Objective: To identify the mutation spectrum and the distribution of minihaplotypes (STR/VNTR) of phenylalanine hydroxylase (PAH) gene and explore the correlations between genotype and phenotype of patients with phenylketonuria (PKU) in Beijing area of China.
Method: (1) Fifty cases with PKU were involved in this study. PKU was identified by the Neonatal Screening Center of Beijing. All 13 exons and their flanking intronic sequences of PAH gene of these patients were amplified and then subjected to SSCP analysis and direct sequencing. (2) The distribution of polymorphic locus of short tandem repeat (STR) and variable number tandem repeat (VNTR) was analyzed by PCR and denaturing gel electrophoresis. (3) The correlations between genotype and phenotype were studied by analysis of the matching rate between the expected and observed phenotypes. The predicted phenotype was determined on the basis of the sum of the assigned values of the two mutant alleles.
Results: (1) A total of 34 different mutations were detected with the relative frequency of 95% among 50 PKU patients. The prevalent mutations in this study were: R243Q (20%), EX6-96A > G (11%), Y356X (9%), and V399V (7%). The next common mutations were R111X (5%), R413P (5%), R252Q (3%) and A434D (3%). Thirty-four detected mutations were distributed throughout the whole PAH gene, except exon 1, 8 and 13. Exon 7 and 11, with the mutant rate 34% and 19% respectively, seemed to be the hot mutant areas/regions of PAH gene. (2) The minihaplotypes (STR/VNTR) of 34 mutations were identified in this research. The STR and VNTR showed 8 and 3 alleles, respectively. Among them, 244 bp (44%) and 240 bp (34%) were the prevalent STR alleles. Meanwhile, the VNTR3 (83%) was the most common VNTR allele in PKU patients. (3) A better consistency (81.5%) between expected and observed phenotypes was revealed by analysis of correlation between genotype and phenotype. Especially in classic PKU, the consistency rate was up to 87.5%.
Conclusion: (1) The frequency distribution of common PAH gene mutations in Beijing region was close to that of Tianjin and Yunnan regions, while it was different from that of Southern regions of China, such as Guangzhou, especially Taiwan. The PAH mutation with a highly heterogeneous trait was also demonstrated in this study. (2) STR and VNTR minihaplotype will prove helpful to trace the origins of PAH mutations and to analyze the genetic drift. However, the most minihaplotypes of the STR/VNTR are similar, so it is necessary to associate some other polymorphic loci with the STR/VNTR minihaplotype to analyze the different mutations. (3) The fact that a better consistency existed between phenotypes and genotype with most PKU patients suggested that the study of the genotype of PKU patients would be helpful to the individualized treatment and to genetic counseling for their families.