Alternative splicing is part of the expression program of a wide number of genes implicated in cell growth and differentiation. Although the occurrence of inappropriate alternative splicing in tumors has started to emerge, the underlying molecular mechanisms have been, thus far, largely unexplored. We have investigated the alternative splicing pattern of the CD44 gene in specimens of nonfamilial colon adenocarcinomas at different stages of tumor progression. In the same patients, we have assessed by Northern blotting analysis the mRNA levels of different heterogeneous nuclear ribonucleoproteins and SR factors, all involved in pre-mRNA splicing and, more in general, in mRNA maturation. The results of this analysis highlight a general rule for the mode of splicing of the CD44 pre-mRNA. Moreover, we found that the mRNA levels of different SR proteins in tumor specimens are different from, and usually lower than, those detected in samples of nonpathological tissue adjacent to the tumor. Quantitative analysis demonstrates that, in tumors, the mRNA levels of ASF, SRp40, SRp55, and SRp75, when normalized to those of heterogeneous nuclear ribonucleoprotein A1, are lower than those of SRp20 and SRp30. Interestingly, this reduction is more drastic in patients showing a more altered CD44 splicing pattern and seems to be related to the propensity to develop metastases.
This work was supported by a grant from the Associazione Italiana per la Ricerca sul Cancro (AIRC). C. G. was supported by a fellowship from the Italian Ph.D. program.