New Gene Mutation Found in Myeloproliferative Neoplasms

Roxanne Nelson

December 12, 2013

NEW ORLEANS — Researchers might be one step closer to deciphering the underlying mechanisms of myeloproliferative neoplasm (MPN). A previously unrecognized gene has turned out to be a primary driver of MPN in patients without JAK2 and MPL mutations.

One new study shows that frequent mutations in the calreticulin gene CALR occur in MPN (abstract LBA-1). A related study shows that somatic mutations in CALR exist in the majority of patients with JAK2 wildtype MPN (abstract LBA-2).

Both studies were presented here at the American Society of Hematology (ASH) 55th Annual Meeting, and published online simultaneously in the New England Journal of Medicine.

These 2 studies "are critically important contributions to hematology," said ASH president Janis Abkowitz, MD, hematology division head, professor of medicine, and adjunct professor of genome sciences at the University of Washington in Seattle, during a press call held in advance of the meeting.

Using different strategies, the 2 teams of researchers showed that CALR is a major driver in MPN patients without JAK2 and MPL mutations, who comprise about 68% of patients with essential thrombocythemia and 88% of patients with primary myelofibrosis. "This is a huge contribution to the understanding of pathogenesis," Dr. Abkowitz noted.

New Data "Fill a Gap"

The most common MPNs are chronic myeloid leukemia (CML), polycythemia vera, essential thrombocythemia, and primary myelofibrosis. The genetic basis of CML has been known for more than 3 decades, but the specific genetic backbone contributing to the pathogenesis of the other disorders was not known until 2005, Ross L. Levine, MD, writes in an editorial accompanying the published studies.

Previous studies have identified other novel mutations in patients with MNP, which have been observed in patients with JAK2 or MPL mutations and in those without the mutations, and occur in other myeloid cancers, writes Dr. Levine, who is from the Memorial Sloan-Kettering Cancer Center and the Weill Cornell Medical College in New York City.

Conversely, CALR mutations are seen exclusively in patients without JAK2 or MPL mutations who have essential thrombocythemia or primary myelofibrosis, he points out. They are never observed in patients with polycythemia vera or in patients with JAK2 or MPL mutations.

"The new mutations occur in a specific C-terminal region of CALR and always generate a frameshift mutation," Dr. Levine explains. These CALR mutations "are observed in purified stem or progenitor cells from patients with myeloproliferative neoplasms and remain stable during disease evolution."

There are still unanswered questions about the genetic basis of these tumors, he concludes. However, these 2 new studies "fill a gap in our knowledge by identifying the most common mutation in patients without JAK2 or MPL mutations and allow us to better appreciate the genetic complexity of the different myeloproliferative syndromes."

Better Survival, Lower Thrombosis Risk

In the first study, led by Thorsten Klampfl, PhD, from the CeMM Research Center for Molecular Medicine of the Austrian Academy of Sciences in Vienna, researchers performed whole-exome sequencing and identified somatically acquired mutations in 6 patients with primary myelofibrosis but without JAK2 or MPL mutations (N Engl J Med. Published online December 10, 2013).

The analysis revealed recurrent somatic insertions and deletions in exon 9 of CALR encoding calreticulin. The researchers resequenced 1107 samples from MPN patients, and found that CALR mutations were absent in polycythemia vera, and that CALR mutations and JAK2 and MPL mutations were mutually exclusive in essential thrombocythemia and primary myelofibrosis.

In patients with wildtype JAK2 or MPL, the researchers identified CALR mutations in 67% of those with essential thrombocythemia and 88% of those with primary myelofibrosis.

"We detected 36 different types of mutations in CALR," said Dr. Klampfl during his presentation, adding that CALR mutations appear to have a positive influence on survival.

Patients with mutated CALR had better survival than those with mutated JAK2, as well as a lower risk for thrombosis, he explained.

The most prominent types were a 52 bp deletion, found in 53% of all cases with mutant CALR, and a 5 bp insertion, found in 32% of all cases with mutant CALR. The reserchers also evaluated 19 patients with wildtype CALR-exon 9 for mutations in the other exons of the gene, but all were negative.

CALR mutations and mutations in both JAK2 and MPL appear to be mutually exclusive, Dr. Klampfl and colleagues note. In addition, CALR mutations were almost completely absent in patients with other types of myeloid neoplasms, except for a few patients with RARS-T, a myeloid neoplasm with both myelodysplastic and myeloproliferative features.

Incorporated Into Routine Testing

In the related study, Jyoti Nangalia, MBBChir, FRCPath, from the Cambridge Institute for Medical Research and Wellcome Trust/MRC Stem Cell Institute in the United Kingdom, and colleagues conducted exome sequencing in specimens obtained from 151 patients with MPN (N Engl J Med. Published online December 10, 2013). CALR mutation status was evaluated in an additional 1345 hematologic cancers, 1517 other cancers, and 550 control samples.

They identified 1498 somatic mutations with a median of 6.5 mutations in patients with polycythemia vera, 6.5 in patients with essential thrombocythemia, and 13.0 in patients with myelofibrosis. A total of 148 CALR mutations with 19 distinct variants were detected.

Dr. Nangalia and colleagues observed that CALR and JAK2/MPL mutations were mutually exclusive, as did Dr. Klampfl's team, and 97% of patients harbored a mutation in 1 of these 3 genes. In their extended follow-up screen, CALR mutations were detected in 71% of patients with essential thrombocythemia, 56% of those with idiopathic myelofibrosis, 86% of those with postessential thrombocythemia myelofibrosis, and 8% of those with myelodysplasia. However, CALR mutations were not observed in other myeloid, lymphoid, or solid cancers.

Compared with patients with JAK2-mutated MPNs, patients harboring CALR mutations presented with higher platelet counts (P = .0003), lower hemoglobin levels (P = .02), and a higher incidence of transformation to myelofibrosis (P = .03).

These results reveal a novel biologic pathway as a target for tumorigenic mutations and will simplify the diagnosis of MPN patients. "We predict it will be incorporated into routine testing," said Dr. Nangalia.

The study by Dr. Klampfl's team was supported by grants from the MPN Research Foundation and AssociazioneItaliana per la RicercasulCancro. The study by Dr. Nangalia's team was supported by the Kay Kendall Leukaemia Fund, the Wellcome Trust, Leukemia and Lymphoma Research, Cancer Research UK, the National Institute for Health Research Cambridge Biomedical Research Centre, the Cambridge Experimental Cancer Medicine Centre, and the Leukemia and Lymphoma Society of America. Several coauthors report financial relationships with other organizations. Dr. Levine has disclosed no relevant financial relationships.

N Engl J Med. Published online December 10, 2013. Klamf abstract, Nangalia abstract, Editorial

American Society of Hematology (ASH) 55th Annual Meeting: Abstracts LBA-1 and LBA-2. Presented December 10, 2013.


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