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1.  Correction of Cystathionine β-synthase Deficiency in Mice by Treatment with Proteasome Inhibitors 
Human mutation  2013;34(8):1085-1093.
Cystathionine beta-synthase (CBS) deficiency is an inborn error of metabolism characterized by extremely elevated levels of plasma total homocysteine. The vast majority of CBS-deficient patients have missense mutations located in the CBS gene that result in the production of either misfolded or unstable protein. Here, we examine the effect of proteasome inhibitors on mutant CBS function using two different mouse models of CBS deficiency. These mice lack mouse CBS and express a missense mutant human CBS enzyme (either p.I278T or p.S466L) that has less than 5% of normal liver CBS activity, resulting in a 10–30 fold elevation in plasma homocysteine levels. We show that treatment of these mice with two different proteasome inhibitors can induce liver Hsp70, Hsp40, and Hsp27, increase levels of active CBS, and lower plasma homocysteine levels to within the normal range. However, response rates varied, with 100% (8/8) of the p.S466L animals showing correction, but only 38% (10/26) of the p.I278T animals. In total, our data shows that treatment with proteostasis modulators can restore significant enzymatic activity to mutant misfolded CBS enzymes and suggests that they may be useful in treating certain types of genetic diseases caused by missense mutations.
PMCID: PMC3941476  PMID: 23592311
genetic disease; missense mutation; chaperone; inborn error; aminoaciduria
2.  Increasing the therapeutic index of 5-fluorouracil and 6-thioguanine by targeting loss of MTAP in tumor cells 
Cancer Biology & Therapy  2012;13(11):1082-1090.
Methylthioadenosine phosphorylase (MTAP), a key enzyme in the catabolism of 5′-deoxy-5′-methylthioadenosine (MTA), catalyzes the formation of adenine and 5-methylthioribose-1-phosphate. MTAP is expressed in all cells throughout the body, but a significant percentage of human tumors have lost MTAP expression, thereby making MTAP-loss a potential therapeutic target. Here, we have tested an MTAP-targeting strategy based on the idea that MTAP-expressing cells can be protected from toxic purine and uracil analogs by addition of MTA, but MTAP-deleted tumor cells cannot. Addition of as little as 10 μM MTA could entirely protect isogenic MTAP+, but not MTAP-, HT1080 cells from toxicity caused by the chemotherapy agents 6-thioguanine (6TG) or 5-fluorouracil (5FU). Inhibitor studies showed that MTA protection requires functional MTAP activity. Addition of adenine protected both MTAP+ and MTAP- cells from 6TG and 5FU, consistent with the idea that adenine produced from the MTAP reaction competes with 6TG and 5FU for a rate limiting pool of phosphoribosyl-1-pyrophosphate (PRPP), which is required for the conversion of purine and uracil bases into nucleotides. Extracellular MTA can also protect mouse mesothelioma cells from killing by 6-TG or the drug L-alanosine in an MTAP-dependent manner. In addition, MTA can protect non-transformed MTAP+ mouse embryo fibroblasts from 6TG toxicity. Taken together, our data suggest that the addition of MTA to anti-purine-based chemotherapy may greatly increase the therapeutic index of this class of drugs if used specifically to treat MTAP- tumors.
PMCID: PMC3461815  PMID: 22825330
chemotherapy; mesothelioma; methionine; osteosarcoma; purine
3.  Hydroxychloroquine Destabilizes Phospho-S6 in Human Renal Carcinoma Cells 
PLoS ONE  2015;10(7):e0131464.
mTOR inhibitors are used to treat metastatic renal cell cancer (RCC), but most patients eventually become resistant. One possible mechanism for resistance is upregulation of autophagy, a pathway that helps recycle intracellular proteins and promotes cell survival. Hydroxychloroquine (HCQ), a potent autophagy inhibitor used to treat malaria and autoimmune disorders, is currently being studied in the context of cancer treatment. Here, we have investigated the effects of HCQ on three different renal carcinoma derived cell lines. We found that HCQ treatment inhibits RCC cell growth, promotes apoptosis, inhibits mitochondrial oxygen consumption, and increases rates of glycolysis. To understand the molecular mechanism behind these effects, we examined various nodes in the mTOR pathway and compared the effects of HCQ with the effects of the mTOR inhibitor RAD001. A key downstream readout of the pathway, phospho-S6 protein, was inhibited by both HCQ and RAD001. However, the upstream kinase, P70S6K was only inhibited by RAD001 and not HCQ, suggesting that the block by HCQ was downstream of P70S6K. Treatment with the proteasome inhibitor bortezomib restored phospho-S6 levels, suggesting that the reduction of phospho-S6 is caused by increased degradation of phospho-S6, but not total S6. Surprisingly, treatment with other autophagy inhibitors did not exhibit the same effects. Our findings suggest that HCQ causes the down-regulation of phospho-S6 in RCC cell lines via a novel mechanism that is not shared with other autophagy inhibitors.
PMCID: PMC4489871  PMID: 26134285
4.  Serum Amino Acid Levels as a Biomarker for Renal Cell Carcinoma 
The Journal of urology  2011;186(4):1206-1212.
Prognosis in renal cell carcinoma (RCC) is dependent on tumor stage at presentation, with significant differences in survival between early and late stage disease. Currently, there are no screening tests or biomarkers identified for the early detection of kidney cancer. Here, we investigate if serum amino acid profiles are a potentially useful biomarker in patients with RCC.
Materials and Methods
The concentrations of 26 different amino acids were determined in serum taken pre-operatively from 189 RCC patients and 104 age and sex matched controls.
Statistically significant changes were observed in patient levels of 15 different amino acids, with 13 being decreased and two being elevated. A logistic regression model utilizing eight amino acids including cysteine, ornithine, histidine, leucine, tyrosine, proline, valine and lysine was created to distinguish cases from controls. A receiver operator curve based on this model had an area under the curve of 0.81. This same model also had predictive value in predicting overall survival and tumor recurrence in RCC patients.
Our findings suggest that serum amino acid levels may be useful as a screening tool for the identification of individuals with RCC and predicting patient outcomes.
PMCID: PMC3180900  PMID: 21849193
Kidney Cancer; biomarker; serum; amino acids
5.  Mice Heterozygous for Germline Mutations in Methylthioadenosine Phosphorylase (MTAP) Die Prematurely of T-cell Lymphoma 
Cancer research  2009;69(14):5961-5969.
Large homozygous deletions of 9p21 that inactivate CDKN2A, ARF, and MTAP are common in a wide variety of human cancers. The role for CDKN2A and ARF in tumorigenesis is well established, but whether MTAP loss directly affects tumorigenesis is unclear. MTAP encodes the enzyme methylthioadenosine phosphorylase, a key enzyme in the methionine salvage pathway. To determine if loss of MTAP plays a functional role in tumorigenesis, we have created an MTAP-knockout mouse. Mice homozygous for a MTAP null allele (MtaplacZ) have an embryonic lethal phenotype dying around day 8 post-conception. Mtap/MtaplacZ heterozygotes are born at Mendelian frequencies and appear indistinguishable from wild-type mice during the first year of life, but they tend to die prematurely with a median survival of 585 days. Autopsies on these animals reveal that they have greatly enlarged spleens, altered thymic histology, and lymphocytic infiltration of their livers, consistent with lymphoma. Immunohistochemical staining and FACS analysis indicate that these lymphomas are primarily T-cell in origin. Lymphoma infiltrated tissues tend to have reduced levels of Mtap mRNA and MTAP protein, and unaltered levels of methyldeoxycytidine. These studies show that Mtap is a tumor suppressor gene independent of CDKN2A and ARF.
PMCID: PMC2757012  PMID: 19567676
Cancer; Tumor Suppressor Gene; Methionine; Embryonic Lethal
6.  Suppression of Tumor Formation by a COX-2 Inhibitor and a PPARγ Agonist in an in vivo Mouse Model of Spontaneous Breast Cancer 
Activation of cyclooxygenase-2 (COX-2) and inhibition of peroxisome proliferators-activated receptor γ (PPARγ) have been observed in human and animal models of breast cancer. Both inhibition of COX-2 and activation of PPARγ can inhibit proliferation of breast cancer cells in vitro. Here we examine the effects of the COX-2 inhibitor celecoxib and the PPARγ agonist F-L-Leu on mouse breast tumor cells in vitro and in vivo.
Experimental Design
We created and characterized a mouse mammary adenocarcinoma cell (MMAC1) line from C3 (1)-SV40 T-antigen mice to study COX-2 and PPARγ expression and response to celecoxib and F-L-Leu in vitro. To study the in vivo effects, C3 (1) SV40 T-antigen mice were given either control diet or diets containing three different concentrations of celecoxib and F-L-Leu as well as a combination of both agents. Mice were then followed for tumor formation up to one year.
MMAC1 cells express both COX-2 and PPARγ mRNA and exhibited cooperative growth inhibition with a combination of celecoxib and F-L-Leu. In mice, the median age of death due to mammary tumors was significantly delayed in celecoxib treated animals at all three concentrations, but was not significantly affected by F-L-Leu treatment alone. A combination of celecoxib and F-L-Leu was significantly more effective than celecoxib alone.
Our findings suggest that a combination of a COX-2 inhibitor and PPARγ agonist can delay breast cancer in a mouse model and suggest these agents should be studied in the context of human populations with high breast cancer risk.
PMCID: PMC2587271  PMID: 18676768
breast cancer; PPARγ; COX-2
7.  Expression of mutant human cystathionine beta-synthase rescues neonatal lethality but not homocystinuria in a mouse model 
Human molecular genetics  2005;14(15):2201-2208.
Cystathionine β-synthase (CBS) deficiency is a recessive genetic disorder in humans characterized by elevated levels in total plasma homocysteine (tHcy) and frequent thrombosis in humans. The I278T mutation is the most common mutation found in human CBS deficient patients. The T424N mutation was identified as a mutation in human CBS that could restore function to I278T in S. cerevisiae. In this report, we have engineered mice that express human I278T and I278T/T424N proteins from a metallotheinein driven transgene. These transgene-containing mice were then bred to CBS knockout animals (Cbs-) to generate mice that express only human I278T or I278T/T424N protein. Both the I278T and the I278T/T424N transgenes are able to entirely rescue the previously described neonatal mortality phenotype despite the animals having a mean tHcy of 250 μM. The transgenic Cbs-/- animals exhibit facial alopecia, have moderate liver steatosis and are slightly smaller than heterozygous littermates. In contrast to human CBS deficiency, these mice do not exhibit extreme methioninemia. The mutant proteins are stable in the liver, kidney and colon, and liver extracts have only 2-3% of the CBS enzyme activity found in wild type mice. Surprisingly, the I278T/T424N enzyme had exactly the same activity as the I278T enzyme indicating that T424N is unable to suppress I278T in mice. Our results show that elevated tHcy per se is not responsible for the neonatal lethality observed in Cbs-/- animals and suggests that CBS protein may have a function in addition to its role in homocysteine catabolism. These transgenic animals should be useful in the study of homocysteine related human disease.
PMCID: PMC1283068  PMID: 15972722
8.  Quantitation of Cellular Metabolic Fluxes of Methionine 
Analytical chemistry  2014;86(3):1583-1591.
Methionine is an essential proteogenic amino acid. In addition, it is a methyl donor for DNA and protein methylation and a propylamine donor for polyamine biosyn-thesis. Both the methyl and propylamine donation pathways involve metabolic cycles, and methods are needed to quantitate these cycles. Here, we describe an analytical approach for quantifying methionine metabolic fluxes that accounts for the mixing of intracellular and extracellular methionine pools. We observe that such mixing prevents isotope tracing experiments from reaching the steady state due to the large size of the media pools and hence precludes the use of standard stationary metabolic flux analysis. Our approach is based on feeding cells with 13C methionine and measuring the isotope-labeling kinetics of both intracellular and extracellular methionine by liquid chromatography−mass spectrometry (LC-MS). We apply this method to quantify methionine metabolism in a human fibrosarcoma cell line and study how methionine salvage pathway enzyme methylthioadenosine phosphorylase (MTAP), frequently deleted in cancer, affects methionine metabolism. We find that both transmethylation and propylamine transfer fluxes amount to roughly 15% of the net methionine uptake, with no major changes due to MTAP deletion. Our method further enables the quantification of flux through the pro-tumorigenic enzyme ornithine decarboxylase, and this flux increases 2-fold following MTAP deletion. The analytical approach used to quantify methionine metabolic fluxes is applicable for other metabolic systems affected by mixing of intracellular and extracellular metabolite pools.
PMCID: PMC4060246  PMID: 24397525
9.  The Herp Protein Pathway is Not Involved in the Pro-Amyloidogenic Effect of Hyperhomocysteinemia 
Journal of Alzheimer's disease : JAD  2010;20(2):10.3233/JAD-2010-1394.
Diet-induced high circulating levels of homocysteine, also known as hyper-homocysteinemia (HHcy), is associated with an acceleration of Alzheimer’s disease-like amyloidosis. Herp is a homocysteine-responsive stress protein, which has been shown to increase the formation of amyloid-β (Aβ) via interaction with presenilins in vitro. The aim of our paper was to investigate the functional role that Herp plays in HHcy-induced amyloidosis. Amyloidosis secondary to diet-induced HHcy in Tg2576 mice is associated with an increase of Herp protein and mRNA levels. By contrast, no other stress-related proteins are altered by the same diet regimen. Compared to wild type animals, brains from a genetically induced HHcy mouse model did not manifest any significant change in Herp levels. Cells stably over-expressing human AβPP Swedish mutant incubated with high levels of homocysteine had an increase in Aβ formation, but no change in Herp level. Finally, over-expression of Herp did not result in any significant modification of Aβ levels. We conclude that the Herp protein pathway is unlikely to be directly involved in the pro-amyloidotic effect of HHcy.
PMCID: PMC3877940  PMID: 20164556
Amyloid-β; amyloid-β metabolism; endoplasmatic reticulum stress; Tg2576; Tg-278Cbs−/−
10.  Cystathionine β-Synthase p.S466L Mutation Causes Hyperhomocysteinemia in Mice 
Human Mutation  2008;29(8):1048-1054.
Missense mutations in the cystathionine β-synthase (CBS) gene are the most common cause of clinical homocystinuria in humans. The p.S466L mutation was identified in a homocystinuric patient, but enzymatic studies with recombinant protein show this mutant to be highly active. To understand how this mutation causes disease in vivo, we have created mice lacking endogenous mouse CBS and expressing either wild-type (Tg-hCBS) or p.S466L (Tg-S466L) human CBS under control of zinc inducible metallothionein promoter. In the presence of zinc, we found that the mean serum total homocysteine (tHcy) of Tg-S466L mice was 142 ± 55 µM compared to 16 ± 13 µM for hCBS mice. Tg-S466L mice also had significantly higher levels of total free homocysteine and S-adenosylhomocysteine in liver and kidney. Only 48% of Tg-S466L mice had detectable CBS protein in the liver, whereas all the Tg-hCBS animals had detectable protein. Surprisingly, CBS mRNA was significantly elevated in Tg-S466L animals compared to Tg-hCBS, implying that the reduction in p.S466L protein was occurring due to posttranscriptional mechanisms. In Tg-S466L animals with detectable liver CBS, the enzyme formed tetramers and was active, but lacked inducibility by S-adenosylmethionine (AdoMet). However, even in Tg-S466L animals that had in vitro liver CBS activity equivalent to Tg-hCBS animals there was significant elevation of serum tHcy. Our results show that p.S466L causes homocystinuria by affecting both the steady state level of CBS protein and by reducing the efficiency of the enzyme in vivo. Hum Mutat 29(8), 1048–1054, 2008. © 2008 Wiley-Liss, Inc.
PMCID: PMC2630375  PMID: 18454451
inborn error; metabolism; mouse model; homocystinuria; CBS
11.  Germline Mutations in Mtap Cooperate with Myc to Accelerate Tumorigenesis in Mice 
PLoS ONE  2013;8(6):e67635.
The gene encoding the methionine salvage pathway methylthioadenosine phosphorylase (MTAP) is a tumor suppressor gene that is frequently inactivated in a wide variety of human cancers. In this study, we have examined if heterozygosity for a null mutation in Mtap (MtaplacZ) could accelerate tumorigenesis development in two different mouse cancer models, Eμ-myc transgenic and Pten+/−.
Mtap Eμ-myc and Mtap Pten mice were generated and tumor-free survival was monitored over time. Tumors were also examined for a variety of histological and protein markers. In addition, microarray analysis was performed on the livers of MtaplacZ/+ and Mtap+/+ mice.
Survival in both models was significantly decreased in MtaplacZ/+ compared to Mtap+/+ mice. In Eµ-myc mice, Mtap mutations accelerated the formation of lymphomas from cells in the early pre-B stage, and these tumors tended to be of higher grade and have higher expression levels of ornithine decarboxylase compared to those observed in control Eµ-myc Mtap+/+ mice. Surprisingly, examination of Mtap status in lymphomas in Eµ-myc MtaplacZ/+ and Eµ-myc Mtap+/+ animals did not reveal significant differences in the frequency of loss of Mtap protein expression, despite having shorter latency times, suggesting that haploinsufficiency of Mtap may be playing a direct role in accelerating tumorigenesis. Consistent with this idea, microarray analysis on liver tissue from age and sex matched Mtap+/+ and MtaplacZ/+ animals found 363 transcripts whose expression changed at least 1.5-fold (P<0.01). Functional categorization of these genes reveals enrichments in several pathways involved in growth control and cancer.
Our findings show that germline inactivation of a single Mtap allele alters gene expression and enhances lymphomagenesis in Eµ-myc mice.
PMCID: PMC3694069  PMID: 23840755
12.  Severe hyperhomocysteinemia promotes bone marrow-derived and resident inflammatory monocyte differentiation and atherosclerosis in LDLr/CBS-deficient mice 
Circulation research  2012;111(1):37-49.
This study examined the causative role of hyperhomocysteinemia (HHcy) in atherogenesis and its effect on inflammatory monocyte (MC) differentiation.
Methods and Results
We generated a novel HHcy and hyperlipidemia mouse model, in which cystathionine β-synthase (CBS) and low-density lipoprotein receptor (LDLr) genes were deficient (Ldlr−/− Cbs−/+). Severe HHcy (plasma homocysteine (Hcy)=275 µM) was induced by a high methionine diet containing sufficient basal levels of B vitamins. Plasma Hcy levels were lowered to 46 µM from 244 µM by vitamin supplementation, which elevated plasma folate levels. Bone marrow (BM)-derived cells were traced by the transplantation of BM cells from enhanced green fluorescent protein (EGFP) transgenic mice after sub-lethal irradiation of the recipient. HHcy accelerated atherosclerosis and promoted Ly6Chigh inflammatory MC differentiation of both BM- and tissue-origins in the aortas and peripheral tissues. It also elevated plasma levels of TNF-α, IL-6 and MCP-1; increased vessel wall MC accumulation; and macrophage maturation. Hcy-lowering therapy reversed HHcy-induced lesion formation, plasma cytokine increase, and blood and vessel inflammatory MC (Ly6Chigh+middle) accumulation. Plasma Hcy levels were positively correlated with plasma levels of pro-inflammatory cytokines. In primary mouse splenocytes, L-Hcy promoted rIFNγ-induced inflammatory MC differentiation, as well as increased TNF-α, IL-6, and superoxide anion production in inflammatory MC subsets. Antioxidants and folic acid reversed L-Hcy-induced inflammatory MC differentiation and oxidative stress in inflammatory MC subsets.
HHcy causes vessel wall inflammatory MC differentiation and macrophage maturation of both BM- and tissue-origins leading to atherosclerosis via an oxidative stress related mechanism.
PMCID: PMC3412115  PMID: 22628578
inflammatory monocyte; atherosclerosis; hyperhomocysteinemia
13.  The TSC1/2 Complex Controls Drosophila Pigmentation through TORC1-Dependent Regulation of Catecholamine Biosynthesis 
PLoS ONE  2012;7(11):e48720.
In Drosophila, the pattern of adult pigmentation is initiated during late pupal stages by the production of catecholamines DOPA and dopamine, which are converted to melanin. The pattern and degree of melanin deposition is controlled by the expression of genes such as ebony and yellow as well as by the enzymes involved in catecholamine biosynthesis. In this study, we show that the conserved TSC/TORC1 cell growth pathway controls catecholamine biosynthesis in Drosophila during pigmentation. We find that high levels of Rheb, an activator of the TORC1 complex, promote premature pigmentation in the mechanosensory bristles during pupal stages, and alter pigmentation in the cuticle of the adult fly. Disrupting either melanin synthesis by RNAi knockdown of melanogenic enzymes such as tyrosine hydroxylase (TH), or downregulating TORC1 activity by Raptor knockdown, suppresses the Rheb-dependent pigmentation phenotype in vivo. Increased Rheb activity drives pigmentation by increasing levels of TH in epidermal cells. Our findings indicate that control of pigmentation is linked to the cellular nutrient-sensing pathway by regulating levels of a critical enzyme in melanogenesis, providing further evidence that inappropriate activation of TORC1, a hallmark of the human tuberous sclerosis complex tumor syndrome disorder, can alter metabolic and differentiation pathways in unexpected ways.
PMCID: PMC3492411  PMID: 23144943
14.  Inhibition of betaine-homocysteine S-methyltransferase in rats causes hyperhomocysteinemia and reduces liver cystathionine β-synthase activity and methylation capacity 
Methylation of homocysteine (Hcy) by betaine-homocysteine S-methyltransferase (BHMT) produces methionine, which is required for S-adenosylmethionine (SAM) synthesis. We have recently shown that short term dietary intake of S-(-δ-carboxybutyl)-DL-homocysteine (CBHcy), a potent and specific inhibitor of BHMT, significantly decreases liver BHMT activity and SAM concentrations but does not have an adverse affect on liver histopathology, plasma markers of liver damage or DNA methylation in rats. The present study was designed to investigate the hypothesis that BHMT is required to maintain normal liver and plasma amino acid and glutathione profiles, and liver SAM and lipid accumulation. Rats were fed an adequate (4.5 g/kg methionine and 3.7 g/kg cystine), cysteine-devoid (4.5 g/kg methionine and 0 g/kg cystine), or methionine-deficient (1.5 g/kg methionine and 3.7 g/kg cystine) diet either with or without CBHcy for 3 or 14 d. All rats fed CBHcy had increased total plasma Hcy (2- to 5-fold), and reduced liver BHMT activity (>90%) and SAM concentrations (>40%). CBHcy treatment slightly reduced liver glutathione levels in rats fed the adequate or cysteine-devoid diet for 14 d. Rats fed the methionine-deficient diet with CBHcy developed fatty liver. Liver cystathionine β-synthase activity was reduced in all CBHcy-treated animals, and the effect was exacerbated as time on the CBHcy diet increased. Our data indicate that BHMT activity is required to maintain adequate levels of liver SAM and low levels of total plasma Hcy, and might be critical for liver glutathione and triglyceride homeostasis under some dietary conditions.
PMCID: PMC3156413  PMID: 21840473
rat; betaine-homocysteine S-methyltransferase; betaine; homocysteine; glutathione
15.  Brain Phenotype of Transgenic Mice Overexpressing Cystathionine β-Synthase 
PLoS ONE  2012;7(1):e29056.
The cystathionine β-synthase (CBS) gene, located on human chromosome 21q22.3, is a good candidate for playing a role in the Down Syndrome (DS) cognitive profile: it is overexpressed in the brain of individuals with DS, and it encodes a key enzyme of sulfur-containing amino acid (SAA) metabolism, a pathway important for several brain physiological processes.
Methodology/Principal Findings
Here, we have studied the neural consequences of CBS overexpression in a transgenic mouse line (60.4P102D1) expressing the human CBS gene under the control of its endogenous regulatory regions. These mice displayed a ∼2-fold increase in total CBS proteins in different brain areas and a ∼1.3-fold increase in CBS activity in the cerebellum and the hippocampus. No major disturbance of SAA metabolism was observed, and the transgenic mice showed normal behavior in the rotarod and passive avoidance tests. However, we found that hippocampal synaptic plasticity is facilitated in the 60.4P102D1 line.
We demonstrate that CBS overexpression has functional consequences on hippocampal neuronal networks. These results shed new light on the function of the CBS gene, and raise the interesting possibility that CBS overexpression might have an advantageous effect on some cognitive functions in DS.
PMCID: PMC3257219  PMID: 22253703
16.  Chemical Genetic Screening for Compounds that Preferentially Inhibit Growth of Methylthioadenosine Phosphorylase (MTAP) Deficient Saccharomyces Cerevisiae 
Methylthioadenosine phosphorylase (MTAP), a key enzyme in the methionine salvage pathway, is inactivated in a variety of human cancers. Since all human tissues express MTAP, it would be of potential interest to identify compounds that selectively inhibit the growth of MTAP deficient cells. To determine if MTAP inactivation could be targeted, we have performed a differential chemical genetic screen in isogenic MTAP+ and MTAP− S. cerevisiae. A low molecular weight compound library containing 30,080 unique compounds was screened for those that selectively inhibit growth of MTAP− yeast using a differential growth assay. One compound, containing a 1,3,4-thiadiazine ring, repeatedly showed a differential dose response, with MTAP− cells exhibiting a four-fold shift in IC50 compared to MTAP+ cells. Several structurally related derivatives of this compound also showed enhanced growth inhibition in MTAP− yeast. These compounds were also examined for growth inhibition of isogenic MTAP+ and MTAP− HT1080 fibrosarcoma cells, and four of the five compounds exhibited evidence of modest, but significant, increased potency in MTAP− cells. In summary, these studies show the feasibility of differential growth screening technology and have identified a novel class of compounds that can preferentially inhibit growth of MTAP− cells.
PMCID: PMC3019245  PMID: 21131597
Methionine Salvage Pathway; Drug screening; Yeast; Genetic-chemical interaction
17.  Cystathionine Beta-Synthase Deficiency Causes Fat Loss in Mice 
PLoS ONE  2011;6(11):e27598.
Cystathionine beta synthase (CBS) is the rate-limiting enzyme responsible for the de novo synthesis of cysteine. Patients with CBS deficiency have greatly elevated plasma total homocysteine (tHcy), decreased levels of plasma total cysteine (tCys), and often a marfanoid appearance characterized by thinness and low body-mass index (BMI). Here, we characterize the growth and body mass characteristics of CBS deficient TgI278T Cbs−/− mice and show that these animals have significantly decreased fat mass and tCys compared to heterozygous sibling mice. The decrease in fat mass is accompanied by a 34% decrease in liver glutathione (GSH) along with a significant decrease in liver mRNA and protein for the critical fat biosynthesizing enzyme Stearoyl CoA desaturase-1 (Scd-1). Because plasma tCys has been positively associated with fat mass in humans, we tested the hypothesis that decreased tCys in TgI278T Cbs−/− mice was the cause of the lean phenotype by placing the animals on water supplemented with N-acetyl cysteine (NAC) from birth to 240 days of age. Although NAC treatment in TgI278T Cbs−/− mice caused significant increase in serum tCys and liver GSH, there was no increase in body fat content or in liver Scd-1 levels. Our results show that lack of CBS activity causes loss of fat mass, and that this effect appears to be independent of low serum tCys.
PMCID: PMC3214081  PMID: 22096601
18.  Methionine-deficient diet induces post-transcriptional down-regulation of Cystathionine β-Synthase 
Elevated plasma total homocysteine (tHcy) is a risk factor for a variety of human diseases. Homocysteine is formed from methionine and has two primary metabolic fates: remethylation to form methionine or commitment to the transulfuration pathway by the action of cystathionine β-synthase (CBS). Here, we have examined the metabolic response in mice of a shift from a methionine-replete (M+) to a methionine-free (M−) diet.
Methods/Principle Findings
We found that shifting three-month old C57BL6 mice to a M− diet caused a transient increase in tHcy, as well as an increase in the tHcy/methionine ratio. Since CBS is a key regulator of tHcy, we examined CBS protein levels and found that within 3 days on methionine-deficient diet, animals had a 50% reduction in the levels of liver CBS protein and enzyme activity. Examination of CBS mRNA and studies of transgenic animals that express CBS from a heterologous promoter indicate that this reduction is occurring post-transcriptionally. Loss of CBS protein was unrelated to intracellular levels of S-adenosylmethionine, a known regulator of CBS activity and stability.
Our results imply that methionine deprivation induces a metabolic state in which methionine is effectively conserved in tissue by shutdown of the transsulfuration pathway via an S-adenosylmethionine-independent mechanism that signals a rapid down-regulation of CBS protein.
PMCID: PMC2956870  PMID: 20036517
Metabolism; Genetics; Amino Acids; Cardiovascular Disease
19.  Dietary intake of S-(α-carboxybutyl)-DL-homocysteine induces hyperhomocysteinemia in rats 
Betaine homocysteine S-methyltransferase (BHMT) catalyzes the transfer of a methyl group from betaine to homocysteine forming dimethylglycine and methionine. We previously showed that inhibiting BHMT in mice by intraperitoneal injection of S-(α-carboxybutyl)-DL-homocysteine (CBHcy) results in hyperhomocysteinemia. In the present study, CBHcy was fed to rats to determine whether it could be absorbed and cause hyperhomocysteinemia as observed for the intraperitoneal administration of the compound in mice. We hypothesized that dietary administered CBHcy will be absorbed and will result in the inhibition of BHMT and cause hyperhomocysteinemia. Rats were meal-fed every 8 hours an L-amino acid-defined diet either containing or devoid of CBHcy (5 mg/meal) for 3 days. The treatment decreased liver BHMT activity by 90% and had no effect on methionine synthase, methylenetetrahydrofolate reductase, phosphatidylethanolamine N-methyltransferase and CTP:phosphocholine cytidylyltransferase activities. In contrast, cystathionine β-synthase activity and immunodetectable protein decreased (56 and 26%, respectively) and glycine N-methyltransferase activity increased (52%) in CBHcy-treated rats. Liver S-adenosylmethionine levels decreased by 25% in CBHcy-treated rats and S-adenosylhomocysteine levels did not change. Further, plasma choline decreased (22%) and plasma betaine increased (15-fold) in CBHcy-treated rats. The treatment had no effect on global DNA and CpG island methylation, liver histology and plasma markers of liver damage. We conclude that CBHcy mediated BHMT inhibition causes an elevation in total plasma homocysteine that is not normalized by the folate-dependent conversion of homocysteine to methionine. Further, metabolic changes caused by BHMT inhibition affect cystathionine β-synthase and glycine N-methyltransferase activities, which further deteriorate plasma homocysteine levels.
PMCID: PMC2929918  PMID: 20797482
BHMT; betaine; rat; homocysteine; sulfur amino acids
20.  Testing computational prediction of missense mutation phenotypes: Functional characterization of 204 mutations of human cystathionine beta synthase 
Proteins  2010;78(9):2058-2074.
Predicting the phenotypes of missense mutations uncovered by large-scale sequencing projects is an important goal in computational biology. High-confidence predictions can be an aid in focusing experimental and association studies on those mutations most likely to be associated with causative relationships between mutation and disease. As an aid in developing these methods further, we have derived a set of random mutations of the enzymatic domains of human cystathionine beta synthase. This enzyme is a dimeric protein that catalyzes the condensation of serine and homocysteine to produce cystathionine. Yeast missing this enzyme cannot grow on medium lacking a source of cysteine, while transfection of functional human CBS into yeast strains missing endogenous enzyme can successfully complement for the missing gene. We used PCR mutagenesis with error-prone Taq polymerase to produce 948 colonies, and compared cell growth in the presence or absence of a cysteine source as a measure of CBS function. We were able to infer the phenotypes of 204 single-site mutants, 79 of them deleterious and 125 neutral. This set was used to test the accuracy of six publicly available prediction methods for phenotype prediction of missense mutations: SIFT, PolyPhen, PMut, SNPs3D, PhD-SNP, and nsSNPAnalyzer. The top methods are PolyPhen, SIFT, and nsSNPAnalyzer, which have similar performance. Using kernel discriminant functions, we found that the difference in position-specific scoring matrix values is more predictive than the wild-type PSSM score alone, and that the relative surface area in the biologically relevant complex is more predictive than that of the monomeric proteins.
PMCID: PMC3040297  PMID: 20455263
mutations; phenotype prediction; cystathionine beta synthase
21.  Hyperhomocysteinemia promotes inflammatory monocyte generation and accelerates atherosclerosis in transgenic cystathionine β-synthase deficient mice 
Circulation  2009;120(19):1893-1902.
Hyperhomocysteinemia (HHcy) is an independent risk factor for cardiovascular diseases. Monocytes display inflammatory and resident subsets, and commit to specific functions in atherogenesis. In this study, we examined the hypothesis that HHcy modulates monocyte heterogeneity and leads to atherosclerosis.
Methods and Results
We established a novel atherosclerosis susceptible mouse model with both severe HHcy and hypercholesterolemia, in which the mouse cystathionine β-synthase (CBS) and apolipoprotein E (apoE) genes are deficient, and an inducible human CBS transgene is introduced to circumvent the neonatal lethality of the CBS deficiency (Tg-hCBS apoE−/− Cbs−/− mice). Severe HHcy accelerated atherosclerosis and inflammatory monocyte/macrophage accumulation in lesions and increased plasma TNFα and MCP-1 levels in Tg-hCBS apoE−/− Cbs−/− mice fed a high fat diet. Furthermore, we characterized monocyte heterogeneity in Tg-hCBS apoE−/− Cbs−/− mice and another severe HHcy mouse model (Tg-S466L Cbs−/−) with a disease relevant mutation (Tg-S466L) that lacks hyperlipidemia. HHcy increased monocyte population and selective expansion of inflammatory Ly-6Chi and Ly-6Cmid monocyte subsets in blood, spleen and bone marrow of Tg-S466L Cbs−/− and Tg-hCBS apoE−/− Cbs−/− mice. These changes were exacerbated in Tg-S466L Cbs−/− mice with aging. Addition of L-homocysteine (100–500 μM), but not L-cysteine, maintained the Ly-6Chi subset and induced the Ly-6Cmid subset in cultured mouse primary splenocytes. Homocysteine-induced differentiation of Ly-6Cmid subset was prevented by catalase plus SOD, and the NAD(P)H oxidase inhibitor, apocynin.
HHcy promotes differentiation of inflammatory monocyte subsets and their accumulation in atherosclerotic lesions via NAD(P)H oxidase-mediated oxidant stress.
PMCID: PMC2783582  PMID: 19858416
atherosclerosis; inflammation; leukocytes; HHcy; oxidant stress
22.  Activation of Mutant Enzyme Function In Vivo by Proteasome Inhibitors and Treatments that Induce Hsp70 
PLoS Genetics  2010;6(1):e1000807.
Missense mutant proteins, such as those produced in individuals with genetic diseases, are often misfolded and subject to processing by intracellular quality control systems. Previously, we have shown using a yeast system that enzymatic function could be restored to I278T cystathionine β-synthase (CBS), a cause of homocystinuria, by treatments that affect the intracellular chaperone environment. Here, we extend these studies and show that it is possible to restore significant levels of enzyme activity to 17 of 18 (94%) disease causing missense mutations in human cystathionine β-synthase (CBS) expressed in Saccharomyces cerevisiae by exposure to ethanol, proteasome inhibitors, or deletion of the Hsp26 small heat shock protein. All three of these treatments induce Hsp70, which is necessary but not sufficient for rescue. In addition to CBS, these same treatments can rescue disease-causing mutations in human p53 and the methylene tetrahydrofolate reductase gene. These findings do not appear restricted to S. cerevisiae, as proteasome inhibitors can restore significant CBS enzymatic activity to CBS alleles expressed in fibroblasts derived from homocystinuric patients and in a mouse model for homocystinuria that expresses human I278T CBS. These findings suggest that proteasome inhibitors and other Hsp70 inducing agents may be useful in the treatment of a variety of genetic diseases caused by missense mutations.
Author Summary
Genetic diseases are often caused by missense mutations: single nucleotide changes that cause a single incorrect amino acid to be substituted into the underlying protein. Most missense mutations cause the encoded protein to fold incorrectly and therefore not function properly. Examples of missense mutation diseases include CBS deficiency, Li-Fraumeni syndrome, and methylenetetrahydrofolate reductase deficiency, which are caused by alterations in the CBS, TP53, and MTHFR genes, respectively. In the work presented here, we show that it is possible to restore substantial enzymatic function to disease-causing human mutant CBS, p53, and MTHFR proteins by treatments that alter the intracellular folding environment. All of these treatments result in elevation of the Hsp70 protein, a molecular chaperone protein involved in helping proteins fold properly. One way to rescue mutant protein function is by using a drug that inhibits the function of the proteasome, the intracellular machine responsible for degrading misfolded proteins. Our findings suggest that drugs of this class may be potentially useful in the treatment of human genetic diseases caused by missense mutations.
PMCID: PMC2795852  PMID: 20066033
23.  Valproic acid increases expression of methylenetetrahydrofolate reductase (MTHFR) and induces lower teratogenicity in MTHFR deficiency 
Journal of cellular biochemistry  2008;105(2):467-476.
Valproate (VPA) treatment in pregnancy leads to congenital anomalies, possibly by disrupting folate or homocysteine metabolism. Since methylenetetrahydrofolate reductase (MTHFR) is a key enzyme of folate interconversion and homocysteine metabolism, we addressed the possibility that VPA might have different teratogenicity in Mthfr+/+ and Mthfr+/− mice and that VPA might interfere with folate metabolism through MTHFR modulation. Mthfr+/+ and Mthfr+/− pregnant mice were injected with VPA on gestational day 8.5; resorption rates and occurrence of neural tube defects (NTDs) were examined on gestational day 14.5. We also examined the effects of VPA on MTHFR expression in HepG2 cells and on MTHFR activity and homocysteine levels in mice. Mthfr+/+ mice had increased resorption rates (36%) after VPA treatment, compared to saline treatment (10%), whereas resorption rates were similar in Mthfr+/− mice with the 2 treatments (25–27%). NTDs were only observed in one group (VPA-treated Mthfr +/+). In HepG2 cells, VPA increased MTHFR promoter activity and MTHFR mRNA and protein (2.5- and 3.7-fold, respectively). Consistent with cellular MTHFR up-regulation by VPA, brain MTHFR enzyme activity was increased and plasma homocysteine was decreased in VPA-treated pregnant mice compared to saline-treated animals. These results underscore the importance of folate interconversion in VPA-induced teratogenicity, since VPA increases MTHFR expression and has lower teratogenic potential in MTHFR deficiency.
PMCID: PMC2574752  PMID: 18615588
folate; methylenetetrahydrofolate reductase; neural tube; congenital defects; valproic acid
24.  Chemical chaperone rescue of mutant human cystathionine β-synthase 
Molecular genetics and metabolism  2007;91(4):335-342.
Missense mutations in the cystathionine beta-synthase (CBS) gene, such as I278T, are responsible for CBS deficiency, the most common inherited disorder in sulfur metabolism. Expression of human mutant CBS proteins in S. cerevisiae reveals that most disease causing mutations severely inhibit enzyme activity and cannot support growth of yeast on cysteine-free media. Here we show that the osmolyte chemical chaperones glycerol, trimethyl-N-oxide, dimethylsulfoxide, proline or sorbitol, when added to yeast media, allows growth on cysteine-free media and causes increased enzyme activity from I278T and three other mutant CBS proteins. Rescuable mutants are ones that are predicted to cause a decrease in solvent accessible surface area. The increase in enzyme activity is associated with stabilization of the tetramer form of the enzyme. This effect is not specific to yeast, as addition of the chaperone glycerol resulted in increased I278T activity when the enzyme is produced either in E. coli or in a coupled in vitro transcription/translation reaction. However, no stimulation of specific activity was observed when chaperones were added directly to purified I278T indicating that the presence of chemical chaperones is required during translation. We also found that by mixing different chaperones we could achieve rescue at significantly lower chaperone concentrations. Taken together, our data show that chemical chaperones present during the initial folding process can facilitate proper folding of several mutant CBS proteins and suggest it may be possible to treat some inborn errors of metabolism with agents that enhance proper protein folding.
PMCID: PMC2040066  PMID: 17540596
Homocystinuria; Methionine metabolism; osmolytes; chaperone; mutation
25.  Chemical chaperone rescue of mutant human cystathionine β-synthase 
Molecular Genetics and Metabolism  2007;91(4):335-342.
Missense mutations in the cystathionine beta-synthase (CBS) gene, such as I278T, are responsible for CBS deficiency, the most common inherited disorder in sulfur metabolism. Expression of human mutant CBS proteins in Saccharomyces cerevisiae reveals that most disease causing mutations severely inhibit enzyme activity and cannot support growth of yeast on cysteine-free media. Here, we show that the osmolyte chemical chaperones glycerol, trimethylamine-N-oxide, dimethylsulfoxide, proline or sorbitol, when added to yeast media, allows growth on cysteine-free media and causes increased enzyme activity from I278T and three other mutant CBS proteins. Rescuable mutants are ones that are predicted to cause a decrease in solvent accessible surface area. The increase in enzyme activity is associated with stabilization of the tetramer form of the enzyme. This effect is not specific to yeast, as addition of the chaperone glycerol resulted in increased I278T activity when the enzyme is produced either in Escherichia coli or in a coupled in vitro transcription/translation reaction. However, no stimulation of specific activity was observed when chaperones were added directly to purified I278T indicating that the presence of chemical chaperones is required during translation. We also found that by mixing different chaperones we could achieve rescue at significantly lower chaperone concentrations. Taken together, our data show that chemical chaperones present during the initial folding process can facilitate proper folding of several mutant CBS proteins and suggest it may be possible to treat some inborn errors of metabolism with agents that enhance proper protein folding.
PMCID: PMC2040066  PMID: 17540596
Homocystinuria; Methionine metabolism; Osmolytes; Chaperone; Mutation

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