The complexity of newborn screening follow-up in phenylketonuria.

In the United States, and most developed nations, the newborn screening (NBS) panel covers many primary disorders of metabolism, including phenylketonuria (PKU). When an elevated phenylalanine level is identified, the infant is evaluated for PKU and should also be tested for tetrahydrobiopterin (BH4) deficiency. A neonate presented with a phenylalanine level of 254 μmol/L (reference range <138 μmol/L) on newborn screening. The infant's confirmatory phenylalanine was 118 μmol/L (reference range <77 μmol/L). Her urine pterin profile was normal, and initially she had no measurable activity of red blood cell (RBC) dihydropteridine reductase (DHPR). Subsequent study revealed normal levels of CSF tetrahydrobiopterin and neurotransmitter metabolites, and by 18 months of age, her RBC DHPR activity was detectable at 0.5 nmol/min/mgHgb (reference range 0.8-3.9). Sequencing of the QDPR gene for DHPR revealed c.1A>T nucleotide substitution in exon 3 expressed as "p.MET1?" Phenylalanine hydroxylase (PAH) gene sequencing revealed compound heterozygosity for L249F and A300S. Although initial testing suggested the child was affected with DHPR deficiency, further analysis, finding increasing levels of DHPR activity and PAH compound mutant heterozygosity, indicated that the primary disorder is mild hyperphenylalaninemia with carrier status for DHPR deficiency. This is an example of newborn screening results leading to confusing findings requiring extensive biochemical studies and genotyping in order to arrive at the appropriate diagnosis.