Abstract: Magnetic reconnection, topological changes in magnetic fields, is a fundamental process in magnetized plasmas. It is associated with energy release in regions of magnetic field annihilation, but this is only one facet of this process. Astrophysical fluid flows normally have very large Reynolds numbers and are expected to be turbulent, in agreement with observations. In strong turbulence, magnetic field lines constantly reconnect everywhere and on all scales, thus making magnetic reconnection an intrinsic part of the turbulent cascade. We note in particular that this is inconsistent with the usual practice of magnetic field lines as persistent dynamical elements. A number of theoretical, numerical, and observational studies starting with the paper done by Lazarian and Vishniac [Astrophys. J. 517, 700–718 (1999)] proposed that 3D turbulence makes magnetic reconnection fast and that magnetic reconnection and turbulence are intrinsically connected. In particular, we discuss the dramatic violation of the textbook concept of magnetic flux-freezing in the presence of turbulence. We demonstrate that in the presence of turbulence, the plasma effects are subdominant to turbulence as far as the magnetic reconnection is concerned. The latter fact justifies a magnetohydrodynamiclike treatment of magnetic reconnection on all scales much larger than the relevant plasma scales. We discuss the numerical and observational evidence supporting the turbulent reconnection model. In particular, we demonstrate that the tearing reconnection is suppressed in 3D, and unlike the 2D settings, 3D reconnection induces turbulence that makes magnetic reconnection independent of resistivity. We show that turbulent reconnection dramatically affects key astrophysical processes, e.g., star formation, turbulent dynamo, and acceleration of cosmic rays. We provide criticism of the concept of “reconnection-mediated turbulence” and explain why turbulent reconnection is very different from enhanced turbulent resistivity and hyper-resistivity and why the latter have fatal conceptua l flaws.

Abstract: TP53 aberrations, including somatic mutations of TP53 gene or 17p deletion leading to the loss of the TP53 locus, are a major predictive factor of resistance to fludarabin based chemotherapy in chronic lymphocytic leukemia (CLL) and remain an adverse prognostic factor in the chemofree era. Therefore, detection of TP53 alteration before each new line of treatment is required for theranostic stratification. In order to better characterize the distribution and combination of the TP53 variants in CLL, we collected the TP53 sequencing data of 343 patients harboring TP53 mutations from centers of the French Innovative Leukemia Organization-CLL (FILO) and established a large data base of 573 TP53 mutations. Mutations were identified through NGS sequencing (covering exon 2 to 11) allowing the detection of low frequency variants down to 1% VAF. Several distinct low VAF mutations were orthogonally confirmed by digital PCR. TP53 variants were analyzed through UMD_TP53 data gathering 90 000 TP53 mutations from all type of cancers. IGHV mutational status and FISH analysis were available for 224 and 176 patients respectively. Using ACMG criteria from the UMD_TP53 database, we confirmed that 523 could be classified as pathogenic, 42 were likely pathogenic and 8 were VUS (Variants of Unknown Significance). As expected, the mutation distribution along the p53 protein exhibited a clustering of variants in the DNA binding domain of the protein. We also confirmed the presence of a specific hotspot at codon 234 (6%) which is noticeable in other CLL cohorts but absent in solid tumors. 431 TP53 variants led to the expression of a mutant protein whereas the remaining 142 led a TP53 null phenotype. For 8 patients without 17p deletion and a mutation VAF larger than 50%, SNP analysis indicate that these tumors had a copy number neutral loss of heterozygosis at 17p with a duplication of the mutant allele leading to homozygous mutations of TP53. When focusing on the allele burden of TP53 mutations, 264/573 (46%) variants had an allele frequency 〈 10%. Even if they were predominantly found in polymutated cases, presence of only low VAF ( 〈 10%) mutations was evidenced in 74 (21%) patients (50 patients with a single TP53 mutation and 24 patients with more than one). All these cases would have been missed by conventional sequencing. Among the 343 patients, 113 (33%) were poly-mutated and harbored more than one pathogenic TP53 variants (2 to 11 variants per patient): 57 (16,7 %) had 2 variants, 32 (9,3%) had 3, 10 had 4 (3%) and 14 patients (4%) had 5 to 11 variants. Using both long range sequencing and in silico analysis, we could show that all these variants were distributed in different alleles supporting an important intratumoral heterogeneity and a strong selection for TP53 loss of function during tumor progression in these patients. Null variants were rarely found as single alteration: only 46 patients (13,4%) patients harbored a single null mutation. Null mutations were predominantly found in patients with multiclonal mutations (87% with 4 or more). Median size of variants significantly decreased with the number of mutations and most of low VAF (less than 10%) variants were found in multiclonal combinations. Multiclonal mutations were predominantly found in previously treated patients (41% treated versus 10 % untreated) but whether all these variants preceded treatment and were further selected is currently unknown. We observed that 71,5 % of patients were IGHV unmutated and multiclonal mutations were surprisingly more frequent in mutated IGHV cases than in unmutated ones. Only 50% of cases carried a 17p deletion, highlighting again the importance of testing for TP53 mutations in addition to FISH analysis. Presence or absence of 17p deletion was unrelated to the number of TP53 mutations. Taken together these observations suggest that the TP53 mutational landscape in CLL is very complex and can involve multiple mechanisms, converging to a total loss of TP53 function and tumor progression. NGS provides a powerful tool for detecting all these alterations including variants with low VAF and should become a standard for CLL screening prior to each line of treatment. Disclosures Leblond: Amgen: Honoraria, Speakers Bureau; Abbvie: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Janssen: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Gilead: Honoraria, Speakers Bureau; Astra Zeneca: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Roche: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees, Speakers Bureau. Letestu:Abbvie: Membership on an entity's Board of Directors or advisory committees, Other: speaker fee, expert contracts; Janssen: Membership on an entity's Board of Directors or advisory committees, Other: speaker fee, expert contracts; Roche: Membership on an entity's Board of Directors or advisory committees, Other: speaker fee, expert contracts; Alexion: Membership on an entity's Board of Directors or advisory committees, Other: speaker fee, expert contracts. Cymbalista:Abbvie: Honoraria; Roche: Research Funding; Sunesis: Research Funding; Gilead: Honoraria; Janssen: Honoraria; AstraZeneca: Honoraria.

Abstract: A hotspot mutation within the DNA-binding domain of IKZF3 (IKZF3-L162R) has been identified as a putative driver in chronic lymphocytic leukemia (CLL); however, its functional effects are unknown. We recently confirmed its role as a CLL driver in a B cell-restricted conditional knock-in model. IKZF3 mutation altered mature B cell development and signaling capacity, and induced CLL-like disease in elderly mice (~40% penetrance). Moreover, we found IKZF3-L162R acts as a gain-of-function mutation, altering DNA binding specificity and target selection of IKZF3, and resulting in overexpression of multiple B-cell receptor (BCR) genes. Consistent with the murine data, RNA-sequencing analysis showed that human CLL cells with mut-IKZF3 [n=4] have an enhanced signature of BCR-signaling gene expression compared to WT-IKZF3 [n=6, all IGHV unmutated] (p & lt;0.001), and also exhibited general upregulation of key BCR-signaling regulators. These results confirm the role of IKZF3 as a master regulator of BCR-signaling gene expression, with the mutation contributing to overexpression of these genes. While mutation in IKZF3 has a clear functional impact on a cardinal CLL-associated pathway, such as BCR signaling, we note that this driver occurs only at low frequency in patients (~3%). Because somatic mutation represents but one mechanism by which a driver can alter a cellular pathway, we examined whether aberrant expression of IKZF3 could also yield differences in BCR-signaling gene expression. We have observed expression of the IKZF3 gene to be variably dysregulated amongst CLL patients through re-analysis of transcriptomic data from two independent cohorts of human CLL (DFCI, Landau et al., 2014; ICGC, Ferreira et al., 2014). We thus examined IKZF3 expression and BCR-signaling gene expression, or the 'BCR score' (calculated as the mean expression of 75 BCR signaling-associate genes) in those cohorts (DFCI cohort, n=107; ICGC cohort, n=274). Strikingly, CLL cells with higher IKZF3 expression (defined as greater than median expression) had higher BCR scores than those with lower IKZF3 expression ( & lt;median) (p=0.0015 and p & lt;0.0001, respectively). These findings were consistent with the notion that IKZF3 may act as a broad regulator of BCR signaling genes, and that IKZF3 overexpression, like IKZF3 mutation, may provide fitness advantage. In support of this notion, our re-analysis of a gene expression dataset of 107 CLL samples (Herold Leukemia 2011) revealed that higher IKZF3 expression associated with poorer prognosis and worse overall survival (P=0.035). We previously reported that CLL cells with IKZF3 mutation appeared to increase in cancer cell fraction (CCF) with resistance to fludarabine-based chemotherapy (Landau Nature 2015). Instances of increase in mut-IKZF3 CCF upon treatment with the BCR-signaling inhibitor ibrutinib have been reported (Ahn ASH 2019). These studies together suggest an association of IKZF3 mutation with increased cellular survival following either chemotherapy or targeted treatment. To examine whether higher expression of IKZF3 was associated with altered sensitivity to ibrutinib, we performed scRNA-seq analysis (10x Genomics) of two previously treatment-naïve patients undergoing ibrutinib therapy (paired samples, baseline vs. Day 220). We analyzed an average of 11,080 cells per patient (2000 genes/cell). Of note, following ibrutinib treatment, remaining CLL cells expressed higher levels of IKZF3 transcript compared to pretreatment baseline (both p & lt;0.0001), whereas no such change was observed in matched T cells (n ranging between 62 to 652 per experimental group, p & gt;0.05), suggesting that cells with high expression of IKZF3 were selected by ibrutinib treatment. Moreover, we showed that ibrutinib treatment resulted in consistent upregulation of BCR-signaling genes (e.g., CD79B, LYN, GRB2, FOS, RAC1, PRKCB and NFKBIA) (n ranging between 362 to 1374 per experimental group, all p & lt;0.0001), which were likewise activated by mutant IKZF3. Altogether, these data imply that IKZF3 mutation or overexpression may influence upregulation of BCR-signaling genes and enhance cellular fitness even during treatment with BCR-signaling inhibitors. We highlight our observation that IKZF3 mutation appears to be phenocopied by elevated IKZF3 expression, and suggest that alterations in mRNA or protein level that mimic genetic mutations could be widespread in human cancers. Disclosures Kipps: Pharmacyclics/ AbbVie, Breast Cancer Research Foundation, MD Anderson Cancer Center, Oncternal Therapeutics, Inc., Specialized Center of Research (SCOR) - The Leukemia and Lymphoma Society (LLS), California Institute for Regenerative Medicine (CIRM): Membership on an entity's Board of Directors or advisory committees, Research Funding, Speakers Bureau; Celgene: Honoraria, Research Funding; Gilead: Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Genentech/Roche: Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; VelosBio: Research Funding; Oncternal Therapeutics, Inc.: Other: Cirmtuzumab was developed by Thomas J. Kipps in the Thomas J. Kipps laboratory and licensed by the University of California to Oncternal Therapeutics, Inc., which provided stock options and research funding to the Thomas J. Kipps laboratory, Research Funding; Ascerta/AstraZeneca, Celgene, Genentech/F. Hoffmann-La Roche, Gilead, Janssen, Loxo Oncology, Octernal Therapeutics, Pharmacyclics/AbbVie, TG Therapeutics, VelosBio, and Verastem: Membership on an entity's Board of Directors or advisory committees. Wu:BionTech: Current equity holder in publicly-traded company; Pharmacyclics: Research Funding.

Abstract: Amongst the novel putative drivers identified by large-scale sequencing studies of chronic lymphocytic leukemia (CLL) is the ribosomal protein RPS15. Mutated in 5.3% of CLL, it co-occurs with heterozygous TP53 alterations in 36% of RPS15-mutated samples. Mutation of this mediator of ribosome maturation and translation is associated with poor disease prognosis and enriched in cohorts with del(17p) and relapsed CLL, suggesting a role in disease progression and therapeutic resistance. However, the impact of RPS15 mutation on B cell function and CLL development, in the presence or absence of TP53 mutation, has yet to be characterized. To this end, we developed overexpression HG3 CLL cell lines modeling four common RPS15 mutations (G134R, H137Y, S138F, and S139F) and a conditional knock-in mouse model of the S138F mutation with and without heterozygous Trp53 deletion (generated by crossing Rps15 and Trp53 mutant mice with Cd19-Cre mice). To characterize the impact of RPS15 mutation on transcription, we performed RNA-sequencing on splenic B cells from 3-month-old Rps15WT, Rps15Het and Rps15Hom mice (3 per cohort). We identified 255 and 670 upregulated and 596 and 777 downregulated genes in the Rps15MT vs Rps15WT mice (Rps15Het and Rps15Hom, respectively; log2FC & gt;0.5, p & lt;0.05). Gene set enrichment analysis (GSEA) revealed strong enrichment for MYC target genes that was also evident upon RNA-sequencing of the HG3 RPS15-S138F MT vs WT overexpression lines, and of 3 primary untreated CLLs with heterozygous RPS15 mutation (compared to 3 RPS15WT CLLs of similar genetic background). Pathway analysis of differentially expressed signatures across murine, cell line and primary CLL models revealed a common enrichment in translational machinery, such as mRNA splicing/processing, rRNA processing, and snRNP assembly (normalized enrichment score & gt;1, nominal p-value & lt;0.05). To evaluate whether RPS15 mutant proteins incorporate into ribosomes, we performed polysome profiling of the HG3 lines. All overexpressed RPS15-WT and MT proteins were observed to integrate into the small ribosomal subunit and mature ribosomes, potentially impacting translation. Next, ribosome profiling of HG3 RPS15-WT and S138F cells revealed 2,334 genes with differential translation efficiency (TE) between RPS15-S138F vs WT cells and 2,425 genes between RPS15-S138F vs WT in TP53 knock-out cells (log2FC & gt;0.5, p & lt;0.05). GSEA of differentially translated genes in RPS15 MT- vs WT cells revealed a strong enrichment for TP53-related genes, consistent with the activation of stress pathways by RPS15 mutant expression. RPS15 MT- vs WT cells with TP53-deletion, however, exhibited a strong increase in TE of MYC target genes and components of the ribosomal machinery. This finding suggests that loss of TP53 surveillance allows RPS15 MT cells to induce MYC-mediated changes in mRNA processing and translation - potentially setting the stage for oncogenesis. To determine whether Rps15 mutation can drive CLL-like disease, we engineered 6 novel mouse lines with B cell restricted expression of alterations through crossing with CD19-Cre mice: Rps15WT, Rps15Het, and Rps15Hom mutant mice alone or co-expressing Trp53 deletion. We detected circulating CLL-like (B220+CD5+) cells in 5 of 30 (17%) Rps15Het mice by 20 months of age, but not in 30 age-matched Rps15WT mice. We also detected CLL-like cells in 6 of 30 (20%) Trp53+/- mice by 17 months, indicating that Trp53 deletion alone can induce CLL-like disease. Interestingly, we found CLL-like cells in 2 of 30 Rps15Het/Trp53+/- mice as early as 15 months of age. The cohorts of Rps15Hom and Rps15Hom/Trp53+/- mice, however, have been monitored for 18 months of age with no disease occurrences, indicating that a double dosage of Rps15 mutation may be detrimental to disease formation. Altogether, Rps15 heterozygous mutation can drive CLL development in mice, and our early data hint that co-mutation with Trp53 may shorten the latency of CLL-like disease. Overall, RPS15 mutant protein can incorporate into the ribosome and induce changes in mRNA translation, resulting in MYC activation predominantly in the context of TP53 loss. Our mouse studies indicate that mut-Rps15 drives CLL development, with a more aggressive disease course when combined with Trp53 deletion. Our results collectively suggest that RPS15 and TP53 co-mutation drives CLL development through translational dysregulation and MYC-mediated signaling. Disclosures Neuberg: Pharmacyclics: Research Funding; Celgene: Research Funding; Madrigak Pharmaceuticals: Current equity holder in publicly-traded company. Getz:Broad Institute: Patents & Royalties: MuTect, ABSOLUTE, MutSig, MSMuTect, MSMutSig, POLYSOLVER and TensorQTL; Pharmacyclics: Research Funding; IBM: Research Funding; Scorpion Therapeutics: Consultancy, Current equity holder in publicly-traded company, Other: Founder. Wu:BionTech: Current equity holder in publicly-traded company; Pharmacyclics: Research Funding.

Abstract: Patients with chronic lymphocytic leukemia (CLL) are prone to infectious complications, including Ears, Nose and Throat (ENT) infections due to the humoral immunodepression and/or to the immunosuppression related to the therapy. However, specific CLL infiltration in non-lymphoid regions of the head and neck causing ENT symptoms but unrelated to an infection is not well described. Extra-nodal localizations of CLL cells, including involvement of the mucosa of the rhinopharynx is uncommon and poorly reported. Here, we retrospectively analyzed the clinical, histopathologic and molecular features of 25 CLL patients with specific head and neck involvement. To date, this is the largest cohort reporting this new entity of CLL also named Nasal Associated Lymphoid Tissue (NALT) CLL. All patients had proven CLL prior to symptoms. Median time between ENT manifestation and CLL was 3 years [1-11 years]. Symptoms included chronic coughing (44%), antero-posterior nasal discharge (44%), nasal congestion (33%) and pharyngitidis (33%). ENT examination evidenced cervical lymphadenopathies in 68 % of cases, a granular aspect of the mucosa of the pharynx in 56%, enlarged tonsil (37%) or adenoids (37%). All patients underwent a biopsy of the mucosa of the nose or the throat. Histology and immunochemistry analysis demonstrated an infiltration of small lymphocytes CD20+, CD5+ and CD23+, consistent with a phenotype of CLL. The infiltration was diffuse in 50% of biopsies and perivascular in 38%. Patients with NALT-CLL had a poor prognosis: the majority was IGHV unmutated (n=18/25, 72%). Furthermore, they all required a treatment according to IWCLL criteria few time after the first ENT symptoms (median time 2 years), indicating that NALT-CLL is associated with a more progressive disease. To characterize the genetic background of NALT-CLL, we analyzed the cytogenetic data and NGS sequencing of 13 CLL-associated mutations from peripheral CLL cells. The karyotype was normal in only 2/24 cases (8%) and complex ( & gt;3 abnormalities) in 5/24 cases (20%). Half of the patients had a trisomy 12(12/24; 50%), while 13q, 17p and 11q deletions were found in 29%, 8% and 4% respectively. Eleven patients harbored one mutation (44%) while 9 patients had 2 to 5. Mutation of the NOTCH1 pathway was found in half of the cases (11/25 NOTCH1 and 2/25 FBXW7 mutated cases). TP53 and SF3B1 mutations occurred respectively in 20% (5/25 cases) and in 12% (3/25 cases). To gain insight into the molecular mechanism associated with involvement of the rhinopahrynx, we studied the expression of 70 genes related to cell migration and cell adhesion pathways using a qPCR array in the peripheral CLL cells of patients with NALT (n=4) compare to no NALT-CLL (n=4). Genes significantly up regulated with a fold change & gt;2 included the chemokine receptors CCR7 (p=0.05) and CCR5 (p=0.04), the receptor involved in leukocyte trafficking CXCR3 (p=0.01) or the chemoattractant chemokine-like factor CKLF. By targeted qPCR, we confirmed the up regulation of CCR7 (p=0.002), CXCR3 (p=0.04) and CCR5 (p=0.03) in the cohort NALT-CLL patient (n=25) as compared to 20 age-matched CLL patients without head and neck symptoms (77% with unmutated IGHV, 30% with NOTCH1 mutation and 30% with trisomy 12). Up regulation of those targets was independent of the presence of NOTCH1/trisomy12 aberration or of the IGHV mutation status. These results suggest an increased migratory capacity of the leukemic CLL cells into the rhynopharynx mucosa related to a higher expression of these receptors involved in cell trafficking and migration. In line with these results, immunohistochemistry analysis of 5 patients with nasal involvement showed a strong staining of the CCR7 marker on the membrane of CLL cells infiltrating the mucosa. Interestingly, the staining of CCL21, the cognate ligand of CCR7, was positive in the vessels of the mucosa, suggesting that the recruitment and the transendothelial migration of CLL cells into the mucosa occur through a local secretion of CCL21 by the vessels. In summary, we report here a new presentation of CLL associated with symptomatic and specific ENT localization. CLL cells are predominantly IGHV unmutated, harbor NOTCH1 mutation and/or trisomy12 and show a higher expression of the chemokine receptors CCR7, CCR5 and CXCR3. We are currently studying the expression of those receptors by flow cytometry and the enhanced migratory capacity toward CCL21 through an in vitro chemotaxis assay. Disclosures Letestu: AbbVie: Research Funding, Speakers Bureau; Roche: Speakers Bureau; Janssen: Research Funding, Speakers Bureau. Cymbalista: Abbvie: Honoraria, Membership on an entity's Board of Directors or advisory committees; ASTRA ZENECA: Honoraria, Membership on an entity's Board of Directors or advisory committees; Janssen: Honoraria, Membership on an entity's Board of Directors or advisory committees; Lilly-LOXO: Honoraria, Membership on an entity's Board of Directors or advisory committees; Roche: Honoraria, Membership on an entity's Board of Directors or advisory committees.

Abstract: Introduction:Long-term outcome of patients with IPSS low or int 1 (LR) MDS treated by IST with horse or rabbit (hATG or rATG) +/-CsA is not well described and patient selection criteria for such a therapy remain ill-defined in daily practice, including after resistance to erythropoiesis stimulating agents (ESA). Methods: We report the outcome of 15 consecutive LR-MDS patients treated in our institution between 2005 and 2013, with h or rATG (15 mg/kg/d for 5 days and 3.5 mg/kg/d for 5 days, respectively), +/- CsA 2 mg/kg BID for 6 months (m). The use of h or rATG formulation was based on institutional practice and availability. Patients with altered renal function or vascular contra-indication to CsA (mostly due to age) only received ATG. Bone marrow (BM) blasts were counted over 500 cells in cases of very low BM cellularity and over 1,000 cells otherwise. FISH analysis of chromosomes 5 and 7 was performed in case of cytogenetic failure. IST response probability score (ISTRPS) was calculated according to Saunthararajah, Blood 2003. Overall response rate (ORR), including complete response (CR), assessed at 12 m, and progressive disease (PD) were defined according to IWG 2006 criteria. Transfusion independence (TI) was defined as independence from RBC transfusion for at least 8 weeks. Results: Median age was 60 years (y) (range 46-77). Median follow-up was 8.1 y. All patients were RBC-transfusion dependent (RCTD). Median RCTD duration was 4.9 m (0-42). Platelet counts were 〈 50 109/L and 〈 20 109/L in 85% and 50% of patients, respectively. Median ANC was 0.95 109/L (0.18-4.5). WHO classification was RA, RCMD, RAEB-1 and non assessable due to very low BM cellularity in 5, 6, 2 and 2 patients. Median BM blast percentage was 1% (0-8). BM cellularity was reduced in 9/14 (64%) evaluable patients. Cytogenetics according to IPSS-R was very good, good and intermediate, in 2 (13%), 10 (67%) and 1 (7%) respectively and failed in 2 (13%). IPSS-R, assessable in 14 patients, was low, int and poor in 7, 6, 1. 7/14 patients had a DRB15 allele. ISTRPS was high in 4/15 patients. A PNH clone was detected by flow in 4 patients. A large granular lymphocyte population, a TCR clonal rearrangement and clinical or serological features of an associated immune disorder were present in 7/11 (63%), 4/10 (40%) and 5/14 (36%) patients, respectively. 6/14 patients had received at least one prior therapy for MDS including ESA in 5 and CsA, androgens, steroids, thalidomide, lenalidomide in 2, 3, 3, 1, 1 patients, respectively. 10 patients received hATG (66%), 5 (34%) rATG, combined to CsA in 7/15 (46%). In the 14 patients evaluable for response (early death, n=1), ORR including TI was 7/14 (50%) (4 CR and 3 HI; 6 TI). Median time to TI was 5.5 m (1-8). Median TI duration was 6.2 y. The only significant prognostic factors of TI were BM blast counts (60% if BM blasts ≤2% versus 0 if BM blasts 〉 2%, P=0.04) and RCTD duration (62.5% if RCTD ≤6 m versus 0 if RTCD 〉 6 m, P=0.02). There was a trend for a higher TI rate with lower platelet counts (P=0.13) and high ISTRPS (P=0.14). By contrast age, using a 60y cut-off, and presence of a DRB15 allele were not predictive of TI (P=0.58 and 0.31) in this cohort. Two relapsed patients were retreated with hATG +CsA and achieved a second durable response (1 CR (45m+) and 1 TI (9m+)). One non-responding patient, who subsequently developed full-blown PNH, responded to CsA and eculizumab. 5-y cumulative incidence of PD and AML were 42% and 6% respectively and median OS was 5.6 y. Median OS was not reached in responders versus 22 m in non-responders (P=0.004 by log-rank test). A single toxic death from a hemorrhagic stroke occurred very early in a patient heavily pretreated for MDS-associated vasculitis. PNH clone expansion and cytogenetic evolution occurred in 2 (both responders) and 3 patients (2 responders). Conclusion: Lower-risk MDS patients responding to IST have a good long-term outcome. After relapse, durable responses to a second course of IST can be achieved. In addition to RCTD ≤6 m, BM blasts ≤2% emerged as an IPSS-R component strongly predicting response to IST. Age 〉 60 y was not associated with worse response. Therefore, IST should be considered as a treatment option for selected lower-risk MDS patients with very low blast counts, irrespective of age. Disclosures Off Label Use: ATG and cyclosporin A are not approved for the treatment of MDS..

Abstract: TP53 aberrations are a major predictive factor of resistance to chemoimmunotherapy in chronic lymphocytic leukemia (CLL), and an assessment of them before each line of treatment is required for theranostic stratification. Acquisition of subclonal TP53 abnormalities underlies the evolution of CLL. To better characterize the distribution, combination, and impact of TP53 variants in CLL, 1,056 TP53 variants collected from 683 patients included in a multicenter collaborative study in France were analyzed and compared to UMD_CLL, a dataset built from published articles collectively providing 5,173 TP53 variants detected in 3,808 patients. Our analysis confirmed the presence of several CLL-specific hotspot mutations, including a two-base pair deletion in codon 209 and a missense variant at codon 234, the latter being associated with alkylating treatment. Our analysis also identified a novel CLL-specific variant in the splice acceptor signal of intron 6 leading to the use of a cryptic splice site, similarly utilized by TP53 to generate p53psi, a naturally truncated p53 isoform localized in the mitochondria. Examination of both UMD_CLL and several recently released large-scale genomic analyses of CLL patients confirmed that this splice variant is highly enriched in this disease when compared to other cancer types. Using a TP53-specific single-nucleotide polymorphism, we also confirmed that copy-neutral loss of heterozygosity is frequent in CLL. This event can lead to misinterpretation of TP53 status. Unlike other cancers, CLL displayed a high proportion of patients harboring multiple TP53 variants. Using both in silico analysis and single molecule smart sequencing, we demonstrated the coexistence of distinct subclones harboring mutations on distinct alleles. In summary, our study provides a detailed TP53 mutational architecture in CLL and gives insights into how treatments may shape the genetic landscape of CLL patients.

Abstract: What's New? For patients with chronic lymphocytic leukemia, possessing multiple chromosomal abnormalities means bad news. Could there be a way to predict genomic instability before it happens? These authors investigated whether mutations in TP53 might presage the onset of chromosomal abnormalities. They conducted a longitudinal study, in which they followed 31 CLL patients over a period of several years. It turned out that mutations in TP53 occur early and precede the genomic instability that generates chromosomal abnormalities and hinders recovery. Thus, searching for TP53 mutations could identify those patients likely to develop additional chromosomal defects and poor outcomes.