Polycythemia Vera-From “Disorder” to “Cancer”

Cancers known as myeloproliferative neoplasms (MPNs) illustrate how our understanding and classification of diseases can evolve over time. MPNs are blood cancers in which the bone marrow cells that produce blood cells develop and function abnormally. “I would view them most accurately as a type of chronic leukemia,” says Ruben Mesa, MD, chair of the Division of Hematology and Medical Oncology and deputy director of the Mayo Clinic Cancer Center in Arizona. The three main types of MPNs are polycythemia vera (PV), essential thrombocythemia (ET), and myelofibrosis.

“Initially,” explains Dr. Mesa, “they were called myeloproliferative disorders.” At that time it was unclear whether these conditions should actually be considered cancers. Eventually, however, it was determined that they were clonal (the abnormal cells could all be traced back to the same, initial abnormal cell) and could all progress to a fast-growing type of leukemia known as acute myeloid leukemia. In 2008 the World Health Organization changed the name to myeloproliferative neoplasms (abnormal growths), and MPNs are now considered cancers.

A recent advance in the understanding of these cancers is the identification of gene mutations that contribute to their growth. “There are some relatively newly described genetic changes, such as changes in a gene known as JAK2, that are very common across these diseases,” says Dr. Mesa. “The mutations in JAK2, which is an on/off switch for cell growth and proliferation, are present in almost everyone who has PV and about half of those with ET and myelofibrosis.”

Asked about the effect of these findings, Dr. Mesa says, “First it truly helps with the diagnosis, particularly of individuals who have more-borderline cases. The second and probably even more important effect is that it has led to a tremendous amount of additional scientific scrutiny into the diseases and has fueled the development of many potential targeted therapies.”

Targeted therapies are drugs that interfere with specific biological pathways that contribute to cancer growth. One targeted therapy has already been approved for the treatment of certain patients with myelofibrosis: the JAK inhibitor Jakafi® (ruxolitinib), which was approved in 2011.

Development of new approaches to treatment will provide important benefits to people with MPNs. “About half of those with ET and PV have symptoms that are inadequately controlled with current therapy options,” says Dr. Mesa. “The disease-related symptoms—such as itching, fatigue, night sweats, difficulties with concentration, and other issues—can significantly affect quality of life. And prior to JAK2 inhibition, the majority of people with myelofibrosis had very poor therapeutic options.”

Asked if people with MPNs should receive care from someone who specializes in these cancers, Dr. Mesa replies, “Well, these are diseases with a lot of subtlety in terms of management. The busy general oncology practitioner sees very few of these patients. So I do think that it is helpful to have someone involved in the broader team who has much more of a focus in this area. There’s a network of folks who focus on MPNs around the country. The NCI [National Cancer Institute] helps fund the MPD Research Consortium, with its base at the Mount Sinai School of Medicine [Icahn School of Medicine at Mount Sinai] in New York. We’re a network of many sites in the US and in Europe for clinical trials and things of that nature.”

Although MPNs are not common cancers, they are cancers for which important progress is being made. “I would say that if I were a patient with MPN, I would be very hopeful,” concludes Dr. Mesa. “There’s never been a period of time where there’s been more scientific energy and focus on these diseases. In addition, the things that we’re learning about the MPNs have implications for many other cancers.”