Aminoacidopathies or also called inborn errors of metabolism are rare inherited diseases caused by inability to metabolize certain amino acids. Such conditions can cause several issues in the body, particularly concerning physical development. Proper differentiation of the various categories of aminoacidopathies should be of great help in enabling the identification and treatment of the diseases at the appropriate time. This article is relevant in details about different aminoacidopathies, clinical descriptors, and management, information which is very useful for family members and health workers.
Aminoacidopathies are a group of hereditary metabolic disorders caused by an enzyme deficiency that prevents the body from breaking down specific amino acids. Either the membrane transport system for amino acids or the activity of a particular enzyme in the metabolic pathway exhibit anomalies. The first newborn screening test was introduced in the early 1960s for phenylketonuria (PKU), an aminoacidopathy. States now mandate screening testing for up to 26 amino acids. Hereditary abnormalities in the metabolism of amino acids have been linked to over 100 illnesses. Because of the accumulation of toxic amino acids and/or metabolic byproducts in the blood, aminoacidopathy may lead to serious medical problems.
Phenylketonuria
About 1 in 15,000 infants have phenylketonuria (PKU), which is inherited as an autosomal recessive condition. The enzyme phenylalanine hydroxylase (PAH), which catalyzes the conversion of phenylalanine to tyrosine, is absent in the typical form of PKU, indicating a metabolic abnormality. Phenylalanine levels in the absence of the enzyme are typically higher than 1200mol/L. The maximum acceptable level of phenylalanine in a baby is 120 μm/L (2 mg/dL). Blood levels in untreated classic PKU can reach up to 2.4mM/L.
Significant brain issues can result from persistently elevated amounts of phenylalanine and several of its metabolites, such as phenylpyruvic acid, phenylpyruvate (often referred to as phenylketone), and phenyllactic acid.
PKU patients have all of these substances in their blood and urine, which gives their urine a distinct musty smell. When phenylalanine levels fall between 600 and 1200μmol/L, partial shortages of PAH activity are usually categorized as moderate PKU, or as non-PKU mild hyperphenylalaninemia when phenylalanine levels fall between 180 and 600 μmol/L and no phenylketone buildup is present.
Phenylalanine or its metabolic byproducts have a deleterious effect on the brain, resulting in microcephaly and impaired mental development in babies and children with this hereditary disease. If the condition is identified at birth and the child is fed a diet with extremely low phenylalanine levels, brain harm can be prevented. Furthermore, untreated PKU mothers nearly often give birth to microcephalic, mentally handicapped children. It is possible to prevent the fetal consequences of maternal PKU if the mother follows a diet low in phenylalanine from the time of conception until delivery.
There are cases of hyperphenylalaninemia that are not caused by insufficient PAH enzyme. The inadequacy in these instances is a shortage of the enzymes required for the production and regeneration of tetrahydrobiopterin (BH4). Tyrosine, tryptophan, and phenylalanine are three aromatic amino acids that must be hydroxylated by enzymes, and BH4 is one such cofactor. A BH4 deficit causes phenylalanine levels in the blood to rise and tyrosine and tryptophan to be insufficiently converted into neurotransmitters. Even while cofactor deficiencies only cause 1%–5% of cases of increased phenylalanine levels, they still need to be found in order to treat the active cofactor and the neurotransmitter precursors 5-OH tryptophan and L-DOPA appropriately.
Every state currently examines babies for blood phenylalanine levels at around three days of age. Further testing is required to confirm or rule out PKU if the screening test results are abnormal. Early detection and therapy implementation are made possible by newborn screening. Maintaining the blood level of phenylalanine between 2 and 10 mg/dL (120–600 μmol/L) is the aim of PKU treatment. Since normal growth requires some phenylalanine, the suggested course of treatment is a diet that contains some phenylalanine but in considerably lower proportions than usual. Avoid foods high in protein, including meat, fish, poultry, eggs, cheese, and milk. Rather, measured servings of fruits, vegetables, grains, and carbohydrates are typically advised, coupled with an alternative milk.
The first medication to aid in PKU management, Kuvan (sapropterin dihydrocholoride), was approved by the US Food and Drug Administration (FDA) in December 2007.
Through an increase in PAH enzyme activity, the medication aids in the reduction of phenylalanine levels. Only patients with some PAH activity who continue to eat a diet low in phenylalanine and have regular phenylalanine level monitoring are candidates for Kuvan.
Tests for PKU
The Guthrie test is a semiquantitative bacterial inhibition assay for phenylalanine that makes use of the compound’s capacity to promote bacterial growth in an inhibitor-containing culture medium. A little disk of the filter paper is punched out and placed on an agar gel plate containing Bacillus subtilis and 2-thienylalanine after newborn infant blood is collected on it. While B-2-thienylalanine inhibits bacterial growth, agar gel can encourage bacterial growth. When excess phenylalanine seeps out of the impregnated filter paper disk, the inhibition is broken down and the bacteria proliferate. The Guthrie assay has a sensitivity range of 180–240 µmol/L (3–4 mg/dL) for serum phenylalanine. Since the late 1960s, the test has been routinely utilized as one of the primary newborn screening tests in North America and Europe. Newer methods that can detect a greater range of congenital illnesses, including tandem mass spectrometry, are increasingly replacing it in many places in recent years. The Guthrie test may also yield false-negative results if the baby is not at least 24 hours old, which guarantees enough time for the development of enzyme and amino acid levels, and if the sample is not obtained prior to the administration of antibiotics or the transfusion of blood or blood products.
A microfluorometric test for the direct determination of phenylalanine in dried blood filter disks is another method for PKU screening. This approach is more automation-friendly, produces quantitative data, and is unaffected by the use of antibiotics. The process is based on the fluorescence of a copper-phenylalanine-ninhydrin complex that forms when a dipeptide is present. Tricholoroacetic acid (TCA) pretreatment of the filter paper specimen is required for the test.
The extract is then combined with a solution of leucylalanine, succinate, and ninhydrin in the presence of copper tartrate in a microtiter plate. The complex’s fluorescence is measured at 530 nm and 360 nm, respectively, for excitation and emission wavelengths.
Verification is necessary for any positive screening test findings. High-performance liquid chromatography (HPLC) is the standard technique for measuring blood phenylalanine quantitatively, although enzymatic and fluorometric approaches are also available. Tandem mass spectrometry (MS/MS) is being used to test infants for genetic abnormalities. An analytical method for determining the mass-to-charge ratio of charged particles is mass spectrometry. By creating a mass spectrum that represents the masses of the sample’s constituents, it is most frequently used to determine the composition of a physical sample. The ratio of phenylalanine to tyrosine (Phe/Try) can be computed since it is possible to identify both the rise in phenylalanine and the fall in tyrosine levels observed in PKU. When metabolite ratios are used instead of individual levels, the measurement’s specificity is increased and the false-positive rate for PKU is reduced to less than 0.01%. With its higher sensitivity, the MS/MS approach can identify phenylalanine at lower levels, making it possible to diagnose PKU as early as the first day of life. A single specimen can identify over 25 distinct genetic abnormalities, hence MS/MS is replacing the numerous steps that are now involved in newborn screening programs.
In 2005, a rapid diagnostic method for PKU in neonates was created by utilizing gas chromatography–mass spectrometry (GC/MS) in conjunction with microwave-assisted silylation. Under microwave radiation, amino acids are quickly isolated from neonatal blood samples using N, O-bis (trimethylsilyl)-trifluoroacetamide. Next, GC/MS is used to evaluate the derivatives.
DNA analysis can now be used for prenatal diagnosis and carrier status determination in PKU families. PKU is caused by numerous distinct mutations at the PAH gene (more than 400 have been found).
Tyrosinemia
The excretion of tyrosine and tyrosine catabolites in urine is a characteristic of aminoacidopathies metabolism of phenylalanine and tyrosine. abnormalities of tyrosine catabolism. Tyrosinemia comes in three different forms, each with unique symptoms brought on by an enzyme shortage.
About 1 in 100,000 infants have type I tyrosinemia, the most severe form of this aminoacidopathy. The fifth of the five enzymes required to break down tyrosine, fumarylacetoacetate hydrolase, is deficient in type I tyrosinemia. Failure to thrive, diarrhea, vomiting, jaundice, a smell similar to cabbage, a swollen abdomen, swelling in the legs, and an increased risk of bleeding are all signs of type I tyrosinemia. In addition to neurological issues, liver and renal failure, and a higher chance of developing cirrhosis or liver cancer in later life are also possible outcomes of type I tyrosinemia.
Tyrosine aminotransferase deficiency is the cause of type II tyrosinemia. There are less than 1 in 250,000 babies with type II tyrosinemia. The first of five enzymes called tyrosine aminotransferase breaks down tyrosine into smaller molecules that are either eliminated by the kidneys or used in processes that generate energy. In addition to having excessive tearing, photophobia (abnormal sensitivity to light), eye pain and redness, and painful skin lesions on the palms and soles of the feet, about half of people with type II tyrosinemia are mentally retarded.
With only a few examples documented, type III tyrosinemia is an uncommon illness brought on by a 4-hydroxyphenylpyruvate dioxygenase enzyme deficiency. The liver contains greater quantities of this enzyme than the kidneys do. It is one of the group of enzymes required to degrade tyrosine as well. Patients with type III tyrosinemia often exhibit occasional loss of balance and coordination, convulsions, and minor mental retardation.
Using MS/MS in conjunction with a confirmatory test for an elevated level of the aberrant metabolite succinylacetone, an elevated tyrosine level is one of the diagnostic criteria.
Tyrosinemia can be treated with a low-protein diet, a full or partial liver transplant, or the medication nitisinone (NTBC), which blocks the production of fumarylacetoacetic acid and maleylacetoacetic acid, which can be converted to succinyl acetone, a toxin that destroys the liver and kidneys. Since liver transplantation was no longer the first-line treatment for tyrosinemia in 1991, nitisinone has taken its place.
Alkaptonuria
The aminoacidopathies known as alkaptonuria is caused by a loss of the enzyme homogentisate oxidase, which is required for the metabolism of tyrosine and phenylalanine. This condition is inherited and is caused by the autosomal recessive gene, the HGD gene.
Out of every 250,000 births, one has this disease. The patient’s urine turning brownish-black when it comes into contact with air is a common clinical sign of alkaptonuria. Homogentisic acid (HGA) builds up in urine and oxidizes to form this dark pigment, which is the cause of this condition.
There are no symptoms in the beginning for patients with alkaptonuria; however, as the disease progresses, the high concentration of HGA slowly builds up in connective tissue, resulting in ochronosis (pigmentation of these tissues), an arthritis-like degeneration due to homogentisic acid accumulation in the cartilage, dark spots on the sclera (white of the eye), and pigment deposition in the cartilage of the ears, nose, and tendons of the extremities.
One test for alkaptonuria is urinalysis. In people with alkaptonuria, adding ferric chloride to the urine will cause it to turn black. High-dose vitamin C has been found to reduce the accumulation of brown pigment in the cartilage and may halt the progression of arthritis as a treatment for alkaptonuria.
Maple Syrup Urine Disease
The three necessary branched-chain amino acids—leucine, isoleucine, and valine—cannot be properly metabolized when the enzyme branched-chain ketoacid decarboxylase is absent or has significantly decreased activity. This condition is known as maple syrup urine disease (MSUD). An autosomal recessive genetically inherited condition is MSUD. Since the mid-1970s, multiple state screening programs have included newborn screening for MSUD, with a reported frequency of 1 in 150,000 births in the general population. The most noticeable aspect of this genetic illness is the distinctive smell of burnt sugar or maple syrup on the skin, breath, and urine. An accumulation of branched-chain amino acids and the ketoacids that go along with them in the blood, urine, and cerebrospinal fluid (CSF) is the outcome of this enzyme deficiency.
At birth, infants with MSUD appear normal, but within a week they show signs of malnutrition, lethargy, vomiting, and lack of appetite. Following are CNS symptoms such as respiratory abnormalities, stupor, and muscle rigidity. As the illness worsens, patients experience acidosis, hypoglycemia, convulsions, and severe mental retardation. In the absence of therapy, the illness may be fatal. There have been reports of intermediate types of MSUD, in which the decarboxylase activity is about 25% of normal. Leucine, isoleucine, and valine restriction in the diet can often limit the levels of branched-chain amino acids, even though this still causes a permanent rise of them.
A modified Guthrie test is frequently employed in the screening of newborns. 4-azaleucine is a metabolic inhibitor of B. subtilis that is present in the growth media. The hemoglobin is denatured in a microfluorometric assay for the three branched-chain amino acids by treating a filter paper specimen with a methanol and acetone solvent mixture. An extract aliquot is treated to leucine dehydrogenase, and an excitation wavelength of 360 nm is used to quantify the fluorescence of the NADH generated in the ensuing reaction at 450 nm. Leucine levels more than 4 mg/dL are suggestive with MSUD. MS/MS is also utilized in MSUD testing. By measuring the quantity of the decarboxylase enzyme in cells cultivated from amniotic fluid, MSUD can be diagnosed in utero.
Isovaleric Acidemia
A loss of the enzyme isovaleryl-CoA dehydrogenase results in isovaleric acidemia, an autosomal recessive metabolic disease that impairs leucine’s normal metabolism. Because of mutations in the isovalerylCoA dehydrogenase (IVD) gene, isovaleric acidemia affects about 1 in every 250,000 babies born in the US.
The unique smell of perspiring feet, which isovaleric acid accumulation, is a distinguishing symptom of isovaleric acidemia. Isolated acidemia can cause anything from really minor health issues to potentially fatal conditions, but in extreme cases, it can harm the brain and neurological system. A few days after birth, the disorder’s clinical signs and symptoms appear. These include lethargy, vomiting, and failure to thrive, which can lead to seizures, a coma, and possibly even death. Certain individuals who have isovaleric acidemia-causing gene mutations are asymptomatic, meaning they never show any symptoms at all.
A protein-restrictive diet is part of the treatment to reduce the amounts of accumulated isovaleric acid, which is harmful to the central nervous system. Glycine and carnitine supplements may be taken orally since they combine with isovaleric acid to create compounds that are easily eliminated and do not pose a risk.
Using chromotography or MS/MS, neonates’ urine can be checked for isovaleric acidemia. Metabolic acidosis, mild to severe ketonuria, hyperammonemia, thrombocytopenia, and neutropenia are found in the laboratory results.
Homocystinuria
Another inherited autosomal recessive condition related to amino acid metabolism is homocystinuria. Elevated plasma and urine levels of the amino acid methionine and its precursor homocysteine are the consequence of homocystinuria, which is caused by a lack of the enzyme cystathionine-β synthetase, which is required for the metabolism of methionine. This illness affects roughly 1 in 200,000 babies born. The infants appear to be in good health, and any early signs are vague. Osteoporosis, displaced lenses in the eye due to a lack of cysteine synthesis, which is necessary for the creation of collagen, and, usually, mental retardation are associated clinical symptoms in late childhood. If left untreated, this deficiency results in a multisystemic condition of the muscles, CNS, connective tissue, thinning and weakening of bones, and thrombosis due to the toxicity of homocysteine to the vascular endothelium.
Methionine restriction (low protein diet) along with high dosages of vitamin B6 is the treatment. A little fewer than half of the patients benefit from this treatment, and they will always need to take extra vitamin B6. For those who do not respond to standard treatment, trimethylglycine is necessary. A standard dosage of folic acid supplements and occasionally an addition of cysteine to the diet might also be beneficial.
The Guthrie test, which uses L-methionine sulfoximine as a metabolic inhibitor, is used for newborn screening.
Bacterial growth will be caused by elevated plasma methionine levels in afflicted infants. The test utilized as the confirmatory method is HPLC; positive results from the screening test are confirmed if the methionine level is more than 2 mg/dL. Methionine levels are also tested in screening programs using MS/MS. Alternatively, liquid chromatography electrospray-tandem mass spectrometry (LC-MS/MS) can quantify elevated urine total homocysteine levels in large testing volumes and offer a quick turnaround. The foundation of this technique is the analysis of 100 μL of either plasma or urine, to which 2 nmol of homocystine is added as the internal standard. The analysis is carried out in the multiple reaction monitoring mode with detection through the transition from the precursor to the synthesis after sample reduction and deproteinization. Automation is possible, and a batch of 40 specimens can be finished in less than an hour.
Homocysteine elevations are worth looking into when examining cardiovascular risk. Before the age of 30, 50% of people with untreated homocystinuria and noticeably high plasma homocysteine levels (200–300 mmol/L) suffer a thromboembolic event. Additionally, 20% to 30% of patients with atherosclerotic disease had modest homocysteine increase (>15 mmol/L). Apart from the previously mentioned deficiency in cystathionine synthase, other potential causes of hyperhomocystinemia include low levels of folate, insufficient vitamin B12, deterioration in renal function, and a genetic modification in the enzyme methylenetetrahydrofolate reductase (MTHFR), which is responsible for converting homocysteine back to methionine.
Citrullinemia
Citrullinemia is a member of the urea cycle disorders genetic illness class. The body uses the urea cycle, a metabolic pathway, in liver cells to handle excess nitrogen produced during protein utilisation. The extra nitrogen is utilized to create urea, which is subsequently expelled as urine.
The mode of inheritance for citrullinemia is autosomal recessive. The most prevalent form of the condition, type I citrullinemia, affects roughly 1 in 57,000 babies born. The prevalence of type II citrullinemia is highest in the Japanese population, where it affects between 1 in 100,000 and 230,000 persons.
The metabolic abnormality known as type I citrullinemia is brought on by argininosuccinic acid synthetase deficiency, which results in an accumulation of ammonia and the amino acid citrulline in the blood. As ammonia builds up in the body, affected newborns experience clinical symptoms such as vomiting, lethargy, convulsions, loss of appetite, failure to thrive, and coma. Severe brain damage or even death may ensue from delayed treatment. A milder version of type I citrullinemia may occur later in childhood or adulthood, however this is less common.
A mutation in the gene that would normally supply instructions for producing the protein citrin results in type II citrullinemia. Citrin aids in the intracellular transportation of molecules involved in the urea cycle, protein synthesis, and the synthesis and degradation of simple carbohydrates.
Nucleotides, the building blocks of DNA and RNA, are also produced by molecules carried by citrin. When cells are unable to produce citrin, the urea cycle is inhibited and the synthesis of proteins and nucleotides is disturbed. This condition is known as type II citrullinemia. The accumulation of ammonia and other hazardous materials that follows causes neurological system-related clinical symptoms.
In patients with adult-onset type II citrullinemia, these symptoms can be fatal and are known to be brought on by specific drugs, infections, surgeries, and alcohol consumption.
A high-calorie, protein-restrictive diet, arginine supplementation, and the administration of sodium benzoate and sodium phenylacetate are all part of the treatment for citrullinemia.
Argininosuccinic Aciduria
Argininosuccinic aciduria, or ASA, is a hereditary condition that is inherited in an autosomal recessive form. It is classified under the urea cycle disorders. About 1 in 70,000 babies have aminosuccinic aciduria. The absence of the enzyme argininosuccinic acid lyase in newborns with argininosuccinic acidemia inhibits the conversion of argininosuccinic acid to arginine. The amino acid citrulline accumulates in the blood when argininosuccinic acid levels are elevated. Nitrogen builds up in the blood as ammonia due to the disruption of the urea cycle caused by the mutation of the ASL gene, which is the cause of argininosuccinic aciduria. Particular harm to the brain system and eventual liver disease are caused by ammonia. Typically, the signs of argininosuccinic aciduria appear while the baby is still in the hospital. A person with argininosuccinic aciduria may initially exhibit tiredness and an unwillingness to eat.
A high-calorie, protein-restrictive diet, supplementing with arginine, and administering sodium benzoate and sodium phenylacetate are all part of the treatment for ASA. It should be mentioned that argininosuccinic acidemia and citrullinemia cannot be distinguished by the newborn screening test.
Cystinuria
Rather than being the result of a lack of metabolic enzymes, cystinuria is a hereditary autosomal recessive disorder caused by a malfunction in the amino acid transport system. The condition known as cystinuria is characterized by an excess of cystine in the blood due to insufficient reabsorption of the amino acid during the kidneys’ filtration process. Urine that contains cysteine precipitates out and can deposit as stones in the bladder, ureters, or kidneys. Kidney stones frequently repeat during the course of a patient’s lifetime and are either directly or indirectly accountable for all of the disease’s signs and symptoms, such as hematuria, side pain from kidney pain, and urinary tract infections.
The goal of cystine stone prevention is the treatment for cystinuria. In order to lower the amount of cystine in the urine and lessen its precipitation and formation of stones, this is primarily achieved by increasing the volume of pee. A high fluid intake entails consuming at least 4 liters of water daily. Penicillamine is administered when a high fluid intake on a regular basis is insufficient to prevent the formation of stones. Since cystine is generally insoluble on its own, penicillamine and cystine produce a more soluble combination. An alternative to surgery for kidney stone removal is percutaneous nephrolithotripsy (PNL).
The cyanide nitroprusside test, which reacts with sulfhydryl groups to create a reddish-purple color, can be used to detect cystinuria in urine. It is necessary to rule out homocystine-related false-positive results. Urinary quantities of three other amino acids that have structures comparable to cystine—lysine, arginine, and ornithine—were also found in significant amounts in the laboratory. Amino acid levels in plasma or urine can be quantitatively analyzed using ion exchange chromatography.
To conclude, aminoacidopathies are one of the vital genetic disorders resulting from the abnormalities of amino acid metabolism. They can impact heath in various methods, and some of the examples of these conditions include phenylketonuria and maple syrup urine disease. It is also important to know that early diagnosis and proper management are important factors that can significantly enhance the general health of the patient. These disorders make people gain awareness and therefore support those affected by such conditions. To know more about the aminoacidopathies, it is advisable to search for other materials or consult with a doctor.
