Myeloproliferative hypereosinophilic syndrome (M-HES) is a blood disorder in which the bone marrow overproduces eosinophils, a type of white blood cell, due to a genetic mutation in blood-forming stem cells. Unlike other forms of hypereosinophilic syndrome where the cause is unclear or driven by immune cells, M-HES is a clonal disorder, meaning the excess eosinophils all trace back to a single abnormal cell line. The eosinophil count rises above 1,500 per microliter of blood and stays elevated, and without treatment, these cells infiltrate organs and cause progressive damage.
The most common genetic driver is a fusion of two genes called FIP1L1 and PDGFRA, created by a small deletion on chromosome 4. This fusion produces a protein that acts as a permanently “on” growth signal, telling the bone marrow to keep making eosinophils without the usual stop signals. The World Health Organization now classifies cases with this and similar gene fusions under a category called myeloid/lymphoid neoplasms with eosinophilia, essentially recognizing M-HES as a form of blood cancer rather than a mysterious inflammatory condition.
How It Differs From Other Types of HES
Hypereosinophilic syndrome is not a single disease. It breaks into several subtypes based on what’s driving the eosinophil overproduction, and distinguishing between them changes the entire treatment approach.
M-HES is overwhelmingly a male disease: men account for roughly 90% of cases. The lymphocytic variant (L-HES), by contrast, has a more balanced sex ratio, with men making up 55% to 65% of cases. The subtypes also differ in how they present. M-HES patients frequently develop an enlarged spleen, while L-HES tends to show up more with skin problems like eczema or hives. Respiratory symptoms are more common in idiopathic HES, the form where no underlying cause can be identified.
Treatment differs sharply. About 81% of M-HES patients receive a targeted therapy called imatinib, while most patients with idiopathic or lymphocytic HES start on corticosteroids (65% to 85% of cases). This is why getting the subtype right matters so much.
Blood Tests That Point Toward M-HES
Two blood markers are particularly useful for distinguishing M-HES from other subtypes. Serum tryptase, a protein released by mast cells, is elevated in 82% of patients who carry the FIP1L1-PDGFRA fusion gene, compared to only 20% of those without it. Vitamin B12 levels tell a similar story: 93% of fusion-positive patients have elevated B12, versus just 18% of fusion-negative patients. Extremely high vitamin B12 levels (above 2,000 pg/mL) appear exclusively in the fusion-positive group, making this one of the more discriminating markers available before genetic testing results come back.
The definitive diagnosis requires molecular testing. The FIP1L1-PDGFRA fusion is invisible on standard chromosome analysis because the deletion that creates it is too small to see under a microscope. Specialized tests, typically fluorescence in situ hybridization (FISH) or PCR-based assays, are needed to detect it. Testing also screens for rearrangements in related genes like PDGFRB, FGFR1, and JAK2, which can cause similar-looking diseases with different treatment implications.
Bone Marrow Findings
A bone marrow biopsy in M-HES typically shows a hypercellular marrow packed with eosinophils at various stages of development. One characteristic finding is the presence of spindle-shaped mast cell clusters, sometimes appearing as aggregates of 15 or more mast cells. This mast cell involvement is a hallmark that helps pathologists distinguish M-HES from reactive causes of eosinophilia, where the marrow may have more eosinophils but lacks these clonal features.
Organ Damage From Excess Eosinophils
Eosinophils carry granules filled with toxic proteins designed to kill parasites. When billions of excess eosinophils circulate for months or years, those same proteins damage healthy tissue. The heart and skin bear the brunt.
Cardiac involvement is especially common in FIP1L1-PDGFRA-positive disease. The damage unfolds in stages. First, eosinophils infiltrate the heart muscle, causing inflammation. This can progress to scarring of the inner heart lining, a condition called endomyocardial fibrosis. In its most advanced form, fibrosis stiffens the heart so severely that it can no longer fill properly, leading to restrictive cardiomyopathy. Blood clots can form inside the heart chambers, and scarring of the heart’s electrical system can trigger dangerous rhythm disturbances. Symptoms at this stage include shortness of breath, chest pain, cough, and palpitations.
Skin involvement appears in about 37% of HES patients overall. Manifestations range from itchy red bumps and hives on the trunk and limbs to more diffuse problems like widespread skin thickening or reddening. These skin changes are often the first visible sign that something is wrong, appearing before the diagnosis is made.
Treatment With Targeted Therapy
The discovery of the FIP1L1-PDGFRA fusion gene transformed M-HES from a difficult-to-treat condition into one of the most responsive blood cancers in medicine. Imatinib, a drug originally developed for chronic myeloid leukemia, blocks the exact protein that the fusion gene produces.
The response is often dramatic. In early clinical reports, some patients saw their eosinophil counts drop to undetectable levels within 7 to 10 days of starting treatment. Most responding patients achieved this at just 100 mg per day, a fraction of the 400 to 600 mg dose used in other blood cancers. If lower doses don’t work, the dose can be increased to 400 mg daily.
Long-term outcomes are encouraging. In patients with PDGFR-rearranged disease followed for a median of about 10 years, imatinib produced a 96% response rate, an 88% progression-free survival rate at 6 years, and a 90% overall survival rate at 10 years. These numbers make M-HES one of the most treatable forms of eosinophilic blood cancer, provided it’s identified correctly.
When Standard Treatment Isn’t Enough
Not all M-HES cases carry PDGFRA or PDGFRB rearrangements, and not all respond to imatinib. For patients with rearrangements in a gene called FGFR1, a targeted drug called pemigatinib was recently approved for relapsed or refractory disease.
For cases without a targetable gene fusion, options become broader but less precise. Hydroxyurea and interferon-alpha have shown efficacy both as initial treatment and in steroid-resistant disease. Mepolizumab, an antibody that neutralizes the eosinophil growth signal IL-5, proved safe and effective as a steroid-sparing agent in a placebo-controlled trial of 85 PDGFRA-negative patients. It is now FDA-approved for idiopathic HES. For aggressive disease that doesn’t respond to any of these approaches, chemotherapy or stem cell transplantation may be considered, though published experience is limited to small numbers of patients.
The overarching goal across all treatments is the same: bring eosinophil counts down before they cause irreversible organ damage, particularly to the heart. Patients with mild eosinophilia and no signs of organ involvement may be monitored closely without immediate treatment, but once symptoms or lab markers of tissue damage appear, therapy is started promptly.