Reversal of hypopigmentation in phenylketonuria mice by adenovirus-mediated gene transfer

Current Advances and Material Innovations in the Search for Novel Treatments of Phenylketonuria

Phenylketonuria (PKU) is a genetically inherited disease caused by a mutation of the gene encoding phenylalanine hydroxylase (PAH) and is the most common inborn error of amino acid metabolism. A deficiency of PAH leads to increased blood and brain levels of phenylalanine (Phe), which may cause permanent neurocognitive symptoms and developmental delays if untreated. Current management strategies for PKU consist of early detection through neonatal screening and implementation of a restrictive diet with minimal amounts of natural protein in combination with Phe-free supplements and low-protein foods to meet nutritional requirements. For milder forms of PKU, oral treatment with synthetic sapropterin (BH4), the cofactor of PAH, may improve metabolic control of Phe and allow for more natural protein to be included in the patient's diet. For more severe forms, daily injections of pegvaliase, a PEGylated variant of phenylalanine ammonia-lyase (PAL), may allow for normalization of blood Phe levels. However, the latter treatment has considerable drawbacks, notably a strong immunogenicity of the exogenous enzyme and the attached polymeric chains. Research for novel therapies of PKU makes use of innovative materials for drug delivery and state-of-the-art protein engineering techniques to develop treatments which are safer, more effective, and potentially permanent.

State-of-the-art 2023 on gene therapy for phenylketonuria

Phenylketonuria (PKU) or hyperphenylalaninemia is considered a paradigm for an inherited (metabolic) liver defect and is, based on murine models that replicate all human pathology, an exemplar model for experimental studies on liver gene therapy. Variants in the PAH gene that lead to hyperphenylalaninemia are never fatal (although devastating if untreated), newborn screening has been available for two generations, and dietary treatment has been considered for a long time as therapeutic and satisfactory. However, significant shortcomings of contemporary dietary treatment of PKU remain. A long list of various gene therapeutic experimental approaches using the classical model for human PKU, the homozygous enu2/2 mouse, witnesses the value of this model to develop treatment for a genetic liver defect. The list of experiments for proof of principle includes recombinant viral (AdV, AAV, and LV) and non-viral (naked DNA or LNP-mRNA) vector delivery methods, combined with gene addition, genome, gene or base editing, and gene insertion or replacement. In addition, a list of current and planned clinical trials for PKU gene therapy is included. This review summarizes, compares, and evaluates the various approaches for the sake of scientific understanding and efficacy testing that may eventually pave the way for safe and efficient human application.

A porcine model of phenylketonuria generated by CRISPR/Cas9 genome editing

Phenylalanine hydroxylase-deficient (PAH-deficient) phenylketonuria (PKU) results in systemic hyperphenylalaninemia, leading to neurotoxicity with severe developmental disabilities. Dietary phenylalanine (Phe) restriction prevents the most deleterious effects of hyperphenylalaninemia, but adherence to diet is poor in adult and adolescent patients, resulting in characteristic neurobehavioral phenotypes. Thus, an urgent need exists for new treatments. Additionally, rodent models of PKU do not adequately reflect neurocognitive phenotypes, and thus there is a need for improved animal models. To this end, we have developed PAH-null pigs. After selection of optimal CRISPR/Cas9 genome-editing reagents by using an in vitro cell model, zygote injection of 2 sgRNAs and Cas9 mRNA demonstrated deletions in preimplantation embryos, with embryo transfer to a surrogate leading to 2 founder animals. One pig was heterozygous for a PAH exon 6 deletion allele, while the other was compound heterozygous for deletions of exon 6 and of exons 6-7. The affected pig exhibited hyperphenylalaninemia (2000-5000 μM) that was treatable by dietary Phe restriction, consistent with classical PKU, along with juvenile growth retardation, hypopigmentation, ventriculomegaly, and decreased brain gray matter volume. In conclusion, we have established a large-animal preclinical model of PKU to investigate pathophysiology and to assess new therapeutic interventions.

Sustained Correction of a Murine Model of Phenylketonuria following a Single Intravenous Administration of AAVHSC15-PAH

Phenylketonuria is an inborn error of metabolism caused by loss of function of the liver-expressed enzyme phenylalanine hydroxylase and is characterized by elevated systemic phenylalanine levels that are neurotoxic. Current therapies do not address the underlying genetic disease or restore the natural metabolic pathway resulting in the conversion of phenylalanine to tyrosine. A family of hepatotropic clade F adeno-associated viruses (AAVs) was isolated from human CD34⁺ hematopoietic stem cells (HSCs) and one (AAVHSC15) was utilized to deliver a vector to correct the phenylketonuria phenotype in Pah^enu2 mice. The AAVHSC15 vector containing a codon-optimized form of the human phenylalanine hydroxylase cDNA was administered as a single intravenous dose to Pah^enu2 mice maintained on a phenylalanine-containing normal chow diet. Optimization of the transgene resulted in a vector that produced a sustained reduction in serum phenylalanine and normalized tyrosine levels for the lifespan of Pah^enu2 mice. Brain levels of phenylalanine and the downstream serotonin metabolite 5-hydroxyindoleacetic acid were restored. In addition, the coat color of treated mice darkened following treatment, indicating restoration of the phenylalanine metabolic pathway. Taken together, these data support the potential of an AAVHSC15-based gene therapy as an investigational therapeutic for phenylketonuria patients.

Prospects for Cell-Directed Curative Therapy of Phenylketonuria (PKU)

Phenylketonuria (PKU) due to recessively inherited phenylalanine hydroxylase (PAH) deficiency is among the most common inborn errors of metabolism. Dietary therapy begun early in infancy prevents the major manifestations of the disease but shortcomings to treatment continue to exist including lifelong commitment to a complicated and unpalatable diet, poor adherence to diet in adolescence and adulthood, and consequently a range of unsatisfactory outcomes, including neuropsychiatric disorders, frequently develop. Novel treatments that do not strictly depend upon dietary protein restriction are actively sought. This review discusses the potential for and the limitations of permanently curative cell-directed treatment of PKU, including liver-directed gene therapy and gene editing, if initiated during early infancy. A fictional but realistic vignette of a family with a new baby girl recently diagnosed with PKU is presented. What is needed to permanently cure her?

State-of-the-Art 2019 on Gene Therapy for Phenylketonuria

Phenylketonuria (PKU) is considered to be a paradigm for a monogenic metabolic disorder but was never thought to be a primary application for human gene therapy due to established alternative treatment. However, somewhat unanticipated improvement in neuropsychiatric outcome upon long-term treatment of adults with PKU with enzyme substitution therapy might slowly change this assumption. In parallel, PKU was for a long time considered to be an excellent test system for experimental gene therapy of a Mendelian autosomal recessive defect of the liver due to an outstanding mouse model and the easy to analyze and well-defined therapeutic end point, that is, blood l-phenylalanine concentration. Lifelong treatment by targeting the mouse liver (or skeletal muscle) was achieved using different approaches, including (1) recombinant adeno-associated viral (rAAV) or nonviral naked DNA vector-based gene addition, (2) genome editing using base editors delivered by rAAV vectors, and (3) by delivering rAAVs for promoter-less insertion of the PAH-cDNA into the Pah locus. In this article we summarize the gene therapeutic attempts of correcting a mouse model for PKU and discuss the future implications for human gene therapy.

Comparative genomics provides new insights into the remarkable adaptations of the African wild dog (Lycaon pictus)

Within the Canidae, the African wild dog (Lycaon pictus) is the most specialized with regards to cursorial adaptations (specialized for running), having only four digits on their forefeet. In addition, this species is one of the few canids considered to be an obligate meat-eater, possessing a robust dentition for taking down large prey, and displays one of the most variable coat colorations amongst mammals. Here, we used comparative genomic analysis to investigate the evolutionary history and genetic basis for adaptations associated with cursoriality, hypercanivory, and coat color variation in African wild dogs. Genome-wide scans revealed unique amino acid deletions that suggest a mode of evolutionary digit loss through expanded apoptosis in the developing first digit. African wild dog-specific signals of positive selection also uncovered a putative mechanism of molar cusp modification through changes in genes associated with the sonic hedgehog (SHH) signaling pathway, required for spatial patterning of teeth, and three genes associated with pigmentation. Divergence time analyses suggest the suite of genomic changes we identified evolved ~1.7 Mya, coinciding with the diversification of large-bodied ungulates. Our results show that comparative genomics is a powerful tool for identifying the genetic basis of evolutionary changes in Canidae.

Gene Therapy for the Treatment of Neurological Disorders: Metabolic Disorders

Metabolic disorders comprise a large group of heterogeneous diseases ranging from very prevalent diseases such as diabetes mellitus to rare genetic disorders like Canavan Disease. Whether either of these diseases is amendable by gene therapy depends to a large degree on the knowledge of their pathomechanism, availability of the therapeutic gene, vector selection, and availability of suitable animal models. In this book chapter, we review three metabolic disorders of the central nervous system (CNS; Canavan Disease, Niemann-Pick disease and Phenylketonuria) to give examples for primary and secondary metabolic disorders of the brain and the attempts that have been made to use adeno-associated virus (AAV) based gene therapy for treatment. Finally, we highlight commonalities and obstacles in the development of gene therapy for metabolic disorders of the CNS exemplified by those three diseases.

Cellular unfolded protein response against viruses used in gene therapy

Viruses are excellent vehicles for gene therapy due to their natural ability to infect and deliver the cargo to specific tissues with high efficiency. Although such vectors are usually "gutted" and are replication defective, they are subjected to clearance by the host cells by immune recognition and destruction. Unfolded protein response (UPR) is a naturally evolved cyto-protective signaling pathway which is triggered due to endoplasmic reticulum (ER) stress caused by accumulation of unfolded/misfolded proteins in its lumen. The UPR signaling consists of three signaling pathways, namely PKR-like ER kinase, activating transcription factor 6, and inositol-requiring protein-1. Once activated, UPR triggers the production of ER molecular chaperones and stress response proteins to help reduce the protein load within the ER. This occurs by degradation of the misfolded proteins and ensues in the arrest of protein translation machinery. If the burden of protein load in ER is beyond its processing capacity, UPR can activate pro-apoptotic pathways or autophagy leading to cell death. Viruses are naturally evolved in hijacking the host cellular translation machinery to generate a large amount of proteins. This phenomenon disrupts ER homeostasis and leads to ER stress. Alternatively, in the case of gutted vectors used in gene therapy, the excess load of recombinant vectors administered and encountered by the cell can trigger UPR. Thus, in the context of gene therapy, UPR becomes a major roadblock that can potentially trigger inflammatory responses against the vectors and reduce the efficiency of gene transfer.

Tacrolimus: 10 years of successful clinical application

Tacrolimus (FK506) is the first drug from the new class of drugs - calcineurin inhibitors. It has a great potential and can change the current approaches to treatment of different dermatoses. Topical forms of Tacrolimus are activated due to the effect on T-lymphocyte signal transduction, and they also inhibit gene transcription. As a result, the T-cell response to antigens is alleviated. Percutaneous absorption of Tacrolimus in affection foci is higher than in healthy skin. Due to this a smaller amount of the drug is absorbed by the skin as long as the skin is healed. Systemic absorption of Tacrolimus is low in the course of treatment. The maximum clinical experience has been gained in the field of applying Tacrolimus for treatment of atopic dermatitis. In a number of clinical studies, 0.03-0.3% Tacrolimus ointment efficiently reduced the severity of symptoms and course of atopic dermatitis in adults and children. Moreover, topical treatment with Tacrolimus did not involve any substantial toxic effects. The patients most often complained of local irritations in the area where the ointment was applied, which were transient by nature and did not demand the drug withdrawal. Today Tacrolimus is a valuable and safer alternative to glucocorticosteroids for treatment of atopic dermatitis. It can also be used for treating other dermatoses, which demands additional all-round examination.

Long-term correction of murine phenylketonuria by viral gene transfer: liver versus muscle

Current therapy for phenylketonuria (PKU) consists of life-long dietary restriction of phenylalanine (Phe), which presents problems of adherence for patients. Alternative therapies under investigation include, among others, the use of gene therapy to provide copies of wild-type, non-mutant, phenylalanine hydroxylase (PAH) enzyme. Expression of PAH in both liver (the usual metabolic source of this enzyme) and skeletal muscle is under investigation. Liver gene therapy, using a viral vector based on the adeno-associated viruses (AAVs), provided effective clearance of serum Phe that was sustained for 1 year in some mice. In order for PAH expression to be effective in skeletal muscle, the essential metabolic cofactor, tetrahydrobiopterin (BH(4)), must also be provided, either by supplementation or gene therapy. Both these approaches were effective. When transgenic PKU mice that constitutively expressed PAH in muscle were given intraperitoneal supplementation with BH(4), this produced (transient) effective clearance of Phe to normal levels. In addition, use of an AAV vector containing the genes for PAH, and for two key synthetic enzymes for BH(4), provided substantial and long-lasting correction (more than 1 year) of blood Phe levels when injected into skeletal muscle of PKU mice. These two strategies provide promising treatment alternatives for the management of PKU in patients.

Cell encapsulation as a potential nondietary therapy for maternal phenylketonuria

OBJECTIVE

The objective of this work was to determine whether cells overexpressing phenylalanine (Phe) hydroxylase (PAH) can significantly reduce Phe in vitro for potential use as a therapy for preventing maternal phenylketonuria.

STUDY DESIGN

Human 293T and WRL68 cell lines were transiently and stably transfected to overexpress PAH. Cells were encapsulated within microspheres of sodium alginate. Timed measurements of Phe in media were performed using tandem mass spectrometry.

RESULTS

Both nonencapsulated and encapsulated transiently transfected cells overexpressing PAH significantly reduced the Phe concentration in media by approximately 50% in comparison to mock-transfected cells. Cell line clones stably expressing PAH significantly decreased the Phe concentration in the media by up to 85% compared with media alone.

CONCLUSION

Both unencapsulated and encapsulated cells overexpressing PAH significantly reduce Phe in vitro. Studies using phenylketonuria model mice will be important in determining the ability of our therapy to prevent the teratogenic effects of elevated maternal Phe in maternal phenylketonuria.

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.

Administration-route and gender-independent long-term therapeutic correction of phenylketonuria (PKU) in a mouse model by recombinant adeno-associated virus 8 pseudotyped vector-mediated gene transfer

Phenylketonuria (PKU) is an inborn error of metabolism caused by deficiency of the hepatic enzyme phenylalanine hydroxylase (PAH) which leads to high blood phenylalanine (Phe) levels and consequent damage of the developing brain with severe mental retardation if left untreated in early infancy. The current dietary Phe restriction treatment has certain clinical limitations. To explore a long-term nondietary restriction treatment, a somatic gene transfer approach in a PKU mouse model (C57Bl/6-Pahenu2) was employed to examine its preclinical feasibility. A recombinant adeno-associated virus (rAAV) vector containing the murine Pah-cDNA was generated, pseudotyped with capsids from AAV serotype 8, and delivered into the liver of PKU mice via single intraportal or tail vein injections. The blood Phe concentrations decreased to normal levels (< or =100 microM or 1.7 mg/dl) 2 weeks after vector application, independent of the sex of the PKU animals and the route of application. In particular, the therapeutic long-term correction in females was also dramatic, which had previously been shown to be difficult to achieve. Therapeutic ranges of Phe were accompanied by the phenotypic reversion from brown to black hair. In treated mice, PAH enzyme activity in whole liver extracts reversed to normal and neither hepatic toxicity nor immunogenicity was observed. In contrast, a lentiviral vector expressing the murine Pah-cDNA, delivered via intraportal vein injection into PKU mice, did not result in therapeutic levels of blood Phe. This study demonstrates the complete correction of hyperphenylalaninemia in both males and females with a rAAV serotype 8 vector. More importantly, the feasibility of a single intravenous injection may pave the way to develop a clinical gene therapy procedure for PKU patients.

Iron restriction negatively affects bone in female rats and mineralization of hFOB osteoblast cells

We previously reported that severe iron deficiency negatively affects bone microarchitecture. Here we determined whether marginal iron restriction that reflects some human consumption patterns could have similar consequences. Thirty-two weanling female rats were randomly divided into four groups and fed the following diets for 10 weeks: (i) iron-adequate, calcium-adequate (FeA:CaA), (ii) calcium-restricted (FeA:CaR), (iii) iron-restricted (FeR:CaA), and (iv) both calcium- and iron-restricted (FeR:CaR) diets. DEXA analysis revealed that CaR decreased bone mineral density (BMD), and FeR decreased whole-body bone mineral content (BMC). Iron-restricted and calcium-restricted groups had lower BMD than did their adequate counterparts. All treatment-restricted groups had lower BMD in the fourth lumbar (L-4) vertebrae than the FeA:CaA group. Vertebrae BMD was lower in all treatment groups compared to the control group, and for BMC, the CaR groups were lower than the CaA groups and the FeR groups were lower that the FeA groups, and BMC were lower in iron- and calcium-restricted groups. The microarchitecture of the L-4 vertebrae was compromised in FeA:CaR, FeR:CaA, and FeR:CaR: (i) the connectivity density was reduced by FeR and by CaR; and (ii) trabecular number was decreased and trabecular separation was increased by FeR. Cortical thickness of the femur was reduced by both FeR and CaR. Finite element analysis revealed that L-4 vertebrae from the FeR:CaA group had greater internal stress with an applied force than the FeA:CaA group and, thus, would be more likely to break. Chelation of iron in cultured osteoblast cells impaired mineralization but had no impact upon Type I collagen deposition. Iron depletion, similar to that occurring among some human populations, reduced bone strength and microarchitecture based on the in vivo and in vitro results reported here. Impaired mineralization with iron depletion appears to be a possible mechanism for the observed bone abnormalities.

Reversal of gene expression profile in the phenylketonuria mouse model after adeno-associated virus vector-mediated gene therapy

Phenylketonuria (PKU) is an autosomal recessive metabolic disorder caused by phenylalanine hydroxylase (PAH) deficiency. Accumulation of phenylalanine leads to severe mental and psychomotor retardation, and hypopigmentation of skin and hair. We have demonstrated the cognitive outcome of biochemical and phenotypic reversal by the adeno-associated virus vector-mediated gene delivery of a human PAH transgene. In this study, we identified the expression of genes related to pathologic abnormalities of the PKU-affected brain, in which the symptoms of PKU are mainly manifest, and transcriptional changes in effective gene therapy treatment using oligonucleotide array. Therapeutic effectiveness was verified by change in enzyme activity (15+/-5.84%), phenylalanine plasma level (261+/-108 microM), and coat color. Our data indicated that 12 genes were significantly up-regulated in PKU. Four are involved in defense and inflammatory responses of neutrophils (NE, MPO, NGP, and CRAMP), three other overexpressed genes are related to extracellular matrix organization and degradation (COL1A1, COL1A2, and MMP13); the remainder were a nociceptor in sensory neurons (MrgA1), a structural gene of P lysozyme (Lzp-s), an immunoglobulin alpha heavy chain constant region gene (Igh-2), an osteocalcin-related protein precursor (Bglap-rs1), and a membrane-spanning 4 domain, subfamily A, member 3 (Ms4a3). Data demonstrated that elevated genes in the PKU-affected brain could be normalized by human PAH gene delivery. Although we could not precisely link transcript level changes and neurologic pathogenesis, this study provides a more comprehensive understanding of the PKU-affected brain at the molecular level, possibly resulting in better therapeutic approaches.

Characterization of transgenic mice with the expression of phenylalanine hydroxylase and GTP cyclohydrolase I in the skin

Phenylketonuria (PKU) is a metabolic disease causing increased levels of phenylalanine in blood and body fluids. Circulating phenylalanine is normally cleared by phenylalanine hydroxylase (PAH) expressed in the liver. The aim of this study is to exploit the skin as a 'metabolic sink' removing phenylalanine from the blood. We have previously showed that the overexpression of PAH and GTP cyclohydrolase I (GTP-CH), the rate-limiting enzyme in the synthesis of the cofactor for PAH, leads to high levels of phenylalanine clearance in primary human keratinocytes. In this study, we have investigated the 'metabolic sink' strategy in an in vivo model by developing three lines of transgenic mice expressing PAH and GTP-CH in various layers of the skin. The promoters used were keratin 14 (K14), involucrin (INV) and a truncated variant of Keratin 1 (K1). The mice were crossbred to a mouse model of human PKU, the PAH(enu2) mouse, in order to obtain mice that do not express PAH in the liver and the kidney. Transgenic mice containing the INV and K14 promoters expressed PAH and GTP-CH in the epidermis. However, the K1 promoter did not lead to detectable gene expression. Analysis of the mice showed that no phenotypic effect was observed in mice expressing PAH and GTP-CH from the INV promoter. However, low level of phenylalanine clearance was observed in mice expressing PAH and GTP-CH from the K14 promoter, suggesting that the skin can be genetically engineered to function as a 'metabolic sink'.

Wild-type phenylalanine hydroxylase activity is enhanced by tetrahydrobiopterin supplementation in vivo: an implication for therapeutic basis of tetrahydrobiopterin-responsive phenylalanine hydroxylase deficiency

We previously proposed a novel disease entity, tetrahydrobiopterin (BH4)-responsive phenylalanine hydroxylase (PAH) deficiency, in which administration of BH4 reduced elevated levels of serum phenylalanine [J. Pediatr. 135 (1999) 375-378]. Subsequent reports indicate that the prevalence of BH4-responsive PAH deficiency is much higher than initially anticipated. Although growing attention surrounds treatment with BH4, little is known about the mechanism of BH4 responsiveness. An early report indicates that BH4 concentration in rat liver was 5 microM where Km for BH4 of rat PAH was estimated to be 25 microM in an oxidation experiment using a liver slice, suggesting relative insufficiency of BH4 in liver in vivo. In the present study, we developed a breath test for mice using [1-13C]phenylalanine in order to examine the BH4 responsiveness of normal PAH in vivo. The reliability of the test was verified using BTBR mice and its mutant strain lacking PAH activity, Pahenu2. BH4 supplementation significantly enhanced 13CO2 production in C57BL/6 mice when phenylalanine was pre-loaded. Furthermore, BH4 apparently activated PAH in just 5 min. These observations suggest that submaximal PAH activity occurs at the physiological concentrations of BH4 in vivo, and that PAH activity can be rapidly enhanced by supplementation with BH4. Thus, we propose a possible hypothesis that the responsiveness to BH4 in patients with PAH deficiency is due to the fact that suboptimal physiological concentrations of BH4 are normally present in hepatocytes and the enhancement of the residual activity may be associated with a wide range of mutations.

Long-term enzymatic and phenotypic correction in the phenylketonuria mouse model by adeno-associated virus vector-mediated gene transfer

Phenylketonuria (PKU) is an autosomal recessive metabolic disorder caused by a deficiency of phenylalanine hydroxylase (PAH). The accumulation of phenylalanine leads to severe mental and psychomotor retardation, and hypopigmentation of skin and hair. Low-phenylalanine diet therapy can prevent irreversible damage if instituted from birth. However, poor compliance with the strict lifelong dietary therapy leads to various neurologic and behavioral problems. To develop a safe and promising gene therapy method for PKU, we investigated whether a recombinant adeno-associated virus could be used as a PAH gene transfer vector to reduce the excessive phenylalanine level in the PKU mouse model. A recombinant adeno-associated virus vector encoding the human PAH gene (rAAV-hPAH), driven by EF1-alpha promoter, was infused into PAH-deficient mice, Pah(enu2), via the hepatic portal vein. Two weeks after injection, the plasma phenylalanine level dramatically decreased to 360 microM in male PKU mice, accompanied by the coat color changing to black. The mean plasma phenylalanine level of untreated PKU mice was 1800 microM. The PAH enzyme activities of treated mice increased to 10-17% of wild-type mice. No signs of liver toxicity were observed after gene transfer. The biochemical and phenotypic corrections were sustained for up to 25 wk (25-wk detection period). In contrast, the treatment was less effective in female PKU mice. These results indicate that recombinant adeno-associated virus vector-mediated gene therapy can be a useful therapeutic candidate for patients with PKU. Further studies are needed to clarify the differences in PKU pathogenesis in males and females, and to explore alternative administration routes besides hepatic portal vein injection.

Long-term correction of hyperphenylalaninemia by AAV-mediated gene transfer leads to behavioral recovery in phenylketonuria mice

Classical phenylketonuria (PKU) is a metabolic disorder caused by a deficiency of the hepatic enzyme phenylalanine hydroxylase (PAH). If untreated, accumulation of phenylalanine will damage the developing brain of affected individuals, leading to severe mental retardation. Here, we show that a liver-directed PAH gene transfer brought about long-term correction of hyperphenylalaninemia and behavioral improvement in a mouse model of PKU. A recombinant adeno-associated virus (AAV) vector carrying the murine PAH cDNA was constructed and administered to PAH-deficient mice (strain PAH(enu2)) via the portal vein. Within 2 weeks of treatment, the hyperphenylalaninemic phenotype improved and completely normalized in the animals treated with higher vector doses. The therapeutic effect persisted for 40 weeks in male mice, while serum phenylalanine concentrations in female animals gradually returned to pretreatment levels. Notably, this long-term correction of hyperphenylalaninemia was associated with a reversal of hypoactivity observed in PAH(enu2) mice. While locomotory activity over 24 h and exploratory behavior were significantly decreased in untreated PAH(enu2) mice compared with the age-matched controls, these indices were completely normalized in 12-month-old male PKU mice with lowered serum phenylalanine. These results demonstrate that AAV-mediated liver transduction ameliorated the PKU phenotype, including central nervous system dysfunctions.

State-of-the-art 2003 on PKU gene therapy

Phenylketonuria (or PKU) is a well-known and widespread genetic disease for which many countries perform newborn screening, and life-long dietary restriction is still the ultimate and effective therapy. However, the diet is complicated, unpalatable, and expensive. The long-term effects of diet discontinuation in adults, except for the serious adverse effects of maternal hyperphenylalaninemia upon the developing fetus, have not been systematically studied, but cognitive decline and neurologic abnormalities have been anecdotally reported. Thus, alternative approaches for PKU therapy, including gene therapy, must be further explored. Here we summarize past present nonviral and viral gene transfer approaches, both in vitro studies and preclinical animal trials, to delivering the PAH gene into liver or other organs as potential alternatives to life-long phenylalanine-restricted dietary therapy.

Adenovirus and adeno-associated virus vectors

Recombinant adenovirus (rAd) and recombinant adeno-associated virus (rAAV) are among the most extensively used vectors in gene therapy studies to date. These two vectors share some similar features such as a broad host range and ability to infect both proliferating and quiescent cells. However, they also possess their own unique set of properties that render them particularly attractive for gene therapy applications. rAd vectors can accommodate larger inserts, mediate transient but high levels of protein expression, and can be easily produced at high titers. Development of gutted rAd vectors has further increased the cloning capacity of these vectors. The gaining popularity of rAAV use in gene therapy can be attributed to its lack of pathogenicity and added safety due to its replication defectiveness, and its ability to mediate long-term expression in a variety of tissues. Site-specific integration, as occurs with wild-type AAV, will be a unique and valuable feature if incorporated into rAAV vectors, further improving their safety. This paper describes these properties of rAd and rAAV vectors, and discusses further development and vector improvements that continue to extend the utility of these vectors, such as cell retargeting by capsid modification, differential transduction by use of serotypes, and extension of the cloning capacity of rAAV vectors by dual vector heterodimerization.

Comparison of epidermal keratinocytes and dermal fibroblasts as potential target cells for somatic gene therapy of phenylketonuria

Phenylketonuria (PKU) is caused by deficiency of phenylalanine hydroxylase (PAH) and increased levels of phenylalanine. PAH requires the cofactor BH(4) to function and the rate-limiting step in the synthesis of BH(4) is GTP cyclohydrolase I (GTP-CH). The skin is a potential target tissue for PKU gene therapy. We have previously shown that overexpression of PAH and GTP-CH in primary human keratinocytes leads to high levels of phenylalanine clearance without BH(4) supplementation [Gene Ther. 7 (2000) 1971]. Here, we investigate the capacity of fibroblasts, another cell type from the skin, to metabolize phenylalanine. After retroviral gene transfer of PAH and GTP-CH both normal and PKU patient fibroblasts were able to metabolize phenylalanine, however, in lower amounts compared to genetically modified keratinocytes. Further comparative analyses between keratinocytes and fibroblasts revealed a higher copy number of transgenes in keratinocytes and also a higher metabolic capacity.

Prolongation of transgene expression by coexpression of cytokine response modifier a in rodent liver after adenoviral gene transfer

The short duration of expression of the transgenes is a major barrier to the clinical application of adenovirus-mediated gene therapy for hepatic enzyme deficiencies. Previous reports show that Fas-mediated apoptosis has a pivotal role in the rapid elimination of adenovirus-infected hepatocytes. After considering this result and our recent observation that murine hepatocytes can be protected from Fas-mediated apoptosis by expressing cytokine response modifier A (CrmA) in vivo, we hypothesized that CrmA coexpression could also prevent adenovirus-infected hepatocytes from rapid elimination and that this would make prolonged transgene expression achievable in vivo. To examine this, mice with congenital deficiency of lysosomal beta-glucuronidase (GUSB) were infected with recombinant adenoviruses expressing both CrmA and GUSB, and the duration of transgene expression was evaluated. The serum GUSB activity in the mice injected with a recombinant adenovirus expressing GUSB only became undetectable 60 days after the injection, whereas higher than normal GUSB activity was observed for at least 120 days in mice injected with adenoviruses expressing both GUSB and CrmA. Furthermore, we showed that exogenous CrmA expression could prevent the adenovirus-infected hepatocytes from cell death induced by cytotoxic T lymphocytes in vitro. These observations indicate that transgene expression after administration of E1-deleted adenovirus is prolonged by coexpression of the antiapoptotic protein CrmA.

Tacrolimus: a review of its use for the management of dermatoses

The newly developed immunomodulator tacrolimus (FK506) is the first of a new class of agents that have enormous potential to change the way that dermatoses are treated and managed. Tacrolimus has been found to be active in a topical formulation with the latter exerting its effects by acting on the signal transduction pathways inside T cells and inhibiting gene transcription. The result is decreased responsiveness of T cells to antigens. Percutaneous absorption of tacrolimus is higher in diseased skin as opposed to healthy skin and, therefore, the drug will be taken in at progressively lower quantities as lesions heal. There is limited systemic absorption of tacrolimus over the course of therapy. The most extensive experience with tacrolimus has been in treating atopic dermatitis. In numerous trials, tacrolimus ointment 0.03-0.3% has shown to be effective in reducing the symptoms and severity of atopic dermatitis in adults and the paediatric population. Furthermore, there have been no significant toxic effects associated with topical therapy with tacrolimus. The most common complaint is that of local irritation after applying the ointment. This is generally transient and the patient is able to continue with therapy. The other dermatoses where tacrolimus has been used include contact dermatitis, psoriasis and pyoderma gangrenosum.

A structural hypothesis for BH4 responsiveness in patients with mild forms of hyperphenylalaninaemia and phenylketonuria

Deficiencies in the human enzyme phenylalanine hydroxylase (PAH) due to mutations in the PAH gene (PAH) result in the inborn error of metabolism phenylketonuria (PKU). The clinical symptom of this disease is an elevated concentration of L-phenylalanine (L-Phe) in blood serum. To prevent mental retardation due to the buildup of neurotoxic metabolites of L-Phe, patients with severe PKU must be treated with a low-L-Phe diet starting early in their life. Owing to extensive newborn screening programmes and genotyping efforts, more than 400 different mutations have been identified in the PAH gene. Recently, there have been several reports of PKU patients showing a normalization of their L-Phe concentrations upon oral administration of the natural cofactor to PAH, (6R)-L-erythro-5,6,7,8-tetrahydrobiopterin (BH4). In an attempt to correlate the clinical responsiveness to BH4 administration with PKU genotype, we propose specific structural consequences for this subset of PAH mutations. Based on the location and proximity of this subset of mutations to the cofactor-binding site in the three-dimensional structure of PAH, a hypothesis for BH4 responsiveness in PKU patients is presented. It is believed that some of these mutations result in expressed mutant enzymes that are Km variants (with a lower binding affinity for BH4) of the standard PAH enzyme phenotype. Oral administration of excess BH4 thus makes it possible for these mutant enzymes to suppress their low binding affinity for BH4, enabling this subset of PAH mutations to perform the L-Phe hydroxylation reaction. Most of the BH4-responsive PAH mutations map to the catalytic domain of PAH in either of two categories. Residues are located in cofactor-binding regions or in regions that interact with the secondary structural elements involved in cofactor binding. Based on the series of known mutations that have been found to be responsive to BH4, we propose that other subsets of PAH mutations will have a high likelihood of being responsive to oral BH4 administration.

Development of a skin-based metabolic sink for phenylalanine by overexpression of phenylalanine hydroxylase and GTP cyclohydrolase in primary human keratinocytes

Phenylketonuria, PKU, is caused by deficiency of phenylalanine hydroxylase (PAH) resulting in increased levels of phenylalanine in body fluids. PAH requires the non-protein cofactor BH4 and the rate-limiting step in the synthesis of BH4 is GTP cyclohydrolase I (GTP-CH). Here we show that overexpression of the two enzymes PAH and GTP-CH in primary human keratinocytes leads to high levels of phenylalanine clearance without BH4 supplementation. Integration of multiple PAH and GTP-CH transgenes were achieved after optimized retroviral transduction. Phenylalanine clearance was measured ex vivo in primary human keratinocytes cotransduced with PAH and GTP-CH (more than 370 nmol/24 h/106 cells), a level exceeding that of a human liver cell line (HepG2 cells). Cells overexpressing either one of the enzymes alone did not clear significant amounts of phenylalanine. Transfer of the two genes into the same cell was not necessary, since cocultivation of cells transduced separately with PAH and GTP-CH also resulted in phenylalanine clearance. Thus the experiments indicate metabolic cooperation between cells overexpressing PAH and cells overexpressing GTP-CH, possibly due to intercellular transport of synthesized BH4.

Adenovirus-mediated in utero gene transfer in mice and guinea pigs: tissue distribution of recombinant adenovirus determined by quantitative TaqMan-polymerase chain reaction assay

Fetal somatic cell gene therapy could become an attractive solution for some congenital genetic diseases or the disorders which manifest themselves during the fetal period. We performed adenovirus-mediated gene transfer to mice and guinea pig fetuses in utero and evaluated the efficiency of gene transfer by histochemical analysis and a quantitative TaqMan-polymerase chain reaction (TaqMan-PCR) assay. We first injected a replication-deficient recombinant adenovirus containing the Escherichia coli LacZ gene driven by a CAG promoter (AxCALacZ) into pregnant mice through the amniotic space, placenta, or intraperitoneal space of the fetus. Histochemical analysis showed limited transgene expression in fetal tissues. We then administered AxCALacZ to guinea pig fetuses in the late stage of pregnancy through the umbilical vein. The highest beta-galactosidase expression was observed in liver followed by moderate expression in heart, spleen, and adrenal gland. The transgene expression was also present in kidney, intestine, and placenta to a lesser degree. No positively stained cells were observed in lung, muscle, or pancreas except in the vascular endothelium of these organs. Quantitative measurement of recombinant adenoviral DNA by the TaqMan-PCR assay showed that the vast majority of the injected viruses was present in liver. The current study indicated that adenovirus-mediated gene transfer into guinea pig fetus through the umbilical vein is feasible and results in efficient transgene expression in fetal tissues. The experimental procedures using pregnant guinea pigs might serve as a good experimental model for in utero gene transfer. Since our TaqMan-PCR assay detects the LacZ gene, one of the most widely used reporter genes, it may be generally applicable to adenovirus quantification in various gene transfer experiments.