ALBERT

Description: Background Intensive efforts of genome sequencing studies during the past decade identified 〉100 driver genes recurrently mutated in one or more subtypes of myeloid neoplasms, which collectively account for the pathogenesis of 〉90% of the cases. However, approximately 10% of the cases have no alterations in known drivers and their pathogenesis is still unclear. A possible explanation might be the presence of alterations in non-coding regions that are not detected by conventional exome/panel sequencing; mutations and complex structural variations (SVs) affecting these regions have been shown to deregulate expression of relevant genes in a variety of solid cancers. Unfortunately, however, no large studies have ever been performed, in which a large cohort of myeloid malignancies were analyzed using whole genome sequencing (WGS) in an attempt to identify a full spectrum of non-coding alterations, even though its efficacy have been demonstrated in many solid cancers. In this study, we performed WGS in a large cohort of pan-myeloid cancers, in which both coding and non-coding lesions were comprehensively analyzed. Patients and methods A total of 338 cases of myeloid malignancies, including 212 with MDS, 70 with AML, 17 with MDS/MPN, 23 with t-AML/MDS, and 16 with MPN were analyzed with WGS, of which 173 were also analyzed by transcriptome sequencing. Tumor samples were obtained from patients' bone marrow (N=269) or peripheral blood (N=69), while normal controls were derived from buccal smear (N=263) or peripheral T cells (N=75). Sequencing of target panel of 86 genes were performed for all samples. Sequencing data were processed using in-house pipelines, which were optimized for detection of complex structural variations (SVs) and abnormalities in non-coding sequences. Results WGS identified a median of 586,612 single nucleotide variants (SNVs) and 124,863 short indels per genome. NMF-based decomposition of the variants disclosed three major mutational signatures, which were characterized by age-related C〉T transitions at CpG sites (Sig. A), C〉T transitions at CpT sites (Sig. B), and T〉C transitions at ApTpN context (Sig. C). Among these, Sig. C showed a prominent strand bias and corresponds to COSMIC signature 16, which has recently been implicated in alcohol drinking. Significant clustering of SNVs and short indels were interrogated across the genome divided into different window sizes (1Kbp, 10Kbp, 100Kbp) or confining the targets to coding exons and known regulatory regions, such as promoters, enhancers/super enhances, and DNase I hypersensitive sites. Recapitulating previous findings, SNVs in the coding exons were significantly enriched in known drivers, including TP53, TET2, ASXL1, DNMT3A, SF3B1, RUNX1, EZH2, and STAG2. We detected significant enrichment of SNVs in CpG islands, and promoters/enhancers. We also detected a total of 8,242 SVs with a median of 15 SVs/sample, which is more prevalent than expected from conventional karyotype analysis. Focal clusters of complex rearrangements compatible with chromothripsis were found in 8 cases, of which 7 carried biallelic TP53 alterations. NMF-based signature analysis of SVs revealed that large (〉1Mb) deletions, inversions, and tandem duplications and translocations are clustered together and were strongly associated with TP53 mutations, while smaller deletions and tandem duplications, but not inversions, constitute another cluster. As expected, FLT3-ITD (N=15) and MLL-PTD (N=12) were among the most frequent SVs. Unexpectedly, in addition to known SVs associated with t(8;21) (RUNX1-RUNX1T1) (N=6) and t(3;21) (RUNX1-MECOM) (n=1) as well as non-synonymous SNVs within the coding exons (N=30), we detected frequent non-coding alterations affecting RUNX1, including SVs (N=15) and SNVs around splicing acceptor sites (N=5), suggesting that RUNX1 was affected by multiple mechanism, where as many as 38% of RUNX1 lesions were explained by non-coding alterations. Other recurrent targets of non-coding lesions included ASXL1, NF1, and ETV6. Conclusions WGS was successfully used to reveal a comprehensive registry of genetic alterations in pan-myeloid cancers. Non-coding alterations affecting known driver genes were more common than expected, suggesting the importance of detecting non-coding abnormalities in diagnostic sequencing. Disclosures Nakagawa: Sumitomo Dainippon Pharma Co., Ltd.: Research Funding. Usuki:Mochida Pharmaceutical: Speakers Bureau; Astellas Pharma Inc.: Research Funding; Sanofi K.K.: Research Funding; GlaxoSmithKline K.K.: Research Funding; Otsuka Pharmaceutical Co., Ltd.: Research Funding; Kyowa Hakko Kirin Co., Ltd.: Research Funding; Daiichi Sankyo: Research Funding; Celgene Corporation: Research Funding, Speakers Bureau; SymBio Pharmaceuticals Limited.: Research Funding; Shire Japan: Research Funding; Janssen Pharmaceutical K.K: Research Funding; Boehringer-Ingelheim Japan: Research Funding; Sumitomo Dainippon Pharma: Research Funding, Speakers Bureau; Pfizer Japan: Research Funding, Speakers Bureau; Novartis: Speakers Bureau; Nippon Shinyaku: Speakers Bureau; Chugai Pharmaceutical: Speakers Bureau; Takeda Pharmaceutical: Speakers Bureau; Ono Pharmaceutical: Speakers Bureau; MSD K.K.: Speakers Bureau. Chiba:Bristol Myers Squibb, Astellas Pharma, Kyowa Hakko Kirin: Research Funding. Miyawaki:Otsuka Pharmaceutical Co., Ltd.: Consultancy; Novartis Pharma KK: Consultancy; Astellas Pharma Inc.: Consultancy.

Description: Up to 70% of patients with del(5q) MDS may respond to Lenalidomide (LEN). However, the success rates in non-del(5q) cases, while substantial, are much lower (ranging from 20-40% depending on selection criteria). Aside from the presence of del(5q), up front identification of potentially responsive patients is difficult, particularly as the mechanistic underpinnings of LEN response are still under investigation. Initial attempts to prospectively predict LEN sensitivity resulted in a description of response expression signatures, but they have not been robust enough to serve as an actionable diagnostic test. In the outset of this study, we stipulated that apart of clinical selection (low risk MDS, transfusion-dependence, normal/low risk cytogenetics, etc.), analyses of molecular lesions including somatic mutations and chromosomal defects may help to predict LEN responsiveness. To that end, we performed deep targeted NGS (using multiamplicon panel of the top most commonly mutated genes in MDS). In total we analyzed 143 cases of myeloid neoplasms (MDS, MDS/MPN, or MPN) treated with LEN (median duration 6 months) for whom annotated clinical outcomes were available (83 responders vs. 60 refractory cases). Clinical parameters including IPSS-R, cytogenetics (FISH, SNP-array or metaphase cytogenetics) were used to characterize patients, whose responses were assessed by 2006 IWG criteria. Initially, in a combined analysis, we included both del(5q) (N =37) and non-del(5q) patients (N =106). Very low/low, intermediate, high/very high IPSS-R scores were found in 47%, 23%, 34% cases, respectively. Of 143 LEN-treated patients, regimens included LEN (80%), or LEN+5-Aza (20%). Any hematologic improvement (HI), partial response (PI), and complete response (CR) were achieved in 44%, 14% and 42%, respectively. Responses were associated with a better survival (median survival time 6.2 yrs. vs. 3.7 yrs. in refractory cases; P =.003). Non-responders showed significantly lower platelet levels compared to responders (median 169 vs. 89 K/uL; P =.007) but intricate analysis of clinical parameters (age, other blood counts, blasts and IPSS-R score) failed to identify other factors that would help to select potential responders. As expected, when sub-analysis of patients with del(5q) was performed, combined response was achieved in 78% (OR 13.14 [4.34-39.75]; PR 16%, CR 35%) of patients, respectively, while in non-del(5q) the responses were as predicted lower at 51% (P =.004). Of note is that both del(5q) involving and excluding commonly retained regions (CRR; q11.1-q14.2 and/or q34-qter) also was associated with sensitivity (CRR affected; OR=9.9 [1.4-102], vs. CRR not affected; OR=6.3 [1.3-37.6]). When we analyzed impact of karyotype on LEN sensitivity, -7/del(7q), -20/del(20q), complex karyotype and normal cytogenetics did not correlate with response, but in addition to del(5q); the presence of +8 (7/10 responded; OR 12.25 [1.33-113.06]) was significantly associated with responsiveness. Using targeted deep NGS, we confirmed 168 somatic mutations in responders vs. 142 mutations in non-responders (OR .85; .67-1.07). The number of mutational events per patient did not correlate with responses (P =.38). Among genes sequenced mutations in DDX41 (100% vs. 0%; OR infinity [6.7-infinity]) and RUNX1 mutation+deletion (75% vs. 25%; OR=8.1 [1.1-84.6]) were overrepresented in responders vs. refractory cases while in U2AF1 mutationswere more common among non-responders (20% vs. 80%; OR=.075 [.004-.76]). When reverse analysis was performed DDX41 mutations correlated with LEN response (10% mutant cases among responders vs. 0% in refractory; P =.009), while mutations in U2AF1 correlated with LEN failure (2.4% vs. 13.3% of mutant cases in responders and refractory, respectively; P =.02). The presence of all combined or any of the other spliceosomal mutations (SRSF2, SF3B1, ZRSR2, LUC7L2, and PRPF8) did not influence the results of the therapy. All TP53 mutations were found with complex karyotype with del(5q) and 5/7 (71%) TP53 mutant cases were treatment failures (OR .19 [.01-2.41]). In conclusion, in addition to the presence of del(5q), low platelet count and the presence of various molecular lesions (+8 and RUNX1, DDX41 mutations or wild type status of U2AF1) may help to predict responses to LEN. Disclosures Sekeres: Celgene Corporation: Membership on an entity's Board of Directors or advisory committees; TetraLogic: Membership on an entity's Board of Directors or advisory committees; Amgen: Membership on an entity's Board of Directors or advisory committees. Santini:celgene, Janssen, Novartis, Onconova: Honoraria, Research Funding. List:Celgene Corporation: Honoraria, Research Funding.

Description: Interstitial deletion of the long arm of chromosome 5 (del(5q)) is the most common chromosomal abnormality in MDS. The extent of individual defects vary, which may account for observed clinical diversity. Del(5q) pathogenesis has been related to haploinsufficiency of genes contained in the common deleted regions (CDR), including RPS14, miR-145/146a and SPARC. Driver mutations or pathogenic microdeletions were not identified for these genes, suggesting that multiple genes must function in combination to promote clonal evolution and phenotypic heterogeneity. Hence, we performed a comprehensive analysis of somatic mutations in genes located on chromosome 5 (chr5), both in patients with diploid 5q and in those with del(5q), to clarify the role of germline and somatic mutations in disease pathogenesis. In parallel, expression analysis was performed to correlate haploinsufficiency with the frequency of mutational events, in particular for diploid 5q cases. Applying SNP-array karyotyping to samples from 146 patients with del(5q), the lesion was identified in 5q31.1q33.1. Two retained regions (CRRs) were also observed in q11.1q14.2 (CRR1) and q34qter (CRR2). Lower-risk MDS is frequently affected by CDR, while in higher-risk MDS and secondary AML CRR1/2 are commonly co-involved. Using whole exome sequencing, we identified 11 hemizygous mutations located within the deleted area in del(5q) (N=59), while in cases diploid for 5q (N=330), 243 heterozygous mutations were found. One of the mutations discovered on chr5q afflicted a gene G3BP1 (5q33.1), located within the CDR and present in 2 patients. Both were missense mutations (one heterozygous and the other homo/hemizygous). A mutant case showed good responses to lenalidomide even though diploid 5. In addition, other somatic mutations of driver genes including TET2, CUX1 and EZH2 were concomitantly observed. Whole transcriptome sequencing demonstrated hemizygous loss of G3BP1 resulting in haploinsufficiency. G3BP1 was haploinsufficient in 48% of RAEB as well as low-risk MDS cases with del(5q). Overall, defective G3BP1 is associated with shorter overall survival (P

Description: While patients with del(5q) MDS treated with Lenalidomide (LEN) have a response rate as high as 70%, the efficacy of this drug is lower in non-del(5q) cases. Aside from the presence of del(5q), up front identification of potentially responsive patients is difficult, particularly as the mechanistic underpinnings of LEN response have not been elucidated. Although expression signatures of responders were described in 2008, they have not yet been translated into an actionable diagnostic test. Analyses of molecular lesions including somatic mutations and chromosomal defects may predict response to LEN in MDS. We performed deep targeted DNA sequencing on 62 genes in 111 cases of myeloid neoplasms (MDS, MDS/MPN, and MPN) treated with Len for at least 3 months for whom fully annotated clinical outcomes were available. Clinical parameters, FISH, SNP array-based karyotyping and metaphase cytogenetics were also included in our analysis. We assessed response according to IWG 2006 criteria and performed analyses for responses at 3 or 6 months of therapy. Of 111 LEN-treated patients, 77% had lower-risk MDS (IPSS Low /Int-1) and 23% higher-risk disease (IPSS Int-2/High/sAML). Regimens included either LEN alone (52%), or in various combinations (29%) LEN+azacytidine, TLK+LEN (1.8%) or high-dose chemotherapy (7+3)+LEN (0.9%). Any hematologic improvement, cytogenetic response, and complete response (BM) were achieved in 58%, 19% and 18% at 3 months and 84%, 44% and 30% at 6 months, respectively. Responders had better survival, with HR=0.55 (0.32, 0.94; P=.03). The mean age did not differ between responders and non-responders. Using IPSS scoring criteria, there was no difference in proportion of patients with lower-risk disease among responders and non-responders (73% vs. 81%). When IPSS-revised (-R) score was applied, there also was no significant difference between responder and non-responders with very low risk (4% vs. 7%), low risk (30% vs. 41%), intermediate risk (22% vs. 15%), high risk (29% vs. 22%), and very high risk (14% vs.12%). Refractory patients showed significantly lower platelet counts compared to responders (117 vs. 215 K/uL; P=.01). Responders tended to have higher reticulocyte counts prior to therapy compared to non-responders (0.5 vs. 0.3 M/uL; P=.07) and had significantly higher MCVs compared to refractory cases (99 vs. 91fL; P

Description: The molecular pathogenesis of myeloid neoplasms characterized by 5q deletion (del(5q)) has not been completely elucidated. While some pathomorphologic features including e.g., megakaryocytic and erythroid dysplasia, have been associated with specific genes within minimal common deleted regions (CDR), genes responsible for clonal advantage and expansion have not been identified. It is not clear if haploinsufficiency of one or multiple genes within del(5q) is responsible for clonal evolution or whether mutations in those genes or other genes located in other genomic areas are present. Moreover, with the recognition of intra-tumor diversity and hierarchical clonal architecture, it may be possible to establish whether del(5q) or other lesions, including common somatic mutations, constitute the ancestral event in the pathophysiologic cascade. We performed a comprehensive mutational screen in 124 patients with del(5q), including 59 patients studied by whole exome sequencing (WES) and 65 by targeted deep NGS of genes within the deleted area and the other most commonly mutated genes as previously determined in WES cohorts. To identify pathogenic genes, those most consistently found to be haploinsufficient in del(5q) were matched for the presence of mutations in diploid cases. For the purpose of this study haploinsufficiency was quantitated based on the number of cases with del(5q) showing

Description: Background In patients with Myelodysplastic Syndromes (MDS), TP53 mutations associate with high-risk presentation, complex karyotype, acute myeloid leukemia (AML) progression and poor response to hematopoietic stem cell transplantation. These associations highlight the relevance of TP53 as a prognostic and predictive biomarker. Consistent with its role as a tumor suppressor, bi-allelic targeting of the TP53 locus is a frequent but not an obligatory event. Despite the central role of TP53 in MDS, the clinical implications of TP53 mutations in the context of allelic state have not been extensively studied. Methods Under the auspices of the International Working Group for Prognosis in MDS, we sequenced a representative cohort of 3,324 peri-diagnosis MDS patients on a next generation sequencing (NGS) panel optimized for myeloid disease. Conventional G-banding analysis (CBA) was available for 2,931 patients. Focal (~3MB) gains and deletions and regions of NGS-derived copy-neutral loss of heterozygosity (cnLOH) were assessed using an in-house algorithm CNACS. Putative oncogenic mutations in TP53 were characterized by consideration of normal controls and established population databases. A validation cohort of 1,120 samples with independent but comparable molecular and clinical annotation was sourced from a compendium of Japanese MDS data to include JALSG-MDS212, JMDP registry, and regional registries. Results NGS-derived ploidy alterations and CBA show a high genome-wide concordance. From NGS profiles, 11% of patients (n=360) are subject to cnLOH, of which 80 target the TP53 locus. We characterize 490 TP53 mutations in 380 patients, representing 11% of the cohort. Amongst those patients, 22% (n=85) and 21% (n=78) have a deletion or a cnLOH involving the TP53 locus, respectively. Taken together, these segregate patients into two TP53 states: a mono-allelic state where one wild type allele remains (33% of TP53 mutated patients, n=126); and a multi-hit state where TP53 is altered multiple times by either mutations, deletions or cnLOH (67% of TP53 mutated patients, n=254). We find that TP53 state shapes clinical presentation and outcomes. Mono-allelic TP53 patients present with more favorable disease than multi-hit TP53 patients: they are less cytopenic, have lower bone marrow blasts (median 4 vs. 9%, p

Description: Clinical and pathomorphologic diversity in MDS is a reflection of heterogeneity of molecular lesions. Somatic mutations and chromosomal deletions/amplifications affect various pathways in a convergent and divergent fashion, generate phenocopy and can occur in a variety of combinations. Recent technological advances, including high density arrays and the new generation sequencing (NGS) led to the discovery of novel pathway mutations or gene families affected by somatic defects, e.g., cohesin or spliceosomal mutations. We have performed whole exome NGS of paired (tumor/germ line) samples in 222 patients with myeloid neoplasms from the Cleveland Clinic and University of Tokyo. Clinical parameters were studied including age, gender, overall survival (OS), bone marrow blast count, and metaphase cytogenetics. Additionally, we also used in our analysis data sets from 197 AML included in the Cancer Genome Atlas (TCGA). We found 1.4% (6/419) of non-canonical somatic mutations of KIF2Bwhich is a member of kinesin13 family located on the long arm of chromosome 17; 3 cases from our cohort (p.V32M (c.G94A), p.T113M (c.C338T), p.R163C (c.C487T)) and 3 cases from TCGA database (p.T47M (c.C140T), p.T310M (c.C929T), p.H551N (c.C1651A)). By analyzing clonal architecture and intra-tumor heterogeneity in 2 cases (RCMD and RAEB) by targeted deep sequencing, allelic frequencies of KIF2B mutations were more than 45% and larger than for any other concomitant mutations, suggesting that KIF2B mutations might consequently constitute ancestral events followed by subclonal acquisitions of the other mutations. Of note is that 6 non-sense mutations were also reported in lung cancer. Based on SNP-array mapping of chromosomal abnormalities, deletions of 17q involving the KIF2B locus (17q22) was present about 3% (6/215) of myeloid neoplasm. KIF2B defects were frequently detected in higher-risk MDS and AML phenotypes (9%). KIF2B performed an important role in regulation of kinetochore-microtubule attachment. Previous studies showed that the velocity of chromosomes’ movement in KIF2B-deficient cells is reduced 80% comparing to control and fail to perform cytokinesis. In our series, 56% of myeloid neoplasms with KI2B defects had complex cytogenetics and 67% cases of them were also UPD, suggesting that KIF2B defects might lead to inducing abnormal chromosomal movements and segregations. We then, expanded our study to the whole kinesin gene family: 17 somatic mutations and 57 deletions were identified in KIF1A (n=6), KIF23 (n=1), KIF26A (n=1), KIF27 (n=7), KIF1C (n=9), KIF21B (n=2), KIF13A (n=10), KIF14 (n=2), KIF17 (n=15), KIF25 (n=1), KIF3C (n=8), KIF6 (n=2) and CENPE (n=10). All mutations were heterozygous and mutually exclusive. By survival analysis of such mutated cases, a tendency towards worse prognosis was observed (HR; 1.72, 95%CI 0.86-3.37). Analysis of concomitant mutations associated with whole kinesin family mutations or deletions showed that most frequently affected genes are TET2 (n=14), DNMT3A (n=8), IDH1/2 (n=8) and MLL (n=5), all involved in epigenetic regulation. In conclusion, somatic mutations in kinesin family genes are found in myeloid malignancies and might be responsible for another pathogenesis of the disease. KIF2B is most frequently found in myeloid malignancies and associated with aggressive type of MDS. Since knockout mice of multiple kinesin family genes (KIF5A, KIF16B and EG5) were lethal in embryo and all the mutations occur in a heterozygous configuration, it is likely synthetic lethal approach might create therapeutic window between defective malignant cells and healthy controls. Kinesin family of motor proteins may be an emerging novel therapeutic target. In fact some kinesins have been already successfully targeted in solid tumors. Disclosures: Polprasert: MDS foundation: Research Funding. Makishima:AA & MDS international foundation: Research Funding; Scott Hamilton CARES grant: Research Funding. Maciejewski:NIH: Research Funding; Aplastic anemia&MDS International Foundation: Research Funding.

Description: Background Bone marrow staging is important in determining the extent of disease and for making therapeutic decision in B-cell non-Hodgkin lymphoma (B-NHL). Flow cytometry has been widely used in this setting but its value has not been clearly demonstrated. Objectives To determine the usefulness of bone marrow flow cytometry in staging of B-NHL. Materials and Methods We compare the results of bone marrow studies using bone marrow aspirate (BMA), bone marrow biopsy (BMB) and three-color flow cytometry (FCM), in non-leukemic B-NHL patients performed during May 2003 to December 2006 at our institution and determine the concordant rate among these three methods in detecting bone marrow involvement by B-NHL. Results Two hundred and sixty-eight bone marrow samples were analyzed in this study. Diffuse large B-cell lymphoma (DLBCL) was the most common histologic subtype (60.8%). Bone marrow (BM) involvement as confirmed by immunohistochemistry of BMB was observed in 66 samples (24.6%). Prevalence of BM involvement was slightly higher in indolent B-NHL (32.2%) as compared to aggressive B-NHL (20.8%). Agreement among all three methods was observed in 163 samples (60.8%). BMA had the sensitivity of 77.3% and specificity of 73.8%. FCM alone had the sensitivity of 63.6% and specificity of 89.6%. FCM failed to demonstrate clonal B-cells mainly in patients with focal involvement pattern. When FCM was combined with BMA, the sensitivity was increased to 87.9% but the specificity was decreased to 67.8%. They did not perform significantly better than using morphological criteria in BMB alone, which showed sensitivity of 80.3% and specificity of 94.5%. Conclusion Based on these data, we conclude that flow cytometry may not be necessary in bone marrow staging of B-NHL in patients with non-leukemic presentation.

Description: Borjeson−Forssman−Lehmann syndrome (BFLS), a hereditary X-linked disorder characterized by mental retardation, truncal obesity, gynecomastia, hypogonadism and other dysmorphic features, is known to be caused by germline (GL) mutations of plant homeo domain finger protein 6 (PHF6). PHF6 is a highly conserved 41kDa protein showing ubiquitous expression in a variety of tissues, including bone marrow, CD34+ cells and blood leukocytes. Human PHF6 is located on chrXq26.2. Recently, rare somatic nonsense mutations and deletions have been detected in patients with T-ALL and AML and found in some T-ALL cell lines. Patients with BFLS with PHF6 mutations have been reported to develop leukemia, suggesting PHF6 mutations may predispose cancer. Although the actual function and molecular pathogenesis is unknown, PHF6 has been suggested to be a tumor suppressor gene involved in the control of myeloid development. In an index case of a young adult female patient with proliferative CMML with dysmorphic features, we have identified remarkable GL mosaicism for PHF6 mutation (p.K44fs), confirmed by deep sequencing of marrow, CD3+ cells and skin tissue. Subsequently, we screened patients with myeloid neoplasms by targeted multi-amplicon sequencing to determine the prevalence and distribution of PHF6 gene alterations. Sequencing results from 1072 cases were analyzed (728 by targeted deep sequencing and 344 by whole exome sequencing). In total, we identified 21 cases with PHF6 mutations, 13 of which were frameshift or nonsense mutations. Previously, PHF6 have been included in screening panels by Haferlach et al., (Leukemia 2014) and Papaemmanuil et al., (Blood 2013) and somatic mutations were found in 24/944 and 21/738 cases of MDS, respectively. These results along with ours suggest that PHF6 mutations are common driver events. The somatic nature of these defects was confirmed by analysis of non-clonal CD3+ lymphocytes, thus, PHF6 mutations occur at a frequency of 2.0% and are most frequently observed among patients with secondary AML (33%, P=.0021). Gender distribution showed a strong male predominance (76%), likely due to the location of PHF6 on chrX and indicating that retention of a single copy of PHF6 may be protective. SNP-array karyotyping showed that deletions of Xq, involving the PHF6 locus (Xq26), were present in about 1.2% of myeloid neoplasms and affect only female patients. As a family, plant homeo domain (PHD) finger genes are affected by mutations associated with various cancers. JARID1A, PHF23, NSD1 and NSD3 were described to serve as fusion partners with the NUP98 in a subset of AML cases. The most frequent chromosomal aberration observed in conjunction with PHF6 mutations was trisomy-8 (P=.08). The most commonly associated somatic mutations were in RUNX1 (N=7; P=.001), U2AF1 (N=5), ASXL1 (N=5), IDH1 (N=4), and DNMT3A (N=4). Interestingly, 6/7 cases with concomitant PHF6 and RUNX1 mutations showed a poor prognosis AML. Subsequent analysis of clonal architecture using variant allelic frequency calculations and serial samples for these cases suggested that PHF6 may function as a founder driver gene while RUNX1 mutations are acquired as secondary events. Recent studies proposed that PHF6 deficiency leads to impaired cell proliferation, cell cycle arrest at G2/M phase and an increase of DNA damage. To examine DNA damage and quantify double stranded breaks (DSBs) in primary cells from PHF6-mutants, those with wild-type (WT) PHF6 and normal bone marrow we used a flow cytometric anti-γH2AX assay, following induction of DNA damage with Camptothecin. As judged by greater percentages of anti-γH2AX labeled cells, DSBs were more common in mutant cases consistent with more DNA damage present in PHF6 mutant compared to WT MDS and normal bone marrow cells. In conclusion, our results indicate that PHF6 mutations are generally present in more aggressive types of myeloid neoplasms, frequently associated with RUNX1 mutations. Our functional in vitro studies along with recently published reports suggest an association of PHF6 deficiency with genomic instability and thereby provide a basis for a mutator phenotype conveyed by ancestral lesions, consistent with its role as a tumor suppressor gene. Disclosures Sekeres: Celgene: Membership on an entity's Board of Directors or advisory committees; Amgen Corp: Membership on an entity's Board of Directors or advisory committees; Boehringer-Ingelheim Corp: Membership on an entity's Board of Directors or advisory committees.