MLL Exons 4-8 Duplication in a Patient with Calr-Mutated Essential Thrombocythemia

Background: About 25% of individuals with essential thrombocythemia (ET) harbor a somatic mutation of the cal reticulin (CALR) gene which is thought to be the primary driving factor for the myeloid clone in those cases. Different secondary mutations have been described in association with CALR defects, however to our knowledge mixed-lineage leukemia (MLL) gene alterations have not been described before in this setting before progression to MDS or AML.

Case Report: A 68 year old woman with a past medical history significant for thyroid cancer in remission following treatment with radioactive iodine was initially noted to have a platelet count of 629 K/mcL at the occasion of a routine CBC (normal=160-400 K/mcL). Total white blood cell count, differential and hemoglobin levels were normal. Platelet count had been normal at 377 K/mcL about one year prior to presentation. Testing for mutations of the JAK2 and MPL genes came back negative. There was no splenomegaly; initial bone marrow biopsy showed mild hypercellularity with maturing trilineage hematopoiesis and atypical megakaryocytosis without an increase in blasts. A very small population of clonal B-cells was detected (<2% of total). Cytogenetic analysis did not reveal any chromosomal rearrangement. The patient was started on hydroxyurea (HU) 500 mg PO daily one month after initial presentation. There was evidence of mild iron deficiency, which was corrected with administration of IV iron sucrose at a total dose of 400 mg.

Four months after starting HU the platelet count had decreased to 543 K/mcL. Repeat bone marrow biopsy around that time showed mild hypocellularity (10-20%) along with persisting normal maturing trilineage hematopoiesis and atypical megakaryocytosis. There was no increase in reticulin fibers and blast count was normal. Stainable iron was present but no ring sideroblasts were noted. Karyotype was still normal and FISH revealed no evidence of deletion 5q, monosomy 5, deletion 7q, monosomy 7, trisomy 8, 11q23 translocation or 20q deletion. Given the absence of clonal defect and the lack of symptoms, HU was withheld following which the patient was observed closely. The platelet count then increased progressively, reaching a maximum of level of 977 K/mcL about 2 months after discontinuing cytotoxic treatment. The drug was then resumed and testing with the FoundationOne Heme^TM panel was obtained, looking for a select list of base substitutions, insertions, deletions, copy number alterations and other DNA rearrangements for more than 400 genes known to be somatically altered in hematologic malignancies.

This assay revealed the presence of an acquired anomaly of the MLL gene, consisting of a duplication of exons 4-8. Subsequent testing specific for the CALR gene also revealed an exon 9 insertion/deletion. HU was progressively increased, up to a dose of 1000 mg alternating with 1500 mg daily. With this treatment, the platelet count has decreased to 396 K/mcL one year after presentation. Total white blood cell count, differential and hemoglobin have remained normal. The patient is also taking aspirin 81 mg daily and remains clinically stable with no B symptoms, bleeding or thrombotic manifestations.

Discussion: Since mutations of the CALR gene were found to be associated with ET and myelofibrosis, several secondary genetic defects were demonstrated. Those are thought to represent clonal evolution from a primary subset of cells carrying either the JAK2 V617F mutation or a CALR exon 9 alteration. To our knowledge, MLL gene mutations have not been described as early “hits” in myeloproliferative neoplasms. However they have been abundantly documented in myelodysplastic syndrome and acute myeloid leukemia (AML), where they are often a marker of poor prognosis.

Conclusion: This is the first report of MLL exons 4-8 duplication in an individual with CALR-mutated ET. Interestingly, the patient presented here did not exhibit any neutrophilia, dysplasia or increase in bone marrow blasts. It remains unclear if in this setting such a genomic alteration confers an increased risk of progression to MDS or AML.