Assignment of the human dihydropyrimidine dehydrogenase gene (DPYD) to chromosome region 1p22 by fluorescence in situ hybridization

Spatiotemporal Distribution and Function of Mitochondria in Oocytes

Mitochondria are energy provider organelles in eukaryotic cells that contain their own specific genome. This review addresses structural and functional properties of mitochondria, focusing on recent discoveries about the changes in quality and number of mitochondria per cell during oocyte development. We highlight how oocyte mitochondria exhibit stage-specific morphology and characteristics at different stages of development, in sharp contrast to the elongated mitochondria present in somatic cells. We then evaluate the latest transcriptomic data to elucidate the complex functions of mitochondria during oocyte maturation and the impact of mitochondria on oocyte development. Finally, we describe the methodological progress of mitochondrial replacement therapy to rescue oocytes with developmental disorders or mitochondrial diseases, hoping to provide a guiding reference to future clinical applications.

Pharmacogenomics of 5-fluorouracil in colorectal cancer: review and update

BACKGROUND

Colorectal cancer (CRC) is a disease with high morbidity and mortality rates. 5-fluorouracil (5-FU) is the first-line recommended drug for chemotherapy in patients with CRC, and it has a good effect on a variety of other solid tumors as well. Unfortunately, however, due to the emergence of drug resistance the effectiveness of treatment may be greatly reduced. In the past decade, major progress has been made in the field of 5-FU drug resistance in terms of molecular mechanisms, pre-clinical (animal) models and clinical trials.

CONCLUSIONS

In this article we systematically review and update current knowledge on 5-FU pharmacogenomics related to drug uptake and activation, the expression and activity of target enzymes (DPD, TS and MTHFR) and key signaling pathways in CRC. Furthermore, a summary of drug combination strategies aimed at targeting specific genes and/or pathways to reverse 5-FU resistance is provided. Based on this, we suggest that causal relationships between genes, pathways and drug sensitivity should be systematically considered from a multidimensional perspective. In the design of research methods, emerging technologies such as CRISPR-Cas, TALENS and patient-derived xenograft models should be applied as far as possible to improve the accuracy of clinically relevant results.

Single-Cell Transcriptomics of Human Oocytes: Environment-Driven Metabolic Competition and Compensatory Mechanisms During Oocyte Maturation

AIMS

The mechanisms coordinating maturation with an environment-driven metabolic shift, a critical step in determining the developmental potential of human in vitro maturation (IVM) oocytes, remain to be elucidated. Here we explored the key genes regulating human oocyte maturation using single-cell RNA sequencing and illuminated the compensatory mechanism from a metabolic perspective by analyzing gene expression.

RESULTS

Three key genes that encode CoA-related enzymes were screened from the RNA sequencing data. Two of them, ACAT1 and HADHA, were closely related to the regulation of substrate production in the Krebs cycle. Dysfunction of the Krebs cycle was induced by decreases in the activity of specific enzymes. Furthermore, the activator of these enzymes, the calcium concentration, was also decreased because of the failure of influx of exogenous calcium. Although release of endogenous calcium from the endoplasmic reticulum and mitochondria met the requirement for maturation, excessive release resulted in aneuploidy and developmental incompetence. High nicotinamide nucleotide transhydrogenase expression induced NADPH dehydrogenation to compensate for the NADH shortage resulting from the dysfunction of the Krebs cycle. Importantly, high NADP⁺ levels activated DPYD to enhance the repair of DNA double-strand breaks to maintain euploidy.

INNOVATION

The present study shows for the first time that exposure to the in vitro environment can lead to the decline of energy metabolism in human oocytes during maturation but that a compensatory action maintains their developmental competence.

CONCLUSION

In vitro maturation of human oocytes is mediated through a cascade of competing and compensatory actions driven by genes encoding enzymes.

SNPs in predicting clinical efficacy and toxicity of chemotherapy: walking through the quicksand

In the "precision medicine" era, chemotherapy still remains the backbone for the treatment of many cancers, but no affordable predictors of response to the chemodrugs are available in clinical practice. Single nucleotide polymorphisms (SNPs) are gene sequence variations occurring in more than 1% of the full population, and account for approximately 80% of inter-individual genomic heterogeneity. A number of studies have investigated the predictive role of SNPs of genes enrolled in both pharmacodynamics and pharmacokinetics of chemotherapeutics, but the clinical implementation of related results has been modest so far. Among the examined germline polymorphic variants, several SNPs of dihydropyrimidine dehydrogenase (DPYD) and uridine diphosphate glucuronosyltransferases (UGT) have shown a robust role as predictors of toxicity following fluoropyrimidine- and/or irinotecan-based treatments respectively, and a few guidelines are mandatory in their detection before therapy initiation. Contrasting results, however, have been reported on the capability of variants of other genes as MTHFR, TYMS, ERCC1, XRCC1, GSTP1, CYP3A4/3A5 and ABCB1, in predicting either therapy efficacy or toxicity in patients undergoing treatment with pyrimidine antimetabolites, platinum derivatives, irinotecan and taxanes. While formal recommendations for routine testing of these SNPs cannot be drawn at this moment, therapeutic decisions may indeed benefit of germline genomic information, when available. Here, we summarize the clinical impact of germline genomic variants on the efficacy and toxicity of major chemodrugs, with the aim to facilitate the therapeutic expectance of clinicians in the odiern quicksand field of complex molecular biology concepts and controversial trial data interpretation.

Pharmacogenetic landscape of DPYD variants in south Asian populations by integration of genome-scale data

AIM

Adverse drug reactions to 5-Fluorouracil(5-FU) is frequent and largely attributable to genetic variations in the DPYD gene, a rate limiting enzyme that clears 5-FU. The study aims at understanding the pharmacogenetic landscape of DPYD variants in south Asian populations.

MATERIALS & METHODS

Systematic analysis of population scale genome wide datasets of over 3000 south Asians was performed. Independent evaluation was performed in a small cohort of patients.

RESULTS

Our analysis revealed significant differences in the the allelic distribution of variants in different ethnicities.

CONCLUSIONS

This is the first and largest genetic map the DPYD variants associated with adverse drug reaction to 5-FU in south Asian population. Our study highlights ethnic differences in allelic frequencies.

Therapeutic drug monitoring of cancer chemotherapy

Therapeutic drug monitoring is not routinely used for chemotherapy agents. There are Several reasons, but one major drawback is the lack of established therapeutic Concentration ranges. Combination chemotherapy makes the establishment of Therapeutic ranges for individual drugs difficult, the concentration-effect relationship for a single drug may not be the same as when that drug is used in a drug combination. Pharmacokinetic optimization protocols for many classes of cytotoxic compounds exist in specialized centers, and some of these protocols are now part of large multicentre trials. Nonetheless, TDM clearly has the potential to improve the clinical use of chemotherapy gents, most of which have very narrow therapeutic indices and highly variable pharmacokinetics. A substantial body of literature accumulating during the past 15 years demonstrates relationships between systemic exposure to various chemotherapy agents and their toxic or therapeutic effects. This article reviews TDM concepts in addition to tools based on pharmacokinetic modeling of chemotherapy agents. The administered dose of chemotherapy agents is sometimes adjusted individually using either a priori or a posteriori methods. These models can only be applied by using the same dose and schedule as the original study. Bayesian estimation offers more flexibility in blood sampling times and, owing to its precision and to the amount of information provided is the method of choice for ensuring that a given patient benefits from the desired systemic exposure. Moreover, the role and application of Pharmacogenetics as a tool for individualizing chemotherapy is discussed highlighting the agents and mechanisms that have been well studied and defined and their relevance to clinical practice. Finally, this paper address issues critical to the optimal use of TDM in a clinical setting, and the role of clinical pharmacist in this regard. In addition, it discusses future developments in this field that can contribute to improving cancer chemotherapy In terms of patient outcome and survival. J Oncol Pharm Practice (2007) 13: 207—221.

A genomewide association mapping study using ultrasound-scanned information identifies potential genomic regions and candidate genes affecting carcass traits in Nellore cattle

The aim of this study was to identify candidate genes and genomic regions associated with ultrasound-derived measurements of the rib-eye area (REA), backfat thickness (BFT) and rumpfat thickness (RFT) in Nellore cattle. Data from 640 Nellore steers and young bulls with genotypes for 290 863 single nucleotide polymorphisms (SNPs) were used for genomewide association mapping. Significant SNP associations were explored to find possible candidate genes related to physiological processes. Several of the significant markers detected were mapped onto functional candidate genes including ARFGAP3, CLSTN2 and DPYD for REA; OSBPL3 and SUDS3 for BFT; and RARRES1 and VEPH1 for RFT. The physiological pathway related to lipid metabolism (CLSTN2, OSBPL3, RARRES1 and VEPH1) was identified. The significant markers within previously reported QTLs reinforce the importance of the genomic regions, and the other loci offer candidate genes that have not been related to carcass traits in previous investigations.

High-resolution melting analysis of the common c.1905+1G>A mutation causing dihydropyrimidine dehydrogenase deficiency and lethal 5-fluorouracil toxicity

Dihydropyrimidine dehydrogenase (DPD) deficiency is a pharmacogenetic syndrome associated with life-threatening toxicity following exposure to the fluoropyrimidine drugs 5-fluorouracil (5-FU) and capecitabine (CAP), widely used for the treatment of colorectal cancer and other solid tumors. The most prominent loss-of-function allele of the DPYD gene is the splice-site mutation c.1905+1G>A. In this study we report the case of a 73-year old woman with metastatic colorectal cancer who died from drug-induced toxicity after the first cycle of 5-FU-containing chemotherapy. Her symptoms included severe neutropenia, thrombocytopenia, mucositis and diarrhea; she died 16 days later despite intensive care measures. Post-mortem genetic analysis revealed that the patient was homozygous for the c.1905+1G>A deleterious allele and several family members consented to being screened for this mutation. This is the first report in Spain of a case of 5-FU-induced lethal toxicity associated with a genetic defect that results in the complete loss of the DPD enzyme. Although the frequency of c.1905+1G>A carriers in the white population ranges between 1 and 2%, the few data available for the Spanish population and the severity of this case prompted us to design a genotyping procedure to prevent future toxic effects of 5-FU/CAP. Since our group had previously developed a high-resolution melting (HRM) assay for the simultaneous detection of KRAS, BRAF, and/or EGFR somatic mutations in colorectal and lung cancer patients considered for EGFR-targeted therapies, we included the DPYD c.1905+1G>A mutation in the screening test that we describe herein. HRM provides a rapid, sensitive, and inexpensive method that can be easily implemented in diagnostic settings for the routine pre-therapeutic testing of a gene mutation panel with implications in the pharmacologic treatment.

Evaluation of clinical value of single nucleotide polymorphisms of dihydropyrimidine dehydrogenase gene to predict 5-fluorouracil toxicity in 60 colorectal cancer patients in China

Dihydropyrimidine dehydrogenase (DPD) activity could be affected by single nucleotide polymorphisms (SNPs), resulting in either no effect, partial or complete loss of DPD activity. To evaluate if SNPs of DPD can be used to predict 5-FU toxicity, we evaluated five SNPs of DPD (14G1A, G1156T, G2194A, T85C and T464A) by TaqMan real time PCR in 60 colorectal cancer patients. Clinical data demonstrated that there was higher correlation between DPD activity and toxic effects of 5-FU (p<0.05). Six patients were positive for G2194A detection, which were all heterozygous. Two patients had lower DPD activities (< 3) with higher toxic effects (≥ stage III) while one patient was also positive for T85C detection. Ten patients were positive for T85C detection. Two patients were homozygous with lower DPD activities and higher toxic effects. Two patients were positive for the T464A detection, which were heterozygous with lower DPD activity and higher toxic effects and also positive for T85C detection. These data clearly indicated that the T464A and homozygous of the T85C are stronger biomarkers to predict the 5-FU toxicity. Our study significantly indicated that the detection for G2194A, T85C and T464A could predict ~13% of 5-FU severe toxic side effects.

Dihydropyrimidine dehydrogenase polymorphisms and fluoropyrimidine toxicity: ready for routine clinical application within personalized medicine?

Fluoropyrimidines, including 5-fluorouracil (5-FU), are widely used in the treatment of solid tumors and remain the backbone of many combination regimens. Despite their clinical benefit, fluoropyrimidines are associated with gastrointestinal and hematologic toxicities, which often lead to treatment discontinuation. 5-FU undergoes complex metabolism, dihydropyrimidine dehydrogenase (DPD) being the rate-limiting enzyme of inactivation of 5-FU and its prodrugs. Several studies have demonstrated significant associations between severe toxicities by fluoropyrimidines and germline polymorphisms of DPD gene. To date, more than 30 SNPs and deletions have been identified within DPD, the majority of these variants having no functional consequences on enzymatic activity. However, the identification of deficient DPD genotypes may help identify poor-metabolizer patients at risk of developing potentially life-threatening toxicities after standard doses of fluoropyrimidines.

Toxicity profile and efficacy of oral capecitabine as adjuvant chemotherapy for Chinese patients with Stage III colon cancer

PURPOSE

The Xeloda in Adjuvant Cancer Therapy trial, conducted in a white population of patients, established capecitabine (Xeloda) as adjuvant chemotherapy for Stage III colon cancer. Given the ethnical difference in toxicity of adjuvant chemotherapy in colon cancer, this study was designed to evaluate the safety and efficacy of adjuvant capecitabine in Chinese patients with colon cancer.

METHODS

Chinese patients with curatively resected Stage III colon adenocarcinoma, who received adjuvant capecitabine, were entered into a prospective database. Oral capecitabine was given at 1,250 mg/m(2) twice daily, Days 1 to 14, every 21 days, for 8 cycles. Toxicities, laboratory abnormalities, and survival outcomes were evaluated.

RESULTS

Fifty-eight patients were entered into the database between August 2004 and October 2005. The median age was 63.9 years with a male-to-female ratio of 1.15:1. With a median follow-up duration of 20.9 months, 14 patients relapsed and 3 patients died. Disease-free and overall survival at two years was 69 and 97 percent, respectively. Grade 3 toxicities occurred as follows: stomatitis (1.7 percent), diarrhea (0 percent), hand-foot syndrome (41.4 percent), leucopenia (1.7 percent), neutropenia (3.4 percent), and hyperbilirubinemia (1.7 percent). No Grade 4 or 5 toxicity was noted. Compared with the Xeloda in the Adjuvant Cancer Therapy trial, a much higher incidence of serious hand-foot syndrome and a lower rate of severe diarrhea were found in this study.

CONCLUSIONS

A different toxicity profile of adjuvant capecitabine was noted in this study on Chinese patients with colon cancer compared with that reported in the Xeloda in Adjuvant Cancer Therapy trial, whereas the efficacy outcomes were comparable.

Mutational spectrum of dihydropyrimidine dehydrogenase gene (DPYD) in the Tunisian population

Dihydropyrimidine dehydrogenase enzyme (DPD) deficiency is a pharmacogenetic syndrome leading to severe side-effects in patients receiving therapies containing the anticancer drug 5-fluorouracil (5-FU). The aim of this population study is to evaluate gene variations in the coding region of the dihydropyrimidine dehydrogenase gene (DPYD) in the Tunisian population. One hundred and six unrelated healthy Tunisian volunteers were genotyped by denaturing HPLC (DHPLC). Twelve variants in the coding region of the DPYD were detected. Allele frequencies of DPYD*5 (A1627G), DPYD*6 (G2194A), DPYD*9A (T85C), A496G, and G1218A were 12.7%, 7.1%, 13.7%, 5.7%, and 0.5%, respectively. The DPYD alleles DPYD*2A (IVS 14+1g>1), DPYD*3 (1897 del C) and DPYD*4 (G1601A) associated with DPD deficiency were absent from the examined subjects. We describe for the first time a new intronic polymorphism IVS 6-29 g>t, found in an allelic frequency of 4.7% in the Tunisian population. Comparing our data with that obtained in Caucasian, Egyptian, Japanese and African-American populations, we found that the Tunisian population resembles Egyptian and Caucasian populations with regard to their allelic frequencies of DPYD polymorphisms. This study describes for the first time the spectrum of DPYD sequence variations in the Tunisian population.

Should DPD analysis be required prior to prescribing fluoropyrimidines?

Dihydropyrimidine dehydrogenase (DPD) is a key enzyme in the metabolic catabolism of chemotherapeutic agent 5-fluorouracil (5FU) and its derivatives, including capecitabine. Numerous genetic mutations have been identified in the DPD gene locus (DPYD), with a few key variants having functional consequences on enzymatic activity. Deficiencies in DPD activity have been shown to cause 5FU-treated cancer patients to experience severe drug-related toxicities, often requiring extensive medical intervention. We review the performance of assays that assess DPD and DPYD status, with an emphasis on the robustness for routine clinical applications. None of the current strategies are adequate to mandate routine DPD testing prior to starting a fluoropyrimidine-based therapy. However, further research and technological improvements will hopefully allow prospective identification of potentially toxic patients, in order to reduce the number of patients with severe, life-threatening side effects to 5FU treatment.

Increased prevalence of dihydropyrimidine dehydrogenase deficiency in African-Americans compared with Caucasians

PURPOSE

African-American patients with colorectal cancer were observed to have increased 5-fluorouracil (5-FU)-associated toxicity (leukopenia and anemia) and decreased overall survival compared with Caucasian patients. One potential source for this disparity may be differences in 5-FU metabolism. Dihydropyrimidine dehydrogenase (DPD), the initial and rate-limiting enzyme of 5-FU catabolism, has previously been shown to have significant interpatient variability in activity. Several studies have linked reduced DPD activity to the development of 5-FU toxicity. Although the distribution of DPD enzyme activity and the frequency of DPD deficiency have been well characterized in the Caucasian population, the distribution of DPD enzyme activity and the frequency of DPD deficiency in the African-American population are unknown.

EXPERIMENTAL DESIGN

Healthy African-American (n=149) and Caucasian (n=109) volunteers were evaluated for DPD deficiency using both the [2-(13)C]uracil breath test and peripheral blood mononuclear cell DPD radioassay.

RESULTS

African-Americans showed significantly reduced peripheral blood mononuclear cell DPD enzyme activity compared with Caucasians (0.26+/-0.07 and 0.29+/-0.07 nmol/min/mg, respectively; P=0.002). The prevalence of DPD deficiency was 3-fold higher in African-Americans compared with Caucasians (8.0% and 2.8%, respectively; P=0.07). African-American women showed the highest prevalence of DPD deficiency compared with African-American men, Caucasian women, and Caucasian men (12.3%, 4.0%, 3.5%, and 1.9%, respectively).

CONCLUSION

These results indicate that African-Americans, particularly African-American women, have significantly reduced DPD enzyme activity compared with Caucasians, which may predispose this population to more 5-FU toxicity.

Sequence analysis of the 5'-flanking regions of human dihydropyrimidine dehydrogenase gene: identification of a new polymorphism related with effects of 5-fluorouracil

Dihydropyrimidine dehydrogenase (DPD), known as a rate-limiting metabolic enzyme in the catabolism of 5-fluorouracil (5-FU), degrades more than about 80% of the administered 5-FU in human liver. Since it was reported that the anticancer effects of 5-FU were observed in cancer patients with lower DPD activities, many attempts have been conducted to anticipate the expected anticancer effects of 5-FU based on expression of intracanceral DPD. It have been reported that 39 different mutations and polymorphisms in the coding regions of DPD genes have been identified; however, there is no report on polymorphisms in the 5'-flanking region of DPD genes. We investigated polymorphisms in the 5'-flanking regions (3,058 bp), which are considered to control expression of DPD genes, in genomic DNA extracted from 37 kinds of human cancer cells. As the results, out of 37 cancer cells subjected to analysis, DLD- 7 cells had C insertion and 7 strains G deletion, which were hetelozygote. No significant relationship was identified between the DPD activity and the expression levels of DPD mRNA in examined 10 kinds of human cancer cells. However, in DLD-1 cells, which have C-insertion polymorphism in 5'-flanking region of DPD gene, the DPD activity was below detection limit (< or = 0.5 pmol/min/mg protein). Furthermore, 50% of cytosine residue on the CpG site generated by the C insertion was methylated at the 5 position. In this study, we have identified novel polymorphism possibly related the cytotoxicity of 5-FU in the 5'-flanking region of DPD gene. It is suggested that newly identified polymorphism of DPD gene might affect transcription of DPD, thereby providing influence on the clinical outcome of cancer patients treated with 5-FU.

Dihydropyrimidine Dehydrogenase Deficiency, a Pharmacogenetic Syndrome Associated with Potentially Life-Threatening Toxicity Following 5-Fluorouracil Administration

Dihydropyrimidine dehydrogenase (DPD) deficiency is a pharmacogenetic syndrome associated with potentially life-threatening toxicity following the administration of standard doses of 5-fluorouracil. This syndrome derives its importance from the fact that approximately 2 million patients receive the drug worldwide each year. Population studies have suggested that 4%-7% of the American population exhibit dose-limiting toxicity that might be associated with a genetic defect in the DPYD gene that encodes for the DPD enzyme. During the past several years it has become increasingly clear that genetics is a major determinant of the variability in drug response, accounting for the probability of drug efficacy and the likelihood of toxic drug reactions. This article briefly discusses the clinical presentation, laboratory diagnosis, pharmacokinetics, inheritance, and the clinical management options of DPD deficiency. The variability of DPD enzyme activity in population studies and the different DPYD alleles together with new phenotypic and genotypic methods of screening for DPD deficiency will also be reviewed.

Dihydropyrimidine dehydrogenase and the efficacy and toxicity of 5-fluorouracil

The identification of genetic factors associated with either responsiveness or resistance to 5-fluorouracil (5-FU) chemotherapy, as well as genetic factors predisposing patients to the development of severe 5-FU-associated toxicity, is increasingly being recognised as an important field of study. Dihydropyrimidine dehydrogenase (DPD) is the initial and rate-limiting enzyme in the catabolism of 5-fluorouracil (5-FU). Although the role of tumoral levels as a prognostic factor for clinical responsiviness has not been firmly established, there is ample evidence that a deficiency of DPD is associated with severe toxicity after the administration of 5-FU. Patients with a partial DPD deficiency have an increased risk of developing grade IV neutropenia. In addition, the onset of toxicity occurred twice as fast compared with patients with a normal DPD activity. To date, 39 different mutations and polymorphisms have been identified in DPYD. The IVS14+1G>A mutation proved to be the most common one and was detected in 24-28% of all patients suffering from severe 5-FU toxicity. Thus, a deficiency of DPD appears to be an important pharmacogenetic syndrome.

Chromosomal imbalances associated with acquired resistance to fluoropyrimidines in human colorectal cancer cells

The chromosomal aberrations underlying the development of resistance to fluoropyrimidines have not yet been identified. To characterise the genomic changes that induce the development of resistance to fluoropyrimidines, we used comparative genomic hybridisation (CGH) to analyse and compare the parent DLD-1 human colorectal cancer cell line and two cell lines, DLD-1/5-FU and DLD-1/FdUrd, which were resistant to 5-fluorouracil (5-FU) and 5-fluoro-2'-deoxyuridine (FdUrd), respectively. Both resistant cell lines showed a genetic aberration derived from the parental cell line DLD-1. Losses of 3p and 3q were also detected as additional genetic changes in the two resistant cell lines. Both resistant cell lines showed decreased orotate phosphoribosyltransferase (OPRT) activity, which is associated with the activity of the uridine monophosphate (UMP) synthase gene (3q13). These results suggested that the loss of 3q might be a genetic change responsible for the decreased OPRT activity and fluoropyrimidine cytotoxic response in cancer cells. Amplification of 18p11.2-p11.3 containing the thymidine synthase (TS) gene (18p11.32) was observed only in the DLD-1/FdUrd-resistant cell line, which overexpresses TS. These findings suggested that 18p amplification represents a genetic change associated with the overexpression of the TS protein. Our results indicate that chromosomal aberrations identified by CGH could explain, at least in part, acquired fluoropyrimidine resistance.

The effect of dihydropyrimidine dehydrogenase deficiency on outcomes with fluorouracil

The use of fluorouracil has been complicated by unpredictable pharmacokinetics, low response rates and seemingly random toxicity. The variable pharmacology is largely due to inherited differences in expression of the metabolising enzyme dihydropyrimidine dehydrogenase (DPD). This converts fluorouracil to inactive metabolites (catabolic pathway) and ultimately dictates the amount of fluorouracil that is available to be metabolised to cytotoxic nucleotides (anabolic pathway). Absolute and partial DPD deficiency affect around 0.1 and 3% of the Caucasian population, respectively. Administration of conventional doses of fluorouracil to these individuals has resulted in profound bone marrow and gastrointestinal toxicity, especially in those with absolute DPD deficiency. Other forms of toxicity such as myocardial ischaemia have been difficult to attribute directly to DPD deficiency. Efforts to improve outcomes with fluorouracil have included monitoring of fluorouracil concentrations and modifying fluorouracil administration techniques (e.g. from bolus injections to protracted intravenous infusions). In general, these moves have met with limited therapeutic advancement. The recognition that DPD deficiency increases toxicity has lead to the suggestion that genotypic or phenotypic assessment of DPD status prior to initiating fluorouracil may help predict outcomes. The gene that encodes for DPD expression is called DPYD. Approximately 1% of Caucasians are heterozygotes for the DPYD*2A mutation which is the variant allele that is most frequently implicated in DPD deficiency. Screening for this mutation may identify around 60% of individuals with absolute DPD deficiency who are at the greatest risk of toxicity. Another approach is to determine DPD activity in peripheral blood mononuclear cells, with low activity suggesting an increased risk of toxicity. Intratumoral DPD activity may also be assessed with high activity suggesting a poorer response to fluorouracil. Recently, drugs that inhibit DPD (e.g. eniluracil) have become available. These remove much of the variability in fluorouracil pharmacokinetics and may make assessment of DPD activity redundant. Despite the considerable inroads that have been made, further study is needed before the best means of optimising fluorouracil treatment is determined.

Implications of dihydropyrimidine dehydrogenase on 5-fluorouracil pharmacogenetics and pharmacogenomics

A prominent example of the potential application of pharmacogenomics and pharmacogenetics to oncology is the study of dihydropyrimidine dehydrogenase (DPD) in 5-fluorouracil (5-FU) metabolism. 5-FU is currently one of the most widely administered chemotherapeutic agents used for the treatment of epithelial cancers. DPD is the rate-limiting enzyme in the catabolism and clearance of 5-FU. The observation of a familial linkage of DPD deficiency from a patient exhibiting 5-FU toxicity suggested a possible molecular basis for variations in 5-FU metabolism. Molecular studies have suggested there is a relationship between allelic variants in the DPYD gene (the gene that encodes DPD) and a deficiency in DPD activity, providing a potential pharmacogenetic basis for 5-FU toxicity. In the last decade, studies have correlated tumoral DPD activity with 5-FU response, suggesting it may be a useful pharmacogenomic marker of patient response to 5-FU-based chemotherapy. This article reviews the basis and discusses the challenges of pharmacogenetic and pharmacogenomic testing of DPD for the determination of 5-FU efficacy and toxicity.

A comparative analysis of translated dihydropyrimidine dehydrogenase cDNA; conservation of functional domains and relevance to genetic polymorphisms

A pharmacogenetic syndrome caused by molecular defects in the dihydropyrimidine dehydrogenase gene (DPYD ) results in partial to complete loss of dihydropyrimidine dehydrogenase (DPD) enzyme activity with patients exhibiting life-threatening toxicity following administration of routine doses of 5-fluorouracil. To date, more than 19 reported mutations have been putatively associated with DPD deficiency with 16 occurring within the open reading frame of the cDNA. The purpose of this study was to examine the conservation of functional domains (including the uracil, flavine adenine dinucleotide and NADPH binding sites) across three phyla (Chordata, Arthropoda and Nematoda) and the conservation of regions corresponding to the previously reported mutations. Comparative analysis of the uracil and NADPH binding sites in mammals and invertebrates demonstrated 100% amino acid identity between mammals and Drosophila melanogaster. Caenorhabditis elegans demonstrated 89% and 88% identity in these domains, respectively. The mammalian sequences demonstrated 100% identity in two iron sulphur motifs (amino acids 953-964 and 986-997) with significant conservation in D. melanogaster (92% and 92% identity, respectively) and C. elegans (100% and 92% identity, respectively). Comparative amino acid analysis revealed non-conservation in the loci of four DPYD mutations [DPYD*12 (R21Q), DPYD*5 (I543V), DPYD*6 (V732I), DPYD*9A (C29R)]. Seven mutations occurred in highly conserved regions [M166V, DPYD*8 (R235W), DPYD*11 (V335l), DPYD*4 (S534N), DPYD*9B (R886H), D949V, DPYD*10 (V995F)]. In summary, this comparative analysis identified conserved regions which may be critical to enzyme structure and/or function. The conservation of loci where DPYD mutations occur was also examined to evaluate their functional significance on DPD enzyme activity. These data should prove useful in the evaluation of newly discovered mutations in the DPYD gene.

Pharmacogenetics and cancer chemotherapy

Cancer chemotherapy is limited by significant inter-individual variations in responses and toxicities. Such variations are often due to genetic alterations in drug metabolising enzymes (pharmacokinetic polymorphisms) or receptor expression (pharmacodynamic polymorphisms). Pharmacogenetic screening prior to anticancer drug administration may lead to identification of specific populations predisposed to drug toxicity or poor drug responses. The role of polymorphisms in specific enzymes, such as thiopurine S-methyltransferases (TPMT), dihydropyrimidine dehydrogenase (DPD), aldehyde dehydrogenases (ALDH), glutathione S-transferases (GST), uridine diphosphate glucuronosyl-transferases (UGTs) and cytochrome P450 (CYP 450) enzymes in cancer therapy are discussed in this review.

Characterization of the human dihydropyrimidine dehydrogenase gene

Dihydropyrimidine dehydrogenase (DPD) catabolizes endogenous pyrimidines and pyrimidine-based antimetabolite drugs. A deficiency in human DPD is associated with congenital thymine-uraciluria in pediatric patients and severe 5-fluorouracil toxicity in cancer patients. The dihydropyrimidine dehydrogenase gene (DPYD) was isolated, and its physical map and exon-intron organization were determined by analysis of P1, PAC, BAC, and YAC clones. The DPYD gene was found to contain 23 exons ranging in size from 69 bp (exon 15) to 961 bp (exon 23). A physical map derived from a YAC clone indicated that DPYD is at least 950 kb in length with 3 kb of coding sequence and an average intron size of about 43 kb. The previously reported 5' donor splice site mutation present in pediatric thymine-uraciluria and cancer patients can now be assigned to exon 14. All 23 exons were sequenced from a series of human DNA samples, and three point mutations were identified in three racial groups as G1601A (exon 13, Ser534Asn), A1627G (exon 13, Ile543Val), and G2194A (exon 18, Val732Ile). These studies, which have established that the DPYD gene is unusually large, lay a framework for uncovering new mutations that are responsible for thymine-uraciluria and toxicity to fluoropyrimidine drugs.