[Correlation between genotypes and biochemical phenotypes of phenylalanine hydroxylase in patients with phenylketonuria].

Objective: To investigate the correlation between genotypes and biochemical phenotypes of phenylalanine hydroxylase (PAH) in patients with phenylketonuria (PKU).

Methods: Thirteen exons and flanking introns of PAH gene in 102 patients with high blood phenylalanine levels (Phe > 120 umol/L) at initial diagnosis were amplified with polymerase chain reaction and analyzed with single strand conformation polymorphism (SSCP), denaturing high performance liquid chromatography (DHPLC) and DNA sequencing. Correlation between genotypes and biochemical phenotypes was analyzed.

Results: Biochemical assaying has indicated that 69 patients had classical PKU (Phe> 1200 umol/L), 31 were moderate (Phe 600-1200 umol/L), and 2 were mild (Phe 400-600 umol/L). More than 41 mutations and 75 genotypes have been identified. There were 9 (8.8%) homozygous mutations, which included 3 cases with R111X/R111X, 1 case with IVS4-1G>A/IVS4-1G>A, 3 cases with R243Q/R243Q and 2 cases with V399V/V399V. Among these 8 belonged to classic PKU phenotypes, except for a R243Q/R243Q genotype which has led to a moderate phenotype. In 91 patients carrying compound PAH mutations, 61 were classic, 29 were moderate, and 1 was mild. Patients who were heterozygous for R111X/R243Q and EX6-96A>G(Y204C)/R243Q were found with both classic and moderate PKU phenotypes. Certain individuals who have carried 2 null mutant alleles such as R111X/V399V, EX6-96A>G/Y356X and EX6-96A>G/V399V only showed a moderate phenotype. Individuals with R111X/A165D and R176X/A165D genotypes, on the other hand, respectively presented moderate and classic PKU phenotypes.

Conclusion: Ninety percent of our patients are compound heterozygotes. Independent assortment of mutant alleles has resulted in a complex genotype-phenotype correlation. Although in most cases a correlation may be found, caution should still be taken upon genetic counseling. The phenomena where similar or even identical genotype may give rise to different biochemical phenotypes have implied that other factors may also influence the phenylalanine metabolism.