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Introduction: Cytogenetic changes occur in 50% of patients (pts) with Myelodysplastic Syndromes (MDS). Complex aberrations (cA, = 3 or more) are associated with a very poor outcome. In about 50% of the cases with cA aberrations of the TP53 locus are detectable. Those pts show an even worse outcome with a significantly shortened median overall survival (OS) compared to pts with wildtype TP53 (wtTP53). One of the most common cytogenetic aberrations in MDS is an interstitial deletion of the long arm of chromosome 5 (5q). As an isolated aberration, it is associated with a rather favorable prognosis. As part of a cA, 5q deletions however are assumed to even worsen the prognosis further. We wanted to find out in which prevalence 5q deletions and TP53 changes appear together and how those two factors in combination or not influence the OS of pts with MDS and cA. Methods: 218 pts with MDS or sAML and cA were identified and extensively characterized. 126 of them were diagnosed with MDS, 89 with sAML and 3 with CMML. Cytogenetic analysis by chromosome banding (CBA) and fluorescence in situ hybridization (FISH) of the TP53 locus on 17p as well as sequencing of TP53 either by Sanger or by Next Generation Sequencing was available for all pts. Multicolour FISH (mFISH) was available for 146 pts, SNP array analysis for 42 pts. The median number of cytogenetic aberrations was 8 (range 3-50). At the time of first diagnosis with cA the median age was 72 (range: 29-95). Median OS of the entire cohort was 10.7 months (95% CI: 8.0-16.4). Results: In 146 of 218 pts we found alterations of TP53: a single hit mutation in 32 pts, a single deletion in 22 pts, a combined mutation and deletion in 67 pts and more than 1 mutation in 25 pts. The OS of those 146 pts was 6.6 months compared to 22 months of the pts with wtTP53 (p-value Conclusion: Mutations and/or deletions of TP53 show a strong association with del(5q). Both were frequent in our cohort of 218 pts with MDS and cA. There also was a large intersection of 130 pts with both del(5q) and TP53 alteration. The combination of both changes seems to further worsen the already poor prognosis of pts with MDS and cA. Our observation that those two factors appear together frequently supports the hypothesis that the presence of del(5q) may promote the acquisition of cA. This is in accordance with Hsu´s hypothesis that in small clones with a mono-allelic TP53 mutation a del(5q) may favor the loss of heterozygosity of TP53 which could in a next step lead to a higher complexity of cytogenetic aberrations (Hsu et al, 2019). It is remarkable that the presence of del(5q) in combination with a single hit status of TP53 confers the same bad prognosis compared to multi hit TP53 status (figure 1).We will continue analyzing pts with MDS and cA to examine the influence of different TP53 and 5q alterations on the prognosis, the disease progression and median OS of those pts with cA. Figure 1 Disclosures Platzbecker: Novartis: Consultancy, Honoraria, Research Funding; Amgen: Honoraria, Research Funding; Geron: Consultancy, Honoraria; Takeda: Consultancy, Honoraria; Janssen: Consultancy, Honoraria, Research Funding; AbbVie: Consultancy, Honoraria; BMS: Consultancy, Honoraria.

Introduction: Complex aberrant karyotypes (CK, ≥3 cytogenetic aberrations, CA) are associated with an unfavorable prognosis and an increased AML transformation rate in MDS. However, even MDS with CK (CK-MDS) are heterogeneous in terms of genetic profile and prognosis. Recently, we demonstrated that a high number of CA as well as mutations in TP53 (TP53mut) are associated with increased risk in CK-MDS (Haase et al, 2019). However, as there is a strong association between CK-MDS and TP53mut, it is still a matter of debate whether the karyotype and TP53mut are prognostically independent genetic markers. Furthermore, loss of heterozygosity (LOH) of 17p13 (TP53LOH), due to loss of genetic material or to copy number neutral LOH (CN-LOH), is also associated with a poor prognosis. We here aimed to characterize TP53mut andTP53LOH in CK-MDS and to elucidate the impact of cytogenetics, TP53mut and TP53LOH on the outcome of CK-MDS. Methods: We included 178 pts with MDS (N=138), CMML (N=5) and secondary AML after MDS (AML with myelodysplasia-related changes, N=35), all with CK. The median precentage of bone marrow (bm) blasts was 11% (range: 0-90%). The median age was 72 yrs (range: 30-95 yrs). The male:female ratio was 1.23:1. The number of CA was determined by banding analysis in all cases. The karyotype was confirmed by multicolor FISH in 134 cases. TP53LOH was verified by FISH analysis of the TP53 locus in 17p13 (146 analyses) and/or molecular karyotyping (MK, 41 analyses). In 144 cases further FISH probes in addition to TP53 were used. TP53mut was identified by NGS (54 cases) or Sanger sequencing (124 cases). Follow-up data for survival analyses were available for 127 pts with MDS and oligoblastic AML with less than 30% bm blasts. Results: The median number of CA was 7 (range: 3-46), 98/178 pts (55%) showed a TP53mut (median VAF: 34%, range: 8-93%) and 64/178 (36%) a TP53LOH (median FISH clone size: 65%, range: 6-99%), including 9 pts with a CN-LOH in 17p13. The CN-LOH was either identified by MK (5/41 pts (12%) where MK was available showed a CN-LOH, 4/5 with TP53mut) or by NGS (4/54 pts (7%) where NGS was available showed a VAF >70% and normal TP53-FISH). In total, a TP53mut and/or a TP53LOH was identified in 116/178 pts (65%). Overall survival (OS) did not significantly differ between CK-MDS with TP53mut only, TP53LOH only, and TP53mut+TP53LOH (Fig.1). Therefore, we merged TP53mut and TP53LOH to TP53altered in all further analyses. Regarding the cytogenetic characterization of pts with TP53altered, the number of CA was significantly higher in pts with TP53altered than in pts with normal TP53 (median 9 CA (range: 3-46) vs 5 CA (range: 3-24), P The number of CA as well as the TP53 status contributed significantly to OS (Fig.2). The presence of anemia (Hb Conclusions: The presence of ≥5 CA is associated with reduced OS in CK-MDS. A TP53mut as well as a TP53LOH both further segregate outcome. The impact of the clone size of TP53mut and TP53LOH on survival is currently being evaluated. Our data imply that the TP53 status (TP53mut and/or TP53LOH) and the complexity of the karyotype are independent prognostic markers. Based on the presence of anemia, the TP53 status (TP53mut and/or TP53LOH), and the number of CA, the individual risk of CK-MDS can be estimated more accurately. This will allow to better tailor treatment decisions for individual pts with CA. Funding (FS): 2017 SGR 288-GRC Disclosures Germing: Jazz Pharmaceuticals: Honoraria; Novartis: Honoraria, Research Funding; Amgen: Honoraria; Celgene: Honoraria, Research Funding. Kaivers:Jazz Pharmaceuticals: Other: Travel Support. Kröger:Celgene: Honoraria, Research Funding; DKMS: Research Funding; JAZZ: Honoraria; Medac: Honoraria; Neovii: Honoraria, Research Funding; Novartis: Honoraria, Research Funding; Riemser: Research Funding; Sanofi-Aventis: Research Funding. Hertenstein:RS Media: Research Funding. Döhner:Daiichi: Honoraria; Jazz: Honoraria; Novartis: Honoraria; Celgene: Honoraria; Janssen: Honoraria; CTI Biopharma: Consultancy, Honoraria. Bug:Hexal: Membership on an entity's Board of Directors or advisory committees; Celgene Neovii: Other: travel grant; Novartis: Membership on an entity's Board of Directors or advisory committees, Research Funding; Gilead Sciences: Membership on an entity's Board of Directors or advisory committees, Other: Travel grants; Sanofi: Other: travel grants; Celgene: Honoraria, Membership on an entity's Board of Directors or advisory committees, Other: travel grants; Jazz Pharmaceuticals: Honoraria; Pfizer: Membership on an entity's Board of Directors or advisory committees. Nickelsen:Roche Pharma AG: Membership on an entity's Board of Directors or advisory committees, Other: Travel Grants; Celgene: Membership on an entity's Board of Directors or advisory committees, Other: Travel Grant; Janssen: Membership on an entity's Board of Directors or advisory committees. Platzbecker:Celgene: Consultancy, Honoraria; Abbvie: Consultancy, Honoraria; Novartis: Consultancy, Honoraria.