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Phenylketonuria
The pathogenesis of phenylketonuria
Phenylketonuria is an autosomal recessive genetic disorder. Phenylalanine, which is an essential amino acid required by the human body, is partly used for protein synthesis after ingestion and partly converted into tyrosine via the action of phenylalanine hydroxylase, which is necessary for the synthesis of substances such as adrenaline, melanin, and thyroxine. Phenylketonuria is mainly caused by a deficiency of phenylalanine hydroxylase, which prevents the conversion of phenylalanine to tyrosine. This leads to increased levels of phenylalanine in the blood, cerebrospinal fluid, and various tissues. Additionally, as the primary metabolic pathway is blocked, secondary metabolic pathways are enhanced. Under the action of transaminases, phenylalanine undergoes deamination to produce large quantities of phenylpyruvic acid, which is further metabolized to produce phenylacetic acid, phenyllactic acid, and p-hydroxyphenylpyruvic acid, among other byproduct metabolites. These are excreted in large amounts in the urine. The high concentration of phenylalanine and its byproduct metabolites accumulates in brain tissue, leading to damage to brain cells and resulting in a range of clinical symptoms.
Phenylketonuria Test Methods
The examination methods for phenylketonuria primarily include newborn screenings usually performed three to seven days after birth by drawing a blood sample from the heel and placing drops on specialized blood collection filter paper. Once dried, the sample is sent to a screening laboratory to determine the concentration of phenylalanine. If the phenylalanine concentration exceeds the cutoff value, further examinations and confirmatory tests are necessary, with the confirmatory test involving the measurement of phenylalanine concentration. Another method involves the analysis of urinary pterin profile, mainly used to differentiate dihydrobiopterin reductase deficiency. Additionally, gene analysis can be applied to detect mutations in genes such as phenylalanine hydroxylase, aiding in genetic diagnoses and prenatal diagnoses.
Phenylketonuria lacks what enzyme?
Phenylketonuria is an autosomal recessive genetic disorder caused by a mutation in the phenylalanine hydroxylase gene, which leads to reduced enzyme activity and the accumulation of phenylalanine and its metabolites in the body, resulting in disease. Phenylketonuria is the most common congenital disorder of amino acid metabolism, clinically characterized by developmental delays in intelligence, light pigmentation of the skin and hair, and a mousey odor to the urine. It is often due to a deficiency of phenylalanine hydroxylase.
How is phenylketonuria diagnosed?
How is phenylketonuria diagnosed? Firstly, we start with screening, which is usually done when the newborn is 3 to 7 days old by heel prick to collect peripheral blood, which is then dropped onto specialized filter paper for the determination of phenylalanine concentration. If the concentration of phenylalanine is above the cutoff value, further determination of phenylalanine concentration is conducted. The normal concentration is less than 120 umol/L. If the concentration of phenylalanine is greater than 1200 umol/L, it suggests severe phenylketonuria. Moderate levels are above 360 umol/L, and mild levels are above 120 umol/L but less than 360 umol/L. Furthermore, diagnosis can be confirmed through DNA analysis, currently available tests include screening for mutations in the phenylalanine hydroxylase gene.
Clinical symptoms of phenylketonuria
The clinical symptoms of phenylketonuria generally include normal appearance at birth, with symptoms usually appearing between three to six months, and becoming more pronounced by the age of one. The most noticeable initial symptoms involve the nervous system; delayed intellectual development is prominent, with intelligence often below normal, along with behavioral abnormalities such as hyperactivity, depression, restlessness, and withdrawal. There could be minor epileptic seizures, and in some cases, increased muscle tone or exaggerated tendon reflexes. Regarding the skin, several months after birth, due to insufficient melanin synthesis, the child’s hair changes from black to yellow, the skin appears pale, and eczema is also relatively common. Additionally, the urine and sweat contain higher levels of phenylacetic acid, which can result in a distinctive mouse-like urine odor.
What is the probability of phenylketonuria?
Phenylketonuria is an autosomal recessive genetic disorder primarily caused by mutations in the phenylalanine hydroxylase gene, leading to reduced enzyme activity and the accumulation of phenylalanine and its metabolites in the body, resulting in the disease. Phenylketonuria is the most common congenital amino acid metabolism disorder, with clinical manifestations mainly including intellectual developmental delays, light skin and hair pigmentation, and a mouse urine odor. The incidence of the disease varies by race and region. In China, the incidence of phenylketonuria is approximately 1 in 11,000, meaning that about one in 11,000 children suffers from phenylketonuria.
Can phenylketonuria be treated?
Can phenylketonuria be treated? Yes, it can be treated, and treatment should begin immediately once diagnosed. The younger the age at which treatment begins, the better the prognosis. Infants are generally treated with low-phenylalanine formula milk. Once the blood phenylalanine concentration drops to an ideal level, natural foods can gradually be added in small amounts, with a preference for adding breast milk, since it contains only one-third the phenylalanine content of cow's milk. Older infants and children can add foods like milk, porridge, noodles, and eggs. Added foods should follow the principle of being low in protein and phenylalanine. The amount and frequency of these foods should be determined based on the blood phenylalanine concentration. Concentrations that are too high or too low can affect the growth and development of the child. As each child's tolerance to phenylalanine concentration differs, it is necessary to regularly measure the blood phenylalanine concentration during dietary treatment and adjust the diet according to the specific situation of the child, avoiding increased phenylalanine levels or deficiency.
How is phenylketonuria diagnosed?
Phenylketonuria screening is routinely conducted in China for all newborns. This involves pricking the heel to collect peripheral blood, which is then dropped on specialized filter paper and dried. The samples are sent to a screening laboratory for the measurement of phenylalanine concentration. If the concentration exceeds the threshold, further testing for phenylalanine is conducted. Normally, the concentration should be less than 120 micromoles per liter. Additionally, an analysis of the urinary purine profile can be performed.
Mechanism of phenylketonuria
Phenylketonuria is characterized by reduced activity of phenylalanine hydroxylase, which fails to convert phenylalanine to tyrosine, causing extremely elevated levels of phenylalanine in the blood, cerebrospinal fluid, and tissues. This leads to the production of large amounts of phenylpyruvic acid, phenylacetic acid, phenyllactic acid, and p-hydroxyphenylacetic acid via alternative pathways. High concentrations of phenylalanine and its metabolic products can cause brain damage, which manifests as developmental intellectual disabilities, changes in skin and hair pigmentation, and a mousy urine odor, all part of a clinical syndrome.
The pathogenesis of phenylketonuria
Phenylketonuria is an amino acid metabolic disease caused by a deficiency of phenylalanine hydroxylase. Phenylalanine is an essential amino acid for the human body, and it is partially converted into tyrosine by phenylalanine hydroxylase. Due to the reduced activity of phenylalanine hydroxylase, phenylalanine cannot be transformed into tyrosine. This leads to extremely high concentrations of phenylalanine in the blood, cerebrospinal fluid, and tissues. Through alternative metabolic pathways, large amounts of phenylpyruvic acid, phenylacetic acid, phenyllactic acid, and others are produced. High concentrations of phenylalanine and its metabolic products can cause brain damage.