A promising new drug to treat Acute Myeloid Leukemia

    The financial news was abuzz today about a report by Agios, a biotech company, that in a small Phase 1 study, a drug they are developing was remarkably successful with a subset of Acute Myeloid Leukemia (AML) patients. The patients tested had been through at least one round of chemotherapy, and up to four rounds, without success. The Agios drug, AG-221, was tested for how well it would be tolerated by AML patients. The results were very good, in that adverse side effects were few and limited. The big news though was that 6 of the 7 evaluable patients showed complete remission of their leukemia.
    It is rare to see such impressive results in a preliminary study that deals with dose tolerance. In response to the news, the stock price of Agios rose significantly today.
    Some 10% of AML patients might benefit from AG-221, those with a mutation in a metabolic enzyme, isocitrate dehydrogenase-2. Cells with that mutation produce a higher than normal concentration of 2-hydroxyglutarate, which leads to brain cancers (gliomas and blastomas) as well as AML. How that oncometabolite causes cancer is still being examined. In any event, additional clinical trials with AG-221 are on their way.

Tumor, parasite, and ancient dog — all at once

 

This is a story about an extraordinary tumor.

You know that most cancers arise from mutations in a person’s gene complex, though in some cases a viral infection can cause cancer. An example of the latter is the human papillomavirus, or HPV, a virus is transmitted sexually and that can cause cervical cancer in women.

Now comes word that there is a tumor in dogs that is also sexually transmitted, but it is not caused by a virus. Instead infection occurs by the passage, during sex, of some of the cancer cells themselves. By this means they take up residence in a new dog. It is referred to as the Canine Transmissible Venereal Tumor, or CTVT.

After it is passaged it can cause the growth of a tumor in the recipient organism.  Tumor growth commonly occurs in the genital orifice, but can also appear in other places, including the skin. It generally does not metastasize, and quite inexplicably it regresses after a period of a few months. After that the affected dog is immune from further infection. But meanwhile it can have passed the tumor cells on to other dogs through sexual activity.

For all practical purposes, it acts like a parasitic organism, growing in a host and transmitted by a particular behavior; it is not generally harmful to the host organism, and death from the tumor is rare.

This cancer cell has been passed from dog to dog for many generations. In fact, according to genomic analysis, it may have originated some 11,000 years ago from a breed of dog that no longer exists.

It has survived in dogs as if it were a parasitic single celled organism. But it is not a protozoan. It is a cell that originated from a dog, but no longer belongs to a dog.

This is like a HeLa cell that uses host dogs instead of Petri dishes to provide its growth conditions.

I don’t know of anything comparable.

Meanwhile, according to the investigators from England and Australia who did the genomic analysis[1], it is extraordinary that the cell has survived all this time. Its genome has undergone 1.9 million base substitutions, undergone thousands of rearrangements, and has lost 646 genes. The cell has changed dramatically over millennia, while allowing it – or enabling it – to grow and reproduce and survive passage from one dog to another — and produce tumors that do not kill their host organism.

As they write, “Our results provide a genetic identikit of an ancient dog, and demonstrate the robustness of mammalian somatic cells to survive for millennia despite a massive mutation burden”.

These are two “firsts” for me; a tumor that is transmitted from one organism to another by tumor cells themselves, and a mammalian cell that has survived away from its original organism for over 10,000 years.  Extraordinary.

1.         Murchison EP, Wedge DC, Alexandrov LB, et al. Transmissible dog cancer genome reveals the origin and history of an ancient cell lineage. Science 2014;343:437-40.

Cells that lead the metastasis migration — can they be stopped?

A tumor is an abnormal growth of cells, but cancer occurs when some of those cells leave the tumor mass, enter the circulatory system and colonize new parts of the body.  We usually think about single cells breaking away from the pack and setting off on their own to establish these new colonies. But, according to recent research findings, when cells break away from the tumor mass they do so usually as groups of cells.

It now turns out that the leader cells have a unique identity.

This was the result of a study conducted at Johns Hopkins and the UCSF Medical School in San Francisco.[1] The goal now is to selectively target those cells to block the migration, and thereby inhibit the development or extent of cancer.

To investigate the characteristics of the leader cells the researchers made use of a method they recently developed for culturing breast tumors as three-dimensional structures, rather than the more established method of culturing cancer cells as single cells. A three-dimensional structure more closely reflects what occurs in an organism. The method involves embedding small chunks of highly metastatic tumors from mice, containing 200-1000 cells, in a collagen gel.

They observed that within 2-3 days, small strands of cells projected from the tumor mass, like a finger pointing to a new diection. “Because the cells leading these invasive strands were highly protrusive and migratory we refer to them as ‘invasive leader cells’”. Since these cells were so morphologically distinct, they were able to determine, at a cellular level, if their molecular properties were distinct from the tumor mass.

Indeed they were. They contain proteins that distinguish them from the tumor mass, while sharing some molecular features as well. To simplify, I will focus on one of the markers, perhaps the most significant.

That protein is cytokeratin-14 (K14), an intermediate filament protein which contributes to the skeletal structure of cells. 93% of leader cells were positive for K14, while the rest of the tumor cells were K14 negative. So K14 is associated with invasive leader cells.

They then asked whether K14 positive cells could be seen in intact tumors in mice. For this they examined sections of tumors derived from mice. They found that K14+ cells were concentrated at the boundary of tumors with surrounding tissue (stroma), and that those cells led invasive strands into adjacent muscle. The same was true for several different types of breast cancers.

These experiments in culture and in several types of intact tumors were repeated for human tumors, and again the same results were obtained. From the observations on cells in 3D culture, insight into the molecular features of invasive cells in a variety of breast cancers, both in mice human, had emerged.

Now for the clincher. The investigators genetically engineered mice breast cancer cells such that the emergence of  K14 was blocked. These cells were grown in mice and formed tumors. Those tumors were then cultured in the 3D models, side by side with K14+ tumor 3D models. In contrast to the K14+ models, in the K14- models “both K14 and collective invasion were markedly reduced”. Further, among K14- tumors that formed in vivo, there was also a striking reduction in collective invasion at the tumor-stromal border.

These data indicate that K14 does not merely appear in the leading invasive cells as a consequence of differentiation, but that K14 is required for strands of collective invasion to occur.  The authors write, “targeting a basal invasive program expressed in a small minority of tumor cells is sufficient to disrupt the invasive process in an advanced carcinoma”. The prediction is that tumors in which K14 activity is blocked would be less metastatic than normal. Perhaps we will see that data in their next paper.

This is such a wonderful approach because any therapeutic development would not only be effective in a variety of breast cancers, but would also likely be effective for other metastatic cancers as well.


[1] Cheung KJ Gabrielson E, Werb Z, and Ewald AJ (2013) Cell 155:1639-1651.  Collective invasion in breast cancer requires a conserved basal epithelial program.

A new FAKt about metastasis

Most cancer deaths are due to metastasis, where tumor cells detach from their original site of tissue growth and spread to other parts of the body.

Metastasis requires tumor cells to break free from the tissue in which it is growing, crawl through a “basement membrane” (a collagen-based structure that envelops the tissue), crawl across the extracellular matrix, attach to a capillary, and crawl through openings made in the capillary wall to enter the blood stream. Unless tumor cells can enter the blood stream, and depart from it, they cannot migrate to other body sites, and metastasis would be thwarted, just as it would be impossible to travel home from work if the doors to the subway or train station refused to open.

Researchers have just clarified the way in which tumor cells open the “doors “of the capillary and enter the blood stream.  This finding can lead to ways to at least diminish metastasis if not eliminate it.

When tumor cells attach to capillaries, they secrete VEGF, a compound that causes endothelial cells that make up the wall of the capillary, and that are normally very tightly sealed to one another, to separate apart just enough for the tumor cell to crawl through and enter (or leave) the blood stream. VEGF binds to a receptor on the cell surface of endothelial cells; the receptor’s cytoplasmic side has enzymatic activity. Like other growth factor receptors it can add phosphate groups to tyrosine amino acids of selective proteins. 

In this case, phosphorylation of a molecule, cadherin, is promoted. Cadherin is responsible for the endothelial cells being able to adhere tightly to one another. When cadherin is phosphorylated, however, they lose their grip, and the endothelial cells can form opening between them.

However, an important step on the pathway was missing. While VEGF binding to its receptor PROMOTES phosphorylation of cadherin, it does not actually do the phosphorylation of cadherin. Some intermediate is needed.

Researchers from San Diego (Jean et al., 2014) recently identified that intermediate, an enzyme called focal adhesion kinase, or FAK. Focal adhesion kinase plays an important role in supporting attachment between cells, as well as attachment of cells to a substrate. When tumor cells secrete VEGF it causes FAK to accumulate at the endothelial cell junctions. (Another protein, src, also accumulates at the junction, but fails to do so if FAK activity is blocked). Now it is known that FAK does not merely accumulate at the cell junction but actually modifies the properties of the junction through its phosphorylation of cadherin.

 To summarize, when tumor cells secrete VEGF it binds to its receptor on endothelial cells, which then stimulates FAK phosphorylation, and FAK in turn phosphorylates cadherin resulting in a weakening of the adhesion of endothelial cells, allowing tumor cells to pass between them.

This is clearly and important signaling pathway because it allows tumors cells to enter into and exit from the circulatory system. Limiting access to the circulatory system could decrease metastasis and therefore limit that aspect of cancer that most affects mortality.

In mice, that was the case. By genetically eliminating FAK activity in adult endothelial cells the investigators prevented melanoma cell metastasis in mice.

Interestingly, this did not prevent the growth of the tumor, only its metastasis.

Therefore, the VEGF-Receptor-FAK-src-cadherin pathway has emerged as an important set of therapeutic targets, creating new opportunities for preventing cancer deaths. Inhibitors of FAK are already being tested in clinical trials. As the pathway continues to be clarified, new pharmacological agents will surely be sought.

Reference:
Jean, C., Chen, X. L., Nam, J. O., Tancioni, I., Uryu, S., Lawson, C., Ward, K. K., Walsh, C. T., Miller, N. L., Ghassemian, M. et al. (2014) ‘Inhibition of endothelial FAK activity prevents tumor metastasis by enhancing barrier function’, J Cell Biol 204(2): 247-63.

 

 

A New Explanation for Metastasis

Understanding how metastasis occurs may allow the most important approach to preventing cancer deaths; that is because most cancer deaths occur due to metastasis of epithelial tumor cells. The key question is how tumor cells of epithelial tissue, whose general characteristic is one of constant shape and immobility, acquire the ability to change to a crawling shape and move out of the tumor mass, travel into the blood stream, and arrive at a location they can colonize.

It is generally thought that epithelial tumor cells undergo transition to mesenchyme-like cells that can move and crawl to other sites in the body. The transition  is thought to be triggered by products of other cells, like the senescent stroma cells that are neighbors to epithelial tissues, perhaps together with changes at the level of the genome. The ability of transitioned cells to move away from the tumor itself may also depend on the composition of the extracellular matrix the cell encounters.

I call your attention to a new possibility for metastasis suggested in a paper (Lazova at al, 2013) from the Pawelek lab at the Yale School of Medicine, with the assistance of the Denver Police Department Crime Lab. They argue that metastatic cancer cells may be derived from a fusion of mobile cells such as leukocytes together with epithelial cancer cells. The fusion product would have characteristics of the cancer cell, but also characteristics of leukocytes, namely the ability to move, to crawl into and out of blood vessels, and to squeeze into tissues throughout the body. It would also have a novel genetic makeup.

This is an interesting possibility, but a very difficult one to demonstrate. It was first proposed over a hundred years ago. But till now there has been no experimental evidence of metastatic  cancer cells in humans with hybrid properties.

The researchers used an innovative approach. They examined a metastatic brain melanoma taken from a person who had received a bone marrow transplant.  Some time after the transplant the patient developed a melanoma that had then spread to other parts of the body, including the brain.

Here is the interesting part. Since the person had received a bone marrow transplant, his or her own bone marrow cells were all destroyed, meaning that all new white blood cells were derived from the donor. The donor and host have different genes. Therefore if host melanoma cells fused to donor bone marrow-derived cells then it might be possible to show that metastasized melanoma cells also have properties of leukocytes, and contain donor genes as well as host genes.  That would constitute evidence that that the metastasized cells were derived from a fusion of cells. And if that were the case, that could account for the mobile, colonizing properties of the metastatic cells.

This is indeed what the investigators found, giving rise to a new way to view the epithelia-to-mesenchyme transition, and to the properties of metastasized cells. This is important, because once it is understood HOW metastasis occurs, it then becomes possible to develop drugs that can PREVENT metastasis from occurring.

The authors are quick to point out that this is only a single instance, only one type of cancer from only one patient. As they say, “The extent to which this mechanism is operative in other tumors remains to be determined”.

I look forward to the results of future studies; preventing metastasis is the key to limiting cancer deaths. A method to prevent metastasis would have general applicability for a variety of tumors, and would not depend on treating each cancer with medication to counter the specific mutation(s) that caused the tumor.

REFERENCE

Lazova R, LaBerge GS, Duvall E, Spoelstra N, Klump V, Sznol M, Cooper D, Spritz RA, Chang JT, Pawelek JM (2013) PLOS one 8:1-7.        A melanoma brain metastasis with a donor-patient hybrid genome following bone marrow transplantation: First evidence for fusion in human cancer.

Impact of tumor heterogeneity on genomic approach to cancer therapy

            I noted earlier (see The Personalized Cancer Treatment Bandwagon Continues to Grow, June13, 2013) that both in the US and Great Britain, policy decisions have been made to conduct a full-out campaign to develop a personalized medicine approach to cancer treatment. The idea is to assay by genomic analysis, a person’s cancer for the underlying genetic mutation (or mutations) associated with the cancer, and to provide a specific drug therapy directed against the altered protein.  Treatment would be highly specific.

            The first stage of this approach is to sample as many different types of cancers as possible in order to identify as many mutations as possible that may be causative (some may simple be associative but not causative). The second stage is to develop drugs that interfere with the altered protein product of the mutated gene. The third stage is to perform clinical trial to see which of the drugs are most effective with fewest side effects. This could be a lengthy process.

            A review article appeared in Nature recently that identify significant obstacles to the development of personalized cancer medicine (Burrell et al 2013). The main point of the paper is that most cancers are genetically heterogeneous, which means that there are different mutations in different cells of a tumor, and that different metastatic foci may be different with respect to the mutations they carry. That is because the mutation rate in tumors is often higher than in normal cells due to what is called “genomic instability”. “Most solid tumors, and haematopoietic malignancies display at least one form of genomic instability”. Consequently, future genomic analyses will have to search for more than just the dominant mutation, because even if cells carrying that mutation are eliminated therapeutically, that could leave other cancer cells still capable of growing. More than one specific drug would be needed for therapy.

            In addition, the authors warn that even identifying all the mutations may still be insufficient, because among genetically identical cells there is a range of different phenotypic characteristics. “Phenotypic heterogeneity…is determined …also through stochastic events in gene expression and protein stability, epigenetic divergence and micro-environmental fluctuations”.  The particular microenvironment in which a tumor is found can have a significant influence on the gene expression of a tumor.

            Finally, as a tumor evolves with time the genetic pattern may change. Therefore, for personalized medicine to be successful, genomic assays will have to be repeated throughout the course of therapy.

            Personalized cancer medicine may be a lot more complicated than first thought. Universal therapies for cancer (which I have written about here) are looking better and better, and may be closer at hand.  Perhaps that is where the research money and effort should be directed.

Burrell et al, Nature 501:338.  The causes and consequences of genetic heterogeneity in cancer evolution.

A new compound that majorly boosts the effectiveness of immunotherapy

Immunotherapy is one of the more hopeful approaches to cancer treatment, insofar as it uses highly specific monoclonal antibodies against specific types of cancer. Several antibodies are in clinical use, and many more are being developed. But immunotherapy is not always fully effective – yet.
I report here on a compound has been developed that enormously increases the effectiveness of immunotherapy, working like an adjuvant. When you reach the bottom of this blog you will be convinced that we are on the threshold of a major advance in cancer treatment.
One of the keys to immunotherapy involves the stimulation of macrophages to consume antibody-bound cancer cells. Macrophages have evolved the ability to recognize cancer cells and destroy them. However, cancer cells have evolved counter-mechanisms to evade detection by macrophages, so they continue to live. That mechanism even makes it difficult for antibodies raised against specific cancer cell surface molecules to successfully eliminate the cancer cell population.
In a recent study from Stanford medical school, researchers have learned to interfere with the evasion mechanism, and have increased the potential for successful immunotherapy with a wide variety of cancers.(1)
Many cancer cells overexpress a ubiquitous cell surface protein known as CD47, a member of the immunoglobulin superfamily. It functions in apoptosisproliferationadhesion, and migration, among other cell processes.
The cell surface of macrophages has a protein, SIRP-a, or Signal Response Protein-alpha, which, when bound to CD47, prevents the macrophage from enveloping, incorporating and digesting the cancer cell, a process called phagocytosis. The Stanford researchers show that a molecule that interferes with the binding of C47 to SIRP-a allows the macrophages to phagocytize and eliminate cancer cells. The interfering molecule is brilliantly simple, works well, and heralds the possibility of other drugs that could work in a similar manner.
The researchers reasoned that if the SIRP-a molecule binds to CD47, they could isolate that fragment of the whole SIRP-a molecule that binds to C47, and when added back to cells it should compete with the SIRP-a attached to macrophages. That could prevent macrophages from being inhibited, enabling them to phagocytose cancer cells. It turned out to be a little more complicated than that, but in an advantageous way.
First, by using a selection scheme which is not necessary to discuss here, they were able to construct a SIRP-a fragment that binds to CD47 a hundred times better than the natural fragment. It was this molecule, which they called CV1, that was used for further experiments.
CV1 was added to a mixture of fluorescent human lymphoma cells together with human macrophages, and the macrophages were assayed for phagocytosis. Perhaps unexpectedly, phagocytosis was not stimulated, relative to control mixtures of cells. However, when the CV1 was fused to the Fc chain of an immunoglobulin (the constant region base of an antibody), phagocytosis was stimulated. This led the investigators to suppose that CV1 would enhance phagocytosis of cancer cells in the presence of tumor-specific antibodies. That turned out to be the case.
After successful in vitro tests, and toxicity tests in both mice and primates, the investigators jumped to a test in live mice, a test with clinical implications. They engrafted Raji lymphoma cells subcutaneously into mice lacking B cells, T cells, and Natural Killer cells while retaining macrophages. For three weeks, from days 8 to 29, the mice were injected daily with 200 ug of CV1 plus 200 ug of Rituximab, a commercially developed monoclonal antibody approved for treatment of B-cell Non-Hodgkin lymphoma. These were compared to mice treated with saline alone, or with CV1 alone or with Rituximab alone.  Each treatment group consisted of 10-15 mice.
The findings were stunning. Large tumors developed in all of the mice – except for those mice treated with both CV1 and Rituximab. There was a modest reduction in tumor size in the groups treated with CV1 alone or with Rituximab alone, but in the mice treated with combination therapy tumors were either very small or in most cases, not detectable (a bioluminescence assay was used). The effect continued even after treatment was over.
Even more significant, the combination therapy had a major impact on longevity. Mice treated with saline alone or CV1 alone did not survive more than 40 days. Survival for most mice treated with Rituximab was slightly longer, with about 10 percent living for about 3 months. But 75% of the mice treated with combination therapy were still alive at 250 days. Similar results were obtained with another type of cancer and a different monoclonal antibody.  See the treated mice in the figure below. Take special note of the 4th column.
Image

The authors tell us clearly what all of this means: “We have developed reagents that broadly enhance the efficacy of tumor-specific antibodies, and thus, could be used as UNIVERSAL (emphasis mine) adjuvants to antibody therapies”. In other words, this one reagent, CV1, could be used with a number of different cancers and a number of different antibodies. These data bursts open the doors for the widespread and successful use of immunotherapy. I suspect that drug companies will be quickly screening for small molecule compounds that can also inhibit the interactions between CD47 and SIRP-a, drugs that might be easier and less expensive to produce, but with similar effectiveness to CV1.
Of course, these experiments were done with mice, and we must wait for the human clinical test to see whether CV1 will be useful in cancer treatment. If it is, as expected, we will be witness to a major leap in the efficacy and safety of treatments for cancer, and possibly years of additional healthy life for the hundreds of thousand of people who now succumb to cancer.
1.         Weiskopf K, Ring A, Ho C, Volkmer J-P, Levin A, Volkmer A, Özkan E, Fernhoff N, van de Rijn M, Garcia K. 2013. Engineered SIRP-α variants as immunotherapeutic adjuvants to anticancer antibodies. Science 341:88-91.

Chromosome Banding and the Path to Successful Treatment of Leukemia

One reason why the fruit fly has been a favorite model organism for genetics research is that it has only 4 chromosomes, and in some tissue like the salivary gland the chromosomal DNA is replicated many times without cell division, leading to giant chromosomes, large and thick. In addition, those giant, amplified, chromosomes have a distinct banding pattern, somewhat like laser tags, that help distinguish one chromosome from another, and they identify even specific regions within chromosomes. The pattern is thought to result from the complex folding of chromatin within the chromosome.

In contrast, human chromosomes are many (43 pairs) and small, and do not have a banding pattern. But in the 1960’s, in an experiment in which chromosomes removed from a cell and splayed on a microscope slide were treated with a protein-digesting enzyme (trypsin), suddenly a banding pattern emerged.  This was hardly expected, but very welcomed because now specific human chromosomes and chromosome regions could be readily recognized.

The discovery of the banding pattern of chromosomes was one of the lynchpins in working through the first discovery of the genetic cause of cancer in humans, the first “miracle drug” for the treatment of Chronic Myeloid Leukemia, and the ongoing revolution in personalized medicine.

This discovery involved the observation that patients with CML often had an unusual looking chromosome in their leukemic cells. It became known as the Philadelphia chromosome, for the city in which the observation was made.

By observing the banding pattern of that unusual chromosome, Janet Rowley, a young geneticist at the University of Chicago, was able to conclude that the Philadelphia chromosome was actually comprised of a portion of chromosome 22, and portion of chromosome. The two chromosomes had apparently each broken and the broken pieces were rejoined incorrectly, like a piece of a hot dog joining on it broken end to the broken end of a piece of sausage.

The joining point, it turned out – after several years of research – brought together two pieces of unrelated genes, one a regulatory fragment, the other a coding region for a gene for tyrosine kinase, and the two together resulted in a gene that was no longer regulated, belting out a continuous supply of the enzyme, whether needed or not.  Tyrosine kinase proved to be a stimulant for driving the cell division cycle, leading to cancerous growth.

Once the gene was discovered it took 50 years before the remarkably specific drug, Gleevec, entered the market, changing the prospects for leukemia patients from grim to hopeful.

The full history of this remarkable story has been published in an appropriately named book, The Philadelphia Chromosome, by Jessica Wapner. A terrific review of the book appeared in Science magazine on June 21, and I attach it here.  In the same issue Janet Rowley gives a really interesting personal perspective of this great scientific and medical success story. The book review and perspective together offer us a rich picture as a prelude to reading the book.

1. Review of The Philadelphia Chromosome
From Science  2013 342:1412.

Aditi NadkarniTracing a Translocation’s Impact
The reviewer is at the Department of Biology, 1009 Silver Center, New York University, 100 Washington Square East, New York, NY 10003-6688, USA. E-mail: an60@nyu.edu

The Philadelphia Chromosome A Mutant Gene and the Quest to Cure Cancer at the Genetic Level by Jessica Wapner The Experiment, New York, 2013. 327 pp. $25.95. ISBN 9781615190676.

Most of us love an underdog’s success story. Jessica Wapner’s The Philadelphia Chromosome includes several such stories. Wapner, a freelance science writer, describes the path from the first description of a chromosomal abnormality in cancer cells to the successful deployment of a gene-targeted medicine against what had previously been a lethal leukemia. Along the way, she pays homage to various scientific underdogs: a graduate student who spent 10-hour days peering through a microscope for the sheer love of observation; a postdoctoral fellow who risked “the judgment and ridicule of [his] peers” in negating the findings of a world-famous scientist; a junior researcher who doggedly pursued a goal dismissed as unfeasible; a physician who urged a drug company to set aside commercialism for a cure; and, most important, a patient brought back from the brink of death by a new cancer drug. Wapner aptly compares the investigators to “a hundred painters applying brushes to a canvas at some time or another over twenty-five years, driven only by curiosity and, sometimes, a vague hope that their work might eventually be relevant to human cancer.” Without knowing what the end result would look like, she notes, they ended up creating a “scientific masterpiece.”

A half century ago, a diagnosis of cancer was devastating because it meant death was likely imminent. Most available treatments were ineffective and often put patients through immense suffering before they succumbed to their disease. One such cancer was chronic myeloid leukemia (CML), which rendered patients sallow as it rapidly poisoned their immune systems, flooding them with abnormal white blood cells. The survival rate was a mere 14%, and most patients died within six years. In 1960, Peter Nowell and his graduate student David Hungerford noticed a genetic abnormality in a chromosome of CML patients (1), a discovery that eventually led to the idea that rogue genes could be a cause of cancer. The genetic abnormality, the Philadelphia chromosome, would become the first genetic defect identified as a direct culprit in cancer initiation.

Wapner’s narrative begins with the discovery of the Philadelphia chromosome and continues through the 30 years that it took to develop a drug that successfully targeted this genetic abnormality, thus turning a once fatal disease into a manageable condition that can be treated with a pill. Today, 12 years after the drug, imatinib mesylate (Gleevec), entered the market, the survival rate of CML patients is 95.2%. Told in the manner of a thriller with a dash of history, The Philadelphia Chromosome describes the events that went into the “rational design” of that pharmaceutical treatment for CML.

In the early 1970s, geneticist Janet Rowley, using recently developed chromosome banding techniques, observed that the lopped-off head of the Philadelphia chromosome in CML patients had somehow perched onto another chromosome. Going against established views about cancer, Rowley argued that this chromosomal translocation could be the cause of the cancer. Her finding opened the door to the discovery of the cancer-causing oncogene affected by this change in CML patients. It took several years and the concerted efforts of many researchers to uncover the precise mechanism by which the translocated Philadelphia chromosome causes the brakes to fail on a runaway kinase, resulting in uncontrolled cell division—a hallmark of cancer. The discovery of this culprit oncogene, as Wapner describes, ultimately revealed the relation between the Philadelphia chromosome and CML and presented the kinase as a target for therapy. A specific kinase inhibitor could bind to and stop the uncontrolled kinase in its tracks, “[l]ike a gloved hand fitting perfectly over a mouth to block the next breath.”

Much of the second half of the book focuses on the frantic struggle to get the life-saving drug to patients. Here Brian Druker, a clinical oncologist turned researcher, emerges as a hero through his inspiring commitment to patients and his unyielding determination to save people from the clutches of a fierce disease.

Reading Wapner’s account suggests that while the scientists involved in the discovery and development of Gleevec were underdogs in their field, the drug was somewhat of a stepchild for the company. “There were too few patients with CML to make developing this new drug worthwhile for the company.” In short, the drug would not profit the company as much as one that targeted some disease affecting a larger population. This problem of market-driven policy remains a subject of debate. Drug companies are often criticized for shifting their research focus to diseases with a higher incidence to ensure that they have a larger demand for their products.

The author covers a lot of ground in describing the politics of science and dissecting the inner workings of a pharmaceutical company’s drug development process. She withholds judgment, instead providing two sides of the story: In one, a physician coaxes a drug company focused on commercial interest to think about patients. In the other, a drug company, bombarded with sudden demand from the market, rushes to complete the toxicity studies required for Food and Drug Administration clearance. Wapner also describes the impact of the CML community’s 1999 online petition urging increased production of the drug for clinical trials.

In the last chapter, Wapner surveys the current landscape of cancer research, noting hurdles to continued progress such as difficulties in sequencing tumors and the unintended effects of technology transfer agreements on collaborations. She also mentions the pressing issues of poverty, limited access to medical care, and ever-rising drug prices that patients face. In her concluding comments she notes that although targeted drugs have changed cancer treatment and “medicine as a whole,” their failure “to transform other cancers into tolerable chronic conditions has generated skepticism about the future of the approach.”

As a young cancer researcher, I especially appreciate Wapner’s mention of the current abysmal funding situation—which does not reward new or risky ideas and is extremely stingy in supporting those early in their careers. She points out that in 2010, less than 4% of the U.S. National Institutes of Health’s R01 funding went to scientists under age 36. On the contrary, in the 1960s and 1970s (when many of the discoveries described in the book occurred), being a young scientist with novel ideas was an asset. Some key contributions to the Philadelphia chromosome story, as the author notes, came from young researchers given the freedom to pursue their own ideas. Support for such freedom of thought and creativity is now hard to come by in academic bioscience.

Wapner’s use of analogies to describe difficult scientific concepts makes her narrative accessible to lay readers. Still, the jargon in some portions that detail experimental hurdles may overwhelm those lacking firsthand experience in the laboratory. Nonetheless, I appreciate that her artful storytelling does not dilute the complexity of the science or sensationalize the subject. By recounting personal stories of the researchers and patients, Wapner helps readers identify with the story. The Philadelphia Chromosome offers powerful testimony on what the cancer research and pharmaceutical fields are capable of when academic scientists, physicians, and drug companies work together.

PERSPECTIVE by Janet D. Rowley   (Science 2013 342:1412)

Forty years ago, a chromosomal translocation was discovered to cause leukemia and revealed cancer as a genetic disease.It was dubbed the Philadelphia chro-mosome, named after the city where the abnormal chromosome was first described in 1960 (1). Peter Nowell, of the University of Pennsylvania, and David Hungerford, at the Fox Chase Cancer Center, had taken a close look at patients with chronic myeloid leuke-mia (CML) and found that regardless of sex, they had a very small chromosome. It was a turning point in cancer biology, the beginning of a story that would draw new attention to chromosome abnormalities as a cause of cancer, a phenomenon that still influences our under-standing of the disease.

To appreciate the importance of the discovery of Nowell and Hungerford, it is necessary to understand the state of biomedical science in the 1950s. The prevailing view from studies of experimentally induced cancer was that chromosome abnormalities were the result of genomic instability in cancer cells, not the cause. It was assumed that loss of DNA from the Philadelphia (Ph) chromosome (originally thought to be a deletion in chromosome 21) included genes that regulate cell growth, thereby leading to unrestrained proliferation of leukocytes. The situation was complicated because some patients with CML lacked the Ph chromosome and surprisingly, they had a shorter survival than did those with a Ph chromosome (2). Nonetheless, the presence of the Ph chromosome became an important diagnostic tool in hematology, and it appeared to be the exception to the established view that chromosome changes were variable and irrelevant in cancer.

The situation changed dramatically in the 1970s when several new staining techniques revealed chromosomes with unique banding patterns (transverse stripes) that allowed them to be distinguished individually and precisely ( 3, 4). Having learned a banding technique at Oxford University, I returned to the University of Chicago to apply the method to chromosome samples from leukemia patients.

Among these patients were two with acute myeloid leukemia (AML), where banding revealed that a piece of chromosome 8 had broken off and joined chromosome 21. This was the first recurring chromosomal translocation [t(8;21)] to be identified (5) . Were chromosomal changes consistent in other leukemias? It was already known that CML patients in terminal blast crisis showed a gain in middle-size chromosomes; these, I discovered, all turned out to be chromosome 8. What was even more startling was that chromosome 9 had an extra piece of material whose stain-ing resembled that of the missing piece of the Ph chromosome (by then, known to be chromosome 22). This suggested that the Ph chromosome could be the result of a translocation involving the swapped ends of chromosome 9 and chromosome 22. Leukemia cells from the same patients in the chronic phase of CML showed the same (9;22) translocation, whereas non-leukemia cells from their peripheral blood had a normal karyotype. It seemed quite likely that the Ph chromosome was an acquired translocation, a finding I reported 40 years ago (6).

There the matter stood for a decade. In the meantime, Herbert Abelson had isolated a virus that induced B cell leukemia. The ”Abelson” virus could transform normal murine lymphocytes and fibroblasts, and the causative viral factor was a protein with tyrosine kinase activity (v-Abl). The human counterpart of the Abelson viral gene, ABL,was mapped to chromosome 9 (7). Moreover, the only additional DNA found in the Ph chromosome was from chromosome 9 (8).The laborious task of cloning the chromosomal breakpoint in CML revealed that the ABL gene on chromosome 9 was translocated into part of a gene called the breakpoint cluster region (BCR) in chromosome 22, creating a BCR-ABL gene fusion (9,  11).

The only previously cloned translocation breakpoints, namely the t(8;14) in Burkitt lymphoma (B cells), had involved an onco-gene called MYC(the human counterpart of the viral oncogene v-myc), and the immuno-globulin gene on chromosome 14 (12,13).The discovery that oncogenes were involved in translocation breakpoints proved to be a remarkable validation of virology and of cytogenetics, fields that were struggling to show their relevance to human cancer. That CML involved the human oncogene ABL was welcome corroboration. Additional translocations were found to involve oncogenes as well, a few of which encoded tyrosine kinases like ABL; others involved genes that activate transcription factors that function in cell growth, differentiation, and even cell death. It was fortuitous that at the same time, drug companies were developing tyrosine kinase inhibitors. From this focus emerged imatinib (marketed as Gleevec), the compound eventually approved in 2001 to treat CML (and later, for other cancers). Like all tyrosine kinase inhibitors, imatinib prevents the protein (BCR-ABL, in the case of CML) from phosphorylating proteins that promote cancer development (14). This pharmaceutical breakthrough came almost 50 years after the discovery of the Ph chromosome. Imatinib changed CML from a disease with a 3- to 5-year average life span to one where patients have an almost normal life expectancy, especially with the advent of new second- and third-generation tyrosine kinase inhibitors. These later drugs, especially ponatinib, have been designed to be effective despite mutations in the activation domain of the ABL protein.

Whereas translocations were first identified in leukemias, lymphomas, and sarcomas, they are now cropping up in many common epithelial tumors, prostate cancer, and lung cancer, among others. Next-generation sequencing of leukemias and solid tumors has revealed a host of translocations (often small deletions or inversions) (15), some of which involve genes that are targets of drugs already approved for therapy of other conditions. It took only a few years from the discovery of the EML4-ALK translocation in lung cancer to the development of the tyrosine kinase inhibitor crizotinib (16), indicating that the discovery of new translocations may be more rapidly translatable to drug discovery. Although data on the occurrence and types of new translocations, based on karyotype analysis, are more frequently reported for hematologic cancers (75%) than for solid cancers (mainly epithelial) (25%), the proportion of malignancies that have recurring chromosomal translocations is the same in both. Moreover, the genes involved have the same function in both cases

(17). Thus, translocations are remarkably similar in function, though not  necessarily in their frequency in individual cancers.

It is likely that next-generation sequencing will reveal a much higher incidence of gene fusions in solid tumors. But this method is a two-edged sword. It has identified numerous chromosomal translocations and deletions, but which of these lead to altered gene function and which are inconsequential? It will be difficult to distinguish them in the future without characterizing RNA from the tumors.

A goal of personalized medicine is to identify virtually all of the targetable genetic and epigenetic abnormalities in a patient’s tumor through next-generation sequencing and other technologies. To evolve targeted treatments for cancer, we also need a more sophisticated understanding of tumor-specific antigens and chromatin modifications, for example. There likely will be many surprises along the way, and paradigms will be discarded. Nevertheless, the goal will always be the same – to treat disease and benefit the patient.

1. P. C. Nowell, D. A. Hungerford, Science132, 1497 (1960).
2. J. Whang-Peng et al., Blood 32, 755 (1968).
3. T. Caspersson et al., Exp. Cell Res. 60, 315 (1970).
4. A. T. Sumner et al., Nat. New Biol. 232, 31 (1971).5. J. D. Rowley, Ann. Genet. 16, 109 (1973).
6. J. D. Rowley, Nature 243, 290 (1973).
7. N. Heisterkamp et al., Nature 299, 747 (1982).
8. A. de Klein et al., Nature 300, 765 (1982).
9. N. Heisterkamp et al., Nature 306, 239 (1983).
10. J. Groffen et al ., Cell 36, 93 (1984).
11. E. Shtivelman et al., Nature 315, 550 (1985).
12. R. Dalla-Favera et al., Proc. Natl. Acad. Sci. U.S.A. 79, 6497 (1982).
13. R. Taub et al., Proc. Natl. Acad. Sci. U.S.A. 79, 7837 (1982).
14. B. J. Druker et al., N. Engl. J. Med. 344, 1031 (2001).
15. B. Vogelstein et al., Science 339, 1546 (2013).
16. A. T. Shaw et al Lancet Oncol. 12, 1004 (2011).
17. F. Mitelman et al., Nat. Rev. Cancer 7, 233 (2007).

A plausible explanation for the link between obesity and cancer

It is well known that obesity is on the rise throughout the developed world, and that obesity is a risk factor for several types of cancer, but it is not known why that should be. Why, it may be wondered, does extra weight or extra fat lead to a greater incidence of cancer?

We now have an intriguing explanation based on cell and molecular biology that provides at least one possible explanation for increased liver cancer associated with obesity (1).

The work was done with mice, but is likely to apply to humans as well. It needs to be said that the experimental mice were made obese by feeding them a high fat diet. And to prime the mice to develop liver cancer both control mice and experimental mice were treated with a carcinogen.  Under these conditions about 5% of control non-obese mice developed liver cancer while virtually every one of the experimental mice did. This  is a big difference, and allow the researchers to compare the control and experimental in a variety of ways.

The answer they came up with is intriguing for its complexity. Several different processes were linked together sequentially in this study, with each step along the way leading to the next.

Let me describe the sequence of event in fairly simple terms.

First, obesity leads to a change in the microbial flora of the gastrointestinal tract. Gut bacteria in obese mice are enriched in gram-positive Clostridia senillii. They constitute over 12% of fecal bacteria.

Secondly, the bacteria are particularly capable of converting bile salts toa metabolite called deoxycholic acid DCA. Consequently, significantly more DCA is taken into the blood stream by obese mice compared to control mice.

After absorption by the gut into the blood stream, DCA is carried to the liver; the liver is the first stop for all compounds absorbed by the gut, thanks to the hepatic portal vein. The authors note that DCA “is known to cause DNA damage through reactive oxygen species production.”

What the authors found, however, can be considered quite unexpected. DCA activates hepatic stellar cells.  Hepatic stellate cells are a small group of mesenchyme cells with long cytoplasmic processes that give it its stellar name. In that regard they differ from the majority epithelial cells of the liver. Hepatic stellate are readily recognized because they contain small lipid droplets. The cells are quiescent, which means they don’t normally divide. They function in retinol (a  precursor to Vitamin A and retinoic acid) homeostasis, taking up retinol bound to retinol-binding proteins.

When activated by DCA, hepatic stellate cells begin to grow and divide, and also start to move about, a mesenchyme cells do. But then, these cells move into another state, the state of senescence, in which they not only do not divide, but cannot divide again. They can no longer be activated.

More importantly, as senescent cells, they secrete a constellation of proteins, and the authors demonstrate that these proteins  induce the development of hepatic carcinoma cells in liver epithelial cells.  

Now we know how, at least in mice, obesity can lead to increase in liver cancer. But we must giver credit to the remarkable ability of the investigators to fit together the pieces of this mechanism, with solid experimental proof for each step along the way.

With an understanding of the mechanism by which obese mice (and presumably humans) are susceptible to liver cancer, the investigators reasoned that by eliminating the gram positive bacteria from the gut, liver cancer might reduced. They used the commonly available antibiotic vancomycin, which is known to block gram positive bacteria. Their prediction was realized, and liver cancer was significantly reduced in obese mice. Reduced, but not eliminated, suggesting to the authors that there is still more to the story than revealed by this study.

This is a remarkable demonstration of the power of basic research.  With an understanding of HOW the cancer is induced in obese mice it became immediately obvious how to prevent those cancers from developing, in this case, by use of a simple antibiotic. If this story hold true for humans, it may be possible to save many live by the simple expedient of killing or replacing gram-positive clostridia bacteria in the gut. It may not be feasible to treat people with antibiotics for prolonged periods of time, but there might be other approaches that could be used year after year.

I can’t help but wonder whether it might be feasible to replace the clostridia bacteria with another, beneficial, gram positive bacteria like lactobacillus. Maybe it will be possible to use a probiotic approach with yogurt in the diet, or by taking a daily probiotic supplement. Wouldn’t it be wonderful to be able to save so many lives by such a simple and inexpensive approach.

1.         Yoshimoto S, Loo TM, Atarashi K, Kanda H, Sato S, Oyadomari S, Iwakura Y, Oshima K, Morita H, Hattori M, Honda K, Ishikawa Y, Hara E, Ohtani N. 2013. Obesity-induced gut microbial metabolite promotes liver cancer through senescence secretome. Nature 499:97-101.

Image

Data of experiments presented in previous post

Data of experiments presented in previous post

Red indicates tumor growth of control melanoma cells in immune-compromised mice. Blue represents tumor growth in mice injected with melanoma with depleted PDK1, an enzyme that would otherwise reduce pyruvate dehydrogenase activity. In the bottom graph, PDK1 was depleted after tumor growth had begun (at DOX labelled arrow), and reversal of tumor growth was observed. Please read previous post for a fuller explanation.