The CALR gene provides the blueprint for calreticulin, a multi-functional protein found throughout the body. Although present in many cell types, the gene’s mutation is primarily associated with myeloproliferative neoplasms (MPNs), a group of blood cancers. Discovered in 2013, this specific genetic change provided a molecular explanation for many MPN cases lacking previously known driver mutations. Understanding the CALR mutation and how it alters cell behavior is fundamental to diagnosing and classifying these disorders. The mutation status now influences a patient’s outlook and guides treatment decisions.
The Normal Role of Calreticulin and Mutation Types
Calreticulin is predominantly located within the endoplasmic reticulum (ER), where it performs a chaperone function. It ensures newly synthesized proteins fold correctly and helps manage calcium levels inside the ER, which is necessary for proper cellular signaling. The normal protein contains a specific sequence at its end, known as the KDEL motif, which acts as an ER retention signal.
The mutations associated with myeloproliferative neoplasms are acquired, developing during a person’s lifetime in blood-forming cells. These genetic changes are nearly always insertions or deletions (indels) occurring in exon 9 of the gene. This addition or removal of genetic material causes a frameshift, which changes the protein’s reading frame. The resultant abnormal protein loses its KDEL retention signal and develops a new C-terminal tail.
Two variants account for over 80% of all CALR mutations observed in patients. Type 1 is the most common, involving a 52 base-pair deletion, and Type 2 is a 5 base-pair insertion. Type 1 is generally more frequent than Type 2. Although more than 50 different CALR mutations exist, they all result in the same pathological outcome: a novel protein end rich in positively charged amino acids.
Pathogenesis: How the Mutant Protein Drives Cell Growth
The disease process begins when the altered calreticulin protein, having lost its ER retention signal, traffics to the surface of blood-forming cells. There, it binds to the thrombopoietin receptor, known as MPL. The MPL receptor normally requires the hormone thrombopoietin to activate its signaling pathway.
The mutant CALR protein’s positively charged tail allows it to bind directly to the MPL receptor without the natural hormone. This binding forces the MPL receptors to cluster, triggering constitutive activation of the downstream JAK-STAT signaling pathway. This continuous “always-on” signal acts like a permanent growth switch inside the cell.
This uncontrolled signaling drives the excessive proliferation and survival of the abnormal blood cell clone. The constitutive activation of the JAK-STAT pathway leads to the overproduction of specific blood cell lines, the defining characteristic of myeloproliferative neoplasms. The CALR mutation functions as a gain-of-function event, transforming normal cell signaling into a constant signal for uncontrolled growth.
Clinical Context: Associated Myeloproliferative Neoplasms
The CALR mutation is a primary driver mutation in Essential Thrombocythemia (ET) and Primary Myelofibrosis (PMF). It is the second most common driver mutation in these disorders, after the JAK2 V617F mutation. Its discovery allowed for the classification of patients who tested negative for JAK2 and MPL mutations.
In Essential Thrombocythemia, the CALR mutation is associated with a distinct clinical phenotype. Patients typically present with significantly higher platelet counts compared to those with the JAK2 mutation, and tend to have lower white blood cell counts and hemoglobin levels. In Primary Myelofibrosis, the mutation is associated with the proliferation of megakaryocytes and the eventual scarring of the bone marrow.
The CALR mutation is almost always mutually exclusive of the JAK2 and MPL mutations, meaning a patient usually harbors only one of these three primary driver mutations. The diagnostic workup for MPNs often involves a sequential search for these mutations. The CALR test is typically performed on patients whose initial JAK2 testing is negative, confirming the molecular pathogenesis of the neoplasm.
Diagnostic Testing for CALR Status
Testing for the CALR mutation is an established procedure for diagnosing and classifying myeloproliferative neoplasms. It is typically ordered as part of the molecular diagnostic workup for patients with suspected ET or PMF, especially after the absence of the JAK2 mutation is confirmed. The sample required is usually peripheral blood, though a bone marrow sample may also be used.
The standard detection method is a molecular technique, most commonly polymerase chain reaction (PCR). PCR-based methods are fast and sensitive, focusing on exon 9 to identify the characteristic insertions and deletions. For comprehensive analysis, next-generation sequencing (NGS) may be employed to detect all known and rare CALR indel variants. Identifying the specific mutation type (Type 1 or Type 2) is important because the variant can have different prognostic implications.
Prognostic Implications and Therapeutic Strategy
The presence of a CALR mutation carries specific implications for prognosis. Patients with CALR-mutated Essential Thrombocythemia and Primary Myelofibrosis generally experience a more favorable disease course compared to those with the JAK2 V617F mutation. This includes a lower risk of developing harmful blood clots (thrombosis), a major complication of MPNs.
The prognostic benefit may vary by mutation type; the Type 1 deletion is often associated with a better outcome than the Type 2 insertion. This status is important for risk stratification, helping clinicians determine if a patient falls into a low-risk or high-risk category for disease progression. Conventional therapeutic strategies include interferon-alpha, which has shown effectiveness in reducing the mutant CALR allele burden in some patients.
The unique mechanism of CALR-driven disease has opened the door for highly targeted therapeutic development. Since the mutant protein creates a novel C-terminal tail exposed on the cell surface, it acts as a specific target for new agents. Studies are exploring monoclonal and bispecific antibodies designed to bind specifically to the mutant CALR protein, disrupting its interaction with the MPL receptor. This approach aims to selectively eliminate the abnormal cell clone while sparing healthy cells.