Mouse models of hypoxanthine phosphoribosyltransferase deficiency

A novel mouse model for genetic variation in 10-formyltetrahydrofolate synthetase exhibits disturbed purine synthesis with impacts on pregnancy and embryonic development

Genetic variants in one-carbon folate metabolism have been identified as risk factors for disease because they may impair the production or use of one-carbon folates required for nucleotide synthesis and methylation. p.R653Q (1958G>A) is a single-nucleotide polymorphism (SNP) in the 10-formyltetrahydrofolate (formylTHF) synthetase domain of the trifunctional enzyme MTHFD1; this domain produces the formylTHF which is required for the de novo synthesis of purines. Approximately 20% of Caucasians are homozygous for the Q allele. MTHFD1 p.R653Q has been proposed as a risk factor for neural tube defects (NTDs), congenital heart defects (CHDs) and pregnancy losses. We have generated a novel mouse model in which the MTHFD1 synthetase activity is inactivated without affecting protein expression or the other activities of this enzyme. Complete loss of synthetase activity (Mthfd1S(-/-)) is incompatible with life; embryos die shortly after 10.5 days gestation, and are developmentally delayed or abnormal. The proportion of 10-formylTHF in the plasma and liver of Mthfd1S(+/-) mice is reduced (P < 0.05), and de novo purine synthesis is impaired in Mthfd1S(+/-) mouse embryonic fibroblasts (MEFs, P < 0.005). Female Mthfd1S(+/-) mice had decreased neutrophil counts (P < 0.05) during pregnancy and increased incidence of developmental defects in embryos (P = 0.052). These findings suggest that synthetase deficiency may lead to pregnancy complications through decreased purine synthesis and reduced cellular proliferation. Additional investigation of the impact of synthetase polymorphisms on human pregnancy is warranted.

Despite 2009 guidelines, few women report being counseled correctly about weight gain during pregnancy

OBJECTIVE

The purpose of this study was to determine the information that pregnant women report receiving when being counseled about weight gain and the risks of inappropriate gain.

STUDY DESIGN

With the use of a self-administered questionnaire at prenatal clinics in Hamilton, Ontario, Canada, a cross-sectional survey was conducted of women who had had at least 1 prenatal visit, who could read English, and who had a live singleton gestation.

RESULTS

Three hundred ten women completed the survey, which was a 93.6% response rate. Although 28.5% (95% confidence interval, 23.5-33.6%) reported that their health care provider had made a recommendation about how much weight they should gain, only 12.0% (95% confidence interval, 8-16.1%) of the women reported having achieved the recommended weight gain in accordance with the 2009 guidelines. One quarter of the women reported being told that there were risks with inappropriate gain.

CONCLUSION

Despite the recent 2009 publication of the gestational weight gain guidelines, only 12% of women reported being counseled correctly, which suggests an urgent need for improved patient education.

Engineering the mouse genome to model human disease for drug discovery

Genetically engineered mice (GEM) have become invaluable tools for human disease modeling and drug development. Completion of the mouse genome sequence in combination with transgenesis and gene targeting in embryonal stem cells have opened up unprecedented opportunities. Advanced technologies for derivation of GEM models will be introduced and discussed.

Multifocal atrophy of cerebellar internal granular neurons in lesch-nyhan disease: case reports and review

The neuropathologic findings in 31 cases (aged 6 months to 33 years) of Lesch-Nyhan disease (hypoxanthine-guanine phosphoribosyltransferase deficiency) have been previously reported. Herein 2 additional cases, a 10-year-old boy and a 21-year-old man, are described. Both cases had unusual cerebellar abnormalities comprising multifocal internal granular layer atrophy with sparing of the Purkinje layer, one had a slightly small brain, and neither had striatal abnormalities. Careful review of the literature indicates that the most prevalent neuropathologic abnormalities are small cerebrum (13 of 33 cases) and multifocal cerebellar lesions (9 of 33 cases), although these could be underreported. Other authors have disregarded these abnormalities, focusing on the apparently normal basal nuclei, and they have suggested that the clinical neurologic abnormalities are based solely on changes in neurotransmitters. We discuss potential mechanisms of cerebellar damage, suggest that the cerebellar abnormality could in part explain the clinical syndrome, and recommend that cerebellar structure and function should be more carefully studied in Lesch-Nyhan disease.

Complex changes in ecto-nucleoside 5'-triphosphate diphosphohydrolase expression in hypoxanthine phosphoribosyl transferase deficiency

Lesch-Nyhan disease is caused by a deficiency of the purine salvage enzyme, hypoxanthine phosphoribosyl transferase (HPRT). The link between HPRT deficiency and the neuropsychiatric symptoms is unknown. In rat B103 neuroblastoma cell membranes and mouse Neuro2a neuroblastoma cell membranes, nucleoside 5'-triphosphatase (NTPase) activity is substantially reduced, whereas in fibroblast membranes from HPRT knock-out mice, NTPase activity is increased. Candidate genes for these NTPase activity changes are ecto-nucleoside 5'-triphosphate diphosphohydrolases (NTPDases). Therefore, we studied expression of NTPDases in B103 cells, Neuro2a cells and skin fibroblasts by reverse transcriptase polymerase chain reaction and restriction enzyme digestion of amplified cDNA fragments. In B103 cells, expression of NTPDases 1, 3 and 6 decreased, whereas expression of NTPDases 4 and 5 increased in HPRT deficiency. In Neuro2a cells, expression of NTPDases 3-6 increased in HPRT deficiency. In fibroblasts, NTPDase 3 expression decreased, and expression of NTPDases 4-6 increased in HPRT deficiency. Collectively, there are complex decreases and increases in NTPDase isoform expression in HPRT deficiency that depend on the specific cell type and species studied. These changes in NTPDase expression may reflect an (insufficient) attempt of cells to compensate for the changes in nucleotide metabolism caused by HPRT deficiency.

Human embryo and early fetus research

Studies of human embryos and fetuses have highlighted developmental differences between humans and model organisms. In addition to describing the normal biology of our own species, a justification in itself, studies of early human development have aided identification of candidate disease genes mapped by positional cloning strategies, understanding pathophysiology, where human disorders are not faithfully reproduced by models in other species, and, more recently, potential therapies based on human embryonic stem and embryonic germ cells. In this article, we review these applications. We also discuss when and how to study human embryo and early fetuses and some of the regulations of this research.

Decreased GTP-stimulated adenylyl cyclase activity in HPRT-deficient human and mouse fibroblast and rat B103 neuroblastoma cell membranes

Defect of the purine salvage enzyme, hypoxanthine phosphoribosyl transferase (HPRT), results in Lesch-Nyhan disease (LND). It is unknown how the metabolic defect translates into the severe neuropsychiatric phenotype characterized by self-injurious behavior, dystonia and mental retardation. There are abnormalities in GTP, UTP and CTP concentrations in HPRT-deficient cells. Moreover, GTP, ITP, XTP, UTP and CTP differentially support Gs-protein-mediated adenylyl cyclase (AC) activation. Based on these findings we hypothesized that abnormal AC regulation may constitute the missing link between HPRT deficiency and the neuropsychiatric symptoms in LND. To test this hypothesis, we studied AC activity in membranes from primary human skin and immortalized mouse skin fibroblasts, mouse Neuro-2a neuroblastoma cells and rat B103 neuroblastoma cells. In B103 control membranes, GTP, ITP, XTP and UTP exhibited profound stimulatory effects on basal AC activity that approached the effects of hydrolysis-resistant nucleotide analogs. In HPRT- membranes, the stimulatory effects of GTP, ITP, XTP and UTP were strongly reduced. Similarly, in human and mouse skin fibroblast membranes we also observed a decrease in GTP-stimulated AC activity in HPRT-deficient cells compared with the respective controls. In mouse Neuro-2a neuroblastoma membranes, AC activity in the presence of GTP was below the detection limit of the assay. We discuss several possibilities to explain the abnormalities in AC regulation in HPRT deficiency that encompass various species and cell types.

Altered membrane NTPase activity in Lesch-Nyhan disease fibroblasts: comparison with HPRT knockout mice and HPRT-deficient cell lines

Lesch-Nyhan disease (LND) is a rare disorder caused by a defect of an enzyme in the purine salvage pathway, hypoxanthine phosphoribosyl transferase (HPRT). It is still unknown how the metabolic defect translates into the complex neuropsychiatric phenotype characterized by self-injurious behavior, dystonia and mental retardation. There are abnormalities in purine and pyrimidine nucleotide content in HPRT-deficient cells. We hypothesized that altered nucleotide concentrations in HPRT deficiency change G-protein-mediated signal transduction. Therefore, our original study aim was to examine the high-affinity GTPase activity of G-proteins in membranes from primary human skin and immortalized mouse skin fibroblasts, rat B103 neuroblastoma cells and mouse Neuro-2a neuroblastoma cells. Unexpectedly, in membranes from human fibroblasts, B103- and Neuro-2a cells, V(max) of low-affinity nucleoside 5'-triphosphatase (NTPase) activities was decreased up to 7-fold in HPRT deficiency. In contrast, in membranes from mouse fibroblasts, HPRT deficiency increased NTPase activity up to 4-fold. The various systems analyzed differed from each other in terms of K(m) values for NTPs, absolute V(max) values and K(i) values for nucleoside 5'-[beta,gamma-imido]triphosphates. Our data show that altered membrane NTPase activity is a biochemical hallmark of HPRT deficiency, but species and cell-type differences have to be considered. Thus, future studies on biochemical changes in LND should be conducted in parallel in several HPRT-deficient systems.

CAG-polyglutamine-repeat mutations: independence from gene context

Several neurological disorders have been attributed to the inheritance of long CAG-polyglutamine repeats. Unlike classical mutations, whose deleterious effects are totally dependent on the context of the gene in which they reside, these translated CAG repeat mutations have been shown to cause neurotoxicity and neuronal intranuclear inclusions when expressed outside their natural gene context. We provide a description of mice with different lengths of repeat in the foreign context of the murine Hprt locus, focusing on aspects of the phenotype that provide an insight into the mechanism by which this unusual mutation might cause toxicity.

Ectopically expressed CAG repeats cause intranuclear inclusions and a progressive late onset neurological phenotype in the mouse

The mutations responsible for several human neurodegenerative disorders are expansions of translated CAG repeats beyond a normal size range. To address the role of repeat context, we have introduced a 146-unit CAG repeat into the mouse hypoxanthine phosphoribosyltransferase gene (Hprt). Mutant mice express a form of the HPRT protein that contains a long polyglutamine repeat. These mice develop a phenotype similar to the human translated CAG repeat disorders. Repeat containing mice show a late onset neurological phenotype that progresses to premature death. Neuronal intranuclear inclusions are present in affected mice. Our results show that CAG repeats do not need to be located within one of the classic repeat disorder genes to have a neurotoxic effect.

Age-dependent selection against hypoxanthine phosphoribosyl transferase-deficient cells in mouse haematopoiesis

The basis of a previously observed difference in the level of contribution of hypoxanthine phosphoribosyltransferase-deficient cells between the haematopoietic and non-haematopoietic tissues of chimaeric and heterozygous mice has been clarified by studying two populations of female mice that differ only in that one is heterozygous for a null allele at the hprt locus and the other is wild type at this locus. Both populations are heterozygous for an electrophoretic variant allele at the X-linked Pgk-1 locus, so that X-chromosome inactivation generates cells expressing different isozymes of phosphoglycerate kinase which can be assayed to monitor cell selection. The results show that hypoxanthine phosphoribosyltransferase deficiency itself, rather than an effect of another X-linked gene, causes a reduced level of contribution to haematopoietic tissues. Further, the extent of the depletion increases significantly with age, and this effect is due to a progressive reduction in the level of contribution to haematopoietic tissues rather than to an increase in the level of contribution to non-haematopoietic tissues.

Production of a model for Lesch-Nyhan syndrome in hypoxanthine phosphoribosyltransferase-deficient mice

The inherited disease Lesch-Nyhan syndrome, which is caused by a deficiency of the enzyme hypoxanthine phosphoribosyltransferase (HPRT), is characterized by behavioural alterations, including self-injurious behaviour and mental retardation. Although HPRT-deficient mice have been generated using the embryonic stem cell system, no spontaneous behavioural abnormalities had been reported. We examined whether mice were more tolerant of HPRT deficiency because they were more reliant on adenine phosphoribosyltransferase (APRT) than HPRT for their purine salvage. The administration of an APRT inhibitor to HPRT-deficient mice induced persistent self-injurious behaviour. This combined genetic and biochemical model will facilitate the study of Lesch-Nyhan syndrome and the evaluation of novel therapies.