.6 mmol/1, and should always be confirmed in the laboratory.

• The key additional investigation is the presence or absence of ketosis.

• Hypoketotic hypoglycaemia has a limited differential diagnosis that can usually be resolved on history and examination:

• Hyperinsulinism (endogenous or exogenous)• Fat oxidation defects (e.g. MCAD)• Liver failure

Investigation

Lactate

• Lactate is a weak acid which can be used directly as a fuel for the brain and is readily produced during anaerobic respiration.

• Secondary causes of lactate level anomalies (e.g. hypoxia, sepsis, shock, liver failure, poor sampling, etc.) are much more common than primary metabolic causes.

• Cerebrospinal fluid (CSF) lactate is raised in mitochondrial disorders, central nervous system (CNS) sepsis and seizures.

Investigation

Ammonia• Hyperammonaemia may result from poor

sampling (squeezed sample) and/or delays in processing.

• The level of ammonia may give a clue to the cause.

Differential diagnosisAmmonia concentration(mic mol/1)

Normal< 40

Sick patient, fat oxidation defect, OA, liver failure, UCD40 to< 150

Fat oxidation defect, OA, liver failure, UCD150 to< 250

OA, liver failure, UCD250 to< 450

Liver failure, UCD450 to> 2000

OA, organic acidaemia; UCD, urea cycle defect.

• Transient hyperammonaemia of the newborn (THAN) is characterized by very early onset,

• usually in the first 36 hours before feeding is truly established.

• It is associated with low glutamine.

• THAN is managed as other urea cycle defects but has an excellent prognosis if treated early as the hyperammonaemia is secondary to blood bypassing the liver (e.g. patent ductusvenosus), rather than a block in the urea cycle.

Investigation

• Amino acids

• Amino acids are measured in both blood and urine. The latter reflects renal threshold, e.g.

• generalized aminoaciduria of a proximal renal tubulopathy or the specific transporter defect of cystinuria (Cystine, Ornithine, Arginine, Lysine).

Investigation

• An increase in the serum levels of a specific amino acid may be missed if only a urine sample is analysed and the renal threshold has yet to be breached.

• Plasma amino acids are useful in the work up of a number of metabolic disorders, and are essential in monitoring some metabolic disorders.

• ↑leucine, isoleucine and valine- maple syrup urine disease (MSUD)

• ↑ Glutamine, ↑ arginine in UCDs

• ↑ Alanine- lactic acidosis

• ↑ Glycine- non-ketotic hyperglycinaemia, OAs

• ↑ Phenylalanine- phenylketonuria

• ↑ Tyrosine- tyrosinaemia

Organic acids

• These are measured in urine only and are diagnostic in many organic acidaemias, e.g.

• increased propionate in propionic acidaemia, increased isovalerate in isovaleric acidaemia, etc.

Organic acids

• Organic acids are also essential in the diagnosis of other disorders.

• ↑ Orotic acid - UCDs, mitochondrial.

• ↑ Succinylacetone- tyrosinaemia type I

• ↑ Dicarboxylic acids- fat oxidation defects, medium-chain triglyceride feeds, mitochondrial.

Investigation

Acylcarnitines

• Carnitine conjugates with acyl-CoA intermediates proximal to the block in fat oxidation defects.

• The chain length of the acylcarnitines formed is diagnostic of where the block lies, e.g. medium-chain (MCAD), very long-chain (VLCAD), etc.

• Likewise, conjugation with organic acids allows diagnosis of organic acidaemias, e.g. propionylcarnitine.

Investigation

Urate• Urate is the end product of the breakdown of purines. • Raised levels in plasma may indicate:• Increased production (eg Lesch-Nyhan syndrome, GSD

type I, rhabdomyolysis) or • Decreased excretion (familial juvenile hyperuricaemic

nephropathy, FJHN). • It is essential to measure a concurrent urinary urate

because urate clearance in children is so efficient that plasma levels may be in the upper normal range in Lesch-Nyhan syndrome, whereas urinary levels are grossly elevated.

• In FJHN the reverse is true with high plasma urate, but low urinary urate.

Investigation

Acute patient screening• Specific metabolites are used to screen acute

patients for specific disorders or groups of disorders.

DisorderMetabolite

Peroxisomal disorders, e.g. Zellwegersyndrome, adrenoleukodystrophy

Very-long-chain fatty acids

Congenital disorders of glycosylationTransferrin isoelectric focusing

Purine disordersUrate

Smith-Lemli-Opitz syndrome7 -Dehydrocholesterol

Mucopolysaccharidoses andmucolipidoses

Urinary glycosaminoglycansand oligosaccharides

GalactosaemiaUrinary reducing substances

Purine and pyrimidine disordersUrinary purine and pyrimidinemetabolites

Secondary investigations include:

• Neuroimaging- basal ganglia signal change in mitochondrial disorders

• Neurophysiology - mitochondrial, peroxisomal

• Echocardiogaraphy - especially hypertrophic cardiomyopathy, mitochondrial, fat oxidation, Pompe (GSD type II), storage disorders

• ECG - fat oxidation, mitochondrial

• EEG- metabolic encephalopathy, e.g. MSUD, hyperammonaemia

Investigation

Enzymology

• Definitive diagnosis is confirmed on enzymology.

• The sample requirement depends on which tissues express the enzyme, e.g. galactosaemia (blood), OTC deficiency (liver), mitochondrial (muscle).

Investigation

• White cell enzymes are often requested in patients with potential neurodegenerative or storage disorders.

• The neurodegeneration panel includes GM1 gangliosidosis, arylsulphatase A deficiency, Krabbe and fucosidosis in white cells.

• Where us Tay-Sachs, Sandhoff, Sly mucopolysaccharidosis (MPS VII) and mannosidosis in plasma;

Investigation

• The organomegaly panel includes GM1 gangliosidosis, Gaucher, Niemann-Pick A and B, mannosidosis, fucosidosis and Wolman disease in white cells.

• while Sly (MPS VII), and mannosidosis in plasma.

Investigation

Acute management

• Stop feeds

• Promote anabolism- give 10% dextrose with appropriate electrolyte additives (add insulin rather than reduce% dextrose if hyperglycaemiq.

• Correct biochemical disturbance along standard guidelines, e.g. hypernatraemia, low phosphate, etc.

• Clear toxic metabolites

- Dialysis- lactate, organic acids, ammonia, leucine

- Drugs- UCDs: phenylbutyrate, sodium benzoate; OAs: carnitine, glycine.

Supplement enzyme cofactors- e.g. biotin, thiamine, riboflavin

Specific treatment• Dietary restriction• Drugs, e.g. nitisinone in tyrosinaemia• Enzyme replacement therapy in Gaucher, Fabry,

Pompe, Hurler, and Hunter syndromes• Substrate deprivation therapy in Niemann Pick C and

Gaucher• Transplantation (liver, bone marrow)• Hepatocyte transfer (future)• Gene therapy (future)• Genetic counselling(future prenatal counselling)• Screen siblings if indicated

SCREENING

• Principles of screening

• Screening for defined disorders aims to prevent avoidable morbidity and mortality.

• The sensitivity of a screening test is the rate of true-positives, and its specificity is the rate of true-negatives.

• The aim is not to miss any cases with the minimum of false-positives.

• The necessary requirements for including a condition in a screening program are:

• Important health problem• Accepted treatment• Facilities available for diagnosis and treatment• Latent or asymptomatic disease• Suitable test• Natural history understood• Agreed case definition• Early treatment improves prognosis• Economic• Case-finding may need to be continuous

• Neonatal blood spots are collected on day 5-8 .

• Laboratories still using the Guthrie test for PKU,

• which relies on phenylalanine-dependent bacterial growth,

• may give false-negatives if the baby is receiving antibiotics.

• The feeding status is also requested to ensure adequate protein intake, but newer techniques are able to detect PKU reliably on day 1 (routine screening day in the USA).

Current UK program• Universal• Congenital hypothyroidism (thyroid-stimulating

hormone)• Phenylketonuria (phenylalanine)• Haemoglobinopathies (sickle-cell disease and

thalassaemia)• Some regions• MCAD• Cystic fibrosis (to become universal)• Galactosaemia• Homocystinuria• Duchenne muscular dystrophy

Thanks