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Expression levels of miR-29 family members (i.e. miR-29a, miR-29b, & miR-29c) are deregulated in various neoplastic diseases, including acute myeloid leukemia (AML), known to affect DNA-methylation profiles by targeting epigenetic modifiers, & have been shown to be important for normal hematopoietic stem cell function. Mir-29 is organized in two distinctively regulated bi-cistronic clusters: the miR-29a/b-1 cluster & the miR-29b-2/c cluster. Here we evaluated the biological associations & clinical impact of the differential expression of pre-miR-29a/b-1 & pre-miR-29b-2/c clusters in AML. We analysed121 AML patients (pts) (median age 63 years [y], range 37-75 y) who have been consolidated with hematopoietic stem cell transplantation following non-myeloablative conditioning (nma-HCT; Fludarabin 30 mg/m2 on day -4 till -2 & 2 Gy total body irradiation) between 2000 & 2014 with pretreatment bone marrow material (BM) available. Disease status at nma-HCT was first (CR1 62%) or second complete remission (CR2 18%) or CR with incomplete peripheral recovery (CRi 20%). The mutation status (mut) of the ASXL1, CEBPA, DNMT3A IDH1, IDH2, NPM1, & TP53 gene & the FLT3-ITD & EVI1 expressionstatusas well as common surface marker expressions were assessed at diagnosis. European LeukemiaNet (ELN) classification was favorable (25%), intermediate-I (23%), intermediate-II (21%), adverse (27%) or unknown (4%). Pretreatment pre-miR-29a/b-1 & pre-miR-29b-2/c clusters expressionin bone marrow (BM)was measured by quantitative reverse transcription polymerase chain reaction & normalized to 18S. The median normalized gene expression defined high & low pre-miR-29a/b-1 & pre-miR-29b-2/c clusterexpressers. Median follow-up was 4.4y for pts alive. At diagnosis a high pre-miR-29a/b-1 expression did not associate with clinical characteristics. High pre-miR-29a/b-1 expressers were less likely to be TP53 mut (p=.01). Pts with high pre-miR-29b-2/c expression at diagnosis had higher BM blast counts (p=.01), were more likely to have a normal cytogenetics (CN, p=.03) & were less likely to be TP53 (p=.004) or ASXL1 mutated (p=.03). When we combined the expression status information of the two miR-29 clusters we found that AML blasts of pts with high expression of both clusters were less likely to be CD34 (p=.05) or CD117 (p=.04) positive & more likely to be CD11b positive (p=.05). These pts more often had CN-AML (p=.04) & better ELN genetic risk (p=.03). High expressers of both miR-29 clusters were also more likely to be DNMT3A mut (p=.01) & less likely to be EVI1 positive (p=.007). Noteworthy, none of the pts with high expression of both clusters had a TP53 (p=.16) or ASXL1 mutation (p=.08). Pts with a high expression of both miR-29 clustershad a significant longer relapse free survival (RFS, p=.01, Figure 1a) & overall survival (OS, p=.03) compared to pts with low expression of one or both miR-29 clusters. In conclusion, high expression of pre-miR-29a/b-1 & pre-miR-29b-2/c associated with different clinical & genetic characteristic at AML diagnosis. High expressers of both clusters were more often DNMT3A mutated, a gene targeted by miR-29. Furthermore, none of these patients harbored TP53 mutations, a gene known to be indirectly activated by miR-29 family members. These findings provide new insights into the miR-29 associated AML biology, which may contribute to the observed impact on AML pts outcomes. While we observed a trend for better survival for each miR-29 cluster, pts with high expression of the pre-miR-29a/b-1 & the pre-miR-29b-2/c clusterhad significantly longer RFS & OS. Figure 1 Figure 1. Disclosures Poenisch: Mundipharma: Research Funding.

The presence of the translocation t(9;22)(q34;q11), leading to the BCR::ABL1 fusion transcript, is the hallmark of chronic myeloid leukemia (CML). Nevertheless, atypical presentation at diagnosis can be challenging. However, although most patients with CML are diagnosed with the e13a2 or e14a2 BCR::ABL1 fusion transcripts, about 5% of them carry rare BCR::ABL1 fusion transcripts, such as e19a2, e8a2, e13a3, e14a3, e1a3, and e6a2. In particular, the e6a2 fusion transcript has been associated with clinically aggressive disease frequently presenting in accelerated or blast crisis phases. To date, there is limited evidence on the efficacy of front-line second-generation tyrosine kinase inhibitors for this genotype. Here, we report two patients, in whom the diagnosis of CML was challenging. The use of primers recognizing more distant exons from the common BCR::ABL1 breakpoint region correctly identified the atypical BCR::ABL1 e6a2 fusion transcript. Treatment with the second-generation tyrosine kinase inhibitor nilotinib was effective in our patient expressing the atypical e6a2 BCR::ABL1 fusion transcript.