Lizhen Wu

Somatic hypermutation (SHM) creates point mutations in the variable regions of immunoglobulin genes in activated B cells to support antibody affinity maturation¹. SHM is initiated by activation-induced deaminase (AID), and the single-stranded DNA substrate of this cytosine deaminase is thought to arise during RNA polymerase II (PolII) transcription, perhaps in the context of PolII stalling^2,3. The factors and mechanisms that mediate AID chromatin recruitment and generate its putative stalled PolII substrate during SHM are unknown. Negative elongation factor (NELF) stabilizes PolII in a paused state near the transcription start site. Release of NELF from PolII and the PolII elongation-versus-termination decision are regulated by opposing super elongation complex (SEC) kinase and integrator–protein phosphatase 2A (INT–PP2A) activities⁴. Here we demonstrate that NELF, the SEC components MLLT1 and MLLT3 and the INT–PP2A phosphatase module are required for SHM. We show that NELF can associate with PolII in the AID target window in the gene body and that, in the absence of NELF, AID fails to deaminate its targets despite efficient recruitment to chromatin and transcription by PolII. MLLT1 and MLLT3 interact with AID through their histone-binding YEATS domain and are required for AID enrichment at its target sites. We propose a physiological function for the NELF–PolII association in gene bodies in creating the stalled PolII substrate for AID action during SHM.

Background: Recently, several studies have examined the association between preoperative sarcopenia and prognosis evaluation in patients with hepatocellular carcinoma (HCC) undergoing hepatectomy. However, their conclusions remain ambiguous and controversial. Thus, we conducted a meta-analysis to assess the prognostic role of preoperative sarcopenia in patients with HCC undergoing hepatectomy. Methods: We searched the existing literature reporting on the prognostic value of preoperative computed tomography (CT)-assessed sarcopenia for the survival of patients with HCC undergoing hepatectomy. The pooled hazard ratios (HRs) and corresponding 95% confidence intervals (CIs) of overall survival (OS) and disease-free survival (DFS) were used to assess the prognostic value of preoperative sarcopenia in HCC patients. The associations between preoperative sarcopenia and clinicopathological characteristics were also evaluated. Results: A total of six studies with 1,420 patients (including 458 sarcopenia and 962 non-sarcopenia patients) were included in the meta-analysis. The results showed that preoperative sarcopenia was significantly associated with poor OS (HR =1.572, 95% CI: 1.342–1.840, P=0) and shorter DFS (HR =1.544, 95% CI: 1.178–2.024, P=0.002) in patients with HCC undergoing hepatectomy. Preoperative sarcopenia was also significantly related to larger diameter tumors (WMD =0.598, 95% CI: 0.216–0.980, P=0.002). The results of the sensitivity analysis were stable in this meta-analysis. Egger’s tests revealed that there was no significant publication bias. Conclusions: Sarcopenia appears to have significant adverse impacts on postoperative outcomes in patients with hepatocellular carcinoma following hepatectomy. However, further large-scale prospective studies are needed to confirm our findings.

Lipid incorporation from endoplasmic reticulum (ER) to lipid droplet (LD) is important in controlling LD growth and intracellular lipid homeostasis. However, the molecular link mediating ER and LD cross talk remains elusive. Here, we identified Rab18 as an important Rab guanosine triphosphatase in controlling LD growth and maturation. Rab18 deficiency resulted in a drastically reduced number of mature LDs and decreased lipid storage, and was accompanied by increased ER stress. Rab3GAP1/2, the GEF of Rab18, promoted LD growth by activating and targeting Rab18 to LDs. LD-associated Rab18 bound specifically to the ER-associated NAG-RINT1-ZW10 (NRZ) tethering complex and their associated SNAREs (Syntaxin18, Use1, BNIP1), resulting in the recruitment of ER to LD and the formation of direct ER–LD contact. Cells with defects in the NRZ/SNARE complex function showed reduced LD growth and lipid storage. Overall, our data reveal that the Rab18-NRZ-SNARE complex is critical protein machinery for tethering ER–LD and establishing ER–LD contact to promote LD growth.