Is SOX-17 Making us See Red?

Submitted by: Kebaneilwe Lebani

School of Chemistry and Molecular Biosciences
University of Queensland

26 April 2009

Cancer is a myriad of different diseases in which the cells or tissues of the body gain abnormal function and lose normal growth control mechanisms. The prevalence of cancer is high and its increase is being driven by increased incidences of cancer world wide. Research into cancer etiology and carcinogenesis as well as diagnostic and staging tools are therefore imperative for prognosis and therapeutic purposes.

Colorectal cancer (CRC) is one of the better understood malignancies and as such, it has availed a good model for the study of the mechanisms underlying carcinogenesis. The development of a readily evident precursor lesion as well as the formation of the adenomatous polyp are two unique features that make CRC models advantageous in the study of tumourigenesis (1). Concomitant to the stated features of the CRC model, pre-neoplastic adenomas in the colon have been found to be polyclonal lesions, meaning that genetics events alone cannot explain the hyper-proliferative state of the colon and thus other events in conjunction with genetic alterations are implicated in the progression of the lesions into colorectal carcinomas (2). Epigenetic modifications are one of the mechanisms that have been found to mediate initiation of colorectal carcinogenesis. The complementary effects of the epigenetic changes together with genetic events seem to drive the progression to CRC through oncogenic pathway addiction (3).

Epigenetics refers to heritable changes in gene expression that are facilitated by mechanisms that do not alter the primary DNA sequence (4). The changes in gene expression can be brought about by DNA methylation or histone modification of CpG island-associated promoters of tumour suppressor genes (5). This review shall however, concentrate on DNA methylation as an initiator and progression determinant of CRC. DNA methylation involves the addition of a methyl group to 5’cytosine residues in CpG dinucleotides by DNA methyl transferases (3). CpG islands have an increased quantity of the CpG dinucleotides and as such, susceptible gene promoter regions are prone to aberrant hypermethylation in de novo epigenetic events and thus lead to inappropriate gene silencing that can initiate tumourigenesis either through summative epigenetic modifications or in concurrence with genetic events (3,6). It is pivotal therefore, that tissue and tumour specific DNA methylation patterns be readily identified to aid in the detection and diagnosis of cancers in an effort to provide better therapeutic and prognostic resolutions.

In an effort to identify a tumour-specific molecular marker for CRC, Zhang et al (6) conducted a study in which they investigated the normal function of Sox-17 as well as the effects of epigenetic inactivation of SOX-17 in colorectal cancer models. Sox-17 is a SRY related High Mobility Group (HMG) box transcription factor which together with Sox-7 and Sox-18 constitute the Sox group F genes. Sox-17 plays a critical role in the regulation of development of several cells and tissues. Its role in the formation and development of the gut definitive endoderm and its implication as a tumour suppressor through repression of Wnt pathway signaling, were its identifying factors as a candidate gene for investigation with regard to CRC (6).

In their research, Zhang et al (6) worked with normal colon samples, 7 CRC cell lines and a CRC cell line called DKO in which 2 methyl transferases had been genetically crippled and they were able to show that;

SOX-17 is a candidate for aberrant gene silencing in CRC

Silencing of SOX-17 is associated with its promoter CpG island hypermethylation

Restoration of SOX-17 expression suppresses tumour cell growth

SOX-17 inhibits β-catenin/TCF-driven transcription

(7).

The identification of Sox-17 as a candidate gene that can serve as a hypermethylation marker for colorectal cancer is a step of monumental proportion in the quest to develop molecular strategies for CRC staging as well as detection given the implication of aberrant de novo methylation of Sox-17 in early carcinogenesis. Efforts will now have to go towards refining methylation determination methods that can be routinely used for patients undergoing testing for CRC. The bisulfite treatment method used in the research by Zhang et al although accurate, has high analytical technique requirements and as such, robust and easy to use commercial kits will have to be developed to determine Sox-17 hypermethylation. Zhang et al also worked on tumour samples as well normal colon samples but in order to make testing and assessment feasible it is important that non-invasive procedures be used to provide samples for hypermethylation analysis. A possible sample for the analysis could be stool samples in which DNA could be isolated from colon epithelial cells that are regularly sloughed off with stool given the relatively high turnover of gut epithelial cells. Superseding detection of Sox-17 hypermethylation, the research by Zhang et al also opens up avenues for treatment of early CRC. The results of the study showed that tumour cell growth could be curbed by restoration of SOX-17 expression. One of the ways that this was achieved, was through treatment of the cell with DAC, a drug that inhibits DNA methyl transferase (DNMT) mediated hypermethylation of promoter CpG islands. While DNMT inhibition may seem like a rational process that may be applied to early CRC treatment, there are numerous concerns that long term administration of the inhibitors may lead to chromosome instability which could result in inadvertent genetic mutations (8).The resolution therefore relies on the development of possible strategies that can facilitate competent and safe gene re-expression in vivo. Progress is being made and it will probably not be too long until these research findings along with further studies find clinical applications for patients.