Chronic lymphocytic leukemia (CLL) is characterized by the existence of subsets of patients with (quasi)identical, stereotyped B cell receptor immunoglobulins (BcR IG). Patients in certain major stereotyped subsets often display remarkably consistent clinicobiological profiles, suggesting that the study of BcR IG stereotypy in CLL has important implications for understanding disease pathophysiology and refining clinical decision-making. Nevertheless, several issues remain open, especially pertaining to the actual frequency of BcR IG stereotypy and major subsets, as well as the existence of higher-order connections between individual subsets. In order to address these issues, we investigated clonotypic IGHV-IGHD-IGHJ gene rearrangements in a series of 29,856 patients with CLL, by far the largest series worldwide. We report that the stereotyped fraction of CLL peaks at 41% of the entire cohort and that all 19 previously identified major subsets retained their relative size and ranking, while 10 new ones emerged; overall, major stereotyped subsets had a cumulative frequency of 13.5%. Higher-level relationships were evident between subsets, particularly for major stereotyped subsets with unmutated IGHV genes (U-CLL), for which close relations with other subsets, termed 'satellites', were identified. Satellite subsets accounted for 3% of the entire cohort. These results confirm our previous notion that major subsets can be robustly identified and are consistent in relative size, hence representing distinct disease variants amenable to compartmentalized research with the potential of overcoming the pronounced heterogeneity of CLL. Furthermore, the existence of satellite subsets reveals a novel aspect of repertoire restriction with implications for refined molecular classification of CLL.

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Introduction Lymphoma patients are at increased risk of thromboembolic events (TE), however, thromboprophylaxis in these patients is largely under utilized. Actual guidelines recommend different models for thromboembolic risk estimation in cancer patients. Proposed models are of limited use in lymphoma patients as their development is not based on specific characteristics for this patient population. Previously, we developed and internally validated a simple model, based on individual clinical and laboratory patient characteristics that would classify lymphoma patients at risk for a TE. The variables independently associated with the risk for thromboembolism were: previous venous and/or arterial events, mediastinal involvement, BMI>30 kg/m2, reduced mobility, extranodal localization, development of neutropenia and hemoglobin level < 100g/L. For patients classified at risk in derivation cohort (n=1236), the model revealed positive predictive value of 25.1%, negative predictive value of 98.5%, sensitivity of 75.4%, and specificity of 87.5%. The diagnostic performance measures retained similar values in the internal validation cohort (n=584). The aim of this study was to perform external validation of the previously developed thrombosis lymphoma (Throly) score. Methods The study population included patients with a confirmed diagnosis of non-Hodgkin lymphoma (NHL), Hodgkin lymphoma (HL), and chronic lymphocytic leukemia (CLL)/ small lymphocytic lymphoma (SLL) from 8 lymphoma centers from USA, France, Spain, Croatia, Austria, Switzerland, Macedonia, and Jordan. During 2015 to 2016, data were prospectively collected for venous TE events from time of diagnosis to 3 months after the last cycle of therapy for newly diagnosed and relapsed patients who had completed a minimum of one chemotherapy cycle. The score development and validation were done according to TRIPOD suggested guidelines. Sensitivity analyses were carried out to test the model robustness to possible different settings, according to in/out patient settings and according to different countries included. Results External validation cohort included 1723 patients, similar to the developed group and consisted of 467 indolent NHL, 647 aggressive NHL, 235 CLL/SLL and 366 HL patients, out of which 121 (7%) patients developed venous thromboembolic events. For patients classified at risk in external validation cohort, the model resulted in positive and negative predictive values of 17% and 93%, respectively. Based on new available information from this large prospective cohort study this model was revised to include the following variables: diagnosis/clinical stage, previous VTE, reduced mobility, hemoglobin level < 100g/L and presence of vascular devices. In the new score we divided patients in two groups: low risk patients, score value ≤ 2; and high risk patients, score value > 2. For patients classified at risk by the revised model, the model produced positive predictive value of 22%, negative predictive value of 96%, sensitivity of 51%, and specificity of 72%. In sensitivity analysis, the final model proved its robustness in different settings of major importance for lymphoma patients. The final model presented good discrimination and calibration performance. Concordance C statistics was 0.794 (95% CI 0.750-0.837). Conclusions Revised Thrombosis Lymphoma - ThroLy score is more specific for lymphoma patients than any other available score targeting thrombosis risk in solid cancer patients. We included biological characteristic of lymphoma, indolent vs aggressive, as well as data about dissemination of disease, localized vs advanced stage, reflecting specificity of lymphomas comparing to other types of cancer. Also, we pointed out significance of central vascular devices as risk factor having considered the role of vascular damage during insertion as a potential trigger for activation of the clotting cascade. This score is user friendly for daily clinical practice and provides a very good predictive power to identify patients who are candidates for pharmacological thromboprophylaxis. Disclosures Cheson: AbbVie, Roche/Genentech, Pharmacyclics, Acerta, TG Therapeutics: Consultancy. Ghielmini:Roche: Consultancy, Honoraria, Research Funding, Speakers Bureau. Jaeger:Gilead: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding; Celgene: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees; Novartis: Membership on an entity's Board of Directors or advisory committees, Research Funding; AOP Orphan: Membership on an entity's Board of Directors or advisory committees; Roche: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding; Janssen: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees; MSD: Research Funding; Bioverativ: Membership on an entity's Board of Directors or advisory committees; AbbVie: Consultancy, Honoraria; Mundipharma: Membership on an entity's Board of Directors or advisory committees; Takeda-Millenium: Membership on an entity's Board of Directors or advisory committees; Takeda-Millenium: Membership on an entity's Board of Directors or advisory committees; Amgen: Membership on an entity's Board of Directors or advisory committees; GSK: Membership on an entity's Board of Directors or advisory committees; Infinity: Membership on an entity's Board of Directors or advisory committees.

5173 It is predicted that the inherited genetic background in the individual patients with myeloproliferative neoplasm (MPN) influences the disease susceptibility and the phenotype expression of the MPN. Recently, several groups suggested that JAK2V617F positive MPN are acquired preferentially on a specific constitutional germline JAK2 46/1 haplotype which is tagged by the “C” allele of single nucleotide polymorphism (SNP) rs12343867 (C/T), and designate the genetic basis for predisposition to MPN. They try to explain the pathomechanism for the acquisition of V617F mutation trough the “hypermutability hypothesis” suggesting as a main mechanism the genomic instability at the JAK2 locus. But, subsequent data, showed equal distribution of this SNP among JAK2V617F negative MPN, indicating that it is a potential common inherited susceptibility factor for MPN. Moreover, only few studies investigated the potential role of the JAK2 46/1 haplotype at the MPN phenotype in context of the clinical presentation and the complication of the diseases. In order to extend further those observations we conduct a retrospective study. First, we assess the frequency of JAK2 46/1 haplotype in a group of patients with MPN in comparison with population controls. As second we evaluate the association of 46/1 with the JAK2V617F mutational status and the clinical characteristics in the series of patients with different entities of MPN that were diagnosed and treated at the University Clinic of hematology-Skopje, Republic of Macedonia. The study group consisted of 212 adult (>15 years) patients with MPNs that were diagnosed and followed at the University Clinic of Hematology- Skopje. According to the 2008 World Health Organization criteria 79 patients were diagnosed as Polycythemia vera (PRV), 95 as Essential thrombocythemia (ET), 10 as Myelofibrosis primaria (MF) and 28 were classified as atypical MPN (aMPN). The 46/1 tag SNP rs12343867 (C/T) was genotyped using the TaqMan SNP genotyping assay (Applied Biosystems, Foster City, CA, USA) according to the manufacturer's instructions. The JAK2 V617F mutation was analyzed by fluorescent allele-specific PCR followed by CE on ABI 310 Genetic analyzer. The incidence of 46/1-linked C allele was significantly higher in all MPN entities [PRV (0.538), ET (0.437), MF (0,464), and in aMPN (0.55)] in comparison with healthy controls (0.290); (P<0,01 for all comparisons). The frequency of the JAK2V617F mutation ranged from 89%in PRV, 67% in ET, 60% in MF to 46,4% in the aMPN. The frequency of the JAK246/1 C allele was significantly higher in the JAK2V617F positive patients with PRV, MF and aMPN; (p<0,01 for all comparisons) except in ET patients, in which genotype distributions were similar among JAK2V617F positive and JAK2V617F negative patients (genotype: CC 7/14%, CT 22/29%, TT 67/57%; C-allele frequency 41/43%; p=0,76) Correlations of the clinical features at diagnosis and long-term prognosis between the two JAK2 46/1 different MPNs groups revealed comparability regarding all tested parameters such as blood counts, NAP score, rate of thrombotic and hemorrhagic complications, disease transformation and survival Our results confirmed latest observations that JAK2 46/1 haplotype is a susceptibility factor for developing ET independent of JAK2V617F mutational status. They also showed that the JAK2 46/1 haplotype does not affect the clinical course and prognosis of the different entities of MPN neoplasm. Our findings indicate that JAK2 46/1 haplotype predispose for development of MPN trough “the fertile ground hypothesis” which suggest that cells that are carrying the haplotype gain selective advantages in situations when oncogenic mutations occur. Disclosures: No relevant conflicts of interest to declare.

The mutational status of the immunoglobulin variable heavy chain genes (VH) is a major prognostic indicator in patients with CLL. More recently, additional prognostic categories have been identified by recognizing disease subsets which utilize unique VH genes. Examples include the V3-21 and V3-72 genes which are invariably associated with progressive and stable disease, respectively. The V1–69 gene is the most frequently rearranged VH gene in CLL and is almost always unmutated. We therefore investigated whether CLL patients that express this VH gene differ in clinical course and outcome with respect to CLL patients expressing other unmutated VH genes. The study group consisted of 106 consecutive CLLs that had been diagnosed at our institution prior to 2000. The 25 V1–69 cases (23.6%) were identified by allele-specific VH gene fingerprinting and all were found to be unmutated by nucleotide sequencing. Forty of the remaining cases were randomly selected for VH gene nucleotide sequence analysis; 19 cases expressed another unmutated VH gene, 14 cases expressed a mutated VH gene and the VH gene sequence could not be determined in 7 cases. The V1–69 cases did not differ significantly from cases with other unmutated VH genes with respect to age (median age 60 years for both subgroups), gender (male: female=21:4 for V1–69 cases and 13:6 for other VH unmutated cases, P=n.s.) and median follow-up (V1–69 cases: median 56 months, range 6 to 109; other VH unmutated cases: median 59 months, range 20 to 97, P=n.s.). The percentage of cases presenting at an advanced stage was higher in the V1–69 subgroup (36% of cases in Rai high risk category) than in the subgroup with other unmutated VH genes (5% of cases in Rai high risk category, P=0.027), whereas no significant difference was observed with respect to Ly counts and total tumor burden at diagnosis. Interestingly, 80% of the V1–69 cases compared to 42% of cases with other unmutated VH genes had received treatment immediately following diagnosis, but median time to treatment was not significantly different between the two subgroups (1 month vs. 9 months, respectively, P=0.085). To date 24 V1–69 cases (96%) and 15 cases with other unmutated VH genes (79%) have died, with median survival times of 56 and 60 months, respectively (P=0.9). For comparison, median survival of the 14 VH mutated cases from this series was 125 months (P&lt;0.0001 with respect to both subgroups with unmutated VH genes). We therefore conclude that chronic lymphocytic leukemia patients with a V1–69 gene rearrangement constitute a uniform group with unfavorable prognosis, but their survival does not differ significantly from patients with other unmutated VH genes. Figure Figure