Mutation Carrier Testing in Lesch-Nyhan Syndrome Families: HPRT Mutant Frequency and Mutation Analysis with Peripheral Blood T Lymphocytes

Genotypic and phenotypic spectrum in attenuated variants of Lesch-Nyhan disease

Lesch-Nyhan disease and its attenuated variants are caused by deficiency of the purine salvage enzyme, hypoxanthine-guanine phosphoribosyltransferase (HGprt). All patients exhibit excessive production of uric acid, which increases the risk for nephrolithiasis, renal failure, gouty arthritis and tophi. The mildest phenotype includes only problems related to overproduction of uric acid. The most severe clinical phenotype includes prominent neurological abnormalities and the universal feature is self-injurious behavior. In between the mildest and most severe syndromes is a broad spectrum of phenotypes with varying degrees of neurological, neurocognitive and behavioral abnormalities. The effect of HPRT1 gene mutations on residual HGprt enzyme activity is the most relevant factor contributing to disease phenotype. Attenuated clinical phenotypes are associated with residual enzyme function, whereas the most severe phenotype is usually associated with null activity. In cases of gouty arthritis with urate overproduction, a careful evaluation for motor impairments or neurocognitive abnormalities may help to identify attenuated variants of Lesch-Nyhan disease for better management.

Effects of calorie restriction on the age-dependent accumulation of mutations in the small intestine of lacZ-transgenic mice

To understand the effect of calorie restriction on genome maintenance systems, the age-dependent accumulation of mutations in animals maintained on high and low calorie diets was examined using lacZ-transgenic mice. Mice were fed a diet of 95 kcal/w or 65 kcal/w from 2 to 17 months of age. The mutation frequencies in the lacZ gene in epithelial tissues from the small intestine were examined at 12 and 17 months. Mutation frequencies were found to be lower in mice fed with a low calorie diet than in mice fed with a high calorie diet at the two age points. The molecular nature of the mutations was examined with DNA sequencing. It showed a predominance of transversions from G:C to T:A, and this is a typical type of mutation induced by reactive oxygen species. The fraction of this type of mutation among the different types of mutations detected was not affected by calorie restriction. The percentage of the other types of mutation was not influenced either. These results suggest that calorie restriction reduces the age-dependent accumulation of mutations by stimulating or inducing various types of DNA protection and repair systems rather than protecting cells against any specific type of DNA alteration.

Hypoxanthine-guanine phosophoribosyltransferase (HPRT) deficiency: Lesch-Nyhan syndrome

Deficiency of hypoxanthine-guanine phosphoribosyltransferase (HPRT) activity is an inborn error of purine metabolism associated with uric acid overproduction and a continuum spectrum of neurological manifestations depending on the degree of the enzymatic deficiency. The prevalence is estimated at 1/380,000 live births in Canada, and 1/235,000 live births in Spain. Uric acid overproduction is present inall HPRT-deficient patients and is associated with lithiasis and gout. Neurological manifestations include severe action dystonia, choreoathetosis, ballismus, cognitive and attention deficit, and self-injurious behaviour. The most severe forms are known as Lesch-Nyhan syndrome (patients are normal at birth and diagnosis can be accomplished when psychomotor delay becomes apparent). Partial HPRT-deficient patients present these symptoms with a different intensity, and in the least severe forms symptoms may be unapparent. Megaloblastic anaemia is also associated with the disease. Inheritance of HPRT deficiency is X-linked recessive, thus males are generally affected and heterozygous female are carriers (usually asymptomatic). Human HPRT is encoded by a single structural gene on the long arm of the X chromosome at Xq26. To date, more than 300 disease-associated mutations in the HPRT1 gene have been identified. The diagnosis is based on clinical and biochemical findings (hyperuricemia and hyperuricosuria associated with psychomotor delay), and enzymatic (HPRT activity determination in haemolysate, intact erythrocytes or fibroblasts) and molecular tests. Molecular diagnosis allows faster and more accurate carrier and prenatal diagnosis. Prenatal diagnosis can be performed with amniotic cells obtained by amniocentesis at about 15–18 weeks' gestation, or chorionic villus cells obtained at about 10–12 weeks' gestation. Uric acid overproduction can be managed by allopurinol treatment. Doses must be carefully adjusted to avoid xanthine lithiasis. The lack of precise understanding of the neurological dysfunction has precluded development of useful therapies. Spasticity, when present, and dystonia can be managed with benzodiazepines and gamma-aminobutyric acid inhibitors such as baclofen. Physical rehabilitation, including management of dysarthria and dysphagia, special devices to enable hand control, appropriate walking aids, and a programme of posture management to prevent deformities are recommended. Self-injurious behaviour must be managed by a combination of physical restraints, behavioural and pharmaceutical treatments.

HPRT deficiency in a two-month-old child presenting acute renal failure and gout with a new deletion of two bases in exon 3 of the HPRT gene

We describe an HPRT deficiency in a 2-month-old child who presented acute renal failure and gout with normal mental and motor development for age. The patient was diagnosed with Lesch-Nyhan disease and showed a new mutation, a deletion of two bases in exon 3 of the HPRT gene (c.269-270delAT).

Real-time PCR and linkage studies to identify carriers presenting HPRT deleted gene

Lesch-Nyhan syndrome (LNS) is an X-linked genetic disorder resulting in hyperuricemia, choreoathetosis, mental retardation, and self-injurious behavior. It is caused by loss of activity of the ubiquitous enzyme hypoxanthine-guanine-phosphoribosyltransferase (HPRT). The biochemical analysis of residual HPRT activity in patients' red blood cells is the first step in LNS diagnosis, and it precedes molecular study to discover the specific mutation. Unfortunately, biochemical diagnosis of healthy carriers is difficult because HPRT enzymatic activity in blood cells is similar in LNS carriers and in healthy people; genetic tests can help reveal mutations at the genomic or cDNA level, whereas gross deletions involving the first or last exons of HPRT gene are not detectable. Until now, a test based on 6-thioguanine-resistant phenotype of HPRT mutant cells from LNS patients is the only method accepted for the diagnosis of any kind of mutation in carriers. In this work, we introduce a new approach to identify carriers of large deletions in HPRT gene using real-time PCR. Results were validated in a blinded manner with a linkage study and with results obtained in Italian families previously analyzed with selective medium test. Real-time PCR analysis clearly confirmed the results obtained by selective medium; linkage data strengthened real time results, allowing us to follow the allele with the mutated HPRT through the family pedigree. We hope that the real-time PCR approach will provide a useful and reliable method to diagnose LNS carriers of large deletions in HPRT gene.

Eighteen-year follow-up of a patient with partial hypoxanthine phosphoribosyltransferase deficiency and a new mutation

Hypoxanthine phosphoribosyltransferase (HPRT) deficiency is an inherited disorder. Complete deficiency of HPRT activity is phenotypically expressed as the devastating Lesch-Nyhan syndrome. Partial HPRT deficiency usually causes hyperuricemia, precocious gout, and uric acid nephrolithiasis. We describe an 18-year follow-up of a 5-year old boy with partial HPRT deficiency and report a novel mutation in his HPRT gene. He presented with overproduction of uric acid and passage of uric acid renal stones, and without gout or neurological and behavioral abnormalities. Treatment with allopurinol, adequate hydration, urinary alkalization, and a low-purine diet was started. No subsequent nephrolithiasis has occurred. After 18-year of this therapy his physical and neuropsychological status were normal, merely his glomerular filtration rate (GFR, normal 97-137 mL min(-1)/1.73 m(2)) fell from normal to 65.1 mL min(-1). The most likely cause of initial renal impairment in our patient is uric and/or xanthine crystalluria. A missense and transition mutation 169A>G (57ATG>GTG, 57met>val) in exon 3 of the patient's HPRT gene was identified and the mother was the carrier of the mutation. As far as we are aware, the identified mutation has not previously been reported. We named the mutant HPRT Maribor.

Lesch-Nyhan disease in a female with a clinically normal monozygotic twin

Lesch-Nyhan disease (LND) is an inborn error of purine metabolism caused by defective activity of the enzyme hypoxanthine guanine phosphoribosyl transferase (HPRT, EC 2.4.2.8), resulting from mutation in the corresponding gene on the long arm of the X chromosome (Xq26). The classic phenotype occurs almost exclusively in males and is characterized by hyperuricemia, mental retardation, severe dystonia, and self-injurious behavior. Heterozygous carrier females are usually clinically normal. However, a small number of clinically affected females have been described. In all previous cases there was a mutation in one HPRT allele and non-random inactivation of the X chromosome carrying the normal HPRT gene. We have analyzed a female MZ twin pair discordant for Lesch-Nyhan disease. The mother and both twins are heterozygous carriers of a HPRT splicing mutation (IVS8 + 4A > G; c.609 + 4A > G) and all three express the mutant allele at similar frequencies in peripheral blood T cells. The mother and one sister are clinically normal. In the affected twin, the clinical phenotype is classical for Lesch-Nyhan disease, despite the fact that HPRT activity in the blood was also normal. X inactivation analysis showed a skewed pattern in the fibroblasts of the affected twin sister, with the X chromosome carrying the normal HPRT allele preferentially inactivated. As in many other reported cases of X-linked diseases, the discordant phenotype of the two monozygous twin sisters suggests that the process responsible for monozygotic twinning can trigger skewed X inactivation.