A study of bone turnover markers in prepubertal children with phenylketonuria.

Abstract

To investigate the effect of low-phenylalanine diets onbone mineralisation status, we compared biochemicalbone formation and resorption markers in prepubertalchildren with phenylketonuria with those of age-mat-ched healthy controls.Dietary phenylalanine (Phe) restriction is the onlyknown strategy of preventing neurological impairmentand mental retardation in patients with phenylketonuria(PKU). In general, most phenylketonuric patients on alow-Phe diet can achieve normal growth and intellectualdevelopment. It is suggested, however, that this form ofdiet may influence bone metabolism, especially inchildhood and adolescence when growth and boneturnover are at their most intensive. Some authors havedescribed decreased bone mineral density and osteope-nia in patients suffering from PKU [1, 2, 3,5].Measurements of bone mineral density (BMD) reflectonly bone mineral status but not the dynamics of boneturnover. Non-invasive biochemical markers whichshow global skeletal activity have lately been developedand validated for the assessment of bone formation andbone resorption processes. Among them, osteocalcin(OC), bone alkaline phosphatase (BALP) and collagentype 1 cross-linked C-telopeptide (CTX) are consideredto be clinically useful. Recently, the novel cytokineosteoprotegerin (OPG), belonging to the tumournecrosis factor receptor family has been established asan endogenous inhibitor of osteoclastogenesis andresorption processes.Our study population consisted of 37 childrenwith PKU under surveillance at the Department ofPaediatrics at the Institute of Mother and Child inWarsaw. Patients were divided into two groups. Thosein group A (n =12; median age 4.5 years; range 3–10years) followed their therapeutic diet strictly and hadmean serum Phe concentrations close to the referencerange (189.4±64.2 lmol/l) and those in group B(n =25; median age 6.0 years; range 3–10 years) did notadhere to their diet and had increased Phe concentra-tions (649.2±140.6 lmol/l). The mean serum Pheconcentration was calculated for the last 3 years of thelife of each patient. Children with PKU were fed with anaminoacid mixture PAM, Milupa-PKU 2 or Phenyl-free. These patients have normal values for calcium andphosphate. Healthy children sent to our laboratory forroutine analytical control (n =27; median age 5.9 years;range 4–9 years) were the reference group. The wholegroup of investigated children was ethnically homoge-nous. Venous blood samples were collected after anovernight fast, centrifuged and serum levels of Phe,calcium and phosphate were determined. Remainingserum samples were frozen and collected for measure-ment of BALP, OC, CTX and OPG. Phe was assayedfluorometrically according to McCaman and Robbins;calcium and phosphate by standard procedures with aCobas Integra analyser (Roche, Switzerland). Serum OCand CTX were measured by immuno-enzymatic ELISAassays (Osteometer, Denmark). For determination ofBALP, the Alkphase-B kit from Metra Biosystems(USA) and for OPG, the kit from Biomedica (Austria)were used. The differences were evaluated by ANOVAwith Bonferroni correction. The significance was set atP <0.05.We observed lower levels of bone formation andbone resorption markers in group A than in group B,but these differences are not statistically significant(Table 1). However, OC, CTX and OPG concentrationswere significantly lower in both groups of PKU childrenin comparison to the healthy age-matched controls.There are only a few papers presenting values of boneturnover markers in PKU subjects. Perez-Duenas et al.[6] observed the same levels of OC and BALP in a groupof younger patients and significantly lower serum BALP