Biochemical algorithm to identify individuals with ALPL variants among subjects with persistent hypophosphatasaemia.

BACKGROUND: Hypophosphatasia (HPP) is a rare and underdiagnosed condition characterized by deficient bone and teeth mineralization. The aim of this study was first, to evaluate the diagnostic utility of employing alkaline phosphatase (ALP) threshold levels to identify adults with variants in ALPL among individuals with persistently low ALP levels and second, to determine the value of also including its substrates (serum pyridoxal-5'-phosphate-PLP-and urinary phosphoetanolamine-PEA) for this purpose in order to create a biochemical algorithm that could facilitate the diagnostic work-up of HPP. RESULTS: The study population comprised 77 subjects with persistent hypophosphatasaemia. They were divided into two groups according to the presence (+GT) or absence (-GT) of pathogenic ALPL variants: 40 +GT and 37 -GT. Diagnostic utility measures were calculated for different ALP thresholds and Receiver Operating Characteristic (ROC) curves were employed to determine PLP and PEA optimal cut-off levels to predict the presence of variants. The optimal threshold for ALP was 25 IU/L; for PLP, 180 nmol/L and for PEA, 30 µmol/g creatinine. Biochemical predictive models were assessed using binary logistic regression analysis and bootstrapping machine learning technique and results were then validated. For ALP < 25 UI/L (model 1), the area under curve (AUC) and the 95% confidence intervals (CI) was 0.68 (95% CI 0.63-0.72) and it improved to 0.87 (95% CI 0.8-0.9), when PEA or PLP threshold levels were added (models 2 and 3), reaching 0.94 (0.91-0.97) when both substrates were included (model 4). The internal validation showed that the addition of serum PLP threshold levels to the model just including ALP improved significantly sensitivity (S) and negative predictive value (NPV) - 100%, respectively- with an accuracy (AC) of 93% in comparison to the inclusion of urinary PEA (S: 71%; NPV 75% and AC: 79%) and similar diagnostic utility measures as those observed in model 3 were detected when both substrates were added. CONCLUSIONS: In this study, we propose a biochemical predictive model based on the threshold levels of the main biochemical markers of HPP (ALP < 25 IU/L and PLP > 180 nmol/L) that when combined, seem to be very useful to identify individuals with ALPL variants.

Can we identify individuals with an ALPL variant in adults with persistent hypophosphatasaemia?

BACKGROUND: Hypophosphatasia (HPP) is an inborn error of metabolism characterized by low levels of serum alkaline phosphatase (ALP). Scarce evidence exists about features that should signal the potential association between hypophosphatasaemia and HPP in adults. The aim of this study is to estimate the prevalence of ALPL variants in subjects with persistent hypophosphatasaemia and determine the associated clinical and laboratory features. For this cross-sectional study, laboratory records of 386,353 subjects were screened by measurement of ALP activity. A total of 85 (0.18%) subjects with persistent hypophosphatasaemia (≥2 serum alkaline phosphatase-ALP-measurements ≤35 IU/L and none > 45 IU/L) were included (secondary causes previously discarded). ALPL genetic testing and a systematized questionnaire to retrieve demographic, clinical and laboratory data were performed. Descriptive analysis and logistic regression models were employed to identify the clinical and laboratory characteristics associated with ALPL variants. RESULTS: Forty subjects (47%) had a variant(s) in ALPL. With regard to clinical characteristics, the presence of an ALPL variant was significantly associated only with musculoskeletal pain (OR: 7.6; 95% IC: 1.9-30.9). Nevertheless, a trend to present more dental abnormalities (OR: 3.6; 95% IC: 0.9-13.4) was observed. Metatarsal stress fractures were also more frequent (4 vs 0; p < 0.05) in this group. Regarding laboratory features, median ALP levels were lower in subjects with ALPL variants (26 vs 29 IU/L; p < 0.005). Interestingly, the threshold of ALP levels < 25 IU/L showed a specificity, positive predictive value and positive likelihood ratio of 97.8, 94.4% and 19.8 to detect a positive ALPL test, respectively. CONCLUSIONS: In subjects with persistent hypophosphatasaemia -secondary causes excluded- one out of two presented ALPL variants. Musculoskeletal pain and ALP levels < 25 IU/L are associated with this variant(s). In this scenario, ALP levels < 25 IU/L seem to be very useful to identify individuals with the presence of an ALPL variant.

Clasificación de la deficiencia de hipoxantina-guanina fosforribosiltransferasa (HPRT) basadas en el estudio de 22 pacientes pertenecientes a 18 familias españolas / Hypoxanthine-guanine phosphoribosyltransferase (HPRT) deficiency. A new classification based in the study of 22 patients belonging to 18 Spanish families

La deficiencia de HPRT es un trastorno que se hereda ligado al cromosoma X y se caracteriza por producción excesiva de ácido úrico y alteraciones neurológicas variables. La deficiencia completa de HPRT se denomina síndrome de Lesch-Nyhan y se manifiesta por coreatetosis, espasticidad, retraso mental y automutilación. En otros pacientes con déficit de HPRT la deficiencia parece ser “parcial” y los síntomas neurológicos pueden variar de ausentes a severos (síndrome de Kelley-Seegmiller). La deficiencia de HPRT se clasifica en dos grupos –síndrome de Lesch-Nyhan y síndrome de Kelley-Seegmiller–, pero esta clasificación es confusa. Nosotros hemos analizado nuestra experiencia con 22 pacientes con déficit de HPRT pertenecientes a 18 familias españolas diagnosticados en el Hospital “La Paz” de Madrid en los últimos 16 años. Los estudios clínicos, bioquímicos, enzimáticos y moleculares realizados en estos pacientes nos han permitido delinear una nueva clasificación de la deficiencia de HPRT. Esta clasificación según un espectro continuo puede ser de utilidad para un mejor conocimiento de la enfermedad (AU)

The spectrum of hypoxanthine-guanine phosphoribosyltransferase (HPRT) deficiency. Clinical experience based on 22 patients from 18 Spanish families.

The enzyme hypoxanthine-guanine phosphoribosyltransferase (HPRT) catalyzes the reutilization of hypoxanthine and guanine to the purine nucleotides IMP and GMP, respectively. HPRT deficiency is an X-linked disorder characterized by uric acid overproduction and variable neurologic impairment. The complete deficiency of HPRT is diagnostic of Lesch-Nyhan syndrome manifested by choreoathetosis, spasticity, mental retardation, and self-injurious behavior. In some HPRT-deficient patients the enzyme defect appeared to be "partial" and the neurologic symptoms mild to severe (Kelley-Seegmiller syndrome). This has prompted the classification of HPRT deficiency in 2 distinct groups: Lesch-Nyhan syndrome and Kelley-Seegmiller syndrome, which has created much confusion. A spectrum of clinical consequences of HPRT deficiency has been recognized in small series of patients, but the complete spectrum of the neurologic disorder has not been described in a single series of patients examined by the same observers. We analyzed our experience with 22 patients belonging to 18 different families with HPRT deficiency diagnosed at "La Paz" University Hospital in Madrid over the past 16 years. The clinical spectrum of these HPRT-deficient Spanish patients was similar to the different phenotypes occasionally reported in the literature, in some cases diagnosed as Lesch-Nyhan "variants." The clinical, biochemical, enzymatic, and molecular genetic studies on these 22 patients allowed us to delineate a new classification of HPRT deficiency. Based on the neurologic symptoms, dependency for personal care, HPRT activity in hemolysate and in intact erythrocytes, and predicted protein size, patients were classified into 4 groups: Group 1 (2 patients), normal development with no neurologic symptoms, HPRT activity was detectable in hemolysates and in intact erythrocytes, and the mutation did not affect the predicted protein size. Group 2 (3 patients) mild neurologic symptoms that did not prevent independent lives, HPRT activity was detectable in intact erythrocytes, and the protein size was normal. Group 3 (2 patients), severe neurologic impairment that precluded an independent life, no residual HPRT activity, and normal protein size. Group 4 (15 patients), clinical characteristics of Lesch-Nyhan syndrome (some may not show self-injurious behavior), no residual HPRT activity, and in most (7 of 8 patients in whom the mutation could be detected) the mutation affected the predicted protein size. This classification of HPRT deficiency into 4 groups may be more useful in terms of accuracy, reproducibility, assessment for treatment trials and prognosis. The study of this Spanish series allows us to conclude that HPRT deficiency may be manifested by a wide spectrum of neurologic symptoms; the overall severity of the disease is associated with mutations permitting some degree of residual enzyme activity; and mutation analysis provides a valuable tool for prognosis, carrier identification, and prenatal diagnosis.

Effect of eprosartan and losartan on uric acid metabolism in patients with essential hypertension.

OBJECTIVE: The influence of angiotensin II AT-1 receptor antagonists on uric acid metabolism, and the potential differences among them with regard to this effect, remains to be precisely established. This study was designed to compare the effects of losartan and eprosartan on uric acid metabolism in patients with mild to moderate essential hypertension. DESIGN: Randomized, double-blind, parallel-group study in hypertensive patients. SETTING: Outpatient clinic. PATIENTS: Following a 2- to 3-week single-blind placebo run-in period, 60 patients with sitting diastolic blood pressure > or = 95 and < or = 114 mmHg were randomized. Fifty-eight patients completed the study. INTERVENTIONS: Patients were randomized to receive losartan 50 mg or eprosartan 600 mg once daily for 4 weeks. MAIN OUTCOME MEASURES: The primary endpoint was the change in the ratio of urinary uric acid/creatinine in the period 0-4 h of a 24 h urine collection after 4 weeks of treatment. Secondary endpoints included 24 h urinary uric acid excretion, as well as serum urate and anti-hypertensive efficacy. RESULTS: Mean urinary uric acid/creatinine changes from baseline were 0.14 (day 1) and 0.11 (week 4) for losartan and -0.04 for eprosartan (at both day 1 and week 4; P < 0.01 between groups at both time-points). The mean increase in 24 h urinary uric acid excretion with losartan was 0.7 mmol/24 h (25% increase from baseline) at both day 1 and week 4. No significant difference was observed in the change of serum urate levels versus baseline between both treatment groups after 4 weeks (- 23.4 and - 19.5 micromol/l for losartan and eprosartan, respectively). Patients with hyperuricaemia in both treatment groups showed similar modifications of uric acid metabolism compared with non-hyperuricaemic subjects. Blood pressure control (sitting diastolic blood pressure < 90 mmHg or < 100 mmHg with a decrease of at least 10 mmHg from baseline) was achieved in 22 patients (73%) with eprosartan and in 16 (53%) with losartan. CONCLUSIONS: Losartan increased uric acid excretion in hypertensive patients, whilst eprosartan did not Neither AT-1 receptor antagonist substantially modified serum urate concentrations.

Purine metabolism in female heterozygotes for hypoxanthine-guanine phosphoribosyltransferase deficiency.

BACKGROUND: Female carriers of the X-linked recessive disorder hypoxanthine-guanine phosphoribosyltransferase (HPRT) deficiency show somatic cell mosaicism, and this may cause an increased synthesis of purines. We have examined whether urinary oxypurines could be useful for carrier diagnosis. METHODS: Carrier testing was performed in 35 women belonging to 16 unrelated Spanish families with at least one subject affected by the Lesch-Nyhan syndrome (11 families, 14 patients) or the Kelley-Seegmiller syndrome (five families, six patients) by means of HPRT and adenine phosphoribosyltransferase activities in hair follicles and/or molecular studies. Plasma and 24-h urinary concentrations of hypoxanthine, xanthine and uric acid were measured while subjects were on a purine-restricted diet. RESULTS: Mean plasma urate concentrations and 24-h urinary hypoxanthine, xanthine and uric acid excretion rates were significantly higher in 22 heterozygotes than in 13 non-carriers (P < 0.02). Daily urinary oxypurine excretion rates were also significantly higher in heterozygotes than in 12 normal women (P = 0.0011). Cumulative 5-day radioactivity excretion after [8-14C]-adenine infusion was markedly increased in 10 carrier women compared with five normal women (P = 0.0369). The sensitivity of 24-h urinary hypoxanthine and xanthine excretion rates was 86% and 77%, respectively, and the specificity 100% for both tests. CONCLUSION: Female heterozygotes for HPRT deficiency show an enhanced purine nucleotide degradation and purine overproduction. An elevated hypoxanthine and/or xanthine excretion rate differentiated most heterozygotes for HPRT deficiency from non-carrier women and thus could be useful for carrier diagnosis.