Mutant mice exhibiting heritable hyperphenylalaninemia have been isolated after ethylnitrosourea mutagenesis of the germ line. We describe one mutant pedigree in which phenylalanine hydroxylase activity is severely deficient in homozygotes and reduced in heterozygotes while other biochemical components of phenylalanine catabolism are normal. In homozygotes, injection of phenylalanine causes severe hyperphenylalaninemia and urinary excretion of phenylketones but not hypertyrosinemia. Severe chronic hyperphenylalaninemia can be produced when mutant homozygotes are given phenylalanine in their drinking water. Genetic mapping has localized the mutation to murine chromosome 10 at-or near the Pah locus, the structural gene for phenylalanine hydroxylase. This mutant provides a useful genetic animal model affected in the same enzyme as in human phenylketonuria.Disorders ofphenylalanine catabolism, resulting in phenylketonuria (PKU) and hyperphenylalaninemia (HPH), were among the first heritable errors of metabolism discovered in the human (1). The rate-limiting step in mammalian phenylalanine catabolism is hydroxylation to produce tyrosine. This reaction, catalyzed by phenylalanine hydroxylase (PAH) (2), requires the reduced pteridine cofactor tetrahydrobiopterin (3), which is synthesized from GTP (4) through a number of intermediates and is maintained in its reduced form by quinonoid dihydropteridine reductase (q-DHPR) (5). Mutations reducing the activity of PAH, q-DHPR, or the enzymes involved in tetrahydrobiopterin synthesis result in HPH because of a block in phenylalanine hydroxylation (6). In humans, PKU is defined as a condition resulting from mutations that abolish or severely reduce PAH activity (7). Other defects in phenylalanine catabolism are termed HPH. Extensive research has been undertaken to characterize these disorders (early work is reviewed in refs. 8 and 9 with recent summations in refs. 10 and 11). Laboratory mice with defined PKU and HPH mutations would be helpful in evaluating features of these diseases by permitting investigations not acceptable with human subjects. To produce such mutants we have used the alkylating agent N-ethyl-N-nitrosourea, which induces mutations in the mouse germ line at a frequency near 10-3 per locus (12). We refer to all mutants with deficiencies in phenylalanine catabolism by their common phenotype, HPH. We have screened the progeny of 347 gametes and have isolated four mutant pedigrees exhibiting the HPH phenotype. One of these, HPH-1, was detected by its neonatal HPH phenotype and is deficient in GTP cyclohydrolase activity (13)(14)(15). The others were detected by their impaired ability to clear a phenylalanine challenge (described in ref. 13). One of these, HPH-5, we now report to be deficient in PAH. MATERIALS AND METHODSBiochemical Determinations. Liver homogenates were prepared for PAH assay as described in ref. 16 methyltetrahydropterin, and the enzyme extract. The background rates of NADH oxidation were determined in the absence ofphenylalanine and ind...

The germline supermutagen, N-ethyl-N-nitrosourea (ENU), has a variety of effects on mice. ENU is a toxin and carcinogen as well as a mutagen, and strains differ in their susceptibility to its effects. Therefore, it is necessary to determine an appropriate mutagenic, non-toxic dose of ENU for strains that are to be used in experiments. In order to provide some guidance, we have compiled data from a number of laboratories that have exposed male mice from inbred and non-inbred strains or their F(1) hybrids to ENU. The results show that most F(1) hybrid animals tolerate ENU well, but that inbred strains of mice vary in their longevity and in their ability to recover fertility after treatment with ENU.

The quaking (qk) locus on mouse chromosome 17 has been defined by a single viable quaking allele. Three new alleles of quaking were selected after ENU mutagenesis by their failure to complement the quaking phenotype. The qk k2 allele was induced on wild-type chromatin and the qk ktl and qk ki4 alleles were induced on f-chromatin. Each is a recessive embryonic lethal mutation. They fail to complement each other and are not complemented by the deletion, Tt OH . Homozygotes and hemizygotes die at 8-9-5 days gestation, but not at a single precise time. Because the classical quaking mutation complements the lethality of these new alleles, but they fail to complement its quaking phenotype (myelination defect), we conclude that the quaking + function is required for embryonic survival as well as for myelination.

ABSTRACT. A mutation, resulting in a deficiency of liver GTP-cyclohydrolase activity, has been induced in the laboratory mouse. Mice homozygous for this mutation exhibit hyperphenylalaninemia under the following conditions: 1 ) early in life and 2) throughout life when exposed to phenylalanine. A phenylalanine loading regimen was used to discriminate between mutant and wild type mice on the basis of the resultant phenylalanine and tyrosine serum levels. Subjecting mice to this regimen reveals several distinguishing characteristics. Mutant mice exhibit approximately 2-fold higher peak phenylalanine levels than wild-type mice. In wild-type mice the hyperphenylalaninemic state is transient and rapidly abates while in mutant mice it is persistent and remains for a prolonged period. Mutant mice exhibit normal serum tyrosine levels after a loading challenge, while wild-type mice experience an increase in tyrosine levels. The loading regimen was also used to gauge the response of mutant hyperphenylalaninemic mice to exposure to chemical compounds required for normal phenylalanine catabolism (i.e. pteridine cofactors of the phenylalanine hydroxylase reaction). Mutant mice exposed to native enzyme cofactor or cofactor precursors exhibit a sharp decline in serum phenylalanine levels relative to their uninjected counterparts coupled with a tyrosine increase. By contrast, mutant mice exposed to nonprecursor compounds that are structurally related to the native cofactor, experience no diminution of serum phenylalanine levels. (Pediatr Res 23: 63-67, 1988)

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