CALR Mutation: Associated Diseases and Prognosis

CALR mutations are strongly linked to two blood cancers collectively known as myeloproliferative neoplasms: essential thrombocythemia and primary myelofibrosis. These mutations appear in roughly two-thirds to nearly 90 percent of patients who lack the more common JAK2 mutation, making CALR the second most frequent driver gene in these diseases. Prognosis varies depending on the specific type of CALR mutation, the disease it accompanies, and how it interacts with other genetic changes, but in general, a CALR mutation carries a more favorable outlook than a JAK2 mutation for similar conditions.

How CALR Mutations Were Discovered

Calreticulin is a protein that normally lives inside the endoplasmic reticulum of cells, where it helps fold other proteins and regulate calcium levels. For decades, it was studied mainly in that context. The breakthrough came in late 2013, when two independent research groups reported that mutations in the CALR gene were present in the majority of myeloproliferative neoplasm patients who did not carry JAK2 or MPL mutations. One group, using exome sequencing, identified novel somatic CALR mutations in 26 out of 31 JAK2- or MPL-unmutated patients.¹ The other found CALR mutations in 67 percent of essential thrombocythemia patients and 88 percent of primary myelofibrosis patients who were negative for JAK2 and MPL.² These findings filled a longstanding gap: clinicians had known that a significant minority of myeloproliferative neoplasm patients carried no detectable driver mutation, and CALR turned out to be the missing piece for most of them.

Which Diseases Carry CALR Mutations

The two diseases most tightly associated with CALR mutations are essential thrombocythemia, in which the body produces too many platelets, and primary myelofibrosis, in which scar tissue gradually replaces normal bone marrow. CALR mutations appear in around 20 to 25 percent of all essential thrombocythemia cases and 25 to 30 percent of all primary myelofibrosis cases. Among only those patients who are negative for JAK2 and MPL mutations, CALR mutations account for 70 to 84 percent of cases.³

One disease conspicuously absent from the list is polycythemia vera, a related myeloproliferative neoplasm driven almost entirely by JAK2 mutations. CALR mutations are essentially absent in polycythemia vera.² Outside the classic myeloproliferative neoplasms, CALR mutations show up occasionally in myelodysplastic syndromes, some overlap syndromes like MDS/MPN with ring sideroblasts and thrombocytosis, and rare other myeloid cancers, but the frequencies are low.⁴ They do not appear in lymphoid cancers or common solid tumors.

How Mutant CALR Drives Disease

All disease-causing CALR mutations share a unifying feature: they are insertions or deletions in exon 9 of the gene that shift the reading frame, producing a protein with a completely altered tail end. This new tail loses the signal that normally keeps calreticulin inside the endoplasmic reticulum, allowing the mutant protein to escape to the cell surface. Once there, mutant calreticulin latches onto the thrombopoietin receptor (also called MPL) and switches on the JAK-STAT signaling pathway without needing the normal thrombopoietin signal.⁵ In other words, the mutant protein hijacks the same growth-signaling pathway that JAK2 mutations activate, just from a different entry point. This explains why CALR and JAK2 mutations are almost always mutually exclusive: both accomplish the same downstream effect, so a cell that already has one rarely needs the other.²

Type 1 and Type 2 Variants

More than fifty different CALR mutations have been catalogued, but two dominate. Type 1 is a 52-base-pair deletion, and type 2 is a 5-base-pair insertion. Together, these two variants account for over 80 percent of all CALR mutations in essential thrombocythemia.⁶ The remaining mutations are rarer variants that are usually grouped as “type 1-like” or “type 2-like” based on how much of the original protein structure they preserve. Type 1 mutations remove a larger chunk of the normal C-terminal domain, while type 2 mutations leave more of it intact. This structural difference matters for prognosis, especially in myelofibrosis, where type 1 and type 1-like mutations carry a meaningfully better outlook than type 2.

Prognosis in Essential Thrombocythemia

Patients with CALR-mutated essential thrombocythemia tend to be younger, have higher platelet counts, and show lower white blood cell counts and hemoglobin levels compared to those with JAK2-mutated disease.⁷ The most clinically important difference is in blood clotting risk. Patients with JAK2-mutated essential thrombocythemia have roughly twice the rate of thrombotic events compared to CALR-mutated patients.⁸ This is a substantial difference, since thrombosis is the complication patients fear most and the one that drives many treatment decisions. One study found that the thrombosis risk in CALR-mutated essential thrombocythemia was similar to that of patients who carried none of the three driver mutations at all.⁸

Another notable finding is that CALR-mutated essential thrombocythemia behaves quite differently from JAK2-mutated essential thrombocythemia when it comes to transformation. None of the CALR-mutated patients in one large study developed polycythemia vera, while the 15-year cumulative risk was 29 percent for JAK2-mutated patients.⁷ This has led researchers to argue that CALR-mutated and JAK2-mutated essential thrombocythemia are really two distinct diseases that happen to present with high platelet counts. The risk of progression to myelofibrosis, however, is not as clearly different. Some studies found similar rates between the two, while at least one found a slightly higher progression risk in CALR-mutated patients.⁹ Overall survival in CALR-mutated essential thrombocythemia is generally at least as good as, and often better than, JAK2-mutated disease, driven largely by the lower clotting risk.

Prognosis in Primary Myelofibrosis

The survival advantage of CALR mutations is more dramatic in primary myelofibrosis. A meta-analysis of non-Asian populations found that JAK2-positive myelofibrosis patients had more than double the risk of death compared to CALR-positive patients, with a combined hazard ratio of about 2.4.¹⁰ A separate meta-analysis confirmed the survival advantage and additionally found that CALR-mutated patients had a lower risk of developing an enlarged spleen and lower thrombosis rates compared to JAK2-mutated patients.¹¹ Neither meta-analysis found a significant difference in the rate of transformation to acute leukemia between the two groups.

The survival benefit is not uniform across all CALR mutations, though. A large study of over 1,000 primary myelofibrosis patients at the Mayo Clinic showed that the advantage is concentrated in type 1 and type 1-like CALR mutations. Patients with type 2-like CALR mutations, MPL mutations, JAK2 mutations, or no detectable driver mutation (“triple-negative”) all had similarly worse outcomes, with hazard ratios ranging from about 1.8 to 2.6 compared to type 1-like CALR.¹² This makes the type 1 versus type 2 distinction more than academic; it directly affects treatment planning and how aggressively clinicians consider options like stem cell transplantation.

How CALR Status Fits Into Risk Scoring

Because driver mutation status carries so much prognostic weight, modern risk-scoring systems for myelofibrosis now incorporate genetic information alongside traditional clinical factors like blood counts, age, and symptoms. The MIPSS70 system, designed for transplant-age patients, lists the absence of a CALR type 1 mutation as a significant risk factor for shorter survival, along with clinical features and the presence of certain high-risk additional mutations in genes like ASXL1, EZH2, and SRSF2.¹³ The GIPSS model goes further by relying entirely on genetic data, including karyotype and mutation profile, without clinical variables. In practice, clinicians often use GIPSS to identify patients at the highest or lowest risk levels, where the genetic information alone is decisive, and then apply more comprehensive scoring like MIPSS70 for patients who fall in the intermediate categories.¹⁴

For patients, the practical upshot is straightforward. If you have primary myelofibrosis with a type 1 CALR mutation and no additional high-risk genetic changes, your disease is more likely to be classified as lower risk, and your hematologist may recommend a watch-and-wait approach or less aggressive therapy. If you carry a type 2 CALR mutation, a JAK2 mutation, or are triple-negative, the scoring models tend to push toward earlier intervention and closer monitoring.

Testing for CALR Mutations

Several laboratory methods can detect CALR mutations, each with different strengths. Sanger sequencing was the first widely used method and can identify any mutation in exon 9, including rare or novel variants, but it is less sensitive at detecting mutations when only a small fraction of cells carry the change. Fragment analysis and high-resolution melt analysis do a better job at picking up low-level mutations.¹⁵ Next-generation sequencing panels offer the best sensitivity, detecting mutations down to about 1 percent allele frequency, and are increasingly standard in hematology workups because they simultaneously check for CALR, JAK2, MPL, and a panel of other relevant genes in a single test.¹⁵ Screening PCR assays designed specifically for the two common CALR mutations offer a fast and affordable option with high specificity but can miss rarer variants.¹⁶

In practice, if your hematologist suspects a myeloproliferative neoplasm and your JAK2 test comes back negative, CALR testing is typically the next step. A positive result not only confirms a diagnosis but immediately tells the clinician something about expected behavior and prognosis, which is why molecular testing has become an essential part of the diagnostic workup rather than an optional add-on.

CALR Mutations in Younger Patients

Myeloproliferative neoplasms in children and young adults are rare, but when they occur, CALR mutations play a proportionally larger role. A study comparing pediatric and young adult patients to older adults found that CALR mutations were roughly twice as common in the younger group, at about 20.5 percent versus 10.5 percent.¹⁷ Younger patients were also more likely to carry no detectable driver mutation at all and had a higher prevalence of familial myeloproliferative neoplasms. In pediatric primary myelofibrosis specifically, CALR mutations were found in half of tested patients, with a notable predominance of type 2 mutations.¹⁸ The preponderance of type 2 variants in children contrasts with adult primary myelofibrosis, where type 1 is more common, and it raises questions about whether pediatric disease follows the same prognostic rules as adult disease. So far, the data are too sparse to answer that definitively.

CALR Mutations as Clonal Hematopoiesis

Not every person who carries a CALR mutation has a blood cancer. The mutation can exist at very low levels in the blood of people who are otherwise healthy, a state known as clonal hematopoiesis of indeterminate potential. In a large Danish population study of nearly 20,000 people, 32 individuals were found to carry CALR mutations without any diagnosis of a hematologic malignancy.¹⁹ What made this study particularly interesting was the discovery that CALR-positive individuals had measurably lower kidney function compared to mutation-negative people, with a difference of about 5 to 6 units of estimated filtration rate. The effect was more pronounced in those with type 2 mutations or higher mutation levels, and it persisted over 11 years of follow-up.¹⁹ This is early evidence that CALR mutations may have consequences beyond the blood system itself, though the clinical significance of a modest kidney function difference in otherwise healthy people remains unclear.

Therapies Targeting Mutant CALR

Current treatment for CALR-mutated myeloproliferative neoplasms relies on the same drugs used for JAK2-mutated disease, primarily JAK inhibitors like ruxolitinib. When CALR-mutated patients with myelofibrosis start ruxolitinib, they tend to be younger and more frequently have a cytopenic phenotype, meaning lower blood counts rather than elevated ones, compared to JAK2-mutated patients.²⁰ The cytopenic profile can make treatment more challenging, since JAK inhibitors themselves can worsen anemia and low platelet counts.

Researchers are working on therapies that directly target the mutant calreticulin protein, which would spare normal cells. One promising approach involves a monoclonal antibody, called 4D7 in preclinical work, that recognizes the unique tail end created by the frameshift mutation. This antibody binds specifically to cells co-expressing mutant calreticulin and the thrombopoietin receptor, blocks the abnormal JAK-STAT signaling, and inhibits the uncontrolled growth of mutant cells. In laboratory models, it blocked the proliferation and differentiation of patient-derived progenitor cells carrying both type 1 and type 2 CALR mutations but had no effect on JAK2-mutated cells, and it prolonged survival in bone marrow transplant models of CALR-driven disease.²¹ The researchers characterized this as a way to target a mutation that would normally be considered “undruggable,” since the mutant protein does not have an obvious enzymatic pocket for a small molecule to block.

Vaccine and T-cell-based immunotherapy approaches have also been explored, but with more mixed results so far. One study that attempted to isolate T-cell receptors targeting mutant CALR fragments found no evidence that these fragments were naturally displayed on the surface of cancer cells in the context of the immune system’s recognition molecules, at least for the specific immune types studied.²² That does not rule out immunotherapy entirely, but it suggests that the immune system does not naturally “see” mutant calreticulin as easily as hoped, which complicates vaccine design.

Calreticulin on the Surface of Solid Tumor Cells

While CALR gene mutations are primarily a blood cancer phenomenon, the calreticulin protein itself plays a broader role in cancer biology. When cells are under stress or dying, particularly after certain types of chemotherapy or radiation, calreticulin migrates to the outer surface of the cell membrane. Surface-exposed calreticulin acts as an “eat me” signal, flagging the damaged cell for engulfment by immune cells called dendritic cells, which then present pieces of the cancer cell to the rest of the immune system. Higher levels of surface calreticulin on tumor cells have been associated with stronger anti-tumor immune responses and better outcomes across several solid tumors and blood cancers.²³ This is conceptually distinct from the somatic CALR mutations in myeloproliferative neoplasms. In the solid tumor context, the calreticulin protein itself is normal; it is the location and amount of the protein on the cell surface that matter. The connection is worth knowing, though, because researchers studying mutant calreticulin’s immune properties in blood cancers are drawing on this broader understanding of how calreticulin interacts with the immune system.