Calreticulin mutated prefibrotic-stage myelofibrosis and PMF represent an independent clone from coexisting CLL

To the editor:

JAK2^V617F-positive myeloproliferative neoplasms (MPNs), especially polycythemia vera (PV), may occur concomitantly with chronic lymphocytic leukemia (CLL). This association has not been described with a variant of essential thrombocythemia (ET), prefibrotic-stage myelofibrosis (preF-MF) or primary myelofibrosis (PMF) or with calreticulin (CALR)-mutated MPNs. JAK2^V617F has been reported in MPNs, but not in CLL clones.^1,2  We previously reported 3 unrelated women with concomitant PV and CLL, in which these 2 diseases used different X chromosome alleles, indicating their origin from different pre-JAK2^V617F hematopoietic stem cells.²  Herein, we present additional observations in 2 patients suggesting an independent relation between MPN and coexisting CLL clones.

The first is a 67-year-old man with thrombocytosis who presented 9 years ago with a platelet count of 707 × 10⁶/μL, a leukocyte count of 9.55/μL (lymphocytes 19%), and a hemoglobin level of 13.5 g/dL, but no history of thromboembolic events, aquagenic pruritus, erythromelalgia, lymphadenopathy, or splenomegaly. There was no evidence of reactive thrombocytosis or detectable JAK2^V617F or the thrombopoietin receptor (cMPL) mutations. Marrow was 90% cellular and consistent with preF-MF (Figure 1A-B). Although no increased lymphocytes were appreciated by marrow morphological evaluation, marrow and blood flow cytometry revealed a small CD5, CD19, dim CD20, CD23, dim κ light chain-positive B-cell clone accounting for ∼4% of lymphoid cells. This clone resembled CLL but was deemed to be of unknown clinical significance. Imaging studies failed to detect any evidence of lymphoproliferative disorder. The propositus remained asymptomatic and in hematological remission on hydroxyurea for the subsequent 9 years. On 3 successive occasions, the same small B-lymphocyte clone was consistently detectable at a low level in blood. After 9 years, the patient developed progressive lymphocytosis (white blood cells of 25.12 × 10³/μL, with 72% lymphocytes and 23% neutrophils, platelet count of 281 × 10⁶/μL, and hemoglobin level of 12 g/dL) and a moderately enlarged spleen but was asymptomatic with no lymphadenopathy. Marrow showed persistent morphology of preF- MF, with expansion of the previous B-cell clone to 30% of marrow cells (Figure 1C) and increased fibrosis, now judged “moderate” (2+). Flow cytometry again revealed the same B-cell clone, now comprising 48% of leukocytes and consistent with CLL.

The second case is a 69-year-old man with a 14-year history of CLL who recently was referred to us. A year ago, while his CLL was in complete remission after chemotherapy, he developed transfusion-dependent anemia and progressive splenomegaly and underwent a splenectomy resulting in a decreased but persistent red cell transfusion requirement. On referral, we found leukoerythroblastic peripheral blood morphology with dacrocytes, and his marrow also indicated PMF, without JAK2 or cMPL mutations.

Somatic CALR mutations have been identified in patients with ET and MF who lacked JAK2^V617F or cMPL mutations.^3,4  We detected a CALR 52-bp deletion type 1 mutation³  in the granulocytes of both patients, but the CD19-positive sorted CLL cells had no detectable CALR mutation (Figure 1D-F). It is not known if CALR mutations in MF and ET are disease-originating mutations or more similar to the JAK2^V617F-positive MPNs, wherein other somatic or germ line mutations are thought to precede the MPN phenotypes.⁵ 

CALR mutations are reported to be associated with a lower risk in MPNs and thus are likely to be acquired earlier,^3,4  raising the possibility that preF-MF and CLL may arise from the same pluripotent stem cell. However, the lack of CALR mutation in these patients’ CLL cells suggests that they represent independent clones. However, as the X chromosome–based clonality assays can be performed only in women, a common precursor origin of MPN and CLL clones cannot be completely excluded in these male patients.