Juvenile myelomonocytic leukaemia (JMML) is a rare myelodysplastic/myeloproliferative haematological neoplasm that occurs predominantly in young children, particularly children with one of a number of predisposing inherited conditions [1-3]. Both neurofibromatosis type 1 (NF1 mutation, about 10% of cases of JMML) and Noonan's syndrome (PTPN11 mutation) predispose to JMML.
Most affected children are under the age of 3 years but age of presentation ranges from early infancy to adolescence. Boys are affected about twice as often as girls. Children present with systemic symptoms and clinical features resulting from anaemia, neutropenia and thrombo cytopenia. Hepatomegaly and splenomegaly are usually marked and tonsillar enlargement is common. Lung infiltration leads to cough, tachypnoea and sometimes death from pulmonary insufficiency . In addition, there may be eczema and lymphadenopathy. Children with neurofibromatosis type 1 may have café-au-lait spots and children with Noonan's syndrome, abnormal facies and cardiac anomalies.
Haematological and pathological features
There is anaemia and thrombocytopenia. The anaemia is normocytic (the majority), macrocytic or microcytic (a small minority). The white cell count (WBC) is moderately to markedly elevated (Figures 14.1, 14.2, 14.3). The neutrophil count is usually increased. The monocyte count is increased, a count of at least 1 × 109/l being one of the criteria for diagnosis of this condition. There are increased numbers of neutrophil precursors including some blast cells (usually less than 5% and by definition always less than 20%). Eosinophilia and basophilia are less prominent than in Philadelphia (Ph)-positive chronic myeloid leukaemia. There are usually circulating nucleated red cells.
Peripheral blood film from a child with JMML showing a neutrophil, a promyelocyte and abnormal monocytes. MGG, high power.
Peripheral blood film from a child with JMML showing a neutrophil and highly abnormal monocytes (same child as Figure 14.1). MGG, high power.
Peripheral blood film from a child with JMML showing a dysplastic neutrophil and several blast cells. MGG, high power.
The bone marrow is hypercellular, due to neutrophilic and to a lesser extent monocytic hyperplasia (Figure 14.4). Dysery -thropoiesis and dysgranulopoiesis are common but Auer rods are not seen. Megakaryocytes are often decreased. Blast cells (plus promonocytes) are, by definition, less than 20%.
Bone marrow aspirate film from a child with JMML showing increased granulocyte and monocyte precursors and a dysplastic (binucleated and hypogranular) neutrophil. MGG, high power.
Haemoglobin F is often increased for age and there may be other features also ...