Sherlock (grazie!) came across and sent me a study by Carol Ann Huff and William Matsui recently published in the “Journal of Clinical Oncology” (June 10 2008) and titled “Multiple myeloma cancer stem cells.”. The abstract can be viewed here: http://tinyurl.com/4yx38s
The full study tells us that most myeloma cells are mature and quiescent and lack the ability to clone themselves. The fact that the majority of plasma cells are quiescent suggests that tumor growth is restricted to a specialized cell population.
A bit of history. In the 1970s Salmon and Hamburger showed that more than 86% of tumor samples from patients with multiple myeloma were capable of colony formation, and clonogenic growth occurred at a frequency of 1 in 100 to 100,000 cells. This could be explained by one of the following hypotheses: 1. only a small, functionally unique, subset of cancer cells was able to clone itself or 2. all myeloma cells can clone themselves, but only a few express this property at any point in time.
From what I wrote in my second paragraph, we can figure out that Huff and Matsui believe that hypothesis 1 is correct. Based on scientific data, they suggest that myeloma stem cells are clonotypic B cells: The ability of clonotypic B cells to recapitulate multiple myeloma in immunodeficient mice suggests that these cells represent the cancer stem cell in multiple myeloma. This part wasn’t easy to follow, but basically some features of clonotypic B cells are similar to those of healthy adult stem cells, such as resistance to toxic injury, and the continual risk of relapse among patients treated with standard therapies suggest that myeloma stem cells should also be relatively drug resistant. They can also self-renew and give rise to differentiated effectors (ie, plasma cells).
The scientists tested various novel chemotherapy drugs recently approved for the treatment of myeloma. The myeloma cancer stem cells were relatively resistant to both standard cytotoxic compounds and novel agents in vitro compared with the myeloma plasma cells. This suggests that these drugs work against the bigger population of myeloma cells, the ones that don’t have a cloning ability, but have no effect on the smaller population of stem cells. Nothing new here.
For the more scientifically-minded, here are a few comparisons between myeloma stem cells and normal ones: it appears that myeloma stem cells display properties common to normal stem cells, such as expression of membrane-bound drug transporters, intracellular detoxification enzymes, and quiescence. Thus, the chemoresistance of cancer stem cells may be mediated by multiple processes similar to those that protect normal stem cells.
The paragraphs that follow deal with therapeutic ways to target myeloma stem cells. For instance, as we know, the aberrant functioning of the Notch, Wnt and Hedgehog pathways is fundamental for the well-being of myeloma stem cells. These pathways therefore represent a good target. Let me add that we have non toxic ways to affect these pathways: curcumin, cyclopamine (by the way, I just read that a new water-soluble form has been developed!), zerumbone, DMAPT…
Then we are immersed in a discussion concerning telomerase activity…mamma mia, I confess I had to resort to parts of my brain that I never thought I possessed (!) in order to attempt to understand this section…not easy stuff! But, in essence, telomerase activity is an important process in myeloma, and its inhibition means that myeloma stem cells end up not being able to clone themselves. So, telomerase becomes another target.
Another promising target seems to be SOX2 (I wrote a post and page about SOX2 a while ago, by the way), an embryonic transcription factor that is normally turned off after embryonic stem cells differentiate; however, in both MGUS and myeloma patients it becomes reactivated (hah! Figures…).
Anyway, even if you don’t understand what this all means (as I don’t, to be honest), the point is this: SOX2 antibodies are present in folks with MGUS but not in those with myeloma. If you are lucky enough to possess those antibodies, you are less likely to develop myeloma. So targeting SOX2 could be another way to injure the myeloma stem cells, since, as Huff and Matsui write, SOX2 is a feature of clonogenic myeloma cells, and stimulation of anti-SOX2 immunity could limit clonogenic tumor growth of primary samples in vitro.
The development of new evil-stem-cell-focused treatments won’t happen overnight. That much is clear. New trial designs that incorporate novel end points will be needed to study myeloma stem-cell–targeted therapies. One potential strategy is to incorporate these approaches with existing therapies to determine whether they prevent tumor regrowth and prolong the duration of remissions after cytoreduction with chemotherapeutic or novel agents.
The researchers admit that the exact phenotype of the clonogenic cell has not been definitively established and controversy remains. Resolution of the controversy will probably depend on how well patients respond to stem-targeted treatments (read: on long-term outcomes…). Time…time…
The study ends as follows: growing knowledge regarding the basic biology of multiple myeloma, such as the identification of prognostic categories based on cytogenetic alterations or transcriptional profiling may allow multiple myeloma to serve as a model system to address general questions regarding cancer stem-cell biology.
As a myeloma patient, I confess that I (selfishly) don’t care that much about setting up a model system. I care much more about getting the promising, non toxic, stem-cell-targeting treatments into clinical trials as soon as possible. I’m ready and willing to try them!
So, where do I sign?!!!