The Recently Suggested MDS/MPN Subgroup “Refractory Anemia with Ring Sideroblasts Associated with Marked Thrombocytosis (RARS-T)” Demonstrates Unique Clinical, Cytogenetic, and Molecular Aspects: A Comprehensive Analysis of 57 Cases Strictly Defined According to WHO Criteria

In the new WHO 2008 classification, “refractory anemia with ring sideroblasts associated with marked thrombocytosis” (RARS-T) represents a provisional entity defined by platelets ≥450 ×10⁹/l (being lowered from 600 ×10⁹/l; WHO 2001), proliferation of large megakaryocytes, bone marrow (BM) blasts <5%, and ring sideroblasts ≥15% of nucleated erythropoiesis. The separation of RARS-T from other myeloproliferative/myelodysplastic neoplasms is still under debate.

To further characterize this subtype and to evaluate whether its separate position in the WHO classification is justified from biologic/genetic aspects, we analyzed 57 patients with a diagnosis of RARS-T (strictly defined according to WHO 2008 criteria) for peripheral blood parameters, BM morphology, cyto-/molecular genetics, and clinical profiles. The study cohort consisted of 34 females and 23 males (median age, 76 years, range, 51–92 yrs; 52 de novo; 5 therapy-associated). At the time of analysis, all pts were at diagnosis or therapy naïve. Patients with a sole del(5q) or >5% of blasts were excluded according to WHO criteria.

All BM samples underwent May Giemsa Gruenwald and iron stainings. Chromosomal banding analysis (and FISH if needed) were performed in 56/57 cases. PCR was done for the following markers: JAK2V617F (investigated: n=47), MPLW515 (n=46), NRAS (n=24), TET2 mutations (TET2mut, n=14), MLL-PTD (n=13), FLT3-ITD (n=12), and CBL (n=16).

Median WBC count was 7.9 ×10⁹/l (range, 3.1–60.0 ×10⁹/l), median hemoglobin (Hb) level was 10 g/dl (range, 6–13 g/dl), and median platelet count was 572 ×10⁹/l (range, 454-1, 737 ×10⁹/l). The median ring sideroblast count was 60% (range, 18–92%). Karyotypes (KT) were as follows: normal KT: n=52 (52/56; 93%); +8: n=2; -Y: n=1. The most frequent mutation was the JAK2V617F (18/47; 38%); an MPLW515 mutation was detected in 3/46 (7%). From the 46 pts being analyzed both for the JAK2 as for the MPLmut, 21 (45.6%) were observed with one of both markers; there was no coincidence of the JAK2 and the MPL mutations. Furtheron, 5/14 (36%) had a TET2 mutation. Coincidences of molecular markers were observed in 3 pts who had a JAK2V617F and a TET2mut in parallel (TET2mut: 3/10; 30% in JAK2mut pts; vs. 2/4 in JAK2 wildtype pts; n.s.). No patient had a JAK2V617F and MPLW515 in parallel. There was no mutation of the NRAS, MLL-PTD, FLT3, or CBL genes in pts investigated for these markers. A positive JAK2V617F mutated status correlated significantly with higher platelets (p=0.038; T-test), whereas no significant correlations were observed for the respective medians taken as thresholds for leukocytes (≥7.9 ×10⁹/l vs. <7.9 ×10⁹/l vs.), Hb (≥10.0 g/dl vs. <10.0 g/dl), or ring sideroblast percentages (≥60% vs. <60%). All 3 pts with MPLW515mut had platelets ≥600×10⁹/l. Cytogenetic aberrations were independent from the JAK2mut status (normal karyotype: 17/45 JAK2mut; 38%; vs. aberrant KT: 1/2 JAK2mut; n.s.) and the MPLmut status (normal KT: 3/44 MPLmut; 7%; vs. aberrant KT: 0/2; n.s.). Higher WBC count (≥7.9 ×10⁹/l) was correlated to a higher Hb level (≥10 g/dl) (p=0.47) and to higher platelets (≥600 ×10⁹/l) (p=0.011). The patients with RARS-T had a favorable outcome with 84.6% being alive at 2.5 years.

Investigation of 57 patients strictly fulfilling the criteria of the WHO 2008 classification was able to confirm the unique profile of RARS-T in all aspects: patients with the RARS-T had a normal karyotype (>90% of all cases), had no prognostically adverse cytogenetic alterations, and frequently showed mutations of the JAK2 (V617F) or MPL (W515K/L) genes (45.6% in total). The molecular profile was even more homogeneous in RARS-T cases with ≥600 ×10⁹/l platelets (the WHO threshold from 2001) due to significantly higher proportions of JAK2V617F positive cases when compared to cases with platelets between 450 and <600 ×10⁹/l. However, from clinical aspects, patients with RARS-T had a favorable outcome in our study independent of the molecular state or the number of platelets. These data support to include RARS-T as definite subtype in the next edition of the WHO classification. The frequent occurrence of TET2 mutations in our cohort has to be noted for future diagnostic and classification approaches. Therefore, in cases suspicious for RARS-T but without evidence of a JAK2V617F, molecular screening should be performed including analysis for alterations of the TET2 and MPL genes.

Haferlach:MLL Munich Leukemia Laboratory: Employment, Equity Ownership, Research Funding. Kern:MLL Munich Leukemia Laboratory: Employment, Equity Ownership. Schnittger:MLL Munich Leukemia Laboratory: Employment, Equity Ownership. Haferlach:MLL Munich Leukemia Laboratory: Employment, Equity Ownership.