Philadelphia chromosome and/or bcr-abl mRNA-positive primary thrombocytosis: morphometric evidence for the transition from essential thrombocythaemia to chronic myeloid leukaemia type of myeloproliferation.

AIMS

The incidence, bone marrow morphology and genetic features of bcr+ essential thrombocythaemia were investigated.

METHODS AND RESULTS

Sixty-four consecutive patients meeting the criteria of essential thrombocythaemia have been investigated for bcr-abl rearrangement and chimera mRNA expression. Reverse transcriptase-polymerase chain reaction indicated bcr-abl expression in six patients, in two of whom large fraction of the blood and bone marrow cells proved to be positive for Philadelphia chromosome (Ph) by fluorescent in-situ hybridization (FISH) and conventional cytogenetic analysis. In the remaining four patients FISH analysis could not detect Ph+ cells among the blood cells, but in one of these four patients conventional cytogenetic analysis indicated a very small fraction (2%) of Ph+ mitoses in the bone marrow (bcr+ essential thrombocythaemia patients). In three of these four patients, X-chromosome-linked clonality assay showed that the disease is of uncommitted stem cell origin. During an average of 57 month long follow-up no transformation to chronic myeloid leukaemia type of disease or acceleration/blastic crisis could be observed in the four bcr+ essential thrombocythaemia patients. They did not differ significantly from typical essential thrombocythaemia patients in quantitative indices of bone marrow cellularity or the size of megakaryocytes. In these two parameters as well as in the total nucleolus organizer region area per nucleus, however, significant differences could be detected between these four as well as typical chronic myeloid leukaemia patients. Statistical analysis of the morphometric data obtained from all six Ph+ and bcr+ essential thrombocythaemia patients combined indicated a shift of the bone marrow morphology towards the chronic myeloid leukaemia type of myeloproliferation.

CONCLUSIONS

These investigations indicate that bcr+ essential thrombocythaemia is infrequent among essential thrombocythaemia patients, and this condition resembles essential thrombocythaemia more than chronic myeloid leukaemia. Various expansions of the Ph+ clone appear to lead to either essential thrombocythaemia or, rather, chronic myeloid leukaemia type of myeloproliferation; however, data in the present study do not indicate that bcr+ essential thrombocythaemia would be a form fruste variant of chronic myeloid leukaemia.