MOLECULAR-CLONING, SEQUENCING AND EXPRESSION OF THE MESSENGER-RNA ENCODING HUMAN CDX1 AND CDX2 HOMEOBOX - DOWN-REGULATION OF CDX1 AND CDX2 MESSENGER-RNA EXPRESSION DURING COLORECTAL CARCINOGENESIS

Defining the molecular mechanisms involved in cancer formation and pro gression is still a major challenge in colorectal-cancer research. Our strategy was to characterize genes whose expression is altered during colorectal carcinogenesis. To this end, the phenotype of a colorectal tumour was previously established by partial sequencing of a large nu mber of its transcripts and the genes of interest were selected by dif ferential screening on high-density filters with mRNA of colorectal ca ncer and normal adjacent mucosa. Fifty-one clones were found over-expr essed and 23 were underexpressed in the colorectal-cancer tissues of t he 5 analyzed patients. Among the latter, clones 6G2 and 32D6 were fou nd of particular interest, since they had significant homology with se veral homeodomain-containing genes. The highest degree of similarity w as with the murine Cdx1 for 6G2, and with the murine Cdx2 and hamster Cdx3 for 32D6. Using a RT-PCR approach, complete sequence of both type s of homeobox-containing cDNA was obtained. The amino-acid sequence of the human Cdx1 is 85% identical to the mouse protein, and human Cdx2 has 94% identity with the mouse Cdx2 and hamster Cdx3. Tissue distribu tion analysis of Cdx1 and Cdx2 mRNA showed that both transcripts were specifically expressed in small intestine, in colon and rectum. Twelve tissue samples from colorectal adenocarcinomas and the corresponding normal mucosa were analyzed by Northern blot. Expression of the 2 type s of mRNA was either reduced or absent in 10 of them. Several colon-ca ncer cell lines were also analyzed. Cdx2 mRNA was absent from LS174T c ells and Cdx1 mRNA was absent in PF11, TC7 and SW480 cells; none was d etected in HT29 cells. It was concluded that decrease in human Cdx1 an d/or Cdx2 expression is associated with colorectal tumorigenesis. (C) 1997 Wiley-Liss, Inc,.