The U1A/U2B″/SNF family of proteins found in the U1 and U2 spliceosomal snRNPs is highly conserved. In spite of the high degree of sequence and structural conservation, modern members of this protein family have unique RNA binding properties. These differences have necessarily resulted from evolutionary processes, and therefore we reconstructed the protein phylogeny in order to understand how and when divergence occurred, and how protein function has been modulated. Contrary to the conventional understanding of an ancient human U1A/U2B″ gene duplication, we show that the last common ancestor of bilaterians contained a single ancestral protein (URB). The gene for URB was synthesized and the protein was overexpressed, purified, and we assessed RNA binding modern snRNA sequences. We find that URB binds human and Drosophila U1 snRNA SLII and U2 snRNA SLIV with higher affinity than do modern homologs, suggesting that both Drosophila SNF and human U1A/U2B″ have evolved into weaker binders of one or both RNAs.
SNF is a protein that is found in the U1 and U2 snRNPs (small nuclear ribonucleoproteins) of Drosophila. Its mammalian counterparts are two homologous proteins, U1A and U2B''. In vivo, these proteins segregate to the U1 and U2 snRNPs, respectively, where they bind distinct RNA hairpins. The RNA binding properties and mechanism of U1A have been studied extensively, but much less is known about SNF and U2B'' binding to their RNA targets. By comparing thermodynamic aspects of SNF-RNA interactions with those of U1A-RNA interactions, we find that SNF binds its RNA targets in a manner that is distinct from that of U1A. In vitro, SNF is able to bind both Drosophila U1 stem-loop II and U2 stem-loop IV with high affinity, although it binds stem-loop II more tightly than it binds stem-loop IV. Intriguingly, SNF is unable to bind human U2 stem-loop IV, which suggests that both the protein and RNAs have coevolved to interact with each other such that a single protein can bind RNAs that are more commonly bound by two distinct proteins.
The human U2B” protein is one of the unique proteins that comprise the U2 snRNP, but it is also a representative of the U1A/U2B” protein family. In the U2 snRNP, it is bound to Stem-Loop IV (SLIV) of the U2 snRNA. We find that in vitro it binds not only to human SLIV, but also to Stem-Loop II (SLII) from human U1 snRNA and to Drosophila U2 snRNA SLIV. The thermodynamics of these binding interactions show a striking similarity, leading to the conclusion that U2B” has a relaxed specificity for its RNA targets. The binding properties of U2B” are distinct from those of human U1A and of Drosophila SNF, despite its high homology to those proteins, and so provide important new information on how this protein family has modulated its target preferences.
IMPORTANCE Immune checkpoint inhibitors (ICIs) have transformed the treatment paradigm for an ever-increasing number of cancers. However, their use has also led to the emergence of immunerelated adverse events, such as ICI-induced inflammatory arthritis. A reproducible, reliable, and accessible modality is needed to assess and distinguish early ICI-induced inflammatory arthritis and help in management. Magnetic resonance imaging (MRI) of joints may be helpful for early diagnosis, guiding therapeutic decision-making, and identifying patients at high risk for erosive disease.OBJECTIVE To assess the role of MRI of joints in patients with ICI-induced inflammatory arthritis. DESIGN, SETTING, AND PARTICIPANTSThis retrospective case series included patients enrolled at the National Institutes of Health Clinical Center in Bethesda, Maryland. Patients were evaluated by the rheumatology consultation service between December 27, 2016, and May 28, 2019. A retrospective health record review was performed to determine demographic characteristics, clinical characteristics of inflammatory arthritis and malignant tumors, and imaging findings. Inclusion criteria were patients who were enrolled on various institutional review board-approved protocols of ICIs, developed joint-related symptoms, and had MRI data for at least 1 joint. Data were analyzed from EXPOSURES Undergoing MRI of at least 1 joint. MAIN OUTCOMES AND MEASURESAll MRIs were reviewed for synovitis, tenosynovitis, bone marrow edema, and soft tissue conditions. RESULTS A total of 8 patients (mean [SD] age, 58.8 [5.2] years; 6 women and 2 men) between the ages of 50 and 65 years who were undergoing ICI therapy for a variety of malignant tumors were included in this study. Only 1 patient was receiving combined ICI therapy. The results of 13 separate MRI examinations were reviewed. The most commonly performed MRIs were of the hands and wrists (9 MRIs), followed by knee examinations (3 MRIs). Tenosynovitis and synovitis were frequently seen in the hands and wrists. Bone marrow edema and erosions were also found in 3 patients, suggesting early damage. In larger joints (ie, knees and ankles), joint effusions and synovial thickening were characteristic. Most patients (5 patients) were treated with corticosteroids and had good responses.In patients with high-risk features on MRI imaging (eg, bone marrow edema, erosions), diseasemodifying antirheumatic drug therapy was also discussed as a treatment option.CONCLUSIONS AND RELEVANCE These findings suggest that advanced imaging may help to distinguish ICI-induced inflammatory arthritis from other causes of joint pain, aid in identifying (continued)
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