purification of ligation product, amplification and library purification for multi-gene snp detection in gastric cancer

SNP-Guided 5-FU Therapy in Gastric Cancer Using Ion Torrent Sequencing

Gastric cancer is a heavy burden in the field of global public health. According to the Global Cancer Statistics 2020, published by the International Agency for Research on Cancer (IARC), gastric cancer is the fifth most diagnosed cancer and the fourth leading cause of cancer-related death. Worldwide, the incidence of age-standardized rate in East Asia is the highest in both males and females1. The occurrence of gastric cancer is insidious, which means that patients often do not have any obvious and specific symptoms in the early stage. Among all gastric cancer patients, in countries without routine screening, 80%-90% of patients are either diagnosed at an advanced stage when the tumor cannot be operated on or relapses within 5 years after the operation2.
For advanced or metastatic gastric cancer, chemotherapy is the main treatment, which can improve the survival rate and quality of life of patients. For the initial therapy of patients with metastatic gastric cancer, a platinum-fluoropyrimidine regimen is the principal choice for a first-line chemotherapeutic regimen3. Fluoropyrimidine mainly includes 5-fluorouracil (5-FU) and oral fluoropyrimidine derivatives, such as capecitabine and tegafur. The main target of 5-FU is folate metabolism-related enzymes, which inhibit DNA synthesis and slow the growth of tumor tissue. Adverse drug reactions limit their utility, with diarrhea, mucositis, myelosuppression, and hand-foot syndrome among the most frequent side effects. It has been reported that therapeutic response and adverse drug reactions are closely related to factors in the folate metabolic pathway. Notably, the homozygous mutation of rs1801131 has been identified as an indicator for hand-foot syndrome (p = 4.1 x 10-6, OR =9.99, 95% CI: 3.84-27.8)4. Although fluoropyrimidines are extensively used in anticancer chemotherapy, their chemoresistance is a common emergency, causing therapeutic failure in gastric cancer treatment. For instance, the overall response rate is only 10%-15% among patients with advanced colorectal cancer treated with 5-FU only5. Also, fluoropyrimidines have toxicity that cannot be ignored. The toxicity reactions induced by 5-FU mainly include diarrhea, hand-foot syndrome, stomatitis, neutropenia, thrombocytopenia, neurotoxicity, and even death6. Severe treatment-related toxicity occurs in 10%-30% of patients treated with fluoropyrimidines, and fatal toxicity occurs in 0.5%-1% of these patients7.
A quality-of-life study of patients with advanced gastric cancer found that the response rate was less than 50% for those who received 5-FU-based chemotherapy8. Therefore, understanding the factors related to the sensitivity of 5-FU-based chemotherapy is particularly important for precise treatment to maximize the response rate and effectiveness while minimizing toxicity. Considering that 5-FU is closely related to folate metabolism, the genetic variants of enzymes in folate metabolic pathway may be one of the factors. About 90% of human sequence variation is attributed to single base mutations in DNA, known as single nucleotide polymorphisms (SNPs)9. When SNPs change the enzyme properties of folate metabolism, it may lead to individual differences in efficacy, toxicity and chemoresistance to fluorouracil in gastric cancer patients.
Methylenetetrahydrofolate reductase (MTHFR) is mainly used to convert 5,10-methylenetetrahydrofolate (5,10-MTHF) into 5-methyltetrahydrofolate. 5-FdUMP,a metabolite of 5-FU, forms inactive ternary with 5,10-MTHF and thymidylate synthase (TS), inhibiting activity of TS and leading to deficiency of dTMP10. Accumulation of 5,10-MTHF can enhance the inhibition effect of 5-FU on TS, which is correlated with the activity of MTHFR. MTHFR rs1801131 and rs1801133 are the most common polymorphisms, which are related to lower enzyme activity (a decrease of 75% for rs1801133 and 30% for rs181131) and accumulation of 5,10-MTHF11.
Dihydrofolate reductase (DHFR) is the key enzyme in folate metabolism and DNA synthesis. DHFR reduces dihydrofolate, using NADPH, to tetrahydrofolate (THF) which is used to carry one-carbon unit. SNPs of DHFR may affect its expression, change the activity and abundance of THF, and further affect folate metabolism and sensitivity of 5-FU. DHFR rs1650697 point mutation occurs in the major promoter of DHFR gene, which increases the DHFR expression12. A study found that rs442767 is associated with the efficacy and toxicity of antifolate antitumor drugs, such as pemetrexed and methotrexate. Regarding the SNP rs442767, a GT genotype signifies the inheritance of a G allele and a T allele at the same locus on the homologous chromosomes from each parent. Similarly, the genotypes GG and TT denote the inheritance of either two G alleles or two T alleles, correspondingly. Compared with genotype GT+TT, GG is related to decreased event-free survival and increased risk13. This suggests that rs442767 may lead to certain potential influence on 5-FU.
Methionine synthase (MTR) catalyzes the re-methylation of homocysteine to methionine, which plays an important role in folate metabolism. MTR rs1805087 is the most common polymorphism of MTR gene. MTR rs1805087 substitutes glycine for aspartic acid at the potentially functional site of protein, which may decrease the activity of MTR. Subjects with the G allele had increased plasma folate level and decreased plasma homocysteine level14. On the contrary, a study showed that rs1805087 has no statistically significant association with the efficacy of 5-FU. But this study focused on colorectal cancer and the sample size was small. The relationship between rs1805087 and the efficacy of 5-FU in gastric cancer patients remains to be explored15.
Gamma-glutamyl hydrolase (GGH) is a lysosomal enzyme that regulates intracellular folate concentrations. Pteroylglutamic acid is the synonym of folic acid that is made up of pterin, p-aminomethylbenzoic acid and glutamic acid. There are two forms of folic acid in organisms, monoglutamate folate and polyglutamated folate. THF-polyglutamate is enzymatically converted to monoglutamic folate by GGH, successively releasing either mono-Glutamate (mono-Glu) or di-Glutamate (di-Glu)16. A study about GGH expression in patients with locally advanced gastric cancer, showed high GGH expression could reduce 5,10-MTHF and TS, which means that only a small dosage of 5-FU is needed to achieve the TS inhibitory effect in these patients17. GG is a prognostic biomarker in patients with locally advanced gastric cancer treated with postoperative adjuvant chemotherapy with S-1, a prodrug of 5-FU, and plays an important role in maintaining intracellular homeostasis of folic acid18. GGH rs11545078 is missense variant and alters Thr-127 to Ile-127. A study focusing on substrate specificity of GGH suggests that rs11545078, compared to wild-type, results in higher Km and lower catalytic efficiency for methotrexate and the structure is similar to folic acid19. Together, exploring the relationship between rs11545078 and clinical outcomes of 5-FU is a promising strategy to understand drug resistance.
Solute carrier family 19 member 1 (SLC19A1), also named reduced folate transporter, is a typical facilitative transmembrane protein that imports reduced folates that mammalian cells lack the ability to de novo synthesize, which is recognized to estimate the response of tumor to 5-FU20. However, only a few studies have been performed regarding the association between 5-FU and SLC19A1 polymorphism21. In patients with non-small cell lung cancer receiving pemetrexed which is a folate analog, the rs1051298 on the SLC19A1 gene contributed to increase the risk of all adverse drug reaction and decrease overall survival22,23. SLC19A1 rs1051298, a 3&#39;untranslated region variant about folate metabolism, may help explain some of the individual differences about 5-FU therapy. Here the aim is to evaluate the association between rs1051298 and 5-FU resistance among patients with gastric cancer.
A kit, based on semiconductor sequencing, is used for qualitative gene detection in vitro (Figure 1), which can detect 7 selected SNPs in 5 genes, the rs1801131 and rs1801133 mutations of MTHFR gene, rs1650697 and rs442767 mutations of DHFR gene, rs1805087 mutation of MTR gene, rs11545078 mutation of GGH gene and rs1051298 of SLC19A1 gene in tumor tissue samples of gastric cancer patients. Firstly, the sample nucleic acid was extracted, and the target fragment was specifically amplified by PCR. A universal sequencing adapter was added at both ends of the DNA fragment to construct a library that can be used for sequencing. Then amplification of the sequencing library by PCR to form a sequencing template was done. The positive template was enriched to meet the sequencing requirements. Using the semiconductor sequencing system, by fixing the DNA strand in the tiny hole of the semiconductor chip. DNA polymerase takes the single-stranded DNA as the template and synthesizes the complementary DNA strand according to the principle of complementary base pairing. Each time a base is extended in a DNA chain, a proton will be released, causing local pH changes. An ionic sensor detects pH changes and converts chemical signals into digital signals, so that bases can be interpreted in real time, and finally the base sequence of each DNA segment can be obtained. Bioinformatics analysis was used to match these sequences to the reference map of the human genome. When gastric cancer related genes mutate, their corresponding DNA base sequences will change, so as to obtain the mutation information of related genes.
The result can display gene mutation status and provide reference for clinicians to select appropriate types and dosages of chemotherapy drugs and predict the drug resistance for gastric cancer patients. However, the test results are only for clinical reference and are not recommended to be the only basis for individualized treatment of patients. Clinicians should make a comprehensive judgment based on the patient&#39;s condition, drug indications, treatment reactions and other laboratory test indicators.

Author Spotlight: Genetic Profiling for Fluorouracil Response in Gastric Cancer

Multi-Gene Single Nucleotide Polymorphism Detection in Gastric Cancer Based on Ion Semiconductor Sequencing Platform

Our research team is devolving into the elements that dictate the sensitivity and resistance to 5-fluorouracil, a key chemotherapeutic agent for gastric cancer, emphasizing genetic inferences. We are dedicated to uncovering how genetic differences impact drug reactions and assessing the feasibility of tailoring treatment to individual genetic profiles. This protocol is tailored to detect single-nucleotide polymorphisms in patients with gastric cancer, elucidating the correlation between genetic mutations and their sensitivity to fluorouracil chemotherapy.This breakthrough enables the prediction of patients'responses to fluorouracil, facilitating the customization of treatment strategies. This protocol stands out by offering high sensitivity for various sample types, including blood, reducing invasiveness with multiple samples. Utilizing semiconductor sequencing simplifies operations and consequently time by directly detecting ion changes by passing fluorouracils.

Purification of Ligation Product, Amplification and Library Purification for Multi-Gene SNP Detection in Gastric Cancer

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