Phenylalanine hydroxylase (PAH) is central to phenylalanine metabolism and defects in this gene cause phenylketonuria. This ENU-induced V106A single amino acid substitution results in a mild phenylketonuria phenotype. Decreased levels of PAH protein were detected by western blot, but some catalytic activity was found, albeit reduced to approximately one quarter that of controls (Sarkissian et al., 2000).
Dr. Alexandra Shedlovsky, University of Wisconsin , Madison
Phenylketonuria (PKU) in humans results from a deficiency in phenylalanine hydroxylase (PAH), which normally converts phenylalanine to tyrosine. If untreated, this condition results in severe and irreversible mental retardation. Hyperphenylalaninemia is a less severe hyperphenylalaninemia than phenylketonuria. This conservative V106A point substitution in the N-terminal region of PAH results in a more mild phenotype than that of the Pahenu2 mutation. Homozygous Pahenu1 mice show hyperphenylalanemia with delayed clearing of IP injected phenylalanine dose. (McDonald et al., 1990.)
Consistent with the milder phenotype, homozygotes do not display the pronounced hypopigmentation, or learning and memory deficits found in Pahenu2 homozygotes. Additionally, homozygous Pahenu1 females do not display the severe maternal effect of smaller and fewer litters and failed survival of pups that homozygous Pahenu2 females do. While Pahenu2 homozygotes have serum phenylalanine levels 10-20 times higher than normal and brain phenylalanine levels 10 times higher than normal, Pahenu1 homozygotes have near normal serum phenylalanine levels and elevated brain phenylalanine levels less than twice normal levels. Western blots of liver extracts detect almost no PAH protein from Pahenu1 homozygotes, yet plasma PAH enzyme activity is higher than that of controls, indicative of rapid degradation of the mutant PAH. Conversely, Pahenu2 homozygotes have PAH protein evident by Western blot but almost no enzyme activity, consistent with disruption to the catalytic domain. Pahenu1/Pahenu2 compound heterozyogtes provide a model with intermediate phenotype that has proven useful in treatment assessment. (Sarkissian et al., 2000.)
The coat color of the background strain, BTBR T+ Itpr3tf/J (Stock No. 002282) is black and tan (at/at). This strain is also homozygous for the tufted allele (Itpr3tf) resulting in various molting patterns in the mouse coat.
ENU mutagenesis of BTBR T+ Itpr3tf/J (Stock No. 002282) males created this T364C missense mutation in exon 3 of the phenylalanine hydroxylase gene that causes a phenylalanine to serine substitution at amino acid 263 (F263S). To breed away from other ENU-induced mutations, this allele was backcrossed to BTBR T+ Itpr3tf, which is homozygous for at, Itpr3tf, Disc1del, and Cox7a2ll, but wildtype for brachyury. Mice were bred to homozygosity for Pahenu1, maintained by sibling intercrossing homozygotes, and imported into The Jackson Laboratory in 1994 from Dr. Alexandra Shedlovsky and Dr. William Dove at the McArdle Laboratory for Cancer Research, University of Wisconsin. Embryos were initially cryopreserved after 3 additional sibling intercrosses.
|Allele Name||phenylalanine hydroxylase enu 1|
|Allele Type||Chemically induced (ENU)|
|Allele Synonym(s)||PAHenu2; Pahhph-5|
|Gene Symbol and Name||Pah, phenylalanine hydroxylase|
|Strain of Origin||BTBR|
|Molecular Note||A T364C missense mutation in exon 3 results in a valine to alanine substitution at amino acid 106 (V106A).|
|Mutations Made By|| |
Dr. William Dove, University of Wisconsin- Madison