Chromosomal Aberration Network In Myeloproliferative Neoplasms

Abstract 318

The classical myeloproliferative neoplasms (MPNs) comprise of three entities: polycythemia vera (PV), essential thrombocythemia (ET) and primary myelofibrosis (PMF). Despite distinct phenotypic features of MPN entities they share characteristics like clonal hematopoiesis, risk for thrombosis and bleeding and tendency to transform to secondary acute myeloid leukemia (post-MPN AML). In order to investigate the genetic lesions associated with MPN a large single-center cohort of 311 MPN patients was analyzed for chromosomal aberrations using high resolution Affymetrix SNP 6.0 arrays. The cohort included 150 patients with PV, 90 with ET, 68 with PMF and 3 with post-MPN AML. Of the 311 patients, 144 (46%) had a normal karyotype and 167 (54%) harbored 1 to 8 detectable chromosomal aberrations. We found 51 gains, 102 deletions and 143 uniparental disomies (UPDs). A total of 13 recurrent chromosomal defects (more than three events) were detected. We investigated if either the number of chromosomal aberrations in a patient or specific types of lesions associate with a certain patient group defined by clinical criteria. Chromosomal aberrations were equally distributed among the three MPN entities and only 9pUPD showed significant clustering with PV. We did not detect an association between the number of chromosomal aberrations and disease duration. Patients positive or negative for JAK2 mutations did not differ significantly in the frequency of chromosomal aberrations (except of the association of 9pUPD with JAK2 positive MPN). Patients with complex karyotype were significantly older than patients with normal karyotype (P<0.001). Transformation to post-MPN AML is an important complication in MPN. To investigate associations between chromosomal changes and transformation, we included additional 19 post-MPN AML patients from another center into the study (total N=22). Patients in the post-MPN AML group harbored significantly more chromosomal lesions (P<0.001). Recurrent aberrations of chromosomes 1q, 7p, 7q, 5q, and 3q strongly associated with post-MPN AML. When we reviewed the clinical data of patients in chronic phase MPN harboring the leukemia-associated aberrations, they showed features of disease progression, and some transformed to AML at a later follow-up. We were able to map a common deleted region (CDR) on chromosome 4 to the tet oncogene family member 2 (TET2), a gene frequently deleted in myeloid disorders. On chromosome 7p we mapped a CDR to the Ikaros transcription factor (IKZF1) and a 7q CDR mapped to a novel putative tumor suppressor, the cut-like homeobox 1 gene (CUX1). Interestingly, in one patient who carried a UPD of chromosome 7q we did not detect a mutation in the CUX1 gene but an R288Q mutation was found in the EZH2 gene. Chromosome 7 aberrations in our cohort were strongly linked to post-MPN AML. Our results show that at least three chromosome 7 genes (IKZF1, CUX1, and EZH2) are relevant in leukemic transformation. In addition to chromosome 7, we found gains of chromosome 1q equally relevant in post-MPN AML. We mapped the common 1q amplification to a 3.5 Mbp region that contained the MDM4 gene. Mdm4 is a known negative regulator of p53 and was frequently shown amplified in various cancers. This result prompted us to investigate the relevance of the p53 pathway in post-MPN AML and we sequenced TP53 in all 22 leukemic patients and found mutations in 6 cases (27.3%). Interestingly, none of the patients with TP53 mutation carried an MDM4 amplification. Taken together, 10 out of 22 post-MPN AML cases (45.5%) had evidence of a p53-related defect. To gain deeper insight into the pathways involved in transformation to post-MPN AML we sequenced genes commonly affected in de novo AML, and found two patients with mutations in FLT3, two patients with RUNX1 mutations, two patients with either IDH1 or IDH2 mutations. We conclude that lesions known to play an important role in de novo AML are present only in a fraction of post-MPN AML patients. In this study we show that aberrations of the p53 pathway together with the chromosome 7 lesions affecting IKZF1 and CUX1 are present in 64% of all post-MPN AML patients. Our data give insight into the genetic complexity and heterogeneity of MPN patients in chronic phase as well as in post-MPN AML. The marked genetic heterogeneity of MPN patients will render targeted therapies challenging and underlines the requirement of personalized treatments.