Structural and functional analysis of SMAD4 gene promoter in malignant pancreatic and colorectal tissues: detection of two novel polymorphic nucleotide repeats

Background: The tumor suppressor gene SMAD4 (DPC4) encodes for the common intracellular mediator of the TGF-β superfamily pathway, which regulates numerous cellular processes, such as cell proliferation, cell differentiation, apoptosis, cell fate and migration. This study was aimed to investigate the presence of genetic variants in SMAD4 gene promoter in malignant pancreatic and colorectal tissue and to analyze their functional consequences.

Methods: The study was performed on genomic DNA isolated from malignant tissue samples obtained on surgery from 50 patients with pancreatic carcinoma and 50 patients with colorectal cancer. Screening for mutations within an 800bp-long fragment of the SMAD4 gene promoter was performed by DNA sequencing and two mononucleotide repeats, at positions -462 and -4, were found to be polymorphic in malignant tissue. The exact number of thymidines in the tracts -462T(15) and -4T(12) was determined by PCR with fluorescently labeled primers followed by capillary electrophoresis. Functional analysis of -462T(15)/-4T(12) haplotypes was performed by luciferase reporter assays.

Results: Haplotype -462T(14)/-4T(10) was found in 85% of pancreatic cancer tissues, but it was not present in any of colorectal cancer tissues. Statistically significant reduction (p<0.001) in activity was observed in the haplotype -462T(14)/-4T(10) in comparison with the haplotypes -462T(15)/-4T(12) and -462T(14)/-4T(11).

Conclusion: Results of this study indicate that novel genetic variant -4T(10) in the SMAD4 gene promoter affects its activity and that element -4T(12) may play a role in transcriptional regulation of SMAD4 gene expression. Obtained results, though preliminary, also indicate that SMAD4 gene promoter haplotype -462T(14)/-4T(10) may represent a genetic marker of potential relevance for pancreatic and colorectal cancer. The findings of this study should be confirmed by further investigation in these two and other tumors, on larger number of patients and with different tumor stages. Translational research aimed at investigating potential application of mononucleotide repeats -462T(15) and -4T(12) in SMAD4 gene promoter as molecular markers in cancer may also prove useful.