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Mutations in ET treated with imetelstat
tracked with the driver mutation. Of note, ASXL1 muta- tions were also responsive to imetelstat treatment, although one ASXL1-mutated patient later lost response and transformed to myelofibrosis with an acquired TET2 mutation. This is in contrast to a study on imetelstat in myelofibrosis patients that reported a lack of response among patients with ASXL1 mutations.18
With regard to the additional mutations, we observed several patterns of response. The parallel decrease of one or more mutations with the driver mutation in five of nine patients suggests that coexistence of mutations in the same clone or subclone was frequent in our cohort of patients. Unfortunately, we were not able to track the clonal architecture or coexistence of mutations within a cell due to the lack of additional cell material.
Discrepant patterns of response were seen in patients with multiple mutations that were responsive or persist- ent, with DNMT3A, SF3B1 and TP53 mutations persisting or increasing over time, suggesting the presence of inde- pendent clones. It has been reported that DNMT3A muta- tions are often present in preleukemic clones and persist during therapy in myeloid malignancies, e.g., in acute myeloid leukemia.30,31 In this study, the four patients with DNMT3A mutations were 78, 80, 84 and 87 years old at study entry, and had had ET for 1, 2, 25 and 20 years, respectively. Since they were significantly older than the patients without DNMT3A mutations (mean age at study entry 82 years vs. 64 years, P<0.05), antecedent age-related clonal hematopoiesis (ARCH/CHIP) may be a contribut- ing factor.32-34 Individuals with ARCH/CHIP have a high risk of developing a hematologic malignancy. Experiments in mouse models carrying loss-of-function mutations in DNMT3A or TET2 suggest a competitive advantage and enhanced self-renewal capacity of the mutant stem cells leading to clonal expansion.34
Of the other non-responsive mutations in this study, mutations affecting the splicing factor SF3B1 are uncom- mon events in ET, reported to occur in 5% or fewer.12,14,35 They have been considered as “adverse mutations” based on their negative impact on myelofibrosis-free and overall survival.12,14 TP53 mutations in MPN were described to be present for several years at low allelic burden and, after loss of the wild-type TP53 allele, clones expanded rapidly resulting in leukemic transformation.8,36 Hence, the pres- ence of TP53 mutations may be a warning of leukemic transformation in MPN.10
The presence of additional mutations per se, specific mutations and the total number of additional mutations have been associated with inferior response to treatment with interferon-α and ruxolitinib.15-18,37 In contrast, imetel- stat treatment led to a high proportion of MMR in patients with or without additional mutations, although the latter patients had more reduction of mutant allele burden.
Furthermore, initial mutant allele burden may have an impact on response as high-level additional mutations at study entry correlated with shorter duration of response.
Overall, this detailed molecular analysis of heavily pre- treated and resistant ET patients reveals high individual patient complexity, with half of the patients harboring up to five additional somatic mutations at study entry. These results raise the question of whether additional mutations were acquired prior to diagnosis or whether mutational events were induced during treatment with prior thera- pies. Additional studies are needed to address this ques- tion.
In conclusion, treatment with imetelstat led to rapid and sustained hematologic and molecular responses and addi- tional mutant allele burdens were also reduced. However, additional mutations significantly reduced the depth of response and had an impact on duration of response. Of acquired mutations with known adverse prognosis and/or risk for transformation to myelofibrosis or acute myeloid leukemia, ASXL1, EZH2 and U2AF1 mutations were responsive to imetelstat, while SF3B1 and one of two TP53 mutations persisted. These data emphasize imetel- stat’s potential to inhibit neoplastic clones in patients with ET.
Disclosures
EOL has received research funding and honoraria from Geron; BB has acted as a consultant for Geron; OO has received research funding from Geron; MMcD is a current employee of Abbvie; AR has received research funding and honoraria from Geron and honoraria from Janssen; DSS has participated in advisory board work for and received honoraria from Gilead; GB has received research funding and honoraria from Geron and honoraria from Janssen. The other authors have no competing interests.
Contributions
EOL and GMB designed the study and wrote the manuscript; MH performed research; EOL, MH, BB and GMB analyzed and interpreted data; OGO, GS, OO, MAM, AR, DSS and GMB provided clinical data; BB performed statistical analysis and all authors read and approved the manuscript.
Acknowledgments
The authors would like to thank the patients, caregivers and staff who participated in this study, and Ingrid Helsen, Dania Hiltbrunner and Barbara Hügli for technical assistance at the Laboratory of Hematopoiesis and Molecular Genetics, Department of BioMedical Research, University of Bern.
Funding
This investigator-initiated and -driven study was supported by research funding from Geron to GMB and EOL.
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