Cervical Stem Cell

Introduction
In the past several years many scientists have been attracted by the field of stem cells. This promising new area of research has brought hope and challenge in the fight against degenerative diseases. Significant progress has been achieved in hematology, embryology, and neurobiology through the isolation and characterization of the stem cells. However, newer fields in stem cell biology such as cervical stem cells are still not well known and only few studies have been reported. Nonetheless, a better knowledge of cervical stem cells is crucial in the study and treatment of cervical cancer and the human Papilloma virus (HPV) infection.

Biology of the Cervix and cervical stem cell identification
The first step to better understand cervical stem cells (CSCs) and their applications is to study the biology of the cervix where the CSCs are found. The cervix is a strong muscle located at the opening of the womb into the vagina and is normally closed with just a small opening for the sperm (Figure 1). The cervix is composed of two types of epitheliums. The ectocervical squamous epithelium is on the outer surface of the cervix and the endocervical columnar epithelium is found on the lining of the cervix (5, 6). The cells of the squamous epithelium are skin-like cells which can give rise to the squamous cell cervical cancer (Figure 2). On the other hand, the endocervical columnar epithelium is composed of columnar cells and subcolumnar cells also known as reserved cells (Figure 3), which can give rise to an adenocarcinoma of the cervix (2, 6). Another important area of the cervix is the transformation zone (Figure 1) that is located at the opening to the vagina where the cells are more likely to become cancerous (2,3).

The identification of stem cells in the cervix is based on morphological studies. By definition stem cells should be capable of self-renewal and differentiation into mature cells of a specific tissus by mean of assymetric or symmetric division. In the case of asymmetry, cell division enables the stem cell to form one identical daughter cell and one differentiated cell. The study of reserve cells shows similar properties because these cells are undifferentiated, omnipotent cells that have the ability to differentiate into squamous cells (6).  Thus, the area where the cervical stem cells are more likely to be found is at the basal layer of the ectocervix and the endocervix where the reserve cells can be found (Figure 2 & 3). These reserve cells can differantiate into squamous cells, columnar cells, and granular cells, which enable them to maintain a necessary number of differentiated cells using a slow cycling process (5, 2).  However, scientists are still debating as to what is the exact nature of the the cervical stem cells.

Some scientists stated that subcolumnar reserve cells appear to be the best candidate for cervical stem cell using morphological studies whereas the basal cells of the squamous epithelium (Figure 3) show a high level of differentiation and seem less likely to be CSCs (6). 

Recent studies have identified two markers for CSCs, p63 and cytokeratin 17 that can help in the identification and characterization of the cervical stem cells. The first marker p63 was found in cells of different tissues such as the bronchial, prostate and recently in cervical reserve cells. It is an important factor in the regulation and maturation of the cells (6). Cytokeratin 17 is more specific in the identification of CSCs located in the epithelium the cervix (Figure 4). Another study showed two more markers present on the surface of CSCs which are reported as CD49 and CD 71 (1).

Some scientists stated that subcolumnar reserve cells appear to be the best candidate for cervical stem cell using morphological studies whereas the basal cells of the squamous epithelium (Figure 3) show a high level of differentiation and seem less likely to be CSCs (6). 

Recent studies have identified two markers for CSCs, p63 and cytokeratin 17 that can help in the identification and characterization of the cervical stem cells. The first marker p63 was found in cells of different tissues such as the bronchial, prostate and recently in cervical reserve cells. It is an important factor in the regulation and maturation of the cells (6). Cytokeratin 17 is more specific in the identification of CSCs located in the epithelium the cervix (Figure 4). Another study showed two more markers present on the surface of CSCs which are reported as CD49 and CD 71 (1).

CSCs associated with Cancer and human papilloma virus (HPV)
The ability to identify and characterize CSCs is crucial for understanding and treating cervical cancer and human papilloma virus, which have recently been shown to specifically target CSCs (6, 9). In fact, the relationship between cervical cancer and HPV has been well defined in past studies but it is just recently that the link with CSCs was established (6). Dr. Michael Dean reported that recent identification of a cell subset in solid tumors support the existence of cervical cancer stem cells (7). Other experiments have shown that numerous premalignant lesions of the uterine cervix are similar in marker expression to the progenitor reserve cells.  Cells not expressing the keratin 17 markers did not show any transformation into cervical cancer (8). To support this hypothesis some researchers argued that stem cells have already the ability to self-renew and the fact that they live long make them more susceptible to mutational changes. Therefore, a better understanding of these changes could help design new gene silencing therapy to fight cervical cancer, which was reported to be the third most important cause of cancer death in women (9). The role of HPV in the development of cervical cancer has been well studied and reports have confirmed the effect of the HPV on the stem cell carcinogenesis. Two viral oncogenes E6 and E7 from the HPV were shown to be part of the molecular mechanism along side with the integration of the viral DNA in the host genome (5). An experiment conducted in 2004 by a group of researchers at NIH has isolated cervical cell with virus like particles (VLP) showing that a small population of cervical stem cell has a VLP binding activity which has increased (1). However, binding of VLP to CSCs alone was not enough for the initiation of cancer: it was necessary for the viral DNA to be integrated into the host genome (4). However, the mechanism by which the cell carcinogenesis is induced is still not well understood.

Conclusion
Many questions on the cervical cancer stem cells remain. This area on cancer stem cell biology is evolving and is critical for the design of new therapeutic techniques and approaches. Clinical trials are already underway, such as use of virus-like particles to design vaccines against the HPV to prevent the virus from targeting the cervical stem cells. Other scientists are working on gene silencing therapeutics that will specifically target the cervical cancer stem cells (5). Nevertheless, the identification and characterization of CSCs is the key to help fight cervical cancer and HPV. 

1) Baege A, Schlegel R, Baker C. Identification and Isolation of Putative Cervical Stem Cells Based on Cell Surface Phenotype. NCI, NIHGeorgetownUniversityMedicalCenter. 2004. 

2) Elson DA, Rebeccah R. Riley, Ama Lacey, Gudmundur Thordarson, Frank J. Talamantes and Jeffrey M. Arbeit. Sensitivity of the Cervical Transformation Zone to Estrogen-induced Squamous Carcinogenesis. Cancer Res2000;60:1267-1275.

3) Hulman G, Pickles CJ, Gie CA, Dowling FM, Stocks PJ, Dixon R. Frequency of Cervical Intraepithelial Neoplasia Following Large Loop Excision of the Transformation Zone. J Clin. Pathol. 1998;51:375–377.

4) Shera KA, Shera CA, McDougall JK.  Small Tumor Virus Genomes Are Integrated Near Nuclear Matrix Attachment Regions in Transformed Cells. J Virol. 2001;75:12339–12346.

5) Ledwaba T, Dlamini Z, Naicker S, Bhoola K. Molecular Genetics of Human Cervical Cancer: Role of Papillomavirus and the Apoptotic Cascade. Biol Chem. 2004;385:671-682.

6)  Martens JE, Arends J, Van der Linden PJ, De Boer BA, Helmerhorst TJ. Cytokeratin 17 and p63 are markers of the HPV Target Cell, the Cervical Stem Cell. Hum Pathol. 2005;36:932-933; author reply 934-935.

7) Dean M.  Cancer Stem Cells: Redefining the Paradigm of Cancer Treatment Strategies. Mol Intervention. 2006;6:140-148

8) Pruszczynski M,  Smedts F, Ramaekers F, Troyanovsky S, Robben H, Lane B, Leigh I, Plantema F, Vooijs P.  Basal-cell Keratins in Cervical reserve cells and a Comparison to their Expression in Cervical Intraepithelial Neoplasia.  Am J Pathol. 1992;140:601-612.

9) Roopa L, Hareesh BN, Nameer K, Liu YG, Lia F, Dean E, Rajeshwar RT. Induction of Aromatase Expression in Cervical Carcinomas: Effects of Endogen on cervical Cancer Cell Proliferation. Cancer Res. 2005; 65:11164-11173.

Acknowledgements

This review was prepared by the following graduate students in the Stem Cell Biology Class, Graduate School of Biomedical Sciences, University of Medicine and Dentistry of New Jersey: Meriem Bendaoud, Rasha Nayal (in alphabetical order).

Teaching Assistant: Reema Patel

The review was edited by two stem cell biologists.