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

Analysis of Energy Metabolism and Lipid Spatial Distribution in Hypoxic-Ischemic Encephalopathy Revealed by MALDI-MSI

Background: Neonatal hypoxic-ischemic encephalopathy (HIE) is a major cause of neonatal death and neurodevelopmental disorders, and its pathological mechanisms are closely related to disturbed energy metabolism and lipid remodeling. Exploring the spatial heterogeneity of metabolomics is essential to analyze the pathological process of HIE. Methods: In this study, we established a neonatal mouse hypoxic-ischemic brain damage (HIBD) model by the modified Rice method, and analyzed various metabolic pathways such as the tricarboxylic acid (TCA) cycle, purine metabolism, and lipid metabolism in the ischemic edema area, with contralateral and control brain tissues using matrix-assisted laser desorption mass spectrometry imaging (MALDI-MSI) with a spatial resolution of 50 μm. Results: In the HIBD model, key metabolites of the tricarboxylic acid (TCA) cycle (citrate, succinate, L-glutamate, glucose, aspartate, and glutamine) were significantly enriched in the edematous area compared with the control (fold change: 1.52-2.82), which suggests a blockage of mitochondrial function; ATP/ADP/AMP levels were reduced by 53-73% in the edematous area, and xanthine was abnormally accumulated in the hippocampus of the affected side, suggesting energy depletion and altered purine metabolism; lipid remodeling showed regional specificity: some unsaturated fatty acids, such as docosahexaenoic acid, were abnormally accumulated in the hippocampus. In contrast, pentadecanoic acid levels were reduced across the entire brain in the HIBD model, with a more pronounced decrease in the ipsilateral hippocampus, suggesting impaired membrane stability. Conclusions: The neonatal mouse HIBD model exhibits reprogramming of energy metabolism, characterized by a blockage in the tricarboxylic acid (TCA) cycle and ATP depletion, along with an abnormal spatial distribution of lipids. By targeting xanthine metabolic pathways, restoring mitochondrial function, and intervening in region-specific lipid remodeling, brain energy homeostasis may be improved and neurological damage attenuated. Further studies should validate the clinical feasibility of xanthine and lipid imbalance as diagnostic markers of HIBD and explore the critical time window for metabolic intervention to optimize therapeutic strategies.

Identification of Reference Gene for Quantitative Gene Expression in Early-Term and Late-Term Cultured Canine Fibroblasts Derived from Ear Skin

Fibroblasts are cells that reside within the fibrous or loose connective tissues of most mammalian organs. For research purposes, fibroblasts are often subjected to long-term culture under defined conditions, during which their properties can significantly change. It is essential to understand and document these changes to obtain reliable outcomes. For the quantification of specific gene expressions, the most reliable and widely used technique is quantitative real-time polymerase chain reaction (qRT-PCR). Here, we assessed the impact of a reference gene's stability on a qRT-PCR analysis of long-term cultured canine skin fibroblasts. After successfully isolating the fibroblasts from canine skin tissues, they were cultured and evaluated for proliferation and β-galactosidase activity at different passage numbers. With extended culture, the fibroblasts showed a long doubling time and elevated β-galactosidase activity. Using three widely used algorithms, geNorm, Normfinder, and Bestkeeper, we identified HPRT1, YWHAZ, and GUSB as the most stable reference genes for both early- and late-passage fibroblasts. Conventional reference genes such as GAPDH were found to be less stable than those genes. The normalization of Vimentin by the stable genes showed statistical differences, whereas normalization by an unstable gene did not. Collectively, this study indicates that using stable reference genes is essential for accurately and reliably measuring gene expression in both early- and late-passage fibroblasts. These findings provide valuable insights into internal controls for gene expression studies and are expected to be utilized for analyzing gene expression patterns in molecular biology research.

Validation of reliable reference genes for qPCR of CD4+ T cells exposed to compressive strain

For accurate interpretation of quantitative real-time PCR (qPCR) data, stable reference genes are essential for normalization of target genes. To date, there is no information on reliable housekeeping genes in CD4+ T cells in a three-dimensional (3D) matrix under pressure stimulation. This in vitro study describes for the first time a method for pressure stimulation of CD4+ T cells in a 3D matrix in the context of orthodontic tooth movement (OTM) and identifies a set of reliable reference genes. CD4+ T cells were isolated from murine spleen and activated with anti-CD3/-CD28 Dynabeads (Thermo Fisher, Langenselbold, Germany) on standard cell culture plates or in 3D scaffolds with or without compressive strain. Expression stability of nine potential reference genes was examined using four mathematical algorithms. Gene expression of Il2 was normalized to all potential reference genes to highlight the importance of correct normalization. Cell proliferation and the expression of the surface markers CD25 and CD69 were also determined. The 3D matrix did not inhibit proliferation after immunological activation of T cells and embedded the cells sufficiently to expose them to pressure load. Expression of ubiquitin C (Ubc) and hypoxanthine phosphoribosyltransferase (Hprt) was the most stable under all conditions tested. A combination of these two genes was suitable for normalization of qPCR data. Normalization of Il2 gene expression showed highly variable results depending on the reference gene used. Pressure reduced cell proliferation and the number of CD69-positive T cells. This study provides a basis for performing valid and reliable qPCR experiments with CD4+ T cells cultured in 3D scaffolds and exposed to compressive forces simulating OTM.

Analysis of Polyphenol Extract from Hazel Leaf and Ameliorative Efficacy and Mechanism against Hyperuricemia Zebrafish Model via Network Pharmacology and Molecular Docking

Hazel leaf, a by-product of hazelnuts, is commonly used in traditional folk medicine in Portugal, Sweden, Iran and other regions for properties such as vascular protection, anti-bleeding, anti-edema, anti-infection, and pain relief. Based on our previous studies, the polyphenol extract from hazel leaf was identified and quantified via HPLC fingerprint. The contents of nine compounds including kaempferol, chlorogenic acid, myricetin, caffeic acid, p-coumaric acid, resveratrol, luteolin, gallic acid and ellagic acid in hazel leaf polyphenol extract (ZP) were preliminary calculated, among which kaempferol was the highest with 221.99 mg/g, followed by chlorogenic acid with 8.23 mg/g. The inhibition of ZP on α-glucosidase and xanthine oxidase activities was determined via the chemical method, and the inhibition on xanthine oxidase was better. Then, the effect of ZP on hyperuricemia zebrafish was investigated. It was found that ZP obviously reduced the levels of uric acid, xanthine oxidase, urea nitrogen and creatinine, and up-regulated the expression ofOAT1 and HPRT genes in hyperuricemia zebrafish. Finally, the targeted network pharmacological analysis and molecular docking of nine polyphenol compounds were performed to search for relevant mechanisms for alleviating hyperuricemia. These results will provide a valuable basis for the development and application of hazel leaf polyphenols as functional ingredients.

The pleiotropic contribution of genes in dopaminergic and serotonergic pathways to addiction and related behavioral traits

Introduction

Co-occurrence of substance use disorders (SUD) and other behavioral conditions, such as stress-related, aggressive or risk-taking behaviors, in the same individual has been frequently described. As dopamine (DA) and serotonin (5-HT) have been previously identified as key neurotransmitters for some of these phenotypes, we explored the genetic contribution of these pathways to SUD and these comorbid phenotypes in order to better understand the genetic relationship between them.

Methods

We tested the association of 275 dopaminergic genes and 176 serotonergic genes with these phenotypes by performing gene-based, gene-set and transcriptome-wide association studies in 11 genome-wide association studies (GWAS) datasets on SUD and related behaviors.

Results

At the gene-wide level, 68 DA and 27 5-HT genes were found to be associated with at least one GWAS on SUD or related behavior. Among them, six genes had a pleiotropic effect, being associated with at least three phenotypes: ADH1C, ARNTL, CHRNA3, HPRT1, HTR1B and DRD2. Additionally, we found nominal associations between the DA gene sets and SUD, opioid use disorder, antisocial behavior, irritability and neuroticism, and between the 5-HT-core gene set and neuroticism. Predicted gene expression correlates in brain were also found for 19 DA or 5-HT genes.

Discussion

Our study shows a pleiotropic contribution of dopaminergic and serotonergic genes to addiction and related behaviors such as anxiety, irritability, neuroticism and risk-taking behavior, highlighting a role for DA genes, which could explain, in part, the co-occurrence of these phenotypes.

HPRT1 Deficiency Induces Alteration of Mitochondrial Energy Metabolism in the Brain

Alterations in function of hypoxanthine guanine phosphoribosyl transferase (HPRT), one of the major enzymes involved in purine nucleotide exchange, lead to overproduction of uric acid and produce various symptoms of Lesch-Nyhan syndrome (LNS). One of the hallmarks of LNS is maximal expression of HPRT in the central nervous system with the highest activity of this enzyme in the midbrain and basal ganglia. However, the nature of neurological symptoms has yet to be clarified in details. Here, we studied whether HPRT1 deficiency changes mitochondrial energy metabolism and redox balance in murine neurons from the cortex and midbrain. We found that HPRT1 deficiency inhibits complex I-dependent mitochondrial respiration resulting in increased levels of mitochondrial NADH, reduction of the mitochondrial membrane potential, and increased rate of reactive oxygen species (ROS) production in mitochondria and cytosol. However, increased ROS production did not induce oxidative stress and did not decrease the level of endogenous antioxidant glutathione (GSH). Thus, disruption of mitochondrial energy metabolism but not oxidative stress could play a role of potential trigger of brain pathology in LNS.

Identifying genes associated with genetic control of color polymorphism in the pearl oyster Pinctada margaritifera var. cumingii (Linnaeus 1758) using a comparative whole genome pool-sequencing approach

For hundreds of years, the color diversity of Mollusca shells has been a topic of interest for humanity. However, the genetic control underlying color expression is still poorly understood in mollusks. The pearl oyster Pinctada margaritifera is increasingly becoming a biological model to study this process due to its ability to produce a large range of colors. Previous breeding experiments demonstrated that color phenotypes were partly under genetic control, and while a few genes were found in comparative transcriptomics and epigenetic experiments, genetic variants associated with the phenotypes have not yet been investigated. Here, we used a pooled-sequencing approach on 172 individuals to investigate color-associated variants on three color phenotypes of economic interest for pearl farming, in three wild and one hatchery populations. While our results uncovered SNPs targeting pigment-related genes already identified in previous studies, such as PBGD, tyrosinases, GST, or FECH, we also identified new color-related genes occurring in the same pathways, like CYP4F8, CYP3A4, and CYP2R1. Moreover, we identified new genes involved in novel pathways unknown to be involved in shell coloration for P. margaritifera, like the carotenoid pathway, BCO1. These findings are essential to possibly implement future breeding programs focused on individual selection for specific color production in pearl oysters and improve the footprint of perliculture on the Polynesian lagoon by producing less but with a better quality.

Gene Dose-Dependent and Additive Effects of ABCG2 rs2231142 and SLC2A9 rs3733591 Genetic Polymorphisms on Serum Uric Acid Levels

This study aimed to evaluate whether the single nucleotide polymorphisms of ATP-binding cassette subfamily G member 2 (ABCG2) and solute carrier family 2 member 9 (SLC2A9) affect individual blood uric acid levels using pyrosequencing. ABCG2 (rs2231142, rs72552713, rs2231137), SLC2A9 (rs3734553, rs3733591, rs16890979), and individual uric acid levels were prospectively analyzed in 250 healthy young Korean male participants. Prominent differences in uric acid levels of the alleles were observed in the SLC2A9 rs3733591 polymorphism: wild-type (AA) vs. heterozygote (AG), 0.7 mg/dL (p < 0.0001); AA vs. mutant type (GG), 1.32 mg/dL (p < 0.0001); and AG vs. GG, 0.62 mg/dL (p < 0.01). In ABCG2 single nucleotide polymorphisms (SNPs), the statistically significant differences in uric acid levels were only found in rs2231142 between CC vs. AA (1.06 mg/dL; p < 0.001), and CC vs. CA (0.59 mg/dL; p < 0.01). Serum uric acid levels based on the ABCG2 and SLC2A9 diplotype groups were also compared. The uric acid levels were the lowest in the CC/AA diplotype and highest in the AA/AG diplotype. In addition, the SNP SLC2A9 rs3733591 tended to increase the uric acid levels when the ABCG2 rs2231142 haplotypes were fixed. In conclusion, both the ABCG2 rs2231142 and SLC2A9 rs3733591 polymorphisms may additively elevate blood uric acid levels.

HPRT1 Most Suitable Reference Gene for Accurate Normalization of mRNA Expression in Canine Dermal Tissues with Radiation Therapy

Reference genes are crucial in molecular biological studies as an internal control for gene re-search as they exhibit consistent expression patterns across many tissue types. In canines, radiation therapy is the most important therapeutic tool to cure various diseases like cancer. However, when using radiation for therapeutic strategy, radiation exposure to healthy tissues leads to some possible side effects such as acute radiation-induced skin injury and alters gene expression. Therefore, the analysis of a change in reference gene expression during the skin recovery process after radiation therapy is essential in healthy canine tissue. In the present study, we analyzed eight reference genes (ACTB, GAPDH, YWHAZ, GUSB, HPRT1, RPL4, RPS5, and TBP) in canine dermal tissues at 0, 1, 2, 3, 4, 5, 7, and 9 weeks of radiation exposure that affected the skin condition of canines. The stability of reference genes is determined by evaluating radiation therapy’s effect on healthy canine dermal tissue. Epidermal marker, Keratin 10 expression varies each week after irradiation, and HPRT1 is found to be the most suitable for normalization of mRNA expression in radiation-exposed canine dermal tissues. Changes in the gene expression level were evaluated by using a reliable tool such as quantitative real-time polymerase chain reaction (qRT-PCR). In order to achieve a valid qRT-PCR result, the most stable reference genes used for normalization after the radiation exposure process are important. Therefore, the current study was designed to evaluate the most stable reference gene for the post-irradiation canine tissues. After radiation exposure, the alternation of reference gene expression was estimated by three algorithms (geNorm, Normfinder, and Bestkeeper). The RG validation programs (GeNorm and NormFinder) suggested that HPRT1, RPL4, and TBP were suitable for normalization in qRT-PCR. Furthermore, three algorithms suggested that HPRT1 was the most stable reference gene for normalization with qRT-PCR results, regardless of before and after radiation exposure. Whereas GAPDH was found to be the most unstable reference gene. In addition, the use of stable or unstable reference genes for the normalization of Keratin 10 expression showed statistical differences. Therefore, we observed that, to obtain accurate and suitable PCR results of the canine tissues with and without radiation exposure, the HPRT1 reference gene is recommended for normalization with its high stability. Additionally, the use of RGs such as HPRT1, RPL4, and TBP for normalization in qRT-PCR experiments is recommended for post-radiation canine tissues to generate more accurate and reliable data. These results will provide fundamental information regarding internal controls for gene expression studies and can be used for the analysis of gene patterns in regenerative medicine.

Hypoxanthine Reduces Radiation Damage in Vascular Endothelial Cells and Mouse Skin by Enhancing ATP Production via the Salvage Pathway

An effective method that can protect radiation-damaged tissues from apoptosis and promote tissue repair has not been reported to date. Hypoxanthine (Hx) is an intermediate metabolite in the purine degradation system that serves as a substrate for ATP synthesis via the salvage pathway. In this study, we focused on the transient decrease in intracellular ATP concentration after radiation exposure and examined the protective effect of Hx against radiation-induced tissue damage. Human umbilical vein endothelial cells were X irradiated, and the cell viability and incidence of apoptosis and DNA double-strand breaks (DSBs) were evaluated at different Hx concentrations. We found that in the presence of 2-100 μM Hx, the percentages of DSBs and apoptotic cells after 2, 6 and 10 Gy dose of radiation significantly decreased, whereas cell viability increased in a concentration-dependent manner. Moreover, the addition of Hx increased the levels of AMP, ADP, and ATP in the cells at 2 h postirradiation, suggesting that Hx was used for adenine nucleotide synthesis through the salvage pathway. Administration of a xanthine oxidoreductase inhibitor to a mouse model of radiation dermatitis resulted in increased blood Hx levels that inhibited severe dermatitis and accelerated recovery. In conclusion, the findings provide evidence that increasing the levels of Hx to replenish ATP could be an effective strategy to reduce radiation-induced tissue damage and elucidating the detailed mechanisms underlying the protective effects of Hx could help develop new protective strategies against radiation.

The Mutagenic Impact of Environmental Exposures in Human Cells and Cancer: Imprints Through Time

During life, the DNA of our cells is continuously exposed to external damaging processes. Despite the activity of various repair mechanisms, DNA damage eventually results in the accumulation of mutations in the genomes of our cells. Oncogenic mutations are at the root of carcinogenesis, and carcinogenic agents are often highly mutagenic. Over the past decade, whole genome sequencing data of healthy and tumor tissues have revealed how cells in our body gradually accumulate mutations because of exposure to various mutagenic processes. Dissection of mutation profiles based on the type and context specificities of the altered bases has revealed a variety of signatures that reflect past exposure to environmental mutagens, ranging from chemotherapeutic drugs to genotoxic gut bacteria. In this review, we discuss the latest knowledge on somatic mutation accumulation in human cells, and how environmental mutagenic factors further shape the mutation landscapes of tissues. In addition, not all carcinogenic agents induce mutations, which may point to alternative tumor-promoting mechanisms, such as altered clonal selection dynamics. In short, we provide an overview of how environmental factors induce mutations in the DNA of our healthy cells and how this contributes to carcinogenesis. A better understanding of how environmental mutagens shape the genomes of our cells can help to identify potential preventable causes of cancer.

Untargeted metabolomics changes on Gammarus pulex induced by propranolol, triclosan, and nimesulide pharmaceutical drugs

The presence of pharmaceuticals and personal care products (PPCPs) in natural water resources due to incomplete removal in Wastewater Treatment Plants (WWTPs) is a serious environmental concern at present. In this work, the effects of three pharmaceuticals (propranolol, triclosan, and nimesulide) on Gammarus pulex metabolic profiles at different doses and times of exposure have been investigated by liquid chromatography coupled to high-resolution mass spectrometry (LC-HRMS). The complex data sets generated in the different exposure experiments were analyzed with the ROIMCR procedure, based on the selection of the MS regions of interest (ROI) data and on their analysis by the Multivariate Curve-Resolution Alternating Least Squares (MCR-ALS) chemometrics method. This approach, allowed the resolution and identification of the metabolites present in the analyzed samples, as well as the estimation of their concentration changes due to the exposure experiments. ANOVA Simultaneous Component Analysis (ASCA) and Partial Least Squares Discriminant Analysis (PLS-DA) were then conducted to assess the changes in the concentration of the metabolites for the three pharmaceuticals at the different conditions of exposure. The three tested pharmaceuticals changed the concentrations of metabolites, which were related to different KEGG functional classes. These changes summarize the biochemical response of Gammarus pulex to the exposure by the three investigated pharmaceuticals. Possible pathway alterations related to protein synthesis and oxidative stress were observed in the concentration of identified metabolites.

Proteome analysis of endometrial tissue from patients with PCOS reveals proteins predicted to impact the disease

Polycystic ovary syndrome (PCOS) is a complex disease that causes an ovulatory infertility in approximately 10% of reproductive-age women. We searched for candidate proteins that might contribute to endometrial receptivity defects in PCOS patients, and result in adverse reproductive outcomes. Shotgun proteomics approach was used to investigate the proteome profile of the endometrium at the luteal phase in PCOS patients compared to healthy fertile individuals. Biological process and pathway analyses were conducted to categorize the proteins with differential expressions. Confirmation was performed for a number of proteins via immunoblotting in new samples. 150 proteins with higher abundance, and 46 proteins with lower abundance were identified in the endometrial tissue from PCOS patients compared to healthy fertile individuals. The proteins with higher abundance were enriched in protein degradation, cell cycle, and signaling cascades. Proteins with lower abundance in PCOS patients were enriched in extracellular matrix (ECM) composition and function, as well as the salvage pathway of purine biosynthesis. Metabolism was the most affected biological process with over 100 up-regulated, and approximately 30 down-regulated proteins. Our results indicate significant imbalances in metabolism, proteasome, cell cycle, ECM related proteins, and signaling cascades in endometrial tissue of PCOS, which may contribute to poor reproductive outcomes in these patients. We postulate that the endometria in PCOS patients may not be well-differentiated and synchronized for implantation. Possible roles of the above-mentioned pathways that underlie implantation failure in PCOS will be discussed. Our findings need to be confirmed in larger populations.

Identification of reliable reference genes for quantitative real-time PCR in ovary and uterus of laying hens under heat stress

The main stage in real-time quantitative PCR is a quantification of gene transcriptomes, in which suitable use of reliable reference genes is critical to normalize accurately. To determine the most stable reference genes in laying hens under heat stress, from a panel of nine typical candidate reference genes, the mRNA transcript of ACTB, HMBS, HPRT1, RPL13, RPL32, 18SrRNA, TBP, TFRC, and YWHAZ was evaluated in the ovary and uterus of both control and heat-stress groups of laying hens. Forty 23-week-old White Leghorn laying hens were housed in two rooms. The control (n = 20) and heat-stress (n = 20) groups were maintained at 21-23 °C and 36-38 °C for 8 weeks respectively. Analysis of this set of genes was done with BestKeeper, geNorm, and NormFinder software programs to find the most stable ones. Candidate reference genes ranked in the uterus of heat-stress and control groups of hens included YWHAZ, HPRT1, HMBS, RPL13, TFRC, ACTB, TBP, RPL32, and 18SrRNA; those in the ovary were YWHAZ, HPRT1, TFRC, HMBS, RPL13, TBP, RPL32, ACTB, and 18SrRNA. The overall results indicated that the most stable genes are YWHAZ, HPRT1, HMBS, RPL13, TFRC, TBP, ACTB, RPL32, and 18SrRNA respectively. In addition, the combination of YWHAZ, HPRT1, and HMBS is suggested as the most stable reference group of genes for more accurate quantitative data normalization in the ovarian and uterine tissues of laying hens under control and heat stress conditions. Lay summary Heat stress influences the expression of many genes in the reproductive tissues of birds. Accurate evaluation of these changes via real-time quantitative PCR depends on the determination of reliable reference genes. In this study, nine candidate housekeeping genes were evaluated, and the most stable were YWHAZ, HPRT1, HMBS, RPL13, TFRC, TBP, ACTB, RPL32, and 18SrRNA.

Engineering functional BMP-2 expressing teratoma-derived fibroblasts for enhancing osteogenesis

Bone morphogenetic protein 2 (BMP-2) is considered an effective growth factor for bone formation, and is used for making osteo-inductive scaffolds, but the related clinical investigations have shown low success rates. In this study, we genetically manipulated teratoma-derived fibroblast (TDF) cells by simultaneous introduction of BMP-2 and herpes simplex virus-thymidine kinase (HSV-tk) encoding genes. Self-production of BMP-2 in TDF cells strongly enhanced the alkaline phosphatase (ALP) activity, calcium content, and elevated the mRNA expression of osteogenic marker genes during in vitro osteogenesis. The bone formation volume was also remarkably enhanced in calvarial and femoral critical-size defect models. Ganciclovir (GCV) treatment induced apoptosis in TDF cells co-expressing HSV-tk and BMP-2, implying that HSV-tk suicide gene can modulate the side-effects of stem cell therapy, e.g., development of uncontrollable teratoma and tumor formation. Altogether, our findings revealed a safe and highly efficient technique with potential therapeutic applications for bone regeneration.

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.

Mechanistic Modelling of DNA Repair and Cellular Survival Following Radiation-Induced DNA Damage

Characterising and predicting the effects of ionising radiation on cells remains challenging, with the lack of robust models of the underlying mechanism of radiation responses providing a significant limitation to the development of personalised radiotherapy. In this paper we present a mechanistic model of cellular response to radiation that incorporates the kinetics of different DNA repair processes, the spatial distribution of double strand breaks and the resulting probability and severity of misrepair. This model enables predictions to be made of a range of key biological endpoints (DNA repair kinetics, chromosome aberration and mutation formation, survival) across a range of cell types based on a set of 11 mechanistic fitting parameters that are common across all cells. Applying this model to cellular survival showed its capacity to stratify the radiosensitivity of cells based on aspects of their phenotype and experimental conditions such as cell cycle phase and plating delay (correlation between modelled and observed Mean Inactivation Doses R(2) > 0.9). By explicitly incorporating underlying mechanistic factors, this model can integrate knowledge from a wide range of biological studies to provide robust predictions and may act as a foundation for future calculations of individualised radiosensitivity.

Identification of Suitable Reference Genes for Normalization of Real-Time Quantitative Polymerase Chain Reaction in an Intestinal Graft-Versus-Host Disease Mouse Model

BACKGROUND

With the development of real-time quantitative polymerase chain reaction (RT-qPCR) and intensive research on acute graft-versus-host disease (GVHD), selecting the best reference gene for normalization of RT-qPCR analysis in a GVHD model becomes more and more important. In this study, we aimed to identify suitable reference genes for mRNA studies in an intestinal GVHD mouse model after bone marrow transplantation (BMT).

METHODS

BALB/c recipients received 7.5 Gy total body irradiation (TBI) followed by injection of 5 × 10(6) bone marrow cells, without infusion of spleen cells for BMT, with infusion of 5 × 10(5) or 2.5 × 10(6) spleen cells for mild or moderate GVHD, respectively. Healthy mice were chosen as normal control subjects. Duodenum, jejunum, ileum, colon, and small intestine were collected at days 7, 14, 21, and 28 after transplantation. Transcription levels of 9 candidate genes, B2M, SDHA, HPRT, ACTB, GAPDH, HMBS, TBP, YWHAZ, and RPLP0, in each tissue were measured with the use of RT-qPCR. Combined data from these tissues in each group were defined as all samples. The expression stability of these genes was analyzed with the use of Genorm, Normfinder, Bestkeeper, and ΔCt.

RESULTS

Our results showed that in all samples, ACTB and HMBS displayed the highest and lowest expression levels, respectively. Genorm identified HRPT and SDHA as the most stable reference genes, whereas Normfinder and ΔCt method showed HPRT as the most stably expressed gene. Bestkeeper ranked YWHAZ and HPRT as the top 2 most suitable genes. In conclusion, HPRT was recommended as the most suitable reference gene after comprehensive ranking, suggesting that it could be used as an internal control for mRNA studies in intestinal GVHD after BMT.

Crystal structures of Apo and GMP bound hypoxanthine-guanine phosphoribosyltransferase from Legionella pneumophila and the implications in gouty arthritis

Hypoxanthine-guanine phosphoribosyltransferase (HGPRT) (EC 2.4.2.8) reversibly catalyzes the transfer of the 5-phophoribosyl group from 5-phosphoribosyl-alpha-1-pyrophosphate (PRPP) to hypoxanthine or guanine to form inosine monophosphate (IMP) or guanosine monophosphate (GMP) in the purine salvage pathway. To investigate the catalytic mechanism of this enzyme in the intracellular pathogen Legionella pneumophila, we determined the crystal structures of the L. pneumophila HGPRT (LpHGPRT) both in its apo-form and in complex with GMP. The structures reveal that LpHGPRT comprises a core domain and a hood domain which are packed together to create a cavity for GMP-binding and the enzymatic catalysis. The binding of GMP induces conformational changes of the stable loop II. This new binding site is closely related to the Gout arthritis-linked human HGPRT mutation site (Ser103Arg). Finally, these structures of LpHGPRT provide insights into the catalytic mechanism of HGPRT.

β2-Microglobulin is an appropriate reference gene for RT-PCR-based gene expression analysis of hematopoietic stem cells

Real-time reverse transcription polymerase chain reaction (RT-PCR) is regarded as one of the most useful and powerful tools for characterizing hematopoietic stem cells (HSCs), because samples of extremely small cell numbers can be analyzed. The expression levels determined by RT-PCR are based on relative quantification; therefore, the selection of an appropriate reference gene with a relatively stable expression level under most conditions is crucial. Here, we determined that beta2-microglobulin (B2m) is an appropriate reference gene for analyzing mouse HSCs by a novel method using single-cell RT-PCR. Clonally sorted HSCs were subjected to RT reactions with exogenous RNA fragments and then to real-time PCR. Next, the relative gene expression levels of 4 well-known housekeeping genes were quantified in each single cell sample based on the threshold cycle of exogenous RNA. The analysis revealed that B2m expression was reproducibly detected in almost all HSCs and that B2m had the most stable expression level among the compared genes, even after the cells had been cultured under various conditions. Thus, our results indicate that B2m can reliably be used as a reference gene for the relative quantification of expression levels in HSCs across various conditions. Furthermore, our work proposes a novel method for the selection of appropriate reference genes.

Quantitative expression proteomics and phosphoproteomics profile of brain from PINK1 knockout mice: insights into mechanisms of familial Parkinson's disease

Parkinson's disease (PD) is an age-related, neurodegenerative motor disorder characterized by progressive degeneration of dopaminergic neurons in the substantia nigra pars compacta and presence of α-synuclein-containing protein aggregates. Mutations in the mitochondrial Ser/Thr kinase PTEN-induced kinase 1 (PINK1) are associated with an autosomal recessive familial form of early-onset PD. Recent studies have suggested that PINK1 plays important neuroprotective roles against mitochondrial dysfunction by phosphorylating and recruiting Parkin, a cytosolic E3 ubiquitin ligase, to facilitate elimination of damaged mitochondria via autophagy-lysosomal pathways. Loss of PINK1 in cells and animals leads to various mitochondrial impairments and oxidative stress, culminating in dopaminergic neuronal death in humans. Using a 2-D polyacrylamide gel electrophoresis proteomics approach, the differences in expressed brain proteome and phosphoproteome between 6-month-old PINK1-deficient mice and wild-type mice were identified. The observed changes in the brain proteome and phosphoproteome of mice lacking PINK1 suggest that defects in signaling networks, energy metabolism, cellular proteostasis, and neuronal structure and plasticity are involved in the pathogenesis of familial PD. Mutations in PINK1 are associated with an early-onset form of Parkinson's disease (PD). This study examines changes in the proteome and phosphoproteome of the PINK1 knockout mouse brain. Alterations were noted in several key proteins associated with: increased oxidative stress, aberrant cellular signaling, altered neuronal structure, decreased synaptic plasticity, reduced neurotransmission, diminished proteostasis networks, and altered metabolism. 14-3-3ε, 14-3-3 protein epsilon; 3-PGDH, phosphoglycerate dehydrogenase; ALDOA, aldolase A; APT1, acyl-protein thioesterase 1; CaM, calmodulin; CBR3, carbonyl reductase [NADPH] 3; ENO2, gamma-enolase; HPRT, hypoxanthine-guanine phosphoribosyltransferase; HSP70, heat-shock-related 70 kDa protein 2; IDHc, cytoplasmic isocitrate dehydrogenase [NADP+]; MAPK1, mitogen-activated protein kinase 1; MEK1, MAP kinase kinase 1; MDHc, cytoplasmic malate dehydrogenase; NFM, neurofilament medium polypeptide; NSF, N-ethylmaleimide-sensitive fusion protein; PHB, prohibitin; PINK1, PTEN-induced putative kinase 1; PPIaseA, peptidyl-prolyl cis-trans isomerase A; PSA2, proteasome subunit alpha type-2; TK, transketolase; VDAC-2, voltage-dependent anion-selective channel protein 2.

Effect of commercial or depurinized milk on rat liver growth-regulatory kinases, nuclear factor-kappa B, and endonuclease in experimental hyperuricemia: comparison with allopurinol therapy

Hyperuricemia is a biochemical hallmark of gout, renal urate lithiasis, and inherited purine disorders, and may be a result of enormous ATP breakdown or purine release as a result of cardiovascular disease, hypertension, kidney disease, eclampsia, obesity, metabolic syndrome, psoriasis, tumor lysis syndrome, or intense physical training. The beneficial role of dairy products on hyperuricemia management and prevention is well documented in the literature. The primary aim of our experimental study was to examine the effect of milk dietary regimen (commercial 1.5% fat UHT milk or patented depurinized milk) compared with allopurinol therapy on experimental hyperuricemia induced by oxonic acid in rats. Principal component analysis was applied on a data set consisting of 11 variables for 8 different experimental groups. Among the 11 parameters measured (plasma uric acid and the liver parameters NFκB-p65, Akt kinase/phospho-Akt kinase, ERK kinase/phospho-ERK kinase, IRAK kinase/phospho IRAK kinase, p38/phospho-p38, and DNase), Akt/phospho Akt and ERK/phospho-ERK signaling were extracted as the most discriminating. We also compared the content of various potentially toxic compounds (sulfur compounds, ketones, aldehydes, alcohols, esters, carboxylic acids, and phthalates) in untreated commercial milk and depurinized milk. Of all the compounds investigated in this study that were observed in commercial milk (24 volatile organic compounds and 4 phthalates), 6 volatile organic compounds were not detected in depurinized milk. For almost all of the other compounds, significant decreases in concentration were observed in depurinized milk compared with commercial milk. In conclusion, a depurinized milk diet may be recommended in nutritional treatment of primary and secondary hyperuricemia to avoid uric acid and other volatile, potentially toxic compounds that may slow down liver regeneration and may induce chronic liver diseases.

Molecular characterization and structure analysis of HPRT in a Chinese patient with Lesch-Nyhan disease

Lesch-Nyhan disease (LND) is caused by deficiency of hypoxanthine guanine phosphoribosyltransferase (HPRT). The aim of the present study is to characterize the molecular deficiency of a clinical diagnosed Chinese patient with attenuated variant of LND. The coding region and the intron-exon boundaries of HPRT1 gene were sequenced by standard methods, and HPRT activity was assayed by HPLC method. Structure analysis was performed to estimate the consequence of the mutant of HPRT1 gene. A new mutation c.245T>G (p.Ile82Ser) was identified in this patient, and heterozygous mutation was found in the patient's mother. The activity of HPRT in the patient was completely undetectable. Structure study indicates that the mutation of p.Ile82Ser may lead to loss of hydrophobic side chain and disrupt its normal conformation of HPRT protein. It is helpful for diagnosis of LND that sequencing analysis of HPRT1 gene is performed in male infant and juvenile with hyperuricaemia and neurologic dysfunction in Chinese.

Late diagnosis of Lesch-Nyhan disease variant

A 30-year-old man was referred for investigation and management of hyperuricaemia. History included recurrent nephrolithiasis and chronic gout with poor response to medical management. Hypoxanthine-guanine phosphoribosyltransferase (HGPRT) enzyme activity was investigated and found to be deficient confirming the diagnosis of Lesch-Nyhan disease. Hyperuricaemia was treated with allopurinol. To prevent nephrolithiasis, the patient was instructed to avoid dehydration and aim for a minimum urine output of 2 L/day. Urinary alkalinisation with potassium citrate was started. The patient was referred for genetic counselling. This case discusses the genetics, pathophysiology, clinical manifestations, diagnosis and management of HGPRT deficiency.

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

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

Genotype-phenotype correlations in Lesch-Nyhan disease: moving beyond the gene

Lesch-Nyhan disease and its attenuated variants are caused by mutations in the HPRT1 gene, which encodes the purine recycling enzyme hypoxanthine-guanine phosphoribosyltransferase. The mutations are heterogeneous, with more than 400 different mutations already documented. Prior efforts to correlate variations in the clinical phenotype with different mutations have suggested that milder phenotypes typically are associated with mutants that permit some residual enzyme function, whereas the most severe phenotype is associated with null mutants. However, multiple exceptions to this concept have been reported. In the current studies 44 HPRT1 mutations associated with a wide spectrum of clinical phenotypes were reconstructed by site-directed mutagenesis, the mutant enzymes were expressed in vitro and purified, and their kinetic properties were examined toward their substrates hypoxanthine, guanine, and phosphoribosylpyrophosphate. The results provide strong evidence for a correlation between disease severity and residual catalytic activity of the enzyme (k(cat)) toward each of its substrates as well as several mechanisms that result in exceptions to this correlation. There was no correlation between disease severity and the affinity of the enzyme for its substrates (K(m)). These studies provide a valuable model for understanding general principles of genotype-phenotype correlations in human disease, as the mechanisms involved are applicable to many other disorders.

The effect of depurinized milk draught diet on rat serum uric acid, lipid status and haematological parameters

Hyperuricaemia and gout are closely related, but hyperuricaemia is an independent risk factor for endothelial damage, autoinflammation and haemodynamic abnormalities. Milk, generally known as a 'purine-free diet', is an essential protein source for patients suffering from hyperuricaemia and gout. As milk still contains different purine ribonucleotides, the new product, depurinized milk, almost free of purine nucleotides and uric acid, was produced. The potential effect of depurinized milk diet on serum uric acid (SUA) level, lipid parameters and blood haematological parameters was explored in rats after 72 h and 15 days, in relation to standard laboratory chow or the untreated milk diet. The beneficial effect on SUA was achieved when depurinized milk draught was given instead of standard chow for 72 h [28.39 ± 4.76 μm; p < 0.001 vs. standard diet (STD) 47.6 ± 6.12, vs. untreated milk diet 31.55 ± 8.50; p < 0.05] or as a supplement for STD for 15 days experiment (35.38 ± 6.40 μm; p < 0.05 vs. STD only 48.05 ± 4.32; vs. untreated milk + STD 46.02 ± 9.48). Depurinized milk diet significantly decreased the low density lipoproteins/high density lipoproteins (LDL/HDL) ratio (p < 0.001), triglycerides (p < 0.05) and leucocyte count (p < 0.001), while both milk draughts enhanced haemoglobin concentration (p < 0.01). In conclusion, considering the detrimental effect of persisting hyperuricaemia, the depurinized milk draught may meet the demand of healthy dairy product for population under hyperuricaemic risk.

Structural and functional studies of the human phosphoribosyltransferase domain containing protein 1

Human hypoxanthine-guanine phosphoribosyltransferase (HPRT) (EC 2.4.2.8) catalyzes the conversion of hypoxanthine and guanine to their respective nucleoside monophosphates. Human HPRT deficiency as a result of genetic mutations is linked to both Lesch-Nyhan disease and gout. In the present study, we have characterized phosphoribosyltransferase domain containing protein 1 (PRTFDC1), a human HPRT homolog of unknown function. The PRTFDC1 structure has been determined at 1.7 Å resolution with bound GMP. The overall structure and GMP binding mode are very similar to that observed for HPRT. Using a thermal-melt assay, a nucleotide metabolome library was screened against PRTFDC1 and revealed that hypoxanthine and guanine specifically interacted with the enzyme. It was subsequently confirmed that PRTFDC1 could convert these two bases into their corresponding nucleoside monophosphate. However, the catalytic efficiency (k(cat)/K(m)) of PRTFDC1 towards hypoxanthine and guanine was only 0.26% and 0.09%, respectively, of that of HPRT. This low activity could be explained by the fact that PRTFDC1 has a Gly in the position of the proposed catalytic Asp of HPRT. In PRTFDC1, a water molecule at the position of the aspartic acid side chain position in HPRT might be responsible for the low activity observed by acting as a weak base. The data obtained in the present study indicate that PRTFDC1 does not have a direct catalytic role in the nucleotide salvage pathway.

Hsp90 modulates CAG repeat instability in human cells

The Hsp90 molecular chaperone has been implicated as a contributor to evolution in several organisms by revealing cryptic variation that can yield dramatic phenotypes when the chaperone is diverted from its normal functions by environmental stress. In addition, as a cancer drug target, Hsp90 inhibition has been documented to sensitize cells to DNA-damaging agents, suggesting a function for Hsp90 in DNA repair. Here we explore the potential role of Hsp90 in modulating the stability of nucleotide repeats, which in a number of species, including humans, exert subtle and quantitative consequences for protein function, morphological and behavioral traits, and disease. We report that impairment of Hsp90 in human cells induces contractions of CAG repeat tracks by tenfold. Inhibition of the recombinase Rad51, a downstream target of Hsp90, induces a comparable increase in repeat instability, suggesting that Hsp90-enabled homologous recombination normally functions to stabilize CAG repeat tracts. By contrast, Hsp90 inhibition does not increase the rate of gene-inactivating point mutations. The capacity of Hsp90 to modulate repeat-tract lengths suggests that the chaperone, in addition to exposing cryptic variation, might facilitate the expression of new phenotypes through induction of novel genetic variation.

Pyrophosphate activation in hypoxanthine--guanine phosphoribosyltransferase with transition state analogue

Isotope-edited difference Raman and FTIR studies complemented by ab initio calculations have been applied to the transition state analogue complex of HGPRT.ImmHP.MgPP(i) to determine the ionic states of the 5'-phosphate moiety of ImmHP and of PP(i). These measurements characterize electrostatic interactions within the enzyme active site as deduced from frequency shifts of the phosphate groups. The bound 5'-phosphate moiety of ImmHP is dianionic, and this phosphate group exists in two different conformations within the protein complex. In one conformation, a hydrogen bond between the 5'-phosphate of ImmHP and the OH group of Tyr104 in the catalytic loop appears to be stronger. With the stronger H-bond, the OH of Tyr104 approaches one of the P..O bonds from the bridging oxygen side to cause distortion of the PO(3) moiety, as indicated by a lowered symmetric P..O stretch frequency. The asymmetric stretch frequencies are similar in both phosphate conformations. Bound PP(i) in this complex is fully ionized to P(2)O(7)(4-). Bond frequency changes for bound PP(i) indicate coordination to Mg(2+) ions but show no indication of significant P..O bond polarization. Extrapolation of these results to reaction coordinate motion for HGPRT suggests that bond formation between C1' of the nucleotide ribose and the oxygen of PP(i) is accomplished by migration of the ribocation toward immobilized pyrophosphate.

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.

Biochemical and structural characterization of the hypoxanthine-guanine-xanthine phosphoribosyltransferase from Pyrococcus horikoshii

The 6-oxopurine phosphoribosyltransferase (HPRT, EC 2.4.2.8) from the hyperthermophile Pyrococcus horikoshii was expressed in Escherichia coli and purified. Steady-state kinetic studies indicated that the enzyme is able to use hypoxanthine, guanine and xanthine. The first two substrates showed similar catalytic efficiencies, and xanthine presented a much lower value (around 20 times lower), but the catalytic constant was comparable to that of hypoxanthine. The enzyme was not able to bind to GMP-agarose, but was able to bind the other reverse reaction substrate, inorganic pyrophosphate, with low affinity (K(d) of 4.7+/-0.1 mM). Dynamic light scattering and analytical gel filtration suggested that the enzyme exists as a homohexamer in solution.

Hypoxanthine-guanine phosphoribosyltransferase deficiency: biochemical and molecular findings in six Argentine patients

Hypoxanthine-guanine phosphoribosyltransferase (HPRT) deficiency is an inborn error of purine metabolism responsible for Lesch-Nyhan Disease (LND) and its partial phenotypes, HPRT-related hyperuricemia with neurologic dysfunction (HRND) and hyperuricemia alone. We report here the recognition of six Argentine patients, two with LND and four with HRND. All patients presented elevated excretion of uric acid, hypoxanthine, and xanthine and decreased HPRT enzyme activities <1 nmol/h/mg Hb. The molecular analysis demonstrated in the two LND patients a novel inherited transition mutation, c.203T >C (L68P), in one subject and a germline transition mutation, c.209G >A (G70E), in the other. In the HRND patients a novel transversion mutation, c.584 A >C (Y195S), was found in three related patients and an inherited transition mutation, c.143G >A (R48H), in the fourth subject.

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

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

The effect of normothermic and hypothermic hypoxia-ischemia on brain hypoxanthine phosphoribosyl transferase activity

OBJECTIVES

Cerebral hypoxia-ischemia leads to the depletion of ATP. Hypoxanthine, a degradation product of ATP, can be salvaged by hypoxanthine phosphoribosyl transferase (HPRT) and used to reform high-energy purines. Hypothermia conserves ATP in hypoxia-ischemia, possibly by preserving HPRT activity. We hypothesized that cerebral hypoxia-ischemia would decrease the activity of this enzyme, and that this reduction would be attenuated by moderate hypothermia.

METHODS

Three groups of rabbits were evaluated. Normothermic rabbits were exposed to 8 minutes of hypoxia, 8 minutes of cerebral ischemia, and 30 minutes or 4 hours of cerebral reperfusion. Hypothermic rabbits were cooled to a brain temperature of 33-34 degrees C throughout identical injury and reperfusion periods. Control rabbits underwent the same preparation, without hypothermia or injury. HPRT activity in the cortex, hippocampus, thalamus, caudate, and cerebellum was measured spectrophotometrically.

RESULTS

There were no significant differences (p>0.05) in enzymatic activity when comparing the three groups of animals, regardless of reperfusion time or brain temperature. Within the control group, some regional differences in enzyme activity were noted.

DISCUSSION

The results indicate that brain HPRT activity is unaffected by hypoxia-ischemia, even after 4 hours of reperfusion and regardless of brain temperature. This study supports the importance of this enzyme in the conservation of brain purines after neurologic injury.

Molecular and functional analysis of hypoxanthine-guanine phosphoribosyltransferase from Arabidopsis thaliana

Hypoxanthine-guanine phosphoribosyltransferase (HGPT) occurs in both eukaryotic and prokaryotic organisms. However, the molecular and functional properties of plant HGPT are not well understood. In this study, it was found that the putative HGPT proteins from dicot and monocot plant species exhibited significant identities to their homologs from other cellular organisms. Ectopic expression of the HGPTs from Arabidopsis, soybean or wheat complemented HGPT deficiency in the hpt1 mutant of Saccharomyces cerevisiae. Recombinant Arabidopsis HGPT (AtHGPT) catalyzed both forward and reverse reactions in in vitro biochemical assays. The relative catalytic efficiency for the synthesis of guanosine monophosphate (GMP) was significantly greater than that for the production of guanine from GMP. Further investigations led to identification of the candidate residues that may form the pyrophosphate (PPi) binding loop in AtHGPT. AtHGPT expression level was dynamically regulated in Arabidopsis organs and during leaf development and senescence and seed germination. AtHGPT knockout mutant germinated more slowly than wild type control, whereas its overexpression mutant exhibited accelerated germination. Collectively, the data suggest that functional HGPTs are expressed in higher plants. In Arabidopsis, HGPT plays an active role in the salvage of purine bases and its activity is required for efficient seed germination.

Normal HPRT coding region in a male with gout due to HPRT deficiency

A deficiency of the enzyme hypoxanthine-guanine phosphoribosyltransferase (HPRT; EC 2.4.2.8) is associated with a spectrum of disease that ranges from gouty arthritis (OMIM 300323) to the more severe Lesch-Nyhan syndrome (OMIM 300322). To date, all cases of HPRT deficiency have shown a mutation within the HPRT cDNA. In the present study of an individual with gout due to HPRT deficiency, we found a normal HPRT cDNA sequence. This is the first study to provide an example of HPRT deficiency which appears to be due to a defect in the regulation of the gene.

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.

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.

Interactions at the dimer interface influence the relative efficiencies for purine nucleotide synthesis and pyrophosphorolysis in a phosphoribosyltransferase

Enzymes that salvage 6-oxopurines, including hypoxanthine phosphoribosyltransferases (HPRTs), are potential targets for drugs in the treatment of diseases caused by protozoan parasites. For this reason, a number of high-resolution X-ray crystal structures of the HPRTs from protozoa have been reported. Although these structures did not reveal why HPRTs need to form dimers for catalysis, they revealed the existence of potentially relevant interactions involving residues in a loop of amino acid residues adjacent to the dimer interface, but the contributions of these interactions to catalysis remained poorly understood. The loop, referred to as active-site loop I, contains an unusual non-proline cis-peptide and is composed of residues that are structurally analogous with Leu67, Lys68, and Gly69 in the human HPRT. Functional analyses of site-directed mutations (K68D, K68E, K68N, K68P, and K68R) in the HPRT from Trypanosoma cruzi, etiologic agent of Chagas' disease, show that the side-chain at position 68 can differentially influence the K(m) values for all four substrates as well as the k(cat) values for both IMP formation and pyrophosphorolysis. Also, the results for the K68P mutant are inconsistent with a cis-trans peptide isomerization-assisted catalytic mechanism. These data, together with the results of structural studies of the K68R mutant, reveal that the side-chain of residue 68 does not participate directly in reaction chemistry, but it strongly influences the relative efficiencies for IMP formation and pyrophosphorolysis, and the prevalence of lysine at position 68 in the HPRT of the majority of eukaryotes is consistent with there being a biological role for nucleotide pyrophosphorolysis.

Functional roles for amino acids in active site loop II of a hypoxanthine phosphoribosyltransferase

A flexible loop of amino acids (loop II) closes over the active site of hypoxanthine phosphoribosyltransferase (HPRT) as the enzyme approaches the transition state [Biochemistry 37 (1998) 17120]. Formerly, the deletion of much of loop II from the HPRT of Trypanosoma cruzi resulted in a 2-3 order of magnitude reduction in k(cat) values with relatively modest changes in the Michaelis constants for substrates [Biochim. Biophys. Acta 1537 (2001) 63-70]. However, the contributions of individual loop II residues to catalysis remained poorly understood or have been disputed. Herein, saturation mutagenesis was used to generate relatively random sets of mutations in the 12 residues of active site loop II in the HPRT from T. cruzi and steady-state kinetics was used to investigate reactions catalyzed by the mutants. The results of analyses of 18 different mutations in an evolutionarily invariant Ser-Tyr dipeptide are consistent with interactions, between main chain nitrogen atoms of these residues and the O1A atom of phosphoribosylpyrophosphate (PRPP) or pyrophosphate (PPi), being essential for efficient enzyme chemistry. The results of analyses of 55 mutations in the nine other amino acids in loop II are inconsistent with these residues participating directly in enzyme chemistry, but are consistent with several of their side chains influencing loop flexibility and folding, as well as the efficiency for nucleotide formation relative to pyrophosphorolysis.

Analysis of 6-(2,2-Dichloroacetamido)chrysene interaction with the hypoxanthine phosphoribosyltransferase from Trypanosoma cruzi

Selective inhibition is needed for drugs targeting the hypoxanthine phosphoribosyltransferase of Trypanosoma cruzi, etiologic agent of Chagas' disease. 6-(2,2-Dichloroacetamido)chrysene, was shown herein to be a selective inhibitor of the trypanosomal enzyme. SAR analysis revealed that the 6-amido moiety was essential, but the dichloroaceto moiety was not essential for achieving the low K(i) for this inhibitor. Understanding the molecular basis for these interactions could facilitate the design of selective inhibitors without a chrysene moiety.

Mice deficient for cytosolic thymidine kinase gene develop fatal kidney disease

The thymidine kinase (Tk) gene codes for a cytosolic protein involved in the pyrimidine nucleotide salvage pathway. A functional Tk gene is not necessary for cells in culture, and a naturally occurring Tk deficient phenotype has not been described in humans or animal models. In order to determine the biological significance of the Tk gene, we created Tk(-/-) knockout (KO) mice through homologous recombination in mouse embryonic stem cells. Tk KO mice have shortened life spans compared with their wild-type or Tk heterozygous (HET) siblings. All Tk KO mice develop sclerosis of kidney glomeruli and die before one year of age of kidney failure. Among other changes in KO animals, the most consistent is a switch from exclusively mucous secretion to predominantly serous secretion in the sublingual salivary gland. HET parents can produce KO mice at a frequency approaching Mendelian inheritance. Other observations in KO animals include an elevated level of serum thymidine, a significant decrease in the cloning efficiency of splenic lymphocytes, an increase in the frequency of hypoxanthine guanine phosphoribosyl transferase gene mutant lymphocytes, and histological alteration in the lymphoid structure of the spleen. In addition, KO animals sporadically exhibit inflammation of the arteries, which taken together with the lymphocyte and spleen abnormalities, suggest an abnormal immune system. Alterations in Tk KO mice indicate that the pyrimidine nucleotide salvage pathway is indispensable in vivo.