Fine structure of the human hypoxanthine phosphoribosyltransferase gene

The diagnosis and treatment of disorders of nucleic acid/nucleotide metabolism associated with epilepsy

Epilepsy is a prevalent paroxysmal disorder in the field of neurology. Among the six etiologies of epilepsy, metabolic causes are relatively uncommon in clinical practice. Metabolic disorders encompass amino acid metabolism disorders, organic acid metabolism disorders, and other related conditions. Seizures resulting from nucleic acid/nucleotide metabolism disorders are even more infrequent. This review provides an overview of several studies on nucleic acid/nucleotide metabolism disorders associated with epilepsy, including adenosine succinate lyase deficiency, Lesch-Nyhan syndrome, and aminoimidazole carboxamide ribonucleotide transformylase/inosine monophosphate cyclohydrolase (ATIC) deficiency, among others. The potential pathogenesis, phenotypic features, diagnostic pathways, and therapeutic approaches of these diseases are discussed in this review. The goal is to help clinicians make an accurate diagnosis when encountering rare nucleic acid/nucleotide metabolism disorders with multi-system symptoms and manifestations of epilepsy.

Potential molecular link between the β-amyloid precursor protein (APP) and hypoxanthine-guanine phosphoribosyltransferase (HGprt) enzyme in Lesch-Nyhan disease and cancer

Lesch-Nyhan disease (LND) is a rare X-linked inherited neurogenetic disorders of purine metabolic in which the cytoplasmic enzyme, hypoxanthine-guanine phosphoribosyltransferase (HGprt) is defective. Despite having been characterized over 60 years ago, however, up to now, there is no satisfactory explanation of how deficits in enzyme HGprt can lead to LND with the development of the persistent and severe self-injurious behavior. Recently, a role for epistasis between the mutated hypoxanthine phosphoribosyltransferase 1 (HPRT1) and the β-amyloid precursor protein (APP) genes affecting the regulation of alternative APP pre-mRNA splicing in LND has been demonstrated. Furthermore, there were also some reported cases of LND developing thrombosis while APP is an important regulator of vein thrombosis and controls coagulation. Otherwise, the surface expression of HGprt enzyme was also observed in several somatic tissue cancers while APP and the APP-like protein-2 (APLP2) are deregulated in cancer cells and linked to increased tumor cell proliferation, migration, and invasion. The present review provides a discussion about these findings and suggests a potential molecular link between APP and HGprt via epistasis between HPRT1 and APP genes affecting the regulation of alternative APP pre-mRNA splicing. As a perspective, expression vectors for HGprt enzyme and APP are constructed as described in Ref. # 24 (Nguyen KV, Naviaux RK, Nyhan WL (2020) Lesch-Nyhan disease: I. Construction of expression vectors for hypoxanthine-guanine phosphoribosyltransferase (HGprt) enzyme and amyloid precursor protein (APP). Nucleosides Nucleotides Nucleic Acids 39: 905–922), and they could be used as tools for clarification of these issues. In addition, these expression vectors, especially the one with the glycosyl-phosphatidylinositol (GPI) anchor can be used as a model for the construction of expression vectors for any protein targeting to the cell plasma membrane for studying intermolecular interactions and could be therefore useful in the vaccines as well as antiviral drugs development (studying intermolecular interactions between the spike glycoprotein of the severe acute respiratory syndrome coronavirus 2, SARS-CoV-2, as well as its variants and the angiotensin-converting enzyme 2, ACE2, in coronavirus disease 2019 (COVID-19) [43],[44], for example).

Lesch-Nyhan disease: I. Construction of expression vectors for hypoxanthine-guanine phosphoribosyltransferase (HGprt) enzyme and amyloid precursor protein (APP)

Lesch-Nyhan disease (LND) is a rare X-linked inherited neurogenetic disorder of purine metabolism in which the enzyme, hypoxanthine-guanine phosphoribosyltransferase (HGprt) is defective. Despite having been characterized over 50 years ago, it remains unclear precisely how deficits in HGprt enzyme activity can lead to the neurological syndrome, especially the self-injury of LND. Several studies have proposed different hypotheses regarding the etiology of this disease, and several treatments have been tried in patients. However, up to now, there is no satisfactory explanation of the disease and for many LND patients, efficacious treatment for persistent self-injurious behavior remains unreachable. A role for epistasis between mutated hypoxanthine phosphoribosyltransferase 1 (HPRT1) and amyloid precursor protein (APP) genes has been recently suggested. This finding may provide new directions not only for investigating the role of APP in neuropathology associated with HGprt-deficiency in LND but also for the research in neurodevelopmental and neurodegenerative disorders in which the APP gene is involved in the pathogenesis of diseases and may pave the way for new strategies applicable to rational antisense drugs design. It is therefore necessary to study the HGprt enzyme and APP using expression vectors for exploring their impacts on LND as well as other human diseases, especially the ones related to APP such as Alzheimer's disease in which the physiologic function and the structure of the entire APP remain largely unclear until now. For such a purpose, we report here the construction of expression vectors as the first step (Part I) of our investigation.

Phenotypic and molecular spectrum of Korean patients with Lesch-Nyhan syndrome and attenuated clinical variants

Lesch-Nyhan syndrome (LNS) is an X-linked recessive disorder caused by mutations in the HPRT1 gene. The clinical features and mutation spectrum of 26 Korean LNS patients from 23 unrelated families were retrospectively reviewed. The HPRT1 gene was analyzed by direct sequencing of genomic DNA. The median age at diagnosis was 2.3 years (range, 4 months-22.6 years) and the initial presenting features included developmental delay, orange colored urine, and self-injurious behaviors. Most patients were wheelchair-bound and suffered from urinary complications and neurologic problems such as self-mutilation and developmental delay. Twenty different mutations in HPRT1 were identified among 23 independent pedigrees, including six novel mutations. The most common mutation type was truncating mutations including nonsense and frameshift mutations (45%). Large deletions in the HPRT1 gene were identified in exon 1, exons 5-6, exons 1-9, and at chr X:134,459,540-134,467,241 (7702 bp) including the 5'-untranslated region, exon 1, and a portion of intron 1. In conclusion, this study describes the phenotypic spectrum of LNS and has identified 20 mutations from 23 Korean families, including six novel mutations in Korean patients with LNS.

Understanding the structure-function relationship of HPRT1 missense mutations in association with Lesch-Nyhan disease and HPRT1-related gout by in silico mutational analysis

The nucleotide salvage pathway is used to recycle degraded nucleotides (purines and pyrimidines); one of the enzymes that helps to recycle purines is hypoxanthine guanine phosphoribosyl transferase 1 (HGPRT1). Therefore, defects in this enzyme lead to the accumulation of DNA and nucleotide lesions and hence replication errors and genetic disorders. Missense mutations in hypoxanthine phosphoribosyl transferase 1 (HPRT1) are associated with deficiencies such as Lesch-Nyhan disease and chronic gout, which have manifestations such as arthritis, neurodegeneration, and cognitive disorders. In the present study, we collected 88 non-synonymous single nucleotide polymorphisms (nsSNPs) from the UniProt, dbSNP, ExAC, and ClinVar databases. We used a series of sequence-based and structure-based in silico tools to prioritize and characterize the most pathogenic and stabilizing or destabilizing nsSNPs. Moreover, to obtain the structural impact of the pathogenic mutations, we mapped the mutations to the crystal structure of the HPRT protein. We further subjected these mutant proteins to a 50 ns molecular dynamics simulation (MDS). The MDS trajectory showed that all mutant proteins altered the structural conformation and dynamic behavior of the HPRT protein and corroborated its association with LND and gout. This study provides essential information regarding the use of HPRT protein mutants as potential targets for therapeutic development.

Novel mutation in the human HPRT1 gene and the Lesch-Nyhan disease

Lesch-Nyhan disease (LND) is a rare X-linked inherited neurogenetic disorder of purine metabolism in which the enzyme, hypoxanthine-guanine phosphoribosyltransferase (HGprt) is defective. The authors report a novel point mutation that led to HGprt-related neurological dysfunction (HND) in a family in which there was a missense mutation in exon 6 of the coding region of the HPRT1 gene: g.34938G>T, c.403G>T, p.D135Y. Molecular diagnosis is consistent with the genetic heterogeneity of the HPRT1 gene responsible for HGprt deficiency. It allows fast, accurate carrier detection and genetic counseling.

Lesch-Nyhan disease in two families from Chiloé Island with mutations in the HPRT1 gene

Lesch-Nyhan disease (LND) is a rare X-linked inherited neurogenetic disorder of purine metabolism in which the enzyme, hypoxanthine-guanine phosphoribosyltransferase (HGprt) is defective. The authors report two independent point mutations leading to splicing errors: IVS 2 +1G>A, c.134 +1G>A, and IVS 3 +1G>A, c.318 +1G>A in the hypoxanthine-phosphoribosyltransferase1 (HPRT1) gene which result in exclusion of exon 2 and exon 3 respectively, in the HGprt enzyme protein from different members of two Chiloé Island families. Molecular analysis has revealed the heterogeneity of genetic mutation of the HPRT1 gene responsible for the HGprt deficiency. It allows fast, accurate carrier detection and genetic counseling.

Human HPRT1 gene and the Lesch-Nyhan disease: Substitution of alanine for glycine and inversely in the HGprt enzyme protein

Lesch-Nyhan disease (LND) is a rare X-linked inherited neurogenetic disorder of purine metabolism in which the enzyme, hypoxanthine-guanine phosphoribosyltransferase (HGprt) is defective. The authors report three novel independent mutations in the coding region of the HPRT1 gene from genomic DNA of (a) a carrier sister of two male patients with LND: c.569G>C, p.G190A in exon 8; and (b) two LND affected male patients unrelated to her who had two mutations: c.648delC, p.Y216X, and c.653C>G, p.A218G in exon 9. Molecular analysis reveals the heterogeneity of genetic mutation of the HPRT1 gene responsible for the HGprt deficiency. It allows fast, accurate detection of carriers and genetic counseling.

Mutation in the Human HPRT1 Gene and the Lesch-Nyhan Syndrome

Lesch-Nyhan syndrome (LNS) is a rare X-linked inherited neurogenetic disorder of purine metabolism in which the enzyme, hypoxanthine-guanine phosphoribosyltransferase (HGprt) is defective. The authors report a novel mutation which led to HGprt-related neurological dysfunction (HND) in two brothers from the same family with a missense mutation in exon 6 of the coding region of the HPRT1 gene: c.437T>C, p.L146S. Molecular diagnosis discloses the genetic heterogeneity of the HPRT1 gene responsible for HGprt deficiency. It allows fast, accurate carrier detection and genetic counseling.

Lesch-Nyhan Syndrome in a Family with a Deletion Followed by an Insertion within the HPRT1 Gene

Lesch-Nyhan syndrome (LNS) is a rare X-linked inherited neurogenetic disorder of purine metabolism in which the enzyme, hypoxanthine-guanine phosphoribosyltransferase(HGprt) is defective. The authors report a novel mutation which led to LNS in a family with a deletion followed by an insertion (INDELS) via the serial replication slippage mechanism: c.428_432delTGCAGinsAGCAAA, p.Met143Lysfs*12 in exon 6 of HPRT1 gene. Molecular diagnosis discloses the genetic heterogeneity of HPRT1 gene responsible for HGprt deficiency. It allows fast, accurate carrier detection and genetic counseling.

Epigenetic Regulation in Amyloid Precursor Protein with Genomic Rearrangements and the Lesch-Nyhan Syndrome

Recently, epigenetic regulation of alternative APP pre-mRNA splicing in the Lesch-Nyhan syndrome (LNS) has been studied (see Ref. 7) and showed for the first time, the presence of several APP-mRNA isoforms encoding divers APP protein isoforms ranging from 120 to 770 amino acids (with or without mutations and/or deletions). Here, by continuing on this work, I identified, for the first time new APP-mRNA isoforms with a deletion followed by an insertion (INDELS) in LNS and LNVs patients: c.19_2295delinsG166TT…GAGTCC…CTTAGTC…TCT489,p.Leu7Valfs*2;c.19_2295 delinsG169TT…GAGACC…CTTGGTC…TCT492,p.Leu7Valfs*2;and c.16_2313delinsG84CC…CAT616,p.Leu7Hisfs*45. A role of genomic rearrangements of APP gene via the Fork Stalling and Template Switching (FoSTeS) mechanism leading to INDELS was suggested. Epistasis between mutated HPRT1 and APP genes could be one of the factors of epigenetic modifications responsible for genomic rearrangements of APP gene. My findings accounted for epigenetic mechanism in the regulation of alternative APP pre-mRNA splicing as well as for epigenetic control of genomic rearrangements of APP gene may provide therefore new directions not only for investigating the role of APP in neuropathology associated with HGprt-deficiency in LNS and LNVs patients but also for the research in neurodevelopmental and neurodegenerative disorders by which APP gene involved in the pathogenesis of the diseases such as autism, fragile X syndrome (FXS), and Alzheimer's disease (AD) with its diversity and complexity, especially for sporadic form of AD (SAD). An accurate quantification of various APP-mRNA isoforms in brain tissues for detection of initial pathological changes or pathology development is needed and antisense drugs are the potential treatments.

The human β-amyloid precursor protein: biomolecular and epigenetic aspects

Beta-amyloid precursor protein (APP) is a membrane-spanning protein with a large extracellular domain and a much smaller intracellular domain. APP plays a central role in Alzheimer’s disease (AD) pathogenesis: APP processing generates β-amyloid (Aβ) peptides, which are deposited as amyloid plaques in the brains of AD individuals; point mutations and duplications of APP are causal for a subset of early-onset familial AD (FAD) (onset age <65 years old). However, these mutations in FAD represent a very small percentage of cases (∼1%). Approximately 99% of AD cases are nonfamilial and late-onset, i.e., sporadic AD (SAD) (onset age >65 years old), and the pathophysiology of this disorder is not yet fully understood. APP is an extremely complex molecule that may be functionally important in its full-length configuration, as well as the source of numerous fragments with varying effects on neural function, yet the normal function of APP remains largely unknown. This article provides an overview of our current understanding of APP, including its structure, expression patterns, proteolytic processing and putative functions. Importantly, and for the first time, my recent data concerning its epigenetic regulation, especially in alternative APP pre-mRNA splicing and in the control of genomic rearrangements of the APP gene, are also reported. These findings may provide new directions for investigating the role of APP in neuropathology associated with a deficiency in the enzyme hypoxanthine-guanine phosphoribosyltransferase (HGprt) found in patients with Lesch-Nyhan syndrome (LNS) and its attenuated variants (LNVs). Also, these findings may be of significance for research in neurodevelopmental and neurodegenerative disorders in which the APP gene is involved in the pathogenesis of diseases such as autism, fragile X syndrome (FXS) and AD, with its diversity and complexity, SAD in particular. Accurate quantification of various APP-mRNA isoforms in brain tissues is needed, and antisense drugs are potential treatments.

Small Duplication of HPRT 1 Gene May Be Causative For Lesh-Nyhan Disease in Iranian Patients

Deficiency of hypoxanthine-guanine phosphoribosyltransferase (HGPRT) is a rare inborn error of purine metabolism and is characterized by uric acid overproduction along with a variety of neurological manifestations that depend on a degree of the enzymatic deficiency. Inheritance of HPRT deficiency is X-linked recessive; thus, males are generally more affected and heterozygous females are carriers (usually asymptomatic). Human HPRT is encoded by a single structural gene on the long arm of the X chromosome at Xq26. More than 300 mutations in the HPRT1 gene have been detected. Diagnosis can be based on clinical and biochemical findings as well as enzymatic and molecular testing. Molecular diagnosis is the best way as it allows for faster and more accurate carrier and prenatal diagnosis. In this report, a new small duplication in the HPRT1 gene was found by sequencing, which has yet to be reported.

Treatment of Lesch-Nyhan disease with S-adenosylmethionine: experience with five young Malaysians, including a girl

BACKGROUND

Lesch-Nyhan disease (LND) is a rare X-linked recessive neurogenetic disorder caused by deficiency of the purine salvage enzyme hypoxanthine phosphoribosyltransferase (HPRT, EC 2.4.2.8) which is responsible for recycling purine bases into purine nucleotides. Affected individuals have hyperuricemia leading to gout and urolithiasis, accompanied by a characteristic severe neurobehavioural phenotype with compulsive self-mutilation, extrapyramidal motor disturbances and cognitive impairment.

AIM

For its theoretical therapeutic potential to replenish the brain purine nucleotide pool, oral supplementation with S-adenosylmethionine (SAMe) was trialed in 5 Malaysian children with LND, comprising 4 related Malay children from 2 families, including an LND girl, and a Chinese Malaysian boy.

RESULTS

Dramatic reductions of self-injury and aggressive behaviour, as well as a milder reduction of dystonia, were observed in all 5 patients. Other LND neurological symptoms did not improve during SAMe therapy.

DISCUSSION

Molecular mechanisms proposed for LND neuropathology include GTP depletion in the brain leading to impaired dopamine synthesis, dysfunction of G-protein-mediated signal transduction, and defective developmental programming of dopamine neurons. The improvement of our LND patients on SAMe, particularly the hallmark self-injurious behaviour, echoed clinical progress reported with another purine nucleotide depletion disorder, Arts Syndrome, but contrasted lack of benefit with the purine disorder adenylosuccinate lyase deficiency. This first report of a trial of SAMe therapy in LND children showed remarkably encouraging results that warrant larger studies.

Epigenetic regulation in amyloid precursor protein and the Lesch-Nyhan syndrome

Lesch-Nyhan syndrome (LNS) is a neurogenetic disorder of purine metabolism in which the enzyme, hypoxanthine-guanine phosphoribosyltransferase (HPRT) is defective. A major unsolved question is how the loss of HPRT enzyme function affects the brain to cause the neurobehavioural syndrome in LNS and its attenuated variants (LNVs). To address this issue, a search for a link between LNS and the amyloid precursor protein (APP) is developed. Here, I identified, for the first time in fibroblasts from normal subjects as well as from LNS and LNV patients: (a) several APP-mRNA isoforms encoding divers APP protein isoforms ranging from 120 to 770 amino acids (with or without mutations and/or deletions) accounted for epigenetic mechanisms in the regulation of alternative APP pre-mRNA splicing and (b) five novel independent polymorphisms in the APP promoter: -956A>G, -1023T>C, -1161A>G, -2224G>A, -2335C>T relative to the transcription start site. A role for epistasis between mutated HPRT and APP genes affecting the regulation of alternative APP pre-mRNA splicing in LNS is suggested. An accurate quantification of various APP isoforms in brain tissues for detection of initial pathological changes or pathology development is needed. My findings may provide new directions not only for investigating the role of APP in neuropathology associated with HPRT-deficiency in LNS but also for the research in neurodevelopmental and neurodegenerative disorders by which various APP isoforms involved in the pathogenesis of the diseases such as Alzheimer's disease.

RNA stability in human liver: comparison of different processing times, temperatures and methods

The accuracy of information garnered by real-time quantitative polymerase chain reaction (RT-qPCR), an important technology for elucidating molecular mechanisms of disease, is dependent on tissue quality. Thus, this study aimed to determine the effects of intra-operative manipulation, extended processing times, different temperatures or storage in RNAlater on RNA quality in liver samples for tissue banking. Liver samples, flash-frozen or in RNAlater, were collected over a time course (during surgery before blood arrest up to 1 day after surgery) with samples kept either at room temperature (RT) or on ice. This study showed that at the longest time-point at RT, the RNA quality decreased significantly by 20%. However, relative gene expressions of FOS, GUSB, MYC, HIF1α and GFER were in general not significantly different when the time-points were compared. In conclusion, samples should be kept on ice during processing, and either RNAlater or snap-freezing should be utilised for storage. Further, intra-operative manipulation and extended postoperative processing time generally does not change relative gene expression levels for the 5 genes studied, making such sampling suitable for RT-qPCR analysis. Thus, if relative gene expression of a gene of interest is stable, these guidelines will lead to increased accrual of samples to the tissue bank.

Lesch-Nyhan syndrome: mRNA expression of HPRT in patients with enzyme proven deficiency of HPRT and normal HPRT coding region of the DNA

Inherited mutation of the purine salvage enzyme, hypoxanthine guanine phosphoribosyltransferase (HPRT) gives rise to Lesch-Nyhan syndrome (LNS) or Lesch-Nyhan variants (LNV). We report a case of two LNS affected members of a family with deficiency of activity of HPRT in intact cultured fibroblasts in whom mutation could not be found in the HPRT coding sequence but there was markedly decreased HPRT expression of mRNA.

Novel mutations in the human HPRT gene

Inherited mutation of a purine salvage enzyme, hypoxanthine guanine phosphoribosyltransferase (HPRT), gives rise to Lesch-Nyhan Syndrome (LNS) or HPRT-related gout. Here, we report five novel independent mutations in the coding region of the HPRT gene from five unrelated male patients manifesting different clinical phenotypes associated with LNS: exon 2: c.133A > G, p.45R > G; c.35A > C, p.12D > A; c.88delG; exon 7: c.530A > T, p.177D > V; and c.318 + 1G > C: IVS3 + 1G > C splice site mutation.

Next generation tools for high-throughput promoter and expression analysis employing single-copy knock-ins at the Hprt1 locus

We have engineered a set of useful tools that facilitate targeted single copy knock-in (KI) at the hypoxanthine guanine phosphoribosyl transferase 1 (Hprt1) locus. We employed fine scale mapping to delineate the precise breakpoint location at the Hprt1(b-m3) locus allowing allele specific PCR assays to be established. Our suite of tools contains four targeting expression vectors and a complementing series of embryonic stem cell lines. Two of these vectors encode enhanced green fluorescent protein (EGFP) driven by the human cytomegalovirus immediate-early enhancer/modified chicken beta-actin (CAG) promoter, whereas the other two permit flexible combinations of a chosen promoter combined with a reporter and/or gene of choice. We have validated our tools as part of the Pleiades Promoter Project (http://www.pleiades.org), with the generation of brain-specific EGFP positive germline mouse strains.

Human gene targeting favors insertions over deletions

Gene targeting is a powerful technique for manipulating the human genome, but few studies have directly compared the targeting frequencies of various types of vector constructs. Here we show that similar targeting constructs are able to insert nucleotides at the homologous chromosomal target locus more efficiently than they can delete nucleotides, and combination insertion/deletion vectors appear to target at intermediate frequencies. This holds true for deletions ranging from 1 to 334 bp and insertions ranging from 1 to 1332 bp. In addition, vectors designed to inactivate the human hypoxanthine phosphoribosyltransferase gene (HPRT) by deleting nucleotides often produced rearrangements at the target locus that in many cases were due to insertions of multimerized vector constructs, effectively converting a deletion vector into an insertion vector. These findings were obtained when adeno-associated virus vectors were used to efficiently deliver single-stranded DNA targeting constructs, but the same phenomenon was also observed when transfecting linearized double-stranded plasmids. Thus human cells distinguish between deletion and insertion vectors and process their recombination intermediates differently, presumably at the heteroduplex stage, with implications for the design of gene-targeting vectors and the evolution of human genomes.

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.

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.

Artificial chromosome-based transgenes in the study of genome function

The transfer of large DNA fragments to the mouse genome in the form of bacterial, yeast or phage artificial chromosomes is an important process in the definition of transcription units, the modeling of inherited disease states, the dissection of candidate regions identified by linkage analysis and the construction of in vivo reporter genes. However, as with small recombinant transgenes, the transferred sequences are usually integrated randomly often with accompanying genomic alterations and variable expression of the introduced genes due to the site of integration and/or copy number. Therefore, alternative methods of integrating large genomic transgenes into the genome have been developed to avoid the variables associated with random integration. This review encourages the reader to imagine the large variety of applications where artificial chromosome transgenes can facilitate in vivo and ex vivo studies in the mouse and provides a context for making the necessary decisions regarding the specifics of experimental design.

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.

Tissue-specific expression of a BAC transgene targeted to the Hprt locus in mouse embryonic stem cells

The hypoxanthine phosphoribosyltransferase (Hprt) locus has been shown to have minimal influence on transgene expression when used as a surrogate site in the mouse genome. We have developed a method to transfer bacterial artificial chromosomes (BACs) as a single copy into the partially deleted Hprt locus of embryonic stem cells. BACs were modified by Cre/loxP recombination to contain the sequences necessary for homologous recombination into and complementation of the partially deleted Hprt locus. Modified BACs were shown to undergo homologous recombination into the genome intact, to be stably transmitted through the germ line of transgenic mice, and to be expressed in the proper tissue-specific manner. This technology will facilitate many studies in which correct interpretation of data depends on developmentally appropriate transgene expression in the absence of rearrangements or deletions of endogenous DNA.

Distinct mechanisms lead to HPRT gene mutations in leukemic cells

Leukemias are considered malignant clonal disorders arising from the accumulation of mutations in hematopoietic cells; the majority of these mutations are thought to be acquired somatically. Measurement of mutation frequency (Mf) at the hypoxanthine phosphoribosyltransferase (HPRT) locus has been developed as a method for estimating genomic instability. We investigated the Mf in 16 leukemic cell lines to determine whether these cell lines showed evidence of genomic instability. Although some leukemic cell lines had markedly elevated Mfs, the Mfs at the HPRT locus in leukemic cell lines were not always higher than those of B-lymphoblastoid cell lines and T lymphocytes from normal individuals. We were able to identify the HPRT mutation for 159 of 160 individual HPRT mutants. The HPRT mutations were characterized at a molecular level and classified as either gross chromosomal rearrangements (GCRs) or point mutations, such as single-nucleotide substitutions, insertions, or deletions. With rare exceptions, individual leukemic cell lines showed either point mutations or GCR, but not both. Of note, all the cell lines that primarily showed point mutations are known to be defective in mismatch repair machinery.

A novel missense mutation, c.584A > C (Y195S), in two unrelated Argentine patients with hypoxanthine-guanine phosphoribosyl-transferase deficiency, neurological variant

The hypoxanthine-guanine phosphoribosyl-transferase (HPRT) deficiency is an inborn error of purine metabolism, responsible for classic Lesch-Nyhan disease and its neurological and hyperuricemic variants. We report a novel mutation in the HPRT gene, c.584A > C (Y195S), in two unrelated Argentine patients affected with the neurological variant with no HPRT activity in lysed erythrocytes. Using PCR plus DNA sequencing and/or restriction enzyme digestion we were able to confirm the diagnosis and identify new cases and potential carriers.

A recurrent large Alu-mediated deletion in the hypoxanthine phosphoribosyltransferase (HPRT1) gene associated with Lesch-Nyhan syndrome

We identified the identical large genomic deletion in the hypoxanthine phosphoribosyltransferase (HPRT1) gene in two Japanese patients with Lesch-Nyhan (LN) syndrome. This deletion spanned from an Alu sequence in the promoter region to another Alu-sequence in intron 1, a length of 2,969 base pairs including exon 1. In order to ask whether this deletion was a recurrent mutation, we developed a simple alternative method to determine the separate origin of the HPRT1 mutation of the patients as assessed with an apparent mtDNA polymorphism. Considering that an LN syndrome-causing mutation is not transmitted from patient to offspring as LN syndrome is a fatal disease in childhood and that mtDNA is maternally inherited, HPRT1 mutations and mtDNA would be co-transmitted from carrier mother to offspring since both appeared in females. Two bases were different in the hypervariable region I of the mtDNA between the two patients, indicating the separate origin of their mtDNA over at least several thousand years as calculated based on the molecular evolution rate in this region. We thus conclude that the identical deletion found in HPRT1 of the two patients was derived from recurrent events of genomic recombination. Given that the same Alu-mediated deletion of HPRT1 has not been reported among somatic mutations at the same locus, this region of the HPRT1 gene flanked by Alu-sequences is likely a mutational hot spot in the germline but not in somatic cells. In addition, we also report novel LN-syndrome-conferring mutations in intron 6 (IVS6+1G --> C) and intron 8 (IVS7-9T --> G) that resulted in exclusions of exon 6 and exon 8, respectively.

Introduction of single base substitutions at homologous chromosomal sequences by adeno-associated virus vectors

Adeno-associated virus (AAV) vectors can modify homologous chromosomal sequences at high rates. This gene targeting transduction pathway is distinct from the integrating and episomal pathways used in gene addition approaches. In previous studies, AAV vectors were used to introduce small insertion and deletion mutations at homologous chromosomal loci. Here we show that AAV-mediated gene targeting can also be used to introduce all possible types of single base substitution mutations at the endogenous single-copy hypoxanthine phosphoribosyl transferase locus. Southern blot and sequence analysis showed that the point mutations were introduced with high fidelity. We also show that AAV vectors can repair chromosomal alkaline phosphatase genes containing point mutations. Our results suggest that AAV vectors can be used to introduce single base substitutions at high frequencies in normal human cells, including the correction of point mutations responsible for genetic diseases.

Evidence that silencing of the HPRT promoter by DNA methylation is mediated by critical CpG sites

The strong correlation between promoter hypermethylation and gene silencing suggests that promoter methylation represses transcription. To identify methylation sites that may be critical for maintaining repression of the human HPRT gene, we treated human/hamster hybrid cells containing an inactive human X chromosome with the DNA demethylating agent 5-azadeoxycytidine (5aCdr), and we then examined the high resolution methylation pattern of the HPRT promoter in single cell-derived lines. Reactivation of HPRT correlated with complete promoter demethylation. In contrast, the 61 5aCdr-treated clones that failed to reactivate HPRT exhibited sporadic promoter demethylation. However, three specific CpG sites remained methylated in all unreactivated clones, suggesting these sites may be critical for maintaining transcriptional silencing of the HPRT gene. Re-treatment of partially demethylated (and unreactivated) clones with a second round of 5aCdr did not increase the frequency of HPRT reactivation. This is consistent with mechanisms of methylation-mediated repression requiring methylation at specific critical sites and argues against models invoking overall levels or a threshold of promoter methylation. Treatment of cells with the histone deacetylase inhibitor, trichostatin A, failed to reactivate HPRT on the inactive X chromosome, even when the promoter was partially demethylated by 5aCdr treatment, suggesting that transcriptional repression by DNA methylation is unlikely to depend upon a trichostatin A-sensitive histone deacetylase.

Mutations induced in the HPRT gene by X-irradiation during G(1) or S: analysis of base pair alterations, small deletions, and splice errors

Reverse transcriptase PCR was performed with mRNA obtained from HPRT mutants that had base pair alterations, or small deletions or insertions <20bp. The frequencies of mutants yielding RT-PCR products (mRNA) were the same when human EJ30 cells were irradiated in G(1) or S (3-4-fold higher for 6 than 3Gy). However, the frequencies of mutants that did not yield RT-PCR products were approximately 10-fold higher in the cells irradiated in G(1) than in those irradiated in S. Sequence analysis of RT-PCR products and genomic DNA showed that 40% of the RT-PCR products had splice errors (one or more exons not spliced into mRNA), with 64% of them due to 1-17bp deletions. Also, the distributions of molecular alterations in exons, acceptor sites, and donor sites for mutants having splice errors (observed in this study and reported by others) were similar to those reported for mutants not yielding RT-PCR products (isolated from Russian cosmonauts). In addition, we have found previously that large deletions which eliminated 1-9 exons were preferentially induced in G(1). Therefore, we postulate that the preferential induction of mutants not yielding mRNA is due primarily to splice errors that result from deletions preferentially induced during G(1). These splice errors would then result either in no message or a message that is rapidly degraded.

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.

Genetic disorders and urolithiasis

A recent analysis of the McKusick's On-Line Mendelian Inheritance in Man (OMIM) database revealed over 30 genetic or putatively genetic conditions in which urolithiasis contributes to the disease pathology at least to some extent. There is wide clinical, biochemical, and genetic heterogeneity in many of these conditions.

Red blood cell enzymes and their clinical application

Red blood cell enzyme activities are measured mainly to diagnose hereditary nonspherocytic hemolytic anemia associated with enzyme anomalies. At least 15 enzyme anomalies associated with hereditary hemolytic anemia have been reported. Some nonhematologic disease can also be diagnosed by the measurement of red blood cell enzyme activities in the case in which enzymes of red blood cells and the other organs are under the same genetic control. Progress in molecular biology has provided a new perspective. Techniques such as the polymerase chain reaction and single-strand conformation polymorphism analysis have greatly facilitated the molecular analysis of erythroenzymopathies. These studies have clarified the correlation between the functional and structural abnormalities of the variant enzymes. In general, the mutations that induce an alteration of substrate binding site and/or enzyme instability might result in markedly altered enzyme properties and severe clinical symptoms.

Human gene targeting by viral vectors

Stable transduction of mammalian cells typically involves random integration of viral vectors by non-homologous recombination. Here we report that vectors based on adeno-associated virus (AAV) can efficiently modify homologous human chromosomal target sequences. Both integrated neomycin phosphotransferase genes and the hypoxanthine phosphoribosyltransferase gene were targeted by AAV vectors. Site-specific genetic modifications could be introduced into approximately 1% of cells, with the highest targeting rates occurring in normal human fibroblasts. These results suggest that AAV vectors could be used to introduce specific genetic changes into the genomic DNA of a wide variety of mammalian cells, including therapeutic gene targeting applications.

Phosphate depletion enhances the stability of the amphotropic murine leukemia virus receptor mRNA

Through its specific receptor, the amphotropic murine leukemia virus (MLV) infects cells from many mammals, including humans. We have previously demonstrated that levels of human amphotropic MLV receptor (pit2) mRNA varied considerably in different human cell lines. Removal of phosphate from the culture medium led to increases in the amount of pit2 mRNA and the quantity of a 71-kDa protein specifically recognized by antibodies against Pit2. To determine if the increases in pit2 mRNA and protein levels were due to a transcriptional effect, the pit2 promoter region was cloned. This region was characterized and found to contain a functional TATA-less promoter that under our experimental conditions does not respond to phosphate depletion. Instead, pit2 mRNA was found to be more stable in response to Pi depletion. These results suggest that the increase in pit2 mRNA levels observed in response to Pi depletion occurs at a posttranscriptional level and is due to enhanced mRNA stability.

Replication timing properties of the human HPRT locus on active, inactive and reactivated X chromosomes

X chromosome inactivation is associated with a highly asynchronous pattern of DNA replication at most X-linked loci in females. We studied the human HPRT locus, which is subject to X inactivation and expressed from only the active homolog, with the goal of comparing replication properties between the active and inactive homologs in this region using a fluorescence in situ hybridization approach. We found that in normal female lymphoblasts this locus is replicated in a highly asynchronous manner across a broad, discrete 500-600 kb zone with earliest replication appearing at the gene coding sequence. This general timing profile is maintained in normal male lymphoblasts, as well as in hamster x human hybrid cells containing the active human X chromosome. However, the inactive human X chromosome in the hamster cell background does not appear to function in a fully equivalent manner to the normal inactive X chromosome in female cells. Furthermore, reactivation of the inactive human X chromosome in a hamster x human hybrid system by 5-azacytidine treatment and HAT selection restores early replication at the HPRT gene itself, but does not change the overall domain behavior.

Comparison of the frequency of T-cell receptor mutants and thioguanine resistance induced by X-rays and ethylnitrosourea in cultured human blood T-lymphocytes

We have investigated two assays for measuring the induction of mutations using human T-lymphocytes isolated from leukocyte residue buffy coats obtained from normal donors. Variant cell frequency of T-cells defective in the T-cell receptor (TCR) gene expression was measured using a 2-color flow cytometry, and 6-thioguanine-resistant (TGr) cells were determined using a cloning technique at the HPRT gene after treatment with 250 kVp X-rays or ethylnitrosourea (ENU). The frequencies of TCR mutant cells as well as those of TGr cells increased with increasing doses of X-rays or concentrations of ENU studied. For TCR mutants, the induced mutation frequencies at D37 (giving 37% survival) were 31.7 x 10(-4) and 11.0 x 10(-4) for X-rays and ENU, respectively. For TGr T-cells, the induced mutation frequencies at D37 for the same mutagens were 14.4 x 10(-6) and 75.5 x 10(-6), respectively. Over the dose range studied the relationship appears to be linear between the mutation induction of TCR and that of TGr for X-rays or ENU. However, X-rays may induce more TCR mutants against less induction TGr T-cells, and ENU may cause a reverse result. The sensitivity of the assay of each biological endpoint in human blood T-lymphocytes may be different.

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.

Mutation induction in gamma-irradiated primary human bronchial epithelial cells and molecular analysis of the HPRT- mutants

We have examined various radiobiological parameters using commercially-available primary normal human bronchial epithelial (NHBE) cells, which can be subcultured more than 20 population doublings, and have established the mutation system in order to characterize the molecular changes in gamma-irradiated primary cells. The survival curve, obtained after irradiation of cells with 137Cs gamma-rays, indicates that the D0, Dq, and n values are 1.34 Gy, 1.12 Gy, and 2.3, respectively. The induction of HPRT- mutation was dose-dependent and the mutant fraction increased in a non-linear fashion. Since the doubling number of NHBE cells is limited, DNA was extracted directly from the single mutant colonies and alteration in the HPRT gene locus was analyzed using multiplex PCR technique. Among spontaneous mutants, the proportion with total and partial deletions of the gene was 10.0% (2/20) and 60.0% (12/20), respectively, while 30.0% (6/20) did not have any detectable changes in the nine exons examined. On the other hand, the fraction of total deletion increased by more than 2-fold among mutants induced by gamma-rays in that 26.3% (10/38) of them showed the total gene deletions. Twenty-five out of 38 gamma-induced mutants (65.8%) had partial deletions and 3 mutants (7.9%) had no detectable alteration. The present results showed that gamma-irradiation efficiently induced HPRT gene mutation in primary human epithelial cells and that most of the induced mutants suffered larger deletions compared to that observed in spontaneous mutants. This system provides an useful tool for determination of mutagenicity and understanding the molecular mechanisms of environmental carcinogens in primary human bronchial cells.

A conserved downstream element defines a new class of RNA polymerase II promoters

Although many TATA-less promoters transcribed by RNA polymerase II initiate transcription at multiple sites, the regulation of multiple start site utilization is not understood. Beginning with the prediction that multiple start site promoters may share regulatory features and using the P-glycoprotein promoter (which can utilize either a single or multiple transcription start site(s)) as a model, several promoters with analogous transcription windows were grouped and searched for the presence of a common DNA element. A downstream protein-binding sequence, MED-1 (Multiple start site Element Downstream), was found in the majority of promoters analyzed. Mutation of this element within the P-glycoprotein promoter reduced transcription by selectively decreasing utilization of downstream start sites. We propose that a new class of RNA polymerase II promoters, those that can utilize a distinctive window of multiple start sites, is defined by the presence of a downstream MED-1 element.

Ke 6 gene. Sequence and organization and aberrant regulation in murine polycystic kidney disease

Ke 6 gene is a newly identified gene located in the major histocompatibility complex and is a candidate steroid dehydrogenase gene because of structural homology and regulatory similarities with mammalian steroid dehydrogenases. We report here the complete nucleotide sequence and intron-exon organization of the Ke 6 gene and cloning of the alternatively spliced Ke 6b transcript. We find that Ke 6 gene expression is down-regulated in pcy mice which is a murine model of polycystic kidney disease (PKD). Thus far, Ke 6 gene expression is down-regulated in all murine models of PKD we have examined. Abnormal steroid metabolism as a possible cause of PKD is discussed.

Expression of the human Achaete-scute 1 gene in olfactory neuroblastoma (esthesioneuroblastoma)

Olfactory neuroblastoma (ONB) is a rare neuronal malignancy of the olfactory mucosal. Markers used in the diagnosis of ONB do not distinguish ONB from other neuronal tumors or tumors with neuroendocrine features thus making the diagnosis of ONB difficult. Using a modified RT-PCR technique, we show that the human homologue of the Drosophila achaete-scute gene HASH1 is expressed in 6 primary and one metastatic ONB specimens, whereas Olfactory Marker Protein (OMP) is not. Previous studies have shown that HASH1 is expressed in immature olfactory neurons and is required for their development. OMP, whose function is unknown, is expressed exclusively in mature olfactory neurons. Together, these data suggest that ONB is derived from immature olfactory neurons of neuroectodermal origin. Analysis of RNA expression in primary tumor specimens and in an established cell line make this an ideal system to study olfactory growth and differentiation. Furthermore, these studies represent the first molecular genetic analysis of this rare and unusual neuronal tumor.

Southern analysis reveals a large deletion at the hypoxanthine phosphoribosyltransferase locus in a patient with Lesch-Nyhan syndrome

Whole genomic hprt clones were used in Southern analysis to screen the integrity of the hprt gene in a family that includes a patient with HPRT enzyme deficiency causal to Lesch-Nyhan syndrome. A 5 kb DNA sequence deletion was found to have its endpoints in the first and third introns. The probes identified the carrier status of female family members, aided by an RFLP carried by the mother's normal X-chromosome.

A new retroviral vector for detecting mutations and chromosomal instability in mammalian cells

A retroviral vector carrying both forward (neo) and backward (herpes simplex virus thymidine kinase or HSV-TK gene) selection markers was constructed as a substrate for mutational assay in mammalian cells. The cells infected with this virus are first selected with G418, mutagenized and then selected with the anti-herpes drug acyclovir (ACV). Since HSV-TK, but not the host TK, is capable of converting ACV to a toxic metabolite, cells retaining the intact HSV-TK gene fail to survive, while the cells carrying a mutated HSV-TK gene or which have lost the gene can form colonies in the presence of ACV, making it possible to detect the genetic defects in a positive manner. It is also possible to discriminate between small mutations and large deletions by checking the presence of the linked marker, neo. As a model experiment, we prepared an uncloned pool of rat fibroblast cells (CREF) infected with this virus and irradiated them with increasing doses of ultraviolet light. Dose-dependent increases in the number of ACV-resistant colonies were observed. Structural analysis of the HSV-TK gene in these clones revealed point mutations or small deletions in the majority of the cases. Since it requires no pre-existing genetic markers in the host cells, this system may be used for a wide variety of mammalian cells and provides a useful tool to assess both their susceptibility to various mutagens and their genomic instability.

Pulsed field analysis of hprt T-cell large deletions: telomeric region breakpoint spectrum

In order to determine a large deletion breakpoint spectrum, 25 independent hprt T-lymphocyte mutants with deletions extending from hprt into the telomeric or centromeric flanking chromosomal region were analyzed by pulsed field gel electrophoresis (PFGE). PFGE was used to determine deletion sizes which allowed localization of breakpoints external to hprt to specific chromosomal positions in mutants containing an intra-hprt breakpoint. A breakpoint spectrum based on 19 large deletion mutants is reported for the Xq26 chromosomal region telomeric to hprt. A potential cluster of breakpoints (4/19) was observed approximately 60 kb from hprt. In addition, maximum recoverable deletion size was at least 3.5 Mb. Three of the 25 mutants analyzed appeared to be complex deletion events.

5'-flanking sequences of the human HPRT gene direct neuronal expression in the brain of transgenic mice

Total deficiency of hypoxanthine phosphoribosyltransferase (HPRT) in humans causes the neurological disorder Lesch-Nyhan syndrome. The HPRT gene is expressed at basal levels in all tissues but at higher levels in the brain, the relevance and mechanism of which is unknown. To determine if cis-acting DNA elements play a role in the tissue-differential pattern of expression, we generated transgenic mice carrying different sequences of the human HPRT (hHPRT) promoter fused to the bacterial lacZ gene. We show that a 1.6 kb fragment of the hHPRT promoter contains essential information to direct beta-galactosidase expression preferentially to the basal ganglia, cerebral cortex, hippocampus, and several other areas of the forebrain. At least two elements within the 1.6 kb fragment appear to be required for neuronal expression. A 182 bp element (hHPRT-NE) represents one of these sequences and is involved not only in conferring neuronal specificity but also in repressing transgene expression in non-neuronal tissues. These studies provide molecular insight into the mechanism of increased HPRT expression in the brain.

Sequence analysis of the promoter region of the rat somatostatin receptor subtype 1 gene

Somatostatin receptor (SSTR) subtype genes are differentially expressed in brain and various peripheral tissues. RNA blotting and semiquantitative PCR analyses have revealed low levels of SSTR1 mRNA in the gastrointestinal tract and relatively high levels in GH3 anterior pituitary cells. As a first step in the investigation of the regulation of SSTR1 gene expression, we isolated a genomic fragment that contains the promoter region and determined the transcriptional initiation site. The SSTR1 gene lacks introns and TATA and CAAT motifs, but possesses several consensus recognition sequences for the transcription factors GCF and AP-2. The presence, also, of two Pit-1 binding sites could explain the high SSTR1 mRNA levels in GH3 cells.

Clonality in chronic myeloproliferative disorders defined by X-chromosome linked probes: demonstration of heterogeneity in lineage involvement

The restriction fragment length polymorphisms (RFLP) of the X-chromosome phosphoglycerate kinase (PGK) and hypoxanthine phosphoribosyltransferase (HPRT) genes were used to study the clonal basis of the chronic myeloproliferative disorders (CMPD). Analyses were performed on granulocyte and T-lymphocyte fractions obtained from 24 females; 13 had essential thrombocythaemia (ET), eight polycythaemia vera (PV) and three myelofibrosis with myeloid metaplasia (MMM). All 24 of these patients had monoclonal patterns of X-inactivation in the granulocyte fraction. For the T-lymphocyte fraction, non-clonal patterns of X-inactivation were observed in 8/13 patients with ET, 7/8 with PV and 1/3 with MMM, while the remaining eight subjects were found to have monoclonal patterns of X-inactivation. Our findings suggest that the majority of the CMPD in these patients originated from a relatively committed progenitor cell without the capacity to differentiate into T cells, and convincingly demonstrated heterogeneity of lineage involvement.