Low-Burden TP53 Mutations Occur in Chronic Phase of Myeloproliferative Neoplasms Regardless of Hydroxyurea Administration, Disease Type, and JAK2 Status

Myeloproliferative neoplasms (MPN) are characterized by abnormal proliferation of myeloid lineages and a tendency toward leukemic transformation. Inactivation of tumor suppressor TP53 has been repeatedly associated with MPN transformation into secondary acute myeloid leukemia (sAML), but no fully expanded TP53 mutated clones in chronic phase of MPN were reported. The link between TP53 mutations and widely used cytoreductive treatment by hydroxyurea (HU) still remains controversial. To which extent TP53 mutations represent a risk of disease progression is not known. We aimed to search for low-burden TP53 mutated subclones in chronic-phase-MPN patients and to correlate presence of these clones with therapy, disease course, and other clinical features.

In total we analyzed 220 patients by ultra-deep next generation sequencing (NGS). We detected TP53 mutations in 39 (18 %) patients with variant allelic frequency of 0.1-16.3 % at the first examination. The analysis of 136 patients treated with hydroxyurea or other drugs (anagrelide (ANA), interferon α (IFN)) for more than 4 years, as well as a group of 84 patients untreated by cytoreductive drugs, showed that TP53 mutations occurrence in chronic phase is independent of hydroxyurea use, disease type, and JAK2/CALR/MPL status. Mutations were found in 17/72 (24 %) HU treated patients, in 11/64 (17 %) patients treated by other drugs, and in 11/84 (13 %) untreated patients. Median size of mutated clones was 0.5 % and was not influenced by previous treatment. In 10 patients we found more than one mutation.

In patients harboring TP53 mutations, retrospective samples were examined if available to explore the clonal evolution of TP53 mutated clones. The respective TP53 mutations were found in 13/20 cases analyzed; out of them, in 4 cases the mutation was found even in a diagnostic sample. Follow-up samples were examined in 28 patients with TP53 mutations and the mutation burden changed during the monitored time in majority of patients; however, the expansion into dominant clone was observed in one patient only. When all data from retrospective and prospective analyses taken together (30 patients), the median follow-up was 7.2 years. TP53 mutation burden tended to increase in 14 patients. In 6 patients the mutation burden remained stable and in 4 patients it fluctuated. In 6 patients the mutated clone size decreased; out of them in 2 originally very low-burden mutations (0.2 %) were not detectable in samples taken 15 and 3.4 years later. We did not observe any correlations of different patterns in TP53 mutation changes with therapy or other clinical characteristics (disease type, driver mutation, time from diagnosis, treatment response).

Further, we assessed the TP53 mutation impact on overall survival and leukemic transformation. The mutations did not negatively affect disease progression or overall survival either from diagnosis or from mutation identification. sAML developed in 2 patients with TP53 mutations 17.9 (treated with IFN) and 8.3 (treated with HU) years from diagnosis. In the latter patient, the sAML developed from a different clone as it was TP53-wt, JAK2-wt, although 2 TP53 mutated clones within JAK2 mutated clone were detected in chronic phase. On the other hand, we have observed another interesting case where the TP53 mutation burden grew rapidly from 10 % up to nearly 100 % during the follow-up. In contrast to published data, this patient did not show any clinical signs of disease progression for 2 years after the expansion and died of MPN unrelated cause.

In summary, using highly sensitive method we showed that low-burden TP53 mutations are present in MPN chronic phase. Neither their presence nor their size is associated with previous therapy and has impact on overall survival or leukemic transformation. Monitoring of TP53 mutations during the disease course showed that their clonal development is rather variable; nevertheless, TP53 minor mutations may represent a pool for future clonal evolution.

Supported by MZ CR-RVO (FNBr, 65269705), MUNI/A/1028/2015, H2020 692298, MZO AZV 15-31834A, 15-30015A, MEYS LQ1601 and LM2015064.