RIT1 mutations in myeloid neoplasms

Abstract Background: Constitutive activation of the RAS/MAPK cascade due to mutations in canonical RAS-family proteins, upstream tyrosine kinases, or downstream regulators is a hallmark of myeloid malignancies. Ras-like without CAAX 1 (RIT1) is a non-canonical RAS GTPase, with germline mutations accounting for 5–10% of Noonan syndrome cases. Somatic RIT1 mutations have also been reported in myeloproliferative neoplasms (MPNs) and myelodysplastic syndromes (MDS); however, their pathologic and clinical implications remain poorly defined. Methods: We queried the University of Pennsylvania's institutional archive of ~16,000 blood, bone marrow aspirate, and paraffin-embedded tissue samples tested for hematologic malignancy-associated mutations from April 2015 - June 2025 to identify cases of myeloid neoplasms with pathogenic/likely pathogenic or uncertain significance variants in RIT1. All included variantswere confirmed to be somatic based on time of acquisition, low variant allele fraction (VAF) or VAF decline in remission. Subjects were annotated for diagnosis, treatment, death, and disease progression (for acute myeloid leukemia (AML) - relapsed or refractory disease; for MDS - development of MDS with excess blasts (MDS-EB1, -EB2) or AML; for MPN - evolution to myelofibrosis (MF) or AML; and for chronic myelomonocytic leukemia (CMML) - progression to CMML-2 or AML). Co-mutations and outcomes were summarized by disease type and overall. We built a Cox proportional hazards model controlling for age and diagnosis, with RIT1 acquisition as a time-varying covariate, to test the association between RIT1 acquisition and progression or death. Results: Somatic RIT1 variants were identified in 38 subjects (10 AML, 10 CMML, 8 MPN, 6 MDS, and 4 MDS/MPN), with a median age of 62 years (range: 33–85) and a median duration of follow-up of 2.2 years (range: 0.3–21.4 years). No subjects had clinical features of Noonan syndrome. Of the 29 subjects tested for RIT1 at presentation, 18 had RIT1 variants identified at diagnosis, while 11 had them later in the disease course. Median RIT1 variant allele fraction (VAF) at diagnosis was 11.5% (range: <7%–50%); those acquiring RIT1 did so at a median of 1.1 years (range: 0.2–7.1 years), with a median VAF of 21% (range: < 7–45%). In all subjects, a single RIT1 variant was detected, with 33/38 (86.8%) involving hotspots within the switch II region of the GTPase domain: A77T (n=2), E81G (n=2), F82L/I/C/V (n=17), and M90I/V (n=12). Common co-occurring mutations throughout the duration of follow up included ASXL1 (n= 24 patients, 61.5%), SRSF2 (n=18, 46.2%), and TET2 (n=13, 33.3%). Other RAS/MAPK signaling pathway genes—including NRAS (n=8), KRAS (n=2), PTPN11 (n=3), and CBL (n=5)—were co-mutated in 18 patients (46.2%). Eight subjects (20.5%) had co-occurring JAK2 V617F mutations (4 with MPN, 2 with CMML, and 2 with MDS). Of the 8 MPN subjects (6 with myelofibrosis and 2 with polycythemia vera), 4 had JAK2 mutations (2 of which diminished with the rise of the RIT1-mutant clone), 1 had a CALR mutation, and 3 had triple-negative disease. A pattern of combined epigenetic (ASXL1 or TET2) and SRSF2 mutations at time of RIT1 detection predominated in CMML (7/10) and was also observed in AML (3/10), MDS (2/6), and MPN (3/8). Median time to progression or death in subjects exhibiting this pattern was shorter (1.1 vs 3.2 years, log-rank p = 0.021). Amongst the cohort with available baseline RIT1 testing, acquisition of RIT1 as a time-dependent covariate was not significantly associated with increased hazard of progression or death (HR 1.1; CI 0.8–10.8; p = 0.10). 25 of 38 subjects died (66%) at a median time of 2.2 years (range 0.1 – 20.1 years). These were predominantly patients with higher-grade neoplasms (MDS, CMML, AML). Cases where RIT1 occurred early without other co-occurring variants had a more indolent course resembling chronic-phase MPN. Conclusion: RIT1 variants span the myeloid malignancy spectrum, often arising in molecular contexts analogous to canonical RAS variants. Co-mutational profiles mirrored those associated with RAS-driven disease, including epigenetic and splicing events (ASXL1, TET2, and SRSF2) and late clonal drivers (RIT1), and were associated with poorer prognosis. Early acquisition of RIT1 without co-mutations appears to confer a more indolent course, potentially representing a molecularly distinct subset of triple-negative MPN-like disease and will be subject of future study.