Chromosomal Abnormalities and TET2 Involvement in Therapy-Related Myelodysplastic Syndromes (t-MDS) and Acute Myeloid Leukemias (t-AML).

Abstract 2600

Poster Board II-576

In t-MDS/t-AML the chromosomal pattern is not only essential for prognostic evaluation and choice of therapy, but it also reflects the etiology of the disease, being strictly related to the type of previous anticancer chemotherapy. In fact, it has been highlighted that different alternative genetic pathways and cooperating genetic abnormalities play a pivotal role in the pathogenesis of t-MDS/t-AML (Pedersen-Bjergaard et al, 2006). Even more recently, it has been revealed that deletions or mutations of the TET2 gene, a tumour suppressor gene mapped at 4q24, are shared by many disparate myeloid disorders and are an early event in their pathogenesis (Delhommeau et al, 2009). TET2 mutations, identified by DNA sequencing, occur in 24% of t-MDS/t-AML patients, while its deletion, as identified by FISH, occurs in only 5% of patients.

Based on these findings, the present study employed FISH to establish the incidence of band 4q24 deletions/structural defects in a series of 89 t-MDS/t-AML examined between January 1993 and January 2009 and to estimate whether TET2 deletion was correlated with a peculiar genetic pathway or with particular clinical data. There were forty-five females and forty-four males, whose median age was 58 years (range 25–78). Twenty patients had previously been affected with Hodgkin's lymphoma, eighteen with breast cancer, twelve with essential thrombocytemia, seven with polycythemia vera, seven with non Hodgkin's lymphomas, three with ovary cancer and twenty-two with solid tumours. Nine patients had received radiotherapy (RT) only, forty-seven chemotherapy only and thirty-three both treatment modalities. Overall, alkylating agents (AA) were given to fifty-seven patients, topoisomerase inhibitors (TI) to twenty-three and antracyclines (A) to five patients. Patients treated with AA developed t-MDS after a median time of 62 months (range 55–76) and t-MDS had a median duration of 6 months (range 4–14). In contrast, patients treated with TI and A developed t-AML without a preceding t-MDS after a median time of 18 months (range 12–26). At our observation, eighty-one patients presented with t-AML, (according to WHO twenty-one patients were diagnosed as M0, nineteen as M1, ten as M2, three as M3, four as M4, three as M5 and four as M7) and eight patients as t-MDS (according to WHO five patients were classified as RA and three as RAEB-2). At diagnosis, 76 patients (85%) presented clonal cytogenetic abnormalities involving chromosome 5 only (23.6%), chromosome 7 only (28.9%), both chromosomes (25%) and recurring balanced rearrangements (15.7%). A structural defect of chromosome 4 was revealed by conventional cytogenetics (CC) in three patients. A der(4)t(1;4)(p22;q23), a deleted chromosome 4 and a t(3;4)(q21;q24) were revealed in one patient each. Up to now FISH with the 144B4 (mapped at 14q22.3), 810D13, 571L19, 414I7 (all mapped at 4q23), 356L5 and 16G16 (both covering the TET2 gene at band 4q24), 642P17, 788K3, 752J12 (all mapped at 4q24) and 66J16 (mapped at 4q25) probes was carried out in 13 patients. All these probes were obtained from BACPAC Resources Center at C.H.O.R.I. (Oakland, USA), labelled and applied as previously reported. The cut-off values for interphase FISH (i-FISH) were obtained from the analysis of 300 nuclei from ten normal samples and were fixed at 10%. The patient with the unbalanced t(1;4) translocation showed that 88% of interphase and mitotic cells had lost the 356L5, 16G16, 788K3 and 642P17 probes and had maintained the 752J12 and 66J6 probes. So, this patient presented a loss of the TET2 gene and of the 788K3 and 642P17 probes even if the breakpoint of the chromosomal translocation was localized at band 4q25. The other two patients presented a cryptic deletion of the 356L5, 16G16 and 788K3 probes.

In conclusion, our data confirm that in t-MDS/t-AML i) specific chromosomal defects are strictly related to the type of chemotherapy administered for a previous cancer and flag the alteration of disparate molecular pathways; ii) TET2 deletion as investigated by FISH is a rather rare event and does not seem to be correlated with any specific defect.