The JAK2-V617 Mutation Status Identifies Subtypes of Refractory Anemia with Ringed Sideroblasts Associated with Marked Thrombocytosis.

Sideroblastic erythropoiesis is a feature of pure refractory anemia with ringed sideroblasts (RARS), but may also be associated with megakaryocytic proliferation and marked thrombocytosis (RARS-T). Recently, the identification of a JAK2-V617F mutation in small series of patients has suggested that RARS-T is a JAK2 mutation-related disorder. The aim of our study was to correlate the presence of JAK2-V617F with clinical and hematological features in a larger cohort of patients with RARS-T. An allele-specific PCR for JAK2-V617F genotyping was carried out on DNA samples extracted from bone marrow biopsies obtained from 28 patients (mean age 71 years, mean follow-up 72 months). Our assay measures the allelic ratio of the mutated JAK2 allele, i.e. the percentage of JAK2 loci with the mutated dT-nucleotide (%T).

We detected the JAK2-V617F mutation in 12/28 patients (43%) with a predominance of the female gender (8/12). In 7 patients, we found %T<50% (range 16 to 43) at first presentation compatible with a heterozygous JAK2-V617F mutation. Since we are analyzing a mixture of cells, we cannot exclude the presence of a small population of cells that are already homozygous for JAK2-V617F. In 4 cases, the mutant allele predominated (%T> 50%), demonstrating cells homozygous for JAK2-V617F at initial diagnosis. In one patient the allelic ratio of mutated JAK2 increased from 0 to 43 and 98 when biopsies obtained 8 and 16 months after the initial diagnosis were analyzed. At the time when JAK2 V617F was negative, the patient fulfilled the criteria of RARS but not yet of RARS-T since the platelet count was < 500 x 10^9L. The acquisition of the JAK2V617 mutation was accompanied by a progression into RARS-T. In another patient, the ratio increased from 19 to 72 within 14 months. In both cases, the increased ratio of mutated JAK2 was associated with rising platelet counts.

On histological evaluation, all JAK2-V617F positive cases displayed a megakaryocyte morphology in keeping with a typical myeloproliferative phenotype. In 4 of the negative cases, a predominance of small hypolobated forms indicated myelodysplastic features.

With regard to the mutation status, no differences were found for platelet counts, spleen size, marrow fiber score or vascular events. However, V617-positive patients had significantly higher erythrocyte values (p = 0,003), lower MCV (p = 0,0003) and higher leukocyte counts (p = 0,034). The mean survival time of patients in the positive group was 5.14 years versus 6.43 years in the negative group. The mutation-positive patients had a relative death risk which was 82.3% lower than that of the positive group (relative risk of JAK2-V617F positive versus negative 0.177; 95% confidence interval: 0.040–0.789; p = 0,023). Our results suggest that RARS-T may be divided into two biologically distinct subgroups according to the JAK2-V617F mutation status with a more favorable prognosis for the positive disease. An evolution from V617 heterozygosity to homozygosity during the course of RARS-T may occur in a subset of patients.