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

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.

Low-Dose Gene Therapy for Murine PKU Using Episomal Naked DNA Vectors Expressing PAH from Its Endogenous Liver Promoter

Limited duration of transgene expression, insertional mutagenesis, and size limitations for transgene cassettes pose challenges and risk factors for many gene therapy vectors. Here, we report on physiological expression of liver phenylalanine hydroxylase (PAH) by delivery of naked DNA/minicircle (MC)-based vectors for correction of homozygous enu2 mice, a model of human phenylketonuria (PKU). Because MC vectors lack a defined size limit, we constructed a MC vector expressing a codon-optimized murine Pah cDNA that includes a truncated intron and is under the transcriptional control of a 3.6-kb native Pah promoter/enhancer sequence. This vector, delivered via hydrodynamic injection, yielded therapeutic liver PAH activity and sustained correction of blood phenylalanine comparable to viral or synthetic liver promoters. Therapeutic efficacy was seen with vector copy numbers of <1 vector genome per diploid hepatocyte genome and was achieved at a vector dose that was significantly lowered. Partial hepatectomy and subsequent liver regeneration was associated with >95% loss of vector genomes and PAH activity in liver, demonstrating that MC vectors had not integrated into the liver genome. In conclusion, MC vectors, which do not have a defined size-limitation, offer a favorable safety profile for hepatic gene therapy due to their non-integration in combination with native promoters.

Genetic Manipulation of Brown Fat Via Oral Administration of an Engineered Recombinant Adeno-associated Viral Serotype Vector

Recombinant adeno-associated virus (rAAV) vectors are attractive vehicles for gene therapy. Gene delivery to the adipose tissue using naturally occurring AAV serotypes is less successful compared to liver and muscle. Here, we demonstrate that oral administration of an engineered serotype Rec2 led to preferential transduction of brown fat with absence of transduction in the gastrointestinal track. Among the six natural and engineered serotypes being compared, Rec2 was the most efficient serotype achieving high level transduction at a dose 1~2 orders lower than reported doses for systemic administration. Overexpressing vascular endothelial growth factor (VEGF) in brown fat via oral administration of Rec2-VEGF vector increased the brown fat mass and enhanced thermogenesis. In contrast, knockdown VEGF in brown fat of VEGF (loxP) mice via Rec2-Cre vector hampered cold response and decreased brown fat mass. Oral administration of Rec2 vector provides a novel tool to genetically manipulate brown fat for research and therapeutic applications.

Phenylketonuria: translating research into novel therapies

Phenylketonuria (PKU) is an inborn error of metabolism of the amino acid phenylalanine. It is an autosomal recessive disorder with a rate of incidence of 1 in 10,000 in Caucasian populations. Mutations in the phenylalanine hydroxylase (PAH) gene are the major cause of PKU, due to the loss of the catalytic activity of the enzyme product PAH. Newborn screening for PKU allows early intervention, avoiding irreparable neurological damage and intellectual disability that would arise from untreated PKU. The current primary treatment of PKU is the limitation of dietary protein intake, which in the long term may be associated with poor compliance in some cases and other health problems due to malnutrition. The only alternative therapy currently approved is the supplementation of BH4, the requisite co-factor of PAH, in the orally-available form of sapropterin dihydrochloride. This treatment is not universally available, and is only effective for a proportion (estimated 30%) of PKU patients. Research into novel therapies for PKU has taken many different approaches to address the lack of PAH activity at the core of this disorder: enzyme replacement via virus-mediated gene transfer, transplantation of donor liver and recombinant PAH protein, enzyme substitution using phenylalanine ammonia lyase (PAL) to provide an alternative pathway for the metabolism of phenylalanine, and restoration of native PAH activity using chemical chaperones and nonsense read-through agents. It is hoped that continuing efforts into these studies will translate into a significant improvement in the physical outcome, as well as quality of life, for patients with PKU.

Recovery of neurogenic amines in phenylketonuria mice after liver-targeted gene therapy

Phenylketonuria (PKU) is a common genetic disorder arising from a deficiency of phenylalanine hydroxylase. If left untreated, the accumulation of phenylalanine leads to brain damage and neuropsychological dysfunction. One of the abnormalities found in hyperphenylalaninemic patients and a mouse model of PKU is an aminergic deficit in the brain. We previously showed correction of hyperphenylalaninemia and concomitant behavioral recovery in PKU mice after liver-targeted gene transfer with a viral vector. Here, we addressed whether such a functional recovery was substantiated by an improved amine metabolism in the brain. After gene transfer, brain dopamine, norepinephrine, and serotonin levels in the PKU mice were significantly elevated to normal or near-normal levels, along with systemic improvement of phenylalanine catabolism. The results of biochemical analyses validated the efficacy of PKU gene therapy in the central nervous system.

Complete restoration of phenylalanine oxidation in phenylketonuria mouse by a self-complementary adeno-associated virus vector

BACKGROUND

Classical phenylketonuria (PKU) arises from a deficiency of phenylalanine hydroxylase (PAH) that catalyses phenylalanine oxidation in the liver. Lack of PAH activity causes massive hyperphenylalaninemia and consequently severe brain damage. Preclinical studies showed that conventional adeno-associated virus (AAV) vectors could correct hyperphenylalaninemia in a mouse model of PKU, although limitations such as very large dose requirement and relative inefficiency in female animals were recognized.

METHOD

An AAV8-pseudotyped vector was constructed with a self-complementary AAV (scAAV) genome for efficient liver transduction and expression. Following vector injection to PKU mice, blood Phe was periodically measured by an enzymatic fluorometric assay. In vivo Phe oxidation was evaluated by a non-invasive breath test using [1-(13) C]Phe. Vector copy number in the host tissues was determined by quantitative polymerase chain reaction.

RESULTS

A single injection of 1 × 10(11) -1 × 10(12) particles of the scAAV8 vector resulted in a reduction of blood Phe to normal or near-normal levels for more than 1 year in both genders. The treated animals showed normal level of in vivo Phe oxidation. The presence of > 1 copy of vector DNA per diploid genome in the liver was associated with normal blood Phe in the AAV-treated PKU mice.

CONCLUSIONS

Complete phenotypic correction of PKU mice was achieved by the scAAV8 vector for the longest duration reported to date. The vector overcame the female-specific disadvantage in AAV-mediated liver transduction; thus, it offers a promising platform of long-lasting gene therapy for PKU.

Phenylketonuria: a 21st century perspective

Phenylketonuria is the most prevalent inherited defect in amino acid metabolism. Owing to mutations in the gene encoding the enzyme phenylalanine hydroxylase, the essential amino acid phenylalanine cannot be hydroxylated to tyrosine and blood and tissue concentrations of phenylalanine increase. Untreated, phenylketonuria causes severe mental retardation, epilepsy and behavioral problems. The combined effect of neonatal screening and treatment has, however, meant that phenylketonuria is now a biochemical rather than a clinical diagnosis. Treatment consists of stringent dietary restriction of natural protein intake and supplementation of amino acids other than phenylalanine by a chemically manufactured protein substitute. Although clinical outcome on a phenylalanine-restricted diet is good, neuropsychological deficits are now known to exist in dietary-treated patients with phenylketonuria, and quality of life, nutritional condition and psychosocial outcome could probably also be improved. The need for new therapeutic approaches is being met by supplementation with tetrahydrobiopterin or large neutral amino acids, whilst development of the use of phenylalanine ammonia lyase, and, in the longer term, gene therapy and chaperone treatment holds promise. This Review provides an overview of the history of phenylketonuria, the challenges of treatment today and the treatment possibilities in the near future.