Novel approaches to cancer treatment in the context of Gene Therapy have been gaining popularity as an alternative to conventional therapies which have proven to lack specificity, resulting in tumour cell resistance, tumour progression and mortality. As a consequence the use of adenoviruses has been widely developed not only as a replication deficient vector for gene therapy but also as a replication competent oncolytic agent designed to selectively target and kill tumour cells. Unfortunately their success in clinical application has been limited, and it has been suggested that a lack of the primary viral attachment receptor 'CAR' could be a barrier to infection by limiting access to target cells. If Ad/CAR binding is the rate limiting step for successful Ad therapy, it is essential to establish a CAR expression profile in normal and tumour tissue, and in tumour progression, to enable more effective targeted therapy. Furthermore, in the context of using adenovirus as an anticancer strategy by exploiting its replicative lysis, it is important to explore whether Ad success is affected by CAR expression and to identify factors downstream of CAR that may be influential in this process.
In the first experimental chapter, an in vivo immunohistochemical analysis of tissue array slides determined CAR expression in a range of normal and tumour tissue. CAR was differentially expressed dependent on cell of origin, with normal stem cells and basal cells displaying very high CAR, signifying its importance in early development and differentiation. Epithelial cells were also high in CAR but its expression was negligible in mesenchymal, lymphoid and neural cells. This trend was also reflected in most tumour tissue albeit with a general decrease in CAR compared to corresponding normal tissue of the same organ. An exception was the blastic tumours which displayed high CAR reflecting their embryonic state of derivation. CAR expression also decreased in high grade, poorly differentiated tumours of the prostate, stomach and breast compared to their well differentiated counterparts.
In the second experimental chapter, a more comprehensive study of breast cancer biopsy specimens was undertaken, to determine both the expression of CAR and the tumour suppressor gene p53 in relation to tumour grade. The rationale being that both loss of CAR expression and p53 mutation (resulting in loss of function), have been associated with tumour progression. It is possible that CAR and p53 interact directly or indirectly and may be modulated by each other. This study revealed a decrease in both CAR and hormone receptor expression and an increase in p53 'mutational' status with increasing tumour grade. These three factors when compared independently to tumour grade are statistically significant associations, implying that CAR expression and hormone responsiveness decrease with tumour progression and p53 function is compromised or lost via mutation. There was also a significant association between CAR expression and hormone receptor status, however a significant association between CAR expression and p53 status within the tumour grades was not found.
Treatment outcome with Ads will also depend on defining factors downstream of CAR attachment that affect adenovirus 'permissivity', which is ultimately measured by viral replication and cell death, relying on the bystander effect to eradicate all tumour cells. The in vitro analysis revealed statistically significant associations between CAR receptor expression, 'infectivity' (virus infection) and permissivity. Cell lines that were more susceptible to Ad5 were generally of epithelial origin, had high CAR, and were easily infected. However there were exceptions and CAR was not the sole determinant in adenovirus cell entry nor in its ability to replicate and kill the cell. Permissivity was also related to p53 status. Thus, although CAR expression may indeed be a limiting factor, it is apparent that a combination of other events contributes to a deficient infection, especially in the deregulated tumour environment.
The results presented in this thesis clearly demonstrate that there is more to the story of 'CAR' which hints that its role in viro-oncotherapy is not limited solely to its function as an attachment receptor for adenovirus but may also involve its function as a cell adhesion molecule and signal transducer. The further elucidation of these aspects of CAR�s potential role in the scheme of tumour biology may alter the course and strategy of cancer therapy in the future.
| Identifer | oai:union.ndltd.org:ADTP/217733 |
| Date | January 2007 |
| Creators | Wiles, Karen Anna, n/a |
| Publisher | University of Otago. Dunedin School of Medicine |
| Source Sets | Australiasian Digital Theses Program |
| Language | English |
| Detected Language | English |
| Rights | http://policy01.otago.ac.nz/policies/FMPro?-db=policies.fm&-format=viewpolicy.html&-lay=viewpolicy&-sortfield=Title&Type=Academic&-recid=33025&-find), Copyright Karen Anna Wiles |
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