We investigated the variation of CALR-mutant burden during follow-up in 105 CALR-mutant MPN and compared it to the variation of JAK2-mutant burden in 226 JAK2-mutant MPN. The median allele burden at last evaluation was significantly higher than at first evaluation in essential thrombocythemia (ET) (49.5% vs 45%, P < .001) but not in primary myelofibrosis (PMF) (52% vs 51%, P 0.398). Median values of slope were positive both in ET (0.071) and in PMF (0.032). In CALR-mutant ET there was a difference between natural and therapy-related slope (P 0.006). In the JAK2-mutated cohort, the median allele burden at last evaluation was not different respect to that at first evaluation, neither in ET (22.9% vs 23.2%, P = 0.216) nor in PMF (50.5% vs 45.0%, P = 0.809), despite a positive slope. Multivariate analysis to evaluate the effect of mutation (CALR vs JAK2) on the slope of mutant burden in not treated pts with a positive slope adjusting for diagnosis (ET vs PMF) showed a trend toward a higher increase of mutant burden in CALR vs JAK2 (β = 0.19, P = 0.061) with no difference between diagnosis (β = 0.419). The findings of this study suggest that clonal expansion in CALR-mutant MPN is faster than that observed in JAK2-mutant MPN.
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