Cbl and TET2 Mutations Are Present in Refractory Ph+ Disorders Including Accelerated and Blast Crisis CML and ALL.

Abstract 2173

Poster Board II-150

Loss of heterozygosity (LOH) due to acquired uniparental disomy (UPD) is a commonly observed chromosomal lesion in myeloproliferative neoplasms (MPN) and myelodysplastic/myeloproliferative neoplasms (MDS/MPN) including chronic myelomonocytic leukemia (CMML). Most recurrent areas of LOH point towards genes harboring mutations. For example, UPD11q23.3 and UPD4q24 were found to be associated with c-Cbl and TET2 mutations, respectively. Cbl family mutations (c-Cbl and Cbl-b) have been associated with atypical MDS/MPN including CMML and juvenile myelomonocytic leukemia (JMML) as well as more advanced forms of MDS and secondary AML (sAML). Ring finger mutants of Cbl abrogate ubiquitination and thereby tumor suppressor function related to inactivation of phosphorylated receptor tyrosine kinases, Src and other phosphoproteins. TET2 mutations are present in a similar disease spectrum. The TET family of proteins is involved in conversion of methylcytosine to methylhydroxycytosine which cannot be recognized by DNMT1. Thereby, the proteins seem to counteract maintenance hypermethylation. In our screen of MDS/MPN, we found c-Cbl and Cbl-b ring finger mutations in 5/58 (9%) of CMML and AML derived from CMML, 2/39 (5%) MDS/MPNu, 4/21 (19%) JMML and 14/148 (9%) RAEB/sAML. In the same cohort, TET2 mutations were present in 37% and 14% of patients with MDS/MPN and MDS, respectively. Of note we did not find any TET2 mutations in JMML. We and others have also noted that TET2 and c-Cbl mutations were also detected in atypical chronic myeloid leukemia. While translocations resulting in BCR/ABL fusion characterize CML, we stipulated that in analogy to other chronic myeloproliferative diseases, TET2 and c-Cbl mutations may be also present in CML and contribute to phenotypic heterogeneity within BCR/ABL associated disorders. In particular, progression of CML to accelerated phase (AP) or blast crisis (BC) could be associated with acquisition of additional lesions. When 22 patients with CML chronic phase (CP) were screened, no TET2 and c-Cbl mutations were found. However, we identified 1 c-Cbl, 2 Cbl-b (6%) and 6 TET2 (12%) mutations in 51 patients with CML-AP (N=18) and CML-BC (N=33) with myeloid and lymphoid/mix 24 and 9 phenotype, respectively. These mutations were mutually exclusive. We also noted that TET2 mutations were present in 1/9 CML in BC with lymphoid phenotype. We subsequently screened Ph+ ALL cases (N=9) and found a TET2 mutation in 1 case but no Cbl family mutations. In contrast when 9 Ph- ALL cases were screened as controls, neither TET2 or Cbl mutations were found. SNP-A analysis revealed 2 cases of LOH involving chromosome 4 (UPD4q24 and del4) in a patient with lymphoid blast crisis and Ph+ ALL, respectively. However, UPD was not found in Cbl family gene regions (11q23.3 or 3q13.11). A homozygous deletion of Cbl-b region was seen in a CP patient. Cbl family mutations were associated with a more complex karyotype than TET2 mutations (67% vs. 17% cases with abnormal phenotype). Patients with Cbl family mutations were resistant to imatinib which was effective in only 2 out of 6 patients with TET2mutations. Dasatinib was effective in 2 patients with TET2 mutation. Median over all survival of progressed CML was 47, 49 and 48 months in patients with Cbl, TET2 or no mutations, respectively. In conclusion, our results indicate that Cbl family mutations can occur as secondary lesions in myeloid type aggressive CML (AP and myeloid BC), but not in lymphoid types. TET2 mutations were identified in both lymphoid BC and Ph1+ALL, as well as myeloid BC and AP. In contrast to CMML or JMML in which a vast majority of mutations are homozygous, all Cbl family mutations were heterozygous (no LOH). Similarly, all but two TET2 mutations were heterozygous (1 hemizygous in del4 and 1 homozygous case in UPD4q), suggesting that additional cooperating lesions affecting corresponding pathways may be present. These mutations likely represent secondary lesions which contribute to more either progression (CML) or more aggressive features (Ph+ ALL) and characterize disease refractory to therapy with imatinib.