Expression of phenylalanine hydroxylase (PAH) in erythrogenic bone marrow does not correct hyperphenylalaninemia in Pahenu2 mice

Cary Harding, Mark Neff, Kelly Jones, Krzysztof Wild, Jon A. Wolff

Research output: Contribution to journalArticle

10 Citations (Scopus)

Abstract

Background: Treatment of many inherited liver enzyme deficiencies requires the removal of toxic intermediate metabolites from the blood of affected individuals. We propose that circulating toxins can be adequately cleared and disease phenotype influenced by enzyme expressed in tissues other than the liver, such as bone marrow. Our specific hypothesis was that phenylalanine hydroxylase (PAH) expressed in bone marrow would lower blood phenylalanine levels in hyperphenylalaninemic Pahenu2 mice, a model of human phenylketonuria (PKU). Methods: Germline-modified marrow PAH-expressing mice were developed using a transgene that contained the mouse liver PAH cDNA under the transcriptional control of a human β-globin promoter. Marrow PAH-expressing mice were bred to Pahenu2 mice to generate progeny that lacked liver PAH activity but expressed PAH in bone marrow. Results: Marrow PAH expression did not affect the health, function, or reproductive capacity of transgenic animals. Hyperphenylalaninemia persisted in transgenic Pahenu2 homozygous mice despite PAH activity in marrow lysates, and was not altered following supplementation with tetrahydrobiopterin (BH4), a required cofactor for PAH. PAH activity measured in intact marrow cells was significantly lower than in marrow lysates; no such difference was measured in isolated hepatocytes vs. liver homogenate. Conclusions: Marrow PAH expression did not correct hyperphenylalaninemia in Pahenu2 mice. Phenylalanine clearance may have been limited by the natural perfusion rate of the marrow compartment, by insufficient PAH expression in marrow, or by other cellular factors affecting phenylalanine metabolism in intact marrow cells. Differences in PAH activity measured in intact marrow cells vs. cell lysates suggest that hepatocytes and PAH-expressing marrow cells are fundamentally different in their ability to metabolize phenylalanine. The efficacy of bone-marrow-directed gene therapy as a metabolic sink in the treatment of phenylketonuria may be limited, although further experiments with greater marrow PAH expression levels will be necessary to definitively prove this conclusion.

Original languageEnglish (US)
Pages (from-to)984-993
Number of pages10
JournalJournal of Gene Medicine
Volume5
Issue number11
DOIs
StatePublished - Nov 2003

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Phenylalanine Hydroxylase
Phenylketonurias
Bone Marrow
Phenylalanine
Liver
Hepatocytes
Genetically Modified Animals

Keywords

  • β-globin gene
  • Bone marrow gene therapy
  • Hyperphenylalanimemia
  • Inborn erros of metabolism
  • Phenylalanine hydroxylase
  • Phenylketonuria

ASJC Scopus subject areas

  • Genetics

Cite this

Expression of phenylalanine hydroxylase (PAH) in erythrogenic bone marrow does not correct hyperphenylalaninemia in Pahenu2 mice. / Harding, Cary; Neff, Mark; Jones, Kelly; Wild, Krzysztof; Wolff, Jon A.

In: Journal of Gene Medicine, Vol. 5, No. 11, 11.2003, p. 984-993.

Research output: Contribution to journalArticle

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title = "Expression of phenylalanine hydroxylase (PAH) in erythrogenic bone marrow does not correct hyperphenylalaninemia in Pahenu2 mice",
abstract = "Background: Treatment of many inherited liver enzyme deficiencies requires the removal of toxic intermediate metabolites from the blood of affected individuals. We propose that circulating toxins can be adequately cleared and disease phenotype influenced by enzyme expressed in tissues other than the liver, such as bone marrow. Our specific hypothesis was that phenylalanine hydroxylase (PAH) expressed in bone marrow would lower blood phenylalanine levels in hyperphenylalaninemic Pahenu2 mice, a model of human phenylketonuria (PKU). Methods: Germline-modified marrow PAH-expressing mice were developed using a transgene that contained the mouse liver PAH cDNA under the transcriptional control of a human β-globin promoter. Marrow PAH-expressing mice were bred to Pahenu2 mice to generate progeny that lacked liver PAH activity but expressed PAH in bone marrow. Results: Marrow PAH expression did not affect the health, function, or reproductive capacity of transgenic animals. Hyperphenylalaninemia persisted in transgenic Pahenu2 homozygous mice despite PAH activity in marrow lysates, and was not altered following supplementation with tetrahydrobiopterin (BH4), a required cofactor for PAH. PAH activity measured in intact marrow cells was significantly lower than in marrow lysates; no such difference was measured in isolated hepatocytes vs. liver homogenate. Conclusions: Marrow PAH expression did not correct hyperphenylalaninemia in Pahenu2 mice. Phenylalanine clearance may have been limited by the natural perfusion rate of the marrow compartment, by insufficient PAH expression in marrow, or by other cellular factors affecting phenylalanine metabolism in intact marrow cells. Differences in PAH activity measured in intact marrow cells vs. cell lysates suggest that hepatocytes and PAH-expressing marrow cells are fundamentally different in their ability to metabolize phenylalanine. The efficacy of bone-marrow-directed gene therapy as a metabolic sink in the treatment of phenylketonuria may be limited, although further experiments with greater marrow PAH expression levels will be necessary to definitively prove this conclusion.",
keywords = "β-globin gene, Bone marrow gene therapy, Hyperphenylalanimemia, Inborn erros of metabolism, Phenylalanine hydroxylase, Phenylketonuria",
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AU - Wolff, Jon A.

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AB - Background: Treatment of many inherited liver enzyme deficiencies requires the removal of toxic intermediate metabolites from the blood of affected individuals. We propose that circulating toxins can be adequately cleared and disease phenotype influenced by enzyme expressed in tissues other than the liver, such as bone marrow. Our specific hypothesis was that phenylalanine hydroxylase (PAH) expressed in bone marrow would lower blood phenylalanine levels in hyperphenylalaninemic Pahenu2 mice, a model of human phenylketonuria (PKU). Methods: Germline-modified marrow PAH-expressing mice were developed using a transgene that contained the mouse liver PAH cDNA under the transcriptional control of a human β-globin promoter. Marrow PAH-expressing mice were bred to Pahenu2 mice to generate progeny that lacked liver PAH activity but expressed PAH in bone marrow. Results: Marrow PAH expression did not affect the health, function, or reproductive capacity of transgenic animals. Hyperphenylalaninemia persisted in transgenic Pahenu2 homozygous mice despite PAH activity in marrow lysates, and was not altered following supplementation with tetrahydrobiopterin (BH4), a required cofactor for PAH. PAH activity measured in intact marrow cells was significantly lower than in marrow lysates; no such difference was measured in isolated hepatocytes vs. liver homogenate. Conclusions: Marrow PAH expression did not correct hyperphenylalaninemia in Pahenu2 mice. Phenylalanine clearance may have been limited by the natural perfusion rate of the marrow compartment, by insufficient PAH expression in marrow, or by other cellular factors affecting phenylalanine metabolism in intact marrow cells. Differences in PAH activity measured in intact marrow cells vs. cell lysates suggest that hepatocytes and PAH-expressing marrow cells are fundamentally different in their ability to metabolize phenylalanine. The efficacy of bone-marrow-directed gene therapy as a metabolic sink in the treatment of phenylketonuria may be limited, although further experiments with greater marrow PAH expression levels will be necessary to definitively prove this conclusion.

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