Abstract
The natural history of Philadelphia negative chronic myeloproliferative disorders (Ph- CMPD) is characterized by a chronic phase that, in a fraction of cases, may progress to acute leukaemia. The JAK2V617F mutation occurs in 70–90% of polycythaemia vera (PV) and 30–50% essential thrombocythaemia (ET) and idiopathic myelofibrosis (IMF). The aim of this study was to verify the prevalence of JAK2V617F mutation in acute myeloid leukaemia (AML) evolved from Ph- CMPD. Bone marrow samples were derived from 5 PV, 10 ET and 5 IMF patients at the time of transformation to AML. In four cases, the corresponding chronic phase was also available. For comparison, 111 Ph- CMPD in chronic phase were also investigated, including 57 ET, 35 PV and 19 IMF. For each patient, BM samples were simultaneously investigated for JAK2V617F mutation by cDNA sequencing and mutation specific PCR. The JAK2V617F mutation was identified in 5/20 (25%) AML secondary to Ph- CMPD. In all mutated cases, the normal JAK2 sequence could be detected in addition to the mutated allele. JAK2V617F mutation occurred in 3/5 (60%) AML transformed from PV, 2/5 (40%) AML transformed from IMF, and 0/11 AML transformed from ET. In four AML transformed from ET, the corresponding chronic phase sample was also available. In three of these patients, both the ET and the AML phase tested negative for JAK2V617F mutation. One patient carried JAK2V617F mutation in the ET phase, but not in the AML phase. This same patient was investigated for karyotype and p53 mutations in both ET and AML phases. During the ET phase, the karyotype was 45,XY,-4,-20 and p53 mutations were absent. In the AML phase, the karyotype was 47,XY,+mar and p53 analysis revealed a Ser→Thr mutation at codon 303. JAK2V617F mutation was detected in 67/111 (60%) chronic phase Ph- CMPD, including 25/57 (44%) ET, 29/35 (83%) PV and 13/19 (68%) IMF. Several observations suggest that JAK2V617F mutation may not play a substantial role in leukaemic transformation of PV, IMF and ET. First, the prevalence of JAK2V617F mutation in AML transformation from PV and IMF is similar to that observed in chronic phase PV and IMF. Second, AML cases with JAK2V617F mutation carry a normal JAK2 allele in addition to the mutated allele, indicating that the JAK2V617F mutation burden does not increase during leukaemogenesis. Third, in our series, all cases of AML post ET are devoid of JAK2V617F mutation, a prevalence that is markedly different from that expected on the basis of JAK2V617F mutation analysis in chronic phase ET. Fourth, sequential analysis of the ET case that carried JAK2V617F mutation at ET diagnosis but not at the time of AML transformation, suggests that a JAK2V617F negative subclone may emerge during AML transformation of JAK2V617F positive ET. The possibility that AML in such case derives from a subclone that is genetically different from the predominant chronic phase population is also supported by the appearance during the AML phase of a less complex karyotype, characterized by the disappearance of chromosome 4 and 20 deletions, that were present in the corresponding ET chronic phase.
The Philadelphia Chromosome in an Unusual Case of Myeloproliferative Disease
