Growth and bone characteristics of phenylketonurics. Comparative analysis of treated and untreated phenylketonuric children

Molecular epidemiology, genotype-phenotype correlation and BH4 responsiveness in Spanish patients with phenylketonuria

Phenylketonuria (PKU), the most common inborn error of amino acid metabolism, is caused by mutations in the phenylalanine-4-hydroxylase (PAH) gene. This study aimed to assess the genotype-phenotype correlation in the PKU Spanish population and the usefulness in establishing genotype-based predictions of BH4 responsiveness in our population. It involved the molecular characterization of 411 Spanish PKU patients: mild hyperphenylalaninemia non-treated (mild HPA-NT) (34%), mild HPA (8.8%), mild-moderate (20.7%) and classic (36.5%) PKU. BH4 responsiveness was evaluated using a 6R-BH4 loading test. We assessed genotype-phenotype associations and genotype-BH4 responsiveness in our population according to literature and classification of the mutations. The mutational spectrum analysis showed 116 distinct mutations, most missense (70.7%) and located in the catalytic domain (62.9%). The most prevalent mutations were c.1066-11G>A (9.7%), p.Val388Met (6.6%) and p.Arg261Gln (6.3%). Three novel mutations (c.61-13del9, p.Ile283Val and p.Gly148Val) were reported. Although good genotype-phenotype correlation was observed, there was no exact correlation for some genotypes. Among the patients monitored for the 6R-BH4 loading test: 102 were responders (87, carried either one or two BH4-responsive alleles) and 194 non-responders (50, had two non-responsive mutations). More discrepancies were observed in non-responders. Our data reveal a great genetic heterogeneity in our population. Genotype is quite a good predictor of phenotype and BH4 responsiveness, which is relevant for patient management, treatment and follow-up.

6R-tetrahydrobiopterin treated PKU patients below 4 years of age: Physical outcomes, nutrition and genotype

BACKGROUND AND AIMS

Phenylalanine-restricted diets have proven effective in treating phenylketonuria. However, such diets have occasionally been reported to hinder normal development. Our study aimed to assess whether treating 0-4-year-old phenylketonuric patients with 6R-tetrahydrobiopterin might prevent growth retardation later in life.

METHODS

We conducted a longitudinal retrospective study which examined anthropometric characteristics of phenylketonuric patients on 6R-tetrahydrobiopterin therapy (22 subjects), and compared them with a group of phenylketonuric patients on protein-restricted diets (44 subjects). Nutritional issues were also considered. We further explored possible relationships between mutations in the PAH gene, BH4 responsiveness and growth outcome.

RESULTS

No significant growth improvements were observed in either the group on 6R-tetrahydrobiopterin treatment (height Z-score: initial= -0.57 ± 1.54; final=-0.52 ± 1.29; BMI Z-score: initial=0.17 ± 1.05; final=0.18 ± 1.00) or the diet-only group (height Z-score: initial=-0.92 ± 0.96; final= -0.78 ± 1.08; BMI Z-score: initial=0.17 ± 0.97; final=-0.07 ± 1.03) over the 1-year observation period. Furthermore, we found no significant differences (p>0.05) between the two groups at any of the time points considered (0, 6 and 12 months). Patients on 6R-tetrahydrobiopterin increased their phenylalanine intake (from 49.1 [25.6-60.3] to 56.5 [39.8-68.3] mgkg(-1)day(-1)) and natural protein intake (from 1.0 [0.8-1.7] to 1.5 [1.0-1.8] g kg(-1)day(-1)), and some patients managed to adopt normal diets. Higher phenylalanine and natural protein intakes were positively correlated with better physical outcomes in the diet-only group (p<0.05). No correlation was found between patient genotype and physical outcomes, results being similar regardless of the nutritional approach used. We did not detect any side effects due to 6R-tetrahydrobiopterin administration.

CONCLUSIONS

Our study indicates that treating 0-4-year-old phenylketonuric patients with 6R-tetrahydrobiopterin is safe. However, poor developmental outcomes were observed, despite increasing the intake of natural proteins. Genotype could be a valid predictor of tetrahydrobiopterin-responsiveness, since patients who carried the same genotype responded similarly to the 6R-tetrahydrobiopterin loading test. On the other hand, harbouring 6R-tetrahydrobiopterin responsive genotypes did not predispose patients to better physical outcomes.

Dopamine and the origins of human intelligence

A general theory is proposed that attributes the origins of human intelligence to an expansion of dopaminergic systems in human cognition. Dopamine is postulated to be the key neurotransmitter regulating six predominantly left-hemispheric cognitive skills critical to human language and thought: motor planning, working memory, cognitive flexibility, abstract reasoning, temporal analysis/sequencing, and generativity. A dopaminergic expansion during early hominid evolution could have enabled successful chase-hunting in the savannas of sub-Saharan Africa, given the critical role of dopamine in counteracting hyperthermia during endurance activity. In turn, changes in physical activity and diet may have further increased cortical dopamine levels by augmenting tyrosine and its conversion to dopamine in the central nervous system (CNS). By means of the regulatory action of dopamine and other substances, the physiological and dietary changes may have contributed to the vertical elongation of the body, increased brain size, and increased cortical convolutedness that occurred during human evolution. Finally, emphasizing the role of dopamine in human intelligence may offer a new perspective on the advanced cognitive reasoning skills in nonprimate lineages such as cetaceans and avians, whose cortical anatomy differs radically from that of primates.

Radiological assessment of the hand and wrist in phenylketonuria and hyperphenylalaninaemia

A radiograph of the left hand and wrist was taken in 141 children and young adults with phenylketonuria and hyperphenylalaninaemia. Ten (7.1%) had a bony abnormality and, of these, six (4.3%) showed evidence of reduced bone density. Only one patient had evidence of spiculation of the lower ulnar metaphysis previously described in phenylketonuria. Bone maturity was also assessed in 130 patients by the Tanner-Whitehouse (TW2) method. In 28 patients (21.5%) bone age was less than chronological age by 1 year or more. Bone age delay was greater in younger children on a more restricted diet with a lower intake of natural protein.

Growth and skeletal maturation in children with phenylketonuria

Growth and skeletal maturation was evaluated in 82 children participating in the German Collaborative Study of Children Treated for Phenylketonuria (PKU). Height, weight, head circumference and bone age were recorded at regular intervals for the first 6 years of life. The mean SD score (SDS) for height was not significantly different from zero at study entry, but decreased mainly during the second year of life to a nadir of -0.78 in boys and -0.54 in girls at 2.5 years. During the subsequent years, a significant trend towards a regain of height SDS was noted in both sexes. Weight-for-height SDS was close to zero in both sexes, with a significant continuous increasing trend throughout the observation period. Head circumference SDS decreased in boys during the first year of life from -0.28 to -0.68, whereas girls showed only a minor change. During the further follow-up period, head circumference SDS remained at approximately -0.3 in boys and 0.0 in girls. While the mean verbal and performance IQ of the total study population at 5 and 6 years of age did not differ from a group of 212 healthy non-PKU children, patients with a head circumference SDS less than the population median at 2 years of age exhibited poorer cognitive abilities at school age than those patients with a relative head size greater than the population median. The children with a head circumference less than the median at 2 years had smaller head sizes already at birth; in addition, the change in relative head size during the first 2 years was correlated significantly with cognitive abilities at school age in boys. Mean bone age was identical to chronological age at each time point of observation. The rate of maturation was one year of bone age per year of chronological age. No correlation between phenylalanine intake or phenylalanine concentrations and the rates of body or head growth or skeletal maturation could be established. We conclude that despite adequate weight gain, moderate growth retardation occurred during the first 2 years of life in this group of children treated for PKU. Growth was more compromised in boys than in girls and tended to be compensated during later follow-up. Early infantile head circumference and growth appear to be predictors of cognitive development.

Bony changes of PKU neonates unrelated to phenylalanine levels

In 1962 bone abnormalities were described radiographically in phenylketonuria patients. Later, observations were made on PKU infants during the neonatal period, which allowed differentiation between inherited alterations in bone development from those changes due to dietary restriction. Similar changes have been described in other aminoacidurias. Wrist radiographs and serum phenylalanine levels were obtained on 73 patients first seen between 1965 and 1990. All radiographs were taken on the day of referral, when the patient was between 6 and 57 days old. Forty-nine patients were less than 28 days old. Bone abnormalities were present in 56 of 73 (77%) of the children, as compared to 0 of 16 in a control group. The presence or absence of bone abnormality is unrelated to serum phenylalanine level, and to the age of the children at referral. This finding suggests that the cause of these mesodermal changes is not a deviation of a single amino acid in the infant, but that they are caused by an intrauterine amino acid imbalance.