C054 | C-CBL MUTATIONS AT RESIDUE 371 CONFER ADVERSE PROGNOSIS IN MYELODYSPLASTIC SYNDROMES AND CHRONIC MYELOMONOCYTIC LEUKEMIA

Background: In myeloid neoplasm, c-CBL missense mutations, affecting linker region and zinc finger domains (LR/ZF, 352-420 residues), impair the ubiquitination function while preserving PI3K/AKT activation, and are linked to a myeloproliferative phenotype. Among hits occurring at LR/ZF domains, X-ray diffraction analysis of c-CBL protein structure showed that mutations at residue 371 can result in different conformational changes (Kim et al. 2024), possibly affecting disease phenotypes. Thus, we focus our analysis on carriers of these hits. Methods: By analyzing 3301 MDS and 622 MDS/MPN we identified a cohort of 83 patients harboring LR/ZF c-CBL mutations (33 MDS [1%], 50 MDS/MPN [8%]; 93 hits) from Cleveland Clinic, University of Rome Tor Vergata, and published sources. Results: When we compared patients with 371mut residue (11 patients) vs other LR/ZF hits (70 patients; 2 carried both mutation types), we observed, in the first cohort, younger age (71vs75 years, p=0.0344) and no differences in terms of complete blood count or blast percentage. Furthermore, the first cohort was enriched in high-risk karyotypic abnormalities (del(7)/del(7q): 18vs1%, p=0.014; del(17): 9vs0%, p=0.014) and myeloproliferative and unfavorable mutations (CALR: 9vs0%, p=0.011; KIT: 18vs3%, p=0.029; SETBP1: 36vs7%, p=0.004; TP53: 18vs1%, p=0.006; Figure 1A-B). Among 14 patients with LR/ZF mutations with evaluable response to hypomethylating agents, CR rate was 22.2% vs 0% in 371mut patients (3 patients). Furthermore, c-CBL-371mut showed a higher VAF-based clonal hierarchy (ancestral hits in 63% vs 25%, p=0.0312). Survival analysis, including a comparator cohort of 1209 c-CBLwt MDS-MDS/MPN, showed a correlation between c-CBL-371mut and dismal outcome independently of age, IPSS-M and diagnosis (p=0.027; HR:2.744, 95%CI: 1.119-6.732; Figure 1C). As proof of concept, to investigate the biological underpinning of these differences, we utilized BeatAML (Tyner et al. 2018) public database and compared transcriptome analysis of patients harboring c-CBL 371 mutation (1 patient) vs LR/ZF mutation (1 patient harboring p.R420Q mutation) vs FLT3 mutation (Figure 1D). Remarkably, FLT3- and c-CBL 371mut cases exhibited similar expression of genes involved in proliferation pathways (JAK2, c-CBL, FLT3, LYN, FYN, STAT5B, AKT1, MTOR, KRAS, NRAS), enhanced when compared to the one harboring c-CBL 420mut. Conclusions: Our study suggests that c-CBL mutations occurring at 371 confer a negative impact on prognosis.