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Although there has been great progress in the development of animal models of phenylketonuria (PKU), findings in humans with PKU are important in elucidating PKU pathophysiology because of fundamental differences between animals and humans in postnatal brain development. There is strong evidence that phenylalanine (Phe) itself is the toxic agent in patients with defects of the Phe hydroxylase enzyme, and not the increase in Phe metabolites. As shown by various groups (Kaufman, 1976), oxidative Phe metabolites do not reach sufficient intracerebral concentrations to interfere with the development of enzymes. Another argument against the possible effects of Phe degradation products is the existence of a rare inborn error described as "chemical PKU," which was found in two sisters who had no symptoms of untreated PKU even though they had elevated concentrations of phenylpyruvic, o-hydroxyphenylacetic, and phenyllactic acid in the blood and a low normal level of Phe in the urine (Wadman, Ketting, de Bree, et al., 1975; Trefz, Blau, Aulehla-Scholz, unpublished).
Reversible and Irreversible Effects
The target organ in untreated PKU is the brain, but it is necessary to differentiate between reversible and irreversible effects on brain function. Effects that are reversible may be more evident in severely retarded patients (Baumeister, Baumeister, 1998) or in infants whose diets are changed. In normally developed patients, neurophysiological tests must be used to detect brain dysfunctions, such as impaired attention or problems in information processing (Schmidt, Rupp, Burgard, et al., 1994). However, the long-term effects of high concentrations of Phe on the normal brain cannot be predicted and may be influenced by various life factors (Weglage, Oberwittler, Marquardt, et al., 2000).
The Hypotheses: Influence on Myelin, Protein, and Neurotransmitter Metabolism
This abstract is focused on the theory that high concentrations of Phe interfere with normal brain development and that these damaging effects may be caused by other genetic factors rather than by defects in the Phe hydroxylase gene. Recent studies of patients with PKU using magnetic resonance spectroscopy have confirmed high intracerebral concentration of Phe. This implies an imbalance of large neutral amino acids (LNAA) that can be compensated in part by LNAA supplementation (Pietz, Kreis, Rupp, et al., 1999).
This discussion of the pathophysiology of PKU is based on three hypotheses: (1) high concentrations of Phe interfere with myelin metabolism, which not only influences nerve function but also affects normal development of the brain; (2) high Phe or deficiencies in LNAA then create problems in intracerebral protein synthesis and (3) in the synthesis of neurotransmitters.
Neuropathological studies of untreated patients with PKU 50 years ago showed that they had decreased myelin content and decreased amounts of myelin components. It was later demonstrated that dendritic branching of oligodendrocytes decreases in untreated patients (Bauman, Kemper, 1982). Further studies in animals showed that decreased myelination is not caused by demyelination but by hypomyelination. More recent studies (Dyer, Kendler, Philibotte, et al., 1996; Dyer, Kendler, Jean-Guillaume, et al., 2000) using the genetic mouse model indicate that high Phe may interfere not only with myelin metabolism but also with cell differentiation of oligodendrocytes, which adopt a nonmyelinating phenotype. This hypothesis, transferred to humans, would explain hypomyelination and gliosis in untreated patients with PKU.
Since myelin is a proteolipid, hypomyelination and gliosis may also be due to a primary effect on cerebral protein synthesis (Kaufman, 1976). The problem, however, is not to show where high Phe interferes with various enzymatic developments but rather to show where it does not. Thus, the specific effect of amino acid imbalance on intracerebral protein synthesis, and in consequence on brain development, is difficult to quantify. Lowered levels of neurotransmitters have been found in the cerebral spinal fluid as well as in certain brain areas of humans with PKU (McKean, 1972). More recent studies in animals have shown that the functioning of the dopaminergic neurons in the prefrontal cortex, an area vulnerable to Phe/tyrosine imbalances (Diamond, 1996), is influenced by the availability of tyrosine (Tam, Roth, 1997). There are no studies, however, showing that neurotransmitter depletion in patients with PKU leads to irreversible changes in the developing brain. Thus, it is more likely that imbalances of LNAA in the brain leading to lowered neurotransmitters are responsible for reversible changes, especially those seen in the brain of untreated or late-treated patients.
None of these theories alone, however, explains the etiology of brain damage and the heterogeneity of the clinical picture of persons with untreated PKU. Since there is an absence of genotype/phenotype correlation in some patients, other factors (genes?) must be moderating the natural course of the disease. Between 5 and 10 percent of patients with classical PKU have normal intelligence (Hanley, Platt, Bachman, et al., 1999), even though they were never treated by being placed on a low Phe diet.
Normal Intelligence in Untreated PKU: The Possible Role of the Intracerebral LNAA Carrier
The best explanation for this may be the LNAA carrier in the brain and the gene(s) that control it. In vivo magnetic resonance spectroscopy has demonstrated different intracerebral concentrations of Phe after loading with Phe (Weglage, Möller, Wiederman, et al., 1998). Different plasma Phe to brain Phe ratios may explain different intellectual outcomes in patients whose PKU phenotype is otherwise comparable (Weglage, Wiedermann, Möller, et al., 1998; Moats, Koch, Moseley, et al., 2000). Möller and colleagues (1998) found blood levels of around 150 µmol/L in three women with classical PKU, compared with 400 to 730 µmol/L in most patients with PKU. The only explanation is that K m variants of Phe in the cerebral LNAA carrier protected the three women from the devastating effects of high intracerebral Phe.
Further investigation of the role of LNAA and the gene(s) expressing this carrier may therefore be very useful. A fuller understanding of the carrier mechanism not only may provide better insight into the pathophysiology of PKU but also may be a key to alternative treatments. One of these might be optimizing supplementation of other amino acids, as shown by Pietz and colleagues (1999). Another might be the development of a pharmacologic modifier of this carrier that would prevent excess Phe from crossing the blood-brain barrier. A more clinical approach would be to test the effect of the natural cofactor tetrahydrobiopterin on elevated blood Phe in patients with classical or mild PKU. Kure and colleagues (1999) found that some enzyme mutations may be responsive to diet supplementation with tetrahydrobiopterin, as was found in one of our patients with mild PKU (Trefz, Blau, Aulehla-Scholz, et al., unpublished).
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