In a previous article (How To Read A Cancer Paper Part 1 - ), I looked at the pitfalls of in vitro studies. These are studies performed in the lab that look at cancer cells under glass (actually a Petri dish rather than a test tube as such). These studies - and there are tens of thousands of them - are often performed to see how cancer cells respond to specific anti-cancer agents. And, as I said in the article:
...you have to take these test tube studies with a huge pinch of salt. In the test tube substance X kills cancer, but that really doesn’t mean much in the real world. At most it gives you reason to carry on looking at substance X in more detail, but that’s about it.
The obvious alternative is to look at in vivo studies, which are discussed in the second article in the series. These are studies that use animals (normally mice or rats), with real tumours rather than cells grown under glass. While these are better, there are still major pitfalls, which I discuss in the article. One problem that I highlight is that the tumours that are grown are derived from standard cell lines. These are cancer cells that have been grown for year after year after year under lab conditions. The intention is to have some standard cell lines that are indicative of the tumour type or sub-type (e.g. androgen responsive prostate cancer, triple negative breast cancer etc). A key point I make in that article is:
Furthermore, cancer cells mutate and adapt, so cells that have been taken from a patient biopsy twenty or thirty years ago and then kept under glass for generation after generation will have changed in order to survive in the test tube environment.
A new study, published in the Proceedings of the National Academy of Sciences, looks at precisely this issue and comes to a disturbing conclusion. The experimenters were specifically looking at cancer cells that are drug-resistant. This is of major clinical importance as the fact is that we still don't fully understand why some cells can become resistant to anti-cancer drugs. Using state of the art technology, the researchers took a panel of standard cell lines and checked which genes are expressed. They did this both in the test tube and also from tumours grown in animals from these same standard cell lines. They then compared these with a set of cells taken directly from patient biopsies. The results are clear:
No correlation was found between clinical samples and established cancer cell lines....all of the cell lines, grown either in vitro or in vivo, bear more resemblance to each other, regardless of the tissue of origin, than to the clinical samples they are supposed to model.
In other words, the standard cell lines, originally from different types of tumour, have come to resemble each other genetically, whether used in the test tube or implanted in animals. They resemble each other more than they resemble 'live' samples taken from cancer patients.
Now a basic knowledge of evolutionary biology should tell you why this should be so. Cancer is an evolutionary disease - it adapts and changes and mutates. And, not surprisingly, the researchers see this in action:
As expected, we found up-regulation of genes that would facilitate survival across all cultured cancer cell lines evaluated.
In plain English, this means that cancer cells cultured in laboratory conditions for many generations end up adapting and changing to the new environment. They main gain or lose characteristics that they originally had when they were harvested from living tissue.
Is this important?
Unfortunately this is not some obscure topic of minor academic interest. New anti-cancer agents - from naturally derived substances like curcumin, quercetin or resveratrol, or new molecules designed to target specific pathways or otherwise attack cancer cells - are first tested against these standard cells lines. If these cell lines don't resemble real cancer samples, then we could be either rejecting treatments that don't kill these standard cells without knowing how they'd work against real cancers, or deciding to focus on drugs that are highly effective against the standard cells and yet are disappointing when faced with real live tumours. Either prospect is appalling.
The fact that this paper has been published in a prestigious journal is good, but whether it's acted upon or not is an open question.
For the moment it means we have to be even more cautious when interpreting the results of papers that look at results based on these standard cell lines. It means being more sceptical, even when we're desperate for positive results. At the very least it means that there's one thing we have to do when looking at research and that is to see whether the tested agents are tested against live patient samples as well as the standard cells. This is harder to do for the researchers, but it's vital information.
I'll let the researchers have the last word on this:
Although cultured cells can be used to study many aspects of cancer biology and response of cells to drugs, this study emphasizes the necessity for new in vitro cancer models and the use of primary tumour models in which gene expression can be manipulated and small molecules tested in a setting that more closely mimics the in vivo cancer microenvironment so as to avoid radical changes in gene expression profiles brought on by extended periods of cell culture.