the MDS cell cycle. Moreover, the expression of PI-PLCs and Beta-Globin could be associated with a favourable clinical response to the combination therapy, hinting at a specific contribution of lenalidomide on erythroid activation, whilst the frequent demethy- lation of PI-PLCbeta1 promoter could be specifically linked to azacitidine. At baseline, the most frequent gene mutations were ASXL1 (59%), TET2 (41%), RUNX1 (35%) and SRSF2 (29%). Interestingly, all patients showing SRSF2 mutations evolved into AML. Moreover, all patients maintaining a stable disease during the therapy had increasing variant allele frequencies, showing a genetic instability. Finally, microRNA analysis revealed specific clusters associated with the treatment: 14 microRNAs were over- expressed and 28 were under-expressed as compared to baseline levels. Interestingly, down-regulation of 5 microRNAs was signifi- cantly associated with the lack of response. Conclusions: Our results show that the combination of azacitidine and lenalidomide can affect PI-PLC signalling, possibly regulating MDS cell cycle, myeloid and erythroid differentiation. Moreover, the therapy can induce a change in the specific gene mutation and microRNA profile that, if confirmed by larger studies, could be important to better evaluate the response to this therapy. 87 MUTATIONAL AND CLONAL DYNAMICS DURING PROGRESSION FROM MDS TO SAML BY WHOLE-EXOME AND TARGETED-DEEP SEQUENCING M. Martín Izquierdo 1 , M. Abáigar 1 , J.M. Hernández-Sánchez 1 , D. Tamborero 2 , M. Díez-Campelo 3 , M. Hernández-Sánchez 1 , F. Ramos 4 , M. Megido 5 , C. Aguilar 6 , E. Lumbreras 1 , A. Redondo-Guijo 3 , I. Recio 7 , C. Olivier 8 , R. Benito 1 , N. López-Bigas 2 , M.C. del Cañizo 3 , J.M. Hernández-Rivas 3 1 Centro de Investigación del Cáncer, Unidad de Diagnóstico Molecular y Celular del Cáncer, Salamanca, Spain; 2 IMIM Hospital del Mar Medical Research Institute and Universitat Pompeu Fabra, Research Program on Biomedical Informatics, Barcelona, Spain; 3 IBSAL- Hematology Department, Hospital Universitario de Salamanca, Salamanca, Spain; 4 Hospital de León, IBIOMED- University of León- Hematology Department, León, Spain; 5 Hematology Department, Hospital del Bierzo, Ponferrada, Spain; 6 Hematology Department, Hospital Santa Bárbara, Soria, Spain; 7 Hematology Department, Hospital Nuestra Señora de Sónsoles, Ávila, Spain; 8 Hematology Department, Hospital General de Segovia, Segovia, Spain Introduction: Myelodysplastic syndromes (MDS) are hemato- logical disorders at high risk of progression to acute myeloid leukemia (sAML). Due to recent high-throughput sequencing studies, the mutational dynamics and clonal evolution underlying disease progression have just begun to be understood. However, large longitudinal sequencing genomic studies are still required. Methods: Sixty-eight serially collected samples (34MDS/CMML patients evolving to sAML) were studied at two time-points: at diagnosis and at progression to a sAML. At diagnosis, patients were classified into: 18 RAEB-1/2, 9 RCMD and 7 CMML. Whole-exome sequencing (WES) was carried out on 40 diagnosis/ progression-matched samples. To validate mutations and precise variant allele frequencies (VAFs) estimation, targeted- deep sequencing (TDS) using a custom MDS/AML-related capture enrichment panel (Illumina ® ) of 117 genes was performed in 30 out of 40 of the initial cohort. Moreover, a total of 28 paired-samples from a cohort of 14 patients were analyzed by TDS. Results: Combining both WES and TDS approaches, a total of 143 mutations in 50 different genes were identified at the sAML stage. The most recurrently mutated genes were SRSF2 (41%), TET2 (41%), STAG2 (28%), SF3B1 (21%), ASXL1 (21%), TP53 (21%) and NRAS (21%). However, it should be noted that 68% genes were mutated only in less than 10% of the patients, highlighting the great heterogeneity that exists in mechanisms of transformation. To study the mutational dynamics during disease progression we compared VAFs of mutations detected at both time-points (sAML to MDS/CMML stage) in each patient. We identified 4 different clonal dynamics: mutations that were initially present but increased VAF (type-1), decreased (type-2), were newly acquired (type-3) or persisted with similar allelic burden (type-4) at sAML stage. Interestingly, most of type-1 mutations were detected in STAG2 gene. Thus, mutational burden of STAG2 were markedly increased (6/8 patients) at sAML progression. Moreover, type-3 mutations, only detected at the sAML-stage, were predominantly identified in FLT3 (3/4) and NRAS (5/6). Conversely, type-4 mutations were present in MDS-related genes such as SRSF2 (9/12), SF3B1 (3/6) and TET2 (8/12). Most of mutations in these genes showed no changes during progression to sAML. Conclusions: Progression from MDS to sAML could be explained by different mutational processes, as well as by the occurrence of unique and complex changes in the clonal architecture of the disease during the evolution. Mutations in genes such as STAG2, FLT3 or NRAS could play an important role during disease progression. [FP7/2007-2013] n°306242-NGS-PTL; BIO/SA52/14;FEHH 2015-16 (MA). 88 TRACKING OF THE SOMATIC MUTATIONS IN MDS PATIENTS DURING DISEASE RESTAGING IMPROVES PREDICTION OF ONCOMING RELAPSE OR DISEASE PROGRESSION K. Polgarova 1,2 , V. Kulvait 2 , K. Vargova 3 , L. Minarik 1,2 , N. Dusilkova 2,3 , Z. Zemanova 4 , A. Jonasova 1 , T. Stopka 1,2,3 1 First Internal Clinic – Clinic of Haematology,1st Faculty of Medicine & General University Hospital- Charles University Prague, Prague, Czech Republic; 2 1st Faculty of Medicine & General University Hospital- Charles University Prague, Biocev, Prague, Czech Republic; 3 1st Faculty of Medicine & General University Hospital- Charles University Prague, Institute of Pathological Physiology, Prague, Czech Republic; 4 1st Faculty of Medicine & General University Hospital- Charles University Prague, Center of Oncocytogenetics, Prague, Czech Republic Myelodysplastic syndromes (MDS) are clonal stem cell disorders with dysplasia, ineffective hematopoiesis and acute myeloid leukemia (AML) development. 90% of patients with MDS carry somatic mutations in bone marrow (BM). We herein asked whether the treatment with azacitidine (AZA) selects some of the somatic mutations and whether some mutation-bearing clones are sensi- tive to AZA, and finally how AZA influence the clonal architecture and how these changes reflect the clinical course. We analyzed in duplicates the 98 BM samples of 39 high-risk AZA- treated patients before treatment and in different restaging points using Illumina Myeloid TruSight panel. To ensure reliable data, samples were sequenced twice and germinal variants were filtered using data from in parallel-sequenced CD3 + non-malignant cell fraction. MDS subtypes were as follows: RAEB2 (n = 18, 46%), RAEB1 (n = 13, 33%), AML/MDS (n = 5, 13%), CMML (n = 2, 4%), and RCMD (n=1, 2%). 18% of patients had complex karyotypes, 46% were progressions of 5q-. MedianOS was 24 months, medianPFS was 16 months; CR/mCR was reached in 41% (n = 16); 64% patients (n = 25) progressed to AML. 92% of patients bear somatic mutation in at least one out of 54 analyzed genes with median 3 mutations (range 1–9) per patient. The most frequently mutated genes at diagnosis were: TP53 (n = 11), TET2 (n = 10), CUX1 (n = 9), BCORL1 (n = 8) and ASXL1 (n = 7). Out of the mutations observed before AZA therapy, 40% remained stable during the disease course including TET2 (80%), Poster Presentations – 14th International Symposium on Myelodysplastic Syndromes / Leukemia Research 55 S1 (2017) S45–S167 S55