Mast cell activation syndromes (MCAS) are a group of disorders characterized by the accumulation and/or increased activation of mast cells and release of various mediators in different tissues and organs. Mast activation syndromes may be clonal or non-clonal, partly based on the presence or absence of a mutation in the KIT gene called KIT D816V.1
Systemic mastocytosis (SM) is one of these disorders. It is a rare, clonal mast cell disease that can be difficult to diagnose. Nearly all SM patients (about 95%) have the KIT D816V mutation.2-4 This mutation can be detected in bone marrow, peripheral blood or extracutaneous organs.5
Historically, there have been barriers preventing consistent use of a bone marrow biopsy, including the limited sensitivity of the molecular assays needed to detect the mutation. This has improved with the availability of high sensitivity real-time qPCR (Polymerase Chain Reaction) assays, such as droplet digital PCR, which are able to detect the presence of this mutation more reliably in peripheral blood.6
At Blueprint Medicines, we designed the PROSPECTOR study (NCT0481136), an ongoing multi-center global prospective screening study, to better understand the epidemiology and the frequency of the KIT D816V mutation in peripheral blood in patients who presented with symptoms of systemic mast cell activation, but had not been diagnosed with SM. The primary endpoint of the study was the proportion of patients with presence of the KIT D816V mutation in peripheral blood who had evidence of systemic mast cell activation (MCA).
Of the 381 people randomized, 15 had detection of the KIT D816V mutation in peripheral blood. The frequency of patients positive for the KIT D816V mutation in this population suggests patients with MCA symptoms may be at a higher risk of clonal mast cell disease such as SM.
The research also uncovered some surprising findings. There was evidence that the digital droplet PCR using peripheral blood may not be adequate to fully identify the KIT D816V mutation in patients with lower mast cell burden. For example, there were some patients who tested negative for the KIT D816V mutation but showed increased serum tryptase without increased alpha tryptase gene expression, suggestive of clonal mast cell disease.
“We may need higher-sensitivity assays or cell enrichment procedures to increase the sensitivity of the assay from peripheral blood,” remarked Daniel Shaheen, Senior Medical Director of Global Medical Affairs at Blueprint Medicines. “That was a surprising but important finding, not just for the interpretation of the study, but also as it relates to the use of droplet digital PCR for clinical diagnosis, and for therapeutic monitoring.”
PROSPECTOR was also the first prospective, multi-center study to evaluate alpha tryptase gene copy number in patients with systemic mast cell activation. Increased alpha tryptase gene copies, also known as hereditary alpha tryptasemia, or HAT, have been associated with SM.
“It looks like we see around a quarter to a third of patients with an increased alpha tryptase gene copy number,” Shaheen said. “So, we’ve enriched our ability to find a population of patients with HAT, and we have also increased the likelihood of finding patients with clonal mast cell disease. We have yet to explore what this may mean for patients, or how to potentially incorporate it into current diagnostic algorithms.”
As the study continues, Shaheen and the research team hope to perform additional post hoc, sub-group analyses based on the preliminary findings, in particular examining whether the KIT D816V mutation is associated with any specific clinical manifestations, including specific symptoms of mast cell activation.
“Did patients with the mutation have specific clinical experiences? For example, did they always experience anaphylaxis from bee stings? Was the mutation associated with a higher grade or severity of anaphylaxis? We have the opportunity to retrospectively examine some of these associations to better characterize the impact of the D816V KIT mutation,” commented Shaheen.
The PROSPECTOR Study is a small step towards better understanding the genetic determinants of SM, ultimately leading to a faster diagnosis for those who may have struggled with symptoms for several years without a clear diagnosis.