Case Report: First longitudinal study of a patient with CALR positive clonal hematopoiesis of indeterminate potential developing into pre-fibrotic myelofibrosis

Abstract

Initial diagnosis of overt myeloproliferative neoplasms (MPNs) represents the juncture during clonal evolution when symptoms or complications prompt an afflicted individual to seek medical attention. In 30-40% of the MPN subgroups essential thrombocythemia (ET) and myelofibrosis (MF), somatic mutations in the calreticulin gene (CALR) are drivers of the disease resulting in constitutive activation of the thrombopoietin receptor (MPL). In the current study, we describe a healthy CALR mutated individual during a 12 year follow-up from initial identification of CALR clonal hematopoiesis of indeterminate potential (CHIP) to the diagnosis of pre-MF. The pre-diagnostic exponential development dynamics of the malignant clone demonstrated close correlation with the platelet counts, neutrophil-to-lymphocyte (NLR) ratio, and inversely correlated to hemoglobin and erythrocyte counts. Backward extrapolation of the growth rate indicated the potential for discovery of the malignant clone many years prior to presentation of overt disease, opening a window of opportunity for early treatment intervention. We did not find any additional mutations associated with MPNs and the current case report provides novel information regarding the development of a driver mutation and the association with blood cell counts prior to clinical manifestation of symptoms suggesting that pre-diagnostic dynamics may supplement future diagnostic criteria for early diagnosis and intervention in MPN patients.

Keywords: CALR; MPN; case report; clonal hematopoiesis; myelofibrosis.

Figure 1

(A) Normocellular bone marrow not diagnostic for myeloproliferative neoplasia, only a few megakaryocytes with myeloproliferative features in sample from 2017. (B) Hypercellular bone marrow with clusters of atypical megakaryocytes with myeloproliferative features without fibrosis in sample from 2022. (C) Reticulin staining in sample from 2017 reflecting a normal bone marrow. (D) Loose networks of reticulin staining grade 0-1 in bone marrow sample from 2022. (A, B) Hematoxylin and eosin staining; (C, D) Reticulin staining; magnification x400).

Figure 2

Exponential growth of CALR mutant allele burden from CH to MPN (A) Graph illustrating the mutant allele burden of CALR type 1 from September 2010 to November 2022 demonstrating exponential growth of the mutant allele burden and the platelet count. Note the close correlation between the CALR mutant allele burden and the platelet count once it exceeds 5%, including the transient drop even from 19% mutant alleles to 15% indicating a biological phenomenon rather than a technical discrepancy. The left Y-axis indicates the mutant allele burden, where black dots indicate CALR data points. The right Y-axis describes the number of platelets/µl and the red circles measurements of platelets. The dashed lines indicate the upper and lower boundary for the reference range of platelet count. (B) Backward extrapolation used to estimate when the mutation occurred. A heterozygous mutation in 1 out of 100.000 (dashed blue line) or 200.000 (dashed red line) stem cells.

Figure 3

Significant correlation of longitudinal CALR mutant allele burden with (A) platelet counts, (B) the neutrophil-to-lymphocyte ratio, (C) hemoglobin and (D) erythrocyte count. R squared and p values from Person correlation are shown in graphs. NLR, Neutrophil-to-lymphocyte ratio; Hb, Hemoglobin; eryt, Erythrocyte count.