Maternal Phenylketonuria Syndrome Research Articles

Characteristics and outcomes of pregnancies among women with phenylketonuria from the NBS Connect registry

Women with phenylketonuria (PKU) should maintain blood phenylalanine (phe) concentration within the recommended range before and during pregnancy to prevent maternal PKU syndrome (MPKUS) in their offspring. Women who gave birth to children with MPKUS symptoms were more likely to report elevated phe concentration before pregnancy, and barriers to accessing components of their dietary management during pregnancy, including blood phe testing, medical food, modified low-protein foods, and healthcare visits with PKU specialists.

Maternal-fetal outcomes of pregnancies in women treated at an inborn errors of metabolism unit

IntroductionIntermediate Inborn Errors of Metabolism (IEM) are a group of inherited diseases that include phenylketonuria (PKU), tyrosinemia II (TSII), organic acidaemias and ornithine transcarbamylase deficiency (OTCD), among others. They are increasingly more common in adults due to improved management. This has allowed more affected women to consider having children with good prospects. However, pregnancy may worsen metabolic control and/or increase maternal-fetal complications. The objective is to analyse the characteristics and outcomes of pregnancies of our patients with IEM. MethodsRetrospective descriptive study. Pregnancies of women with IEM attended to at the adult IEM referral unit of the Hospital Universitario Virgen del Rocío were included. The qualitative variables were described as n(%) and the quantitative as P50 (P25–P75). Results24 pregnancies were recorded: 12 newborns were healthy, 1 inherited their mother’s disease, 2 had maternal phenylketonuria syndrome, 1 was stillborn (gestational week 31 + 5), 5 were spontaneous abortions and 3 were voluntarily terminated. The gestations were divided into metabolically controlled and uncontrolled. ConclusionsPregnancy planning and multidisciplinary management through to postpartum is essential to ensure maternal and fetal health. The basis of treatment in PKU and TSII is a strict protein-limited diet. Events that increase protein catabolism in organic acidaemias and DOTC should be avoided. Further investigation of pregnancy outcomes in women with IEM is needed.

Resultados materno-fetales de las gestaciones de mujeres atendidas en una unidad de errores innatos del metabolismo

IntroductionIntermediate Inborn Errors of Metabolism (IEM) are a group of inherited diseases that include phenylketonuria (PKU), tyrosinemia II (TSII), organic acidaemias and ornithine transcarbamylase deficiency (OTCD), among others. They are increasingly more common in adults due to improved management. This has allowed more affected women to consider having children with good prospects. However, pregnancy may worsen metabolic control and/or increase maternal-foetal complications. The objective is to analyse the characteristics and outcomes of pregnancies of our patients with IEM. MethodsRetrospective descriptive study. Pregnancies of women with IEM attended to at the adult IEM referral unit of the Hospital Universitario Virgen del Rocío were included. The qualitative variables were described as n(%) and the quantitative as P50(P25-P75). Results24 pregnancies were recorded: 12 newborns were healthy, 1 inherited their mother's disease, 2 had maternal phenylketonuria syndrome, 1 was stillborn (gestational week 31 + 5), 5 were spontaneous abortions and 3 were voluntarily terminated. The gestations were divided into metabolically controlled and uncontrolled. ConclusionsPregnancy planning and multidisciplinary management through to postpartum is essential to ensure maternal and foetal health. The basis of treatment in PKU and TSII is a strict protein-limited diet. Events that increase protein catabolism in organic acidaemias and DOTC should be avoided. Further investigation of pregnancy outcomes in women with IEM is needed.

Impact of pregnancy planning and preconceptual dietary training on metabolic control and offspring's outcome in phenylketonuria.

To prevent maternal phenylketonuria (PKU) syndrome low phenylalanine concentrations (target range, 120-360 μmol/L) during pregnancy are recommended for women with PKU. We evaluated the feasibility and effectiveness of current recommendations and identified factors influencing maternal metabolic control and children's outcome. Retrospective study of first successfully completed pregnancies of 85 women with PKU from 12 German centers using historical data and interviews with the women. Children's outcome was evaluated by standardized IQ tests and parental rating of child behavior. Seventy-four percent (63/85) of women started treatment before conception, 64% (54/85) reached the phenylalanine target range before conception. Pregnancy planning resulted in earlier achievement of the phenylalanine target (18 weeks before conception planned vs. 11 weeks of gestation unplanned, p < 0.001) and lower plasma phenylalanine concentrations during pregnancy, particularly in the first trimester (0-7 weeks of gestation: 247 μmol/L planned vs. 467 μmol/L unplanned, p < 0.0001; 8-12 weeks of gestation: 235 μmol/L planned vs. 414 μmol/L unplanned, p < 0.001). Preconceptual dietary training increased the success rate of achieving the phenylalanine target before conception compared to women without training (19 weeks before conception vs. 9 weeks of gestation, p < 0.001). The majority (93%) of children had normal IQ (mean 103, median age 7.3 years); however, IQ decreased with increasing phenylalanine concentration during pregnancy. Good metabolic control during pregnancy is the prerequisite to prevent maternal PKU syndrome in the offspring. This can be achieved by timely provision of detailed information, preconceptual dietary training, and careful planning of pregnancy.

Preventing maternal phenylketonuria (PKU) syndrome: important factors to achieve good metabolic control throughout pregnancy

BackgroundInsufficient metabolic control during pregnancy of mothers with phenylketonuria (PKU) leads to maternal PKU syndrome, a severe embryo-/fetopathy. Since maintaining or reintroducing the strict phenylalanine (Phe) limited diet in adults with PKU is challenging, we evaluated the most important dietary and psychosocial factors to gain and sustain good metabolic control in phenylketonuric women throughout pregnancy by a questionnaire survey with 38 questions concerning therapy feasibility. Among them, the key questions covered 5 essential items of PKU care as follows: General information about maternal PKU, PKU training, diet implementation, individual metabolic care, personal support. In addition, all participating PKU mothers were asked to estimate the quality of their personal metabolic control of the concluded pregnancies. 54 PKU mothers with 81 pregnancies were approached at 12 metabolic centers in Germany and Austria were included. According to metabolic control, pregnancies of PKU women were divided in two groups: group “ideal” (not more than 5% of all blood Phe concentrations during pregnancy > 360 µmol/l; n = 23) and group “suboptimal” (all others; n = 51).ResultsThe demand for support was equally distributed among groups, concerning both amount and content. Personal support by the direct social environment (partner, family and friends) (“suboptimal” 71% vs “ideal” 78%) as well as individual metabolic care by the specialized metabolic center (both groups around 60%) were rated as most important factors. The groups differed significantly with respect to the estimation of the quality of their metabolic situation (p < 0.001). Group “ideal” presented a 100% realistic self-assessment. In contrast, group “suboptimal” overestimated their metabolic control in 53% of the pregnancies. Offspring of group “suboptimal” showed clinical signs of maternal PKU-syndrome in 27%.ConclusionThe development of training programs by specialized metabolic centers for females with PKU in child bearing age is crucial, especially since those mothers at risk of giving birth to a child with maternal PKU syndrome are not aware of their suboptimal metabolic control. Such programs should provide specific awareness training for the own metabolic situation and should include partners and families.

Maternal phenylketonuria in Turkey: outcomes of 71 pregnancies and issues in management.

Untreated phenylketonuria (PKU) in pregnancy causes a severe embryopathy called maternal PKU syndrome. Here, we aimed to assess management issues and pregnancy outcomes in the first published series of PKU pregnancies from the developing world. Data were collected retrospectively in a single center from 71 pregnancies and 45 live births of 32 women with PKU, 11 of whom were diagnosed in adulthood after having an affected child. Microcephaly, intellectual disability, and dysmorphic facies were more prevalent in offspring of untreated than treated pregnancies with classical PKU (100% vs. 0%, 91% vs. 0%, and 73% vs. 23% with p < 0.001, p < 0.001, and p = 0.037, respectively). In treated pregnancies, phenylalanine levels were higher during weeks 6-14 than other periods of gestation (4.38 vs. 3.93, 2.00 and 2.28mg/dl; p < 0.05). Poor compliance correlated with higher phenylalanine levels (ρ = - 0.64, p = 0.019) and fluctuations (ρ = - 0.66, p = 0.014).Conclusion: More frequent phenylalanine measurements during late first trimester are crucial to improve outcomes in treated pregnancies. In order to prevent untreated pregnancies via detecting undiagnosed adults, countries where significantly many women of childbearing age were not screened as newborns may consider pre-pregnancy PKU screening. Microcephaly in the newborn should prompt screening for PKU in the mother. What Is Known •Untreated phenylketonuria during pregnancy causes maternal phenylketonuria syndrome in the newborn. •Effective treatment throughout pregnancy can prevent adverse fetal outcomes. What Is New: •Metabolic control is related to frequency of follow-up and worsens during late first trimester. Closer follow-up during this period may improve metabolic control. •In order to prevent untreated pregnancies, pre-pregnancy phenylketonuria screening may be considered if many women of childbearing age were not screened as newborns.

Maternal phenylketonuria syndrome: studies in mice suggest a potential approach to a continuing problem

BackgroundUntreated phenylketonuria (PKU), one of the most common human genetic disorders, usually results in mental retardation. Although a protein-restricted artificial diet can prevent retardation, dietary compliance in adults is often poor. In pregnant PKU women, noncompliance can result in maternal PKU syndrome, where high phenylalanine (Phe) levels cause severe fetal complications. Enzyme substitution therapy using Phe ammonia lyase (PAL) corrects PKU in BTBR Phe hydroxylase (Pahenu2) mutant mice, suggesting a potential for maternal PKU syndrome treatment in humans.MethodsWe reviewed clinical data to assess maternal PKU syndrome incidence in pregnant PKU women. We treated female PKU mice (on normal diet) with PAL, stabilizing Phe at physiological levels, and mated them to assess pregnancy outcomes.ResultsPatient records show that, unfortunately, the efficacy of diet to prevent maternal PKU syndrome has not significantly improved since the problem was first noted 40 years ago. PAL treatment of pregnant PKU mice shows that offspring of PAL-treated dams survive to adulthood, in contrast to the complete lethality seen in untreated mice, or limited survival seen in mice on a PKU diet.ConclusionPAL treatment reduced maternal PKU syndrome severity in mice and may have potential for human PKU therapy.

Paying more attention to the prevention and treatment of maternal phenylketonuria

Since 1981, neonatal screening of hyperphenylalaninemia(HPA)has been carrying on in China.The newborns with phenylketonuria(PKU) have been good growthing and developing.Some of them already take part in the marriage period.The fetal brain developmental disorders and various teratogenesis could be leaded if the pregnant women with PKU have high blood concentration of phenylalaninine, namely, maternal PKU syndrome would happen.Now, the international latest progress, including mechanism, prevention, tetrahydrobiopterin support treatment of maternal PKU and the comprehensive management for adult female patients with PKU were summarized, and would like to provide some references for preventing the occurrence of maternal PKU syndrome. Key words: Maternal phenylketonuria; Hyperphenylalaninemia; Tetrahydrobiopterin deficiency; Sapropterin dihydrochloride

Phenylketonuria

Phenylketonuria remains one of the most common inborn errors in the United Kingdom. It is detected on the newborn heel-prick screening sample allowing early treatment with a strict low phenylalanine diet supplemented with artificial amino acids, and appropriate vitamin and minerals. Although the long-term prognosis is good, there is an increasing body of evidence highlighting subtle problems in neuropsychological function with slower reaction times and poorer executive function than peers. White matter changes clearly seen on brain magnetic resonance imaging may have some relationship to these neuropsychological difficulties but their significance is not clearly understood. The diet, although successful, is difficult to follow lifelong and with its attendant risks of nutritional deficiencies needs careful specialist management. In view of these challenges new treatments such as sapropterin (a tetrahydrobiopterin analogue) and large neutral amino acids are currently being used in phenylketonuria and a human trial has started using ammonia lyase as enzyme replacement therapy. Maternal phenylketonuria syndrome remains a risk for those who conceive whilst blood phenylalanine is elevated and females must be counselled early in childhood to avoid this risk.

Methylome repatterning in a mouse model of Maternal PKU Syndrome

Maternal PKU Syndrome (MPKU) is an embryopathy resulting from in utero phenylalanine (PHE) toxicity secondary to maternal phenylalanine hydroxylase deficient phenylketonuria (PKU). Clinical phenotypes in MPKU include mental retardation, microcephaly, in utero growth restriction, and congenital heart defects. Numerous in utero toxic exposures alter DNA methylation in the fetus. The PAHenu2 mouse is a model of classical PKU while offspring born of hyperphenylalaninemic dams model MPKU. We investigated offspring of PAHenu2 dams to determine if altered patterns of DNA methylation occurred in response to in utero PHE exposure. As neurologic deficit is the most prominent MPKU phenotype, methylome patterns were assessed in brain tissue using methylated DNA immunoprecipitation and paired-end sequencing. Brain tissues were assessed in E18.5–19 fetuses of PHE unrestricted PAHenu2 dams, PHE restricted PAHenu2 dams, and heterozygouswt/enu2 control dams. Extensive methylome repatterning was observed in offspring of hyperphenylalaninemic dams while the offspring of PHE restricted dams displayed attenuated methylome repatterning. Methylation within coding regions was dominated by noncoding RNA genes. Differential methylation of promoters targeted protein coding genes. To assess the impact of methylome repatterning on gene expression, brain tissue in experimental and control animals were queried with microarrays assessing expression of microRNAs and protein coding genes. Altered expression of methylome-modified microRNAs and protein coding genes was extensive in offspring of hyperphenylalaninemic dams while minimal changes were observed in offspring of PHE restricted dams. Several genes displaying significantly reduced expression have roles in neurological function or genetic disease with neurological phenotypes. These data indicate in utero PHE toxicity alters DNA methylation in the brain which has downstream impact upon gene expression. Altered gene expression may contribute to pathophysiology of neurologic presentation in MPKU.

Co-administration of creatine plus pyruvate prevents the effects of phenylalanine administration to female rats during pregnancy and lactation on enzymes activity of energy metabolism in cerebral cortex and hippocampus of the offspring.

Phenylketonuria (PKU) is the most frequent inborn error of metabolism. It is caused by deficiency in the activity of phenylalanine hydroxylase, leading to accumulation of phenylalanine and its metabolites. Untreated maternal PKU or hyperphenylalaninemia may result in nonphenylketonuric offspring with low birth weight and neonatal sequelae, especially microcephaly and intellectual disability. The mechanisms underlying the neuropathology of brain injury in maternal PKU syndrome are poorly understood. In the present study, we evaluated the possible preventive effect of the co-administration of creatine plus pyruvate on the effects elicited by phenylalanine administration to female Wistar rats during pregnancy and lactation on some enzymes involved in the phosphoryltransfer network in the brain cortex and hippocampus of the offspring at 21days of age. Phenylalanine administration provoked diminution of body, brain cortex an hippocampus weight and decrease of adenylate kinase, mitochondrial and cytosolic creatine kinase activities. Co-administration of creatine plus pyruvate was effective in the prevention of those alterations provoked by phenylalanine, suggesting that altered energy metabolism may be important in the pathophysiology of maternal PKU. If these alterations also occur in maternal PKU, it is possible that pyruvate and creatine supplementation to the phenylalanine-restricted diet might be beneficial to phenylketonuric mothers.

Maternal Hyperphenylalaninemia: Rapid achievement of metabolic control predicts overall control throughout pregnancy

Women with hyperphenylalaninemia are at risk of having offspring affected with the maternal phenylketonuria syndrome. Here we analyze the effect of the intervention of a nutritionist on plasma phenylalanine control in Maternal Hyperphenylalaninemia. We analyzed a retrospective cohort of 35 completed pregnancies in 20 women with Maternal Hyperphenylalaninemia who visited the metabolic nutritionist during the pregnancy to achieve metabolic control. Women who promptly achieved metabolic control had lower plasma phenylalanine concentrations for the remainder of the pregnancy when compared to women who did not achieve prompt control, and this difference reached statistical significance. The achievement of plasma phenylalanine concentrations within the desired target range by the time of the second visit to the nutritionist is a strong predictor of the ability to maintain the desired target range of plasma phenylalanine for the remainder of the pregnancy. Furthermore, we demonstrate that phenylalanine tolerance increases significantly by trimester in women with classical and variant hyperphenylalaninemia.

Undiagnosed phenylketonuria in parents of phenylketonuric patients, is it worthwhile to be checked?

In our phenylketonuria (PKU) cohort of 120 patients, we uncovered a couple of cases of undiagnosed mild phenylketonuria (mPKU)/hyperphenylalaninemia (mHPA) in maternal parents of the PKU cohort. This finding prompted us to evaluate the risk of either mild phenylketonuria or mild hyperphenylalaninemia in the parent population whose children were diagnosed with hyperphenylalaninemia (HPA). Taking into account the phenylalanine hydroxylase (PAH) mutation carrier frequency and the PAH mild mutation rate, we estimated that the prevalence of the parental mPKU/mHPA varied widely, from 1/74 in Turkey to 1/708 in Lithuania. The benefits of the parental detection procedure described here are the prevention of further maternal PKU syndrome, the follow-up of the newly detected patients and the accuracy of the genetic counseling provided to these families. This very simple procedure should be incorporated into neonatal PKU management of the hospitals in countries where a routine systematic neonatal screening is operational.

Undiagnosed maternal phenylketonuria: own clinical experience and literature review

Phenylketonuria (PKU) is an autosomal recessive inborn error of phenylalanine (Phe) metabolism resulting from deficiency of phenylalanine hydroxylase (PAH). Most forms of PKU are caused by mutations in the PAH gene. Untreated PKU is associated with an abnormal phenotype, which includes growth failure, seizures, global developmental delay and severe intellectual impairment. The maternal PKU (MPKU) syndrome is caused by high blood Phe concentrations during pregnancy and presents with serious foetal anomalies, especially microcephaly, congenital heart disease and mental retardation. However, since the introduction of newborn screening programs and with early dietary intervention, children born with PKU can now expect to lead relatively normal lives. We present the case of a 33-year-old woman who had been diagnosed as having PKU only after a pregnancy with MPKU embryopathy, to emphasize that undiagnosed maternal phenylketonuria still exists. On that ground, we reviewed updated literature on the pathogenesis of this syndrome, possibility of prophylaxis and treatment.

Progress toward cell‐directed therapy for phenylketonuria

Phenylketonuria (PKU) is one of the most common inborn errors of metabolism with an annual incidence of approximately 1:16,000 live births in North America. Contemporary therapy relies upon lifelong dietary protein restriction and supplementation with phenylalanine-free medical foods. This therapy is expensive and unpalatable; dietary compliance is difficult to maintain throughout life. Non-adherence to the diet is associated with learning disabilities, adult-onset neurodegenerative disease, and maternal PKU syndrome. The fervent dream of many individuals with PKU is a more permanent cure for this disease. This paper will review ongoing efforts to develop viable cell-directed therapies, in particular cell transplantation and gene therapy, for the treatment of PKU.

Protective Effect of Recombinant Adeno-Associated Virus 2/8-Mediated Gene Therapy from the Maternal Hyperphenylalaninemia in Offsprings of a Mouse Model of Phenylketonuria

Phenylketonuria (PKU) is an autosomal recessively inherited metabolic disorder caused by a deficiency of phenylalanine hydroxylase (PAH). The accumulation of phenylalanine leads to severe mental and psychomotor retardation, and the fetus of an uncontrolled pregnant female patient presents with maternal PKU syndrome. We have reported previously on the cognitive outcome of biochemical and phenotypic reversal of PKU in a mouse model, Pahenu2, by the AAV serotype 2-mediated gene delivery of a human PAH transgene. However, the therapeutic efficacy had been limited to only male PKU mice. In this study, we generated a pseudotyped recombinant AAV2/8-hPAH vector and infused it into female PKU mice through the hepatic portal vein or tail vein. Two weeks after injection, complete fur color change to black was observed in female PKU, as in males. The PAH activity in the liver increased to 65-70% of the wild-type activity in female PKU mice and to 90% in male PKU mice. Plasma phenylalanine concentration in female PKU mice decreased to the normal value. In addition, the offsprings of the treated female PKU mice can rescue from the harmful effect of maternal hyperphenylalaninemia. These results indicate that recombinant AAV2/8-mediated gene therapy is a potential therapeutic strategy for PKU.

A case of two sisters births from mother with phenylketonuria lacking mental retardation

= Abstract = In this untreated classic phenylketonuria (PKU case mental retardation is severe; however there have been individuals- like the mother of this case- who have escaped mental retardation and all the other potential sequelae of phenylketonuria despite having high blood phenylalanine levels and very poor dietary control. It appears that they have nearly normal brain phenylalanine levels despite high blood phenylalanine (Phe levels. A number of studies have now demonstrated considerable variability in blood vs. brain phenylalanine levels in phenylketonuria patients. Outcome of phenylketonuria appears to be related to brain phenylalanine levels. We report a case of undiagnosed maternal phenylketonuria syndrome. A female infant had low birth weight (2400 g with microcephaly. We examined her family an dd iscovered that her mother was an undiag- nosed phenylketonuria patient with a borderline intelligence quotient ( IQ. The infant's sistersix years oldwas diagnosed with phenylketonuria at the age of four years was mentally retarded and had received an operation for cleft lip and palate. the sister had also had a low birth weight (2300 g. Her sister and mother were compound heterozygotes (mother: R243Q/Y325X; sister: Y325X/P407S. The infant and father were heterozygous carriers (baby: R243Q/-; father: P407S/-. (Korean J Pediatr 2008;51:546-550

Maternal Phenylketonuria Syndrome and Case Management Implications

Well-established dietary protocols have prevented mental retardation for infants born with phenylketonuria (PKU). Dietary protocols for managing females with PKU in their reproductive years exist but are not followed by most of them. Infants who are born to mothers with PKU who are not on dietary treatment usually have serious medical problems, such as mental retardation, heart defects, and other serious congenital anomalies (e.g., orofacial clefting and bladder exstrophy)--a condition known as maternal PKU syndrome. The focus of this article is to review the pathophysiology, associated developmental issues, and existing management protocols used to manage these two separate but highly connected disorders.

Gene therapy for phenylketonuria mouse model using pseudotyped adeno‐associated virus vector

Phenylketonuria (PKU) is an inherited metabolic disorder caused by a deficiency of phenylalanine hydroxylase. The accumulation of phenylalanine leads to severe mental and psychomotor retardation. Phenylalanine restriction diet can prevent irreversible damage if instituted from birth. Recently, we reported the cognitive outcome of biochemical and phenotypic reversal of PKU mouse model, Pahenu2, by the AAV 2-mediated gene delivery of a human PAH transgene (Pediatr Res 56:–284, 2004). However, the therapeutic effectiveness had been observed only in male PKU mice. No normalization in coat color or plasma phenylalanine level was observed in gene therapy on female PKU mice. In this study, we generated pseudotyped AAV2/8 vector encoding the human PAH gene. The rAAV2/8-hPAH vector was infused into female PKU mice via the hepatic portal vein. Six weeks after injection, the coat color change to black was clearly observed in female PKU as in male. Plasma phenylalanine level in female PKU mice decreased to 120_μM/L. The PAH enzyme activities of treated mouse liver increased to 65–70% of wild-type in female PKU mice, to 90–95% in male PKU mice. These corrections were sustained over 6 month. Furthermore, we observed rAAV2/8-hPAH treated female PKU mice can overcome maternal PKU syndrome. These results indicate that recombinant AAV 2/8 mediated gene therapy can be a useful therapeutic candidate for patients with PKU.

214. Dominant-Negative Interference in the Pahenu2 Mouse: Modified Forms of PAH

The common human genetic disorder Phenylketonuria (PKU) is primarily caused by defects in the enzyme phenylalanine hydroxylase (PAH). An animal model for PKU, the BTBR Pahenu2 mouse, has a missense mutation (F263S) that inactivates PAH; the mouse exhibits classic PKU, with elevated blood Phe levels, cognitive deficiencies, and maternal PKU syndrome. We have corrected both the serum Phe levels and maternal PKU in Pahenu2 mice using recombinant AAV vectors containing the mouse PAH gene. Although successful, unusually high vector doses were needed to achieve normal serum Phe levels. More critically, there was a gradual loss of therapeutic effect 20-24 weeks after vector delivery in many animals, particularly females. This was accompanied by changes in liver morphology suggestive of increased metabolic activity.Pahenu2 mice express reduced levels of a mutant PAH protein. The normal enzyme is a tetramer and we have reported evidence fordominant-negative interactions between endogenous mutant subunits and vector-introduced functional subunits. Possible approaches to combating a dominant-negative interaction include the production of ribozymes targeting the endogenous mutant mRNA and expression of modified forms of PAH protein that do not form tetramers. We have shown that a serotype 2 AAV vector expressing both a ribozyme directed against the mouse PAH mRNA and a ribozyme-resistant form of mPAH (mPAH-Hd) were effective at reducing serum Phe levels. We now show that modified forms of the PAH enzyme can, in certain cases, reduce dominant-negative interference. A vector expressing an obligate dimer form of mPAH was still subject to negative interference, indicating this inhibition occurs at the level of the monomer-monomer interaction. We will present in vitro and in vivo data examining the effectiveness of both a monomeric PAH enzyme as well as chimeric enzymes expressing a mouse PAH catalytic core linked to the rat tyrosine hydroxylase tetramerization domain in reducing dominant-negative interactions with missense subunits. The common human genetic disorder Phenylketonuria (PKU) is primarily caused by defects in the enzyme phenylalanine hydroxylase (PAH). An animal model for PKU, the BTBR Pahenu2 mouse, has a missense mutation (F263S) that inactivates PAH; the mouse exhibits classic PKU, with elevated blood Phe levels, cognitive deficiencies, and maternal PKU syndrome. We have corrected both the serum Phe levels and maternal PKU in Pahenu2 mice using recombinant AAV vectors containing the mouse PAH gene. Although successful, unusually high vector doses were needed to achieve normal serum Phe levels. More critically, there was a gradual loss of therapeutic effect 20-24 weeks after vector delivery in many animals, particularly females. This was accompanied by changes in liver morphology suggestive of increased metabolic activity. Pahenu2 mice express reduced levels of a mutant PAH protein. The normal enzyme is a tetramer and we have reported evidence fordominant-negative interactions between endogenous mutant subunits and vector-introduced functional subunits. Possible approaches to combating a dominant-negative interaction include the production of ribozymes targeting the endogenous mutant mRNA and expression of modified forms of PAH protein that do not form tetramers. We have shown that a serotype 2 AAV vector expressing both a ribozyme directed against the mouse PAH mRNA and a ribozyme-resistant form of mPAH (mPAH-Hd) were effective at reducing serum Phe levels. We now show that modified forms of the PAH enzyme can, in certain cases, reduce dominant-negative interference. A vector expressing an obligate dimer form of mPAH was still subject to negative interference, indicating this inhibition occurs at the level of the monomer-monomer interaction. We will present in vitro and in vivo data examining the effectiveness of both a monomeric PAH enzyme as well as chimeric enzymes expressing a mouse PAH catalytic core linked to the rat tyrosine hydroxylase tetramerization domain in reducing dominant-negative interactions with missense subunits.