| To search new molecular biomarkers related with colorectal cancer(CRC), 46 cDNA clones which expressed in lower levels in CRC tissues were obtained by subtractive hybridization in Cancer Institute of Zhejiang University. By homology alignment with Genbank data using BLASTs, the sequences of 32 cDNA clones share high homologies with immunoglobulins, the SNC73 is one of 32 cDNA clones. Sequence analysis revealed that the full-length SNC73 cDNA is 1651 bps in size with an open reading frame of 1152 bps encoding an immunoglobulin a-1 molecule of 494 amino acid residues(Genbank:AF067420). The 142 ammo acid residues at the N-terminal region possess high similarities with the variable regions of immunoglobulins, which contain an 20 amino acid residues' signal peptide, and the 352 aa at the C-terminus are completely identical to the constant region of an IgAl heavy chain(Genbank:AAC82528.1). The SNC73 gene was mapped to human chromosome 14q32 by fluorescence in-situ hybridization. Interestingly, immunoglobulins heavy-chain genes were also located on this chromosome locus. Allthese results suggest that SNC73 is just the heavy chain of IgAl.In-situ hybridization results demonstrated that the transcript of SNC73 was predominantly expressed in epithelial cells of colon mucosa. RT-PCR studies indicated that SNC73 mRNA was present in a variety of human epithelial tumor cell lines, including those from lung, stomach, liver, skin, and breast, suggesting that SNC73 was expressed by non-lymphoid epithelial cells. Moreover, IgAl heavy-chain protein was detected in colon epithelial cells using immunohistochemistry with antibody specific for the heavy chain of IgAl. Based on these results, Zheng shu et al. hypothesized that IgAl might be present in non-lymphoid epithelial cells besides B lymphocytes.The traditional immunological theory believes that the immunoglobulins are synthesized and secreted only by B lymphocytes. Also, the IgA, originally expressed in lymphocytes, is transported into the epithelial cells, passages through the epithelium and enters the intestinal lumen. Unexpectedly, recent studies, however, also suggested that immunoglobulins could be originally expressed in non-lymphoid epithelial cells. First, Kimoto demonstrated that transcripts of Ig genes were present in four non-lymphoid tumor cell lines; Secondly, Qiu et al. reported that immunoglobulins and Ig-like proteins were detected in several human epithelial tumor cell lines; Finally, Cao et al. found that the Tx gene, isolated from mRNA of the naspharyngeal carcinoma cell line, CNE2, shared high similarity with the constant region of Ig light chain, Kappa chain, and then suggested that the Tx gene might be the Kappa chain.All these results suggest that the heavy-chain or light-chain proteins of immunoglobulins might be expressed in non-lymphoid epithelial cells. However, no evidence was provided that the entire IgAl could be expressed and the V(D)J recombination machine was present in non-lymphoid epithelial cells so far. Thus, itprompts us to study the entire IgAl expression and the V(D)J recombination machine in non-lymphoid epithelial cells.Part I. The entire immunoglobulin Al is expressed in non-lymphoid epithelial tumor cell linesThe mRNA and protein of SNC73 were detected in five cultured human epithelial tumor cell lines(including two colorectal cancer cell lines, LoVo and SW480, one liver cancer cell line, SMC7721, one breast cancer cell line, Bcap37, and one cervical cancer cell line, HeLa.) by RT-PCR and immunohistochemistry. In order to avoid DNA contamination in the test samples, the primers specific for SNC73 were designed in two exons separated by an intron. The results showed that both mRNA and protein of SNC73 were expressed in all five tumor cell lines, and the sequencing result demonstrated that the sequence of mRNA was identical to SNC73 (Genbank:AF067420). Meanwhile, the protens of immunoglobulin Al heavy chain(IgHal), IgK and Igk were analyzed in five tumor cell lines by immunohistochemistry respectively. T...
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