The Philadelphia Chromosome by Jessica Wapner: a story of scientific discovery from the bench to the clinic

The Philadelphia Chromosome by Jessica Wapner focuses on the development of the drug Gleevec for the treatment of leukemia. Wapner really hits the sweet spot of science writing: she explains complex processes completely such that an experienced scientist would be interested and yet simply enough that a layman would also understand. The book's primary source material consists of interviews with scientists. This gives the reader details not found in published articles and makes the book very readable, as the story of scientific discovery becomes about the scientists doing the research. The Philadelphia Chromosome highlights how understanding basic biology can lead to significant advances in the clinic.

Reciprocal translocation of chr. 22with chr. 9. Image by
Peter Lowell

The link between cancer and chromosomal rearrangements had been hypothesized since the work of Theodor Boveri in 1902. However, it was not until the mid-20th century that scientists had the right tools to investigate this connection. By arresting and staining cells during cell division, scientists were able to reliably count chromosomes. (As I learned in The Violinist's Thumb, this was one reason that it took so long to learn how many chromosomes humans actually had.) In the 1950s, Nowell and Hungerford used this staining technique to look for chromosomal rearrangements in patients with chronic myelogenous leukemia (CML). They noticed that chromosome 22 was shorter than it should be, suggesting that part of the chromosome was deleted. The shortened chromosome 22 was termed the Philadelphia chromosome for the city in which it was discovered.

Adapted from Montgomery et al., 2004.

In 1973, Janet Rowley, using an improved technique to stain chromosomes from CML patients, noticed that the missing part of chromosome 22 had migrated to the end of chromosome 9 and vice versa (see image above). Today, chromosomes are frequently imaged using spectral karyotyping (example on right), where each chromosome is fluorescently labeled with a unique color, making the identification of chromosomal abnormalities much simpler. Rowley's work showed that the Philadelphia chromosome was the result of a reciprocal translocation, referred to as t(9:22). In simple terms, if you imagine the genome is like a book with each chromosome being a chapter (yes, I am borrowing from Matt Ridley's book), then the Philadelphia chromosome would result from the switching of pages from the end of Chapter 22 with the later part of Chapter 9. A book with such an error would read very differently than the original version of the book.

The Philadelphia chromosome creates
a Bcr-Abl fusion protein.

The next question: how does the Philadelphia translocation change the DNA sequence on the two effected chromosomes? At the time, this proved to be a challenge because DNA cloning was still in its infancy. In 1978, Heisterkamp and Groffen started to investigate the location of the abl oncogene gene in the human genome. They developed an abl probe, which showed that the gene was on chromosome 9. This result raised the question: was the abl oncogene connected to CML and the Philadelphia chromosome? Amazingly, when the abl probe was used in cells from CML patients with the Philadelphia chromosome (Ph+ CML), the probe could be found both on the Philadelphia chromosome as well as the normal copy of chromosome 9. Next, they had to determine what gene was located next to the abl insertion. Looking at the sequence of chromosome 22 from Ph+ CML patients, they found the "breakpoint cluster region" or bcr next to abl. Thus, the Philadelphia chromosome results in a fusion of the bcr-abl genes (see figure above). Later research from the Baltimore lab demonstrated that introducing a bcr-abl fusion gene in mice could cause CML and CML-like symptoms.

Normally, abl encodes a protein tyrosine kinase. Kinases play an important role in cells: turning on target proteins. To ensure that proteins are activated only as needed, kinase activity must be carefully regulated. The Bcr/Abl fusion protein has unregulated kinase activity, which means that all of the target proteins are activated at higher than normal levels. In the case of CML, this leads to unfettered cell growth and proliferation, particularly in white blood cells. This result suggested that being able to turn off the kinase activity could stop the out of control growth of white blood cells in Ph+ CML patients.

The Bcr-Abl fusion protein (green)
is inhibited by STI-571/imatinib (red)
(from Wikipedia commons).

Around this time, increasing numbers of kinases were being linked to disease, especially cancers, which are typically due to unrestricted cell growth. This coincidence grabbed the attention of chemists at the Swiss pharmaceutical company Ciba-Geigy (now known as Novartis); they hypothesized that kinases could be excellent drug targets. This relatively new approach, called "rational drug design", would target the specific cause of a specific cancer, creating a drone strike as opposed to the "carpet bombing" tactic of most chemotherapeutics (p. 108). For the treatment of CML, scientists would need to design a compound to specifically inhibit the Abl kinase, which many doubted would be possible due to the large number of kinases in the cell. After six years of development, the Ciba-Geigy team identified a compound that selectively and specifically inhibited Abl kinase activity. Eventually, this compound landed in the hands of Brian Druker, who found that the Abl inhibitor (called STI-571 or imatinib) could specifically kill cells derived from Ph+ CML patients as well as from mice engineered to express the Bcr-Abl fusion protein.

Much of this section of the book focuses on the conflict between Druker and Ciba-Geigy, who were hesitant to devote resources to a drug for a relatively rare disease. However, the Orphan Drug Act helped tip the scales in Druker's favor. The approval of STI-571 as an orphan drug ensured that the company would have a fast track for FDA approval as well as a longer term for copyright protection. (Interesting side note: Botox was originally approved as an orphan drug to treat a rare muscle disease. Its later approval as a wrinkle smoother meant that the drug company benefited greatly from that status.) In 2001, after very successful phase I clinical trials, the FDA approved STI-571 under the trade name Gleevec. This caused a paradigm shift in cancer research; the new goal was to find driver mutations for each type of cancer. Today, more than 15 tyrosine kinase inhibitors are available for cancer treatment (e.g, erlotinib for lung cancer and lapatinib for breast cancer). Unfortunately, none of the drugs have been as successful as Gleevec; rather, these drugs give incremental improvements in survival rates. Another confounding factor is that diseases, particularly cancers, are rarely caused by single mutations. However, the advent of large scale genome sequencing projects (as covered in my review of Genome) is improving the chances of connecting diseases with specific mutations.

If I had one complaint about the book, it would be that the focus on Druker's career and personal life, while useful in terms of creating a narrative arc, sometimes distracted from other important elements of the story. The book highlights how basic scientific research is an important starting point for successful clinical outcomes. This lesson is critical to remember when funding for basic scientific research is decreasing and the focus on clinically and translationally relevant research is increasingly important for funding.