Abstract： Acute Myeloid Leukemia (AML) is a malignant clonal disease of the blood system, characterized by abnormal proliferation and impaired differentiation of myeloid cells, resulting in the accumulation of leukemia cells and abnormal hematopoietic function. Although targeted therapy has developed rapidly, drug resistance and relapse remain major challenges in AML treatment. Exploring the abnormal proteins driving leukemia development and searching for new therapeutic targets and drugs are key tasks for AML treatment. Enolase-1 (ENO1) is a key enzyme in the glycolysis process, and its abnormal expression drives the development of various cancers. To clarify the role of ENO1 in the progression of AML, we studied the relationship between ENO1 and AML using database and patient transcriptome sequencing datafrom 185 AML patient samples. Our results confirm that ENO1 is highly expressed in AML patients and is associated with poor clinical outcomes. Knockdown of ENO1 in AML cells significantly inhibited their proliferation, cloning, migration, and invasion in vitro, blocked the cell cycle, and promoted apoptosis. We found that ENO1 was closely related to mitogen-activated protein kinase/extracellular signal-regulated kinase (MAPK/ERK) pathway through cell transcriptome sequencing data analysis. ERK pathway inhibitors significantly inhibited proliferation, clonal formation, migration, and invasion of AML cells in vitro, blocked cell cycle, and promoted apoptosis. And knockdown of ENO1 significantly inhibited ERK pathway proteins' phosphorylation. Most importantly, activators of ERK pathway could reverse the effects of ENO1 knockdown on AML cells. We also found that ENO1 knockdown inhibited the proliferation and invasion of AML cells in mice and significantly increase the overall survival rate of mice. Altogether, these results not only reveal the role and mechanism of ENO1 in the development of AML but also highlight ENO1 as a potential therapeutic target in the treatment of AML. Key words: AML, ENO1, Cell proliferation, Invasion

Burkitt's lymphoma (BL) is a rare and highly aggressive B-cell non-Hodgkin lymphoma. Although the outcomes of patients with BL have greatly improved, options for patients with relapsed and refractory BL are limited. Therefore, there is an urgent need to improve BL therapeutics and to develop novel drugs with reduced toxicity. In this study, we demonstrated that enolase 1 (ENO1) is a potential novel drug target for BL treatment. We determined that ENO1 was aberrantly upregulated in BL, which was closely related to its invasiveness and poor clinical outcomes. Furthermore, using RNA interference, we demonstrated that ENO1 depletion significantly inhibited cell proliferation and invasion both in vitro and in vivo. Mechanistically, we established that ENO1 knockdown suppressed the PI3K-AKT and epithelial-mesenchymal transition (EMT) signaling pathways by reducing plasminogen (PLG) recruitment, plasmin (PL) generation, and TGF-β1 activation. Addition of activated TGF-β1 protein to the culture medium of shENO1 cells reversed the inhibitory effects on cell proliferation and invasion, as well as those on the PI3K-AKT and EMT signaling pathways. Notably, our research led to the discovery of a novel ENO1-PLG interaction inhibitor, Ciwujianoside E (L-06). L-06 effectively disrupts the interaction between ENO1 and PLG, consequently reducing PL generation and suppressing TGF-β1 activation. In both in vitro and in vivo experiments, L-06 exerted impressive antitumor effects. In summary, our study elucidated the critical role of ENO1 in BL cell proliferation and invasion and introduced a novel ENO1 inhibitor, which holds promise for improving the treatment of patients with BL in the future.

Fever is a serious condition that can lead to various consequences ranging from prolonged illness to death. Tetrastigma hemsleyanum Diels et Gilg (T. hemsleyanum) has been used for centuries to treat fever, but the specific chemicals responsible for its antipyretic effects are not well understood. This study aimed to isolate and identify the chemicals with antipyretic bioactivity in T. hemsleyanum extracts and to provide an explanation for the use of T. hemsleyanum as a Chinese herbal medicine for fever treatment. Our results demonstrate that kaempferol 3-rutinoside (K3OR) could be successfully isolated and purified from the roots of T. hemsleyanum. Furthermore, K3OR exhibited a significant reduction in rectal temperature in a mouse model of fever. Notably, a 4 μM concentration of K3OR showed more effective antipyretic effects than ibuprofen and acetaminophen. To explore the underlying mechanism, we conducted an RNA sequencing analysis, which revealed that PXN may act as a key regulator in the fever process induced by lipopolysaccharide (LPS). In the mouse model of fever, K3OR significantly promoted the secretion of IL-6 and TNF-α during the early stage in the LPS-treated group. However, during the middle to late stages, K3OR facilitated the elimination of IL-6 and TNF-α in the LPS-treated group. Overall, our study successfully identified the chemicals responsible for the antipyretic bioactivity in T. hemsleyanum extracts, and it answered the question as to why T. hemsleyanum is used as a traditional Chinese herbal medicine for treating fever. These findings contribute to a better understanding of the therapeutic potential of T. hemsleyanum in managing fever, and they provide a basis for further research and development in this field.

Philadelphia chromosome-like acute lymphoblastic leukemia (Ph-like ALL) is a refractory and recurrent subtype of B-cell ALL enriched with kinase-activating rearrangements. Incomplete understanding of the heterogeneity within the tumor cells presents a major challenge for the diagnosis and therapy of Ph-like ALL. Here, we exhibited a comprehensive cell atlas of one Ph-like ALL patient with a novel TPR-PDGFRB fusion gene at diagnosis and relapse by using single-cell RNA sequencing (scRNA-seq). Twelve heterogeneous B-cell clusters, four with strong MKI67 expression indicating highly proliferating B cells, were identified. A relapse-enriched B-cell subset associated with poor prognosis was discovered, implicating the transcriptomic evolution during disease progression. Integrative single-cell analysis was performed on Ph-like ALL and Ph+ ALL patients, and revealed Ph-like specific B-cell subpopulations and shared malignant B cells characterized by the ectopic expression of the inhibitory receptor CLEC2D. Collectively, scRNA-seq of Ph-like ALL with a novel TPR-PDGFRB fusion gene provides valuable insights into the underlying heterogeneity associated with disease progression and offers useful information for the development of immunotherapeutic techniques in the future.

Background: The overactivation of NF-κB signaling is a key hallmark for the pathogenesis of extranodal natural killer/T cell lymphoma (ENKTL), a very aggressive subtype of non-Hodgkin’s lymphoma yet with rather limited control strategies. Previously, we found that the dysregulated exportin-1 (also known as CRM1) is mainly responsible for tumor cells to evade apoptosis and promote tumor-associated pathways such as NF-κB signaling. Methods: Herein we reported the discovery and biological evaluation of a potent small molecule CRM1 inhibitor, LFS-1107. We validated that CRM1 is a major cellular target of LFS-1107 by biolayer interferometry assay (BLI) and the knockdown of CRM1 conferred tumor cells with resistance to LFS-1107. Results: We found that LFS-1107 can strongly suppresses the growth of ENKTL cells at low-range nanomolar concentration yet with minimal effects on human platelets and healthy peripheral blood mononuclear cells. Treatment of ENKTL cells with LFS-1107 resulted in the nuclear retention of IkBα and consequent strong suppression of NF-κB transcriptional activities, NF-κB target genes downregulation and attenuated tumor cell growth and proliferation. Furthermore, LFS-1107 exhibited potent activities when administered to immunodeficient mice engrafted with human ENKTL cells. Conclusions: Therefore, LFS-1107 holds great promise for the treatment of ENKTL and may warrant translation for use in clinical trials. Funding: Yang's laboratory was supported by the National Natural Science Foundation of China (Grant: 81874301), the Fundamental Research Funds for Central University (Grant: DUT22YG122) and the Key Research project of 'be Recruited and be in Command' in Liaoning Province (Personal Target Discovery for Metabolic Diseases).

Acute myeloid leukemia (AML) is a heterogeneous disease and about one third of AML patients carry nucleophosmin (NPM1) mutation. Because 95% mutations give NPM1 an additional nuclear export signaling (NES) and dislocate NPM1 in cytoplasm (NPMc+), relocating NPM1 in nucleus provide an innovative strategy for treating this type of AML. The nuclear export of NPM1 depends on the nuclear protein export receptor XPO1, which recognizes the NES sequence on NPM1. Homoharringtonine (HHT) is a first-line chemotherapy drug of AML, yet the exact mechanism of its anti-AML activity is elusive. In this study, we found that HHT can directly target XPO1 to its NES-binding cleft, bind to Cys528 of XPO1, and inhibits its nuclear transport function. In addition, HHT can block NPMc+ proteins nuclear export and thus make NPMc+ AML cells much more sensitive to HHT treatment. Furthermore, the sensitivity of NPMc+ AML cells to HHT is a universal phenomenon irrespective of the different genetic lesions of AML. Taken together, our findings suggest that XPO1 is a new target of HHT and provide a novel strategy for NPMc+ AML treatment.

： Burkitt's lymphoma is a highly aggressive B-cell non-Hodgkin's lymphoma which is characterized by excessive proliferation of the malignant cells and deregulation of the c-MYC gene. However, the mechanisms underlying the aggressive behavior of Burkitt's lymphoma remain largely elusive. By analyzing the secretome of Burkitt's lymphoma cell line Daudi, we identified alpha-enolase (ENO1) was substantially upregulated in the conditional media of Daudi cells. We confirmed that the expression of ENO1 in patients with Burkitt's lymphoma was increased by qPCR and Western blot assays. CCK-8 and transwell assays revealed that knockdown of ENO1 in Burkitt's lymphoma cell lines Daudi and Raji inhibited cell proliferation and invasion. Further, in the mouse models,hematoxylin-eosin staining and flow cytomery revealed that knockdown of ENO1 inhibited the progression and invasion of tumors and delay the onset time of mice. ENO1 is an important glycolytic enzyme which has been found to be overexpressed in multiple solid tumors and promote cell proliferation, migration and invasion in tumors. We confirmed that knowndown of ENO1 reduced the binding of plasminogen on cell membrane by flow cytometry. In addition, silencing ENO1 effectively blocked the fibrinolytic activity. These results suggested ENO1 might contribute to Burkitt's lymphoma invasion by recruiting and activating plasminogen on the surface of Burkitt's lymphoma cells. We also found PI3K/AKT/mTOR pathway and N-cadherin, vimentin, slug were inhibited in response to the reduction of ENO1. We also found that c-Myc can bind to the promoter of ENO1 and promote the expression of ENO1 in Burkitt's lymphoma cell lines. Altogether, these results not only reveal the role and mechanism of ENO1 in the development of Burkitt's lymphoma, but also highlight ENO1 as a potential therapeutic target in the treatment of Burkitt's lymphoma. Key words: Burkitt's Lmphoma , ENO1, c-MYC, PLG, Invasion

Background Rearrangements involving the fibroblast growth factor receptor 1 (FGFR1) gene result in 8p11 myeloproliferative syndrome (EMS), which is a rare and aggressive hematological malignancy that is often initially diagnosed as myelodysplastic syndrome (MDS). Clinical outcomes are typically poor due to relative resistance to tyrosine kinase inhibitors (TKIs) and rapid transformation to acute leukemia. Deciphering the transcriptomic signature of FGFR1 fusions may open new treatment strategies for FGFR1 rearrangement patients. Methods DNA sequencing (DNA-seq) was performed for 20 MDS patients and whole exome sequencing (WES) was performed for one HOOK3-FGFR1 fusion positive patient. RNA sequencing (RNA-seq) was performed for 20 MDS patients and 8 healthy donors. Fusion genes were detected using the STAR-Fusion tool. Fluorescence in situ hybridization (FISH), quantitative real-time PCR (qRT-PCR), and Sanger sequencing were used to confirm the HOOK3-FGFR1 fusion gene. The phosphorylation antibody array was performed to validate the activation of nuclear factor-kappaB (NF-kappaB) signaling. Results We identified frequently recurrent mutations of ASXL1 and U2AF1 in the MDS cohort, which is consistent with previous reports. We also identified a novel in-frame HOOK3-FGFR1 fusion gene in one MDS case with abnormal monoclonal B-cell lymphocytosis and ring chromosome 8. FISH analysis detected the FGFR1 break-apart signal in myeloid blasts only. qRT-PCR and Sanger sequencing confirmed the HOOK3-FGFR1 fusion transcript with breakpoints located at the 11th exon of HOOK3 and 10th exon of FGFR1, and Western blot detected the chimeric HOOK3-FGFR1 fusion protein that is presumed to retain the entire tyrosine kinase domain of FGFR1. The transcriptional feature of HOOK3-FGFR1 fusion was characterized by the significant enrichment of the NF-kappaB pathway by comparing the expression profiling of FGFR1 fusion positive MDS with 8 healthy donors and FGFR1 fusion negative MDS patients. Further validation by phosphorylation antibody array also showed NF-kappaB activation, as evidenced by increased phosphorylation of p65 (Ser 536) and of IKBalpha (Ser 32). Conclusions The HOOK3-FGFR1 fusion gene may contribute to the pathogenesis of MDS and activate the NF-kappaB pathway. These findings highlight a potential novel approach for combination therapy for FGFR1 rearrangement patients.

Background Acute lymphoblastic leukemia (ALL) is a type of heterogeneous hematopoietic malignancy that accounts for approximately 20% of adult ALL. Although ALL complete remission (CR) rate has increased to 85-90% after induction chemotherapy, 40-50% of patients eventually relapsed. Therefore, it is necessary to improve the outcomes of ALL via accurate diagnosis and individualized treatments, which benefits in part from molecular biomarkers. Here, we identified a new fusion gene, Acyl-CoA Thioesterase 7-Nephrocystin 4 (ACOT7-NPHP4), in a 34-year-old patient with ALL. The fusion gene contributed to chemoresistance to doxorubicin and acted as a new molecular marker. Case presentation A 34-year-old male patient was diagnosed with ALL (common B cell) based on clinical manifestations and laboratory results. Although the patient received two cycles of the hyper-CVAD-L regimen as chemotherapy, the induction treatment failed. Because of the refusal of further treatments, the patient died of rapid progression of ALL one month later. Finally, a new fusion transcript, ACOT7-NPHP4, was detected in the patient's lymphoblastic leukemia cells via RNA sequencing. Conclusion This is the first report of a patient with ALL carrying an ACOT7-NPHP4 fusion gene. These findings may help understand the impact of ACOT7-NPHP4 in clinical molecular monitoring and drug resistance to doxorubicin; furthermore, its leukemogenesis will be essential to explore in future.

Hepatocellular carcinoma (HCC) is the most prevalent subtype of liver cancer with a mortality rate of approximately 3–6/100,000 and is the third leading cause of cancer-related death worldwide. Although several small-molecule drugs have been developed for the treatment of HCC, the choice of an agent for patients who require systemic chemotherapy at an advanced stage is still limited. The Hippo pathway is an evolutionarily conserved tumor suppressive pathway commonly dysregulated in HCC, which makes it a promising target for anti-HCC therapies. Homoharringtonine (HHT) is an FDA-approved anti-leukemia drug with proven strong anti-tumor activity in solid tumors. In this study, we found that HHT could significantly inhibit HCC cell growth by suppressing cell proliferation and colony formation. Moreover, HHT repressed cell invasion and migration remarkably. Additionally, HHT induced cell cycle arrest at S phase and promoted apoptosis. Most importantly, we showed that HHT-induced apoptosis was a consequence of the Hippo pathway activation. Consistently, the MST1/2 inhibitor, XMU-MP-1, could restore cell viability and reverse HHT-induced cell apoptosis. Furthermore, in vivo results confirmed the tumor inhibitory effect of HHT. Taken together, our findings suggest that HHT is a potential alternative therapeutic agent for the treatment of HCC.

长链非编码RNA(long noncoding RNAs,lncRNAs)是一种非编码RNA.随着新一代测序的应用,越来越多的lncRNAs被证实可以参与调控染色质重构、基因印迹、组蛋白修饰和DNA甲基化等多种生物学过程,同时参与肿瘤的发生发展.根据lncRNAs在恶性肿瘤中的作用,lncRNAs可以分为癌基因和肿瘤抑制基因.长链非编码RNA IRAIN是胰岛素样生长因子1受体(IGF1 R)反义表达的印迹基因.在多种肿瘤,例如白血病、乳腺癌、非小细胞肺癌、胰腺癌、肝癌、喉癌和肾癌等中表达异常.本综述结合近期文献,总结IRAIN参与肿瘤的发生发展作用与分子机制,以期为肿瘤的诊断和治疗提供新的理论依据.

利用计算机辅助药物设计,筛选新型染色体区域维持因子(CRM1)共价靶向抑制剂,并探究其对结外NK/T细胞淋巴瘤(ENKTL)细胞增殖的影响.利用基于片段的药物设计方法在LFS-01母核结构的基础上设计新型CRM1抑制剂并利用ADME/T、共价对接等手段进行药物筛选得到小分子化合物LFS-829.MALDI-TOF质谱分析表明,LFS-829对CRM1具有靶向作用.用CCK-8法检测LFS-829对ENKTL细胞系SNK6及HANK-1增殖活性的影响,活细胞工作站观察药物作用下细胞形态的变化.利用免疫荧光技术分析LFS-829对CRM1核输出功能的影响.通过蛋白质免疫印迹实验、双荧光素酶报告基因实验以及酶联免疫吸附技术分析不同浓度的LFS-829作用下NF-κB信号通路的变化.借助流式细胞仪分析检测细胞凋亡,并通过蛋白质免疫印迹实验检测凋亡通路相关蛋白的表达.根据外周血单核淋巴细胞(PBMC)毒性测试、血小板毒性测试以及小鼠急性毒性实验对LFS-829进行安全性评价.结果表明,LFS-829能够与CRM1蛋白疏水活性口袋的半胱氨酸残基共价靶向结合,并选择性杀伤SNK6及HANK-1细胞,72 h的IC50分别为366和158 nmol/L.800 nmol/L LFS-829能够显著抑制CRM1的细胞核输出功能,促使IκB-α的细胞核聚集,下调NF-κB信号通路的转录活性,并显著上调凋亡通路蛋白p53、剪切型Caspase 3和剪切型Caspase 9的表达,诱导细胞凋亡.LFS-829对PBMC以及血小板没有明显的杀伤效果.在300 mg/kg的大剂量作用下,LFS-829未对小鼠造成实质性的组织损伤,安全性良好,具有良好的应用前景.

In previous research, we found that lamprey immune protein (LIP) possessed cytocidal activity against tumor cells, but the mechanism of the selective recognition and killing of tumor cells by LIP was not identified. Superresolution microscopy, crystallographic structural analysis, glycan chip assay, SPR experiments, FACS assays, computational studies and mass spectrometric analysis firmly establish the mode of action of LIP, which involves dual selective recognition and efficient binding. We determined the overall crystallographic structure of LIP at a resolution of 2.25 Å. LIP exhibits an elongated structure with dimensions of 105 Å × 30 Å × 30 Å containing an N-terminal lectin module and a C-terminal aerolysin module. Moreover, the Phe209-Gly232 region is predicted to insert into the lipid bilayer to form a transmembrane β-barrel, in which the hydrophobic residues face the lipid bilayer, and the polar residues constitute the hydrophilic lumen of the pore. We found that LIP is able to kill various human cancer cells with minimal effects on normal cells. Notably, by coupling biochemical and computational studies, we propose a hypothetical mechanism that involves dual selective recognition and efficient binding dependent on both N-linked glycans on GPI-anchored proteins (GPI-APs) and sphingomyelin (SM) in lipid rafts. Furthermore, specific binding of the lectin module with biantennary bisialylated nonfucosylated N-glycan or sialyl Lewis X-containing glycan structures on GPI-APs triggers substantial conformational changes in the aerolysin module, which interacts with SM, ultimately resulting in the formation of a membrane-bound oligomer in lipid rafts. LIP holds great potential for the application of a marine protein towards targeted cancer therapy and early diagnosis in humans.

Sulforaphene (LFS-01) is the major chemical constituent of Raphanus sativus, a medicinal herb used for over a thousand years in traditional Chinese medicine. Here we identified that LFS-01 can selectively eradicate lymphoma cells while sparing normal lymphocytes by triggering concomitant mitophagy and apoptosis. We demonstrated that LFS-01 can retain Nrf2 in the nucleus by covalently modulating CRM1 and consequently upregulate p62/SQSTM1, an essential structural component of the autophagosomes during mitophagic process. We found that LFS-01 treatment also stimulated AMPK and thereby inhibited the mTOR pathway. On the contrary, we revealed that AMPK inhibition can severely impair the LFS-01-mediated mitophagy. Transcriptomic studies confirmed that 15 autophagy-associated genes such as p62/SQSTM1, VCP and BCL2 were differentially expressed after LFS-01 treatment. Furthermore, protein interactome network analysis revealed that the events of apoptosis and the assembly of autophagy vacuole were significant upon LFS-01 exposure. Lastly, we found that LFS-01 exhibited strong efficacy in xenograft mouse model yet with the lack of apparent toxicity to animals. We concluded that LFS-01 triggered mitophagic cell death via CRM1-mediated p62 overexpression and AMPK activation. Our findings provide new insights into the mechanism of action for LFS-01 and highlight its potential applications in treating major human diseases.

目的:研究染色体区域维持因子1 (chromosomal region maintenance 1,CRM1)在套细胞淋巴瘤中的表达情况,并进一步探讨天然产物莱菔素LFS-01抑制套细胞淋巴瘤细胞增殖的分子机制.方法:通过Oncomine数据库挖掘分析CRM1在套细胞淋巴瘤中的表达水平.用LFS-01处理套细胞淋巴瘤JeKo-1细胞后,采用CCK-8法检测细胞增殖活性,激光共聚焦显微镜下观察CRM1蛋白的核转运功能,蛋白质印迹法检测CRM1蛋白表达水平,FCM法检测和透射电子显微镜观察细胞周期和凋亡,蛋白质印迹法检测凋亡通路相关蛋白表达,CCK-8和FCM法检测凋亡抑制剂z-VAD-FAM对LFS-01作用的影响.构建CRM1基因突变的慢病毒稳定感染的JeKo-1细胞,激光共聚焦显微镜和CCK-8法再次检测LFS-01对CRM1核转运和细胞增殖的影响.通过RNAseq数据分析LFS-01作用后JeKo-1细胞转录组水平的变化,并采用CCK-8法检测Toll样受体(Toll-like receptor,TLR)抑制剂TAK-242和LFS-01联用对JeKo-1细胞增殖的影响.结果:CRM1在套细胞淋巴瘤中呈现高表达(P＜0.05).LFS-01能够抑制JeKo-1细胞增殖,24和48 h时的半数抑制浓度(50％ inhibitory concentration,IC50) 分别为5.81 μmol/L和9.09 μmol/L.20.0 μmol/LLFS-01可明显抑制CRM1的核转运功能(P＜0.05);6.0 μmol/L LFS-01可明显下调CRM1的表达水平(P＜0.05);10.0 μmol/L LFS-01可将细胞周期抑制在G2/M期,并诱导细胞凋亡(P值均＜0.05);4μmol/L LFS-01可明显上调凋亡通路蛋白聚腺苷二磷酸核糖聚合酶、剪切型caspase-3和剪切型caspase-9 (P值均＜0.01)的表达.凋亡抑制剂z-VAD-FAM可逆转LFS-01的凋亡诱导作用(P＜0.01).当CRM1基因突变后,LFS-01失去靶向抑制CRM1核转运和细胞增殖的功能(P值均＜0.01).RNAseq分析表明LFS-01对细胞增殖相关信号通路有明显的抑制作用(P＜0.01),并且TLR抑制剂TAK-242联合LFS-01用药具有协同抗肿瘤作用(P＜0.01).结论:LFS-01可诱导套细胞淋巴瘤细胞发生周期阻滞和凋亡,其中CRM1是一个关键蛋白.TLR抑制剂TAK-242和LFS-01具有协同抗肿瘤作用.

The covalent modification of intrinsically nucleophilic cysteine in proteins is crucial for diverse biochemical events. Bioinformatics approaches may prove useful in the design and discovery of covalent molecules targeting the cysteine in proteins to tune their functions and activities. Herein, we describe the Cysteinome, the first online database that provides a rich resource for the display, search and analysis of structure, function and related annotation for proteins with targetable cysteine as well as their covalent modulators. To this end, Cysteinome compiles 462 proteins with targetable cysteine from 122 different species along with 1217 covalent modulators curated from existing literatures. Proteins are annotated with a detailed description of protein families, biological process and related diseases. In addition, covalent modulators are carefully annotated with chemical name, chemical structure, binding affinity, physicochemical properties, molecule type and related diseases etc. The Cysteinome database may serve as a useful platform for the identification of crucial proteins with targetable cysteine in certain cellular context. Furthermore, it may help biologists and chemists for the design and discovery of covalent chemical probes or inhibitors homing at functional cysteine of critical protein targets implicated in various physiological or disease process. The Cysteinome database is freely available to public at http://www.cysteinome.org/.

Type I polyketides are natural products with diverse functions that are important for medical and agricultural applications. Manipulation of large biosynthetic gene clusters containing type I polyketide synthases (PKS) for heterologous expression is difficult due to the existence of conservative sequences of PKS in multiple modules. Red/ET mediated recombination has permitted rapid manipulation of large fragments; however, it requires insertion of antibiotic selection marker in the cassette, raising the problem of interference of expression by leaving "scar" sequence. Here, we report a method for precise seamless stitching of large polyketide biosynthetic gene cluster using a 48.4 kb fragment containing type I PKS involved in fostriecin biosynthesis as an example. rpsL counter-selection was used to assist seamless stitching of large fragments, where we have overcome both the size limitations and the restriction on endonuclease sites during the Red/ET recombination. The compatibility and stability of the co-existing vectors (p184 and pMT) which respectively accommodate 16 kb and 32.4 kb inserted fragments were demonstrated. The procedure described here is efficient for manipulation of large DNA fragments for heterologous expression.