Dok1 expression and mutation in Burkitt's lymphoma cell lines

Dok1 is an adaptor tyrosine kinase substrate with tumor-suppressive activity. The gene encoding Dok1 maps to human chromosome 2p13, which is frequently rearranged in human tumors. We have previously reported a frameshift mutation of this gene and the down-regulation of its expression in chronic lymphocytic leukemia. In this study, we have determined the expression levels of Dok1 in Burkitt's lymphoma (BL) cell lines, lymphoblastoid cell lines from patients with X-linked lymphoproliferative (XLP-LCL), or from control healthy donors. We have also screened for Dok1 gene mutations by heteroduplex analysis and direct sequencing. Dok1 expression was down-regulated in all BL and XLP-LCL cell lines in comparison to the control cells. No Dok1 mutation or polymorphism was found in the coding region of Dok1 in the three types of cells. However, DNA sequence analysis revealed the presence of four nucleotide changes in Dok1 gene, T⁹⁰¹⁷²C (intron 1), C⁸⁹⁴⁸⁷T and ⁸⁹⁴³³InsCTCT (intron 2), and A⁸⁷⁷¹⁴G (3′ UTR). T⁹⁰¹⁷²C and ⁸⁹⁴³³InsCTCT that were detected in about 7% of BL, 9% of XLP-LCL and 4% of normal samples may represent a common polymorphism. C⁸⁹⁴⁸⁷T and A⁸⁷⁷¹⁴G changes were detected in 9 and 6% of analyzed BL lines, respectively, but never in the control and XLP-LCL cells, indicating that these nucleotide substitution occurred during tumor development. Interestingly, the C⁸⁹⁴⁸⁷T variant is associated with a significantly lower level of Dok1 expression compared to the control samples. A positive association was also found between the presence of EBV in BL and the Dok1 genetic variation. Our data show that Dok1 expression and structure are affected in a subset of Burkitt's lymphoma samples, suggesting its possible role in this type of cancer.