TET2 Mutations Are Not Specific for Certain MPN Entities but More Likely Seem to Indicate Disease Progression.

Abstract 438

TET2 mutations have recently been described in various myeloid malignancies. To further evaluate the role of TET2 mutations in myeloproliferative neoplasms (MPN) we have analysed 96 MPN that have been well characterized by cytomorphology, cytogenetics and molecular genetics. The cohort consisted of 53 males and 43 females with a median age of 64.9 years (range: 16.6-86.3 years). Diagnosis was ET (n=22), HES (n=5), PMF (n=12), PV (n=32), MPN unclassifiable (MPN-u) (n=25). The ET, PMF and MPN-u were mainly selected for unmutated JAK2 status. Cytogenetics was availabel in 94/96 cases (98%). All ET and HES cases had a normal karyotype. In MPN-u 3 of 25 (12%), in OMF 3 of 12 (25%) and in PV 4 of 31(12.9%) revealed chromosomal aberrations. In all cases a BCR-ABL rearrangement was excluded. In addition in all cases mutation analysis for JAK2V617F, JAK2exon12, MPLW515 and CBL was performed. The total cohort was composed of 39 cases with JAK2V617F (3 × ET, 6 × PMF, 27 × PV, 3 × MPN-u), 5 cases with JAK2exon12 (all PV), 4 cases with MPLW515 (3 × ET, 1 × PMF), 2 cases with CBL mutation (both MPN-u). TET2 mutations were analyzed by amplification and sequencing of 21 PCR fragments covering the total coding region. Within the total cohort 20/96 cases (20.8%) revealed a TET2 mutation. Two different TET2 mutations in parallel were detected in three cases: one with MPN-u and two PV with homozygous JAK2V617F mutations. Throughout the gene the mutations were distributed as follows: exon4 (n=11), exon6 (n=4), exon7 (n=3), exon11 (n=5). 14 were missense, 3 nonsense and 6 were frameshift mutations. To analyze a further potential gene defect based on a TET2 deletion 15/20 cases from which methanol/acidic acid fixed cells were availabel were also analyzed by FISH (fluorescence in situ hybridization) for TET2 deletions. No deletion was detected in any of these cases. Thus with the exception of three cases with two different mutations all other mutated cases probably have retained one intact TET2 allele. With respect to diagnostic entities the TET2 mutations were distributed as follows, ET: 2/22 (9.1%), HES: 1/5 (20%), PMF: 4/12 (33.3%), PV: 9/31 (29%) and MPN-u: 4/27 (14.8%). With respect to other molecular genetic markers the TET2 mutations were distributed as follows: JAK2V617F: 10/20 (50%) (PV: n=8; PMF: n=2) from which 7/10 had JAK2V617F with a high mutation load (classified on the absence of a JAK2 wildtype allele) (PV: n=7; PMF: n=1), JAK2exon12: 1/5 (PV), MPLW515: 1/4 (PMF), CBL: 1/2 (MPN-u) , FIP1L1-PDGFRA: 1/5 case. Thus, 14/20 TET2 mutated cases (70%) revealed a detectable second mutation, 7 (50%) of which even with a high JAK2V617F mutation load. Taking also cytogenetics into account three further cases revealed aberrations resulting in a total of 17/20 TET2 mutated cases (85%) that had genetic markers in addition. 8/20 (40%) even had two or more genetic events in addition to the TET2 mutation. 2/3 cases with two TET2 mutation also had a very high JAK2V617F load. And five high load JAK2V617F cases had only a 50% TET2 mutation load, indicating that JAK2V617F was the first mutation in these cases followed by TET2 mutation as a second hit. There was no independent correlation of TET2 mutation with any of the analyzed MPN entities (p=0.359, n.s.). On the other hand TET2 mutations are more frequent in cases with further mutations compared to those without any other mutation irrespective of diagnosis (p=0.059). These data indicate that TET2 mutations 1) occur in all different subtypes of MPN and thus are no markers that indicate a specific entity, 2) are associated with other genetic markers that are more specific for certain MPN entities like FIP1L1-PDGFRA for HES, MPL for ET and PMF, JAK2exon12 for PV , 3) seem to be more likely associated with progression of MPN e.g. accumulation of mutations at least in MPN.