Regulation of the human inosine monophosphate dehydrogenase type I gene. Utilization of alternative promoters

Personalized Therapy for Mycophenolate: Consensus Report by the International Association of Therapeutic Drug Monitoring and Clinical Toxicology

ABSTRACT

When mycophenolic acid (MPA) was originally marketed for immunosuppressive therapy, fixed doses were recommended by the manufacturer. Awareness of the potential for a more personalized dosing has led to development of methods to estimate MPA area under the curve based on the measurement of drug concentrations in only a few samples. This approach is feasible in the clinical routine and has proven successful in terms of correlation with outcome. However, the search for superior correlates has continued, and numerous studies in search of biomarkers that could better predict the perfect dosage for the individual patient have been published. As it was considered timely for an updated and comprehensive presentation of consensus on the status for personalized treatment with MPA, this report was prepared following an initiative from members of the International Association of Therapeutic Drug Monitoring and Clinical Toxicology (IATDMCT). Topics included are the criteria for analytics, methods to estimate exposure including pharmacometrics, the potential influence of pharmacogenetics, development of biomarkers, and the practical aspects of implementation of target concentration intervention. For selected topics with sufficient evidence, such as the application of limited sampling strategies for MPA area under the curve, graded recommendations on target ranges are presented. To provide a comprehensive review, this report also includes updates on the status of potential biomarkers including those which may be promising but with a low level of evidence. In view of the fact that there are very few new immunosuppressive drugs under development for the transplant field, it is likely that MPA will continue to be prescribed on a large scale in the upcoming years. Discontinuation of therapy due to adverse effects is relatively common, increasing the risk for late rejections, which may contribute to graft loss. Therefore, the continued search for innovative methods to better personalize MPA dosage is warranted.

GTP metabolic reprogramming by IMPDH2: unlocking cancer cells' fuelling mechanism

Growing cells increase multiple biosynthetic processes in response to the high metabolic demands needed to sustain proliferation. The even higher metabolic requirements in the setting of cancer provoke proportionately greater biosynthesis. Underappreciated key aspects of this increased metabolic demand are guanine nucleotides and adaptive mechanisms to regulate their concentration. Using the malignant brain tumour, glioblastoma, as a model, we have demonstrated that one of the rate-limiting enzymes for guanosine triphosphate (GTP) synthesis, inosine monophosphate dehydrogenase-2 (IMPDH2), is increased and IMPDH2 expression is necessary for the activation of de novo GTP biosynthesis. Moreover, increased IMPDH2 enhances RNA polymerase I and III transcription directly linking GTP metabolism to both anabolic capacity as well as nucleolar enlargement historically observed as associated with cancer. In this review, we will review in detail the basis of these new discoveries and, more generally, summarize the current knowledge on the role of GTP metabolism in cancer.

ANKRD9 is associated with tumor suppression as a substrate receptor subunit of ubiquitin ligase

BACKGROUND

Human ANKRD9 (ankyrin repeat domain 9) expression is altered in some cancers.

METHODS

We tested genetic association of ANKRD9 with gastric cancer susceptibility and examined functional association of ANKRD9 with altered proliferation of MKN45 gastric cancer cells. We then identified ANKRD9-binding partners in HEK 293 embryonic kidney cells using quantitative proteomics, western blotting and complex reconstitution assays. We finally demonstrated ANKRD9's role of recognizing substrates for ubiquitination using in vitro ubiquitylation assay.

RESULTS

ANKRD9 is associated with cancer susceptibility in a comparison of single-nucleotide polymorphisms between 1092 gastric cancer patients and 1206 healthy controls. ANKRD9 depletion accelerates tumor progression by increasing cellular proliferation, piling up, and anchorage-independent growth of MKN45 cells. We discovered that ANKRD9 is a ubiquitin ligase substrate receptor subunit and has an anti-proliferative activity. ANKRD9 associates with CUL5 (not CUL2), ELOB, ELOC, and presumably RNF7 subunits, which together assemble into a cullin-RING superfamily E3 ligase complex. ANKRD9 belongs to the ASB family of proteins, which are characterized by the presence of ankyrin repeats and a SOCS box. In addition to its interactions with the other E3 ligase subunits, ANKRD9 interacts with two isoforms of inosine monophosphate dehydrogenase (IMPDH). These IMPDH isoforms are cognate substrates of the ANKRD9-containing E3 enzyme, which ubiquitinates them for proteasomal degradation. Their ubiquitination and turnover require the presence of ANKRD9.

CONCLUSION

ANKRD9, a previously unidentified E3 substrate receptor subunit, functions in tumor suppression by recognizing the oncoprotein IMPDH isoforms for E3 ubiquitination and proteasomal degradation.

Inosine monophosphate dehydrogenase expression and activity are significantly lower in kidney transplant recipients with diabetes mellitus

BACKGROUND

Inosine 5'-monophosphate dehydrogenase (IMPDH) is a target of the immunosuppressive drug, mycophenolic acid (MPA). A 12-hour clinical pharmacokinetic and pharmacodynamic study was conducted to compare IMPDH1 and IMPDH2 gene expression, IMPDHI and IMPDHII protein levels, and enzyme activity between kidney transplant recipients with respect to diabetes status.

METHODS

Nondiabetic (ND, n = 11) and diabetic (D, n = 9) kidney transplant recipients and on nontransplant nondiabetic (n = 10) and diabetic (n = 10) volunteers were included in the study.

RESULTS

Area under the effect curve values for gene expression: IMPDH1 [ND: 22.1 (13.8-31.3) versus D: 4.5 (2.3-6.5), P < 0.001] and IMPDH2 [ND: 15.3 (11.0-21.7) versus D: 6.1 (4.6-8.6), P < 0.001], protein level: IMPDHI [ND: 1.0 (0.5-1.3) versus 0.5 (0.4-0.7), P = 0.002] and IMPDHII [ND: 1.0 (0.6-1.6) versus D: 0.7 (0.6-0.8) P < 0.001] and enzyme activity [ND: 180 (105-245) versus D: 29.9 (15.3-35.6) µmole·s(-1)·mole(-1) adenosine monophosphate, P < 0.001] was significantly lower in transplant recipients with diabetes. Similar results were observed in nontransplanted volunteers. Kinetic studies of MPA-mediated suppression of IMPDH activity in nontransplanted individuals revealed an approximately 2.5-fold lower half-maximum effective concentration (EC50) for diabetic as compared with nondiabetic [ND: 50.2 (49.8-50.7) versus D: 15.8 (15.6-16.3) nmole/L, P = 0.004] volunteers. This difference was not related to several IMPDH gene variants.

CONCLUSIONS

This study indicates a significantly lower IMPDH gene expression, protein level, and enzyme activity in diabetic patients. Further clinical studies in a larger number of patients are warranted to verify whether MPA dosing must be optimized for kidney transplant recipients with diabetes mellitus.

In vitro and in vivo characterization of AS2643361, a novel and highly potent inosine 5'-monophosphate dehydrogenase inhibitor

Inosine 5'-monophosphate (IMP) dehydrogenase is a critical target in solid organ transplantation. To this end, the development of mycophenolate mofetil (MMF) represents a major advance in transplant medicine. Here, we investigated the in vitro and in vivo pharmacological effects of a novel IMP dehydrogenase inhibitor, AS2643361, in several immunological and non-immunological models. The in vitro inhibitory activity of AS2643361 on immune cell and endothelial cell proliferation and on antibody production from lipopolysaccharide-stimulated B cells, was significantly more potent than that of mycophenolic acid, the active form of MMF, despite the similar potency of these compounds on IMP dehydrogenase. In a rat heterotopic cardiac transplant model, monotherapy using orally administered AS2643361 at 10 or 20mg/kg/day prolonged the median graft survival time from 6 to 16 and 19days, respectively. In dinitrophenol-lipopolysaccharide stimulated rats, oral administration of AS2643361 at 2.5, 5 or 10mg/kg/day resulted in suppression of antibody production. In vivo antibody production against alloantigen was also suppressed by AS2643361 treatment at 5 or 10mg/kg/day. Furthermore, treatment with AS2543361 effectively inhibited balloon injury induced-intimal thickening, which is a major cause of late allograft loss. Overall, the in vivo activity of AS2643361 was over two-fold more potent than that of MMF. In addition, gastrointestinal toxicity, considered a dose-limiting factor for MMF, was reduced with AS2643361 treatment. These results suggest AS2643361 has higher potency and less toxicity than MMF, making it a potential candidate for treatment of acute and chronic rejection in transplant medicine.

Investigating the mechanism of disease in the RP10 form of retinitis pigmentosa

Retinitis pigmentosa (RP) is a disease characterized by its vast heterogeneity. Many genes are associated with RP, and the disease causing mutations identified in these genes are even more numerous. To date there are 15 genes that cause autosomal dominant RP (adRP) alone. The role of some of these genes, while complex and not completely understood, is somewhat intuitive in that they are involved in pathways such as phototransduction. However, the role of other genes in retinal disease is not as predictable due to their ubiquitous function and/or expression. One such gene is inosine monophosphate dehydrogenase 1 (IMPDH1) IMPDH1 is a gene involved in de novo purine synthesis and is ubiquitously expressed. IMPDH1 mutations account for 2% of all adRP cases and are a rare cause of Leiber Congenital Amaurosis. Despite its ubiquitous expression missense mutations in this gene cause only retinal degeneration. This paradox of tissue specific disease in the presence of ubiquitous expression has only recently begun to be explained. We have shown in a recent study that novel retinal isoforms of IMPDH1 exist and may account for the tissue specificity of disease. We have gone on to characterize these retinal isoforms both in our laboratory and in collaboration with Dr. Lizbeth Hedstrom's laboratory at Brandeis University (Waltham, MA) in order to understand more about them. We believe that through clarifying the mechanism of disease in RP10 we will be equipped to consider treatment options for this disease.

Pharmacogenetics of the mycophenolic acid targets inosine monophosphate dehydrogenases IMPDH1 and IMPDH2: gene sequence variation and functional genomics

BACKGROUND AND PURPOSE

Inosine monophosphate dehydrogenases, encoded by IMPDH1 and IMPDH2, are targets for the important immunosuppressive drug, mycophenolic acid (MPA). Variation in MPA response may result, in part, from genetic variation in IMPDH1 and IMPDH2.

EXPERIMENTAL APPROACH

We resequenced IMPDH1 and IMPDH2 using DNA from 288 individuals from three ethnic groups and performed functional genomic studies of the sequence variants observed.

KEY RESULTS

We identified 73 single nucleotide polymorphisms (SNPs) in IMPDH1, 59 novel, and 25 SNPs, 24 novel, in IMPDH2. One novel IMPDH1 allozyme (Leu275) had 10.2% of the wild-type activity as a result of accelerated protein degradation. Decreased activity of the previously reported IMPDH2 Phe263 allozyme was primarily due to decreased protein quantity, also with accelerated degradation. These observations with regard to the functional implications of variant allozymes were supported by the IMPDH1 and IMPDH2 X-ray crystal structures. A novel IMPDH2 intron 1 SNP, G > C IVS1(93), was associated with decreased mRNA quantity, possibly because of altered transcription.

CONCLUSIONS AND IMPLICATIONS

These results provide insight into the nature and extent of sequence variation in the IMPDH1 and IMPDH2 genes. They also describe the influence of gene sequence variation that alters the encoded amino acids on IMPDH function and provide a foundation for future translational studies designed to correlate sequence variation in these genes with outcomes in patients treated with MPA.

Inosine monophosphate dehydrogenase variability in renal transplant patients on long-term mycophenolate mofetil therapy

WHAT IS ALREADY KNOWN ABOUT THIS SUBJECT

* Mycophenolic acid (MPA) is a potent, selective and reversible inhibitor of inosine 5'-monophosphate dehydrogenase (IMPDH), the rate-limiting enzyme for de novo guanosine triphosphate biosynthesis. * The large IMPDH interindividual variability could be responsible for the differences in therapeutic effects and side-effects observed with MPA. * Induction of IMPDH activity has been observed in whole blood during immunosuppressive therapy.

WHAT THIS STUDY ADDS

* Our data were acquired in long-term mycophenolate mofetil-treated renal transplant recipients on different combinations of immunosuppressive agents (ciclosporin, tacrolimus, sirolimus) and with different treatment duration (up to 8.8 years post transplant). * The increasing trend in IMPDH activity that we observed throughout our 12-month observation period was significantly higher in rejecting than in nonrejecting subjects.

AIMS

Long-term mycophenolate mofetil (MMF) therapy may induce inosine 5'-monophosphate dehydrogenase (IMPDH) activity in peripheral blood mononuclear cells (PBMCs), thus decreasing MMF immunosuppressive properties. Pharmacodynamic monitoring was used to investigate whether biological activity is altered after long-term therapy.

METHODS

IMPDH activity was measured in PBMC samples from 54 stable kidney transplant patients, already on MMF (for at least 3 months), before (t(0)) and 2 h after (t(2)) MMF morning dose administration; levels were monitored for up to 15 months, together with total mycophenolic acid (MPA) and free MPA concentrations.

RESULTS

During the 15 months' monitoring, t(0) IMPDH activity in transplant recipients increased from 5.9 +/- 3.7 nmol h(-1) mg(-1)[95% confidence interval (CI) 4.9, 6.9] to 9.0 +/- 3.9 nmol h(-1) mg(-1) (95% CI 7.2, 10.8), with an intra- and interpatient variability of 28% and 42%. Five patients experienced acute rejection during the follow-up: t(0) IMPDH activity was increased during rejection vs. nonrejection, and the trend was significantly higher in rejecting than in nonrejecting subjects for the whole monitoring period.

CONCLUSIONS

Even though a correlation has been found between IMPDH activity and rejection, its efficacy as a predictive tool in long-term transplant outcomes may be affected by high interpatient variability; on the other hand, continuous monitoring of the IMPDH trend could make an effective prognostic parameter of rejection. Other trials also including pre-transplant data on both IMPDH expression and activity are warranted to better assess their role as biomarkers for MPA effect in clinical practice.

Inosine monophosphate dehydrogenase messenger RNA expression is correlated to clinical outcomes in mycophenolate mofetil-treated kidney transplant patients, whereas inosine monophosphate dehydrogenase activity is not

Measurement of the pharmacodynamic biomarker inosine monophosphate dehydrogenase (IMPDH) activity in renal transplant recipients has been proposed to reflect the biological effect better than using pharmacokinetic parameters to monitor mycophenolate mofetil therapy. The IMPDH assays are however labor intensive and this complicates implementation into patient care. Quantification of IMPDH messenger RNA (mRNA) could form an attractive alternative. This study was designed to correlate IMPDH mRNA levels with IMPDH activity and clinical outcome in renal transplant recipients. From a cohort of 101 renal transplant patients, blood samples were drawn pre transplantation and at 4 times after transplantation. IMPDH activity, IMPDH type 1 and type 2 mRNA levels, and mycophenolic acid concentrations were measured and correlated to clinical outcomes. No correlation was found between IMPDH type 1 and type 2 mRNA levels and IMPDH activity in pre- and posttransplant samples. A significant increase in IMPDH mRNA levels was found between day 6 and day 140 after transplantation. IMPDH type 1 and type 2 mRNA levels before transplant showed a trend toward statistically significant higher levels in patients with an acute rejection (P = 0.052 and P = 0.058). After transplant, the IMPDH type 1 and type 2 mRNA levels were significantly lower in patients with an acute rejection (P = 0.026 and P = 0.007). We conclude that IMPDH mRNA levels do not correlate with IMPDH activity but are nevertheless correlated with acute rejections. Furthermore, although the regulation of the expression of the 2 isoforms is presumed to be different, in this study, the changes in the expression of type 1 mRNA closely paralleled those of type 2.

Mycophenolate pharmacokinetics and pharmacodynamics in belatacept treated renal allograft recipients - a pilot study

BACKGROUND

Mycophenolic acid (MPA) is widely used as part of immunosuppressive regimens following allograft transplantation. The large pharmacokinetic (PK) and pharmacodynamic (PD) variability and narrow therapeutic range of MPA provide a potential for therapeutic drug monitoring. The objective of this pilot study was to investigate the MPA PK and PD relation in combination with belatacept (2nd generation CTLA4-Ig) or cyclosporine (CsA).

METHODS

Seven renal allograft recipients were randomized to either belatacept (n = 4) or cyclosporine (n = 3) based immunosuppression. Samples for MPA PK and PD evaluations were collected predose and at 1, 2 and 13 weeks posttransplant. Plasma concentrations of MPA were determined by HPLC-UV. Activity of inosine monophosphate dehydrogenase (IMPDH) and the expressions of two IMPDH isoforms were measured in CD4+ cells by HPLC-UV and real-time reverse-transcription PCR, respectively. Subsets of T cells were characterized by flow cytometry.

RESULTS

The MPA exposure tended to be higher among belatacept patients than in CsA patients at week 1 (P = 0.057). Further, MPA concentrations (AUC0-9 h and C0) increased with time in both groups and were higher at week 13 than at week 2 (P = 0.031, n = 6). In contrast to the postdose reductions of IMPDH activity observed early posttransplant, IMPDH activity within both treatment groups was elevated throughout the dosing interval at week 13. Transient postdose increments were also observed for IMPDH1 expression, starting at week 1. Higher MPA exposure was associated with larger elevations of IMPDH1 (r = 0.81, P = 0.023, n = 7 for MPA and IMPDH1 AUC0-9 h at week 1). The maximum IMPDH1 expression was 52 (13-177)% higher at week 13 compared to week 1 (P = 0.031, n = 6). One patient showed lower MPA exposure with time and did neither display elevations of IMPDH activity nor IMPDH1 expression. No difference was observed in T cell subsets between treatment groups.

CONCLUSION

The significant influence of MPA on IMPDH1 expression, possibly mediated through reduced guanine nucleotide levels, could explain the elevations of IMPDH activity within dosing intervals at week 13. The present regulation of IMPDH in CD4+ cells should be considered when interpreting measurements of IMPDH inhibition.

Expression of IMPDH1 is regulated in response to mycophenolate concentration

Inosine 5'-monophosphate dehydrogenase (IMPDH) catalyzes de novo guanine nucleotide synthesis. Mycophenolic acid (MPA) exerts immunosuppressive effects by inhibiting IMPDH. The aim of this study was to investigate gene expressions of two IMPDH isoforms, during in vivo exposure to MPA. Healthy volunteers (n=5) were given single doses of 100, 250, 500 and 1000 mg mycophenolate mofetil (MMF). Blood was sampled pre-dose and at 1, 2, 4, 6, 8, 12, and 24 h post-dose. The expressions of IMPDH 1 and 2 were quantified in CD4+ cells and whole blood by real-time reverse transcription-PCR. Following MMF doses of 500 mg, the expression of IMPDH 1 and 2 in CD4+ cells was reduced 39% (P=0.043) and 10% (P=0.043), respectively. Smaller reductions (ns) were observed after 1000 mg MMF. Similar trends were demonstrated for whole blood. The largest reductions of IMPDH1 occurred at MPA AUC(0-12 h) of 20 mg h/L. Below this, increasing MPA exposure correlated with larger reductions of IMPDH1 expression (CD4+ cells: r=-0.82, P<0.001, and whole blood: r=-0.50, P=0.04, n=17), while higher MPA exposure seemed to be associated with smaller reductions of expression (CD4+ cells: r=0.42, ns, and whole blood: r=0.77, P=0.039, n=8). The concentration-dependent modulation of IMPDH 1 and 2 expressions by MPA might impact IMPDH activity. Knowledge of the regulation of the two IMPDH isoenzymes in vivo by MPA is of importance considering pharmacodynamic monitoring and optimization of MPA treatment.

Insights from retinitis pigmentosa into the roles of isocitrate dehydrogenases in the Krebs cycle

Here we describe two families with retinitis pigmentosa, a hereditary neurodegeneration of rod and cone photoreceptors in the retina. Affected family members were homozygous for loss-of-function mutations in IDH3B, encoding the beta-subunit of NAD-specific isocitrate dehydrogenase (NAD-IDH, or IDH3), which is believed to catalyze the oxidation of isocitrate to alpha-ketoglutarate in the citric acid cycle. Cells from affected individuals had a substantial reduction of NAD-IDH activity, with about a 300-fold increase in the K(m) for NAD. NADP-specific isocitrate dehydrogenase (NADP-IDH, or IDH2), an enzyme that catalyzes the same reaction, was normal in affected individuals, and they had no health problems associated with the enzyme deficiency except for retinitis pigmentosa. These findings support the hypothesis that mitochondrial NADP-IDH, rather than NAD-IDH, serves as the main catalyst for this reaction in the citric acid cycle outside the retina, and that the retina has a particular requirement for NAD-IDH.

Beyond TPMT: genetic influences on thiopurine drug responses in inflammatory bowel disease

Azathioprine and 6-mercaptopurine are widely used in the management of inflammatory bowel disease (IBD). However, approximately 25% of IBD patients experience toxicity, and up to 10% show resistance to these thiopurine drugs. The importance of genetic variability in determining thiopurine toxicity was first recognized over 25 years ago with the discovery of the thiopurine S-methyltransferase (TPMT) polymorphism and the occurrence of azathioprine-induced myelosuppression in TPMT-deficient patients. In the intervening period, TPMT has become the foremost example of pharmacogenetics, and TPMT deficiency represents one of the few pharmacogenetic phenomena that have successfully made the transition from the research laboratory to diagnostics. While TPMT activity predicts some cases of myelosuppression, deficiency in this enzyme is neither predictive of other adverse drug reactions, nor resistance to thiopurine therapy. As myelosuppression only accounts for approximately 2.5% of adverse reactions in IBD patients, researchers are increasingly turning their attention to other enzymes involved in thiopurine metabolism to find molecular explanations for intolerance and resistance to azathioprine and 6-mercaptopurine. In this review, we summarize the current state of knowledge with regards to TPMT, and also explore genetic variability, beyond TPMT, that may contribute to thiopurine response in IBD patients.

Therapeutic benefit derived from RNAi-mediated ablation of IMPDH1 transcripts in a murine model of autosomal dominant retinitis pigmentosa (RP10)

Mutations within the inosine 5'-monophosphate dehydrogenase 1 (IMPDH1) gene cause the RP10 form of autosomal dominant retinitis pigmentosa (adRP), an early-onset retinopathy resulting in extensive visual handicap owing to progressive death of photoreceptors. Apart from the prevalence of RP10, estimated to account for 5-10% of cases of adRP in United States and Europe, two observations render this form of RP an attractive target for gene therapy. First, we show that while recombinant adeno-associated viral (AAV)-mediated expression of mutant human IMPDH1 protein in the mouse retina results in an aggressive retinopathy modelling the human counterpart, expression of a normal human IMPDH1 gene under similar conditions has no observable pathological effect on retinal function, indicating that over-expression of a therapeutic replacement gene may be relatively well tolerated. Secondly, complete absence of IMPDH1 protein in mice with a targeted disruption of the gene results in relatively mild retinal dysfunction, suggesting that significant therapeutic benefit may be derived even from the suppression-only component of an RNAi-based gene therapy. We show that AAV-mediated co-expression in the murine retina of a mutant human IMPDH1 gene together with short hairpin RNAs (shRNA) validated in vitro and in vivo, targeting both human and mouse IMPDH1, substantially suppresses the negative pathological effects of mutant IMPDH1, at a point where, in the absence of shRNA, expression of mutant protein in the RP10 model essentially ablates all photoreceptors in transfected areas of the retina. These data strongly suggest that an RNAi-mediated approach to therapy for RP10 holds considerable promise for human subjects.

Expression of IMPDH1 and IMPDH2 after transplantation and initiation of immunosuppression

BACKGROUND

Mycophenolic acid (MPA) mediates immunosuppressive effects by inhibiting inosine monophosphate dehydrogenase (IMPDH). Induction of IMPDH activity has been observed in whole blood and erythrocyte samples during immunosuppressive therapy. Information concerning the mechanisms for increased IMPDH activity is limited and the potential implications of induction have been debated.

METHODS

Whole blood, CD4+ cell, and reticulocyte samples were collected from 30 renal transplant patients pre- and posttransplantation. The expressions of two IMPDH isoforms, type 1 and 2, were analyzed by real-time reverse-transcription polymerase chain reaction and quantified using a housekeeping gene index. The IMPDH activity was determined by ultraviolet high-performance liquid chromatography.

RESULTS

Transplantation and the initiation of immunosuppressive therapy was associated with increased IMPDH1 (50-88%, P<0.0005) and decreased IMPDH2 (42-56%, P<0.0005) expression. In CD4+ cells, however, IMPDH2 increased (15%, P=0.009). These changes are probably related to glucocorticoid effects. Two weeks posttransplant, MPA-treated patients displayed elevated IMPDH 1 and 2 in reticulocytes, suggesting enzyme induction in these cells during prolonged MPA therapy. Patients with acute rejection during follow-up demonstrated higher IMPDH2 expression in CD4+ cells pretransplant than nonrejecting patients (median expression 1.26 vs. 0.87 respectively, P=0.017).

CONCLUSIONS

Knowledge of changes in IMPDH 1 and 2 expression after transplantation and initiation of immunosuppression is important considering the action of MPA on IMPDH and the potential for pharmacodynamic monitoring of MPA by measuring IMPDH activity. The expression of IMPDH2 in CD4+ cells pretransplant may be an indicator of immune activation.

IMPDH1 promoter mutations in a patient exhibiting azathioprine resistance

Around 9% of inflammatory bowel disease (IBD) patients are resistant to azathioprine. We hypothesized that these patients may carry mutations within inosine-5'-monophosphate dehydrogenase (IMPDH). To test this hypothesis, we screened 20 azathioprine-resistant patients for variations in the two IMPDH genes (IMPDH1 and IMPDH2) using dHPLC and DNA sequencing. A 9 bp insertion within the IMPDH1 P3 promoter was found in a patient exhibiting severe azathioprine resistance. The insertion is predicted to abolish a cAMP-response element (CRE) and was found to significantly reduce IMPDH1 P3 promoter activity in a luciferase reporter gene assay (P-value <0.001). This in vitro assay suggests the variant promoter has altered function in vivo and consequently may have contributed to the thiopurine resistance observed in this patient. The absence of functional variants within the other patients indicates that if IMPDH genetic variability contributes to azathioprine resistance it does so infrequently.

Expression of inosine monophosphate dehydrogenase type I and type II after mycophenolate mofetil treatment: a 2-year follow-up in kidney transplantation

The objective of the study was to evaluate the effect of mycophenolate mofetil (MMF) on the regulation of inosine monophosphate dehydrogenase (IMPDH) during the first 2 years after renal transplantation. Twelve patients were enrolled, and 10-h time-course evaluations of the effects of MMF were regularly performed during the study. IMPDH activity and gene expression were measured in whole blood and in mononuclear cells, respectively. Type I IMPDH (IMPDH-I) mRNA was increased during the first 3 months following transplantation and reached its maximal level during acute rejection episodes, whereas type II IMPDH mRNA was stable. Furthermore, although no alteration in the predose samples was observed, patients with prolonged MMF treatment exhibited an increase in the induction potency of both IMPDH activity and gene expression. In vitro experiments confirmed that IMPDH-I is inducible, but preferentially in monocytes than in lymphocytes. This finding suggests that the measurement of IMPDH mRNAs may provide reliable information to predict acute rejection.

Real-Time PCR Determination of IMPDH1 and IMPDH2 Expression in Blood Cells

Background: Inosine monophosphate dehydrogenase (IMPDH) catalyzes the rate-limiting step in de novo guanine nucleotide synthesis and is implicated in cell cycle control. Inhibition of this enzyme is associated with immunosuppressive, antiviral, and antitumor activity. IMPDH basal activity increases after initiation of immunosuppressive therapy.

Methods: A real-time reverse-transcription PCR assay was developed and validated for mRNA quantification of the 2 human IMPDH isoforms. Target gene expressions were normalized to the geometric mean of 3 housekeeping genes. Assay utility was tested by analyzing patient samples and cultured cells exposed to immunosuppressive drugs such as the IMPDH inhibitor mycophenolic acid.

Results: The assay was linear over 6 logs of cDNA input and demonstrated specific quantification of IMPDH1 and IMPDH2 expression in cultured cells and patient samples. Limits of detection and quantification were 10 and 103 copies of cDNA per reaction, respectively. Within-run and total between-day CVs were &lt;15% for normalized expression. Changes in IMPDH1 and 2 expression were observed in patient samples after initiation of an immunosuppressive regimen that included calcineurin inhibitors, mycophenolate mofetil, and steroids.

Conclusions: This assay can be used to study the regulation of IMPDH expression and the involvement of the enzymes in immunological and malignant proliferative conditions. This may contribute to the processes of drug development and to the establishment of monitoring strategies for treatment effect and disease activity.

Why do mutations in the ubiquitously expressed housekeeping gene IMPDH1 cause retina-specific photoreceptor degeneration?

PURPOSE

The purpose of this study was to investigate retinal inosine monophosphate dehydrogenase 1 (IMPDH1) transcripts and proteins to gain an understanding of how mutations in IMPDH1 lead to retinal disease. Mutations in IMPDH1 cause the RP10 form of autosomal dominant retinitis pigmentosa (adRP) and are a rare cause of dominant Leber congenital amaurosis (LCA). IMPDH1 is a highly conserved, widely expressed housekeeping gene, the product of which catalyzes the rate-limiting step of de novo guanine synthesis. Despite its conservation and ubiquity, the clinical consequences of missense mutations in IMPDH1 are limited to the retina, and the disease mechanism is currently unknown.

METHODS

A variety of methods were used to address the unique features of IMPDH1 in the retina, including Northern blot analysis, serial analysis of gene expression (SAGE), immunohistochemistry, transcript sequencing, and Western blot analysis.

RESULTS

Results of the experiments showed that IMPDH1 levels are higher in the retina than in any other tissue tested. Specifically, IMPDH1 is found predominately in the inner segment and synaptic terminals of retinal photoreceptors. The predominant transcripts of IMPDH1 in human retina are the result of alternate splicing and alternate start sites of translation. They are significantly different from those in other tissues, and these variant transcripts encode distinct proteins. Further, the proportions of IMPDH1 transcripts and proteins in human retina are different from those in mouse retina.

CONCLUSIONS

Identification of unique retinal isoforms supports the existence of a novel IMPDH1 function in the retina, one that is probably altered by disease-causing mutations. This alone, or coupled with the high levels of IMPDH1 in the retina, may explain the retina-specific phenotype associated with IMPDH1 mutations. Elucidating the functional properties of these unique, human retinal isoforms is crucial to understanding the pathophysiology of IMPDH1 mutations.

Spectrum and frequency of mutations in IMPDH1 associated with autosomal dominant retinitis pigmentosa and leber congenital amaurosis

PURPOSE

The purpose of this study was to determine the frequency and spectrum of inosine monophosphate dehydrogenase type I (IMPDH1) mutations associated with autosomal dominant retinitis pigmentosa (RP), to determine whether mutations in IMPDH1 cause other forms of inherited retinal degeneration, and to analyze IMPDH1 mutations for alterations in enzyme activity and nucleic acid binding.

METHODS

The coding sequence and flanking intron/exon junctions of IMPDH1 were analyzed in 203 patients with autosomal dominant RP (adRP), 55 patients with autosomal recessive RP (arRP), 7 patients with isolated RP, 17 patients with macular degeneration (MD), and 24 patients with Leber congenital amaurosis (LCA). DNA samples were tested for mutations by sequencing only or by a combination of single-stranded conformational analysis and by sequencing. Production of fluorescent reduced nicotinamide adenine dinucleotide (NADH) was used to measure enzymatic activity of mutant IMPDH1 proteins. The affinity and the specificity of mutant IMPDH1 proteins for single-stranded nucleic acids were determined by filter-binding assays.

RESULTS

Five different IMPDH1 variants, Thr116Met, Asp226Asn, Val268Ile, Gly324Asp, and His 372Pro, were identified in eight autosomal dominant RP families. Two additional IMPDH1 variants, Arg105Trp and Asn198Lys, were found in two patients with isolated LCA. None of the novel IMPDH1 mutants identified in this study altered the enzymatic activity of the corresponding proteins. In contrast, the affinity and/or the specificity of single-stranded nucleic acid binding were altered for each IMPDH1 mutant except the Gly324Asp variant.

CONCLUSIONS

Mutations in IMPDH1 account for approximately 2% of families with adRP, and de novo IMPDH1 mutations are also rare causes of isolated LCA. This analysis of the novel IMPDH1 mutants substantiates previous reports that IMPDH1 mutations do not alter enzyme activity and demonstrates that these mutants alter the recently identified single-stranded nucleic acid binding property of IMPDH. Studies are needed to further characterize the functional significance of IMPDH1 nucleic acid binding and its potential relationship to retinal degeneration.

Autosomal dominant retinitis pigmentosa mutations in inosine 5'-monophosphate dehydrogenase type I disrupt nucleic acid binding

Two mutations of IMPDH1 (inosine 5'-monophosphate dehydrogenase type I), R224P and D226N, have recently been found to cause adRP (autosomal dominant retinitis pigmentosa). IMPDH1 catalyses the rate-limiting step in guanine nucleotide biosynthesis and also binds single-stranded nucleic acids. In the present paper, we report the biochemical characterization of the adRP-linked mutations, R224P and D226N, and a potentially pathogenic mutation, V268I. The adRP-linked mutations have no effect on enzyme activity, protein stability or protein aggregation. These results suggest strongly that the mutations do not affect enzyme activity in vivo and thus do not perturb the guanine nucleotide pool. The R224P mutation changes the distribution of enzyme between the nucleus and cytoplasm. This effect was not observed with the D226N mutation, so the relevance of this observation to disease is unclear. In contrast, both mutations decrease the affinity of nucleic acid binding and both fail to co-immunoprecipitate RNA. These observations suggest that nucleic acid binding provides a functional assay for adRP pathogenicity. The putative adRP-linked mutation V268I also disrupts nucleic acid binding, which suggests that this mutation is indeed pathogenic.

Phenotypic characterization of a large family with RP10 autosomal-dominant retinitis pigmentosa: an Asp226Asn mutation in the IMPDH1 gene

PURPOSE

To evaluate the clinical features associated with the RP10 form of autosomal-dominant retinitis pigmentosa in 11 affected members of various ages from one family with a defined IMPDH1 mutation (Asp226Asn).

DESIGN

Prospective, observational case series.

METHODS

Visual function assessment included visual acuity, color vision, visual field, dark adaptometry, full-field electroretinography (ffERG), and multifocal electroretinography (mfERG). Ophthalmologic examinations, fundus photography, and optical coherence tomographic scans were also performed. Blood samples were obtained to screen for basic immune function.

RESULTS

Visual acuity was slightly reduced in the teenage years and substantially reduced in association with cystoid macular edema (CME) at all ages. Color defects were observed in three patients (one teen, two adults). Dark-adapted thresholds were elevated. Visual fields were markedly constricted by age 40 (<or=20 degrees). Rod and cone a-wave and b-wave ffERG responses were small or nondetectable by age 20, with greater rod than cone loss at all ages. The normal to significantly delayed ffERG cone b-wave implicit times in different patients were explained by their mfERG implicit times from the central retina. The amplification factors (log S) and recovery kinetics derived from the full-field rod a-waves were normal. Optical coherence tomography revealed subretinal fluid accumulation in the majority of eyes. Cystoid macular edema was diagnosed in four patients. No unusual immunologic findings were noted.

CONCLUSIONS

The Asp226Asn mutation is associated with a severe, early-onset form of retinal degeneration in members of this family.

Regulation of the interaction of inosine monophosphate dehydrogenase with mycophenolic Acid by GTP

Inosine monophosphate dehydrogenase (IMPDH), a rate-limiting enzyme in the de novo synthesis of guanine nucleotides, is a major therapeutic target. A prototypic uncompetitive inhibitor of IMPDH, mycophenolic acid (MPA), is the active form of mycophenolate mofeteil (CellCept), a widely used immunosuppressive drug. We have found that MPA interacts with intracellular IMPDH in vivo to alter its mobility on SDS-polyacrylamide gels. MPA also induces a striking conformational change in IMPDH protein in intact cells, resulting in the formation of annular aggregates of protein with concomitant inhibition of IMPDH activity. These aggregates are not associated with any known intracellular organelles and are reversible by incubating cells with guanosine, which repletes intracellular GTP, or with GTPgammaS. GTP also restores IMPDH activity. Treatment of highly purified IMPDH with MPA also results in the formation of large aggregates of protein, a process that is both prevented and reversed by the addition of GTP. Finally, GTP binds to IMPDH at physiologic concentrations, induces the formation of linear arrays of tetrameric protein, and prevents the aggregation of protein induced by MPA. We conclude that intracellular GTP acts as an antagonist to MPA by directly binding to IMPDH and reversing the conformational changes in the protein.

Detection of the mycophenolate-inhibited form of IMP dehydrogenase in vivo

IMP dehydrogenase (IMPDH) is the rate-limiting enzyme for de novo GMP synthesis. Its activity is correlated with cell growth, and it is the target of a number of proven and experimental drug therapies including mycophenolic acid (MPA). MPA inhibits the enzyme by trapping a covalent nucleotide-enzyme intermediate. Saccharomyces cerevisiae has four IMPDH genes called IMD1-IMD4. IMD2 is transcriptionally regulated and is the only one that enables yeast to grow in the presence of MPA. We show here that de novo synthesis of the IMD2-encoded protein is strongly induced upon MPA treatment. We also monitor the in vivo formation of a covalent nucleotide-enzyme intermediate for Imd2, Imd3, and Imd4 that accumulates in the presence of MPA. Complete formation of the Imd2 intermediate requires drug concentrations manyfold higher than that required to quantitatively trap the Imd3- or Imd4-nucleotide adducts. Purification of the tagged IMD gene products reveals that the family of polypeptides coassemble to form heteromeric IMPDH complexes, suggesting that they form mixed tetramers. These data demonstrate that S. cerevisiae harbor multiple IMPDH enzymes with varying drug sensitivities and offer an assay to monitor the inhibition of IMPDH in living cells. They also suggest that mixed inhibition profiles may result from heteromeric complexes in cell types that contain multiple IMPDH gene products. The mobility shift assay could serve as a tool for the detection of drug-inactivated IMPDH in the cells of patients receiving MPA therapy.

Regulation of inosine monophosphate dehydrogenase type I and type II isoforms in human lymphocytes

The enzyme inosine monophosphate dehydrogenase (IMPDH) catalyzes the rate-limiting step in the de novo biosynthesis of guanine nucleotides. Inhibition of IMPDH leads to immunosuppression by decreasing guanine nucleotides that are required for the proliferation of lymphocytes. IMPDH activity is mediated by two highly conserved isoforms, type I and type II. We have characterized the mRNA and protein expression of the two isoforms in a variety of human tissues, peripheral blood mononuclear cells (PBMCs), and selected cell lines to investigate their regulation. Type I mRNA was expressed in most tissues with high expression in PBMCs and low expression in thymus. IMPDH type II transcript was also detected in most tissues with low expression in spleen and PBMCs. In PBMCs, induction of both type I and type II mRNAs was observed within 12 hr of mitogenic stimulation. Using type-selective IMPDH antibodies, an increase in the levels of type I and type II proteins was observed after mitogenic stimulation. The effect of two IMPDH inhibitors, MPA and VX-497, was investigated on the expression of type I and type II isoforms. VX-497 is an orally bioavailable, potent and reversible inhibitor of IMPDH, with broad applicability in many viral and immune system-mediated diseases. MPA and VX-497 inhibit both isoforms of IMPDH in vitro. Prolonged treatment of lymphocytes with either VX-497 or MPA did not lead to an increase in type I or type II IMPDH protein levels. These results are discussed in the context of IMPDH being a target for immunosuppressive, anti-viral and anti-cancer therapy.

Targeted disruption of the inosine 5'-monophosphate dehydrogenase type I gene in mice

Inosine 5'-monophosphate dehydrogenase (IMPDH) is the critical, rate-limiting enzyme in the de novo biosynthesis pathway for guanine nucleotides. Two separate isoenzymes, designated IMPDH types I and II, contribute to IMPDH activity. An additional pathway salvages guanine through the activity of hypoxanthine-guanine phosphoribosyltransferase (HPRT) to supply the cell with guanine nucleotides. In order to better understand the relative contributions of IMPDH types I and II and HPRT to normal biological function, a mouse deficient in IMPDH type I was generated by standard gene-targeting techniques and bred to mice deficient in HPRT or heterozygous for IMPDH type II. T-cell activation in response to anti-CD3 plus anti-CD28 antibodies was significantly impaired in both single- and double-knockout mice, whereas a more general inhibition of proliferation in response to other T- and B-cell mitogens was observed only in mice deficient in both enzymes. In addition, IMPDH type I(-/-) HPRT(-/0) splenocytes showed reduced interleukin-4 production and impaired cytolytic activity after antibody activation, indicating an important role for guanine salvage in supplementing the de novo synthesis of guanine nucleotides. We conclude that both IMPDH and HPRT activities contribute to normal T-lymphocyte activation and function.

Functional distinctions between IMP dehydrogenase genes in providing mycophenolate resistance and guanine prototrophy to yeast

IMP dehydrogenase (IMPDH) catalyzes the rate-limiting step in the de novo synthesis of GTP. Yeast with mutations in the transcription elongation machinery are sensitive to inhibitors of this enzyme such as 6-azauracil and mycophenolic acid, at least partly because of their inability to transcriptionally induce IMPDH. To understand the molecular basis of this drug-sensitive phenotype, we have dissected the expression and function of a four-gene family in yeast called IMD1 through IMD4. We show here that these family members are distinct, despite a high degree of amino acid identity between the proteins they encode. Extrachromosomal copies of IMD1, IMD3, or IMD4 could not rescue the drug-sensitive phenotype of IMD2 deletants. When overexpressed, IMD3 or IMD4 weakly compensated for deletion of IMD2. IMD1 is transcriptionally silent and bears critical amino acid substitutions compared with IMD2 that destroy its function, offering strong evidence that it is a pseudogene. The simultaneous deletion of all four IMD genes was lethal unless growth media were supplemented with guanine. This suggests that there are no other essential functions of the IMPDH homologs aside from IMP dehydrogenase activity. Although neither IMD3 nor IMD4 could confer drug resistance to cells lacking IMD2, either alone was sufficient to confer guanine prototrophy. The special function of IMD2 was provided by its ability to be transcriptionally induced and the probable intrinsic drug resistance of its enzymatic activity.

Mutations in the inosine monophosphate dehydrogenase 1 gene (IMPDH1) cause the RP10 form of autosomal dominant retinitis pigmentosa

Autosomal dominant retinitis pigmentosa (adRP) is a heterogeneous set of progressive retinopathies caused by several distinct genes. One locus, the RP10 form of adRP, maps to human chromosome 7q31.1 and may account for 5-10% of adRP cases among Americans and Europeans. We identified two American families with the RP10 form of adRP by linkage mapping and used these families to reduce the linkage interval to 3.45 Mb between the flanking markers D7S686 and RP-STR8. Sequence and transcript analysis identified 54 independent genes within this region, at least 10 of which are retinal-expressed and thus candidates for the RP10 gene. A screen of retinal transcripts comparing retinas from normal mice to retinas from crx-/crx- knockout mice (with poorly differentiated photoreceptors) demonstrated a 6-fold reduction in one candidate, inosine monophosphate dehydrogenase 1 (IMPDH1; EC 1.1.1.205). Since many of the genes known to cause retinitis pigmentosa are under CRX control in photoreceptors, IMPDH1 became a high-priority candidate for mutation screening. DNA sequencing of affected individuals from the two American RP10 families revealed a GAC-->AAC transition in codon 226 substituting an asparagine for an aspartic acid in both families. The identical mutation was also found in a British RP10 family. The Asp226Asn missense mutation is present in all affected individuals tested and absent from unaffected controls. The aspartic acid at codon 226 is conserved in all IMPDH genes, in all species examined, including bacteria, suggesting that this mutation is highly deleterious. Subsequent screening of probands from 60 other adRP families revealed an additional family with this mutation, confirming its association with retinitis pigmentosa and the relatively high frequency of this mutation. Another IMPDH1 substitution, Val268Ile, was also observed in this cohort of patients but not in controls. IMPDH1 is a ubiquitously expressed enzyme, functioning as a homotetramer, which catalyzed the rate-limiting step in de novo synthesis of guanine nucleotides. As such, it plays an important role in cyclic nucleoside metabolism within photoreceptors. Several classes of drugs are known to affect IMPDH isoenzymes, including nucleotide and NAD analogs, suggesting that small-molecule therapy may be available, one day, for RP10 patients.

Differential splicing of Pneumocystis carinii f. sp. carinii inosine 5'-monophosphate dehydrogenase pre-mRNA

Inosine 5'-monophosphate dehydrogenase (IMPDH) is a rate-limiting enzyme in guanine nucleotide metabolism that has drawn attention as a drug target in several organisms. Pneumocystis carinii f. sp. carinii IMPDH mRNA (GeneBank Accession No: U42442) previously identified from cultured organisms yielded a predicted amino acid sequence about 70 amino acids shorter at the amino terminus than IMPDH from other species. Recent research has shown that the amino terminal region is important for enzyme activity, suggesting that the previous putative P. carinii IMPDH might not represent full length, functional enzyme. To test this hypothesis, RT-PCR was performed with total RNA isolated from P. carinii f. sp. carinii. Three IMPDH splicing variants were found and splicing preference was observed: P. carinii isolated from infected rat lung contained primarily splicing variant one (introns two and four deleted), but organisms from spinner flask culture contained primarily splicing variant three (all four introns deleted). Importantly, splicing variant one (GeneBank Accession No: AF196975) contained an open reading frame for 529 amino acids, a size comparable to that of other eukaryotic IMPDH forms. The other variants contained the same open reading frame (454 amino acids) previously reported. Sequence analysis and complementation studies suggest variant one represents the full length, catalytically active form of P. carinii IMPDH. The differential splicing of the enzyme may reflect a mechanism by which the organism regulates the expression of IMPDH in response to environmental stresses.

Biased distribution of inverted and direct Alus in the human genome: implications for insertion, exclusion, and genome stability

Alu sequences, the most abundant class of large dispersed DNA repeats in human chromosomes, contribute to human genome dynamics. Recently we reported that long inverted repeats, including human Alus, can be strong initiators of genetic change in yeast. We proposed that the potential for interactions between adjacent, closely related Alus would influence their stability and this would be reflected in their distribution. We have undertaken an extensive computational analysis of all Alus (the database is at http://dir.niehs.nih.gov/ALU) to better understand their distribution and circumstances under which Alu sequences might affect genome stability. Alus separated by <650 bp were categorized according to orientation, length of regions sharing high sequence identity, distance between highly identical regions, and extent of sequence identity. Nearly 50% of all Alu pairs have long alignable regions (>275 bp), corresponding to nearly full-length Alus, regardless of orientation. There are dramatic differences in the distributions and character of Alu pairs with closely spaced, nearly identical regions. For Alu pairs that are directly repetitive, approximately 30% have highly identical regions separated by <20 bp, but only when the alignments correspond to near full-size or half-size Alus. The opposite is found for the distribution of inverted repeats: Alu pairs with aligned regions separated by <20 bp are rare. Furthermore, closely spaced direct and inverted Alus differ in their truncation patterns, suggesting differences in the mechanisms of insertion. At larger distances, the direct and inverted Alu pairs have similar distributions. We propose that sequence identity, orientation, and distance are important factors determining insertion of adjacent Alus, the frequency and spectrum of Alu-associated changes in the genome, and the contribution of Alu pairs to genome instability. Based on results in model systems and the present analysis, closely spaced inverted Alu pairs with long regions of alignment are likely at-risk motifs (ARMs) for genome instability.

Do the 5'untranslated domains of human cDNAs challenge the rules for initiation of translation (or is it vice versa)?

The validity of the scanning mechanism for initiation of translation has been questioned based on a compilation of human cDNA sequences that showed a high frequency of upstream ATG codons. However, closer scrutiny of those cDNAs upholds the opposite view: the 5'UTRs on most cDNAs are compatible with standard rules for initiation of translation, and those rules can be used to flag anomalous cDNAs that, upon checking, turn out to have been misinterpreted. Some of the problematic 5'UTR sequences that persist, after obvious errors in the cDNA library have been corrected, might derive from transcripts that are not intended to be translated. Examples are given of genes that, for regulatory reasons, produce transcripts that are truncated, or retain an intron, or are otherwise configured in a way that precludes translation. The existence of a cDNA proves that a gene is transcribed, but only that; not every cDNA derives from a functional mRNA. Along with providing practical guidelines for interpreting cDNA sequences, the scanning model provides a theoretical framework for understanding the effects of certain mutations in the 5'UTR that alter the translatability of mRNAs, thereby contributing to cancer and other human diseases.

Assessment of Mycophenolic Acid-induced Immunosuppression: A New Approach

Background: Mycophenolic acid (MPA), a metabolite of mycophenolate mofetil (MMF), is an immunosuppressive agent that inhibits inosine monophosphate dehydrogenase (IMPDH), a key enzyme in the ex novo synthesis of GTP. We measured IMPDH activity in peripheral blood mononuclear cells (PBMCs) from MMF-treated patients to evaluate the efficacy of MMF in individual patients.

Methods: IMPDH activity was measured by 3H released from [2,8-3H]IMP that had been formed in the cells from added [2,8-3H]hypoxanthine in PBMCs of 35 renal transplant recipients treated with cyclosporin A and corticoids plus MMF: 2 g (n = 10), 1.5 g (n = 7), 1 g (n = 10), or 0 g (n = 8) per day. An alternative method, based on the capacity of the patients’ sera to inhibit spontaneous proliferation of the CEM cell line, was also analyzed.

Results: The IMPDH activity of PBMCs in transplanted patients was highly variable. For the method based on CEM cell line proliferation: (a) cell proliferation was inhibited only in MMF-treated patients; (b) there was a clear postdose increase in inhibition; (c) inhibition was not affected by other immunosuppressants in vitro or in vivo; (d) inhibition from predose to predose sample was correlated; and (e) when the MMF dosage was &lt;20 mg · kg−1 · day−1, two groups of patients were identified, one that maintained a high inhibitory capacity in all dose intervals, and one with periods of low inhibitory capacity.

Conclusions: Measurement of the inhibition of CEM cell line proliferation by sera from MMF-treated patients may be useful for evaluating the relative efficacy of MMF treatment in individual patients, especially those receiving low doses of MMF.

Inhibition of T lymphocyte activation in mice heterozygous for loss of the IMPDH II gene

Inosine 5'-monophosphate dehydrogenase (IMPDH) is the rate-limiting enzyme in the de novo synthesis of guanine nucleotides, which are also synthesized from guanine by a salvage reaction catalyzed by the X chromosome-linked enzyme hypoxanthine-guanine phosphoribosyltransferase (HPRT). Since inhibitors of IMPDH are in clinical use as immunosuppressive agents, we have examined the consequences of knocking out the IMPDH type II enzyme by gene targeting in a mouse model. Loss of both alleles of the gene encoding this enzyme results in very early embryonic lethality despite the presence of IMPDH type I and HPRT activities. Lymphocytes from IMPDH II(+/-) heterozygous mice are normal with respect to subpopulation distribution and respond normally to a variety of mitogenic stimuli. However, mice with an IMPDH II(+/-), HPRT(-/o) genotype demonstrate significantly decreased lymphocyte responsiveness to stimulation with anti-CD3 and anti-CD28 antibodies and show a 30% mean reduction in GTP levels in lymphocytes activated by these antibodies. Furthermore, the cytolytic activity of their T cells against allogeneic target cells is significantly impaired. These results demonstrate that a moderate decrease in the ability of murine lymphocytes to synthesize guanine nucleotides during stimulation results in significant impairment in T-cell activation and function.

Impact of transposable elements on the human genome

Presence of transposable elements (TEs) in the human genome has profound effects on genome function, structure and evolution. TE mobility and inter-TE recombination are the origin of a large spectrum of mutations and genome reorganization leading to diseases. From the data provided by the Human Genome Project and from information on the detection and dynamics of TEs within and between species acquired during the last two decades, we now know that these elements are not only involved in mutagenesis but can also participate in many cellular functions including recombination, gene regulation, protein-coding RNA messages and, possibly, cellular stress response and centromere function. TEs also promote a general genome shuffling process that has been important for the evolution of several gene families and for the development of new regulatory pathways.

GTP binding by class II transactivator: role in nuclear import

Class II transactivator (CIITA) is a global transcriptional coactivator of human leukocyte antigen-D (HLA-D) genes. CIITA contains motifs similar to guanosine triphosphate (GTP)-binding proteins. This report shows that CIITA binds GTP, and mutations in these motifs decrease its GTP-binding and transactivation activity. Substitution of these motifs with analogous sequences from Ras restores CIITA function. CIITA exhibits little GTPase activity, yet mutations in CIITA that confer GTPase activity reduce transcriptional activity. GTP binding by CIITA correlates with nuclear import. Thus, unlike other GTP-binding proteins, CIITA is involved in transcriptional activation that uses GTP binding to facilitate its own nuclear import.

Inosine-5'-monophosphate dehydrogenase: regulation of expression and role in cellular proliferation and T lymphocyte activation

Guanine nucleotide synthesis is essential for the maintenance of normal cell growth and function, as well as for cellular transformation and immune responses. The expression of two genes encoding human inosine-5'-monophosphate dehyrogenase (IMPDH) type I and type II results in the translation of catalytically indistinguishable enzymes that control the rate-limiting step in the de novo synthesis of guanine nucleotides. Cellular IMPDH activity is increased more than 10-fold in activated peripheral blood T lymphocytes and is attributable to the increased expression of both the type I and type II enzymes. In contrast, abrogation of cellular IMPDH activity by selective inhibitors prevents T lymphocyte activation and establishes a requirement for elevated IMPDH activity in T lymphocytic responses. In order to assess the molecular mechanisms governing the expression of the IMPDH type I and type II genes in resting and activated peripheral blood T lymphocytes, we have cloned the human IMPDH type I and type II genes and characterized their genomic organization and their respective 5'-flanking regions. Both genes contain 14 highly conserved exons that vary in size from 49 to 207 base pairs. However, the intron structures are completely divergent, resulting in disparities in gene length (18 kilobases for type I and 5.8 kilobases for type II). In addition, the 5'-regulatory sequences are highly divergent; expression of the IMPDH type I gene is controlled by three distinct promoters in a tissue specific manner while the type II gene is regulated by a single promoter and closely flanked in the 5' region by a gene of unknown function. The conservation of the IMPDH type I and type II coding sequence in the presence of highly divergent 5'-regulatory sequences points to a multifactorial control of enzyme expression and suggests that tissue-specific and/or developmentally specific regulation of expression may be important. Delineation of these regulatory mechanisms will aid in the elucidation of the signaling events that ultimately lead to the synthesis of guanine nucleotides required for cellular entry into S phase and the initiation of DNA replication.

Isolation of human promoter regions by Alu repeat consensus-based polymerase chain reaction

Knowledge of the promoter structure is critical for an understanding of the regulation of genes. We demonstrate by analysis of 405 human genes that human promoter regions are flanked by upstream Alu repeat elements, typically at a distance of 0.5-5 kb from their protein-coding areas. We identified common Alu repeat consensus sequences (ARC) among the different members of the Alu subfamilies that can be used as universal anchor sites for polymerase chain reaction (PCR) amplification. Utilizing ARC-specific primers and oligonucleotides specific for the 5' end of a selected target gene, we show that sequences spanning unknown human gene promoter regions can be directly amplified by PCR from genomic DNA. This novel technique, termed ARC-PCR, allowed us to characterize the proximal promoters of the human LTA4 hydrolase and SPARC genes, each within 1 day.