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© The Author(s) 2021., BIRC3 is monoallelically deleted in up to 80% of chronic lymphocytic leukemia (CLL) cases harboring del(11q). In addition, truncating mutations in the remaining allele of this gene can lead to BIRC3 biallelic inactivation, which has been shown to be a marker for reduced survival in CLL. Nevertheless, the biological mechanisms by which these lesions could contribute to del(11q) CLL pathogenesis and progression are partially unexplored. We implemented the CRISPR/Cas9-editing system to generate isogenic CLL cell lines harboring del(11q) and/or BIRC3 mutations, modeling monoallelic and biallelic BIRC3 loss. Our results reveal that monoallelic BIRC3 deletion in del(11q) cells promotes non-canonical NF-κB signaling activation via RelB-p52 nuclear translocation, being these effects allelic dose-dependent and therefore further enhanced in del(11q) cells with biallelic BIRC3 loss. Moreover, we demonstrate ex vivo in primary cells that del(11q) cases including BIRC3 within their deleted region show evidence of non-canonical NF-κB activation which correlates with high BCL2 levels and enhanced sensitivity to venetoclax. Furthermore, our results show that BIRC3 mutations in del(11q) cells promote clonal advantage in vitro and accelerate leukemic progression in an in vivo xenograft model. Altogether, this work highlights the biological bases underlying disease progression of del(11q) CLL patients harboring BIRC3 deletion and mutation., This work was supported by grants from the Spanish Fondo de Investigaciones Sanitarias PI15/01471, PI18/01500, Instituto de Salud Carlos III (ISCIII), European Regional Development Fund (ERDF) “Una manera de hacer Europa”, “Consejería de Educación, Junta de Castilla y León” (SA271P18), “Proyectos de Investigación del SACYL”, Spain GRS 2062/A/19, GRS 1847/A/18, GRS1653/A17,“Fundación Memoria Don Samuel Solórzano Barruso” (FS/23-2018), by grants (RD12/0036/0069) from Red Temática de Investigación Cooperativa en Cáncer (RTICC), Universidad de Salamanca (Programa XIII), Centro de Investigación Biomédica en Red de Cáncer (CIBERONC CB16/12/00233) and SYNtherapy “Synthetic Lethality for Personalized Therapy-based Stratification In Acute Leukemia” (ERAPERMED2018-275); ISCIII (AC18/00093), co-funded by ERDF/ESF, “Investing in your future”. M.Q.Á. and A.E.R.V. are supported with a research grant by FEHH (“Fundación Española de Hematología y Hemoterapia”); M.H.S. holds a Sara Borrell postdoctoral contract (CD19/00222) from the Instituto de Salud Carlos III (ISCIII). C.P.C. was supported by an “Ayuda predoctoral en Oncología” (AECC) and is a recipient of a PFIS grant (FI19/00191) from Instituto de Salud Carlos III; PFIS grant and Sara Borrell postdoctoral contrat are co-founded by Fondo Social Europeo (FSE) “El Fondo Social Europeo invierte en tu futuro”; J.L.O. and R.B.S. are supported by a grant from the University of Salamanca (“Contrato postdoctoral programa II”).

SF3B1 is a highly mutated gene in myelodysplastic syndrome (MDS) patients, related to a specific subtype and parameters of good prognosis in MDS without excess blasts. More than 40% of MDS patients carry at least two myeloid-related gene mutations but little is known about the impact of concurrent mutations on the outcome of MDS patients. In applying next-generation sequencing (NGS) with a 117 myeloid gene custom panel, we analyzed the co-occurrence of SF3B1 with other mutations to reveal their clinical, biological, and prognostic implications in very low/low- and intermediate-risk MDS patients. Mutations in addition to those of SF3B1 were present in 80.4% of patients (median of 2 additional mutations/patient, range 0–5). The most frequently mutated genes were as follows: TET2 (39.2%), DNMT3A (25.5%), SRSF2 (10.8%), CDH23 (5.9%), and ASXL1, CUX1, and KMT2D (4.9% each). The presence of at least two mutations concomitant with that of SF3B1 had an adverse impact on survival compared with those with the SF3B1 mutation and fewer than two additional mutations (median of 54 vs. 87 months, respectively: p = 0.007). The co-occurrence of SF3B1 mutations with specific genes is also linked to a dismal prognosis: SRSF2 mutations were associated with shorter overall survival (OS) than SRSF2wt (median, 27 vs. 75 months, respectively; p = 0.001), concomitant IDH2 mutations (median OS, 11 [mut] vs. 75 [wt] months; p = 0.001), BCOR mutations (median OS, 11 [mut] vs. 71 [wt] months; p = 0.036), and NUP98 and STAG2 mutations (median OS, 27 and 11 vs. 71 months, respectively; p = 0.008 and p = 0.002). Mutations in CHIP genes (TET2, DNMT3A) did not significantly affect the clinical features or outcome. Our results suggest that a more comprehensive NGS study in low-risk MDS SF3B1mut patients is essential for a better prognostic evaluation., This work was supported by grants from the following: Contrato Rio Hortega, CM17/00171; Gerencia Regional de Salud (Castilla y León) para proyectos de investigación año 2018, 1850/A/18; Spanish Fondo de Investigaciones Sanitarias, PI15/01471, PI18/01500; Instituto de Salud Carlos III (ISCIII); European Regional Development Fund (ERDF) “Una manera de hacer Europa”; Consejería de Educación, Junta de Castilla y León (SA271P18); Proyectos de Investigación del SACYL, Spain, GRS1847/A/18, GRS1653/A17; SYNtherapy, Synthetic Lethality for Personalized Therapy-based Stratification In Acute Leukemia (ERAPERMED2018–275); ISCIII (AC18/00093), co-funded by ERDF/ESF, “Investing in your future”, by grants from Red Temática de Investigación Cooperativa en Cáncer (RTICC) (RD12/0036/0069) and Centro de Investigación Biomédica en Red de Cáncer (CIBERONC CB16/12/00233). JMHS is supported by a research grant from Fundación Española de Hematología y Hemoterapia. MM is currently supported by an Ayuda predoctoral de la Junta de Castilla y León from the Fondo Social Europeo (JCYL- EDU/556/2019 PhD scholarship).

The development of Next-Generation Sequencing (NGS) has provided useful diagnostic, prognostic, and therapeutic strategies for individualized management of B-cell precursor acute lymphoblastic leukemia (BCP-ALL) patients. Consequently, NGS is rapidly being established in clinical practice. However, the technology’s complexity, bioinformatics analysis, and the different available options difficult a broad consensus between different laboratories in its daily routine introduction. This collaborative study among Spanish centers was aimed to assess the feasibility, pros, and cons of our customized panel and other commercial alternatives of NGS-targeted approaches. The custom panel was tested in three different sequencing centers. We used the same samples to assess other commercial panels (OncomineTM Childhood Cancer Research Assay, Archer®FusionPlex® ALL, and Human Comprehensive Cancer Panel GeneRead Panel v2®). Overall, the panels showed a good performance in different centers and platforms, but each NGS approach presented some issues, as well as pros and cons. Moreover, a previous consensus on the analysis and reporting following international guidelines would be preferable to improve the concordance in results among centers. Our study shows the challenges posed by NGS methodology and the need to consider several aspects of the chosen NGS-targeted approach and reach a consensus before implementing it in daily practice.

Introduction: Lenalidomide is a potent drug with pleiotropic effects in patients with myelodysplastic syndrome (MDS) with deletion of the long arm of chromosome 5 [del(5q)]. The clinical efficacy of lenalidomide in MDS patients has been extensively reviewed and although the mechanisms of action in del(5q) clone have been previously described, in vivo sequential studies of modulatory effect on T lymphocytes are lacking. Our study was conducted in patients included in the Sintra-REV Clinical Trial: Lenalidomide (Revlimid) phase III, multicenter, randomized, double-blind study versus placebo in patients with low-risk MDS (low and intermediate IPSS-1) with del(5q), with anemia (HB≤12gr/dl) and without transfusion needs. Aim: The aim of this study was to explore the effect of lenalidomide in T-lymphocytes in MDS patients with del(5q) and without transfusion dependence. Materials and Methods: Sequential study was carried out in 26 samples from 13 paired MDS patients with del (5q). Seven out 13 were treated with lenalidomide and achieved a major erythroid and cytogenetic response. Peripheral blood (PB) samples were collected before and one month after treatment in treated-patients and at the same time points for non-treated patients. CD3+ cells were collected from PB samples and total RNA was isolated. SureSelect Strand Specific RNA library (Agilent Technologies) was applied to study changes in RNA levels. Raw reads were aligned against the Human genome GRCh37 using the STAR aligner. Counts were assigned to Ensembl gene IDs through HTseq using its UNION version. Differential gene expression was determined with DESeq2, considering as statistically significant those genes with FDR < 0.05. Pathway over-representation analysis (ORA) was conducted in the Webgestalt suite. Results: 332 genes were differentially expressed in CD3+ lymphocytes one month after lenalidomide treatment in our cohort of patients; 199 of them were over-expressed after the administration of this drug (Fig 1a). Of note, none of them were observed in non-treated patients after one month. The ORA revealed significant differences in the gene expression profile of sixteen cytokines and enrichment of genes of the cell cycle pathway (35 genes). The most relevant up-regulated cytokines were: IL10, TNFSF10, IFNGand IL6. These data explain lenalidomide-induced activation of an antileukemic immune response and secretion of anti-inflammatory cytokines. Although lenalidomide has been reported to reduce the expression of IL6 secreted by myeloid cell derived from MDS clon, we have observed upregulation of this gene in T-lymphocytes. Moreover, our study showed a downregulation of MBP6 that may help to correct the anemia and also attenuate inflammation signaling in MDS patients with del(5q) (Fig 1b). In addition, the most represented up-regulated genes related to cell cycle pathway were: cyclines (CCNB1, CCNB2, CDK1), centromere genes (CENPE, CENPM, CENPU), kinesin family members (KIF18A, KIF23, KIF2C), BUB1 mitotic checkpoint genes (BUB1, BUB1B), and genes involved in cell division (CDC6, CDC7,CDC25A). It has been described that lenalidomide inhibits CDC25A selectively in the del(5q) clone resulting in G2/M arrest and apoptosis. By contrast, our study showed that this gene was upregulated in T-lymphocytes promoting cell cycle and proliferation of these cells (Fig 1b). Conclusions: The immunomodulatory properties of lenalidomide can be summarized in two: a) regulation of antileukemic and anti-inflammatory cytokines production, b) activation of cell cycle and proliferation in T cells. To our knowledge, this is the first report describing RNA expression profiles in PB CD3+ lymphocytes collected from lenalidomide-treated del(5q) patients, contributing to overall understanding of lenalidomide action. Disclosures Sanz: Abbvie Pharmaceuticals: Membership on an entity's Board of Directors or advisory committees; LaHoffman Roche Ltd.: Membership on an entity's Board of Directors or advisory committees; Takeda Pharmaceutical Ltd.: Membership on an entity's Board of Directors or advisory committees; Helsinn: Membership on an entity's Board of Directors or advisory committees. Fenaux:Abbvie: Honoraria, Research Funding; Novartis: Honoraria, Research Funding; Jazz: Honoraria, Research Funding; BMS: Honoraria, Research Funding. Diez-Campelo:Novartis: Consultancy, Membership on an entity's Board of Directors or advisory committees, Research Funding, Speakers Bureau; Takeda: Consultancy, Membership on an entity's Board of Directors or advisory committees, Research Funding, Speakers Bureau; Celgene-BMS: Consultancy, Membership on an entity's Board of Directors or advisory committees, Research Funding, Speakers Bureau. OffLabel Disclosure: Lenalidomide was administered in anemic but not transfusion-dependence patients with low-risk MDS and del(5q)

© 2020 by the authors., The clonal basis of relapse in B-cell precursor acute lymphoblastic leukemia (BCP-ALL) is complex and not fully understood. Next-generation sequencing (NGS), array comparative genomic hybridization (aCGH), and multiplex ligation-dependent probe amplification (MLPA) were carried out in matched diagnosis–relapse samples from 13 BCP-ALL patients to identify patterns of genetic evolution that could account for the phenotypic changes associated with disease relapse. The integrative genomic analysis of aCGH, MLPA and NGS revealed that 100% of the BCP-ALL patients showed at least one genetic alteration at diagnosis and relapse. In addition, there was a significant increase in the frequency of chromosomal lesions at the time of relapse (p = 0.019). MLPA and aCGH techniques showed that IKZF1 was the most frequently deleted gene. TP53 was the most frequently mutated gene at relapse. Two TP53 mutations were detected only at relapse, whereas the three others showed an increase in their mutational burden at relapse. Clonal evolution patterns were heterogeneous, involving the acquisition, loss and maintenance of lesions at relapse. Therefore, this study provides additional evidence that BCP-ALL is a genetically dynamic disease with distinct genetic profiles at diagnosis and relapse. Integrative NGS, aCGH and MLPA analysis enables better molecular characterization of the genetic profile in BCP-ALL patients during the evolution from diagnosis to relapse., This work was supported in part by a grant from the Consejería de Educación, Junta de Castilla y León, Fondos FEDER (SA085U16, SA271P18), Proyectos de Investigación de la Gerencia Regional de Sanidad, SACYL, (GRS 1847/A/18; GRS 2062/A/19), SYNtherapy. Synthetic Lethality for Personalized Therapy-based Stratification In Acute Leukemia (ERAPERMED2018-275); ISCIII (AC18/00093), co-funded by ERDF/ESF, “Investing in your future”, Fundación Castellano Leonesa de Hematología y Hemoterapia (FUCALHH 2017), and Centro de Investigación Biomédica en Red de Cáncer (CIBERONC CB16/12/00233). A grant to AM from the Junta Provincial de Salamanca of the Asociación Española Contra el Cáncer (AECC), a grant to MFC from the Universidad Pedagógica y Tecnológica de Colombia—Vicerrectoría de Investigación y Extensión (Grupo de Investigación en Ciencias Biomédicas UPTC—GICBUPTC, Escuela de Ciencias Biológicas). M. Hernández-Sánchez holds a Sara Borrell post-doctoral contract (CD19/00222) from the Instituto de Salud Carlos III (ISCIII). co-founded by Fondo Social Europeo (FSE) “El Fondo Social Europeo invierte en tu futuro”, MM is currently supported by an “Ayuda predoctoral de la Junta de Castilla y León” by the Fondo Social Europeo (JCYL- EDU/556/2019 PhD scholarship) and a grant to JR and JMR from Instituto Carlos III (PI14/01971).

Chromosome 14q32 rearrangements/translocations involving the immunoglobulin heavy chain (IGH) are rarely detected in chronic lymphocytic leukemia (CLL). The prognostic significance of the IGH translocation is controversial and its mutational profile remains unknown. Here, we present for the first time a comprehensive next‐generation sequencing (NGS) analysis of 46 CLL patients with IGH rearrangement (IGHR‐CLLs) and we demonstrate that IGHR‐CLLs have a distinct mutational profile with recurrent mutations in NOTCH1, IGLL5, POT1, BCL2, FBXW7, ZMYM3, MGA, BRAF and HIST1H1E genes. Interestingly, BCL2 and FBXW7 mutations were significantly associated with this subgroup and almost half of BCL2, IGLL5 and HISTH1E mutations reported were previously identified in non‐Hodgkin lymphomas. Notably, IGH/BCL2 rearrangements were associated with a lower mutation frequency and carried BCL2 and IGLL5 mutations, while the other IGHR‐CLLs had mutations in genes related to poor prognosis (NOTCH1, SF3B1 and TP53) and shorter time to first treatment (TFT). Moreover, IGHR‐CLLs patients showed a shorter TFT than CLL patients carrying 13q−, normal fluorescence in situ hybridization (FISH) and +12 CLL, being this prognosis particularly poor when NOTCH1, SF3B1, TP53, BIRC3 and BRAF were also mutated. The presence of these mutations not only was an independent risk factor within IGHR‐CLLs, but also refined the prognosis of low‐risk cytogenetic patients (13q−/normal FISH). Hence, our study demonstrates that IGHR‐CLLs have a distinct mutational profile from the majority of CLLs and highlights the relevance of incorporating NGS and the status of IGH by FISH analysis to refine the risk‐stratification CLL model., This work was supported by grants from the Spanish Fondo de Investigaciones Sanitarias, PI15/01471, PI18/01500, Instituto de Salud Carlos III (ISCIII), European Regional Development Fund (ERDF) “Una manera de hacer Europa”, “Consejería de Educación, Junta de Castilla y León” (SA271P18), “Proyectos de Investigación del SACYL”, Spain: GRS 1847/A/18, GRS1653/A17, “Fundación Memoria Don Samuel Solórzano Barruso”, by grants (RD12/0036/0069) from Red Temática de Investigación Cooperativa en Cáncer (RTICC) and Centro de Investigación Biomédica en Red de Cáncer (CIBERONC CB16/12/00233). Claudia Pérez‐Carretero was supported by an “Ayuda predoctoral en Oncología” (AECC) and is a recipient of a PFIS grant (FI19/00191) from Instituto de Salud Carlos III; María Hernández‐Sánchez was supported by a grant from FEHH/Janssen (“Sociedad Española de Hematología y Hemoterapia”) and now holds a Sara Borrell post‐doctoral contract (CD19/00222) from the Instituto de Salud Carlos III (ISCIII). PFIS grant and Sara Borrell post‐doctoral contrat is co‐founded by Fondo Social Europeo (FSE) “El Fondo Social Europeo invierte en tu futuro”; Miguel Quijada‐Álamo is supported by an “Ayuda Predoctoral de la Junta de Castilla y León” (JCYL‐EDU/529/2017); Jesús‐María Hernández‐Sánchez and Ana E. Rodríguez‐Vicente are supported by research grants of FEHH (“Fundación Española de Hematología y Hemoterapia”).

Background The presence of multiple symptomatic pulmonary nodules and one cardiac tumour in a child requires urgent diagnosis and treatment. Until a few decades ago, the diagnosis of a cardiac tumour was difficult and was based on a high index of suspicion from indirect signs, and required angiocardiography for confirmation. Echocardiography and other imaging techniques have also helped in the detection of cardiac neoplasms. However, it is not always easy to make the correct diagnosis. Clinical case The case is presented of a 12-year-old boy with pulmonary symptoms, and diagnosed with a cardiac tumour with lung metastases. The presence of numerous pulmonary nodules was confirmed in our hospital. The echocardiogram detected a solid cardiac nodule in the right ventricle. Magnetic resonance imaging confirmed the findings and the diagnosis. Puncture-aspiration of a lung nodule gave the diagnosis of hydatidosis. He underwent open-heart surgery with cardiac cyst resection and treated with anthelmintics. The lung cysts were then excised, and he recovered uneventfully. Discussion This child had multiple pulmonary nodules and a solid cardiac nodule, and was suspected of having a cardiac tumour with pulmonary metastases. However, given the clinical history, background and morphology of pulmonary nodules, another possible aetiology for consideration is echinococcosis. The clinical picture of cardiac hydatidosis and its complications is highly variable. The clinical history is essential in these cases, as well as having a high index of suspicion. Conclusion Hydatidosis should be included in the differential diagnosis of a solid, echogenic, cardiac nodule. The treatment for cardiopulmonary hydatid cysts is surgical, followed by anthelmintics.

ResumenAntecedentesLa presencia de múltiples nódulos pulmonares sintomáticos y uno cardíaco en un niño exigen un diagnóstico y tratamiento urgentes. El diagnóstico de una neoformación cardíaca era difícil hasta hace pocas décadas, y se basaba en un alto índice de sospecha ante signos indirectos, necesitando la angiocardiografía para su confirmación. La ecocardiografía y otros medios de imagen han facilitado la detección de los nódulos cardíacos. Sin embargo, no siempre es fácil acertar con el diagnósticoCaso clínicoNiño de 12 años con síntomas pulmonares. Diagnosticado de tumor cardíaco con metástasis pulmonares. En nuestro hospital se confirmó la presencia de numerosos nódulos pulmonares. El ecocardiograma detectó un nódulo cardíaco sólido ventricular derecho. La resonancia magnética nuclear confirmó los hallazgos, haciéndose el mismo diagnóstico. La punción-aspiración de un nódulo pulmonar fue diagnóstica: hidatidosis. Fue operado a corazón abierto resecando el quiste cardíaco y tratado con antihelmínticos. Posteriormente se extirparon los quistes pulmonares. Se recuperó el paciente sin complicaciones.DiscusiónEn este niño, con múltiples nódulos pulmonares y uno cardíaco sólido, se hizo el diagnóstico de tumor cardíaco con metástasis pulmonares; sin embargo, con la historia clínica, los antecedentes y la morfología de los nódulos pulmonares se debió incluir la equinococosis como posible etiología. El cuadro clínico de los quistes hidatídicos cardíacos y de sus complicaciones es muy variable. En estos casos es fundamental la historia clínica y tener un alto índice de sospecha.ConclusiónEn el diagnóstico diferencial de un nódulo cardíaco sólido, ecodenso, debe incluirse la hidatidosis. El tratamiento de los quistes hidatídicos cardiopulmonares es la cirugía.AbstractBackgroundThe presence of multiple symptomatic pulmonary nodules and one cardiac tumour in a child requires urgent diagnosis and treatment. Until a few decades ago, the diagnosis of a cardiac tumour was difficult and was based on a high index of suspicion from indirect signs, and required angiocardiography for confirmation. Echocardiography and other imaging techniques have also helped in the detection of cardiac neoplasms. However, it is not always easy to make the correct diagnosis.Clinical caseThe case is presented of a 12 year-old boy with pulmonary symptoms, and diagnosed with a cardiac tumour with lung metastases. The presence of numerous pulmonary nodules was confirmed in our hospital. The echocardiogram detected a solid cardiac nodule in the right ventricle. Magnetic resonance imaging confirmed the findings and the diagnosis. Puncture-aspiration of a lung nodule gave the diagnosis of hydatidosis. He underwent open-heart surgery with cardiac cyst resection and treated with anthelmintics. The lung cysts were then excised, and he recovered uneventfully.DiscussionThis child had multiple pulmonary nodules and a solid cardiac nodule, and was suspected of having a cardiac tumour with pulmonary metastases. However, given the clinical history, background and morphology of pulmonary nodules, another possible aetiology for consideration is echinococcosis. The clinical picture of cardiac hydatidosis and its complications is highly variable. The clinical history is essential in these cases, as well as having a high index of suspicion.ConclusionHydatidosis should be included in the differential diagnosis of a solid, echogenic, cardiac nodule. The treatment for cardiopulmonary hydatid cysts is surgical, followed by anthelmintics.

Hermansky-Pudlak syndrome (HPS) is a rare inherited platelet disorder characterized by bleeding diathesis, oculocutaneous albinism (OCA) and a myriad of often-serious clinical complications. We established the clinical and laboratory phenotype and genotype of six unrelated pedigrees comprising ten patients with clinical suspicion of HPS; including platelet aggregation, flow cytometry, platelet dense granule content, electron microscopy and high-throughput sequencing (HTS). The clinical presentation showed significant heterogeneity and no clear phenotype-genotype correlations. HTS revealed two known and three novel disease-causing variants. The Spanish patients carried a homozygous p.Pro685Leufs17* deletion (n = 2) in HPS4, or the novel p.Arg822* homozygous variant (n = 1) in HPS3. In the case of two Turkish sisters, a novel missense homozygous HPS4 variant (p.Leu91Pro) was found. In two Portuguese families, genetic studies confirmed a previously reported nonsense variant (p.Gln103*) in DTNBP1 in three patients and a novel duplication (p.Leu22Argfs*33) in HPS6 in two unrelated patients. Our findings expand the mutational spectrum of HPS, which may help in investigating phenotype-genotype relationships and assist genetic counselling for affected individuals. This approach is a proof of principle that HTS can be considered and used in the first-line diagnosis of patients with biological and clinical manifestations suggestive of HPS.Key messagesWe established the relationships between the clinical and laboratory phenotype and genotype of six unrelated pedigrees comprising ten patients with clinical suspicion of HPS.Molecular analysis is useful in confirming the diagnosis and may offer some prognostic information that will aid in optimizing monitoring and surveillance for early detection of end-organ damage.This approach is a proof of principle that HTS can be considered and used in the first-line diagnosis of patients with biological and clinical manifestations suggestive of HPS. We established the relationships between the clinical and laboratory phenotype and genotype of six unrelated pedigrees comprising ten patients with clinical suspicion of HPS. Molecular analysis is useful in confirming the diagnosis and may offer some prognostic information that will aid in optimizing monitoring and surveillance for early detection of end-organ damage. This approach is a proof of principle that HTS can be considered and used in the first-line diagnosis of patients with biological and clinical manifestations suggestive of HPS.

Myelodysplastic syndromes (MDS) and chronic myelomonocytic leukemia (CMML) are hematological disorders at high risk of progression to acute myeloid leukemia (sAML). Previous high-throughput sequencing studies have provided insight into the mutational dynamics and clonal evolution underlying disease progression. However, large serial sequencing studies are still required to define which type of mutations alone or in combination contribute to leukemic transformation. To assess the mutational profiles and mutational dynamics underlying progression from MDS to sAML, a targeted-deep sequencing (TDS) of 117 MDS/AML related-genes was performed in 110 bone marrow serial samples from 50 MDS/CMML patients who evolved to sAML and 5 patients who did not evolved (controls), at two different time-points: at the time of diagnosis and at sAML progression or after a median of 3 year follow-up, respectively. A total of 269 mutations in 57 different genes were identified at second sampling. At diagnosis, all patients, progressing and not progressing (controls), presented similar number of mutations (p=0.15). Moreover, patients evolving to sAML were then divided by FAB/WHO subtypes at diagnosis (CMML, low-risk and high-risk MDS subgroups) and no differences were observed in the number of mutations (p=0.71) and variant allele frequency (VAF) between each group (p=0.63). It should be noted that mutations in the splicing pathway were significantly more frequent in low-risk MDS patients (89% low-risk MDS vs. 56% high risk MDS, p=0.038). However, after progression, those patients who evolved to sAML displayed a statistically significant increase of mutations (p=0.001) at the leukemic phase, while controls did not at the follow-up sample (p=0.88). This higher number of mutations at second sampling in patients who evolved to sAML, independently of their diagnostic subtype, may be indicative of a higher genomic instability during disease evolution. To study the mutational dynamics and what mutations could be important during disease evolution, the VAFs of mutations detected at both time-points in each patient of transformation cohort were compared. We observed that some mutations identified at the sAML stage (163 mutations) were already present at the MDS stage, at clonal or subclonal levels, and were retained during evolution, for example in genes such as SRSF2 and DNMT3A. However, 106 mutations increased in clonal size or were newly acquired. Interestingly, most of mutations in Ras signaling pathway showed a same pattern: they were not present at time of diagnosis and appeared at sAML. In fact, mutations in this pathway were detected in 25 of 50 patients (50%) included in this cohort and in 22 of them (88%) mutations displayed this dynamic. Therefore, in this study, Ras signaling was the most common pathway involved in the progression from MDS to sAML. Of note, 9 of these patients (18% of the whole cohort) presented, independently of diagnosis, a co-occurring cohesin mutation, that was already present at diagnosis and, in most cases, markedly increased in clonal size at sAML. Thus, the combination of mutations in these two pathways could play an important role during disease evolution. In addition, 22 of 50 patients were treated with a disease-modifying agent (18 azacytidine and 4 lenalidomide) before they progressed to sAML, while the remaining 28 patients received no treatment or supportive care and were considered as non-treated. Thus, we studied the effect of disease-modifying therapy on mutational dynamics in this cohort of patients progressing to sAML. In the treated patients, a higher proportion of newly acquired or increasing mutations at sAML in chromatin modifiers was observed, while in non-treated patients most mutations remained stable (61% vs. 28.6%, p=0.013). By contrast, regarding treatment, no differences were detected in the mutational dynamics of cohesin (p=0.56) or Ras pathway (p=1.00). MDS progression to sAML was characterized by a higher genomic instability, independently of MDS subtypes of patients at diagnosis. Ras signaling was the most frequent affected pathway during disease evolution in this cohort and, interestingly, the co-occurrence of Ras signaling and cohesin mutations could play an important role in the progression. Moreover, mutations in chromatin modifiers genes could be related to the evolution of patients who received disease-modifying treatment before progression to sAML. Disclosures Olivier: Celgene: Honoraria; Jassen: Honoraria. Díez-Campelo:Novartis: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding, Speakers Bureau; Celgene: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding, Speakers Bureau.