The Molecular Basis of Phenylketonuria and Hyperphenylalaninemia in Latvia. Summary of the Doctoral Thesis

Abstract

Phenylketonuria (PKU) is the most common inborn error of amino acid metabolism in Europeans. It is caused by an autosomal recessive deficiency of the hepatic enzyme phenylalanine hydroxylase (PAH) that catalyses the irreversible hydroxylation of phenylalanine to tyrosine. The elevated level of phenylalanine effects the energy production, protein synthesis, and neurotransmitter homeostasis in the developing brain and results in the most important manifestation of PKU - mental retardation. More than 560 different disease-causing mutations in the PAH gene have been identified and reported since the gene was discovered in 1986. Mutations differ in residual enzyme activity, and the genotype could be a good predictor of biochemical phenotype in the majority of patients. The aim of the study was to investigate the molecular basis of phenylketonuria and hyperphenylalaninemia in Latvian patients and evaluate a PAH gene mutation diagnostic strategy in Latvian population. Analysis of the molecular basis of PKU in Latvia has revealed 20 different mutations in the PAH gene. The most common mutation was R408W that accounted for 73% of all PKU chromosomes that gives the high level of homogeneity (“homozygosity”) at the PAH locus in Latvia (j=0.514). Frequencies of remained 19 mutations ranged from 0.7 to 5.7% of all mutant alleles. The majority of mutations (12/20) were severe and responsible for the classic PKU phenotype that was observed in 91% of PKU patients. The evaluation of patients’ genotypes can provide the additional information for BH4-responsiveness. According to the study results 13 of 70 (18%) Latvian PKU patients could potentially benefit from chaperon therapy by sapropterin dihydrochloride while remaining 57 (81%) patients with homozygous R408W mutation should keep the low phenylalanine diet as the only effective form of therapy. Minihaplotype studies have revealed 16 different minihaplotypes associated to PAH gene mutations and 20 different minihaplotypes for normal PAH alleles. The average probability of heterozygosity for minihaplotypes was about 76% for mutant and 92% for normal chromosomes indicating a greater diversity of normal alleles. Statistical analysis has revealed the significant difference in the distribution of normal and mutant alleles for only two minihaplotypes 3/238 (p=0.0000572) and 8/230 (p=0.0133), and the tendency to statistically significant difference (p<0.10) between normal and mutant alleles in the distribution of minihaplotypes 3/242, 7/246 and 8/234. Analysis of the distribution of the PAH gene mutation R408W together with it strong association with a typical East-European VNTR3/STR238 minihaplotype have confirmed the Balto-Slavic origin of mutation R408W and introduction of this mutation to other European populations by people migrations. The three-step PAH gene mutation detection strategy used in the study is the most effective for routine diagnostics in Latvian population with the sensitivity of the method 99%.