Phenylketonuria diagnosis

The detection of restriction fi agment length polymorphisms (RFLPs) facilitates prenatal detection of hereditary disorders such as sickle cell trait, beta-thalassemia, infant phenylketonuria, and Huntington s disease. Detection of RFLPs involves cleavage of double-stranded DNA by restriction endonucleases, which can detect subtle alterations in DNA that affect their recognized sites. Chapter 40 provides further details concerning the use of PCR and restriction enzymes for diagnosis. 

Phenylalanine hydroxylase (PH) which requires tetrahydrobiopterin (BH4) as a cofactor, is defective in cases of phenylketonuria (PKU). This is a rare (prevalence 1 / 15 000 in the United Kingdom) genetic condition characterized by fair complexion, learning difficulties and mental impairment. If PH is either not present in the hepatocytes or is unable to bind BH4 and is therefore non functional, phenylalanine accumulates within the cells. Enzymes in minor pathways which are normally not very active metabolize phenylalanine ultimately to phenylpyruvate (i.e. a phenylketone). To use the traffic flow analogy introduced in Chapter 1, the main road is blocked so vehicles are forced along side roads. Phenylpyruvate is excreted in the urine (phenyl-ketone-uria), where it may be detected but a confirmatory blood test is required for a reliable diagnosis of PKU to be made. 

As discussed above, in the case of phenylketonuria, early intervention can make the difference between mental retardation and a near normal life course for a newborn. Congenital adrenal hyperplasia and maple syrup urine disease are two examples of neonatal hereditary disorders where early diagnosis and medical intervention can make the difference between life and death for the newborn. In addition, in a number of genetic diseases, early diagnosis and treatment can help ameliorate symptoms these include fragile X syndrome, homocystinuria, sickle cell anemia, cystic fibrosis, and many /1-thalassemias. 

Indirect, prenatal diagnosis of phenylketonuria using RFLPs ... 

Fiege B, Bonafe L, Ballhausen D, et al. Extended tetrahydrobiopterin loading test in the diagnosis of cofactor-responsive phenylketonuria A pilot study. Mol Genet Metab 86 s91-s95,... 

C-7) Phenylketonuria (deficiency of phenylalanine hydroxylase). Occasionally, the defect is not in the enzyme but in the ability to regenerate tetrahydrobiop-terin, which is also necessary for the reaction. There is a buildup and excretion of phenylpyruvate in the urine, giving it a mousy odor. Mental retardation is a prominent feature. Diagnosis can be made by routine urine testing for phenylpyruvate or serum testing for elevated phenylalanine levels. The condition is treated with a diet low in phenylalanine. Sometimes, tetrahydrobiopterin deficiency may be treated by supplying biopterin,... 

Chace DH, MilUington DS, Terada N, et al. Rapid diagnosis of phenylketonuria by quantitative analysis for phenylalanine and tyrosine in neonatal blood spots by tandem mass spectrometry. Cfin Chem 1993 39 66B71. 

A 3-month-old boy presents with elevated levels of phenylalanine, para-hydroxyphenylpymvate and phenylpymvate in the serum. His skin color is pale. On your differential diagnosis is phenylketonuria. Which of the following would be consistent in such a case ... 

A 1-year-old girl presents at your clinic the day after you saw the 3-month-old boy. The symptoms are the same so you order a test on phenylalanine hydroxylase to confirm your diagnosis of phenylketonuria. To your surprise the phenylalanine hydroxylase activity is well within the normal range. Which of the following might you check next to support your diagnosis ... 

It all works much better if we consider some simpler cases that are less obvious in ordinary life but still have greater importance for medicine than skin or eye color. Take, for example, the disease phenylketonuria, which 1 mentioned briefly in Chapter 8. If left untreated it produces severe mental retardation and, in many cases, death before the age of 25. It is caused by an incapacity to convert the aminoacid phenylalanine into another aminoacid, tyrosine. It is not, however, a deficiency disease, because its harmful effects are not caused by a shortage of tyrosine, and cannot be avoided by adding tyrosine to the diet. Instead they are caused by the toxic effects on the brain of a substance called phenylpyruvate, which the body produces in its efforts to remove the excess of phenylalanine. The name of the disease reflects the fact that phenylpyravate, which belongs to a general class of chemical substances known as phenylketones, is excreted in the urine of affected people. This provides a simple method of diagnosis, and the disease is treated by carefully controlling the diet so that it provides no more phenylalanine than is needed for normal health. There is then no surplus to be converted into phenylpyruvate. 

A liver biopsy was sent to the special chemistry research laboratory, where it was determined that the level of activity of phenylalanine hydroxylase (PAH) in Piquet s blood was less than 1% of that found in normal infants. A diagnosis of "classic" phenylketonuria (PKU) was made. 

Increases of plasma Phe occur in the disorder phenylketonuria (PKU). Texts on this subject should be consulted There may be occasional increases in plasma Tyr in the newborn infant caused by a transient failure of development of the Tyr metabolic pathway. In this case, the elevation of Tyr in plasma may cause elevations of Phe secondary to the block in Tyr metabolism. Therefore, one should always verify that the plasma Tyr of a newborn infant is normal before making the diagnosis of hyperphenylalaninemia. 

It has been recognized in recent years that it is possible to treat this disease, phenylketonuria. A diagnosis of the disease may be made at an age as early as one month, and the infant then may be fed a diet of protein hydrolysate from which most of the phenylalanine has been removed. Children treated in this way seem to develop in an essentially normal manner. 

The accumulating metabolite is metabolized through alternative pathways, mainly the transaminase reaction. Consequently, imidazole pyruvic acid accumulates in the urine. The imidazole pyruvic, like phenylpyruvic, acid reacts with ferric chloride to yield a blue compound. As a result, the diaper test does not distinguish between phenylketonuria and histidinemia. Yet the diagnosis is of considerable importance because histidinemia is a much more benign disease. Furthermore, histidinemia is not alleviated by withdrawal of phenylalanine from the diet. 

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