RNA and protein components of the spliceosome work together to identify the 5΄ splice site, the 3΄ splice site, and the branchsite (BS) of nascent pre-mRNA. SF3b1 plays a key role in recruiting the U2 snRNP to the BS. Mutations in human SF3b1 have been linked to many diseases such as myelodysplasia (MDS) and cancer. We have used SF3b1 mutations associated with MDS to interrogate the role of the yeast ortholog, Hsh155, in BS selection and splicing. These alleles change how the spliceosome recognizes the BS and alter splicing when nonconsensus nucleotides are present at the −2, −1 and +1 positions relative to the branchpoint adenosine. This indicates that changes in BS usage observed in humans with SF3b1 mutations may result from perturbation of a conserved mechanism of BS recognition. Notably, different HSH155 alleles elicit disparate effects on splicing: some increase the fidelity of BS selection while others decrease fidelity. Our data support a model wherein conformational changes in SF3b1 promote U2 association with the BS independently of the action of the DEAD-box ATPase Prp5. We propose that SF3b1 functions to stabilize weak U2/BS duplexes to drive spliceosome assembly and splicing.

The eukaryotic spliceosome is composed of five small nuclear ribonucleoproteins (snRNPs) and many other associated protein splicing factors. The human U2 snRNP‐associated protein, SF3B1, allows the U2 snRNP to associate with and define the branchpoint region of precursor‐messenger RNA (pre‐mRNA). Recent evidence has shown that mutations to SF3B1 are linked to a variety of different myelodysplastic syndromes (MDS). These mutations are found in regions of high conservation between SF3B1 and the yeast homolog, Hsh155p. We have constructed a genetic system in S. cerevisiae to interrogate the effects of these mutations on splicing and cell proliferation. We used plasmid shuffling to introduce Hsh155p mutant proteins into a HSH155Δ yeast strain and growth assays to assess the impact of the mutations on cell proliferation. Several of the mutations surveyed thus far (H331D, K335N, P369E, D450G) have no obvious effect on yeast growth despite being correlated with MDS in humans. This may imply that the effects of these mutations are subtle and do not allow splicing to become limiting for yeast growth. To further understand the effects of these mutations, a Reverse Transcription‐Polymerase Chain Reaction (RT‐PCR) analysis will be used to compare pre‐mRNA and mRNA levels of several transcripts from Hsh155p mutant strains relative to the WT. The results from these experiments will be critical for establishing yeast as a model system for studying MDS. This research has been supported by startup funds from the U. Wisconsin‐Madison, WARF, and the Department of Biochemistry.

BACKGROUND Cerebral venous sinus thrombosis (VST) is a complication of head injury and can be secondary to sinus compression by depressed skull fractures. Fracture elevation is a treatment option for VST secondary to extrinsic compression, but conservative management may also be effective. Venous sinuses can also be lacerated from skull fractures, resulting in epidural or subdural hematomas. The authors presented a case of sagittal sinus injury and thrombosis from a depressed skull fracture that caused a subgaleal hematoma. The injury was successfully managed conservatively. OBSERVATIONS A 14-year-old boy presented after a head injury with a diastatic, depressed parietal bone fracture. Computed tomography venogram showed disruption and occlusion of the superior sagittal sinus with a subgaleal hematoma in continuity with the injured sagittal sinus. Because of concern for hemorrhage if tamponade on the sinus was removed, the patient was treated nonsurgically. At follow-up, the sinus had recanalized and the fracture had healed. LESSONS Skull fractures with underlying sinus thrombosis can be managed conservatively with good outcome. Careful assessment for venous sinus injury should be made before undertaking fracture elevation to relieve sinus compression.