Molecular analysis of a female Lesch-Nyhan patient

Detailed genetic and clinical analysis of a novel de novo variant in HPRT1: Case report of a female patient from Saudi Arabia with Lesch-Nyhan syndrome

Background: Hypoxanthine-guanine phosphoribosyltransferase (HPRT1) deficiency is an inborn error of purine metabolism responsible for Lesch-Nyhan syndrome (LNS). The disease is inherited in an X-linked recessive manner and predominantly affects male individuals. Female individuals can carry a mutation as heterozygotes, but typically, they are asymptomatic because of the random inactivation of the affected allele. Nevertheless, although rare, heterozygote female individuals may manifest LNS with full characteristics. Herein, we describe a female patient from Saudi Arabia with LNS. Results: The patient (a 4-year-old girl) presented with typical characteristics of the disease, which include global developmental delay, self-mutilation, hyperuricemia, hypotonia, speech delay, spasticity, and seizures. Her general biochemical laboratory results were normal except for high levels of uric acid. The abdominal MRI\MRS, mostly unremarkable, showed bilateral echogenic foci within the renal collecting system. Genetic testing (whole-exome sequencing, iterative variant filtering, segregation analysis, and Sanger sequencing) pointed a novel de novo frameshift variant in HPRT1. X-inactivation assay using HpaII showed the presence of a 100% skewed X chromosome carrying the affected allele. RT-PCR of the cDNA indicated complete loss of the expression of the normal allele. Conclusion: Our study presents a female patient who has a severe case of LNSand found to be the 15th female patient with the disease in the world. The study emphasizethe need for a streamlined protocol that will help an early and accurate diagnosis of female LNS patients to avoid unnecessary interventions that lead to costly patient care.

Novel hypoxanthine guanine phosphoribosyltransferase gene mutations in Saudi Arabian hyperuricemia patients

Over the past decade, a steady increase in the incidence of HPRT-related hyperuricemia (HRH) has been observed in Saudi Arabia. We examined all the nine exons of HPRT gene for mutations in ten biochemically confirmed hyperuricemia patients, including one female and three normal controls. In all, we identified 13 novel mutations in Saudi Arabian HPRT-related hyperuricemia patients manifesting different levels of uric acid. The Lys103Met alteration was highly recurrent and was observed in 50% of the cases, while Ala160Thr and Lys158Asn substitutions were found in two patients. Moreover, in 70% of the patients ≥2 mutations were detected concurrently in the HPRT gene. Interestingly, one of the patients that harbored Lys103Met substitution along with two frameshift mutations at codons 85 and 160 resulting in shortened protein demonstrated unusually high serum uric acid level of 738 μmol/L. Two of the seven point mutations that resulted in amino acid change (Lys103Met and Val160Gly) were predicted to be damaging by SIFT and Polyphen and were further analyzed for their protein stability and function by molecular dynamics simulation. The identified novel mutations in the HPRT gene may prove useful in the prenatal diagnosis and genetic counseling.

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.

Hypoxanthine guanine phosphoribosyltransferase (HPRT) mutations in the Asian population

Mutation of hypoxanthine guanine phosphoribosyltransferase (HPRT) gives rise to Lesch-Nyhan syndrome, which is characterized by hyperuricemia, severe motor disability, and self-injurious behavior, or HPRT-related gout (Kelley-Seegmiller syndrome). The marked heterogeneity of HPRT deficiency is well known, with more than 300 mutations at the HPRT gene locus having been reported (deletions, insertions, duplications, abnormal splicing, and point mutations at different sites of the coding region from exons 1 to 9). We have identified mutations in Asian families with patients manifesting different clinical phenotypes, including rare cases of female subjects, by analyzing all nine exons of the HPRT gene (HPRT1) from genomic DNA and reverse-transcribed mRNA using the polymerase chain reaction technique coupled with direct sequencing. We developed suitable methods to detect the mutations identified from respective families with HPRT deficiency. Then, prenatal genetic diagnoses in HPRT-deficient families were carried out using both mRNA and genomic DNA from chorionic villi or amniotic fluid cells. As shown here in the heterogeneity of HPRT mutations, the spectrum of 70 mutations identified in the Asian population fits the four main conclusions that emerged previously from worldwide analysis.

Molecular analysis of hypoxanthine guanine phosphoribosyltransferase (HPRT) deficiencies: novel mutations and the spectrum of Japanese mutations

Inherited mutation of hypoxanthine guanine phosphoribosyltransferase, (HPRT) gives rise to Lesch-Nyhan syndrome or HPRT-related gout. We have identified a number of HPRT mutations in patients manifesting different clinical phenotypes, by analyzing all nine exons of the HPRT gene (HPRT1) from genomic DNA and reverse transcribed mRNA using the PCR technique coupled with direct sequencing. Recently, we detected two novel mutations: a single nucleotide substitution (430C > T) resulting in a nonsense mutation Q144X, and a deletion of HPRT1 exon 1 expressing no mRNA of HPRT. Furthermore, we summarized the spectrum of 56 Japanese HPRT mutations.

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.

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.

Molecular analysis of HPRT deficiencies: an update of the spectrum of Asian mutations with novel mutations

Inherited mutations of a purine salvage enzyme, hypoxanthine guanine phosphoribosyltransferase (HPRT, EC 2.4.2.8), give rise to Lesch-Nyhan syndrome or HPRT-related gout. We have identified a number of HPRT mutations in Asian patients manifesting different clinical phenotypes, by analyzing all nine exons of the HPRT gene (HPRT1) from genomic DNA and reverse-transcribed mRNA using the PCR technique coupled with direct sequencing. In this study, we update the spectrum of mutations with nine novel mutations. Two missense mutations (T124P and D185G) were detected in patients with HRH (HPRT-related hyperuricemia). In a patient having a severe partial deficiency of HPRT with neurological dysfunction (HRND: HPRT-related neurological dysfunction), a single nucleotide substitution (27+5G > A) causing a splicing error was found in intron 1. The mutation resulted in a remarkably decreased level of normal mRNA, and production of an abnormal mRNA with a 49-bp insert at the 5'-end of intron 1, which caused the frame-shift of an amino acid codon (10fs27X). In six typical Lesch-Nyhan families, we found two 3-bp deletions responsible for single amino acid deletions (V8del and Y28del), two 1-bp deletions (440delA and 635delG) generating a frame-shift, an insertion of two amino acids (159insKV), and a 4,131-bp deletion from introns 4 to 6 resulting in two types of abnormal mRNA. Including these nine mutations, 42 HPRT1 mutations have been identified among 47 Asian families with deficiency of HPRT.

Molecular, biochemical, and genetic characterization of a female patient with Lesch-Nyhan disease

Lesch-Nyhan disease (LND) is a rare X-linked recessive disorder caused by virtually complete deficiency of activity of the purine salvage enzyme hypoxanthine phosphoribosyltransferase (HPRT; EC 2.4.2.8). Human HPRT is encoded by a single structural gene located on the long arm of the X-chromosome (Xq26). The classical LND phenotype occurs almost exclusively in males, manifested in excessive purine production and characteristic neurological manifestations, including compulsive self-mutilation, choreoathetosis, spasticity, and occasionally developmental delay. Heterozygous females are usually phenotypically normal, due to the random inactivation of the X chromosome (Lyonization mechanism). However, six females were reported to be affected with the full biochemical and clinical manifestations of LND. All these cases were heterozygous for an HPRT mutation. Absence of transcription of the normal HPRT allele was attributed in all of them to non-random inactivation of the X chromosome carrying the normal allele. Here we describe an additional LND female, who presented with acute renal failure at the age of two months, in whom absence of transcription of the two HPRT alleles occurred due to as yet undescribed mechanism in LND females: the transcription of one HPRT allele was blocked due to a de novo X chromosome-autosome translocation 46,XX,t(X:2)(q26:p25), with a breaking point encompassing the HPRT gene locus, whereas the transcription of the normal allele was inhibited due to non-random inactivation of the second X-chromosome. Cultured fibroblasts from this patient exhibited the biochemical alterations in purine nucleotide metabolism characteristic of male LND fibroblasts.

Hereditary hyperuricemia and renal disease

Hyperuricemia and gout have long been known to run in families. As well as an apparently multifactorial genetic component to classic gout itself, 2 rather unusual sex-linked single-gene disorders of purine biosynthesis or recycling have been defined: deficiency of the enzyme hypoxanthine-guaninephosphoribosyl transferase (HPRT), and overactivity of PPriboseP synthase. Both result in overproduction of urate, hyperuricemia, and secondary overexcretion that may lead to acute or chronic renal damage. Familial juvenile hyperuricemic nephropathy (FJHN) and autosomal-dominant medullary cystic kidney disease (ADMCKD) are more common but less well-defined hyperuricemic conditions resulting from a decrease in the fractional excretion of filtered urate, with normal urate production. Although having features in common, ADMCKD is distinguished in particular by the presence of medullary cysts. One major group of both disorders is associated with mutations in the gene for uromodulin, but this accounts for only about one third of cases, and genetic heterogeneity is present. Whether the genes involved in these latter disorders contribute to the polygenic hyperuricemia and urate underexcretion of classic gout remains unexplored.

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.

Mutations in the hypoxanthine guanine phosphoribosyltransferase gene (HPRT1) in Asian HPRT deficient families

Inherited mutation of hypoxanthine guanine phosphoribosyltransferase, (HPRT) gives rise to Lesch-Nyhan syndrome or HPRT-related gout. We have identified 34 mutations in 28 Japanese, 7 Korean, and 1 Indian families with the patients manifesting different clinical phenotypes, including two rare cases in female subjects, by the analysis of all nine exons of HPRT from the genomic DNA and reverse transcribed mRNA using PCR technique coupled with direct sequencing.

Identification of a new point mutation in hypoxanthine phosphoribosyl transferase responsible for hyperuricemia in a female patient

A 29-year-old woman was referred to our department because of gout. Routine laboratory data showed hyperuricemia, a high level of plasma oxypurines, increased urinary uric acid excretion, and increased urinary oxypurine excretion, with decreased hypoxanthine phosphoribosyl transferase (HPRT) activity in the erythrocytes. From these findings, the patient was diagnosed with a partial deficiency of HPRT. To determine its properties, a cDNA sequence encoding HPRT and the androgen receptor AR XIST minimal promoter gene, as well as methylation of the AR gene were investigated. The HPRT cDNA sequence revealed a point mutation of G to A in nucleotide 40, which changed codon 14 from GAA (Glu) to AAA (Lys) in the mutant gene. In addition, the HPRT genomic DNA sequence, including the mutation site, revealed the same point mutation, indicating that the patient was heterozygote. Further analysis of the AR gene on the X chromosome suggested nonrandom X-chromosome inactivation, whereas the AR XIST minimal promoter gene was normal. Such results have not been previously reported in a female with partial HPRT deficiency.

Biochemical and molecular study of mentally retarded patient with partial deficiency of hypoxanthine-guanine phosphoribosyltransferase

Nucleotide metabolism was studied in erythrocytes of a mentally retarded child and family members. Partial hypoxanthine-guanine phosphoribosyltransferase (HPRT) deficiency was found in the propositus and an asymptomatic maternal uncle. Studies in crude lysates demonstrated decreased apparent V(max) and slightly decreased apparent K(m) for hypoxanthine in both HPRT-deficient subjects. Genomic DNA analysis revealed a single nucleotide change with leucine-147 to phenylalanine substitution in both subjects; mother and grandmother were heterozygous carriers of the same defect. This new variant has been termed HPRT(Potenza). Increased erythrocyte concentration of NAD and rate of synthesis by intact erythrocytes were found in the patient; increased activities of nicotinic acid phosphoribosyltransferase (NAPRT) and NAD synthetase (NADs) were demonstrated in erythrocyte lysates, with normal apparent K(m) for their substrates and increased V(max). These alterations were not found in any member of the family, including the HPRT-deficient uncle. These findings show multiple derangement of nucleotide metabolism associated with partial HPRT deficiency. The enzyme alteration was presumably not the cause of neurological impairment since no neurological symptoms were found in the HPRT-deficient uncle, whereas they were present in the propositus' elder brother who had normal HPRT activity.

The spectrum of inherited mutations causing HPRT deficiency: 75 new cases and a review of 196 previously reported cases

In humans, mutations in the gene encoding the purine salvage enzyme hypoxanthine-guanine phosphoribosyltransferase (HPRT) are associated with a spectrum of disease that ranges from hyperuricemia alone to hyperuricemia with profound neurological and behavioral dysfunction. Previous attempts to correlate different types or locations of mutations with different elements of the disease phenotype have been limited by the relatively small numbers of available cases. The current article describes the molecular genetic basis for 75 new cases of HPRT deficiency, reviews 196 previously reported cases, and summarizes four main conclusions that may be derived from the entire database of 271 mutations. First, the mutations associated with human disease appear dispersed throughout the hprt gene, with some sites appearing to represent relative mutational hot spots. Second, genotype-phenotype correlations provide no indication that specific disease features associate with specific mutation locations. Third, cases with less severe clinical manifestations typically have mutations that are predicted to permit some degree of residual enzyme function. Fourth, the nature of the mutation provides only a rough guide for predicting phenotypic severity. Though mutation analysis does not provide precise information for predicting disease severity, it continues to provide a valuable tool for genetic counseling in terms of confirmation of diagnoses, for identifying potential carriers, and for prenatal diagnosis.

An unexpected affected female patient in a classical Lesch-Nyhan family

Lesch-Nyhan disease is a genetic disorder of purine metabolism caused by defective activity of the enzyme hypoxanthine-guanine phosphoribosyl transferase (HPRT), resulting from mutation in the corresponding gene on the long arm of the X chromosome (Xq26). The classical phenotype, which includes spasticity, involuntary movements, developmental disability, and self-injurious behavior, occurs exclusively in males, while heterozygous, carrier females are clinically normal. We analyzed an Argentine family in which there were male and female siblings with clinically identical classic features of Lesch-Nyhan disease. The mother and an older daughter were carriers and had normal phenotypes. We identified the HPRT mutation in the family. It is a C --> T transition at position 508 of the cDNA (c.508 C --> T) that changes the CGA codon for Arg(169) to the TGA stop codon (R169X). The female patient was karyotypically normal and heterozygous for the mutation. She inherited the HPRT mutation from her mother, but she also had unexpected nonrandom inactivation of the paternal X chromosome carrying the normal HPRT gene. This additional genetic alteration is the cause of the clinical expression of disease in this female patient.

Hypoxanthine-guanine phosphoribosyltransferase deficiency and erythrocyte synthesis of pyridine coenzymes

Purine and pyridine metabolism were studied in ten Lesch-Nyhan patients, with virtually no hypoxanthine-guanine phosphoribosyltransferase (HPRT) activity in erythrocytes. Increased NAD erythrocyte concentrations were found in all patients. Raised activities of two enzymes catalysing NAD synthesis from nicotinic acid (nicotinic acid phosphoribosyltransferase: NAPRT, and NAD synthetase: NADs) was found in erythrocyte lysates from all patients. The two enzymes had normal apparent Km for their substrates and increased Vmax. The rate of synthesis of pyridine nucleotides from nicotinic acid by intact erythrocytes in vitro was also increased in most patients. These findings suggest that raised NAD concentrations in HPRT- erythrocytes are due to enhanced synthesis as a result of increased enzyme activities.

Brother/sister siblings affected with Hunter disease: evidence for skewed X chromosome inactivation

Hunter disease is an X-linked recessive disorder caused by a deficiency of iduronate-2-sulfatase activity. We describe a pair of brother/sister siblings with a typical feature of Hunter disease (mucopolysaccharidosis type II). They had normal karyotypes but a marked deficiency of iduronate-2-sulfatase activity in both lymphocytes and fibroblasts. The molecular analysis of the iduronate-2-sulfatase gene revealed the R468L(G1403-->T) substitution in their genes. Although the sister's genomic DNA was heterozygous for the mutant allele, the sister's cDNA was found to be homogeneous for this mutation. The mother was found to be a heterozygote. The analysis of X chromosome inactivation by comparison of the methylation patterns of the androgen-receptor (AR) gene which was isolated from the sister's fibroblasts and leucocytes revealed a skewed X chromosome inactivation of the paternal allele. These findings indicate that a skewed X chromosome inactivation of the paternal gene and a point mutation in the maternal gene were responsible for the lack of iduronate-2-sulfatase activity in the sister.

A novel de novo mutation in HPRT gene responsible for Lesch-Nyhan syndrome (HPRT OSAKA)

A virtually complete deficiency of hypoxanthine guanine phosphoribosyltransferase (HPRT) causes Lesch-Nyhan syndrome. A novel mutation of HPRT gene in a Japanese Lesch-Nyhan family has been identified using mRNA and genomic DNA from peripheral blood cells. A single nucleotide substitution of T to C in exon 3 resulted in a mis-sense mutation, CTC (Leu) to CCC (Pro), at codon 65. Utilizing an Mn/I restriction site which was lost in the mutation as an indicator, a family study showed that the mother was normal not having the mutant gene. The mutation was a de novo event that had occurred in the germ cells of the mother or in the proband during the early phase of fetal development.

Molecular analysis of a Japanese family with Lesch-Nyhan syndrome: identification of mutation and prenatal diagnosis

Complete deficiency of hypoxanthine guanine phosphoribosyltransferase (HPRT) causes Lesch-Nyhan syndrome. We examined the HPRT gene mutation for prenatal diagnosis in a Japanese family. A single nucleotide substitution of C to T in exon 3 was identified by direct sequencing analysis of the HPRT gene of a Lesch-Nyhan patient. This substitution resulted in a nonsense mutation, CGA (Arg) to TGA (stop), at codon 51. Utilizing an Xho I restriction site which was lost in the mutation as an indicator, a family study showed that the mother was heterozygous, but the grandmother normal. By the same method, prenatal genetic diagnosis was performed using chorionic villus samples (CVS), and showed that the fetus had the mutant allele.

Novel nonsense mutation in the hypoxanthine guanine phosphoribosyltransferase gene and nonrandom X-inactivation causing Lesch-Nyhan syndrome in a female patient

Lesch-Nyhan (LN) disease is a severe X-linked recessive neurological disorder associated with a loss of hypoxanthine guanine phosphoribosyltransferase activity (HPRT, EC 2.4.2.8). We have studied the second example of a female patient with LN disease. The molecular basis of HPRT deficiency in this patient was a previously undescribed nucleotide substitution in exon 6. In this gene, designated HPRT PARIS, a single nucleotide substitution from T to G at base position 558 changed a tyrosine (TAT) to a codon STOP (TAG) (Y153X). Analysis of the mother revealed a normal sequence of the HPRT cDNA and demonstrated that this mutation arose through a de novo gametic event. Allele-specific amplification of exon 6 from the patient's genomic DNA confirmed the single base substitution and showed that the patient was heterozygous for this mutation. Investigation of X-chromosomal inactivation by comparison of methylation patterns of patient's DNA isolated from fibroblasts, T lymphocytes, and polymorphonuclear cells digested with PstI and BstXI, with or without HpaII, and hybridized with M27 beta probe indicated a nonrandom pattern of X-chromosomal inactivation in which there was preferential inactivation of the maternal allele. The data indicate that nonrandom X-inactivation leading to selective inactivation of the maternal gene and a de novo point mutation in the paternal gene were responsible for the lack of HPRT activity in this patient.

Self-mutilation in a patient with mucolipidosis III

An 8-year-old girl with mucolipidosis III had self-mutilation of the distal phalanges of the second and third digits of her hands. She had neurophysiologic evidence of carpal tunnel syndrome, and consequent insensitivity to pain, which could explain the self-mutilation. Self-mutilation has not been previously described in lysosomal diseases in which carpal tunnel syndrome is frequently observed.

Molecular genetic study of a Japanese family with Lesch-Nyhan syndrome: a point mutation at the consensus region of RNA splicing (HPRTKeio)

Complete deficiency of hypoxanthine guanine phosphoribosyltransferase (HPRT) causes Lesch-Nyhan syndrome. A single nucleotide substitution of G to T at the 3'-end of intron 3 in the splicing consensus region has been identified in one allele of the HPRT gene from a mother predicted to be a heterozygous Lesch-Nyhan carrier. Utilizing a BfaI restriction site which was lost in the mutation as an indicator, family study showed that the mother and her only daughter were heterozygotes but the mother's sister did not have the mutant allele. The mutation generated splicing error and resulted in two types of abnormal mRNA. The major altered mRNA, named Type I, skipped the exon 4 and is predicted to produce a protein deleted of 22 amino acid residues. The other, Type II, having a 9-bp deletion at the 5'-end of exon 4, can result in a protein lacking 3 amino acids, from codon 107 to 109.

Molecular analysis of five independent Japanese mutant genes responsible for hypoxanthine guanine phosphoribosyltransferase (HPRT) deficiency

Five independent mutations in the hypoxanthine guanine phosphoribosyltransferase (HPRT) gene were identified in a partially HPRT deficient patient with gout and in four Lesch-Nyhan patients. Using the polymerase chain reaction (PCR) technique coupled with direct sequencing, the nucleotide sequences of the entire HPRT coding region amplified from the cDNA and also of each exon amplified form the genomic DNA were analyzed. Three independent point mutations in the coding region were detected in the partially HPRT deficient patient (Case 1) and in two Lesch-Nyhan patients (Case 2 and 3), resulting in single amino acid substitutions. The family study of Case 3, utilizing a PvuII restriction site created in the mutant gene, indicated that the mother was a heterozygote, and a sister and a fetal brother had inherited the normal HPRT gene from the mother. In two other mutants causing Lesch-Nyhan syndrome, a portion of the HPRT gene was deleted, and RNA splicing was missing in both mutants. A 4-bp deletion at the 5' end of exon 4 resulted in formation of three different types of abnormal mRNA (Case 4). The other mutant (Case 5) produced abnormal mRNA including 26 bp of intron 8 instead of the deleted 58 bp at the 5' end of exon 9, because of a 74-bp deletion from intron 8 to exon 9.

A review of the molecular basis of hypoxanthine-guanine phosphoribosyltransferase (HPRT) deficiency

Hypoxanthine-guanine phosphoribosyltransferase (HPRT, EC 2.4.2.8) is a purine salvage enzyme that catalyses the conversion of hypoxanthine and guanine to their respective mononucleotides. Partial deficiency of this enzyme can result in the overproduction of uric acid leading to a severe form of gout, whilst a virtual absence of HPRT activity causes the Lesch-Nyhan syndrome which is characterised by hyperuricaemia, mental retardation, choreoathetosis and compulsive self-mutilation. The HPRT-encoding gene is located on the X chromosome in the region q26-q27 and consists of nine exons and eight introns totalling 57 kb. This gene is transcribed to produce an mRNA of 1.6 kb, which contains a protein encoding region of 654 nucleotides. With the advent of increasingly refined techniques of molecular biology, it has been possible to study the HPRT gene of individuals with a deficiency in HPRT activity to determine the genetic basis of the enzyme deficiency. Many different mutations throughout the coding region have been described, but in the absence of precise information on the three-dimensional structure of the HPRT protein, it remains difficult to determine any consistent correlation between the structure and function of the enzyme.

Mutation analysis and prenatal diagnosis in a Lesch-Nyhan family showing non-random X-inactivation interfering with carrier detection tests

A nonsense mutation at the CpG-site in the codon for Arg(169) in the gene for hypoxanthine phosphoribosyltransferase (hprt) was identified by genomic polymerase chain reaction (PCR) and DNA sequencing in cultured fibroblasts from two brothers with Lesch Nyhan's syndrome. The recurrence of mutation at this CpG-site in several unrelated Lesch-Nyhan families suggests that deamination of 5-methylcytosine is a possible mechanism for mutagenesis. The level of hprt-mRNA in the fibroblasts of the patients was similar to that in healthy controls, whereas hprt-enzyme activity was not detectable. The mutation in this family was also identified in five female relatives and prenatally in a male fetus. Unexpectedly, results from hair follicle analyses and fibroblast selection studies in 8-azaguanine and 6-thioguanine medium showed a non-carrier phenotype in three of the female heterozygotes, whereas X-inactivation mosaicism was demonstrated in one heterozygote. A possible explanation for the apparent non-random X-inactivation in this family is the co-existence of the hprt mutation with an undefined X-linked lethal mutation. This observation is of practical relevance for carrier detection in other Lesch-Nyhan families.

A female patient with Lesch-Nyhan syndrome

The authors report the second case of a female with typical Lesch-Nyhan syndrome. She exhibited athetoid movement, self-multilation, mental retardation and spasticity. Laboratory investigations revealed hyperuricaemia, hyperuricosuria and decreased erythrocyte hypoxanthine guanine phosphoribosyl transferase activity. She has normal female external genitalia and karyotype. Her parents are non-consanguineous and there is no family member with gout, nephropathy or any psychoneurological disorder. To prevent self-stimulation, it was necessary to fix the patient's upper extremities to the backrest of her wheelchair. The authors also describe an apparatus that limits elbow flexion.

Alport syndrome, basement membranes and collagen

Alport syndrome, an inherited disorder of the kidney, eye and ear, has fascinated nephrologists, pathologists, and geneticists for nearly a century. With the recent application of molecular biochemical and genetic techniques, this mysterious disease has begun to yield some of its secrets. Alport syndrome can now be viewed as a generalized disorder of basement membranes that appears to result from mutations in an X-chromosome-encoded basement membrane collagen chain. This chain, along with two other novel collagen chains, is absent from Alport basement membranes, in contrast to the classical chains of collagen IV. Phenotypic heterogeneity in Alport syndrome probably arises from allelic mutations at a single genetic locus. The phenomenon of post-transplant anti-glomerular basement membrane nephritis may be a manifestation of specific mutations at the Alport locus that prevent synthesis of the gene's protein product and the establishment of immunological tolerance.