Hypoxia–Immune-Related Gene SLC19A1 Serves as a Potential Biomarker for Prognosis in Multiple Myeloma

Background

Multiple myeloma (MM) remains an incurable malignant tumor of plasma cells. Increasing evidence has reported that hypoxia and immune status contribute to the progression of MM. In this research, the prognostic value of the hypoxia–immune-related gene SLC19A1 in MM was evaluated by bioinformatics analysis.

Method

RNA-sequencing (RNA-seq) data along with clinical information on MM were downloaded from the Gene Expression Omnibus (GEO) database. Consistent clustering analysis and ESTIMATE algorithms were performed to establish the MM sample subgroups related to hypoxia and immune status, respectively, based on the GSE24080 dataset. The differentially expressed analysis was performed to identify the hypoxia–immune-related genes. Subsequently, a hypoxia–immune-gene risk signature for MM patients was constructed by univariate and multivariate Cox regression analyses, which was also verified in the GSE4581 dataset. Furthermore, the mRNA expression of SLC19A1 was determined using qRT-PCR in 19 MM patients, and the correlations between the genetic expression of SLC19A1 and clinical features were further analyzed.

Result

A total of 47 genes were identified as hypoxia–immune-related genes for MM. Among these genes, SLC19A1 was screened to construct a risk score model that had better predictive power for MM. The constructed prognostic signature based on SLC19A1 was verified in the GSE4581 dataset. All independent prognostic factors (age, β₂-microglobulin, LDH, albumin, MRI, and gene risk score) were used to develop a nomogram that showed a better performance for predicting the survival probability of MM patients for 1–5 years. Furthermore, SLC19A1 was highly expressed in newly diagnosed and relapsed MM patients, and high expression of SLC19A1 is correlated with higher bone marrow aspiration plasma cells and β₂-microglobulin levels in MM patients.

Conclusion

In conclusion, our results suggest that SLC19A1 is aberrantly expressed in MM and highly expressed SLC19A1 might be a biomarker correlated with inferior prognosis. More importantly, we identified SLC19A1 as a hypoxia–immune-related gene in MM. Future functional and mechanistic studies will further clarify the roles of SLC19A1 in MM.

SLC19A1 May Serve as a Potential Biomarker for Diagnosis and Prognosis in Osteosarcoma

BACKGROUND

Osteosarcoma is the most frequent primary malignant tumor of bone. SLC19A1 has been explored as a novel biomarker in some cancers. In this research, the diagnostic and prognostic value of SLC19A1 expression in osteosarcoma was evaluated by bioinformatics analysis. Data were sourced from the Gene Expression Omnibus (GEO) database.

METHODS

Gene expression data and clinical materials of patients with osteosarcoma were collected from GSE42352 and GSE21257 datasets. The mRNA expression of SLC19A1 was compared between osteosarcoma cells and mesenchyme stem cells with the Wilcoxon rank-sum test. Moreover, receiver operating characteristic (ROC) curve analysis was performed to determine the diagnostic merit of SLC19A1 for osteosarcoma. The relationship between SLC19A1 and clinicopathological characteristics was analyzed using logistic regression. Besides, the correlation between SLC19A1 and survival rate was assessed using Kaplan-Meier and Cox regression. The biological functions of SLC19A1 were annotated and evaluated through gene set enrichment analysis (GSEA) and gene set variation analysis (GSVA).

RESULTS

SLC19A1 was significantly highly expressed in osteosarcoma cells (p < 0.001). The ROC curve showed an area under the curve of 0.899, which indicated a high diagnostic value. High SLC19A1 expression showed a negative correlation with Huvos grade [odds ratio (OR) = 0.09 for III vs. I, p = 0.014]. Kaplan-Meier survival analysis showed that the overall survival (OS) of the patients with high SLC19A1 expression was significantly poorer than the low SLC19A1 expression group (p = 0.016). The univariate analysis revealed that high SLC19A1 expression was associated with poor OS [p = 0.013, hazard ratio (HR) = 6.74, 95% CI = 1.49 - 30.46]. The multivariate analysis revealed that SLC19A1 expression (p = 0.014, HR = 8.03, 95% CI = 1.52 - 42.51) was independently correlated with OS. GSEA showed that genes in high expression group of SLC19A1 were enriched in KEGG pathways, including "Glyoxylate and dicarboxylate metabolism", "Oxidative phosphorylation", "Aminoacyl tRNA biosynthesis", "Base excision repair", "Pyrimidine metabolism" and "Proteasome". GSVA further suggested their importance in the progression of osteosarcoma.

CONCLUSIONS

SLC19A1 may be a potential biomarker for diagnosis and prognosis in osteosarcoma.

Phase II trial of pemetrexed plus bevacizumab for second-line therapy of patients with advanced non-small-cell lung cancer: NCCTG and SWOG study N0426

PURPOSE

To evaluate the efficacy and toxicity of pemetrexed combined with bevacizumab as second-line therapy for patients with advanced non-small-cell lung cancer (NSCLC) and to correlate allelic variants in pemetrexed-metabolizing genes with clinical outcome.

PATIENTS AND METHODS

Patients with previously treated NSCLC received pemetrexed (500 mg/m(2) intravenous) combined with bevacizumab (15 mg/kg intravenous) every 3 weeks. The primary end point, evaluated using a one-stage Fleming design for detecting a true success rate of at least 70%, was the proportion of patients who were progression free and on treatment at 3 months. Polymorphisms in genes responsible for pemetrexed transport (reduced folate carrier [SLC19A1]) and metabolism (folylpolyglutamate synthase [FPGS] and gamma-glutamyl hydrolase [GGH]) evaluated in germline DNA (blood) were correlated with treatment outcome.

RESULTS

Forty-eight evaluable patients (14 females and 34 males) received a median of four cycles (range, one to 20 cycles). The most common grade 3 or 4 nonhematologic adverse events (AEs) were fatigue (13%), dyspnea (10%), and thrombosis (10%). Grade 3 or 4 hematologic AEs were neutropenia (19%) and lymphopenia (13%). Twenty-four (57%; 95% CI, 41% to 72%) of the first 42 patients met the success criteria. Median overall survival (OS) and progression-free survival (PFS) times were 8.6 and 4.0 months, respectively. The exon 6 (2522)C-->T polymorphism in SLC19A1 correlated with 3-month progression-free status (P = .01) and with PFS (P = .05). The IVS1(1307)C-->T polymorphism in GGH correlated with OS (P = .04).

CONCLUSION

The study did not meet its primary end point. However, the median PFS time of 4 months is promising. Pharmacogenetic studies in larger cohorts are needed to definitively identify polymorphisms that predict for survival and toxicity of pemetrexed.

Alternative transcripts of rat slc19a1: Cloning, genomic organisation, tissue specific promoters and alternative splicing

Recently, the rat genome project revealed the genomic sequence of slc19a1, coding for the methotrexate carrier-1, identical to the reduced folate carrier-1 of humans, on rat chromosome 20. At the same time, we have cloned and analysed the complete or partial cDNAs of now at least six different transcripts from rat liver and kidneys. Alignment with the genomic sequence revealed seven exons. The first two non-coding exons, exon I and Ia were used alternatively in kidneys and liver, respectively, suggesting usage of alternative promoters. Three minor mRNA forms resulted from absent splicing of intron III, a shortened exon III (exon IIIa), and a shortened exon IV (exon IVa). The minor transcripts were predicted to result in translation products with 7 or 6 instead of 12 transmembrane domains (TMDs) and a peptide mass of 38, 39 and 40 kDa instead of 58 kDa.

A80G polymorphism of reduced folate carrier 1 (RFC1) and C776G polymorphism of transcobalamin 2 (TC2) genes in Down's syndrome etiology

CONTEXT AND OBJECTIVE

There is evidence that polymorphisms of genes involved in folate metabolism may be associated with higher risk that mothers may bear a Down's syndrome (DS) child. This study therefore had the objective of investigating the A80G polymorphism of the reduced folate carrier 1 (RFC1) gene and the C776G polymorphism of the transcobalamin 2 (TC2) gene as maternal risk factors for DS among Brazilian women.

DESIGN AND SETTING

Analytical cross-sectional study with control group, at Faculdade de Medicina de São José do Rio Preto (Famerp).

METHODS

Sixty-seven mothers of DS individuals with free trisomy 21, and 113 control mothers, were studied. Molecular analysis of the polymorphisms was performed by means of the polymerase chain reaction with restriction fragment length polymorphism (PCR-RFLP), followed by electrophoresis on 2% agarose gel.

RESULTS

The frequencies of the polymorphic alleles were 0.51 and 0.52 for RFC1 80G, and 0.34 and 0.34 for TC2 776G, in the case and control groups, respectively. Thus, there were no differences between the groups in relation to either the allele or the genotype frequency, for both polymorphisms (P = 0.696 for RFC1 A80G; P = 0.166 for TC2 C776G; P = 0.268 for combined genotypes).

CONCLUSION

There was no evidence of any association between the RFC1 A80G and TC2 C776G polymorphisms and the maternal risk of DS in the sample evaluated.

Polymorphisms and methylation of the reduced folate carrier in osteosarcoma

High-dose methotrexate is a standard component in the treatment of osteogenic sarcoma. Impaired methotrexate uptake associated with decreased reduced folate carrier expression is a common mechanism of methotrexate resistance in osteogenic sarcoma samples. We investigated whether promoter methylation and polymorphisms in the 3' untranslated region are involved in regulating reduced folate carrier expression. In a cohort of 66 osteogenic sarcoma specimens, quantitative methylation-specific polymerase chain reaction and single-strand conformation polymorphism were performed. We found detectable levels of promoter methylation in 84.3% of samples. When related to the reduced folate carrier mRNA levels, a trend was observed that reduced folate carrier expression is lower in samples (median, 0.7) with greater than 10% DNA methylation as compared with those (median, 2.3) with less than 10% DNA methylation. The heterozygous polymorphisms of 2582 T/G and 2617C/T in the 3' untranslated region showed reduced folate carrier expression (median, 0.9) as compared with the wild-type 2582T and 2617C (median, 4.2). The data suggest promoter methylation and polymorphisms in the 3' untranslated region of the reduced folate carrier may be involved in its transcriptional regulation in osteogenic sarcoma. Further study is required to confirm this finding.

The reduced folate carrier (RFC) is cytotoxic to cells under conditions of severe folate deprivation. RFC as a double edged sword in folate homeostasis

The reduced folate carrier (RFC), a bidirectional anion transporter, is the major uptake route of reduced folates essential for a spectrum of biochemical reactions and thus cellular proliferation. However, here we show that ectopic overexpression of the RFC, but not of folate receptor alpha, a high affinity unidirectional folate uptake route serving here as a negative control, resulted in an approximately 15-fold decline in cellular viability in medium lacking folates but not in folate-containing medium. Moreover to explore possible mechanisms of adaptation to folate deficiency in various cell lines that express the endogenous RFC, we first determined the gene expression status of the following genes: (a) RFC, (b) ATP-driven folate exporters (i.e. MRP1, MRP5, and breast cancer resistance protein), and (c) folylpoly-gamma-glutamate synthetase and gamma-glutamate hydrolase (GGH), enzymes catalyzing folate polyglutamylation and hydrolysis, respectively. Upon 3-7 days of folate deprivation, semiquantitative reverse transcription-PCR analysis revealed a specific approximately 2.5-fold decrease in RFC mRNA levels in both breast cancer and T-cell leukemia cell lines that was accompanied by a consistent fall in methotrexate influx, serving here as an RFC transport activity assay. Likewise a 2.4-fold decrease in GGH mRNA levels and approximately 19% decreased GGH activity was documented for folate-deprived breast cancer cells. These results along with those of a novel mathematical biomodeling devised here suggest that upon severe short term (i.e. up to 7 days) folate deprivation RFC transport activity becomes detrimental as RFC, but not ATP-driven folate exporters, efficiently extrudes folate monoglutamates out of cells. Hence down-regulation of RFC and GGH may serve as a novel adaptive response to severe folate deficiency.

Impairments in antifolate transport are common in retinoblastoma tumor samples

BACKGROUND

Many patients with retinoblastoma have a genetic predisposition to cancer and external beam radiation therapy and alkylating agent chemotherapy may increase their risk of secondary malignancy. Identification of effective chemotherapy agents for retinoblastoma that are not associated with an elevated risk of secondary malignancy would be beneficial.

PROCEDURE

Twenty-six specimens of fresh retinoblastoma tumor cells were studied in vitro with a PT430 competitive displacement assay. Differential displacement of the PT430 by methotrexate and not trimetrexate was considered indicative of a defect in reduced folate carrier (RFC)-mediated transport. Elevations in the accumulation of PT430 were considered indicative of dihydrofolate reductase (DHFR) amplification.

RESULTS

In 9 of the 26 (35%) samples, displacement by methotrexate was less than half the displacement by trimetrexate indicative of a defect in the RFC. In 5 of the 26 (19%) samples, trimetrexate did not displace the PT430. In 7 of 26 (27%) samples, the peak PT430 accumulation was suggestive of DHFR overexpression. Overall 9 of 26 (35%) samples had no evidence of a transport defect or DHFR overexpression and would be anticipated to be potentially sensitive to methotrexate. In 15 of the 26 (58%), no defects existed in trimetrexate displacement or DHFR overexpression and would be anticipated to be potentially sensitive to trimetrexate.

CONCLUSION

These results would support consideration of a phase II study to determine the effectiveness of trimetrexate for recurrent intra-ocular retinoblastoma.

A humanized mouse model for the reduced folate carrier

The ubiquitously expressed reduced folate carrier (RFC) or SLC19A1 is recognized to be an essential transport system for folates in mammalian cells and tissues. In addition to its generalized role as a folate transporter, RFC provides specialized tissue functions including absorption across intestinal/colonic epithelia, transport across the basolateral membrane of renal proximal tubules, transplacental transport of folates, and folate transport across the blood-brain barrier. The human RFC (hRFC) gene is regulated by five major upstream non-coding regions (designated A1/A2, A, B, C, and D), each transcribed from a unique promoter. Altogether, at least 14 distinct hRFC transcripts can be envisaged in which different 5' untranslated regions (UTRs) are fused to a common splice acceptor region (positions -1 to -49) within the first coding exon with a common 1776bp coding sequence. The 5' non-coding regions are characterized by alternate transcription start sites, multiple splice forms, and selective tissue distributions. Alternate 5' UTRs impact mRNA stabilities and translation efficiencies, and result in synthesis of modified hRFC proteins translated from upstream AUGs. In this report, we describe production and characterization of transgenic mice (TghRFC1) containing a functional hRFC gene and of humanized mice in which the mRFC gene is inactivated and an active hRFC gene has been introduced. The mice appear to be healthy and to breed well. Analysis of tissue specificity of expression in both the TghRFC1 and humanized hRFC mice by real-time RT-PCR demonstrates that the hRFC gene is expressed with a specificity closely resembling that seen in human tissues. For the humanized hRFC mice, levels of B and A1/A2 5' UTRs predominated in all mice/tissues, thus resembling results in normal human tissues. Lower levels of A and C 5' UTRs were also detected. The availability of humanized mouse models for hRFC will permit investigators to address critical unanswered questions pertinent to human health and disease. These include the ability to analyze the hRFC gene in vivo, to control dietary and other environmental conditions that may impact levels of gene expression, and to control the genetics of the mice in order to assess the effects of hRFC gene alterations on tissue folate uptake and distribution, none of which can be easily achieved in human populations.

Folate receptor and human reduced folate carrier expression in HepG2 cell line exposed to fumonisin B1 and folate deficiency

Fumonisin B1 (FB1) induces apoptosis and decreases the cellular uptake of 5-methyltetrahydrofolate. Two folate transporters (folate receptor, FR, and Reduced Folate Carrier, hRFC1) are involved in the cell uptake of folate. We aimed to study whether FB1 modifies the expression of the FR and the hRFC1 and whether its apoptotic effect is influenced by folate. Incubation of HepG2 cells with FB1 induced apoptosis in concentration and time-dependent manner in complete medium (experimental control medium, ECM), as well as in folate-depleted medium (FDM). FDM increased the toxicity of FB1 as the cells developed apoptosis within 24 h at 1 microM of FB1 instead of 100 microM in ECM. Whereas FR protein expression in cells grown in ECM was significantly inhibited after apoptosis event, protein expression of the hRFC1 was rather increased. The hrfc1 transcription was decreased in the treated cells. Under folate-deficient conditions, dramatic changes were observed on both transcriptional and post-transcriptional expression of the two transporters. FDM alone reduced FR protein expression by 12 +/- 2% and 43 +/- 1% at 48 and 72 h, respectively. The 5-methytetrahydrofolate attenuates apoptosis in a greater extent than the folic acid. However, its effects in preventing decrease of both folate transporters have not been observed. In conclusion, this study shows that the changes in the expression of FR after FB1 addition are probably a consequence of the FB1 toxicity. The response to FB1 by HepG2 cell lines is influenced by folate status and by folate form. 5-methyltetrahydrofolate appears to be more effective in preventing apoptosis than folic acid.

Polymorphisms in Folate, Pyrimidine, and Purine Metabolism Are Associated with Efficacy and Toxicity of Methotrexate in Psoriasis

Methotrexate is the gold standard therapy for moderate to severe psoriasis, but there is marked interpersonal variation in its efficacy and toxicity. We hypothesized that in psoriasis patients, specific common polymorphisms in folate, pyrimidine, and purine metabolic enzymes are associated with methotrexate efficacy and/or toxicity. DNA from 203 retrospectively recruited psoriasis patients treated with methotrexate was collected and genotyped by restriction endonuclease digestion or length polymorphism assays. The reduced folate carrier (RFC) 80A allele and the thymidylate synthase (TS) 3'-untranslated region (3'-UTR) 6 bp deletion were associated with methotrexate-induced toxicity (P=0.025 and P=0.025, respectively). RFC 80A and 5-aminoimidazole-4-carboxamide ribonucleotide transformylase (ATIC) 347G were associated with methotrexate discontinuation (P=0.048 and P=0.038). The TS 5'-UTR 28 bp 3R polymorphism correlated with poor clinical outcome (P=0.029), however, this was not the case when patients with palmoplantar pustular psoriasis were not included in the analysis. Stronger associations between specific polymorphisms and methotrexate-induced toxicity and discontinuation were found in a subanalysis of patients on methotrexate not receiving folic acid supplementation. We have demonstrated preliminary evidence that specific polymorphisms of enzymes involved in folate, pyrimidine, and purine metabolism could be useful in predicting clinical response to methotrexate in patients with psoriasis.

Effect of folate oversupplementation on folate uptake by human intestinal and renal epithelial cells

BACKGROUND

Folic acid [corrected] plays an essential role in cellular metabolism. Its deficiency can lead to neural tube defects. However, optimization of body folate homeostasis can reduce the incidence of neural tube defects and may decrease the risk of Alzheimer and cardiovascular diseases and cancer. Hence, food fortification and intake of supplemental folate are widespread.

OBJECTIVE

We examined the effects of long-term folate oversupplementation on the physiologic markers of intestinal and renal folate uptake processes.

DESIGN

Human-derived intestinal Caco-2 and renal HK-2 epithelial cells were maintained (5 generations) in a growth medium oversupplemented (100 micromol folic acid/L) or maintained under sufficient conditions (0.25 and 9 micromol folic acid/L).

RESULTS

Carrier-mediated uptake of (3)H-folic acid (2 micromol/L) at buffer pH 5.5 (but not buffer pH 7.4) by Caco-2 and HK-2 cells maintained under the folate-oversupplemented condition was significantly (P<0.01) and specifically lower than in cells maintained under the folate-sufficient condition. This reduction in folic acid uptake was associated with a significant decrease in the protein and mRNA levels of the human reduced-folate carrier (hRFC) and a decrease in the activity of the hRFC promoter. It was also associated with a decrease in mRNA levels of the proton-coupled folate transporter/heme carrier protein 1 (PCFT/HCP1) and folate receptor (FR).

CONCLUSIONS

Long-term oversupplementation with folate leads to a specific and significant down-regulation in intestinal and renal folate uptake, which is associated with a decrease in message levels of hRFC, PCFT/HCP1, and FR. This regulation appears to be mediated via a transcriptional mechanism, at least for the hRFC system.

Associations of common polymorphisms in the thymidylate synthase, reduced folate carrier and 5-aminoimidazole-4-carboxamide ribonucleotide transformylase/inosine monophosphate cyclohydrolase genes with folate and homocysteine levels and venous thrombosis risk

BACKGROUND

Folate is important in purine and thymidylate synthesis and, via homocysteine remethylation, facilitates S-adenosylmethionine-dependent transmethylation. Low folate availability leads to hyperhomocysteinemia, which is a risk factor for arterial vascular disease and venous thrombosis. Genetic variation in folate-metabolizing genes may affect folate availability and hence confer a greater risk of venous thrombosis.

METHODS

We genotyped the thymidylate synthase (TYMS) 28-bp repeat and 6-bp deletion, and the reduced folate carrier (RFC1) 80G>A and AICAR transformylase/inosine monophosphate (IMP) cyclohydrolase (ATIC) 346C>G polymorphisms in population-based controls (n=431), and assessed their effect on plasma total homocysteine (tHcy), and serum and red blood cell (RBC) folate. We investigated the associations between these variants and disease risk in a retrospective case-control study on recurrent venous thrombosis (n=173) as well.

RESULTS

None of the genotypes, alone or in combination, were associated with major changes in tHcy. However, the TYMS 28-bp repeat was associated with serum and RBC folate levels. We found no evidence that the genetic variants studied were associated with recurrent venous thrombosis risk.

CONCLUSIONS

The TYMS 28-bp repeat and 6-bp deletion, and RFC1 80G>A and ATIC 346C>G polymorphisms are not associated with tHcy, but we did observe an association between the TYMS 28-bp repeat and serum and RBC folate in a general population. None of the polymorphisms was associated with recurrent venous thrombosis risk.

Human reduced folate carrier: translation of basic biology to cancer etiology and therapy

This review attempts to provide a comprehensive overview of the biology of the physiologically and pharmacologically important transport system termed the "reduced folate carrier" (RFC). The ubiquitously expressed RFC has unequivocally established itself as the major transport system in mammalian cells and tissues for a group of compounds including folate cofactors and classical antifolate therapeutics. Loss of RFC expression or function may have potentially profound pathophysiologic consequences including cancer. For chemotherapeutic antifolates used for cancer such as methotrexate or pemetrexed, synthesis of mutant RFCs or loss of RFC transcripts and proteins results in antifolate resistance due to incomplete inhibition of cellular enzyme targets and insufficient substrate for polyglutamate synthesis. Since RFC was first cloned in 1994, tremendous advances have been made in understanding the complex transcriptional and posttranscriptional regulation of RFC, in identifying structurally and functionally important domains and amino acids in the RFC molecule as a prelude to establishing the mechanism of transport, and in characterizing the molecular defects in RFC associated with loss of transport in antifolate resistant cell line models. Many of the insights gained from laboratory models of RFC portend opportunities for modulating carrier expression in drug resistant tumors, and for designing a new generation of agents with improved transport by RFC or substantially enhanced transport by other folate transporters over RFC. Many of the advances in the basic biology of RFC in cell line models are now being directly applied to human cancers in the clinical setting, most notably pediatric acute lymphoblastic leukemia and osteogenic sarcoma.

Genetic polymorphisms in the one-carbon metabolism pathway and breast cancer risk: a population-based case-control study and meta-analyses

Epidemiological evidence suggests that intake of folate and other B-vitamins and genetic variants in the one-carbon metabolism pathway could influence the risk of breast cancer. Previous studies have focused on 2 polymorphisms in the methylenetetrahydrofolate gene (MTHFR A222V and E429A); however, findings are inconclusive. In a large population-based case-control study in Poland (2,386 cases, 2,502 controls), we investigated the association between breast cancer risk and 13 polymorphisms in 6 one-carbon metabolism genes (MTHFR, MTR, MTRR, CBS, SHMT1 and SLC19A1). Data suggested an association between a nonsynonymous change in the gene coding for methionine synthase (MTR D919G) and reduced breast cancer risk: OR (95% CI) = 0.84 (0.73-0.96) and 0.85 (0.62-1.15) for heterozygous and homozygote variant genotypes, respectively, compared with common homozygotes; p-trend = 0.01, false discovery rate = 0.14. We found no significant associations between other variants and breast cancer risk, including MTHFR A222V or E429A. Meta-analyses including published studies of MTHFR A222V (8,330 cases and 10,825 controls) and E429A (6,521 cases and 8,515 controls) supported the lack of an overall association; however, studies suggested an increase in risk among premenopausal women. In conclusion, this report does not support a substantial overall association between the evaluated polymorphisms in the one-carbon metabolism pathway and breast cancer risk.

Cloning and functional characterization of a folate transporter from the nematode Caenorhabditis elegans

Two putative orthologs to the human reduced folate carrier (hRFC), folt-1 and folt-2, which share a 40 and 31% identity, respectively, with the hRFC sequence, have been identified in the Caenorhabditis elegans genome. Functional characterization of the open reading frame of the putative folt-1 and folt-2 showed folt-1 to be a specific folate transporter. Transport of folate by folt-1 expressed in a heterologous expression system showed an acidic pH dependence, saturability (apparent K(m) of 1.23 +/- 0.18 microM), a similar degree of inhibition by reduced and substituted folate derivatives, sensitivity to the anti-inflammatory drug sulfasalazine (apparent K(i) of 0.13 mM), and inhibition by anion transport inhibitors, e.g., DIDS. Knocking down (silencing) or knocking out the folt-1 gene led to a significant inhibition of folate uptake by intact living C. elegans. We also cloned the 5'-regulatory region of the folt-1 gene and confirmed promoter activity of the construct in vivo in living C. elegans. With the use of the transcriptional fusion construct (i.e., folt-1::GFP), the expression pattern of folt-1 in different tissues of living animal was found to be highest in the pharynx and intestine. Furthermore, folt-1::GFP expression was developmentally and adaptively regulated in vivo. These studies demonstrate for the first time the existence of a specialized folate uptake system in C. elegans that has similar characteristics to the folate uptake process of the human intestine. Thus C. elegans provides a genetically tractable model that can be used to study integrative aspects of the folate uptake process in the context of the whole animal level.

Prognostic role of the reduced folate carrier, the major membrane transporter for methotrexate, in childhood acute lymphoblastic leukemia: a report from the Children's Oncology Group

PURPOSE

The value of measuring expression of individual genes relevant to particular chemotherapy drugs and encoding metabolizing enzymes, transporters, or drug targets, as predictors of treatment response and outcome in pediatric acute lymphoblastic leukemia (ALL), remains controversial.

EXPERIMENTAL DESIGN

In a case-control population of 91 pediatric B-precursor ALL patients [42 relapsed within 4 years (cases) and 49 did not relapse (controls)], we used real-time reverse transcription-PCR to measure transcript levels for 20 genes relevant to chemotherapy with the five major drugs used to treat this disease, including asparaginase, 6-mercaptopurine, methotrexate, prednisone, and vincristine. Results were confirmed in a separate case-control population of 26 patients.

RESULTS

Only the human reduced folate carrier (hRFC) gene, encoding the major membrane transporter for methotrexate, showed a significant difference in median transcript levels between the 42 cases and the 49 controls (P = 0.0278, Wilcoxon test). Using cutoffs for hRFC expression levels (based on Akaike information criterion), there were statistically significant associations between hRFC transcripts and treatment relapse (P = 0.0052). hRFC-B, corresponding to the major hRFC transcript form in ALL, was also measured by real-time reverse transcription-PCR and was prognostic. The association between treatment relapse and hRFC levels was validated in a separate study population of 14 cases and 12 controls from an earlier case-control study (P = 0.0221).

CONCLUSIONS

Our results strongly suggest the prognostic importance of hRFC gene expression to treatment outcomes in pediatric ALL. They validate our previous studies of hRFC transcriptional regulation in pediatric ALL and provide further compelling evidence for the critical role for methotrexate in the successful treatment of this disease.

Transcription regulatory polymorphism -43T>C in the 5'-flanking region of SLC19A1 gene could affect rheumatoid arthritis patient response to methotrexate therapy

The reduced folate carrier (RFC) protein (SLC19A1-gene) has central role in the uptake and intracellular accumulation of folates. In this respect, we investigate whether SLC19A1 genetic variations could affect rheumatoid arthritis (RA) patient response to antifolate treatment. One hundred six unrelated RA patients were enrolled in this study. Polymerase chain reaction (PCR)-single strand conformation polymorphism (SSCP) was used as the screening method for genetic variants. Unusual SSCP patterns were characterized by direct sequencing of the PCR products and subsequently restriction assays were established. Western blot analysis of RFC protein was performed in respect of the identified SLC19A1 genotypes. Patient response to methotrexate (MTX) was evaluated using disease activity for 28 joint indices score, American College of Rheumatology 20% and 50% scores. No mutation was found in the SLC19A1 gene, but three polymorphic variants: the -43T>C in the 5'-flanking sequence to the ATG-transcription start site; and the 80G>A (R27H) and 696C>T (P232P) in the coding gene sequence. The wild type alleles of the three polymorphisms were in strict linkage disequilibrium. Western blot analysis revealed that the non-wild type allele of polymorphism -43T>C is associated with low RFC protein expression levels. Furthermore, the genotypic analysis of the functional polymorphic variant -43T>C revealed to be insufficient to predict patient response to MTX therapy. According to recent literature, several transport systems account for folate membrane transport. Additionally, in previous studies discrepancies have been reported to exist between the same genetic variants and their use in prediction of patient response to MTX therapy. Therefore, the present genotypic-phenotypic association study of a functional polymorphism revealed the need of a complex genotypic analysis in order to predict patient response to folate antagonists' therapy.

No association between common polymorphisms in genes of folate and homocysteine metabolism and the risk of Down's syndrome among French mothers

The cause of the non-disjunction leading to trisomy 21 remains unclear. Recent evidence has suggested that 5, 10-methylenetetrahydrofolate reductase (MTHFR) and/or methionine synthase reductase (MTRR) might contribute to the maternal risk of trisomy 21. The purpose of the present study was to analyse these findings among the French population and to investigate whether common polymorphisms in genes of the folate and homocysteine pathway, including the MTHFR 677C>T, MTHFR 1298A>C, the methionine synthase (MTR) 2756A>G, the cystathionine β-synthase (CBS) 844Ins68 and the reduced folate carrier (RFC-1) 80G>A polymorphisms, contribute to the risk of trisomy 21. The risk was studied by analysing independent and combined genotypes in 119 case mothers and 119 control mothers. The MTHFR 677T, MTHFR 1298C, MTR2756G, MTRR66G, CBSIns68+ and the RFC-1 80G allele frequencies were not significantly different among French case mothers, compared with control mothers. The risk of having a child with trisomy 21 did not appear to be linked to polymorphisms in genes associated with folate and homocysteine metabolism.

Molecular basis for decreased folylpoly-γ-glutamate synthetase expression in a methotrexate resistant CCRF-CEM mutant cell line

A CCRF-CEM mutant, CEM-p, has been shown to exhibit resistance to methotrexate due to decreased methotrexate polyglutamate accumulation. To ascertain the mechanism(s) responsible for this phenotype, we analyzed FPGS and SLC19A1 mRNA expression, examined FPGS promoter activity, and determined nucleotide sequence of the FPGS promoter and full length cDNA from CCRF-CEM and CEM-p cells. We identified in FPGS from CEM-p cells three amino acid substitutions that altered the ATP binding P-loop, glutamate/folate binding, and a conserved domain located at the carboxyl-terminal. Our data demonstrated for the first time the importance of the highly conserved domain (VTGSLHLVGGV) located at the carboxyl-terminal for FPGS activity.

Reduced folate carrier-1 80G>A polymorphism affects methotrexate treatment outcome in rheumatoid arthritis

The folate antagonist methotrexate (MTX) is a drug currently used in the treatment of rheumatoid arthritis (RA). MTX enters the cells through the reduced folate carrier (RFC-1) and is activated to polyglutamates. Previous studies have shown that RFC-1 expression may influence the efficacy of therapy with MTX. The studies suggest that G80A polymorphism in RFC-1 is associated with altered folate/antifolate levels and the subjects carrying homozygous mutant 80AA genotype tend to have higher plasma folate and MTX concentrations and higher erythrocyte polyglutamate levels compared with those with the wild type or heterozygous genotype. It is possible that this polymorphism might influence MTX treatment outcome in patients with RA. In the present study, we examined the association between RFC-1 G80A polymorphism and treatment outcome in patients with RA administered MTX. The study was carried out on 174 patients diagnosed with RA treated with MTX (7.5-15.0 mg weekly) plus low doses of methylprednisone. The RFC-1 80G>A polymorphism (resulting in a histidine-to-arginine substitution at codon 27 of RFC-1) was detected using a polymerase chain reaction-restriction fragment length polymorphism method. The probability of remission of RA symptoms was 3.32-fold higher in carriers of 80AA genotype as compared with patients with 80GG genotype (P=0.021, OR=3.32, 95% CI: 1.26-8.79). The frequency of A allele among MTX responders was 62.1, compared to 47.8% in a group of poor MTX responders (P=0.013, OR=1.78, 95% CI: 1.13-2.81). Moreover, the increase of aminotransferase activity was noted more frequently in carriers of 80AA genotype. The present data suggest that evaluation of RFC-1 gene 80G>A polymorphism may be a useful tool to optimize MTX therapy in patients with RA.

Effects of 5' untranslated region diversity on the posttranscriptional regulation of the human reduced folate carrier

The human RFC (hRFC) gene is regulated by five major 5' non-coding exons, characterized by alternate transcription start sites and splice forms. The result is up to 14 hRFC transcripts for which different 5' untranslated regions (UTRs) are fused to a common coding sequence. By in vitro translation assays with hRFC constructs corresponding to the major transcript forms, most of the forms were translated poorly. Upon expression of the 5'UTR-hRFC constructs in hRFC-null HeLa cells, a range of steady state hRFC proteins and transcripts were detected that reflected relative transcript stabilities and, to a lesser extent, translation efficiencies. Transcripts including 5' UTRs derived from non-coding exon A encoded a modified hRFC protein translated from an upstream initiation site. When this modified hRFC protein was expressed in hRFC-null K562 cells, there were only minor differences in surface targeting, stability, or transport function from wild type hRFC. Our results demonstrate an important role for posttranscriptional determinants of cellular hRFC levels and activity.

Metabolic endophenotype and related genotypes are associated with oxidative stress in children with autism

Autism is a behaviorally defined neurodevelopmental disorder usually diagnosed in early childhood that is characterized by impairment in reciprocal communication and speech, repetitive behaviors, and social withdrawal. Although both genetic and environmental factors are thought to be involved, none have been reproducibly identified. The metabolic phenotype of an individual reflects the influence of endogenous and exogenous factors on genotype. As such, it provides a window through which the interactive impact of genes and environment may be viewed and relevant susceptibility factors identified. Although abnormal methionine metabolism has been associated with other neurologic disorders, these pathways and related polymorphisms have not been evaluated in autistic children. Plasma levels of metabolites in methionine transmethylation and transsulfuration pathways were measured in 80 autistic and 73 control children. In addition, common polymorphic variants known to modulate these metabolic pathways were evaluated in 360 autistic children and 205 controls. The metabolic results indicated that plasma methionine and the ratio of S-adenosylmethionine (SAM) to S-adenosylhomocysteine (SAH), an indicator of methylation capacity, were significantly decreased in the autistic children relative to age-matched controls. In addition, plasma levels of cysteine, glutathione, and the ratio of reduced to oxidized glutathione, an indication of antioxidant capacity and redox homeostasis, were significantly decreased. Differences in allele frequency and/or significant gene-gene interactions were found for relevant genes encoding the reduced folate carrier (RFC 80G > A), transcobalamin II (TCN2 776G > C), catechol-O-methyltransferase (COMT 472G > A), methylenetetrahydrofolate reductase (MTHFR 677C > T and 1298A > C), and glutathione-S-transferase (GST M1). We propose that an increased vulnerability to oxidative stress (endogenous or environmental) may contribute to the development and clinical manifestations of autism.

Absorption of folate by Caco-2 cells is not affected by high glucose concentration

The aim of this work was to investigate the effect of high glucose exposure on the absorption of folate by Caco-2 cells. We verified that apical high glucose did not affect the apical uptake of [(3)H]folate. Both different concentrations of glucose (10-45 mM) and different exposure times (10 min-24 h) were tested. Furthermore, apical high glucose (30 mM) did not affect the intracellular steady-state levels of [(3)H]folate, and simultaneous apical and basolateral high glucose (30 mM) did not change the apical-to-basolateral apparent permeability (P(app)) to [(3)H]folate. Both the apical uptake and the steady-state intracellular levels of [(3)H]folate were strongly reduced by 5-methyltetrahydrofolate, methotrexate, SITS (4-acetamido-4'-isothiocyanato-2,2'-stilbenedisulfonic acid), DIDS (4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid) and indomethacin, but were not affected or only hardly affected by p-aminohippuric acid and fumitremorgin C. Moreover, DIDS and indomethacin significantly reduced (by 50-60%) the apical-to-basolateral P(app) to [(3)H]folate, but [(3)H]folate present in the cells at the end of the experiment was higher in the case of indomethacin. Fumitremorgin C had no effect. The effect of the drugs tested was not changed or only hardly changed by high glucose. In conclusion, absorption of [(3)H]folate is not modulated by either apical or basolateral high glucose exposure in Caco-2 cells. Moreover, our results suggest that the apical uptake of [(3)H]folate by Caco-2 cells involves the Reduced Folate Transporter (but not the Organic Anion Transporter), and that Multidrug Resistance Protein and/or Organic Anion Transporter (but not Breast Cancer Resistance Protein) may mediate apical efflux of [(3)H]folate.

Reduced folate carrier gene G80A polymorphism is associated with an increased risk of gastroesophageal cancers in a chinese population

Low folate intake has been associated with an increased risk of both oesophageal and gastric cancers. Reduced folate carrier (RFC1, also named SLC19A1) is an essential folate transporter and functions as a bidirectional anion exchanger, taking up folate cofactors and exporting various organic anions. A G80A polymorphism in RFC1 gene has been shown to be associated with alterations in folate and homocysteine metabolism in healthy individuals. In this study, we hypothesised that genetic variants in RFC1 may modulate risk of oesophageal cancer (EC) and gastric cancer (GC). To test this hypothesis, we evaluated the associations of the G80A polymorphism of RFC1 with EC and GC risk in a case-control study of 216 EC and 633 GC cases and 673 cancer-free controls in a Chinese population. We found that compared with the 80GG/GA genotypes in the recessive model, the variant homozygote RFC1 80AA was associated with a significantly increased risk of EC (adjusted odds ratio (OR)=1.80, 95% confidence interval (CI)=1.29-2.51), GC (adjusted OR=1.59, 95% CI=1.25-2.02) and EC and GC combined (adjusted OR=1.63, 95% CI=1.30-2.04). In the dominant model, the risk associated with RFC1 80AA was also elevated in EC (OR=1.35, 95% CI=0.91-1.99), GC (OR=1.43, 95% CI=1.07-1.91) and EC and GC combined (adjusted OR=1.40, 95% CI=1.07-1.83), compared with the 80GG genotype. The stratification analyses showed that effects of the RFC1 80AA genotype were more evident in subgroups of relatively older (60 years), female, non-smokers, and non-drinkers both in EC and GC. Although the exact biological mechanism of this association remains to be explored, our findings suggest possible involvement of RFC1 variant in the susceptibility of EC and GC. Further large and functional studies are needed to confirm our findings.

Genetic and nutritional deficiencies in folate metabolism influence tumorigenicity in Apcmin/+ mice

Epidemiological studies indicate that adequate dietary folate is protective against colon cancer, although mechanisms remain largely elusive. We investigated the effects of genetic disruptions of folate transport and metabolism and of dietary folate deficiency in a mouse model of colon cancer, the Apc(min/+) mouse. Apc(min/+) mice with heterozygous knockout of the gene for reduced folate carrier 1 (Rfc1(+/-)) developed significantly fewer adenomas compared to Rfc1(+/+)Apc(min/+) mice [30.3+/-4.6 vs. 60.4+/-9.4 on a control diet (CD) and 42.6+/-4.4 vs. 55.8+/-7.6 on a folate-deficient diet, respectively]. Rfc1(+/-)Apc(min/+) mice also carried a lower tumor load, an indicator of tumor size as well as of tumor number. In contrast, there were no differences in adenoma formation between Apc(min/+) mice carrying a knockout allele for methionine synthase (Mtr(+/-)), an enzyme that catalyzes folate-dependent homocysteine remethylation, and Mtr(+/+)Apc(min/+) mice. However, in both Mtr groups of mice, dietary folate deficiency significantly increased adenoma number (from 32.3+/-3.8 on a CD to 48.1+/-4.2 on a folate-deficient diet), increased plasma homocysteine, decreased global DNA methylation in preneoplastic intestines and increased apoptosis in tissues. There were no genotype-associated differences in these parameters in the Rfc1 group, suggesting that the protection conferred by Rfc1 deficiency is carried out through a different mechanism. In conclusion, genetic and nutritional disturbances in folate metabolism can have distinct influences on tumorigenesis in Apc(min/+) mice; altered levels of homocysteine, global DNA methylation and apoptosis may contribute mechanistically to dietary influence.

Transmembrane domains 4, 5, 7, 8, and 10 of the human reduced folate carrier are important structural or functional components of the transmembrane channel for folate substrates

The human reduced folate carrier (hRFC) facilitates membrane transport of folates and antifolates. hRFC is characterized by 12 transmembrane domains (TMDs). To identify residues or domains involved in folate binding, we used substituted cysteine (Cys) accessibility methods (SCAM) with sodium (2-sulfonatoethyl)methanethiosulfonate (MTSES). We previously showed that residues in TMD11 of hRFC were involved in substrate binding, whereas those in TMD12 were not (Hou, Z., Stapels, S. E., Haska, C. L., and Matherly, L. H. (2005) J. Biol. Chem. 280, 36206-36213). In this study, 232 Cys-substituted mutants spanning TMDs 1-10 and conserved stretches within the TMD6-7 (residues 204-217) and TMD10-11 connecting loop domains were transiently expressed in hRFC-null HeLa cells. All Cys-substituted mutants showed moderate to high levels of expression on Western blots, and only nine mutants including R133C, I134C, A135C, Y136C, S138C, G163C, Y281C, R373C, and S313C were inactive for methotrexate transport. MTSES did not inhibit transport by any of the mutants in TMDs 1, 3, 6, and 9 or for positions 204-217. Whereas most of the mutants in TMDs 2, 4, 5, 7, 8, and 10, and in the TMD10-11 connecting loop were insensitive to MTSES, this reagent inhibited methotrexate transport (25-75%) by 26 mutants in these TMDs. For 13 of these (Y126C, S137C, V160C, S168C, W274C, S278C, V284C, V288C, A311C, T314C, Y376C, Q377C, and V380C), inhibition was prevented by leucovorin, another hRFC substrate. Combined with our previous findings, these results implicate amino acids in TMDs 4, 5, 7, 8, 10, and 11, but not in TMDs 1, 2, 3, 6, 9, or 12, as important structural or functional components of the putative hydrophilic cavity for binding of anionic folate substrates.

Alternative transcripts of rat slc19a1: defective sorting and inefficient expression of alternative splicing products--an addendum

Recently, we have reported about the existence and cloning of alternative transcripts of the solute carrier 19a1 in rat liver and kidney, and have explained their origin from tissue specific promoters and by alternative splicing [DNA Seq. 2005, 16(1), 1-6]. The variant open reading frames (ORFs) of these transcripts were now expressed as fusion proteins with an N-terminal GFP. Transfection of HPCT-1E3 hepatocytoma cells as well as the MDCK kidney epithelial cell line with the GFP-fusion of the major splicing form containing 12 transmembrane domains (TMD) resulted in a strong fluorescence at the plasma membrane, which was stable over at least 3 days. Expression of GFP alone gave a comparable overall fluorescence intensity, although, staining was distributed evenly throughout the cells. Fusions of GFP with the shorter ORFs of all alternative slc19a1 transcripts containing 6-7 predicted TMD were expressed less efficiently and not sorted to the plasma membrane. Instead, these proteins accumulated in intracellular granules, and were, apparently, degraded. Hence, it is unlikely that these minor splicing forms are directly involved in solute transport across the plasma membrane.

5-amino-4-imidazolecarboxamide riboside potentiates both transport of reduced folates and antifolates by the human reduced folate carrier and their subsequent metabolism

Transport is required before reduced folates and anticancer antifolates [e.g., methotrexate (MTX)] exert their physiologic functions or cytotoxic effects. The folate/antifolate transporter with the widest tissue distribution and greatest activity is the reduced folate carrier (RFC). There is little evidence that RFC-mediated influx is posttranscriptionally regulated. We show that [(3)H]MTX influx in CCRF-CEM human childhood T-leukemia cells is potentiated up to 6-fold by exogenous 5-amino-4-imidazolecarboxamide riboside (AICAr) in a AICAr and MTX concentration-dependent manner. Metabolism to more biologically active polyglutamate forms is also potentiated for MTX and other antifolates. That potentiation of influx by AICAr is mediated by effects on the RFC is supported by analyses +/-AICAr showing (a) similarity and magnitude of kinetic constants for [(3)H]MTX influx; (b) similarity of inhibitory potency of known RFC substrates; (c) lack of potentiation in a CCRF-CEM subline that does not express the RFC; and (d) similarity of time and temperature dependence. Potentiation occurs rapidly and does not require new protein synthesis. Effects of specific inhibitors of folate metabolism and the time and sequence of AICAr incubation with cells suggest that both dihydrofolate reductase inhibition and metabolism of AICAr are essential for potentiation. Acute folate deficiency or incubation of CCRF-CEM with AICAr-related metabolites (e.g., adenosine) does not initiate potentiation. AICAr increases growth inhibitory potency of MTX and aminopterin against CCRF-CEM cells when both AICAr and antifolate are present for the first 24 hours of a 120-hour growth period. AICAr is the first small molecule that regulates RFC activity.

G80A reduced folate carrier SNP influences the absorption and cellular translocation of dietary folate and its association with blood pressure in an elderly population

The functional consequences of the G80A RFC SNP on the expressed reduced folate carrier protein were evaluated by looking at the relationship between intake of folate, plasma folate and cellular stores of the vitamin. The effect on homocysteine was also examined. Homocysteine is a thiol that is known to be inversely associated with folate, and which is considered to be both thrombo- and athrogenic. At high levels, homocysteine may also interfere with nitric oxide mediated vasodilation, cause oxidative injury to, and proliferation of the vascular endothelium, and alter the elastic properties of the vascular wall, contributing to increased blood pressure. Participants (119; 52 male, 67 female) from a NSW retirement village were assessed. Independent of gender, the assimilation of folate from dietary sources into red cells showed a significant association for GG (r=0.399; p=0.022) and GA (r=0.564; p<0.0001) subjects, but not homozygous recessive (AA) individuals (r=0.223; p=0.236). The same genotype based pattern of significance was shown for the association between dietary folate and plasma folate (GG: r=0.524; p=0.002, GA: r=0.408; p=0.002). No genotype-related pattern of significance was shown for the association between dietary folate and homocysteine. When examined by gender, some differences were apparent; one-way ANOVA showed that genotype influenced diastolic blood pressure in males (p=0.019), while only females showed a significant correlation between dietary folate and blood pressure within specific genotypes (Systolic pressure GA: r=-0.372; p=0.025, carriage of A: r=0.-0.357; p=0.011. Diastolic pressure GA: r=-0.355; p=0.034, carriage of A: r=0.-0.310; p=0.029). The G80A RFC SNP had an impact on the absorption and cellular translocation of dietary folate and its association with blood pressure in an elderly population.

Coexistence of multiple mechanisms of PT523 resistance in human leukemia cells harboring 3 reduced folate carrier alleles: transcriptional silencing, inactivating mutations, and allele loss

The reduced folate carrier (RFC) is the dominant route for the uptake of various antifolates including PT523, a potent dihydrofolate reductase inhibitor (Ki = 0.35 pM) and an excellent transport substrate of the RFC (Kt = 0.7 μM). Here, we describe the multiple mechanisms of RFC inactivation in human leukemia PT523-resistant cells originally harboring 3 RFC alleles. Cellular exposure to gradually increasing PT523 concentrations resulted in sublines displaying up to 3500-fold resistance to various hydrophilic antifolates that rely on RFC for their cellular uptake. Antifolate-resistant cells lost RFC gene expression (65%-99% loss) due to impaired promoter binding of various transcription factors that regulate RFC gene expression. Additionally, DNA sequencing revealed that PT523-resistant cells contained a cluster of 4 nearly consecutive mutations residing on a single RFC allele including L143P, A147V, R148G, and Q150Stop. Southern blot analysis established the loss of an RFC allele in PT523-resistant cells. These alterations resulted in markedly decreased RFC protein levels (∼80%-99% loss) and consequently impaired [3H]methotrexate transport (87%-99% loss). This study provides the first evidence that acquisition of PT523 resistance in human leukemia cells harboring 3 RFC alleles is due to multiple coexisting alterations including transcriptional silencing, inactivating mutations, and RFC allele loss.

Reduced folate carrier polymorphisms and neural tube defect risk

The reduced folate carrier (RFCI) is essential for folate transport into cells. Low folate is an important cause of neural tube defects (NTDs), and a single-nucleotide polymorphism (H27R) (80G-->A) in the RFCI gene has been reported to be a NTD risk factor. We investigated H27R and a 61 bp tandem repeat polymorphism as potential risk factors for NTDs, using a large homogeneous Irish population by case/control comparison, log-linear analysis, and transmission disequilibrium testing. No association was found between NTDs and H27R in mothers [p = 0.23, odds ratio (OR) 0.87, 95% confidence interval (CI) 0.69-1.09], fathers (p = 0.11, OR 0.83, 95% CI 0.66-1.04), or cases (p = 0.36, OR 0.9, 95% CI 0.72-1.12) when compared to controls or through log-linear modeling for dominant or recessive effects or with the transmission disequilibrium test for preferential allele transmission. Using log-linear models, a significant protective case effect was seen for the 61 bp polymorphism (p = 0.0039, OR 0.21, 95% CI 0.05-0.85). When analyzed by genotype, individuals homozygous for a single copy of the 61 bp sequence were underrepresented in cases as compared to controls, although these results did not reach statistical significance (p = 0.081, OR 0.5, 95% CI 0.23-1.09, goodness of fit p = 0.42). We compared the frequencies of H27R and the 61 bp polymorphism in African-Americans and American-Caucasians. The frequencies of H27R polymorphism differed significantly between the two populations (p = 0.0001). This large study does not confirm previous reports that H27R is a risk factor for NTDs. The previously unstudied 61 bp tandem repeat, however, has a possible protective NTD effect in our Irish population. This requires confirmation in other studies.

Transcriptional regulation of the human reduced folate carrier in childhood acute lymphoblastic leukemia cells

PURPOSE

The transcriptional regulation of the human reduced folate carrier (hRFC), involved in cellular uptake of methotrexate and reduced folates, was studied in childhood acute lymphoblastic leukemia (ALL). The hRFC gene is regulated by six noncoding exons (A1/A2 and A to E) and multiple promoters. In ALL, hRFC-A1/A2 and hRFC-B are the major transcript forms.

EXPERIMENTAL DESIGN

RNAs from 18 ALL lymphoblast specimens and 10 nonobese diabetic/severe combined immunodeficient ALL xenografts were assayed by real-time reverse transcription-PCR for hRFC-A1/A2 and hRFC-B transcripts and for transcripts encoding USF1, GATA1, Sp1, and Ikaros transcription factors. For the xenografts, gel shift and chromatin immunoprecipitation assays assessed transcription factor binding to the hRFC-A1/A2 and hRFC-B promoters. CpG methylation density within a 334-bp region, including the core hRFC-B promoter, was established by bisulfite sequencing. hRFC-A1/A2 and hRFC-B promoter polymorphisms were assayed by DNA sequencing.

RESULTS

For the 28 ALLs, hRFC-A1/A2 and hRFC-B transcripts spanned a 546-fold range. By chromatin immunoprecipitation and gel shift assays, binding was confirmed for USF1 and GATA1 for hRFC-A1/A2, and for Sp1, USF1, and Ikaros for hRFC-B. hRFC transcript levels correlated with those for GATA1 and USF1 for hRFC-A1/A2 and with Sp1 and USF1 transcripts for hRFC-B. CpG methylation in ALL did not correlate with hRFC-B transcripts. In 40 ALL and 17 non-ALL specimens, 2 cosegregating high-frequency polymorphisms (T-1309/C-1217 and C-1309/T-1217; allelic frequencies of 36% and 64%, respectively) were detected in the A1/A2 promoter; none were detected in promoter B. The hRFC-A1/A2 polymorphisms only slightly affected promoter activity.

CONCLUSIONS

Our results show a complex regulation of hRFC in ALL involving the hRFC-A1/A2 and hRFC-B promoters and noncoding exons. Although Sp1, USF1, and GATA1 levels are critical determinants of hRFC transcription in ALL, neither DNA methylation nor promoter polymorphisms contribute to differences in hRFC expression.

G80A reduced folate carrier SNP modulates cellular uptake of folate and affords protection against thrombosis via a non homocysteine related mechanism

Dietary folate is absorbed in the jejunum by the 'Reduced Folate Carrier' binding protein. This protein also sequesters extracellular folate for use by many cells in the body. As several biosynthetic pathways require folate for critical life processes, any change in the properties of this protein could lower folate bioavailability, cellular levels of the vitamin, and thus influence health. Since folate lowers thrombogenic homocysteine, we examined the prevalence of a common genetic polymorphism encoding the Reduced Folate Carrier (G80A RFC) to see if it acts as a risk factor for thrombotic vascular disease via an effect on homocysteine disposition in a cohort of 156 patients. The odds ratio indicates a significant protective effect of the mutant A allele against thrombosis: OR = 0.56(95% CI; 0.34-0.92). chi2; p = 0.022 (Yates corrected chi2; p = 0.031). The polymorphism had no impact on homocysteine, but did increase the level of extracellular to intracellular folate as might be predicted by the biological role of the expressed protein. This, and not homocysteine level, may be what affords protection against thrombosis.

BGC 945, a novel tumor-selective thymidylate synthase inhibitor targeted to alpha-folate receptor-overexpressing tumors

BGC 945 is a cyclopenta[g]quinazoline-based, thymidylate synthase inhibitor specifically transported into alpha-folate receptor (alpha-FR)-overexpressing tumors. Affinity of BGC 945 for the alpha-FR is 70% of the high-affinity ligand folic acid. In contrast to conventional antifolates, BGC 945 has low affinity for the widely expressed reduced-folate carrier (RFC). The K(i) for isolated thymidylate synthase is 1.2 nmol/L and the IC(50) for inhibition of the growth of alpha-FR-negative mouse L1210 or human A431 cells is approximately 7 micromol/L. In contrast, BGC 945 is highly potent in a range of alpha-FR-overexpressing human tumor cell lines (IC(50) approximately 1-300 nmol/L). Pharmacokinetic variables measured following i.v. injection of 100 mg/kg BGC 945 to KB tumor-bearing mice showed rapid plasma clearance (0.021 L/h) and tissue distribution. The terminal half-lives in plasma, liver, kidney, spleen, and tumor were 2, 0.6, 5, 21, and 28 hours, respectively. Tumor BGC 945 concentration at 24 hours was approximately 1 nmol/g tissue, at least 10-fold higher than that in plasma or normal tissues. Inhibition of thymidylate synthase in tissues leads to increased incorporation of 5-[(125)I]-iodo-2'-deoxyuridine ([(125)I]dUrd) into DNA. Forty-eight hours after injection of 100 mg/kg 6RS-BGC 945 ([(125)I]dUrd injected at 24 hours), tumor was the only tissue with incorporation above control level (6-fold). The RFC-mediated thymidylate synthase inhibitor plevitrexed also increased uptake of [(125)I]dUrd in tumor (10-fold) but, in contrast, also caused increased incorporation in other normal tissues such as spleen and small bowel (4.5- and 4.6-fold, respectively). These data suggest that BGC 945 selectively inhibits thymidylate synthase in alpha-FR-overexpressing tumors and should cause minimal toxicity to humans at therapeutic doses.

Polymorphisms in folate metabolic genes and lung cancer risk in Xuan Wei, China

The aim of this study is to investigate the role of genetic polymorphisms in twelve folate metabolism genes on the risk of lung cancer in Xuan Wei, China, where the lung cancer mortality rate is among the highest and is mainly caused by indoor smoky coal emissions. A total of 122 incident primary lung cancer cases and 122 matched controls were enrolled. Three single nucleotide polymorphisms were associated with increased risk of lung cancer including homozygotes of the C allele of CBS Ala360Ala (OR: 4.02; 95% CI: 1.64-9.87), the 222Val allele of MTHFR (OR: 2.32; 95% CI: 1.34-4.03), and the C allele of SLC19A1 Pro232Pro (OR: 1.83; 95% CI: 1.02-3.28). The distribution of CBS and MTHFR haplotypes differed between cases and controls (P=0.002 and P=0.07, respectively). In summary, three genetic variants in folate metabolism genes are associated with an increased risk of lung cancer in Xuan Wei, China.

Role of reduced folate carrier in intestinal folate uptake

Studies from our laboratory and others have characterized different aspects of the intestinal folate uptake process and have shown that the reduced folate carrier (RFC) is expressed in the gut and plays a role in the uptake process. Little, however, is known about the actual contribution of the RFC system toward total folate uptake by the enterocytes. Addressing this issue in RFC knockout mice is not possible due to the embryonic lethality of the model. In this study, we describe the use of the new approach of lentivirus-mediated short hairpin RNA (shRNA) to selectively silence the endogenous RFC of the rat-derived intestinal epithelial cells (IEC-6), an established in vitro model for folate uptake, and examined the effect of such silencing on folate uptake. First we confirmed that the initial rate of [(3)H]folic acid uptake by IEC-6 cells was pH dependent with a markedly higher uptake at acidic compared with alkaline pH. We also showed that the addition of unlabeled folic acid to the incubation buffer leads to a severe inhibition ( approximately 95%) in [(3)H]folic acid (16 nM) uptake at buffer pH 5.5 but not at buffer pH 7.4. We then examined the effect of treating (for 72 h) IEC-6 cells with RFC-specific shRNA on the levels of RFC protein and mRNA and observed substantial reduction in the levels of both parameters ( approximately 80 and 78%, respectively). Such a treatment was also found to lead to a severe inhibition ( approximately 90%) in initial rate of folate uptake at buffer pH 5.5 (but not at pH 7.4); uptake of the unrelated vitamin, biotin, on the other hand, was not affected by such a treatment. These results demonstrate that the RFC system is the major (if not the only) folate uptake system that is functional in intestinal epithelial cells.

Polymorphisms in the reduced folate carrier, thymidylate synthase, or methionine synthase and risk of colon cancer

Folate metabolism supports the synthesis of nucleotides as well as the transfer of methyl groups. Polymorphisms in folate-metabolizing enzymes have been shown to affect risk of colorectal neoplasia and other malignancies. Using data from a population-based incident case-control study (1,600 cases and 1,962 controls), we investigated associations between genetic variants in the reduced folate carrier (RFC), thymidylate synthase (TS), methionine synthase (MTR), and 5,10-methylenetetrahydrofolate reductase (MTHFR) and colon cancer risk. The TS enhancer region (TSER) variant was associated with a reduced risk among men [2rpt/2rpt versus 3rpt/3rpt wild-type; odds ratio (OR), 0.7; 95% confidence interval, 0.6-0.98] but not women. When combined genotypes for both TS polymorphisms (TSER and 3'-untranslated region 1494delTTAAAG) were evaluated, ORs for variant genotypes were generally below 1.0, with statistically significantly reduced risks among women. Neither MTR D919G nor RFC 80G>A polymorphisms were associated with altered colon cancer risk. Because folate metabolism is characterized by interrelated reactions, we evaluated gene-gene interactions. Genotypes resulting in reduced MTHFR activity in conjunction with low TS expression were associated with a reduced risk of colon cancer. When dietary intakes were taken into account, individuals with at least one variant TSER allele (3rpt/2rpt or 2rpt/2rpt) were at reduced risk in the presence of a low folate intake. This study supports findings from adenoma studies indicating that purine synthesis may be a relevant biological mechanism linking folate metabolism to colon cancer risk. A pathway-based approach to data analysis is needed to help discern the independent and combined effects of dietary intakes and genetic variability in folate metabolism.

Efficacy and toxicity of methotrexate in early rheumatoid arthritis are associated with single-nucleotide polymorphisms in genes coding for folate pathway enzymes

OBJECTIVE

To determine associations of methotrexate (MTX) efficacy and toxicity with single-nucleotide polymorphisms (SNPs) in genes coding for folate pathway enzymes in patients with early rheumatoid arthritis (RA).

METHODS

Patients (n=205) with active RA received MTX at an initial dosage of 7.5 mg/week, which was increased to 15 mg/week and combined with folic acid (1 mg/day) after 4 weeks. If the Disease Activity Score in 44 joints (DAS44) was >2.4 at 3 months, MTX was increased to 25 mg/week. MTX efficacy was evaluated at 3 and 6 months and compared for genotypes in 3 analyses: patients with and without good response (DAS44<or=2.4), patients with and without good improvement (DeltaDAS44>1.2), and patients with and without moderate improvement (DeltaDAS44>0.6). The association between MTX-related adverse drug events (ADEs) and genotype was evaluated by comparing genotypes between patients with and without ADEs, specifically pneumonitis, gastrointestinal ADEs, skin and mucosal ADEs, and elevated liver enzyme levels. The following SNPs were analyzed: methylenetetrahydrofolate reductase (MTHFR) 677C>T, MTHFR 1298A>C, dihydrofolate reductase (DHFR) -473G>A, DHFR 35289G>A, and reduced folate carrier 80G>A. In case of significant differences, odds ratios (ORs) were calculated.

RESULTS

At 6 months, MTHFR 1298AA was associated with good improvement relative to 1298C (OR 2.3, 95% confidence interval [95% CI] 1.18-4.41), which increased with increased copies of the MTHFR 677CC haplotype. In contrast, MTHFR 1298C allele carriers developed more ADEs (OR 2.5, 95% CI 1.32-4.72).

CONCLUSION

Patients with MTHFR 1298AA and MTHFR 677CC showed greater clinical improvement with MTX, whereas only the MTHFR 1298C allele was associated with toxicity. In the future, MTHFR genotypes may help determine which patients will benefit most from MTX treatment.

Loss of Sp1 function via inhibitory phosphorylation in antifolate-resistant human leukemia cells with down-regulation of the reduced folate carrier

The reduced folate carrier (RFC) is the dominant influx transporter for antifolates. A major mechanism of antifolate resistance is loss of RFC (SLC19A1) gene expression due to decreased GC-box-dependent transcription. However, despite the poor GC-box binding in multiple antifolate-resistant cell lines, normal Sp1 levels were retained. Here we explored the post-translational modifications that may disrupt Sp1 function. Phospho-affinity purification of nuclear proteins revealed that resistant cells contained approximately 8-fold more phosphorylated Sp1 than parental cells; treatment of nuclear proteins from these cells with alkaline phosphatase restored GC-box binding. As protein kinase A phosphorylates Sp1, resistant cells were treated with various cAMP-reactive agents, revealing no apparent effect on GC-box binding except for the general phosphodiesterase inhibitor IBMX. As cGMP levels also may be affected by IBMX, resistant cells were treated with 8-pCPT-cGMP, resulting in the complete restoration of GC-box binding, luciferase reporter activity, and RFC mRNA levels. This restoration was abolished in the presence of the protein phosphatase 2A inhibitor (PP2A) okadaic acid. Importantly, whereas resistant cells showed multiple phosphorylated Sp1 forms barely detectable in parental cells, treatment with 8-pCPT-cGMP resulted in their elimination; this disappearance, however, was prevented by the copresence of okadaic acid. These findings provide the first evidence that loss of RFC gene expression in antifolate-resistant cells is associated with an inhibitory Sp1 phosphorylation that can be eliminated by a cGMP-dependent activation of PP2A.

[A parental case control study on the association between reduced folate carrier gene polymorphism and neural tube defects]

OBJECTIVE

To study the association between reduced folate carrier gene (RFC1 A80G) polymorphism and the risk for child with neural tube defects (NTDs), and to provide epidemiological evidence for the existence of NTDs genetic marker.

METHODS

RFC1 (A80G) genotypes were detected using RFLP-PCR for blood DNA of 104 families with NTDs-affected children and 100 control families with no history of child-affected birth defects. Case-control study and transmission/disequilibrium test(TDT) for the RFC1 genotype of NTDs pedigree were carried out.

RESULTS

The G allele frequency of children with NTDs was higher than that of controls when compared to A allele( OR = 1. 64, 95% CI :1.08-2.49). The offspring of the GG genotype were associated with a 2.56-fold increased risk of NTDs when compared to the AA genotype (OR = 2.56, 95% CI: 1.04-6.36) in our study population. There was evidence of association between G allele and the risk of parent having a child with NTDs (OR = 1.56, 95% CI: 1.07-2.28) in the TDT analysis.

CONCLUSION

Our findings indicated that there was potential association between offspring RFC1 GG genotype and the risk of NTDs, and the G allele was a possible susceptible gene marker for an increased NTDs risk in the Chinese population.

Localization of a substrate binding domain of the human reduced folate carrier to transmembrane domain 11 by radioaffinity labeling and cysteine-substituted accessibility methods

The human reduced folate carrier (hRFC) mediates the membrane transport of reduced folates and classical anti-folates into mammalian cells. RFC is characterized by 12 transmembrane domains (TMDs), internally oriented N and C termini, and a large central linker connecting TMDs 1-6 and 7-12. By co-expression and N-hydroxysuccinimide methotrexate (Mtx) radioaffinity labeling of hRFC TMD 1-6 and TMD 7-12 half-molecules, combined with endoproteinase GluC digestion, a substrate binding domain was previously localized to within TMDs 8-12 (Witt, T. L., Stapels, S. E., and Matherly, L. H. (2004) J. Biol. Chem. 279, 46755-46763). In this report, this region was further refined to TMDs 11-12 by digestion with 2-nitro-5-thiocyanatobenzoic acid. A transportcompetent cysteine-less hRFC was used as a template to prepare single cysteine-replacement mutant constructs in which each residue from Glu-394 to Asp-420 of TMD 11 and Tyr-435 to His-457 of TMD 12 was replaced individually by a cysteine. The mutant constructs were transfected into hRFC-null HeLa cells. Most of the 50 single cysteine-substituted constructs were expressed at high levels on Western blots. With the exception of G401C hRFC, all mutants were active for Mtx transport. Treatment with sodium (2-sulfonatoethyl) methanethiosulfonate (MTSES) had no effect on hRFC activity for all of the cysteine mutants within TMD 12 and for the majority of the cysteine mutants within TMD 11. However, MTSES inhibited Mtx uptake by the T404C, A407C, T408C, T412C, F416C, I417C, V418C, and S419C mutants by 25-65%. Losses of activity by MTSES treatment for T404C, A407C, T412C, and I417C hRFCs were appreciably reversed in the presence of excess leucovorin, a hRFC substrate. Our results strongly suggest that residues within TMD 11 are likely critical structural and/or functional components of the putative hRFC transmembrane channel for anionic folate and anti-folate substrates.

[Epidemiological study on reduced folate carrier gene(RFC1 A80G) polymorphism and other risk factors of neural tube defects]

OBJECTIVE

To study the reduced folate carrier gene (RFC1) A80G polymorphism and other factors influence on children with neural tube defects (NTDs) and provide the epidemiological evidence for finding genetic marker of NTDs.

METHODS

RFC1(A80G) genotypes were detected using polymerase chain reaction-restriction fragment length polymorphism (RFLP-PCR) for blood DNA of 104 trios with NTDs-affected by child, and the 100 control families without child-affected by any birth defects. We performed the analysis of multifactors logistic regression for RFC1 genotypes and other factors in order to investigate the RFC1 genotype of the nuclear families and maternal periconceptional folic acid supplementation influence on NTDs independently. Transmission/disequilibrium test (TDT) for the RFC1 genotype of NTDs and control pedigree were carried out.

RESULTS

The RFC1 G allele frequency of children with NTDs (64.42%) was higher than that of the control children (52.53%), and there was the significant difference between them (chi(2)=5.9198, P<0.05). We observed that the infants of the GG genotype were associated with a 2.56-fold increased risk of NTDs when compared with the AA genotype (95% CI=1.04-6.36), The risk of mothers who did not take folic acid for having an NTDs-affected infants was 7.69 (95% CI=2.86-21.75). There were significant differences between cases and controls in the other risk factors, such as paternal age (> or =30), maternal fever during the early pregnancy, the history of maternal spontaneous abortion. In the logistic regression analysis, of multifactors the three factors, for example, the offspring of the RFC1 GG genotype (OR=2.91, 95% CI=1.35-6.30), maternal periconceptional folic acid supplementation (OR=4.32, 95% CI=1.62-11.55), maternal fever during the early pregnancy, had the statistic significance for the risk of NTDs. There was the evidence of an association between G allele and the risk of the maternal having a child with NTDs (OR=1.56, 95% CI=1.07-2.28) in TDT analysis.

CONCLUSION

Our findings indicate that the RFC1 genotype (GG) is a possible susceptible gene marker for an increased NTDs risk in this Chinese population, and there is a potential influence on the risk of NTDs in maternal periconceptional folic acid supplementation, and maternal fever during the early pregnancy.

The relationship between folate transport activity at low pH and reduced folate carrier function in human Huh7 hepatoma cells

Transport of folates and antifolates in both hepatocytes and Huh7 human hepatoma cells is characterized by a low-pH optimum. Studies were undertaken to determine the extent to which this transport activity is mediated by the reduced folate carrier (RFC) in Huh7 human hepatoma cells. RFC expression was ablated by chemical mutagenesis and antifolate selective pressure with PT632 resulting in the PT632(R) subline in which RFC mRNA could not be detected. Methotrexate (MTX) influx in these cells at pH 7.4 was reduced by 70%, leaving substantial residual RFC-independent influx while influx of MTX and folic acid at pH 5.5 was not significantly decreased. The influx K(t) for folic acid and MTX at pH 5.5 in PT632(R) cells was 0.36 and 1.5 microM, respectively. The affinity of the low pH transporter in PT632(R) cells was highest for pemetrexed (K(i)=140 nM), very low for PT632 (K(i)=77 microM), and was stereospecific for the natural isomer (6S) of 5-formyltetrahydrofolate. In Huh7 cells transiently transfected with an RFC siRNA, RFC expression was reduced by 60% resulting in a 40% decrease in MTX influx at pH 7.4 but only a very small (5%) reduction in MTX or folic acid influx at pH 5.5. These data indicate that MTX transport in Huh7 cells at neutral pH is mediated largely by RFC while at pH 5.5 the predominant route of transport is independent of RFC.

Karyotypic abnormalities create discordance of germline genotype and cancer cell phenotypes

The nature of mendelian inheritance assumes that all tissues in which a phenotype of interest is expressed have a uniform diploid karyotype, which is often not the case in cancer cells. Owing to nonrandom gains of chromosomes, trisomies are present in many cases of leukemia and other malignances. We used polymorphisms in the genes encoding thiopurine S-methyltransferase (TPMT), gamma-glutamyl hydrolase (GGH) and the reduced folate carrier (SLC19A1) to assess the nature of chromosomal acquisition and its influence on genotype-phenotype concordance in cancer cells. TPMT and GGH activities in somatic cells were concordant with germline genotypes, whereas activities in leukemia cells were determined by chromosomal number and whether the acquired chromosomes contained a wild-type or variant allele. Leukemia cells that had acquired an additional chromosome containing a wild-type TPMT or GGH allele had significantly lower accumulation of thioguanine nucleotides or methotrexate polyglutamates, respectively. Among these genes, there was a comparable number of acquired chromosomes with wild-type and variant alleles. Therefore, chromosomal gain can alter the concordance of germline genotype and cancer cell phenotypes, indicating that allele-specific quantitative genotyping may be required to define cancer pharmacogenomics unequivocally.

Genetic variation of infant reduced folate carrier (A80G) and risk of orofacial defects and congenital heart defects in China

PURPOSE

This study was designed to investigate whether the risks of congenital heart defects (CHD) and orofacial defects were influenced by a polymorphism of the offspring's RFC1 or by an interaction between the RFC1 gene and maternal periconceptional use of folic acid.

METHODS

A case-control study was conducted. A total of 82 families with a child affected by cleft lip with or without cleft palate (CLP), 67 families with a child-affected by CHD, and 100 nonmalformed control families were genotyped using PCR-RFLP. RFC1 G allele was tested through family-based association test.

RESULTS

Among mothers who did not use folic acid, the risks of 4.03 (95% CI = 1.33-12.77) for the G80/G80 genotype and 4.14 (95% CI = 1.06-16.82) for the G80/A80 genotype were observed relative to the A80/A80 genotype for CHD offspring. In family-based association tests (FBAT), offspring carrying the G allele for RFC1 is at increased risk for CHD (Z = 2.140, p < .05). No significant association was found between either RFC1 genotype or maternal folic acid supplementation and the risks of CLP.

CONCLUSIONS

Our findings suggest that the RFC1 G allele is likely to be an important candidate gene in folate transport and to be associated with risk for CHD. This study found modest evidence for a gene-nutrient interaction between offspring RFC1 genotype and periconceptional intake of folic acid on the risk of congenital heart defects.