Structural and functional properties of uridine 5'-monophosphate synthase from Coffea arabica.

In higher eukaryotes and plants, the last two sequential steps in the de novo biosynthesis of uridine 5'-monophosphate (UMP) are catalyzed by a bifunctional natural chimeric protein called UMP synthase (UMPS). In higher plants, UMPS consists of two naturally fused enzymes: orotate phosphoribosyltransferase (OPRTase) at N-terminal and orotidine-5'-monophosphate decarboxylase (ODCase) at C-terminal. In this work, we obtained the full functional recombinant protein UMPS from Coffea arabica (CaUMPS) and studied its structure-function relationships. A biochemical and structural characterization of a plant UMPS with its two functional domains is described together with the presentation of the first crystal structure of a plant ODCase at 1.4 Å resolution. The kinetic parameters measured of CaOPRTase and CaODCase domains were comparable to those reported. The crystallographic structure revealed that CaODCase is a dimer that conserves the typical fold observed in other ODCases from prokaryote and eukaryote with a 1-deoxy-ribofuranose-5'-phosphate molecule bound in the active site of one subunit induced a closed conformation. Our results add to the knowledge of one of the key enzymes of the de novo biosynthesis of pyrimidines in plant metabolism and open the door to future applications.

A Facile Fluorometric Assay of Orotate Phosphoribosyltransferase Activity Using a Selective Fluorogenic Reaction for Orotic Acid.

Orotate phosphoribosyltransferase (OPRT) exists as a bifunctional enzyme, uridine 5'-monophosphate synthase, in mammalian cells and plays an important role in pyrimidine biosynthesis. Measuring OPRT activity has been considered important for understanding biological events and development of molecular-targeting drugs. In this study, we demonstrate a novel fluorescence method for measuring OPRT activity in living cells. The technique utilizes 4-trifluoromethylbenzamidoxime (4-TFMBAO) as a fluorogenic reagent, which produces selective fluorescence for orotic acid. To perform the OPRT reaction, orotic acid was added to HeLa cell lysate, and a portion of the enzyme reaction mixture was heated at 80 °C for 4 min in the presence of 4-TFMBAO under basic conditions. The resulting fluorescence was measured using a spectrofluorometer, which reflects the consumption of orotic acid by the OPRT. After optimization of the reaction conditions, the OPRT activity was successfully determined in 15 min of enzyme reaction time without further procedures such as purification of OPRT or deproteination for the analysis. The activity obtained was compatible with the value measured by the radiometric method with [3H]-5-FU as the substrate. The present method provides a reliable and facile measurement of OPRT activity and could be useful for a variety of research fields targeting pyrimidine metabolism.

Enzyme-substrate interactions in orotate-mimetic OPRT inhibitor complexes: a QM/MM analysis.

Orotate phosphoribosyltransferase (OPRT) catalyses the reversible phosphoribosyl transfer from α-D-5-phosphoribosyl-1-pyrophosphate (PRPP) to orotic acid (OA) to yield orotidine 5'-monophosphate (OMP) during the de novo synthesis of nucleotides. Numerous studies have reported the inhibition of this reaction as a strategy to check diseases like tuberculosis, malaria and cancer. Insight into the inhibition of this reaction is, therefore, of urgent interest. In this study, we implemented a QM/MM framework on OPRT derived from Saccharomyces cerevisiae to obtain insights into the competitive binding of OA and OA-mimetic inhibitors by quantifying their interactions with OPRT. 4-Hydroxy-6-methylpyridin-2(1H) one showed the best inhibiting activity among the structurally similar OA-mimetic inhibitors, as quantified from the binding energetics. Our analysis of protein-ligand interactions unveiled the association of this inhibitory ligand with a strong network of hydrogen bonds, a large contribution of hydrophobic contacts, and bridging water molecules in the binding site. The ortho-substituted CH3 group in the compound resulted in a large population of π-electrons in the aromatic ring of this inhibitor, supporting the ligand binding further.

Human uridine 5'-monophosphate synthase stores metabolic potential in inactive biomolecular condensates.

Human uridine 5'-monophosphate synthase (HsUMPS) is a bifunctional enzyme that catalyzes the final two steps in de novo pyrimidine biosynthesis. The individual orotate phosphoribosyl transferase and orotidine monophosphate domains have been well characterized, but little is known about the overall structure of the protein and how the organization of domains impacts function. Using a combination of chromatography, electron microscopy, and complementary biophysical methods, we report herein that HsUMPS can be observed in two structurally distinct states, an enzymatically active dimeric form and a nonactive multimeric form. These two states readily interconvert to reach an equilibrium that is sensitive to perturbations of the active site and the presence of substrate. We determined that the smaller molecular weight form of HsUMPS is an S-shaped dimer that can self-assemble into relatively well-ordered globular condensates. Our analysis suggests that the transition between dimer and multimer is driven primarily by oligomerization of the orotate phosphoribosyl transferase domain. While the cellular distribution of HsUMPS is unaffected, quantification by mass spectrometry revealed that de novo pyrimidine biosynthesis is dysregulated when this protein is unable to assemble into inactive condensates. Taken together, our data suggest that HsUMPS self-assembles into biomolecular condensates as a means to store metabolic potential for the regulation of metabolic rates.

Inhibition of the de novo pyrimidine biosynthesis pathway limits ribosomal RNA transcription causing nucleolar stress in glioblastoma cells.

Glioblastoma is the most common and aggressive type of cancer in the brain; its poor prognosis is often marked by reoccurrence due to resistance to the chemotherapeutic agent temozolomide, which is triggered by an increase in the expression of DNA repair enzymes such as MGMT. The poor prognosis and limited therapeutic options led to studies targeted at understanding specific vulnerabilities of glioblastoma cells. Metabolic adaptations leading to increased synthesis of nucleotides by de novo biosynthesis pathways are emerging as key alterations driving glioblastoma growth. In this study, we show that enzymes necessary for the de novo biosynthesis of pyrimidines, DHODH and UMPS, are elevated in high grade gliomas and in glioblastoma cell lines. We demonstrate that DHODH's activity is necessary to maintain ribosomal DNA transcription (rDNA). Pharmacological inhibition of DHODH with the specific inhibitors brequinar or ML390 effectively depleted the pool of pyrimidines in glioblastoma cells grown in vitro and in vivo and impaired rDNA transcription, leading to nucleolar stress. Nucleolar stress was visualized by the aberrant redistribution of the transcription factor UBF and the nucleolar organizer nucleophosmin 1 (NPM1), as well as the stabilization of the transcription factor p53. Moreover, DHODH inhibition decreased the proliferation of glioblastoma cells, including temozolomide-resistant cells. Importantly, the addition of exogenous uridine, which reconstitutes the cellular pool of pyrimidine by the salvage pathway, to the culture media recovered the impaired rDNA transcription, nucleolar morphology, p53 levels, and proliferation of glioblastoma cells caused by the DHODH inhibitors. Our in vivo data indicate that while inhibition of DHODH caused a dramatic reduction in pyrimidines in tumor cells, it did not affect the overall pyrimidine levels in normal brain and liver tissues, suggesting that pyrimidine production by the salvage pathway may play an important role in maintaining these nucleotides in normal cells. Our study demonstrates that glioblastoma cells heavily rely on the de novo pyrimidine biosynthesis pathway to generate ribosomal RNA (rRNA) and thus, we identified an approach to inhibit ribosome production and consequently the proliferation of glioblastoma cells through the specific inhibition of the de novo pyrimidine biosynthesis pathway.

Plasmid-based complementation of large deletions in Phaeodactylum tricornutum biosynthetic genes generated by Cas9 editing.

The model diatom Phaeodactylum tricornutum is an attractive candidate for synthetic biology applications. Development of auxotrophic strains of P. tricornutum would provide alternative selective markers to commonly used antibiotic resistance genes. Here, using CRISPR/Cas9, we show successful editing of genes in the uracil, histidine, and tryptophan biosynthetic pathways. Nanopore long-read sequencing indicates that editing events are characterized by the occurrence of large deletions of up to ~ 2.7 kb centered on the editing site. The uracil and histidine-requiring phenotypes can be complemented by plasmid-based copies of the intact genes after curing of the Cas9-editing plasmid. Growth of uracil auxotrophs on media supplemented with 5-fluoroorotic acid and uracil results in loss of the complementing plasmid, providing a facile method for plasmid curing with potential applications in strain engineering and CRISPR editing. Metabolomic characterization of uracil auxotrophs revealed changes in cellular orotate concentrations consistent with partial or complete loss of orotate phosphoribosyltransferase activity. Our results expand the range of P. tricornutum auxotrophic strains and demonstrate that auxotrophic complementation markers provide a viable alternative to traditionally used antibiotic selection markers. Plasmid-based auxotrophic markers should expand the range of genome engineering applications and provide a means for biocontainment of engineered P. tricornutum strains.

LncRNA SNORD3A specifically sensitizes breast cancer cells to 5-FU by sponging miR-185-5p to enhance UMPS expression.

Breast cancer is the most common cancer type in women. Long non-coding RNAs (lncRNAs) have been reported as potential new diagnostic markers, prognostic factors, and therapeutic targets in cancer. However, the specific roles and mechanisms of lncRNAs in breast cancer remain to be elucidated. Here we demonstrated the downregulation of lncRNA SNORD3A in breast cancer cells and tissues and verified its non-protein-coding property. SNORD3A overexpression had no effect on cell proliferation but specifically sensitized breast cancer cells to 5-fluorouracil (5-FU) in vitro and in vivo. Mechanistically, SNORD3A exerts its effect via enhancing uridine monophosphate synthetase (UMPS) protein expression. SNORD3A acts as a competing endogenous RNA for miR-185-5p, leading to UMPS protein upregulation. miR-185-5p overexpression disrupted the effect of SNORD3A on chemosensitization to 5-FU in vitro and in vivo. Moreover, Meis1 overexpression transcriptionally promotes SNORD3A expression, and Meis1 is downregulated in breast cancer cells and tissues. In breast cancer tissues, SNORD3A level positively correlates with Meis1 and UMPS protein levels, whereas miR-185-5p level negatively correlates with UMPS protein level. High SNORD3A transcript and Meis1 and UMPS protein levels predicts a better outcome, but high miR-185-5p level predicts a worse outcome in breast cancer patients receiving 5-FU-based chemotherapy. Our findings indicate that Meis1-regulated SNORD3A specifically sensitizes breast cancer cells to 5-FU via enhancing UMPS expression. The SNORD3A-UMPS axis may serve as a potential biomarker and therapeutic target to improve the efficacy of 5-FU-based chemotherapy for breast cancer patients.

BCL-2 family protein BOK is a positive regulator of uridine metabolism in mammals.

BCL-2 family proteins regulate the mitochondrial apoptotic pathway. BOK, a multidomain BCL-2 family protein, is generally believed to be an adaptor protein similar to BAK and BAX, regulating the mitochondrial permeability transition during apoptosis. Here we report that BOK is a positive regulator of a key enzyme involved in uridine biosynthesis; namely, uridine monophosphate synthetase (UMPS). Our data suggest that BOK expression enhances UMPS activity, cell proliferation, and chemosensitivity. Genetic deletion of Bok results in chemoresistance to 5-fluorouracil (5-FU) in different cell lines and in mice. Conversely, cancer cells and primary tissues that acquire resistance to 5-FU down-regulate BOK expression. Furthermore, we also provide evidence for a role for BOK in nucleotide metabolism and cell cycle regulation. Our results have implications in developing BOK as a biomarker for 5-FU resistance and have the potential for the development of BOK-mimetics for sensitizing 5-FU-resistant cancers.

Orotate phosphoribosyltransferase as a predictor of benefit from S-1 adjuvant chemotherapy for cholangiocarcinoma patients.

BACKGROUND AND AIM: To improve the prognosis of cholangiocarcinoma, we investigated potential biomarkers that may enable the selection of patients for whom postoperative adjuvant chemotherapy is likely effective. METHODS: The cohort of this retrospective study included 170 surgically resected cholangiocarcinoma patients, 26 with gemcitabine adjuvant chemotherapy (GEM group), 36 with S-1 adjuvant chemotherapy (S-1 group), and 103 receiving no adjuvant chemotherapy (NC group). Propensity score matching was performed to adjust patient backgrounds; 36 patients from the NC group then were selected. Immunohistochemistry of orotate phosphoribosyltransferase (OPRT) and human equilibrative nucleoside transporter 1 (hENT1) was performed to determine the correlation between their expression and disease-free survival (DFS). RESULTS: After matching, the backgrounds of these three groups were unbiased. No significant improvement of DFS by adjuvant chemotherapy was observed in the whole cohort. However, among the high-OPRT-expression patients, DFS of GEM, S-1, and NC groups at 5 years was 28.8%, 53.8%, and 25.5%, respectively. The DFS of the S-1 group was significantly longer than that of the NC group (P = 0.034). On the other hand, no significant differences in DFS were observed among the low OPRT expression patients. hENT1 expression was shown to have no predictive value. Multivariate analysis of the high-OPRT-expression patients demonstrated that S-1 adjuvant chemotherapy can reduce tumor recurrence (HR, 0.303; P = 0.013). CONCLUSION: Cholangiocarcinoma patients with high OPRT expression would benefit from postoperative adjuvant S-1 therapy, which increases the DFS. Assessment of OPRT expression may contribute to the optimization of adjuvant chemotherapy for cholangiocarcinoma.

One-step generation of multiple gene knock-outs in the diatom Phaeodactylum tricornutum by DNA-free genome editing.

Recently developed transgenic techniques to explore and exploit the metabolic potential of microalgae present several drawbacks associated with the delivery of exogenous DNA into the cells and its subsequent integration at random sites within the genome. Here, we report a highly efficient multiplex genome-editing method in the diatom Phaeodactylum tricornutum, relying on the biolistic delivery of CRISPR-Cas9 ribonucleoproteins coupled with the identification of two endogenous counter-selectable markers, PtUMPS and PtAPT. First, we demonstrate the functionality of RNP delivery by positively selecting the disruption of each of these genes. Then, we illustrate the potential of the approach for multiplexing by generating double-gene knock-out strains, with 65% to 100% efficiency, using RNPs targeting one of these markers and PtAureo1a, a photoreceptor-encoding gene. Finally, we created triple knock-out strains in one step by delivering six RNP complexes into Phaeodactylum cells. This approach could readily be applied to other hard-to-transfect organisms of biotechnological interest.

Metabolic engineering using iterative self-cloning to improve lipid productivity in Coccomyxa.

We previously developed a self-cloning system that introduces cDNA of the uridine monophosphate synthase gene (cUMPS) of Coccomyxa sp. strain Obi as a selectable marker into uracil-auxotrophic mutants (Ura-) of the same alga. Here, we developed a Cre/loxP-based system for the removal of cUMPS flanked by directly repeated loxP sites from the Coccomyxa genome using the intracellular delivery of purified Cre recombinase to generate an Ura- strain that was used as a host for second-round transformation using cUMPS as the selection marker. Employing this marker-gene-recycling system, Coccomyxa strains devoid of foreign DNA except the 34-bp loxP sequence, which overexpressed an acyl-(acyl-carrier-protein) thioesterase gene, and a type-2 diacylglycerol acyltransferase gene, were constructed by the sequential introduction of two expression cassettes for the respective genes. One of the resulting strains showed 1.4-fold higher lipid productivity than the wild-type strain. This method will be applicable to other eukaryotic microalgae to create marker-free transgenic strains.

QM/MM analysis of effect of divalent metal ions on OPRT action.

The role of Mg2+ cofactor in orotate phosphoribosyltransferase (OPRT) catalyzed synthesis of orotidine monophosphate (OMP) from phosphoribosyl pyrophosphate (PRPP) and orotate (OA) in substrate binding and the influence of the identity of the divalent metal ion on the reaction mechanism were addressed in this study using quantum mechanics/molecular mechanics framework. Energetics of migration and binding of different substrate complexes in the active site cavity was established. A quantitative analysis of various processes indicated the reaction pathway to consist of complexation of Mg2+ with PRPP, migration of Mg2+-PRPP and OA towards the active site, binding of OA to OPRT, and binding of Mg2+-PRPP complex to OA-OPRT complex. The mechanism of the reaction was unaltered by the change in the identity of divalent metal ion. Experimentally reported inhibiting character of Co2+ was explained on the basis of large Co2+-PRPP binding and migration energies. Mg2+, Ca2+, Mn2+, Co2+ and Zn2+ ions were screened computationally to assess their inhibiting/activating characteristics. Trends obtained by our computational investigations were in correspondence with experimentally reported trends.

QM/MM reveals the sequence of substrate binding during OPRT action.

Computational investigation of orotate phosphoribosyltransferase (OPRT) action, an enzymatic reaction between phosphoribosyl pyrophosphate (PRPP) and orotic acid (OA) to yield orotidine 5'-monophosphate (OMP), was carried out. Insights into the pathways of the substrate attack step of the reaction were developed under the quantum mechanics/molecular mechanics framework with S. cerevisiae strain as the representative enzyme bearer. Four pathways were proposed for PRPP and OA binding differing in the sequence of PRPP, OA and Mg2+ ion complexation with OPRT. The formation of Mg2+-OPRT complex was accompanied by a small energy change while the largest stabilization was observed for the formation of Mg2+-PRPP complex supporting the experimental observation of Mg2+-PRPP complex as the true substrate for the reaction. Formation of PRPP-OPRT complex was found to be energetically not probable rendering the pathway requiring Mg2+-OA complex not probable. Further, PRPP migration towards the active site was found to be energetically not favoured rendering the pathway involving Mg2+-OA complexation improbable. Migration of OA and Mg2+-PRPP complex towards the active site was found to be energetically probable with a large stabilization of the system when Mg2+-PRPP complex bound to the OA-OPRT complex. This conclusively proved the sequential binding of OA and Mg2+-PRPP complexes during OPRT action.

Bifunctional activity of fused Plasmodium falciparum orotate phosphoribosyltransferase and orotidine 5'-monophosphate decarboxylase.

Fusion of the last two enzymes in the pyrimidine biosynthetic pathway in the inversed order by having a COOH-terminal orotate phosphoribosyltransferase (OPRT) and an NH2-terminal orotidine 5'-monophosphate decarboxylase (OMPDC), as OMPDC-OPRT, are described in many organisms. Here, we produced gene fusions of Plasmodium falciparum OMPDC-OPRT and expressed the bifunctional protein in Escherichia coli. The enzyme was purified to homogeneity using affinity and anion-exchange chromatography, exhibited enzymatic activities and functioned as a dimer. The activities, although unstable, were stabilized by its substrate and product during purification and long-term storage. Furthermore, the enzyme expressed a perfect catalytic efficiency (kcat/Km). The kcat was selectively enhanced up to three orders of magnitude, while the Km was not much affected and remained at low µM levels when compared to the monofunctional enzymes. The fusion of the two enzymes, creating a "super-enzyme" with perfect catalytic power and more flexibility, reflects cryptic relationship of enzymatic reactivities and metabolic functions on molecular evolution.

Uridine monophosphate synthetase enables eukaryotic de novo NAD+ biosynthesis from quinolinic acid.

NAD+ biosynthesis is an attractive and promising therapeutic target for influencing health span and obesity-related phenotypes as well as tumor growth. Full and effective use of this target for therapeutic benefit requires a complete understanding of NAD+ biosynthetic pathways. Here, we report a previously unrecognized role for a conserved phosphoribosyltransferase in NAD+ biosynthesis. Because a required quinolinic acid phosphoribosyltransferase (QPRTase) is not encoded in its genome, Caenorhabditis elegans are reported to lack a de novo NAD+ biosynthetic pathway. However, all the genes of the kynurenine pathway required for quinolinic acid (QA) production from tryptophan are present. Thus, we investigated the presence of de novo NAD+ biosynthesis in this organism. By combining isotope-tracing and genetic experiments, we have demonstrated the presence of an intact de novo biosynthesis pathway for NAD+ from tryptophan via QA, highlighting the functional conservation of this important biosynthetic activity. Supplementation with kynurenine pathway intermediates also boosted NAD+ levels and partially reversed NAD+-dependent phenotypes caused by mutation of pnc-1, which encodes a nicotinamidase required for NAD+ salvage biosynthesis, demonstrating contribution of de novo synthesis to NAD+ homeostasis. By investigating candidate phosphoribosyltransferase genes in the genome, we determined that the conserved uridine monophosphate phosphoribosyltransferase (UMPS), which acts in pyrimidine biosynthesis, is required for NAD+ biosynthesis in place of the missing QPRTase. We suggest that similar underground metabolic activity of UMPS may function in other organisms. This mechanism for NAD+ biosynthesis creates novel possibilities for manipulating NAD+ biosynthetic pathways, which is key for the future of therapeutics.

Development of new promising antimetabolite, DFP-11207 with self-controlled toxicity in rodents.

To reduce 5-fluorouracil (5-FU)-induced serious toxicities without loss of antitumor activity, we have developed DFP-11207, a novel fluoropyrimidine, which consists of 1-ethoxymethyl-5-fluorouracil (EM-FU; a precursor form of 5-FU), 5-chloro-2,4-dihydroxypyridine (CDHP; an inhibitor of 5-FU degradation), and citrazinic acid (CTA; an inhibitor of 5-FU phosphorylation). In vitro studies of DFP-11207 indicated that it strongly inhibited the degradation of 5-FU by dihydropyrimidine dehydrogenase (DPD) in homogenates of the rat liver, and also inhibited the phosphorylation of 5-FU by orotate phosphoribosyltransferase (OPRT) in tumor tissues in a similar magnitude of potency by CDHP and CTA, respectively. Especially, DFP-11207 inhibited the intracellular phosphorylation of 5-FU in tumor cells in a dose-dependent manner whereas CTA alone did not protect intracellular 5-FU phosphorylation. These results postulate that DFP-11207 rapidly entered into the cell and the free CTA produced from DFP-11207 inhibited the phosphorylation of 5-FU in the cell. Furthermore, following oral administration of DFP-11207, CTA was found to be highly retained in the gastrointestinal (GI) tract compared to other tissues in rats. Interestingly, EM-FU, the prodrug of 5-FU was found to specifically produce 5-FU by various species of liver microsomes. When DFP-11207 was administered to rats, the plasma level of 5-FU was persisted for a long-time with lower Cmax and longer half-life than that from other 5-FU prodrugs. The antitumor activity of DFP-11207 was evaluated in human tumor xenografts in nude rats and found that DFP-11207 showed an antitumor activity in a dose-dependent fashion and its efficacy is equivalent to reference 5-FU drugs. In striking contrast, DFP-11207 manifested no or less 5-FU-related toxicities, such as a decrease in body weights, GI injury, and myelosuppression, especially thrombocytopenia. Taken together, the preclinical evaluation of DFP-11207 strongly indicates that DFP-11207 be a potential new version of the oral fluoropyrimidine prodrug for further clinical development.

Structural investigations on orotate phosphoribosyltransferase from Mycobacterium tuberculosis, a key enzyme of the de novo pyrimidine biosynthesis.

The Mycobacterium tuberculosis orotate phosphoribosyltransferase (MtOPRT) catalyses the conversion of α-D-5-phosphoribosyl-1-pyrophosphate (PRPP) and orotate (OA) in pyrophosphate and orotidine 5'-monophosphate (OMP), in presence of Mg2+. This enzyme is the only responsible for the synthesis of orotidine 5'-monophosphate, a key precursor in the de novo pyrimidine biosynthesis pathway, making MtOPRT an attractive drug target for the development of antitubercular agents. We report the crystal structures of MtOPRT in complex with PRPP (2.25 Å resolution), inorganic phosphate (1.90 Å resolution) and the exogenous compound Fe(III) dicitrate (2.40 Å resolution). The overall structure of the mycobacterial enzyme is highly similar to those described for other OPRTases, with the "flexible loop" assuming a well define conformation and making specific contacts with the Fe(III)-dicitrate complex. The structures here reported add to the knowledge of a potential drug target for tuberculosis, and will provide a useful tool for the structure-based drug design of potent enzyme inhibitors.

Mild orotic aciduria in UMPS heterozygotes: a metabolic finding without clinical consequences.

BACKGROUND: Elevated urinary excretion of orotic acid is associated with treatable disorders of the urea cycle and pyrimidine metabolism. Establishing the correct and timely diagnosis in a patient with orotic aciduria is key to effective treatment. Uridine monophosphate synthase is involved in de novo pyrimidine synthesis. Uridine monophosphate synthase deficiency (or hereditary orotic aciduria), due to biallelic mutations in UMPS, is a rare condition presenting with megaloblastic anemia in the first months of life. If not treated with the pyrimidine precursor uridine, neutropenia, failure to thrive, growth retardation, developmental delay, and intellectual disability may ensue. METHODS AND RESULTS: We identified mild and isolated orotic aciduria in 11 unrelated individuals with diverse clinical signs and symptoms, the most common denominator being intellectual disability/developmental delay. Of note, none had blood count abnormalities, relevant hyperammonemia or altered plasma amino acid profile. All individuals were found to have heterozygous alterations in UMPS. Four of these variants were predicted to be null alleles with complete loss of function. The remaining variants were missense changes and predicted to be damaging to the normal encoded protein. Interestingly, family screening revealed heterozygous UMPS variants in combination with mild orotic aciduria in 19 clinically asymptomatic family members. CONCLUSIONS: We therefore conclude that heterozygous UMPS-mutations can lead to mild and isolated orotic aciduria without clinical consequence. Partial UMPS-deficiency should be included in the differential diagnosis of mild orotic aciduria. The discovery of heterozygotes manifesting clinical symptoms such as hypotonia and developmental delay are likely due to ascertainment bias.

Inducing and Quantifying Clostridium difficile Spore Formation.

The Gram-positive nosocomial pathogen Clostridium difficile induces sporulation during growth in the gastrointestinal tract. Sporulation is necessary for this obligate anaerobe to form metabolically dormant spores that can resist antibiotic treatment, survive exit from the mammalian host, and transmit C. difficile infections. In this chapter, we describe a method for inducing C. difficile sporulation in vitro. This method can be used to study sporulation and maximize spore purification yields for a number of C. difficile strain backgrounds. We also describe procedures for visualizing spore formation using phase-contrast microscopy and for quantifying the efficiency of sporulation using heat resistance as a measure of functional spore formation.