Tests In Support of Diagnosis

When JMML is suspected, tests are first done to rule out more common diagnoses such as acute myeloid leukemia (AML) or chronic myeloid leukemia (CML). If neither of these diagnoses are made, next generation sequencing to detect a Ras mutation in the leukemia cells is then performed as ~95% of JMML patients will have at least one mutation in the RAS pathway.

A bone marrow aspirate and or biopsy will be performed and several tests will be sent including cytogenetics and fluorescent in situ hybridization (FISH).  Cytogenetic testing can detect changes in the number of chromosomes (46 is considered normal) as well as “translocations” where one chromosome becomes fused to another. The diagnosis of AML can be made based on certain cytogenetic findings or if more than 20% of bone marrow cells are considered to be leukemia. JMML patients by definition have less than 20% leukemia cells in their bone marrow.

Genetic Tests for Specific JMML Mutations

Several specific genetic mutations are known to be associated with JMML. A mutation is a “mistake” in the patient’s DNA that can result in a change in normal cellular function. The coding sequence of portions of the patient’s DNA can be examined in the laboratory to look for the known JMML mutations. Mutations in genes including CBL, KRAS, NF1, NRAS, PTPN11, RRAS, RRAS2, and SH2B3 are the most common in JMML. It is important to determine whether any mutation that is detected is present in just the leukemia cells (typically blood or bone marrow) or also in normal cells (typically cheek cells, saliva or skin fibroblasts). This process is called a tumor-normal analysis and can be performed in several academic and commercial laboratories. Certain laboratories can also detect mutations in other genes like SETBP1, DNMT3A, ASXL1, EZH2, RUNX1, and ZRSR2 that can be present in JMML and predict a more aggressive disease course.

In the rare case where JMML is suspected but DNA sequencing does not reveal a Ras mutation, a test called “RNASeq” in a clinical or research laboratory is indicated.

Since the introduction of next-generation sequencing, certain older and less reliable tests including a granulocyte-macrophage colony-stimulating factor (GM-CSF)  hypersensitivity assay and STAT5 phosphorylation are no longer performed in clinical laboratories. These can still be performed in research laboratories but are no longer used to establish a clinical diagnosis of JMML.