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RIT1 is a RAS guanosine triphosphatase (GTPase) that regulates different aspects of signal transduction and is mutated in lung cancer, leukemia, and in the germline of individuals with Noonan syndrome. Pathogenic RIT1 proteins promote mitogen-activated protein kinase (MAPK) hyperactivation; however, this mechanism remains poorly understood. Here, we show that RAF kinases are direct effectors of membrane-bound mutant RIT1 necessary for MAPK activation. We identify critical residues in RIT1 that facilitate interaction with membrane lipids and show that these are necessary for association with RAF kinases and MAPK activation. Although mutant RIT1 binds to RAF kinases directly, it fails to activate MAPK signaling in the absence of classical RAS proteins. Consistent with aberrant RAF/MAPK activation as a driver of disease, we show that pathway inhibition alleviates cardiac hypertrophy in a mouse model of RIT1 mutant Noonan syndrome. These data shed light on the function of pathogenic RIT1 and identify avenues for therapeutic intervention.

The majority of pathogenic mutations in the neurofibromatosis type I (NF1) gene reduce total neurofibromin protein expression through premature truncation or microdeletion, but it is less well understood how loss-of-function missense variants drive NF1 disease. We have found that patient variants in codons 844 to 848, which correlate with a severe phenotype, cause protein instability and exert an additional dominant-negative action whereby wild-type neurofibromin also becomes destabilized through protein dimerization. We have used our neurofibromin cryogenic electron microscopy structure to predict and validate other patient variants that act through a similar mechanism. This provides a foundation for understanding genotype-phenotype correlations and has important implications for patient counseling, disease management, and therapeutics.

RAS GTPases are highly conserved proteins involved in the regulation of mitogenic signaling. We have previously described a novel Cullin 3 RING E3 ubiquitin ligase complex formed by the substrate adaptor protein LZTR1 that binds, ubiquitinates, and promotes proteasomal degradation of the RAS GTPase RIT1. In addition, others have described that this complex is also responsible for the ubiquitination of classical RAS GTPases. Here, we have analyzed the phenotypes of Lztr1 loss-of-function mutants in both fruit flies and mice and have demonstrated a biochemical preference for their RIT1 orthologs. Moreover, we show that Lztr1 is haplosufficient in mice and that embryonic lethality of the homozygous null allele can be rescued by deletion of Rit1. Overall, our results indicate that, in model organisms, RIT1 orthologs are the preferred substrates of LZTR1.

The spindle assembly checkpoint (SAC) functions as a sensor of unattached kinetochores that delays mitotic progression into anaphase until proper chromosome segregation is guaranteed.1,2 Disruptions to this safety mechanism lead to genomic instability and aneuploidy, which serve as the genetic cause of embryonic demise, congenital birth defects, intellectual disability, and cancer.3,4 However, despite the understanding of the fundamental mechanisms that control the SAC, it remains unknown how signaling pathways directly interact with and regulate the mitotic checkpoint activity. In response to extracellular stimuli, a diverse network of signaling pathways involved in cell growth, survival, and differentiation are activated, and this process is prominently regulated by the Ras family of small guanosine triphosphatases (GTPases).5 Here we show that RIT1, a Ras-related GTPase that regulates cell survival and stress response,6 is essential for timely progression through mitosis and proper chromosome segregation. RIT1 dissociates from the plasma membrane (PM) during mitosis and interacts directly with SAC proteins MAD2 and p31comet in a process that is regulated by cyclin-dependent kinase 1 (CDK1) activity. Furthermore, pathogenic levels of RIT1 silence the SAC and accelerate transit through mitosis by sequestering MAD2 from the mitotic checkpoint complex (MCC). Moreover, SAC suppression by pathogenic RIT1 promotes chromosome segregation errors and aneuploidy. Our results highlight a unique function of RIT1 compared to other Ras GTPases and elucidate a direct link between a signaling pathway and the SAC through a novel regulatory mechanism.

RAS proteins are molecular switches that interact with effector proteins when bound to guanosine triphosphate, stimulating downstream signaling in response to multiple stimuli. Although several canonical downstream effectors have been extensively studied and tested as potential targets for RAS-driven cancers, many of these remain poorly characterized. In this study, we undertook a biochemical and structural approach to further study the role of Sin1 as a RAS effector. Sin1 interacted predominantly with KRAS isoform 4A in cells through an atypical RAS-binding domain that we have characterized by X-ray crystallography. Despite the essential role of Sin1 in the assembly and activity of mTORC2, we find that the interaction with RAS is not required for these functions. Cells and mice expressing a mutant of Sin1 that is unable to bind RAS are proficient for activation and assembly of mTORC2. Our results suggest that Sin1 is a bona fide RAS effector that regulates downstream signaling in an mTORC2-independent manner.

RIT1 oncoproteins have emerged as an etiologic factor in Noonan syndrome and cancer. Despite the resemblance of RIT1 to other members of the Ras small guanosine triphosphatases (GTPases), mutations affecting RIT1 are not found in the classic hotspots but rather in a region near the switch II domain of the protein. We used an isogenic germline knock-in mouse model to study the effects of RIT1 mutation at the organismal level, which resulted in a phenotype resembling Noonan syndrome. By mass spectrometry, we detected a RIT1 interactor, leucine zipper-like transcription regulator 1 (LZTR1), that acts as an adaptor for protein degradation. Pathogenic mutations affecting either RIT1 or LZTR1 resulted in incomplete degradation of RIT1. This led to RIT1 accumulation and dysregulated growth factor signaling responses. Our results highlight a mechanism of pathogenesis that relies on impaired protein degradation of the Ras GTPase RIT1.

Aim: To evaluate the efficacy of a fixed‐ratio combination of insulin glargine 100 U/mL plus lixisenatide (iGlarLixi) in people with type 2 diabetes (T2D) using derived time‐in‐range (dTIR). Methods: Participant‐level data from LixiLan‐L, LixiLan‐O and LixiLan‐G were pooled and dTIR (70‐180 mg/dL), derived time‐above‐range (> 180 mg/dL) and derived time‐below‐range (dTBR; < 70 mg/dL) were calculated from participant seven‐point self‐monitored blood glucose profiles. Results: This pooled analysis included data from 2420 participants receiving iGlarLixi (n = 1093), iGlar (n = 836), Lixi (n = 234) or a glucagon‐like peptide‐1 receptor agonist (GLP‐1 RA) (n = 257). Numerically greater improvements in least square (LS) means dTIR were seen from baseline to end of treatment (EOT) with iGlarLixi (25.7%) versus iGlar (15.8%), Lixi (11.7%) or GLP‐1 RA (16.2%). At EOT, the mean (standard deviation) dTBR was 0.71% ± 3.4%, 0.61% ± 3.2%, 0.08% ± 1.0% and 0.0% ± 0.0% for iGlarLixi, iGlar, Lixi and GLP‐1 RA, respectively. In a subgroup analysis, participants aged younger than 65 years (n = 1690) and 65 years or older (n = 713) showed numerically greater improvements in LS means dTIR from baseline to EOT with iGlarLixi versus iGlar, Lixi or GLP‐1 RA. Conclusions: iGlarLixi achieved improvements in dTIR, with low dTBR values, providing further evidence to inform clinical outcomes with the use of iGlarLixi. [ABSTRACT FROM AUTHOR]

PURPOSE: Hospice is underutilized by patients with hematologic malignancies (HM), and when patients are referred, they are typically more ill, hospitalized, and with shorter length of stay (LOS) than patients with solid tumors (ST), limiting research about home hospice care experiences of patients with HM. In this mixed-methods study, we examined the hospice experiences of patients with HM who died at residential care homes (RCHs), home-based settings in which volunteer caregivers and hospice staff provide end-of-life (EOL) care under the social hospice model. METHODS: We queried a registry of 535 hospice patients who died at RCHs between 2005 and 2020 that included quantitative medication administration data as well as qualitative data from hospice intake forms and written volunteer caregiver narratives. Qualitative data were analyzed by collective case study methodology. Quantitative comparisons of LOS and liquid morphine use were performed with matched patients with ST. RESULTS: The registry yielded 29 patients with HM, of whom qualitative data were available for 18 patients. Patients with HM exhibited common EOL symptoms (pain, dyspnea, and agitation). Instances of bleeding were low (22%), and notable HM-specific care concerns were described regarding bone fractures, skin integrity, and delirium. Most (78%) experienced good symptom management and peaceful or comfortable deaths. In only one case were symptoms described as severe and poorly managed. Patients with HM had comparable LOS on hospice and at the RCHs to patients with ST, with no group differences in liquid morphine use. CONCLUSION: In this registry cohort, most patients with HM achieved good symptom management in home care settings with volunteer caregivers and hospice support. Caregivers may require additional counseling and palliative medications for HM-specific EOL symptoms. Patients with blood cancers in a home care hospice setting have unique end-of-life experiences. [ABSTRACT FROM AUTHOR]