Biology & Genetics: Inherited Risk vs. Acquired Mutations

When a family experiences multiple cases of Primary Myelofibrosis (PMF) or other blood cancers, the natural question is: “What exactly was passed down?” In the vast majority of cases, family members do not inherit the disease itself. Instead, they may inherit a genetic background that is more susceptible to acquiring it. Understanding the difference between germline (inherited) and somatic (acquired) mutations is the key to understanding your family’s risk.

Inherited Germline Risk

A germline variant is a piece of DNA you are born with, inherited from your parents and present in every cell in your body ^[1]. In families with multiple cases of MPNs, members often share common “susceptibility alleles”—genetic variations that act as a nudge toward the disease rather than a guarantee ^[2].

• The JAK2 46/1 Haplotype: This is a very common inherited low-penetrance association. It is a specific pattern of DNA that makes your blood-forming cells more likely to develop certain mutations ^[1][3]. Carrying this doesn’t mean you have cancer or will definitely get it; it is a research and population association rather than a clinically actionable prediction of your relatives’ future ^[4].
• TERT Variants: Changes in the TERT gene, which helps manage the “caps” on the ends of your chromosomes, have also been linked to an increased risk of developing an MPN, but again these are low-penetrance risk factors ^[1][5].

The Spark: Acquired Somatic Mutations

The actual disease begins with a somatic mutation. This is a genetic change that happens by chance in a single blood-forming stem cell after you are born ^[6]. It is not inherited from your parents and you cannot pass it to your children ^[7].

In PMF, there are three main “driver” mutations that act as the spark:

• JAK2 V617F: Found in about 60% of PMF patients ^[8].
• CALR: Found in about 25% of patients ^[9].
• MPL: Found in about 7% of patients ^[8].
  (Note: These frequencies are population estimates and can vary based on testing methods.)

If two people in the same family both have the JAK2 V617F mutation, they did not inherit that mutation from each other ^[1]. Instead, they likely inherited the underlying susceptibility that allowed the somatic mutation to develop in both of them independently ^[2].

Rare Exceptions: True Pathogenic Germline Variants

While inherited susceptibility markers are common, “true” inherited MPN—where a single highly penetrant mutation passed from parent to child is the primary cause—is extremely rare ^[10]. These rare cases might involve specific inherited mutations in genes like THPO, MPL, or very specific versions of JAK2, which can cause a hereditary myeloid phenotype ^[10][11]. These are distinct from the common susceptibility alleles and often require specialized genetic counseling to manage ^[12].

The Importance of Proper Testing

If a genetic test on your blood shows a mutation at a 50% Variant Allele Fraction (VAF)—meaning it shows up in exactly half of the DNA tested—it might look like a germline variant you were born with ^[13][14].

However, in blood cancers, blood testing can be misleading. Because the disease itself changes your blood’s DNA, a mutation that looks inherited might actually be an acquired mutation that has simply taken over your blood system ^[14][15].

To prove a mutation is truly inherited (germline), doctors must test a non-hematopoietic (non-blood) sample ^[14]. This usually involves:

• A skin biopsy (culturing skin cells called fibroblasts), which is the preferred confirmatory source ^[15][16].
• Testing hair follicles or saliva (though these are not equally validated in every laboratory and can be affected by sample quality or contamination) ^[17].

Using these “clean” DNA sources allows doctors to see what you were born with versus what your blood cells acquired over time. Without this paired testing, it is impossible to be certain if a variant is a family trait or a symptom of the disease itself ^[13].