Myeloproliferative neoplasms (MPN)

The BCR/ABL1 negative myeloproliferative neoplasms are a heterogeneous group of clonal stem cell disorders. They are represented in the WHO classification by a number of different disease categories: classic MPNS (PV, ET and PM), MPN overlapping with MDS and Eosinophillic neoplasms. Cytogenetic abnormalities are usually not specific and general markers of myeloid malignancy are found. A normal karyotype remains uninformative but detection of a chromosome abnormality can be useful in diagnosing a clonal neoplastic disorder. The distinction of MPN from CML requires exclusion of t(9;22)/BCR/ABL1. This test should be done where morphology is suggestive of a non-classic MPN.
There are no specific cytogenetic abnormalities that will confirm transformation to myelofibrosis or acute leukemia although complex karyotypes involving chromosomes, 5, 7 and 17p are highly suggestive of transformation.
Cytogenetics/FISH is required to define WHO disease groups characterised by specific gene rearrangements (PDGFR or FGFR1 neoplasms) as these have the therapeutic options of TKI.
The most common MPN associated with PDGFRA rearrangement is that associated with FIPILI-PDGFRA formed as a cryptic deletion of 4q12. Presentation is generally as CEL. The karyotype generally tends to be normal or occasionally has a rearrangement of 4q12. A number of possible molecular variants of FIPILI-PDGFRA have been recognised in which there are other fusion genes incorporating part of PDGFRA. A distinctive type of myeloid neoplasm occurs in association with rearrangement of PDGFRB at 5q31-33. Most commonly this is a t(5;12)(ETV6-PDGFRB) rearrangement but other variants have been noted and can be present in CEL,CMML with eosinophillia, A-CML and JMML. Haematological malignancies with FGFR1 rearrangement at 8p11 can be found in both lymphoid and myeloid malignancy but presentation is most often CEL, AML OR T-LBL. In patients who present with CEL there may be subsequent transformation to AML, T or B lymphoblastic leukaemia or MPAL. JAK2 mutations (V617F or exon 12) are found in nearly all PV cases and 50% of ET and PM cases and needs to be done by molecular methods. Other molecular mutations such as MPL, TET2 and KIT are increasingly becoming important as potential therapy targets.
Chronic myeloid leukaemia is a myeloproliferative neoplasm of which the cytogenetic hallmark is the t(9; 22)(q34; q11) Philadelphia chromosome translocation that can be detected by chromosome analysis in 90-95% of cases at diagnosis. This translocation gives rise to a BCR/ABL1 gene fusion gene on the derived 22 (Ph chromosome). The remaining 5-10% of cases have variant translocations or cryptic rearrangement, the latter, which can be detected by FISH or RT-PCR. The BCR-ABL1 gene product is a cytoplasmic protein with enhanced tyrosine kinase activity which leads to constitutive activation of several signal transduction pathways causing CML. BCR/ABL1 tyrosine kinase inhibitors (TKIs ) have been developed as a first line treatment for patients with CML. These include Imatinab and Desatinab but there are also other treatments.
Response to treatment can be determined cytogenetically by a reduction in the proportion of abnormal cells by metaphase G-banded analysis or interphase FISH. Once the patient is in cytogenetic remission response to treatment is more accurately determined by RT-PCR.
The chronic phase of CML can last for many years. Additional chromosome abnormalities at diagnosis do not necessarily have any impact on patient outcome BUT according to ELN guidelines they act as a ‘warning as and these patients may require more stringent monitoring.
In cases where disease progression is suspected then G-banded analysis should be used to score for additional abnormalities to the t(9;22). These include, +Ph,+8,+19,+21,-Y, and i(17q). The presence of additional abnormalities may indicate disease acceleration or transformation to acute leukaemia (blast phase). If the bone marrow morphology is not consistent with transformation and additional abnormalities are found, then be cautious as these changes can be transient and of no clinical significance. If the changes are present in two consecutive samples this is a definition of treatment failure and a possible indication of transformation.
In some cases, new clonal abnormalities are occasionally detected in Ph negative cells in patients treated with TKIs. In particular, trisomy 8, abnormalities of chromosome 7 and sex chromosome abnormalities have been observed. These clones are usually of unknown significance but have occasionally been associated with myelodyspasia so these should be kept in mind when screening Ph negative follow-up cases.
FISH analysis should be done at diagnosis in the case of variant translocations to determine the diagnostic FISH pattern and in any cases where CML is suspected but G-banded analysis is normal. This laboratory routinely FISH all diagnostic CML cases. Deletion of ABL1, BCR or ABL1/BCR from the der(9) was considered an adverse marker prior to the introduction of imatinab therapy.