Hyperphenylalaninemia due to defects in tetrahydrobiopterin metabolism: Molecular characterization of mutations in 6-pyruvoyl-tetrahydropterin synthase
- Univ. of Zurich (Switzerland)
A variant type of hyperphenylalaninemia is caused by a deficiency of tetrahydrobiopterin (BH[sub 4]), the obligatory cofactor for phenylalanine hydroxylase. The most frequent form of this cofactor deficiency is due to lack of 6-pyruvoyl-tetrahydropterin synthase (PTPS) activity, the second enzyme in the biosynthetic pathway for BH[sub 4]. The human liver cDNA for PTPS was previously isolated, and the recombinant protein was found to be active when expressed in Escherichia coli. The authors now have investigated two patients for their molecular nature of this autosomal recessive disorder. Both patients were diagnosed as PTPS deficient, one with the central and one with the peripheral form, on the basis of an elevated serum phenylalanine concentration concomitant with lowered levels of urinary biopterin and PTPS activity in erythrocytes. Molecular analysis was performed on the patients' cultured primary skin fibroblasts. PTPS activities were found in vitro to be reduced to background activity. Direct cDNA sequence analysis using reverse transcriptase-PCR technology showed for the patient with the central form a homozygous G-to-A transition at codon 25, causing the replacement of an arginine by glutamine (R25Q). Expression of this mutant allele in E.coli revealed 14% activity when compared with the wild-type enzyme. The patient with the peripheral form exhibited compound heteroxygosity, having on one allele a C-to-T transition resulting in the substitution of arginine 16 for cysteine (R16C) in the enzyme and having on the second allele a 14-bp deletion ([Delta]14bp), leading to a frameshift at lysine 120 and a premature stop codon (K120[yields]Stop). Heterologous expression of the enzyme with the single-amino-acid exchange R16C revealed only 7% enzyme activity, whereas expression of the deletion allele [Delta]14bp exhibited no detectable activity. All three mutations result in reduced enzymatic activity when reconstituted in E. coli.
- OSTI ID:
- 7199858
- Journal Information:
- American Journal of Human Genetics; (United States), Vol. 54:5; ISSN 0002-9297
- Country of Publication:
- United States
- Language:
- English
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