What is mastocytosis?
Mastocytosis is the term for a diverse group of conditions where a single (or clonal) population of mast cells accumulates in one or more tissues, for example skin, bone marrow, liver, spleen, gastrointestinal tract and lymph nodes.
The severity of symptoms depend on the number of mast cells in the tissues. A high load of mast cells leads to more severe symptoms. Cutaneous mastocytosis causes itching, swelling and blistering of the affected skin, particularly when it is rubbed or scratched.
What is the cause of mastocytosis?
Most forms of mastocytosis are caused by a mutation of the KIT gene on the 4q12 chromosome – a mutation that increases cellular reproduction. The c-KIT gene mutation creates an overgrowth of one cell line of mast cells. This is also called a “clonal expansion” of mast cells and leads to areas of abnormal skin that easily reddens, swells and itches. The c-KIT mutation can also lead to proliferation of mast cells within the bone marrow, resulting in systemic mastocytosis.
In some cases, the genetic disorder is inherited but in most cases it is spontaneous and there is no family history of mastocytosis.
What are mast cells?
Mast cells are normal cells in the body, usually found in the skin and other tissues. Mast cells have a role in the early steps of the body’s coordination of healing responses to an injury. Granules within the mast cells contain histamine and other chemicals.
|Mast cell chemicals|
|histamine||leukotriene C4||prostaglandin D2||carboxypeptidase|
|heparin||cathespin G-like protease||tryptase||tumour necrosis factor-A|
When a mast cell is activated, these chemicals are released into the surrounding skin. Mast cell chemicals are mediators of inflammation, and cause the blood vessels to leak, resulting in localised itching, swelling, redness and sometimes blistering. This is what happens normally in insect bites and is thought to be a protective mechanism. For example, a mosquito injects saliva when it bites. The saliva triggers mast cell activation to a varying degree, depending on the individual's hypersensitivity to the saliva. The unpleasant itch soon persuades the person to try to avoid getting bitten again.
Rubbing an area of skin affected by mastocytosis may also activate the mast cells. The rubbed skin becomes reddened, swollen and itchy within a few minutes (Darier sign). In young children, the rubbed area may later blister.
How is mastocytosis diagnosed?
Cutaneous mastocytosis is usually diagnosed by its clinical appearance and positive Darier sign. However, skin biopsy may be helpful for confirmation.
Mast cells may be difficult to see on standard histology with haematoxylin and eosin staining (H&E). They can be identified with special stains, eg Giemsa, toluidine blue, Astra blue or immuno-histochemical stains like tryptase and CD117 (this stains for c-KIT membrane receptor on the surface of mast cells).
A blood test showing high levels of tryptase suggests systemic involvement.
What are the different types of mastocytosis?
Mastocytosis can be broadly characterised into two groups:
- Localised mastocytosis (localised to a single tissue)
- Systemic mastocytosis (involving one or more tissues)
The World Health Organisation (WHO) classifies mastocytosis as:
- Cutaneous mastocytosis
- Systemic mastocytosis
- Mast cell leukaemia
- Mast cell sarcoma
- Extracutaneous mastocytoma
All forms of cutaneous mastocytosis are characterised by a positive Darier sign.
Approximately two-thirds of cases of cutaneous mastocytosis occur in children.
Cutaneous mastocytosis is classified as follows:
- Solitary mastocytoma
- Maculopapular cutaneous mastocytosis
- Diffuse cutaneous mastocytosis
- Telangiectatic cutaneous mastocytosis (telangiectasia macularis eruptiva perstans)
- Plaque-type mastocytoma
- Solitary and multiple nodular mastocytoma
Solitary mastocytoma and maculopapular mastocytoma are the most common forms of mastocytosis in children.
A solitary mastocytoma present in infancy as an itchy area of reddish or yellowish-brown thickened patch of skin. Mastocytomas generally completely or largely resolve in time.
Maculopapular cutaneous mastocytosis
Maculopapular cutaneous mastocytosis was previously called urticaria pigmentosa. It is the most common form of mastocytosis in adults and children.
Maculopapular mastocytosis in adults is unlikely to resolve with time. It may rarely lead to systemic involvement, where the mastocytosis spreads to more than one tissue (see below).
Diffuse cutaneous mastocytosis
Diffuse cutaneous mastocytosis presenting in infancy usually presents as diffuse redness of the skin (erythroderma), sometimes with widespread blistering of the skin. Adults and adolescents with diffuse cutaneous mastocytosis often have a generalised thickening leathery appearance and texture to most or all of their skin, with a positive Darier sign.
Telangiectatic cutaneous mastocytosis
Telangiectatic cutaneous mastocytosis is also known as telangiectasia macularis eruptiva perstans (TMEP). Telangietatic cutaneous mastocytosis is very persistent and may sometimes lead to systemic involvement. The name relates to extensive telangiectases.
Telangiectatic cutaneous mastocytosis
- Indolent systemic mastocytosis
- Systemic mastocytosis with associated clonal haematological non-mast cell lineage disease (SM-AHNMD)
- Aggressive systemic mastocytosis
Systemic mastocytosis is diagnosed by the presence of a certain number of features: 1 major criteria and 1 or more minor criteria or, alternatively, the presence of 3 minor criteria. Diagnosis requires tissue biopsy and use of special stains.
- Multiple areas of dense infiltrates of mast cells in the bone marrow or other extracutaneous organs, confirmed by special stains on histology, such as mast cell tryptase (>15 mast cell aggregating)
- In mast cell infiltrates in the bone marrow or other extracutaneous organs, >25% of mast cells are spindle shaped or otherwise atypical OR in bone marrow smears >25% of mast cells are spindle shaped or otherwise atypical
- Activating point mutation of KIT in codon 816 is present in extracutaneous organs
- Mast cells in extracutaneous organs found using CD117 co-express either CD2 or CD25, or both, as determined by flow cytometry (an electronic test used to count and sort different types of cell)
- Serum tryptase is persistently >20ng/ml (this is not relevant in patients that have SM-AHNMD)
Systemic mastocytosis occurs more commonly in adults than children (it is extremely rare in children).
Indolent systemic mastocytosis
Indolent systemic mastocytosis is the most common and least serious presentation of systemic mastocytosis in adults – it is characterised by a low mast cell burden (<30%), the presence of mediator-related symptoms such as flushing and diarrhoea, and skin involvement (usually maculopapular cutaneous mastocytosis, see above).
Rarer subsets of indolent systemic mastocystosis
- Isolated bone marrow mastocytosis
- Well-differentiated systemic mastocytosis (round cells; compact, multifocal mast cell infiltrates without expression of CD25; KIT mutation outside of codon 816)
- Smouldering systemic mastocytosis (borderline or benign hypercellular marrow; dysplasia; marrow mast-cell infiltrates >30%; serum tryptase >200 ng/ml; and enlarged spleen, liver, lymph nodes without organ impairment
Systemic mastocytosis with associated clonal haematological non-mast cell lineage disease
SM-AHNMD accounts for one third of all systemic mastocytosis cases. It is assocated with concurrent myeloproliferative neoplasms (in which there is increased bone marrow production of blood cells) and myelodysplasia (in which the bone marrow produces too few blood cells).
Aggressive systemic mastocytosis
Aggressive systemic mastocytosis is due to uncontrolled neoplastic cell proliferation and leads to impaired organ function. It results in flushing, itching, low blood pressure, anaphylaxis, diarrhoea and bleeding from the gastrointestinal tract. These patients may have high serum tryptase levels.
Complications may include:
- Marked cytopaenia (low blood counts)
- Osteolysis or osteoporosis (thin bones, sometimes causing fractures)
- Weight loss
- Enlarged overactive spleen (which can destroy circulating blood cells)
- Enlarged liver with impaired liver function
- Ascites (fluid in the abdominal cavity)
- Portal hypertension (high pressures in the blood system running between the gut and the liver).
Other forms of mast cell proliferation
Mast cell leukaemia
Mast cell leukaemia is a rare disease of circulating malignant mast cells with a very poor prognosis.
Mast cell sarcoma
Mast cell sarcoma is a solid tumour of malignant mast cells.
Extracutaneous mastocytoma is a benign mast cell tumour where a localised area of mast cell proliferation is not in the skin.
How is mastocytosis investigated?
Patients with solitary mastocytoma generally require no work up. However, a full history should be taken, and skin, lymph nodes, liver and spleen should be examined to check for diffuse or systemic mastocytosis.
In these cases the following investigations may be undertaken:
- Skin biopsy
- Full blood count
- Serum biochemistry
- Liver function tests
- Serum tryptase level: tryptase is a mast cell enzyme. Systemic mastocytosis is unlikely with a tryptase level <20 ug/L.
- Abdominal ultrasound
- Bone density scan
- Bone marrow trephine
- Blood and marrow smears
- Flow cytometry (to measure CD2 and CD25)
- Genetic examination (looking for KIT mutation D816V in bone marrow mast cells)
How is mastocytosis treated?
Most localised forms of cutaneous mastocytosis without symptoms require no treatment.
Treatment for symptomatic patients with mastocytosis includes general measures to avoid triggers for histamine release. Special care should be taken with patients with systemic mastocytosis or diffuse cutaneous mastocytosis, especially if they have elevated serum tryptase, when under anaesthetic or having imaging requiring radio-contrast.
|Triggering agents in mastocytosis|
|Infectious diseases with fever|
Antihistamines may also be helpful for control/suppression of histamine-related symptoms in mastocytosis. Both H1 and H2 blockers are useful.
Patients with gastrointestinal symptoms from systemic mastocytosis may be treated with H2 histamine antagonists (cimetidine and ranitidine are some examples), proton pump inhibitors (eg omeprazole, lansoprazole, pantoprazole), or oral sodium cromoglycate.
Oral steroids may be used in some cases of systemic mastocytosis. They act as mast cell stabilisers.
In severe cases of systemic mastocytosis or aggressive forms of mastocytosis the following treatments alone or in combination have been used:
- Miltefosine has been used to inhibit mediator release and mast cell-driven inflammation
- KIT inhibitors midostaurin and dasatinib have been shown to inhibit D816V (one of the specific KIT mutations found in mastocystosis) and mediate KIT activation. Imatinib, masitinib, and bafetinib are unable to block D816V and are ineffective in mastocytosis
- Targeted antibodies against mast cells and mast-cell tryptase (under development)
- Interferon alpha
- Chemotherapy eg cladribine, cytarabine, fludarabine, and hydroxyurea
- Rapidly progressing aggressive systemic mastocytosis, mast cell leukaemia and mast cell sarcoma may be treated with polychemotherapy and/or bone marrow stem cell transplantation