A CALR exon 9 mutation is a specific genetic change in the CALR gene that occurs in a type of bone marrow cancer called myeloproliferative neoplasms. It is found in most people with essential thrombocythemia and about half of those with primary myelofibrosis who do not have a JAK2 mutation. This mutation is not inherited — it develops during a person’s lifetime in a single blood-forming cell.
What Is A CALR Exon 9 Mutation?
The CALR gene provides instructions for making a protein called calreticulin. This protein normally helps fold other proteins and regulates calcium inside cells. It is found in almost every cell of the body.
Exon 9 is one specific section of the CALR gene. A mutation in this section changes how the calreticulin protein is built. The most common change is called a type 1 mutation, which deletes part of the gene. A type 2 mutation inserts a small piece of extra genetic code. Both types produce an abnormal calreticulin protein.
This abnormal protein does something the normal protein does not do. It activates a signaling pathway called JAK-STAT, which tells blood-forming cells to grow and divide. That constant signal is what drives the overproduction of blood cells seen in these conditions.
The mutation is somatic, meaning it happens in a single cell sometime after birth. It is not passed down from parents and cannot be inherited by children.
What Conditions Are Linked To CALR Exon 9 Mutations?
CALR exon 9 mutations are found almost exclusively in myeloproliferative neoplasms (MPNs). These are slow-growing cancers where the bone marrow makes too many blood cells.
The two main conditions linked to this mutation are:
- Essential thrombocythemia (ET): The bone marrow makes too many platelets. CALR mutations are found in roughly 15 to 25 percent of ET cases. They are more common in people who test negative for the JAK2 mutation.
- Primary myelofibrosis (PMF): Scar tissue builds up in the bone marrow, disrupting normal blood cell production. CALR mutations appear in about 20 to 30 percent of PMF cases.
CALR mutations are not found in polycythemia vera, another MPN. They are also not found in healthy people. If a CALR exon 9 mutation is detected, it strongly points to one of these two conditions.
In rare cases, CALR mutations have been reported in other myeloid cancers, but this is uncommon. The mutation is highly specific to MPNs.
How Is A CALR Exon 9 Mutation Detected?
Testing for CALR mutations is done through a blood or bone marrow sample. The most common method is polymerase chain reaction (PCR), which amplifies and detects specific DNA sequences. Some labs use next-generation sequencing, which can identify the exact type of mutation.
Doctors usually order CALR testing when a person has unexplained high platelet counts, an enlarged spleen, or other signs of an MPN. Testing typically happens alongside JAK2 and MPL mutation tests, since these three mutations are the main drivers of MPNs.
Finding a CALR mutation helps confirm the diagnosis and rules out other causes. It also gives doctors information about prognosis, which is discussed below.
What Does A CALR Mutation Mean For Prognosis?
People with CALR mutations generally have a better outlook than those with JAK2 or MPL mutations, especially in primary myelofibrosis. Research consistently shows that CALR-mutated PMF has a lower risk of progression to acute leukemia and longer overall survival compared to JAK2-mutated PMF.
In essential thrombocythemia, the picture is less clear. Some studies suggest that CALR mutations are associated with a lower risk of blood clots than JAK2 mutations. Other studies have not found a significant difference. The evidence here is mixed.
It is important to understand that prognosis depends on many factors beyond mutation status. Age, blood counts, spleen size, and other clinical features all play a role. Doctors use scoring systems that combine these factors to estimate risk.
Having a CALR mutation does not mean a person will definitely develop severe complications. Many people with ET live a normal lifespan with proper monitoring and treatment when needed.
How Are CALR-Mutated MPNs Treated?
Treatment for CALR-mutated MPNs depends on the specific diagnosis and the person’s risk level. Not everyone needs immediate treatment.
For essential thrombocythemia, treatment decisions are based mainly on age and history of blood clots. People at low risk may only need regular blood tests and monitoring. Those at higher risk may receive medications to lower platelet counts.
Common treatments include:
- Low-dose aspirin: Some clinicians recommend this to reduce clot risk, though evidence for its benefit in CALR-mutated ET specifically is limited.
- Hydroxyurea: A medication that reduces blood cell production. It is widely used but not specifically approved for CALR-mutated disease.
- Anagrelide: Another option to lower platelets, typically used when hydroxyurea is not tolerated.
- Interferon alfa: Sometimes used, especially in younger patients or during pregnancy.
For primary myelofibrosis, treatment focuses on managing symptoms and complications. Options include JAK inhibitors like ruxolitinib, which can reduce spleen size and improve symptoms. These drugs do not specifically target the CALR mutation.
No treatment currently cures CALR-mutated MPNs. Stem cell transplant is the only potentially curative option, but it carries significant risks and is usually reserved for high-risk cases.
Researchers are studying drugs that target the abnormal calreticulin protein directly. Early-stage trials are underway, but no such treatment is yet approved.
What Are The Symptoms Of CALR-Mutated MPNs?
Symptoms vary widely. Some people have no symptoms at all and are diagnosed through routine blood work.
When symptoms do occur, they may include:
- Fatigue or weakness
- Easy bruising or bleeding
- Headaches or dizziness
- Enlarged spleen, which can cause abdominal discomfort or early fullness
- Bone pain
- Itching, especially after a warm shower
- Night sweats or fever
These symptoms are not specific to CALR mutations. They can occur with any MPN. The mutation itself does not cause unique symptoms.
Some people with ET develop blood clots in the legs, lungs, or brain. Others may have bleeding problems. These complications are more common in certain risk groups.
Can CALR Exon 9 Mutations Be Inherited?
No. CALR exon 9 mutations are somatic, meaning they are acquired during life, not inherited. They occur in a single blood-forming cell and are not present in the person’s germline DNA.
This means the mutation cannot be passed to children. It also means family members do not need testing unless they have symptoms of an MPN.
In very rare cases, families have been reported with inherited MPNs, but these are linked to different genes, not CALR.
What Is The Difference Between CALR Type 1 And Type 2 Mutations?
Type 1 and type 2 are the two most common CALR exon 9 mutations. They differ in how they change the calreticulin protein.
Type 1 mutations delete part of the gene, removing a section that normally helps the protein bind calcium. Type 2 mutations insert extra genetic code, shifting how the protein is read.
Both types activate the JAK-STAT pathway, but they may behave differently. Some studies suggest that type 1 mutations are more common in primary myelofibrosis, while type 2 mutations are more common in essential thrombocythemia. Type 1 mutations may also be associated with a higher risk of progression to myelofibrosis in people with ET.
The evidence on how much these subtypes affect prognosis is still developing. Doctors do not currently use subtype information to make treatment decisions in most cases.
What Should You Do If You Have A CALR Mutation?
If you have been diagnosed with a CALR exon 9 mutation, the first step is to see a hematologist — a doctor who specializes in blood disorders. They can confirm the diagnosis, assess your risk level, and recommend monitoring or treatment.
Regular blood tests are important to track blood counts and watch for changes. Some people need treatment right away. Others can be monitored for years without intervention.
It helps to understand that CALR-mutated MPNs are generally slow-growing. Many people live for decades with these conditions. The goal of treatment is to manage symptoms and reduce the risk of complications, not to eliminate the mutation.
If you are considering treatment, ask your doctor about the evidence behind each option. Some treatments are supported by strong clinical trials. Others are used based on clinical experience but have less definitive evidence.
Frequently Asked Questions
Is a CALR exon 9 mutation cancer?
It is not cancer by itself, but it is a key driver of myeloproliferative neoplasms, which are considered blood cancers. The mutation causes blood-forming cells to grow abnormally.
Can a CALR mutation disappear on its own?
No. Once a CALR exon 9 mutation develops, it persists in the affected cell line. It does not go away without treatment, and no treatment currently eliminates it.
Does a CALR mutation affect life expectancy?
For many people with essential thrombocythemia, life expectancy is close to normal. In primary myelofibrosis, CALR mutations are linked to better outcomes than JAK2 mutations, but survival varies widely based on other factors.
Should family members be tested for CALR mutations?
No. CALR exon 9 mutations are not inherited, so family members without symptoms do not need testing. Testing is only recommended when there are signs of a blood disorder.
