CML Developed in a Japanese Family Transmitting a Novel Point Mutation in the Thrombopoietin Gene(TPO).

Introduction: Four families are reported to have hereditary thrombocythemia (HT) with a mutation in TPO. Their clinical manifestation is essentially thrombocytosis without leukemia. CML is one of myeloproliferative disorders, and shows leukocytosis and thrombocytosis associated with a proliferation of malignant clone originated from a hematopoietic stem cell (HSC). The incidence of CML is about 5 per 100,000 in Japan. Mutations of cytokine receptor including c-kit, flt-3 and G-CSF receptor are reported as a cause of AML. Especially flt-3 abnormalities are found in about 20% of AML. However, abnormality of c-mpl or TPO is not reported as a cause of leukemia. In this paper, we analyzed a CML case with novel point mutation in the TPO who still had thrombocytosis after cytogenetic complete response.

Case: Japanese, 35 y.o., male, complained leukocytosis. He had a family history of thrombocytosis in 4 individuals over 3 generations. A physical examination revealed a moderate splenomegaly. Laboratory tests at the time of diagnosis were as follows; WBC 141,000/μl (blast 1.8%, promyelo 2.4%, myelo 20.0%, meta 8.2%, stab 24.2%, seg 22.2%, immature eosinophils 1.8%, eosinophil 3.6%, immature basophils 0.4%, basophils 10.4%, mono 1.0%, lymphocytes 4.0%, erythroblast 3%), PLT 641,000/μl and NAP score 53 (nl; 156–271). Bone marrow showed hypercellularity with the increased megakaryocytes(Meg), bcr-abl fusion mRNA positive, Ph1 chromosome positive. After 5 months treatment with STI571, most of clinical findings including karyotype and fusion mRNA turned to be normal, but thrombocyte(PLT) still showed more than 1,000,000/μl. At this time, serum TPO concentration was 8.14 f mole/ml (nl; 0.40 +/− 0.28 f mole/ml, mean +/− SD). Genetic analysis of TPO revealed novel point mutation at splicing donor site of 3′-end of the exon3. A point mutation at splicing donor site is reported to cause an exon-skipping and intron-retention, which induce a malfunction of a suppressive post-transcriptional and translational regulation, and consequent high-level expression of functional TPO protein.

Discussion: TPO was cloned as a c-mpl ligand, which leads to the production of PLTs. Its receptor is a c-mpl proto-oncogene product, which is expressed not only in Meg, but also in HSC. Thus, TPO can stimulate HSC. The c-mpl transgenic mice are reported to have the increased Meg, its committed progenitor and PLT. Knockout mice of TPO presented not only the decreased Meg, but also multi-lineage committed progenitors. Thus, a modulation of c-mpl or its ligand function affects on both Meg and HSC. The c-mpl was cloned as a cellular homolog of a viral oncogene, v-mpl of myeloproliferative leukemia virus (MPL). The MPL causes myeloproliferative leukemia syndrome through v-mpl function in mice. Since v-mpl and c-mpl indicate high homology, it is possible that abnormal c-mpl function causes v-mpl like response. Through continuous stimulation of c-mpl signal, high TPO concentration may have induced a malignant transformation of HSC or supported a survival of an immature malignant clone in the present case. Improvement of thrombocytosis in CML is one of hematological responses to an anti-CML treatment such as STI571. In such a case who had good response other than thrombocytosis, an existence of HT might be considered. On the other hand, during following up HT family, occurrence of CML should be noted.