Inborn errors of metabolism occurs 1 in 5000 births collectively. They can be treated if the inborn error is diagnosed properly and early. They are manly caused by a gene defect that blocks vital metabolic pathways. The can be server, which is mainly due to amount of enzymes that is causing the block or deficiency of the reaction product. This can affect the organs and also have local effect such as lysosomal storage disease. The symptoms can vary, from mild to severe. They affect any organs and occur at any time. To recognise the Inborn Errors of Metabolism, the doctor must be suspicious; for example a baby that show signs of antecedent maternal fever should undergo a blood culture and also undergo simple tests such as Amino acids, Organic acids, Glucose, Electrolytes, Lactate, and Ammonia; which is done in the laboratory.
Inborn errors of metabolism (IEM) are a group of genetic disorders that are rare. These disorders are genetic diseases that are involved in metabolism disorders. A large section of these disorders arise from single genes that encodes for enzymes; that have been defective. These enzymes are important that they catalyses chemical reactions converting substrates to products essential for everyday living. Inborn errors of metabolism disorders can arise from a number of conditions such as prolong exposure and accumulation of substances which are toxic, and the interference of normal functions and the inability to produce and synthesize essential compounds. Inborn errors of metabolism has related to defects in or damage to a developing foetus that may have been caused to genetic changes, that is why Inborn errors of metabolism is sometimes referred as congenital metabolic diseases. Inborn errors of metabolism have also been linked to heritable disorders within biochemistry, for example phenylketonuria (PKU). That is why some times it is referred as inherited metabolic diseases as well. In the perspective of changes of normal mechanical, physical, and biochemical functions, IEM can divided into three useful groups that can be analysed diagnostically. Group 1: Disorders which causes intoxication. Group 2: Disorders involving energy metabolism. And Group 3: Disorders involving complex molecules.
In the early 1900s, a researcher called Sir Archibald Garrod’s based his studies on genetic metabolic disorders and discovered IEM [1, 2]. He was known for his work on the “one gene, one enzyme” hypothesis, based on his studies on the nature and inheritance of alkaptonuria. And gave the name and wrote a book on IEM (The Incidence of Alkaptonuria: a Study in Chemical Individuality.) [1, 2].
IEM can arise from a number of causes, but the major cause is alteration of a specific metabolic reaction [1, 2]. IEM has been shown to develop at a very young age, where epidemiology findings indicated that hundreds of IEM affects about 1 in every 5000 born babies . But as technology advances and improved techniques such as metabolomics, has been easy to develop newborn screening that improves early diagnosis and treatment in a number of IEM disorders . But as these new techniques cost a lot to run and time increases, proving to be unreliable .
The study and the knowledge about inborn errors of metabolism (IEM) have improved due to the latest advancement in technology and techniques [2, 3]. These improvement have led us to the conclusion, for example urea cycle disorders and organic acidemias may and will lead to the accumulation of ammonia, which is a toxic product of amino acid metabolism [2, 3]. Also the latest findings are that inborn errors of metabolism (IEM) may impair brain function due to defects in the mitochondrial respiratory chain and disorders in gluconeogenesis .
There are 10 facts that need to take inconsideration when people with IEM undergo clinical diagnosis .
The metabolic disorders can be set into three useful groups that can easily be distinguished from each other [5, 6, 7]. These three groups are sorted from a pathophysiological point of view [5, 6, 7]. The groups are; Group 1: Disorders which is caused by intoxication [5, 6, 7]. Group 2: Disorders involving energy metabolism [5, 6, 7]. And Group 3: Disorders involving complex molecules [5, 6, 7].
In this group, it describes inborn errors of intermediary metabolism. These inborn errors cause acute of progressive intoxication from long exposure and increase of toxic compounds, forming a metabolic block [5, 6, 7]. In group 1 the inborn errors are manly amino acid catabolism [5, 6, 7]. These include phenylketonuria, where there is a deficiency in the enzyme phenylalanine hydroxylase (PAH); which is needed to metabolise the amino acid phenylalanine to the amino acid tyrosine [5, 6, 7]. Homocystinuria is an inherited disorder of the metabolism of methionine [5, 6, 7]. Tyrosinemia is where body cannot break down the amino acid tyrosine [5, 6, 7]. Also organic acidurias such as methylmalonic acidemia, and propionic acidemia [5, 6, 7].
Sugar intolerances are also classified in group 1 [5, 6, 7]. These include hereditary fructose intolerance caused by a deficiency of liver enzymes that metabolise fructose. Metal toxication also falls under group 1; such as hemochromatosis, where the patient has accumulated a lot of iron [5, 6, 7].
All these metabolic disorders have something in common, in that they do not affect the embryo development, and also show similar symptoms of clinical intoxication [5, 6, 7]. A patient with group 1 disorders may show an acute signs of vomiting, coma and liver failure; or chronic signs which are cardiomyopathy, developmental delay and failure to thrive. Acute symptoms that can worsen are catabolism, fever, and food intake [5, 6, 7].
The analysis in group 1 is easy, and without any complications [5, 6, 7]. It needs the use of chromatography in which the plasma and urine amino acid. Most of group 1 disorders are treatable. Treatment is usually involves special diets and cleansing drugs such as sodium benzoate and penicillamine, to remove the toxins [5, 6, 7].
The inborn error of amino acid synthesis is also included to this group, as they have the same or similar features [7, 8]. They are inborn errors of intermediary metabolism; the analysis requires the plasma and urine where disorders are able to be treated even when the disorder starts with in the uterus, for example 3-phosphoglycerate dehydrogenase deficiency [5, 6, 7, 8].
In this group inborn errors are errors of intermediary metabolism as well. The symptoms are mainly caused by a lack of energy production or utilization. This will involve the liver, myocardium, muscle, and brain [5, 6, 7].
There are two types of disorders involving energy metabolism. 1. involves mitochondrial energy defects. 2. involves cytoplasmic energy defects [5, 6, 7].
Mitochondrial energy defects are more aggressive and are not fully treatable [5, 6, 7]. Mitochondrial energy defects cause lactic acidemias where there is dextrorotatory lactic acid in the circulating blood, resulting to defects of the pyruvate transporter resulting to PKU, pyruvate carboxylase this causes lactic acid to accumulate in the blood, pyruvate dehydrogenase where the patient can show symptoms of severe lethargy, and defects in the Krebs cycle [5, 6, 7]. But some defects such as fatty acid oxidation and ketone body defects are partly treatable [5, 6, 7].
Cytoplasmic energy defects are not as much aggressive then mitochondrial energy defects. A cytoplasmic energy defect causes disorders of glycolysis, glycogen metabolism and gluconeogenesis [5, 6, 7]. And recent study showed it causes disorders of creatine metabolism which it is partly treatable by oral creatine supplementation [5, 6, 7]. Cytoplasmic energy defects can cause disorders that are untreatable, such as errors of the pentose phosphate pathway which will be described in further details [5, 6, 7].
In group 2, the common symptoms are heart muscle disease, hypoglycaemia, myopathy which is a muscular disease, cardiac failure, specific failure of the circulation, sudden death especially in infancy [5, 6, 7].
Mitochondrial disorders and as well as the pentose phosphate pathway defects can obstruct embryo-foetal development and give rise to dysmorphism, dysplasia causing an abnormality in maturation of cells within a tissue and congenital disorder [5, 6, 7, 9]. The analysis and the diagnosis are hard to put through as it requires 4 different tests [5, 6, 7]. 1. Function tests [5, 6, 7]. 2. Enzymatic analyses needing biopsies [5, 6, 7]. 3. Cell culture [5, 6, 7]. 4. Molecular analyses [5, 6, 7].
Group 3 involves cellular organelles [5, 6, 7]. The diseases that fall under group 3 modify the synthesis or the catabolism of complex molecules [5, 6, 7]. There are symptoms that are permanent and progressive, and some symptoms free from intercurrent events [5, 6, 7].
In this group there are disorders such as lysosomal storage disorders, peroxisomal disorders and inborn errors of cholesterol synthesis etc. In this group treatment is difficult would need enzyme replacement therapy, especially for lysosomal disorders [5, 6, 7, 10].
There is a way for testing newborn babies for inborn errors . This process is called Newborn screening. This tests babies at a very early age for four types of diseases that are treatable: genetic, endocrinologic, metabolic and hematologic diseases . Dr Robert Guthrie was a microbiologist who designed a dried blood spot testing, and used it to screen for phenylketonuria . To this present day that spotting test is still being used. As techniques have improved so have the screening . A physician who is screening a newborn baby for a metabolic disorder has follow four groups of clinical circumstances:
The three groups have symptoms that are chronic and aggressive that can be easily ignored or misinterpreted.
If a patient has inborn errors of metabolism (IEM), the person might have symptoms that fall under the Gastrointestinal Symptoms (GI) group . These symptoms include Anorexia, which is an eating disorder [5, 13]. Osteoporosis which untreated could lead to bones fracturing. Chronic vomiting, feeding difficulties, and failure to thrive are also symptoms of (GI). But they are also associated with chronic diarrhoea [5, 13]. This could lead to false and the wrong diagnosis [5, 13].
There are two groups that have been described to have caused chronic diarrhoea and failure to thrive within inborn errors of metabolism:
There are a number of symptoms that fall under this group. For example: Hypotonia, where there is a disorder that causes low muscle tone and strength [5, 14]. Muscular weakness and poor muscle mass [5, 14]. These symptoms are common with many inborn errors of metabolism. These symptoms can be caused by urea cycle defects and many amino acid metabolism disorders [5, 14]. Recent studies have shown that the cause of muscle symptoms can be due to mutations in the monocarboxylate transporter 8 gene, which can develop Allan-Herndon-Dudley syndrome . Allan-Herndon-Dudley syndrome falls under the muscle symptoms group as it causes hypotonia, general weakness of the muscle, reduced muscle mass and delayed development . Further studies showed that this X-linked mental retardation syndrome is involved in the transport of triiodothyronine into neurones and disrupts the blood levels of thyroid hormone .
Patients with inborn errors often have neurological symptoms. These include of neurological abnormalities, in the central and peripheral system. Studies have shown that these neurological symptoms are very frequent with inborn errors [5, 15]. These symptoms include poor feeding, hypotonia, ataxia, and even autistic features [5, 15]. The analysis of inborn errors, due to the screening of neurological symptoms is very difficult due to symptoms that are non specific signs; sings that include for example developmental delay, and hypotonia .
Newborn screening is a technique, used to detect inborn errors [17, 12]. It was first used to detect phenylketonuria (PKU) by a bacterial inhibition assay, developed in the 1961 by Dr Robert Guthrie as already stated. His technique in using dried blood sample was further developed in the mid 1975, where a scientist called Dussault used a method to screening for congenital hypothyroidism . A lot of time and money has been invested into the screening programme, and now they have uncovered new disorders that are related to inborn errors [17, 12]. Disorders such as cystic fibrosis, congenital adrenal hyperplasia, which is a form mutation of genes that produces enzymes that mediating production of cortisol from cholesterol by the adrenal glands. Glucose-6-phosphate dehydrogenase deficiency and many more .
To this present day, for screening newborns, tandem mass spectrometry is used . It is a lot easier for screening and diagnosis. The application of tandem mass spectrometry to newborn screening was first described in 1990 .
The primary aim of newborn screening is to identify patients, manly infants with serious disorders that are treatable . This will make it easy to prevent or improve clinical symptoms of the disease . Tandem mass-spectrometry is very useful in detecting more than one disorder at one time . This can be used to detect early untreatable disorders and also can be beneficial if the screening was not limited to just individual babies, but the whole family as well .
The screening process uses MSMS . MSMS is the method used to measure analytes by both mass and structure . First the compounds are ionised, where the first mass spectrometer selects the ion of interest, where it is sorted by weight . Then the compounds travel through a collision cell, are dissociated to signature fragments, and then pass into a second mass spectrometer where ions are selected for detection. .
Research have been done where most newborn screening programmes use simplifying sample preparation, instead derivatisation of the sample which is the old method . When investigating, the sample might show more than one disorder. But the use of ratio of analytes improves sensitivity and specificity . Specialised biochemical genetic testing is always done to verify which type of disorder the patient has . Theses genetic tests include amino acid analysis, organic acid analysis by gas chromatography/mass spectrometry, and plasma acylcarnitine profile by MSMS .
Well over 40 inborn errors of metabolism can now be detected by newborn screening . This section will look at three inborn errors.
The pentose phosphate pathway (PPP) is an anabolic pathway where is uses a 6 carbon glucose to generate a 5 carbon sugars and reducing equivalents, as shown in Fig. 1. There are three primary functions of this pathway :
There are three inborn error in the pentose phosphate pathway that have been identified .
The enzyme glucose-6-phosphate dehydrogenase (G6PDH), catalyses the reaction that converts glucose-6-phosphate to 6-phosphogluconate. This creates one mole of NADPH each for every mole of glucose-6-phosphate (G6P) that enters the PPP . A deficiency would lead to an error to the first irreversible step of the pathway . This would lead further to a lower production in NADPH, making the cell more acceptable to oxidative stress .
G6PDH is very important for Erythrocytes metabolism . A deficiency could lead Individuals to nonimmune hemolytic anaemia which can be caused by, infection or exposure to certain medications or chemicals . G6PDH deficiency is also linked to favism . It is thought to be an X-linked recessive hereditary disease .
A recent study have shown that a patient with of ribose-5-phosphate isomerise deficiency, had developed progressive leucoencephalopathy and, developmental and speech delay . They did further studies using NMR and found that polyols ribitol and D-arabitol concentration was abnormal in body fluids [21, 22]. They did their studies on fibroblasts and found that the enzyme gene-sequence analysis showed a frame-shift and a missense mutation .
Ribose-5-phosphate isomerase deficiency would mean that the reversible reaction converting ribose-5-phosphate to ribulose-5-phosphate and vice versa will halt . If there was no deficiency ribulose-5-phosphate would be converted to xylulose 5-phosphate, which will provide the substrates for transketolase and further conversion into glycolytic intermediates .
Studies have found that there are two mutant allele one from each parents that results in ribose-5-phosphate isomerise gene that causes the deficiency . So it could be an autosomal recessive inheritance disorder . The best way to do a diagnostic test for Ribose-5-phosphate isomerase deficiency would be to take a urine sample . Polyols ribitol and D-arabitol would be analysed . Also enzyme assay can be used to sequence the ribose-5-phosphate isomerise gene .
Some studies have been done where three unrelated families had Transaldolase deficiency . One patient had aortic coarctation where the aorta narrows . During the patient’s life they found that ammonia was rising. But neurological and intellectual development has been normal. Another patient had HELLP syndrome (hemolysis, elevated liver enzymes and low platelet count) [23, 24].
Children with transaldolase deficiency have been diagnosed have found that the development of intellectual and neurological showed no abnormalities [23, 24]. But there is a strong link to liver cirrhosis which results from increased cell death of hepatocytes and biliary epithelial cells [23, 24].
Transaldolase catalyses the reaction:
Sedoheptulose 7-phosphate + glyceraldehyde 3-phosphate erythrose 4-phosphate + fructose 6-phosphate .
It is a reversible reaction in the pentose phosphate pathway. The deficiency lead to the accumulation of polyols derived from the pathway intermediates: erythritol, arabitol and ribitol .
Studies have shown that all patients were homozygous for these specific mutations, suggesting autosomal recessive inheritance [23, 24]. A simple urine test can be done to diagnose of transaldolase deficiency, mainly because there is a high concentration of arabitol and ribitol in urine. Also enzyme assay can be used to sequence the gene [23, 24]. Liver transplant would be the only option with patients that have severe liver cirrhosis [23, 24].
In the production of insulin glucose enters the ß-cell through a GLUT2 transporter where it is phosphorylated to glucose-6-phosphate by the enzyme glucokinase . The enzyme is used as a control, where it monitors the level of glucose . As blood glucose raises the rate of glucose metabolism also increases, where the cell will undergo glycolysis generating ATP . This increase of ATP concentration causes K+ channels to close, making the membrane depolarised . This depolarisation causes the voltage sensitive Ca2+ channels to open and Ca ions flood in, stimulating insulin secretion by exocytosis from storage granules; this is shown in Fig. 2.
Hyperinsulinism has been diagnoses in all ages but it is very common in childhood . Persistent hyperinsulinemic hypoglycaemia (PHHI) is the one of the main cause of hypoglycaemia especially in young children. Patients who are older, that develop PHHI are due to pancreatic adenoma . Hypoglycaemia when there is an overproduction of insulin by the ß-cells in the pancreas . Hypoglycaemia can produce a variety of symptoms the most dangerous is brain damage which can lead to death, and that is why treatment is vital . PHHI has two histopathological lesions that can be easily distinguished, making PHHI a heterogeneous disorder .
Focal hyperinsulinemic hypoglycaemia (FoPHHI) is caused by loss of heterozygosity which is a somatic event . This causes focal adenomatous hyperplasia, which is a pancreatic lesion . They are treated with pancreatectomy, where they surgically remove part of the pancreas.
Diffuse hyperinsulinemic hypoglycaemia (DiPHHI) is also a heterogeneous disorder, in that fact that it is unable to encode for proteins needed for insulin secretion . This can also be caused autosomal recessive and dominant genes which are rare . Positron emission tomography (PET) is used to distinguish between focal and diffuse PHHI . This gives a 3D image or picture of functional processes in the body . Once a patient is diagnosed with PHHI, they are on treatment straight away with glucose and glucagon .
Hyperinsulinemic hypoglycaemia is due to insulin hypersecretion by the pancreas . The Action insulin causes a decrease in plasma glucose by inhibiting hepatic glucose release from glycogen and gluconeogenesis, and by increasing glucose uptake in muscle and fat .
PHHI is a disorder that is caused by a variety of defects, either in regulation of insulin secretion, unable to transcribe the enzymes needed of even a modified receptor . For example diseases that can affect the ion channels like seizures [27, 28, 29]. Also lack of enzyme production of glucokinase (GK), and glutamate dehydrogenase (GDH) [30, 31].
Epidemiology has found that 1/50,000 patients are born with PHHI [32, 33]. Focal hyperinsulinemic hypoglycaemia is strongly linked to mutation of the sulfonylurea-receptor and the K+ channels, both used to depolarise the cell [32, 33]. Both are found to be located on the chromosome 11p15 [32, 33]. To identify these mutations they would need to be tested in a foetus or embryo before it is born. Sulfonylurea-receptor gene (SUR1) will not respond to diazoxide, which is used as a K+ channels activator .
Studies have found that a high activity of the enzyme glutamate dehydrogenase (GDH) has resulted to hyperinsulism/hyperammonemia syndrome. This would make sense as GDH is needed to produce insulin and this would impair detoxification of ammonia in the liver . The enzyme glucokinase (GK) is also expressed highly, where the affinity is increased for glucose, causing high levels of insulin secretion .
Diagnostic of HI is easy, in the fact that, it can be indicated by the levels of glucose in the blood. Treatment varies from age . Hyperammonemia should be treated as another disease, when a patient has PHHI, when treating hyperinsulism/hyperammonemia syndrome. This can be done by analysis of urine organic acids and plasma acylcarnitines .
Patients who show the FoPHHI can have lesion ranging from 2.5 to 7.5 mm in diameter [37, 38]. People who have DiPHHI found that there was ß-cells that were abnormal . Pancreatic venous catheterization (PVS) and pancreatic arteriography have proven very useful in locating the site of insulin secretion [40, 41]. PVS procedure will have to able to maintain blood glucose level, which is between 2 and 3 mmol/l. Blood sample would then be taken from the pancreas to measure 3 things 1.plasma glucose, 2.insulin and 3.C-peptide levels [40, 41].
Studies have shown that people with FoPHHI tend to have high concentration of plasma insulin and C-peptide levels in some samples and low concentration in others [40, 41]. People who have DiPHHI tend to find that all their sample have high concentration of plasma insulin and C-peptide [40, 41].
The use of [18F]-labelled fluoro-L-DOPA whole-body positron emission tomography (PET), has proved to be very useful in detecting hyperfunctional islet pancreatic tissue, where this can be used on patients with focal lesion .
Recent studies have shown that a new technique have been use to locate focal lesion and separate focal from diffuse forms of HI; this is the tolbutamide test [44, 45].
Brain damage can occur if you are hypoglycaemic, so treatment needs to be quick. Glucagon would be given, where the patient would have to take 1 to 2 mg per day if blood glucose levels are unstable . To treat PHHI, diazoxide would be given, usually at a dose of 15-10 mg/kg/day depending on your age . Normal blood glucose levels should be between 4 and 7 mmol/l, before and after a meal . This could need to be check every time once taking diazoxide [35, 56].
Octreotide treatment, can also be used as it is a hormone inhibitor . But a high could lead to a more severe hypoglycaemia, as it can inhibit glucagon and growth hormone . Patients will find that after treatment with octreotide, they might vomit or have diarrhoea .
Calcium-channel blockers could be used, such as Lercanidipine and Pranidipine . These treatments that have been mentioned are very effective in controlling blood sugar .
If a patient is diagnosed with FoPHHI, the treatment tends to be surgical as drugs are ineffective . They would undergo pancreatectomy. This procedure has its risk as the patient might develop diabetes mellitus . DiPHHI patients have been found to have large nuclei in the ß-cells [35, 47]. And patients with FoPHHI showed no abnormal s-cell nuclei but did show shrunken cytoplasm [49, 50].
Monosaccharide’s such as glucose and fructose have the properties of being hydrophilic . The lipid bilayer has hydrophilic heads and hydrophobic tails, prevent polar molecule such as glucose from diffusing across the membrane [59, 60]. So transport mechanisms are needed. These are hydrophilic pores allowing polar molecules to diffuse in and out of the cell [59, 60].
There are two types of glucose transporters. 1. Sodium-dependent glucose transporters (SGLT), which have been found to be located in the small intestine and the proximal tubule [59, 60]. SGLT uses the difference in concentration of sodium to transport glucose [59, 60]. From high to low concentration of sodium causes the transport of glucose against its own concentration gradient [59, 60]. 2. Facilitative glucose transporters (GLUT), which has been found throughout the body, but manly in muscle and pancreas cells [59, 60]. These transporters transport glucose from high to low concentration [59, 60].
Studies have shown that there are four defects in the transport of monosaccharides [59, 60]. These defects can depend on where the transporters are located within the body and what they transport in and out of the cell [59, 60].
As already stated there are four defects. 1. SGLT2, which is found in renal tubulus cells that can cause renal glucosuria [59, 60]. 2. SGLT1, which is found in the intestine, which can cause glucose-galactose malabsorption [59, 60]. 3. GLUT2, a transporter that carries glucose to the liver kidneys and pancreas [59, 60]. 4. GLUT1 is important, in the fact that it carries glucose to the brain cell (neuron and glia cells)