Rosa J. Torres

Attenuated variants of Lesch-Nyhan disease

Lesch–Nyhan disease is a neurogenetic disorder caused by deficiency of the enzyme hypoxanthine–guanine phosphoribosyltransferase. The classic form of the disease is described by a characteristic syndrome that includes overproduction of uric acid, severe generalized dystonia, cognitive disability and self-injurious behaviour. In addition to the classic disease, variant forms of the disease occur wherein some clinical features are absent or unusually mild. The current studies provide the results of a prospective and multi-centre international study focusing on neurological manifestations of the largest cohort of Lesch–Nyhan disease variants evaluated to date, with 46 patients from 3 to 65 years of age coming from 34 families. All had evidence for overproduction of uric acid. Motor abnormalities were evident in 42 (91%), ranging from subtle clumsiness to severely disabling generalized dystonia. Cognitive function was affected in 31 (67%) but it was never severe. Though none exhibited self-injurious behaviours, many exhibited behaviours that were maladaptive. Only three patients had no evidence of neurological dysfunction. Our results were compared with a comprehensive review of 78 prior reports describing a total of 127 Lesch–Nyhan disease variants. Together these results define the spectrum of clinical features associated with hypoxanthine–guanine phosphoribosyltransferase deficiency. At one end of the spectrum are patients with classic Lesch–Nyhan disease and the full clinical phenotype. At the other end of the spectrum are patients with overproduction of uric acid but no apparent neurological or behavioural deficits. Inbetween are patients with varying degrees of motor, cognitive, or behavioural abnormalities. Recognition of this spectrum is valuable for understanding the pathogenesis and diagnosis of all forms of hypoxanthine–guanine phosphoribosyltransferase deficiency.

Genotype-phenotype correlations in neurogenetics: Lesch-Nyhan disease as a model disorder

Establishing meaningful relationships between genetic variations and clinical disease is a fundamental goal for all human genetic disorders. However, these genotype-phenotype correlations remain incompletely characterized and sometimes conflicting for many diseases. Lesch-Nyhan disease is an X-linked recessive disorder that is caused by a wide variety of mutations in the HPRT1 gene. The gene encodes hypoxanthine-guanine phosphoribosyl transferase, an enzyme involved in purine metabolism. The fine structure of enzyme has been established by crystallography studies, and its function can be measured with very precise biochemical assays. This rich knowledge of genetic alterations in the gene and their functional effect on its protein product provides a powerful model for exploring factors that influence genotype-phenotype correlations. The present study summarizes 615 known genetic mutations, their influence on the gene product, and their relationship to the clinical phenotype. In general, the results are compatible with the concept that the overall severity of the disease depends on how mutations ultimately influence enzyme activity. However, careful evaluation of exceptions to this concept point to several additional genetic and non-genetic factors that influence genotype-phenotype correlations. These factors are not unique to Lesch-Nyhan disease, and are relevant to most other genetic diseases. The disease therefore serves as a valuable model for understanding the challenges associated with establishing genotype-phenotype correlations for other disorders.

Prevalence of Left ventricular hypertrophy in patients with mild hypertension in primary care: Impact of echocardiography on cardiovascular risk stratification

BACKGROUND Left ventricular hypertrophy (LVH) is an important predictor of cardiovascular risk, and its detection contributes to risk stratification. The aims of the present study were to estimate the prevalence of echocardiographic LVH and to evaluate the influence of echocardiography (ECHO) on cardiovascular risk stratification in hypertensive patients presenting in primary care. METHODS In this cross-sectional study, 250 patients recently diagnosed with mild hypertension underwent clinical evaluation including electrocardiography (ECG), microalbuminuria measurement, 24-h blood pressure monitoring and ECHO. Level of cardiovascular risk was stratified, initially using routine procedures including ECG to assess target organ damage and then again after detection of LVH by ECHO. RESULTS The frequency of echocardiographic LVH was 32%, substantially higher than that detected by ECG (9%). Initial cardiovascular risk stratification yielded the following results: 30% low risk, 49% medium risk, 16% high risk, and 5% very high risk subjects. The detection of LVH by ECHO provoked a significant change in the risk strata distribution, particularly in those patients initially classified as being at medium risk. In this group, 40% of subjects were reclassified as high risk subjects according to ECHO information. The new classification was as follows: 23% low risk, 30% medium risk, 42% high risk, and 5% very high risk subjects. CONCLUSIONS A substantial proportion of mildly hypertensive patients presenting in primary care have LVH determined by ECHO. Our results suggest that this procedure could significantly improve cardiovascular risk stratification in those patients with multiple risk factors, but no evidence of target organ damage by routine investigations.

Metabolic syndrome: prevalence, associated factors, and C-reactive protein: the MADRIC (MADrid RIesgo Cardiovascular) Study.

The metabolic syndrome (MS) is defined by the clustering of a number of cardiovascular risk factors. The aims of the present study were to estimate the prevalence of MS in Madrid (Spain) by 2 definitions and to investigate its relationship with several sociodemographic factors and C-reactive protein (CRP) levels. This was a cross-sectional population study, and participants were 1344 subjects aged 31 to 70 years. Clinical evaluation included data on sociodemographic and cardiovascular background, physical examination, fasting glucose, triglycerides, and high-density lipoprotein cholesterol. The CRP levels were determined in a subgroup of 843 subjects. The diagnosis of MS was made according to the 2005 Adult Treatment Panel III (ATP III) and International Diabetes Federation (IDF) definitions. The age- and sex-adjusted prevalence of MS was 24.6% (95% confidence interval [CI], 22.3%-26.9%) using the ATP III definition and 30.9% (95% CI, 28.4%-33.3%) using the International Diabetes Federation definition. The overall agreement rate was 91.5% (kappa = 0.80; 95% CI, 0.76-0.83). Prevalence figures by both definitions were higher in men than in women and increased with age. Male sex, older age, low educational level, and physical inactivity were all determinants of ATP III-defined MS. The presence of MS or any of its components was associated with high CRP levels. In a logistic regression analysis, low educational level and waist circumference were the best predictors for high CRP level. The prevalence of MS in the Madrid region is one of the highest in Europe and confirms the strong Spanish regional variability in this syndrome frequency. Some sociodemographic and lifestyle factors, particularly educational level, are predictors for MS and high CRP levels.

Update on the Phenotypic Spectrum of Lesch-Nyhan Disease and its Attenuated Variants

Congenital deficiency of the enzyme hypoxanthine-guanine phosphoribosyltransferase (HPRT) results in a spectrum of clinical phenotypes. All of these phenotypes are associated with marked overproduction of uric acid and related problems such as hyperuricemia, urate nephrolithiasis, tophi, and gout. The mildest phenotypes include only problems related to overproduction of uric acid. The most severe phenotype is known as Lesch-Nyhan disease, in which the phenotype also includes severe motor handicap, intellectual disability, and self-injurious behavior. In between these two extremes is a continuous spectrum of phenotypes with varying degrees of motor and cognitive handicap but no self-injurious behavior. The pathogenesis of overproduction of uric acid in HPRT deficiency is well-understood, and treatments are available to control it. The pathogenesis of the neurobehavioral problems is less well-understood, and effective treatments for them are lacking.

Serum Urate, Metabolic Syndrome, and Cardiovascular Risk Factors. A Population-Based Study

We studied the associations between serum urate levels (determined in 503 subjects from a population of 1,344 subjects living in northern Madrid) and both the metabolic syndrome (MS) (defined by the Adult Treatment Panel III criteria) and C-reactive protein (CRP, determined in 382 subjects). MS was diagnosed in 25% (95%CI, 21–28%) and was associated with hyperuricemia (p<0.001). There was a graded increase in serum urate levels with increasing number of MS components. Urate concentrations significantly correlated with waist circumference (r=0,455, p<0.01). Serum urate was not independently associated with CRP levels. This study shows that serum urate levels are associated with the presence of MS and each of its features.

Uric acid excretion in healthy subjects: a nomogram to assess the mechanisms underlying purine metabolic disorders

The reference range for urinary uric acid excretion has not been precisely defined. Different urinary variables have been proposed to determine the renal contribution to increased or decreased serum urate concentrations. We examined which urinary variable best indicates uric acid excretion over a wide range of serum urate concentrations. Purine metabolism was studied in 10 healthy male subjects (aged 26-58 years) both at their endogenous normal serum urate levels (normouricemic state) and after the oral administration of allopurinol (300 mg/24 h for 5 days) and ribonucleic acid (4 g/8 h for 4 days) to decrease (hypouricemic state) and increase (hyperuricemic state) serum urate concentrations, respectively. The results from patients with several conditions known to affect uric acid synthesis and/or the renal excretion of uric acid were used to validate a constructed nomogram. Over a wide range of mean serum urate levels (from 2.7 to 9.5 mg/dL) and mean 24-hour urinary uric acid excretion (171 to 1368 mg/[24 h 1.73 m(2)]), the highest correlation coefficient between serum urate and uric acid excretion was obtained for the 24-hour uric acid determination (r = 0.928; P < .001). The constructed nomogram allowed the definition of the mechanism underlying hyperuricemia and hypouricemia in patients with a myriad of diseases known to affect purine metabolism. The urinary variable that best correlates with a wide range of serum urate concentrations is 24-hour urinary uric acid excretion. The constructed nomogram allows the identification of the kidney contribution to a given purine metabolic abnormality.

Normal HPRT coding region in complete and partial HPRT deficiency.

Lesch-Nyhan syndrome is an X-linked recessive inborn error of metabolism due to a virtually complete lack of hypoxanthine-guanine phosphoribosyltransferase (HPRT) activity (OMIM 300322). Partial deficiency of HPRT (OMIM 300323) is characterized by the effects of excess uric acid synthesis and a continuum spectrum of neurological manifestations, without the manifestations of full-blown Lesch-Nyhan syndrome. Both diseases have been associated with mutations in the HPRT gene. These mutations are heterogeneous and disperse throughout the entire HPRT gene. In 2005 Dawson et al. described, for the first time, an individual with gout in whom HPRT deficiency appeared to be due to a defect in gene regulation. In the present study we present four patients with partial HPRT deficiency and one patient with Lesch-Nyhan syndrome who showed a normal HPRT coding sequence and markedly decreased HPRT mRNA expression. This is the first report of a patient with Lesch-Nyhan syndrome due to a defect in HPRT gene expression regulation.

Familial juvenile hyperuricaemic nephropathy (FJHN): linkage analysis in 15 families, physical and transcriptional characterisation of the FJHN critical region on chromosome 16p11.2 and the analysis of seven candidate genes

Familial juvenile hyperuricaemic nephropathy (FJHN) is an autosomal dominant renal disease characterised by juvenile onset of hyperuricaemia, gouty arthritis, and progressive renal failure at an early age. Recent studies in four kindreds showed linkage of a gene for FJHN to the same genomic interval on chromosome 16p11.2, where the gene for the phenotypically similar medullary cystic disease type 2 (MCKD2) has been localised. In this study we performed linkage analysis in additional 15 FJHN families. Linkage of FJHN to 16p11.2 was confirmed in six families, which suggests that, in a large proportion of FJHN kindreds, the disease is likely to be caused by a gene or genes located outside of 16p11.2. Haplotype analysis of the new and previously analysed families provided two non-overlapping critical regions on 16p11.2–FJHN1, delimited by markers D16S499-D16S3036 and FJHN2, delimited by markers D16S412-D16S3116. Considering MCKD2 to be a distinct molecular entity, the analysis suggests that as many as three kidney disease genes may be located in close proximity on 16p11.2. From genomic databases we compiled integrated physical and transcription maps of whole critical genomic region in which 45 known genes and 129 predicted loci have been localised. We selected, analysed and found no pathogenic mutations in seven candidate genes. The linkage and haplotype analysis reported here demonstrates the genetic heterogeneity of FJHN. The report of integrated physical and mostly in-silico predicted transcription maps of the FJHN critical region provides a basis for precise experimental annotation of the current transcript map, which is essential for final identification of the FJHN gene(s).

The Spectrum of Hypoxanthine-Guanine Phosphoribosyltransferase (HPRT) Deficiency

Summary The enzyme hypoxanthine-guanine phosphoribo-syltransferase (HPRT) catalyzes the reutilization ofhypoxanthine and guanine to the purine nucleotidesIMP and GMP, respectively. HPRT deficiency is an X-linked disorder characterized by uric acid over-production and variable neurologic impairment. Thecomplete deficiency of HPRT is diagnostic of Lesch-Nyhan syndrome manifested by choreoathetosis,spasticity, mental retardation, and self-injurious be-havior. In some HPRT-deficient patients the enzymedefect appeared to be “partial” and the neurologicsymptoms mild to severe (Kelley-Seegmiller syn-drome). This has prompted the classification ofHPRT deficiency in 2 distinct groups: Lesch-Nyhansyndrome and Kelley-Seegmiller syndrome, whichhas created much confusion. A spectrum of clinicalconsequences of HPRT deficiency has been recog-nized in small series of patients, but the completespectrum of the neurologic disorder has not been de-scribed in a single series of patients examined by thesame observers.We analyzed our experience with 22 patients be-longing to 18 different families with HPRT deficiencydiagnosed at “La Paz” University Hospital in Madridover the past 16 years. The clinical spectrum of theseHPRT-deficient Spanish patients was similar to thedifferent phenotypes occasionally reported in the lit-erature, in some cases diagnosed as Lesch-Nyhan“variants.” The clinical, biochemical, enzymatic, andmolecular genetic studies on these 22 patients al-lowed us to delineate a new classification of HPRTdeficiency. Based on the neurologic symptoms, de-pendency for personal care, HPRT activity in he-molysate and in intact erythrocytes, and predictedprotein size, patients were classified into 4 groups:Group 1 (2 patients), normal development with noneurologic symptoms, HPRT activity was detectablein hemolysates and in intact erythrocytes, and themutation did not affect the predicted protein size.Group 2 (3 patients) mild neurologic symptoms thatdid not prevent independent lives, HPRT activity wasdetectable in intact erythrocytes, and the protein sizewas normal. Group 3 (2 patients), severe neurologicimpairment that precluded an independent life, noresidual HPRT activity, and normal protein size.Group 4 (15 patients), clinical characteristics ofLesch-Nyhan syndrome (some may not show self-injurious behavior), no residual HPRT activity, and inmost (7 of 8 patients in whom the mutation could bedetected) the mutation affected the predicted pro-tein size.This classification of HPRT deficiency into 4groups may be more useful in terms of accuracy, re-producibility, assessment for treatment trials andprognosis. The study of this Spanish series allows usto conclude that HPRT deficiency may be manifestedby a wide spectrum of neurologic symptoms; theoverall severity of the disease is associated with mu-tations permitting some degree of residual enzymeactivity; and mutation analysis provides a valuable