Hairy-Cell Leukemia Disease-Defining Mutation Identified

Becky McCall

June 13, 2011

June 13, 2011 (London, United Kingdom) — A mutation known as BRAF V600E has been identified in 100% of hairy-cell leukemia (HCL) cases but in none of the peripheral B-cell lymphomas and leukemias tested, suggesting that this mutation is a disease-defining characteristic. The finding "provides valuable insight" into the disease, says an expert, and potentially opens up new avenues for diagnostic testing and opportunities for targeted drug therapy (such as with BRAF inhibitors, which are already being studied in melanoma).

The finding was presented here at the 16th Congress of the European Hematology Association, and was published online June 11 in the New England Journal of Medicine to coincide with its presentation.

Brunangelo Falini, MD, from the Institute of Hematology, University of Perugia, Italy, is lead author of the study; his colleague, Enrico Tiacci, MD, from the same institution, presented the results at the meeting.

Working with cells from a patient with HCL, the researchers identified 5 candidate mutations, including the BRAF V600E mutation, which is known to be oncogenic in other tumors. The researchers then focused on determining whether the BRAF V600E mutation was present in other cases of HCL.

"Since BRAF is a mutated gene in other cancers, we selected the one found in HCL and tested it to see if there was recurrence in other cases," Dr. Falini told Medscape Medical News.

Remarkably, the BRAF mutation identified in the study was found in 100% of the HCL cases tested. This is in stark contrast to mutations found in the genomes of all tumors sequenced so far, where the frequency of recurrence is rarely above 20% to 30%.

"In this case, there is a single spot mutation in the DNA that is present in all patients. Maybe there are some cooperative mutations, but this is emerging as the disease-defining mutation in HCL," the researchers explain.

HCL has been recognized for the past 50 years, and is so named because of the distinctive hairy projections on the tumor cells. This morphology distinguishes HCL from other B-cell lymphomas, but until now the genetic lesion associated with the disease has remained unknown.

"Now we have the specific genetic lesion in addition to immunological and other pathological identifiers," said Dr. Falini.

Researchers took advantage of new genome sequencing technology looking at the exome (1% of the entire genome) to identify the underlying genetic cause of the disease. Genomic DNA was extracted from the index HCL patient's leukemic and nonleukemic cells, and the sequences were read and compared, using a mathematical algorithm developed by the biostatistician, Raul Rabadan, PhD, from Columbia University in New York City, a coauthor of the paper.

In addition to the sample from the index patient, 47 samples from other patients with HCL were evaluated; the same BRAF mutation was found in all 47. Another 195 patients with peripheral B-cell lymphomas and leukemias were evaluated, but none carried the BRAF V600E variant.

"Not only do all the HCL cases investigated have the mutation, but the mutations investigated in other B-cell lymphomas showed that the BRAF mutation we found is restricted to HCL," said Dr. Falini.

One of the important implications of this research is that it might aid in the distinction of HCL from indolent B-cell lymphomas that morphologically resemble HCL because of their hairy-cell projections.

Dr. Tiacci reported that his team has recently developed a very sensitive test, which was not described in the paper.

The test is a simple polymerase chain reaction followed by agarose gel electrophoresis, but it provides a higher level of sensitivity for the very low numbers of leukemic cells in HCL.

"The test can detect up to 0.1% leukemic cells in the blood. We expect this will be added to our current armamentarium in the future to increase the diagnostic accuracy of this condition," explained Dr. Tiacci.

In addition, the researchers demonstrated in vitro that HCL cells are sensitive to BRAF inhibitors, which can inhibit the expression of the BRAF mutation in the patient. "These drugs are already in clinical trials in melanoma, so we hope that they may be utilized in HCL, initially in patients who are unsuitable for conventional therapies."

Now HCL adds a nice paradigm for the development of molecular targeted therapies.

The researchers are gathering more data, which they say continues to confirm the findings of their study.

Dr. Falini reported that he believes uncommon hematological diseases, such as HCL, often provide a new paradigm for the development of drugs and the understanding of genetic lesions. "Chronic myeloid leukemia, acute promyelocytic leukemia, and ALK-positive lymphomas are typical examples, and now HCL adds a nice paradigm for the development of molecular targeted therapies" he said.

Finding "Provides Valuable Insight"

Michael Grever, MD, chair of internal medicine at The Ohio State University Medical Center, in Columbus, said the study is important and commended the authors for their "fine" work. He said the finding would provide a confirmatory diagnostic test to accurately identify patients likely to respond to therapy, and also raises the potential for new molecular targeted therapies.

Dr. Grever noted that the identification of BRAF V600E provides valuable insight into the oncogenic molecular pathogenesis of HCL, and might contribute to the understanding of secondary malignancies. "Despite controversy over the incidence of secondary malignancies in patients with HCL, thyroid cancer has been identified as one form of cancer with an increased incidence. Previously, the suggestion had been made that the potential increased incidence of secondary cancer is related to the immunosuppressive therapy for the hairy-cell leukemia. This mutational hotspot may be relevant to understanding the pathogenesis of thyroid cancer in HCL."

"Of extreme interest, the observation of the activating BRAF kinase with subsequent enhanced phosphorylation of downstream targets, including MEK and ERK, raises the potential for novel therapeutic approaches for patients with HCL who have either not responded to standard therapy or who have had multiple relapses to purine analogs," Dr. Grever told Medscape Medical News.

He added that frequently, HCL is considered "fixed" with standard purine analogs and immunotherapy, but approximately 40% of patients relapse within 10 years of initial therapy. "Our current agents have numerous side effects, and subsequent remissions are often not as durable as the first treatment. Thus, there is definite room for identifying novel agents targeting specific molecular pathways in an effort to prolong patient responses."

Also commenting on the study, Srdan Verstovsek, MD, associate professor at the Department of Leukemia, the University of Texas M.D. Anderson Cancer Center in Houston, said it was "amazing" to find that HCL is a uniform disease based on 1 mutation, similar to chronic myeloid leukemia.

The mutation could be used for a diagnosis.

"I could imagine that the chemotherapy approach might change to a targeted therapy. The current therapy for HCL is cladribine [Litak], which is very effective, but it is obviously much better to treat with a targeted agent, such as a small-molecule inhibitor, with fewer side effects and in the form of pills."

"The findings suggest that the mutation could be used for a diagnosis. Sometimes HCL can be missed because it causes a lot of fibrosis. It would be much easier to have 1 specific molecular test rather than interpretation by pathologists," he concluded.

The study was supported by the Italian Association for Cancer Research and the Hairy Cell Leukemia Research Foundation, USA. Dr. Falini, Dr. Tiacci, and Dr. Raul Rabadan have applied for a patent on the discovery of the BRAF mutation. Dr. Grever and Dr. Verstovsek have disclosed no relevant financial relationships.

N Engl J Med. Published online June 11, 2011. Abstract

16th Congress of the European Hematology Association (EHA). Presented June 11, 2011.


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