The Calreticulin ($CALR$) gene codes for the Calreticulin protein. Mutations within this gene, particularly in a specific region known as Exon 9, are strongly associated with a group of blood cancers called myeloproliferative neoplasms (MPNs). Understanding the nature of the $CALR$ Exon 9 mutation is important for the diagnosis and therapeutic management of these blood disorders.
Function of the Normal CALR Protein
The Calreticulin protein acts as a chaperone within the cell, primarily operating inside the endoplasmic reticulum (ER). The ER is a network of membranes responsible for synthesizing and modifying proteins. Calreticulin resides within the ER lumen, where it assists newly synthesized proteins in attaining their correct three-dimensional structure.
A major part of this quality control process is Calreticulin’s function as a calcium-binding lectin. It recognizes specific sugar chains, or glycans, on nascent proteins, ensuring they are folded correctly before they are allowed to move on to the next stage of cellular processing. If a protein is misfolded, Calreticulin temporarily holds it back, preventing faulty material from leaving the ER.
Beyond its role in protein folding, Calreticulin also acts as a reservoir for calcium ions within the ER. Calreticulin helps regulate the concentration of this ion, which is necessary for a wide range of cellular signaling pathways. This dual function of protein quality control and calcium storage makes Calreticulin an important component.
Genetic Structure of the Exon 9 Mutation
The $CALR$ Exon 9 mutation is a genetic alteration occurring within the last exon of the gene, which codes for the protein’s C-terminus. The mutations are characterized by small insertions or deletions (indels) within the DNA sequence.
These insertions or deletions are not multiples of three bases, which make up a single codon. Consequently, the indel causes a frameshift mutation, shifting the reading frame for all subsequent codons in the gene. This results in a completely new sequence of amino acids at the C-terminus of the Calreticulin protein, replacing its normal structure.
While many variations of $CALR$ Exon 9 mutations have been identified, two specific types account for the vast majority of cases. The most common is the Type 1 mutation, which is a 52-base pair deletion in the DNA sequence. The second most frequent is the Type 2 mutation, characterized by a 5-base pair insertion. Together, these two primary types are found in approximately 85% of all patients who test positive for a $CALR$ mutation.
The resulting mutant protein retains the normal N-terminal domain responsible for its chaperone function. However, the frameshift mutation replaces the normal C-terminus, which typically contains a negatively charged sequence necessary for ER retention, with a novel, positively charged sequence. This altered C-terminus drives the disease mechanism.
Link to Myeloproliferative Neoplasms
The $CALR$ Exon 9 mutation is a driver mutation for two specific types of myeloproliferative neoplasms (MPNs): Essential Thrombocythemia (ET) and Primary Myelofibrosis (PMF). These mutations are typically found in patients who do not have the more common $JAK2$ or $MPL$ mutations, making $CALR$ an important diagnostic marker in the workup for MPNs.
The mechanism by which the mutant $CALR$ protein causes MPNs centers on its interaction with the Thrombopoietin Receptor ($TPO-R$), also known as $MPL$. The novel, positively charged C-terminus of the mutated Calreticulin protein is thought to bind to and activate the $TPO-R$ on the surface of hematopoietic progenitor cells. This interaction is abnormal, as the normal Calreticulin protein does not interact with this receptor.
The binding of the mutant Calreticulin to the $TPO-R$ leads to the receptor’s constitutive activation, meaning it signals continuously without its natural ligand, thrombopoietin. This process results in the uncontrolled activation of the Janus Kinase 2 ($JAK2$) signaling pathway. The overactive signaling pathway promotes the excessive growth and proliferation of blood cell precursors, particularly those that give rise to megakaryocytes and platelets.
Prognostic Significance and Treatment Selection
The presence of a $CALR$ Exon 9 mutation helps guide management and predict disease course. In patients with Essential Thrombocythemia, having a $CALR$ mutation is generally associated with a more favorable prognosis compared to those with the $JAK2$ V617F mutation. $CALR$-mutant ET patients typically experience a lower risk of thrombotic events.
The specific type of $CALR$ mutation can also influence the disease’s progression. The Type 1 mutation, the 52-base pair deletion, is sometimes linked to a higher risk of the disease transforming into Primary Myelofibrosis. Conversely, in patients already diagnosed with PMF, the Type 1 mutation is often associated with a better overall survival compared to the Type 2 mutation or the $JAK2$ mutation.
Patients with $CALR$-mutant ET, who are at a lower risk for thrombosis, may be managed with aspirin. Those at higher risk based on clinical factors may require cytoreductive therapies like hydroxyurea to lower blood counts. Although the mutation is distinct from $JAK2$, therapies like $JAK2$ inhibitors, such as ruxolitinib, have shown similar benefits in symptom control for $CALR$-mutant patients.