TET2 Mutations Are Frequent in RARS-T.

Abstract 3794

Poster Board III-730

Refractory anemia with ring sideroblasts and thrombocytosis (RARS-T) has been considered a provisional subtype within the diagnostic entity of myelodysplastic/myeloproliferative neoplasms (MDS/MPN). Since JAK2 V617F and MPL W515L mutations are present in a significant proportion of RARS-T patients, many investigators consider this entity to be more closely related to classical MPN. However, a significant minority of patients with RARS-T do not display either JAK2 V617F or MPL W515L mutations. We have studied a cohort of patients with RARS-T (N=20) characterized by the presence of ring sideroblasts, reticulin fibrosis and thrombocytosis (>450×10⁹/L), that lack obvious causes of secondary thrombocytosis. While 8/20 patients harbored the JAK2 V617F, and 3/20 the MPL W515L mutations, the molecular pathogenesis for the remaining 9 patients was unexplained. Activation of JAK2 and MPL is associated with aberrant phospho-STAT5. Cases positive only for phospho-STAT5 may harbor other related, so far unidentified mutations. Many groups have recently observed a frequent area of somatic uniparental disomy (UPD) at 4q24, most commonly encountered in patients with chronic myelomonocytic leukemia (CMML), MDS/MPN, some typical MDS, and secondary acute myeloid leukemia (sAML). Overlapping microdeletions and UPD on 4q24 pointed towards possible mutations in the TET2 gene; such mutations were subsequently found in myeloid malignancies, most significantly MPN and MDS/MPN. Based on these findings, and the established correlation of RARS-T with JAK2 V617F and MPL W515L mutations, we evaluated the mutational status of TET2 in RARS-T patients. SNP-A allowed detection of copy neutral loss of heterozygosity (CN-LOH), such as UPD9p, which is associated with the JAK2 V617F mutation, and UPD1p, associated with MPL W515L. SNP-A facilitated detection of previously cryptic lesions; 11/20 patients showed an abnormal SNP-A-based karyotype (only 3 of these defects were also detected by MC). The new lesions seen by SNP-A included various UPD, such as, 1p, 2p, 3q, 6p, 8p, 9p and 10p. The presence of UPD9p/1p was consistent with homozygous JAK2 V617F/MPL W515L mutations. Likely, duplication of mutated alleles constituted a further permissive event during clinical evolution. However, none of the patients showed a somatic LOH at 4q24, suggesting that biallelic TET2 mutations were not involved in the pathogenesis of RARS-T. Simultaneously, lack of UPD11q suggested that CBL mutations were absent. Indeed, Cbl ring finger domain mutational screening revealed no mutations. An aberrant phospho-STAT5 staining pattern was present in all cases that were positive for either JAK2 V617F or MPL W515L mutations (N=10). However, 4 patients demonstrated abnormal megakaryocytic STAT5 phosphorylation, despite the absence of both JAK2 V617F and MPL W515L mutations. Within this group, a monoallelic TET2 mutation, delC 1480Sfs, was identified. In addition, we found a group of 5 patients without either JAK2 V617F or MPL W515L mutations, and also without association of the aberrant phospho-STAT5 staining. One of these patients had a monoallelic TET2 V1718L mutation; interestingly, another patient's specimen showed two novel non-synonymous SNPs: Y867H and P1723S. In total, 2/19 (11%) patients harbored TET2 mutations. These findings indicate involvement of TET2 mutations in RARS-T pathogenesis. RARS-T cases with MPN-associated mutations may not show obligatory phospho-STAT5 staining. The majority of patients were characterized by lack of splenomegaly, decreased white blood cell counts, increased thrombocytosis, and a normal karyotype. In summary, the majority of RARS-T patients harbor JAK2 V617F and MPL W515L mutations that strongly activate STAT5 phosphorylation. We described herein the third most common mutation in RARS-T, which can occur with or without abnormal STAT5 activation.