Gene cloning of a novel apomucin expressed in human gastric surface epithelia and parietal cells

以去糖基化的人胃粘蛋白为抗原，制备了三株小鼠单抗，即So-MU1、So-MU2和So-MU3。本研究以SO-MU1为模板，通过筛选人胃组织中表达的lambdagt11文库，克隆了m-8基因片段。So-MU1全长1095个碱基，含有MUC5AC特有的24个核苷酸串联重复序列。因此，So-MU1是第一个检测到MUC5AC的单抗。用So-MU1检测到的MUC5AC在正常胃表面上皮细胞、胃壁细胞、小肠上皮细胞和胆管上皮细胞中均有表达，而在大肠上皮细胞、肝实质细胞和胰腺腺导管上皮细胞中未检测到。观察到的分布与以前报道的使用针对合成串联重复序列多肽的多克隆抗体的分布部分不同。我们认为So-MU1的反应性不受串联重复序列糖基化的影响。胃化生上皮细胞和结肠息肉的杯状细胞表达MUC5AC。胃癌组织中MUC5AC的表达与其组织学类型有关。结肠癌不表达MUC5AC。胰腺癌，尤其是高分化癌表达MUC5AC。结果提示，Aplomucin亚型表达的转换是导致胰腺癌患者糖类抗原表型异常的分子机制之一。初步研究表明，胰腺癌患者血浆CA19-9由MUC5AC携带，而胃肠癌患者则不表达。利用糖类肿瘤标志物的载脂蛋白抗体，似乎可以提高其诊断的特异性。

Three mouse monoclonal antibodies, i.e. SO-MU1,2, and 3, were established using deglycosylated human gastric mucin as antigen. We used SO-MU1 in the present study.A cDNA fragment, m-8, was cloned by screening of human gastric cDNA expression lambdagt11 library with SO-MU1. It had the size of 1095 bp and contained 24 nucleotide tandem repeats characteristic for MUC5AC.Thus SO-MU1 was the first monoclonal antibody for MUC5AC.MUC5AC detected with SO-MU1 was found in the normal gastric surface epithelial cells, gastric parietal cells, small intestinal epithelial cells, and biliary tract epitheliae, but was not found in the large intestinal epithelial cells, parenchymal liver cells, and pancreatic acinal and ductal epithelial cells. The observed distribution differed partly from that previously reported using the polyclonal antibody raised against the synthetic tandem repeat peptide. We suggested that the reactivity of SO-MU1 was not affected by glycosylation of the tandem repeat domains.Gastric metaplastic epithelial cells and the goblet cells of colon polyp expressed MUC5AC.The MUC5AC expression by gastric cancers was dependent on their histological classification. Colon cancers did not express MUC5AC.Pancreatic cancers, especially the well differentiated cancers, expressed MUC5AC.These results suggested that switching of apomucin subtype expression was one of the molecular mechanism for the carbohydrate antigen phenotype abnormality in the differential disorders.The preliminary study suggested that plasma CA19-9 was carried by MUC5AC in pancreatic cancer patients, but not in gastrointestinal cancer patients. It seemed possible to increase the diagnostic specificity of carbohydrate tumor markers with the use of antibodies against their apoproteins.